Steep-sloped and symmetrical Mayon has recorded historical eruptions back to 1616 that range from Strombolian fountaining to basaltic and andesitic flows, as well as large ash plumes, and devastating pyroclastic flows and lahars. A lava dome that grew during August-October 2014 resulted in rockfalls, pyroclastic flows, and lava flows from the summit crater that led to evacuations in nearby communities (BGVN 41:03). Activity declined during November and December 2014 and remained low throughout 2015. By February 2016 the Alert Level was reduced to 0 (on a 0-5 scale) by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) which monitors the volcano. A seismic swarm in August 2016, and the beginning of a new eruption in January 2018 are covered in this report with information provided primarily by PHIVOLCS.

After a brief seismic swarm in August 2016, Mayon remained quiet until a phreatic explosion on 13 January 2018 sent an ash plume 2,500 m above the summit and scattered ash over numerous nearby communities. The growth of a new lava dome sent lava flows down the flanks and ash plumes multiple kilometers above the summit during subsequent weeks. Lava fountaining produced incandescence at the summit for many weeks. Lava collapse events from the flow fronts sent pyroclastic density currents (PDC's) down multiple ravines during January and February 2018. Lava fountaining activity became nearly continuous at the beginning of February but began to taper off by mid-month. Flows had reached as far as 4.5 km down ravines, and lava-collapse generated pyroclastic density currents reached 5 km from the summit crater. The pyroclastic activity continued through February from the gravity-driven collapsing flow fronts even though fountaining and lava effusion had decreased. Brief periods of fountaining and gravity-driven lava flow were noted throughout March 2018, but activity had essentially ceased by month's end.

Activity during 2016-2017. Very low seismicity of 0-2 volcanic earthquakes per day was typical for January and early February 2016; the largest number recorded was 12 on 9 January. On 12 February 2016, PHIVOLCS noted that seismicity had remained at baseline levels of 0-2 earthquakes per day for the previous six months, indicating that rock fracturing associated with magmatic activity had diminished. Ground deformation information suggested a return to pre-2014 eruption positions, and low levels of SO₂ flux had been consistent since November 2015. They reduced the Alert Level to 0.

Increasing SO₂ flux above 1,000 tons/day beginning in July 2016 was accompanied by ground deformation measurements suggesting renewed inflation. A brief swarm of 146 earthquakes was recorded by the Mayon Volcano Observatory's seismic network from 3-6 August; they were located 10 km away on the SE flank. This change led PHIVOLCS to raise the Alert Level back to 1 on 8 September 2016. Seismicity and SO₂ levels remained very low through the end of 2016, but GPS data suggested continued inflation. Slight inflation was recorded throughout 2017. Rare days of small seismic swarms of more than 10 earthquakes occurred during 2017, but otherwise seismicity and SO₂ flux values remained within background levels.

Activity during January 2018. A sudden phreatic eruption at 1621 local time on 13 January 2018 sent a gray steam-and-ash plume 2,500 m above the summit that drifted SW. The activity lasted for a little under two hours. Traces of ash fell on the Barangays of Anoling (4 km SW), Sua (6 km SW), Quirangay (9 km SW), Tumpa (9 km SW), Ilawod (10 km SW), and Salugan (8 km SW) in the city of Camalig and in the Barangays of Tandarora (26 km WSW), Maninila (8 km SW), and Travesia (10 km SW) in the municipality of Guinobatan. Incandescence at the summit crater was first observed a few hours later. As a result, PHIVOLCS raised the Alert Level from 1 to 2 early the next day.

Two more phreatic explosions occurred the following morning (14 January) at 0849 and 1143 that each produced ash plumes, but they were largely obscured by summit clouds. Minor amounts of ash were reported in Camalig. By the evening, PHIVOLCS had raised the Alert Level again to 3 after three explosions, 158 rockfall events, and the observation of bright incandescence at the summit crater. By 2000 on 14 January they noted the growth of a new lava dome and the beginnings of a lava flow towards the southern flank.

Two lava collapse events on the morning on 15 January each lasted 5-10 minutes. They originated from the lava flow front and produced rockfall and small-volume pyroclastic density currents. Ash plumes drifted SW and rained ash on Travesia, Muladbucad Grande, Maninila, Masarawag, Poblacion, Iraya, Ilawod, Calzada, Inamnan Grande, Inamnan Pequeno, Maguiron, Quitago and Mauraro in the municipality of Guinobatan and on the Baranguays of Cabangan, Anoling, Sua, Tumpa, Quirangay, Gapo, and Sumlang, and Baranguays 1 to 7 in the municipality of Camalig. A degassing event at 1107 produced a grayish to dirty white ash column that rose to a maximum of height of approximately 1,000 m above the summit before drifting WSW.

Lava effusion continued from the summit during 16-21 January 2018 with flows down the Mi-isi and Bonga gullies and occasional short-duration lava fountaining. Tens of daily lava collapse events accompanied the growth of the flow in the Mi-isi gully which had reached about 3 km from the summit by 18 January. Debris from the growing summit dome also descended the Matanag and Buyuan Gullies. Pyroclastic density currents descended the Mi-isi, Matanag, and Buyuan Gullies. Ash plumes rose up to 2 km and drifted SW from the summit crater and caused ashfall in Camalig, Guinobatan, and Polangui (figures 26-28).

Figure 26. Mayon emitted ash and steam along with pyroclastic density currents that flowed down the SW flank on 16 January 2018. View is looking N from S of the airport in Lagazpi City, Philippines, about 12 km S. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 27. Pyroclastic density currents (PDC's) descended the W flank of Mayon on 16 January 2018. Incandescence at the base of the PDC was also visible. Lava was fountaining at the summit and incandescent blocks were rolling down the Mi-isi drainage on the S flank. Image taken near Legazpi city, 12 km S. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 28. Lava flows at Mayon descended the Mi-isi drainage on the S flank and were visible from Legazpi city on 17 January 2018. Courtesy of The Express, photo from European Pressphoto Agency.

Activity increased on 22 January 2018 with lava fountains at the summit reaching 200-500 m high, the lava flow into the Mi-isi drainage extending beyond 3 km, and two new flows in the Bonga gully and upper Buyuan watershed. A dense 5-km-tall ash plume erupted at 1243 during a phreatomagmatic event that lasted for 8 minutes (figure 29). It generated pyroclastic density currents in several drainages within 4 km of the summit vent including Mi-isi, Bonga, Buyuan, Basud, San Andres, Buang, Anoling and other minor drainages. Ash was blown W and fell on the municipalities of Guinobatan, Camalig, Oas, Polangui and Iriga City. Five additional episodes of lava fountaining to 700 m occurred overnight that fed the Mi-isi and Bonga gully flows, and generated ash plumes to 2.5 and 3 km above the summit. This increase in activity led PHIVOLCS to raise the Alert Level to 4. By the following day, more than 50,000 people had evacuated to emergency shelters and civil aviation authorities temporarily closed airports in the cities of Legazpi and Naga.

Figure 29. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite acquired this image of the area around Mayon in the Philippines on 22 January 2018. The image combines natural-color data with thermal infrared bands (7-2-1). The substantial ash plume from the explosion that day rose to 10.9 km altitude and drifted NW and W, and the emerging lava dome appeared as a thermal hotspot at the summit. Courtesy of NASA Earth Observatory.

Numerous episodes of intense lava fountains during the nights of 23-26 January each lasted from a few minutes to more than an hour. They generated 150-600 m high fountains and continued to feed the flows in the Mi-isa and Bonga gullies. Ash plumes also rose from 0.5-5 km above the crater. The Mi-isa gully flow remained at 3 km from the summit, and the Buyuan flow had reached 1 km by 24 January. Pyroclastic density currents in the Mi-isi, Lidong/Basud, and Buyuan drainages were also observed. The PDCs in the Buyuan drainage traveled more than 5 km from the summit crater (figures 30-33).

Figure 30. Ash and steam plumes rose from the summit crater of Mayon while lava flows descended drainages on the S flank as seen from the town of Daraga, 10 km S, on 23 January 2018. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 31. An ash plume rises, likely from a pyroclastic density current, in a drainage on the SE flank of Mayon, a few kilometers N of the town of Daraga on 23 January 2018. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 32. Ash and pyroclastic density currents emerged from the summit of Mayon on 24 January 2018, sending ashfall to nearby communities and filling drainages with pyroclastic debris. Image taken from Daraga, 10 km S. Courtesy of The Express, photo from European Pressphoto Agency.

Figure 33. Lava flows were very active on the S flank of Mayon, visible from about 12 km SSE in Legazpi on 25 January 2018. Courtesy of The Express, AFP/Getty Images.

By the evening of 26 January 2018, the lava fountaining episodes had transitioned into aseismic lava effusion, feeding incandescent flows into the Bonga and Mi-isi gullies on the S flank, and advancing the flow in the Bonga significantly downslope to 1.8 km. Fewer fountaining episodes continued during 27-28 January. Heavy rainfall during 28-29 January remobilized deposits from pyroclastic density currents and generated sediment-laden stream flows in several channels (figure 34) and channel-confined lahars on the Binaan Channel.

Figure 34. Sediment-laden streams posed hazards to residents of Camalig (11 km SW) at Mayon on 28 January 2018 after heavy rains and numerous PDC's had filled the drainages with debris. Courtesy of The Express, photo from European Pressphoto Agency.

A significant increase in lava effusion and fountaining at the summit during the evening of 29 January 2018 fed PDCs into the Mi-isi and Bonga Gullies, and resulted in significant ashfall in Camalig and Guinobatan to the SW. Intermittent lava fountaining to 200 m, flow-front collapses that generated PDC events, low-level ash emissions, and slow lava effusion from the summit crater continued during 30 January-4 February (figures 35 and 36). The Mi-isi and Basud lava flows had advanced to 3.2 and 3.6 km, respectively, from the summit crater by 1 February, and the Bonga-Buyuan flow had advanced 4.3 km by 3 February.

Figure 35. Steam-and-ash plumes rose steadily above Mayon on 31 January 2018. Image taken at the port in Legazpi City, about 15 km S. Courtesy of The Express, Getty Images.

Figure 36. Lava effusion at the summit of Mayon had decreased from a week earlier (see figure 33) by 31 January 2018. Courtesy of The Express, Getty Images.

Activity during February-March 2018. Lava fountaining reached 550 m above the summit crater on 5 February and increased to near-continuous activity the next day. Lava flows and incandescent rockfalls were observed throughout the night in the Mi-isi and Bonga-Buyuan channels. High volumes of incandescent lava flows advanced to 3.2, 4.5, and approximately 3.0 km down the Mi-isi, Bonga-Buyuan and Basud channels. Pyroclastic density currents from the collapsing flow fronts reached 4.6, 4.4, and 4.2 km from the summit crater in the same drainages during 7 February. Near-continuous fountaining accompanied by steam plumes that rose up to 800 m continued through 10 February.

Lava fountaining became sporadic and weak beginning on 11 February. Heavy rainfall during 13 February generated channel-confined lahars in the Anoling channel. By 14 February, lava flows remained at 3.3 km, 4.5 km, and 900 m down the Mi-isi, Bonga and Basud gullies, and PDCs had deposited material to distances of 4.6, 4.5, and 4.2 km in the same drainages. Intermittent lava fountaining continued through 22 February. The fountains generally rose 100-600 m above the summit and were often audible more than 10 km from the summit.

Quieter lava effusion with fewer fountaining events was more typical behavior beginning on 23 February. Numerous episodes of lava-collapse pyroclastic density currents were visually observed on the Mi-isi, Basud, and Bonga-Buyuan Gullies within 2-4 kilometers of the summit crater during the second half of February. Deflation of the lower slopes that began on 20 February was recorded by electronic tiltmeter, consistent with the transition to seismically quieter lava effusion at the summit crater. However, the overall electronic tiltmeter and the continuous GPS data indicated that the volcano was still inflated relative to October and November 2017 levels.

Weak fountaining, lava effusion, and degassing were noted during 25-28 February. The sporadic fountains generated plumes that rose 800 m, and weak effusion continued to feed the flows in the drainages. Gravity-driven lava flow movement and degassing with ash plumes rising 600 m above the summit were the primary activity at Mayon on 1 March, although occasional lava fountaining events were still observed. Based on the decrease in activity at the summit, the decrease in seismicity, continued deflation, and significantly lower SO₂ emissions, PHIVOLCS lowered the Alert Level to 3 on 6 March 2018.

Brief periods of weak fountaining and lava flows were observed during 7-24 March. The fountaining generated dark gray ash plumes that rose 100-300 m above the summit crater before drifting SW, and were sometimes audible more than 10 km from the summit crater. At night, lava flows continued moving downslope within 3.3, 4.5, and 1.9 km of the crater in the Mi-isi, Bonga, and Basud gullies. Steam plumes rose as high as 2.5 km above the summit before drifting SW on 7 March. Intermittent bluish steam-laden plumes rose to 700 m before drifting SW on 14 March. A slight inflation of the lower flanks beginning on 11 March 2018 was recorded by electronic tiltmeters through at least 22 March. Overall deformation data indicated that the edifice was still inflated relative to pre-eruption baselines.

Beginning around 24 March 2018, the primary activity consisted of intermittent lava collapse events in the Mi-isi gully located between 4-5 km from the summit and steam-laden plumes that drifted SW from the summit. Lava flow effusion at the crater was last detected on 18 March. Ground deformation since 20 February 2018 recorded deflation despite short-term episodes of inflation of its lower and middle slopes, and incandescence at the summit had diminished from intense to faint. Lava flows had begun to stabilize, producing fewer rockfalls and infrequent pyroclastic density currents, the last of which was observed on 27 March 2018. This continued decrease in activity led PHIVOLCS to lower the Alert Level to 2 on 29 March 2018.

VAAC, SO₂, and MIROVA information. The Tokyo VAAC reported the first ash emission from Mayon on 13 January 2018 as a plume that rose to 5.2 km altitude and drifted SW. Many subsequent ash emissions were obscured by meteoric clouds and were only occasionally observed in satellite imagery. The ash plume from the large explosion on 22 January was observed in satellite imagery at 10.9 km altitude drifting NW. Numerous daily VAAC reports were issued through February; they were intermittent in March, ending on 23 March 2018. Plumes generally were reported at 5.2-7.6 km altitudes. Small sulfur dioxide plumes were captured by the OMI and OMPS satellite instruments on several days between 22 and 31 January 2018 (figure 37).

Figure 37. SO2 anomalies from Mayon were captured by the OMPS and OMI instruments on the SUOMI and AURA satellites during January 2018. Upper left: 22 January 2018; upper right: 23 January 2018; lower left: 26 January 2018; lower right: 31 January 2018. Courtesy of NASA Goddard Space Flight Center.

The MIROVA project thermal anomaly graph of log radiative power clearly captured the onset of activity at Mayon in mid-January 2018 (figure 38). Thermal activity increased through early February and then slowly decreased through mid-March 2018 when lava effusion ended.

Figure 38. The sudden onset of thermal activity at Mayon is apparent in this MIROVA project graph of log radiative power for the year ending on 11 May 2018. The data is based on the satellite-based MODIS infrared thermal imagery. Thermal activity peaked at the end of January and dropped off gradually through mid-March 2018; it then decreased abruptly after that. Courtesy of MIROVA.

Geologic Background. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often devastated populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); The Express (URL: https://www.express.co.uk); European Pressphoto Agency (EPA) (URL: http://www.epa.eu/); Getty Images (URL: https://www.gettyimages.com/); Agence France Presse (AFP) (URL: https://www.afp.com/).

The large Kusatsu-Shiranesan volcanic complex comprises three overlapping pyroclastic cones and numerous summit craters; it is located about 150 km NW of Tokyo in the Gunma Prefecture of central Japan. Intermittent short-lived historic activity has been reported from the northernmost Shiranesan cone since the beginning of the 19th century. An explosion at the southernmost Motoshiranesan cone in January 2018 resulted in one fatality and several injuries. Information about the event was gathered from the Japan Meteorological Agency (JMA) and various news sources.

Summary of activity during 1976-2014. Small phreatic explosions in the Mizugama and Yugama craters at the northernmost part of the Kusatsu-Shiranesan volcanic complex occurred in 1976, 1982, and 1983 (figure 14). Larger ash-bearing explosions in November and December 1983 sent tephra 30-40 km to communities downwind to the SE from the Yugama and adjacent Karagama craters on the Shiranesan cone. Intermittent increases in seismic activity near the Yugama crater coincided with water discoloration in the crater lake, and possible ejections of debris from hydrothermal activity in 1989 and 1996. Increased hydrothermal activity was noted on the N flank of Yugama during 2013-2014. Seismic swarms, deformation, thermal, and fumarolic activity increased briefly during early June 2014 in the area around the Yugama crater lake, but no eruption was observed. In late June 2014, JMA reported dying vegetation in a forested area 3 km SW of the Motoshiranesan summit area.

Figure 14. Subfeatures of the Kusatsu-Shiranesan volcanic complex as seen in Google Earth imagery, looking N. The northernmost bleached area includes the historically active Yugama, Mizugama, and Karagama craters, part of the Shiranesan cone. In the center of the complex is the Ainomine cone which has a ski area on the S flank. The southernmost edifice is the Motoshiranesan cone which has multiple craters at its summit, including Kagamiike or "Mirror pond". The explosions of 23 January 2018 occurred at Kagamiike and the adjacent crater to the N, in area referred to by JMA as Honkonoyama. Courtesy of Google Earth.

Activity during 2014-2107. Seismicity remained elevated from March to mid-August 2014 around the Yugama crater area. Ground deformation data suggested inflation between March 2014 and April 2015 in that area. Field surveys conducted on 4-5 and 10-11 November 2014 indicated fumarolic areas on the N and NE flanks of the Mizugama crater, but no other significant activity. Short-lived increases in seismicity were observed during January-February 2015. A field survey in May 2015 confirmed ongoing thermal activity on the N and NE wall of the Yugama crater, and the N and NE flank of the Mizugama crater. A small-amplitude, 2-minute-long tremor during late June 2015 was the first since January 2013; it was not accompanied by eruptive activity. The fumarolic activity on the N wall of the Yugama Crater was higher during a field survey in October 2015 than in had been the previous May.

Thermal activity was ongoing at Yugama and Mizugama craters during 2015-2017 along with intermittent fumarolic activity in the same general area, but no significant seismicity was reported. By June 2017 the decrease in the concentration of components derived from high-temperature volcanic gas in the lake, and the stable low-level seismicity in the area, led JMA to lower the warning level from 2 to 1 (on a 5 level scale) on 7 June 2017; they noted that the thermal activity continued around the Yugama crater throughout the rest of the year (figures 15-17).

Figure 15. A minor thermal anomaly persisted inside the NE crater wall at Yagama Crater at Kusatsu-Shiranesan throughout 2015-2017. Both visual (upper) and thermal (lower) images were taken during an overflight on 1 November 2017. View is to the north. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Figure 16. Thermal anomalies persisted on the N and NE flank of the Mizugama crater at Kusatsu-Shiranesan during 2015-2017. These visual (upper) and thermal (lower) images were captured on 1 November 2017. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Figure 17. Daily earthquake frequency at Kusatsu-Shiranesan during 1 January 2011-30 November 2017. Although earthquake counts temporarily increased during March-August 2014 and in January and February 2015, no eruptive activity was reported. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, November 2017).

Activity during January-March 2018. JMA reported that at 0959 on 23 January 2018 an eruption began at Kusatsu-Shiranesan coincident with the onset of volcanic tremor which prompted JMA to raise the Alert Level to 3 (on a scale of 1-5); there had been no prior indications of an impending eruption. Skiers at the popular Kusatsu Kokusai ski resort, located on the Ainomine cone, took video showing a plume of tephra and ejected bombs rising from vents around the Kagamiiki and adjacent crater at the summit of the Motoshiranesan cone (see Information Contacts for Mainichi for video link). Motoshiranesan is immediately adjacent S of the Ainomine cone and about 2 km SSE of the Yagama Crater on the Shiranesan cone where all previous historical activity had been reported (figures 14 and 18).

Figure 18. Locations and images of the active vents at Kusatsu-Shiranesan during the eruptive event of 23 January 2018. Upper left: View is looking W at the Motoshiranesan summit craters. The crater with the pond in Box 1 is Kagamiike (yellow Japanese characters). Boxes 1 and 2 in the upper left photo are enlarged in the lower photos. Upper right topographic map shows the locations in red of the three vents. The upper red line and dot correspond to the vents shown in the lower right box 2. The lower red bar on the topographic map (near the small pond) corresponds to the vent shown in the lower left image as box 1. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, January 2018).

Photos and video posted in news articles showed tephra shooting tens of meters into the air, drifting E, and blanketing the nearby hillside (figure 19); JMA noted ashfall in Nakanojo-machi, in the Gunma Prefecture, about 8 km E. Tephra hit a gondola, shattering glass and injuring four skiers (figure 20). Material fell through the roof of a lodge, where about 100 people had already been evacuated. Ground Self-Defense Force troops were engaging in ski training at the time of the event; one member died from the impact of large tephra blocks, and seven others were injured.

Figure 19. Tephra from Mount Kusatsu-Shiranesan covers the N flank of the Motoshiranesan cone and much of the Ainomine cone in this view to the W taken on 23 January 2018. Photo by Suo Takeuma, AP, courtesy of CNN (Japanese man killed by falling rocks after volcano erupts at ski resort, 23 January 2018).

Figure 20. Fist-sized tephra blocks and ash ejected from the eruption of Mount Kusatsu-Shiranesan cover the floor of a damaged gondola at the Kusatsu Kokusai Ski Resort on 24 January 2018, courtesy of The Mainichi Japan (Damaged ski resort gondolas show the power of Gunma Pref. volcanic eruption, 25 January 2018).

The following day, on 24 January 2018, JMA noted that volcanic earthquakes were numerous but decreasing in number, and two 2-3-minute-long periods of volcanic tremor were detected at 1015 and 1049. Minor but elevated seismicity continued through 30 January, punctuated by periods of tremor. The largest fissure where the eruption occurred was oriented E-W, located just inside the N rim of the northernmost crater at the Motoshiranesan summit (figure 21). Kenji Nogami, a professor at the Tokyo Institute of Technology, confirmed that the event appeared to have been "a typical phreatic eruption" (Japan Times).

Figure 21. The largest fissure vent active in the 23 January 2018 explosion at Kusatsu-Shiranesan was still surrounded by ash and tephra when photographed during an overflight on 28 January 2018. The summit ropeway station of the ski area is at the image top just NW of the explosion vent. Courtesy of The Mainichi (Visitor traffic plunges in Kusatsu hot spring resort after deadly eruptions, 30 January 2018).

The Tokyo Volcanic Ash Advisory Center issued a single volcanic ash advisory on 23 January indicating a possible eruption, but it was not identifiable from satellite data. Observations made on 14 February 2018 confirmed the presence of the vents in the Kagamiike and adjacent crater, but there was no evidence of thermal activity and little fumarolic activity in the area (figure 22). Seismicity decreased steadily after the explosion on 23 January 2018 through the end of March 2018 and no further activity was reported (figure 23).

Figure 22. Vents from the 23 January 2018 eruption at Kusatsu-Shiranesan were still visible at the craters on 14 February 2014 during a helicopter overflight by JMA. The upper image, looking W, shows the large vent at the N side of the crater immediately N of the Kagamiike crater, as well as a smaller vent located to the W on the E flank of the adjacent slope. The lower image shows two smaller vents on the inner wall of the adjacent Kagamiike crater. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, February 2018).

Figure 23. Seismicity decreased steadily at Kusatsu-Shiranesan after the explosion on 23 January 2018. Graph shows the number of daily seismic events during 1 January-31 March 2018. Courtesy of JMA (Volcanic activity monthly report, Kusatsu-Shirane, March 2018).

Geologic Background. The Kusatsu-Shiranesan complex, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid-Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the E, S, and SW flanks. The latest eruptive stage began about 14,000 years ago. Historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); The Mainichi (URL: http://mainichi.jp/english/, eruption video URL-https://mainichi.jp/movie/video/?id=121708141#cxrecs_s); The Japan Times (URL: https://www.japantimes.co.jp/); Cable News Network (CNN), Turner Broadcasting System, Inc. (URL: http://www.cnn.com/).

Ethiopia's Erta Ale basaltic shield volcano has had at least one active lava lake since the mid-1960s, and possibly much earlier. Two active craters (North Pit and South Pit) within the larger oval-shaped Summit Caldera have exhibited periodic lava fountaining and lava lake overflows over the years. A new eruptive event located about 3 km SE of the Summit Crater appeared on 21 January 2017. Activity at the eruption site increased during subsequent months, sending lava flows several kilometers NE and SW from a newly formed lava lake. This report discusses activity from February 2017 through March 2018 as the flows traveled as far as 16 km from the main vent. Information comes from satellite thermal and visual imagery, and photographs and reports from ground-based expeditions that periodically visit the site.

Summary of activity, February 2017-March 2018. The 21 January 2017 activity at Erta Ale was the first time a vent outside of the Summit Caldera has been observed (figure 50). The initial vent or vents created multiple lava flows that traveled generally NE and SW from their sources, creating at least one lava lake that persisted for about a year (figure 51). The flows began inside an older caldera at a location about 3 km SE of the South Pit Crater, but eventually overflowed the caldera rim in multiple directions. As the flow fields enlarged, thermal imagery captured hot-spots along the flows that were likely produced by breakouts, skylights into lava tunnels, and hornitos, as well as multiple surges of flows across the growing fields (figure 52). The imagery also showed the locations of the advancing flow fronts which had reached over 5 km SW of the source by August 2017 and over 16 km NE of the source by March 2018, eventually reaching the alluvial plain NE of Erta Ale. Thermal anomaly data indicated that the maximum thermal energy output happened in April 2017, gradually decreasing through March 2018. The far NE front of the northeast flow field was still active at end of March 2018.

Figure 50. The summit of Erta Ale has two oblong NW-trending calderas. The northern Summit Caldera contains the North Pit Crater and the South Pit Crater. The North Pit Crater has had a solidified lava lake with a large hornito emitting magmatic gases and incandescence at night, and the South Pit Crater has had an active lava lake for many years that last overflowed its rim during mid-January 2017. The new eruption began at vents located about 3 km SE of the South Pit Crater near the northern rim of a second caldera referred to here as the Southeast Caldera, on 21 January 2017. The new eruption had not yet begun in this 16 January 2017 image. See figure 46 (BGVN 42:07) for additional images the following week that show the first flows from the new vents. Images copyright by Planet Labs Inc., 3 m per pixel resolution, and used with permission under a Creative Commons license (CC BY-SA 4.0), annotated by GVP.

Figure 51. A new lava lake formed during late January 2017 at the new eruption site about 3 km SE of the South Pit Crater at Erta Ale, inside the Southeast Caldera. This view is likely from the rim of the Southeast Caldera, looking SE or E. Visitors were not able to get closer to the vent due to the active flows for several months. Photo by Stefan Tommasini taken during February 2017, courtesy of Volcano Discovery (Erta Ale volcanic activity: 2017 overview and June update, 27 June 2017).

Figure 52. An active new pahoehoe lava field flowed over older lava flows inside the Southeast Caldera at Erta Ale during February 2017. This photo was likely taken from the northern or western rim of the Southeast Caldera. Photo by Stefan Tommasini, courtesy of Volcano Discovery (Erta Ale volcanic activity: 2017 overview and June update, 27 June 2017).

When the new eruptive episode began, the lava lake at the South Pit Crater drained rapidly to around 80-100 m below the rim, according to visitors to the site a few weeks later. The crater was emitting a strong thermal signal by early March 2017 as the lake level rose again. Visitors in April witnessed a fluctuating lake level rising and falling by up to 20 m every 30 minutes over several days. The thermal signal remained strong at the South Pit Crater through March 2018. Due to significant political instability in the area, ground visits are intermittent, but high-quality photographs were taken in February 2017, December 2017, and January 2018 that show the new lava lake and parts of the new flow fields.

Activity during late January-March 2017. The new eruptive event at Erta Ale began in late January 2017 at the northern end of the Southeast Caldera located; the first lava flows observed were locatedabout 3 km SE from the main Summit Caldera (figure 45 (BGVN 42:07) and figure 50). Two separate vent areas appeared active initially. The northern vent sent lava flows to the NE for several kilometers and to the SW a much shorter distance. The southern vent sent a stream of lava to the S. By the end of January 2017 the North and South Pit Craters at the Summit Caldera were still thermally active, but the signals were much stronger from the new vent areas in the Southeast Caldera (figure 53). A faint thermal signal from about 5 km E of the northern vent suggested the extent of the new flows in that direction.

Figure 53. A Sentinel-2 image from 29 January 2017 shows the initial activity at the new Southeast Caldera vents of Erta Ale (labelled Event 1 and Event 2). Weak thermal signals are apparent from the North and South Pit Craters (Pit Crater Nord, Pit Crater Sud) within the Summit Caldera, and much stronger thermal signals are evident from two areas inside the Southeast Caldera. A faint signal from about 5 km E of the new vents indicates possible flow activity breaking out of lava tubes in that region (Skylight). Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Le point sur l'activité des volcans Etna, Erta Ale, Fuego, Piton de la Fournaise et Bogoslof, 3 février 2017).

A small group of travelers led by Ethiopian geologist Enku Mulugeta visited Erta Ale during the first half of February 2017. They reported that within the main Summit Caldera, the hornito in the North Pit Crater had collapsed and the lava lake in the South Pit Crater was about 80-100 m below the caldera floor level. The eruption in the Southeast Caldera was still very active, and they photographed the sizable new lava field which contained numerous pahoehoe flows, actively spattering hornitos, and a large lava lake (figures 51, 52, and 54). During the following months activity remained high both at the new eruption site and at the Summit Caldera where the lava lake in the South Pit Crater gradually rose back up to about 50 m below the caldera floor. Culture Volcan annotated a series of Sentinel-2 satellite thermal images which show the progression of the lava flows through the following year.

Figure 54. A large new lava field quickly formed inside the Southeast Caldera at Erta Ale after the beginning of the new eruptive event in late January 2017. When photographed here in February 2017, pahoehoe flows had spread outward from a central vent area (glow at top center) for over a kilometer in multiple directions. View is likely to the E from the W rim of the Southeast Caldera. Photo by Stefan Tommasini, February 2017, courtesy of Volcano Discovery (Erta Ale volcanic activity: 2017 overview and June update, 27 June 2017).

By 10 March 2017 only the southern vent area was active inside the Southeast Caldera. It continued to feed the lava field; lava was actively flowing S from the vent towards the W rim of the Southeast Crater, and NE, breaking out from lava tubes which blocked the thermal signal until about 2.6 km NE of the vent (figure 55). Thermal signals from both the North and South Pit Craters were distinct and stronger than in late January.

Figure 55. The thermal signals at both the North and South Pit Craters at Erta Ale were stronger in this 10 March 2017 image than in late January. Only one main source of lava is apparent at the Southeast Caldera. Lava flows directly from the primary vent SW towards the W rim of the caldera, and also surfaces from tunnels about two kilometers NE in an actively moving lava front. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Un point sur l'activité des volcans Etna et Erta Ale, 13 mars 2017).

A site visit to the South Pit Crater on 20 March 2017 demonstrated that the lake level had risen significantly since its drop in early February, and was once again actively convecting (figure 56). By the end of March 2017, satellite thermal imagery made clear the increasing thermal signal at the South Pit Crater, and in the Southeast Caldera, the major increase in effusion to the NE from the main vent. The width of the flow field had increased to about 1,400 m, and the farthest front was about 3,400 m NE from the vent (figure 57). The lava at the source measured about 180 x 75 m in size, suggesting a lava lake; a smaller overflow to the SW appeared to have reached the W rim of the Southeast Caldera by 30 March 2017 near the area where a new flow had first appeared in a 23 January 2017 satellite image (see figure 46, BGVN 42:07).

Figure 56. The South Pit Crater of Erta Ale on 20 March 2017 had risen significantly from its drop in February and was actively convecting. Photo by Jean-Michel Escarpit, courtesy of Cultur Volcan (Un point sur l'activité des volcans Fuego, Manam et Erta Ale, 22 mars 2017).

Figure 57. The thermal signal at the South Pit Crater continued to increase in this 30 March 2017 satellite image of Erta Ale. The main vent in the Southeast Caldera had dimensions of about 180 x 75 m, suggesting a lake had formed. A large increase in the thermally active area to the NE indicated that the flow field was expanding significantly in that direction, with a few small thermal anomalies between the lake and lava field suggesting a number of small flows or lava tube breakouts. Flow activity also continued to the SW reaching the W rim of the Southeast Crater where lava had flowed past the crater rim in late January (see figure 46, BGNV 42:07). Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Un point sur l'activité des volcans Klyuchevskoy et Erta Ale, 31 mars 2017).

Activity during April-May 2017. In the next Sentinel-2 satellite image from 9 April (figure 58), the distance to the farthest front of the lava flow had increased to about 4,600 m from the lava lake, and a new flow had appeared a few hundred meters east of the lake that extended about 1,100 m ENE from its source. Lava also flowed SW from the source to the SW rim of the Southeast Crater, appearing to pond against and flow slightly beyond the rim.

Figure 58. The lava flows continued to extend NE from their source inside the Southeast Crater at Erta Ale in this Sentinel-2 satellite image from 9 April 2017. The farthest edge of the northeast flow front was about 4,600 m from the lake. A new arm of lava flowed more than a kilometer ENE from its source close to the lake. Another thermal signature SW of the lake indicated an accumulation of lava near or slightly spilling over the SW rim of the Southeast Crater. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Le point sur l'activité des volcans Erta Ale et Bogoslof, 16 avril 2017).

A group visited Erta Ale during 11-15 April 2017 in collaboration with Addis Ababa University geologist Enku Mulugeta. They noted that fluctuating lava lake levels at the South Pit Crater were cycling every 30 minutes or so between 40 and 50 m below the caldera floor (figures 59 and 60). Lava tubes from the walls of the crater would feed the lake with fresh lava after it drained. Two coalesced hornitos, about 7 m high, were present in the NE part of the crater, emitting SO2 gas and occasional lava. At the North Crater Pit, noisy degassing of SO2 from several hornitos at the center of the solidified crust was apparent. Observers at the Southeast Caldera could see the lava lake with the top about 10 m below its crater rim, and minor fountaining during the night, but they were not able to get closer than about 700 m due to the active flows.

Figure 59. The lava lake level at the South Pit Crater at Erta Ale during April 2017 was fluctuating by 10-20 m every 30 minutes or so. The high-stand of the lava is shown here. Courtesy of Toucan Photo.

Figure 60. The lava lake level at the South Pit Crater at Erta Ale during April 2017 was fluctuating by 10-20 m every 30 minutes or so. The low stand of the lava is shown here as the lava drains away. Courtesy of Toucan Photo.

By the end of April 2017 satellite thermal imagery indicated that the northeast flow field at the Southeast Caldera extended more than 7 km NE from the lake and was curving towards the E (figure 61). The lava lake was still thermally active, as was the South Pit Crater to the NW.

Figure 61. Sentinel-2 satellite imagery of Erta Ale on 29 April 2017 shows the growth of the northeast lava field from earlier in the month to more than 7 kilometers from its source. The South Pit Crater was still active, as was the source of the northeast lava field. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (L'activité effusive reste soutenue à l'Erta Ale, 3 mai 2017).

Eleven days later, activity was quite different in the Southeast Caldera. Satellite imagery from 9 May 2017 (figure 62) showed a new, relatively narrow but bright lava flow moving NE for 2-3 km originating in a location slightly NE of the original lava lake; activity farther NE had diminished from the previous image. A subsequent image on 18 May looked similar, but by 19 May the narrow flow had been replaced by a much broader area of thermal anomaly in the region immediately E of the source. By 29 May 2017, the source of the lava appeared to have shifted several hundred meters SE of the earlier location, and a strong thermal signal once again extended NE across the northeast flow field from the new source for about two kilometers (figure 63).

Figure 62. A Sentinel-2 satellite image of Erta Ale on 9 May 2017 showed a shift to the NE in the location of the source of the active flows. A new narrow flow had traveled 2-3 km NE from a source located NE of the lava lake. The more distant northeast flow field had a much smaller thermal signature than on 29 April. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Breakout sur le volcan Erta Ale, 11 mai 2017).

Figure 63. A significant shift to the SE in the location of the lava source from a few weeks earlier is apparent in this Sentinel-2 satellite image of Erta Ale captured on 29 May 2017. A strong thermal anomaly trended NE across the northeast flow field for about two kilometers. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Erta Ale: une éruption vraiment exceptionnelle, 11 juin 2017).

Activity during June-August 2017. The rapidly changing flow field was significantly different again less than two weeks later in satellite imagery captured on 8 June 2017. Lava was flowing N, SE, and S across the northeast lava field, extending beyond the rim of the Southeast Caldera to the N and E. Another very strong thermal signal emerged from the SW corner of the Southeast Caldera where lava was flowing W and S outside the caldera rim forming a new southwest lava field (figure 64).

Figure 64. A Sentinel-2 satellite image of Erta Ale on 8 June 2017 shows significant changes in the location of the active flow fields from less than two weeks earlier. The South Pit Crater in the Summit Caldera still had a strong thermal signal suggesting an active lake in the crater. Flows in the Southeast Caldera appeared to be moving N, E, and S across the northeast lava field, and a new area with flows moving S and W from the SW rim of the Southeast Caldera formed the new Southwest lava field. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Erta Ale: une éruption vraiment exceptionnelle, 11 juin 2017).

During June 2017, the most aggressive flow activity contributed to significant growth of the southwest lava field. By 28 June, infrared imaging detected flow fronts 4,500 m SW of the vent; they had extended to about 5,100 m, nearing the base of the SW flank of Erta Ale, by 5 July (figure 65). Flow activity also persisted in the northeast flow field with activity concentrated about 1.5 km NE of the vent on 28 June. Movement increased at the northeast flow field beginning in late June and it had extended to about 3.5 km NE of the lava lake by 5 July 2017.

Figure 65. Lava flow activity at the Southeast Caldera of Erta Ale during June 2017 was concentrated in the growing southwest flow field which had extended about 5,100 m from its lava lake source by 5 July 2017 in this Landsat 8 satellite image. The northeast flow field began extending farther NE during the first week of July, reaching 3,500 m from the lake by 5 July. Courtesy of ESA/Copernicus and NASA/USGS with annotations provided by Culture Volcan (Un point sur l'activité des volcans Copahue et Erta Ale, 8 juillet 2017).

Significant movement to the NE in the northeast flow field was apparent in satellite images beginning on 21 July 2017; the head of the flow had reached about 9.5 km from the lava lake by 28 July 2017, mostly focused in a narrow channel (figure 66). Activity decreased in the southwest flow field during July; the lava front had advanced only a few hundred meters by the end of July from its position on 5 July.

Figure 66. The northeast flow field at Erta Ale lengthened significantly during July 2017; the leading edge was about 9.5 km NE of the lava lake by 28 July 2017, as captured in this Sentinel-2 satellite image. The southwest flow field had extended just a few hundred meters SW from its location on 5 July. The distance between the South Pit Crater and the Southeast Caldera lava lake is about 2.7 km. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Les actus du jour: Katla en alerte jaune et quelques changements à l'Erta Ale, 29 juillet 2017).

During August 2017, lava continued to flow from the Southeast Caldera lava lake in two directions. The northeast flow front extended to 12 km from the vent by 17 August and had reached over 14 km by 7 September. The southwest flow field, while it remained in roughly the same area, had a decreased but still significant thermal signature in early September, suggesting continued but diminished activity throughout the period (figures 67).

Figure 67. During August 2017, lava continued to flow in two directions from the Southeast Caldera lava lake at Erta Ale. The northeast flow field had reached over 14 km from the lake by 7 September 2017 when this Landsat 8 satellite image was taken. The Southwest flow field, while it remained in roughly the same area, still had a significant thermal signature suggesting continued activity. Courtesy of ESA/Copernicus and NASA/USGS with annotations provided by Culture Volcan (volcan Erta Ale: ça continue; Fernandina: c'est moins sûr, 12 septembre 2017).

Activity during September-December 2017. In a Sentinel-2 satellite image from 26 September 2017, it was clear that the South Crater Pit was still thermally active, and that the southwest flow field had largely cooled with only a small area on its NW edge still producing a thermal anomaly (figure 68). In contrast, the northeast flow field had advanced about 1 km in the previous three weeks and was less than a kilometer from the edge of the valley alluvium. It finally reached the edge of the older lava field and began to advance across the alluvium NE of the volcano, more than 16 km from the lava lake, on 16 October 2017 (figure 69). Based on satellite imagery, Cultur Volcan interpreted that activity slowed significantly during November 2017, and while the thermal signal remained strong near the head of the flow, it did not advance significantly across the alluvium.

Figure 68. The South Pit Crater at Erta Ale still had an active lava lake on 26 September 2017 in this Sentinel-2 satellite image. The southwest lava field had largely cooled, with only a small thermal anomaly along it NW edge. The northeast lava field continued to be active; it had advanced about 1 km NE in about three weeks and was about 650 m from the edge of the alluvium. A significant number of hotspots along the northeast lava flow suggest that several skylights existed into lava tubes or there were small breakouts. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Les actus du jour: Heard Island, Erta Ale, Pacaya, Fuego, Sangay, Ol Doynio Lengai, 5 octobre 2017).

Figure 69. Erta Ale's northeast flow field reached the alluvium about 16 km E of the Southeast Caldera lava lake by 16 October 2017, as recorded in this Sentinel-2 satellite image. The distance between the ends of the two easternmost tongues of lava is about 1 km. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Erta Ale: ça y est, le champ de lave entre dans la plaine!, 18 octobre 2017).

Visitors to the South Pit Crater in mid-December 2017 reported that its lava lake continued to be active and its level was about 60 m below the rim. They were also able to visit the Southeast Caldera lava lake, 2.7 km SE of the South Pit Crater, and take photographs from its rim; it was about 200 m long and 100 m wide and filled with slowly convecting lava (figures 70, 71). Satellite imagery from 25 December 2017 showed the active lake at the South Pit Crater, the active lake at the Southeast Caldera, and numerous skylights and overflows along the 16-km-long northeast flow field (figure 72).

Figure 70. The Southeast caldera lava lake at Erta Ale, its surface crusted over with slightly cooled lava, with dimensions of about 200 x 100 m in mid-December 2017. Photograph by FB88, courtesy of Culture Volcan (Un point sur l'activité à l'Erta Ale, 31 décembre 2017).

Figure 71. The Southeast Caldera lava lake at Erta Ale was slowly convecting during mid-December 2017. Photographed by FB88, courtesy of Culture Volcan (Un point sur l'activité à l'Erta Ale, 31 décembre 2017).

Figure 72. Sentinel-2 satellite imagery from 25 December 2017 of Erta Ale showed the active lake at the South Pit Crater (Summit lava lake), the active lake at the Southeast Caldera (Rift-Zone lava lake), and numerous skylights and overflows along the 16-km-long northeast flow field. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Un point sur l'activité à l'Erta Ale, 31 décembre 2017).

Activity during January-March 2018. By mid-January 2018 thermal activity was concentrated a few kilometers back from the front of the northeast flow, about 12 km from the lava lake (figure 73). A Volcano Discovery tour group visited during 13-26 January 2018 and was able to access and photograph both the North and South Pit Craters and the new lake and flow fields around the Southeast Caldera with ground-based and aerial drone photography (figures 74-84).

Figure 73. By 19 January 2018, thermal activity at Erta Ale's northeast flow field was concentrated a few kilometers back from the front of the flow, about 12 km from the Southeast Caldera lava lake. The South Pit Crater and Southeast Caldera lava lakes are visible on the left. Small hot-spots near the Southeast Caldera lava lake could be hornitos or skylights into lava tubes. Courtesy of ESA/Copernicus with annotations provided by Culture Volcan (Le point sur l'activité des volcans Erta Ale, Kadovar (Mis à jour) et Nevados de Chillan, 21 janvier 2018).

Figure 74. In this aerial view taken in January 2018 by a drone of the central part of Erta Ale's Summit Caldera, steam plumes rose from the North Pit Crater (left) and South Pit Crater (right). The fresh black lava around the South Pit Crater overflowed onto the caldera floor in January 2017 shortly before the beginning of the eruptive events in the Southeast Caldera a few kilometers to the south. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 75. The North Pit Crater inside the Summit Caldera at Erta Ale contained a large collapsed vent in January 2018 that formed after the magma drained away from the crater in January 2017. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 76. The lava lake in the South Pit Crater of Erta Ale's Summit Caldera was tens of meters below the rim in January 2018. Magma drained away and parts of the crater walls collapsed in January 2017, followed by repeated filling and draining of the lava lake during 2017. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 77. This aerial view by drone shows the large lava lake that formed at Erta Ale's Southeast Caldera during 2017; it was still slowly convecting in January 2018. The lake dimensions were about 100 x 200 m. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 78. Recently cooled black crust is overrun and consumed by molten lava that quickly cools and crusts over in Erta Ale's Southeast Caldera lava lake in January 2018. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 79. Lava appears to flow into the Southeast Caldera lava lake at Erta Ale from a vent at the far edge and slowly spread across the lake during January 2018. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 80. Lava splashes as it flows into the Southeast Caldera lava lake at Erta Ale in January 2018. Photograph by Anastasia Ganuschenko taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 81. Downwelling consumes lava inside the Southeast Caldera lava lake at Erta Ale in January 2018. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 82. Incandescence is visible inside a hornito that formed through lava spattering along the new flows in the Southeast Caldera at Erta Ale in January 2018. Photograph by Anastasia Ganuschenko taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 83. Many layers of fresh Pahoehoe lava flows were cool enough to walk on in some areas of the Southeast Caldera lava fields in January 2018. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

Figure 84. Fresh lava flows were easily distinguished from older ones by their silver hue and dark black crust at Erta Ale's Southeast Caldera lava fields in January 2018. Photograph by Stefan Tommasini taken during 13-26 January 2018, courtesy of Volcano Discovery.

By late March 2018 no thermal signal appeared in satellite imagery at the site of the Southeast Caldera lava lake, although the South Pit Crater was still visible. A large increase in the area of fresh flows and multiple thermal anomalies were present at the flow front of the northeast lava field 14-16 km from the former lava lake (figure 85). During the second half of March, the flow progressed several hundred meters out into the alluvial plain.

Figure 85. Sentinel-2 satellite imagery captured on 15 March 2018 showed a large increase in the area of fresh lava flows at the NE front of the northeast lava field at Erta Ale when compared with an image from 19 January 2018. Over the next ten days, images showed the narrow finger of lava that just touches the alluvium in this image creep about a kilometer out into the alluvial plain. Courtesy of Courtesy of ESA/Copernicus, published by Cultur Volcan (Les actus volcaniques du jour: Erta Ale, Maly-Semiachik, Suwanose-Jima et Ebeko, 28 mars 2018).

MIROVA thermal anomaly data. The MIROVA thermal anomaly data captures information about the distance of the anomalies from the summit as well as the radiative power released from Erta Ale. Both sets of information agree well with observations from the Sentinel-2 and Landsat satellite data. The plot of distance from the summit (figure 86) shows that during August 2016-mid-January 2017 the thermal anomalies were located very close to the summit point, representing heat flow from both the South and North Pit Craters within the Summit Caldera. Beginning on 21 January 2017, the jump in location of the anomalies corresponded with the beginning of the eruption in the Southeast Caldera. The MIROVA thermal anomalies progressed farther from the summit point during March and April 2017, when the northeast flow field was lengthening to the NE. The thermal signal jumps back closer to the summit point in early May corresponding to when new breakouts were spotted near the Southeast Caldera lava lake; the flows again traveled away from the lake during June and July 2017. Active lava flows from mid-August 2017 through March 2018 were visible in satellite imagery 12-16 km from the lava lake, which is reflected in the MIROVA data (figure 86).

Figure 86. MIROVA data showing the distance from the summit point of thermal anomalies at Erta Ale. Upper graph is the year ending 18 July 2017. Lower graph is the year ending 9 March 2018. They correspond well with locations of thermal anomalies that appear in numerous satellite images during that time. Note the distance scale change. See text and earlier figures for details. Courtesy of MIROVA.

The MIROVA data for the radiative power released from Erta Ale during August 2016-March 2018 also corresponds well with satellite and ground observations (figure 87). The levels of radiative power were moderate and constant during August 2016 to mid-January 2017 when only the lava lake and hornitos at the South and North Pit Craters were active (see also figure 47, BGVN 42:07). A moderate spike in the radiative power corresponds to the overflow of the South Pit Crater during 16-20 January 2017, followed by a large spike in radiative power on 21 January when the eruption started in the Southeast Caldera. This was followed by an extended period of increased radiative power as extensive flow fields formed in the Southeast Caldera. The graph is also able to distinguish the movement of the flows from near the Southeast Caldera lava lake to farther away and then near again during March-June 2017. The radiative power graph from 10 March 2017-9 March 2018 clearly shows a gradual decrease in the amount of radiative power over the period, suggesting a decline in flow activity, which corresponds well to satellite observations.

Figure 87. MIROVA plots of radiative power at Erta Ale for 18 July 2016-18 July 2017 (upper) and 9 March 2017-9 March 2018 (lower). Note the different y-axis scales for VRP due to the large spike on 21 January 2017 at the beginning of the Southeast Caldera eruptive episode. The plots record both the movement of the flow fields away from and closer to the summit point during March-June 2017, and then the gradual decrease in radiative energy from May 2017 through early March 2018. Courtesy of MIROVA.

Geologic Background. Erta Ale is an isolated basaltic shield that is the most active volcano in Ethiopia. The broad, 50-km-wide edifice rises more than 600 m from below sea level in the barren Danakil depression. Erta Ale is the namesake and most prominent feature of the Erta Ale Range. The volcano contains a 0.7 x 1.6 km, elliptical summit crater housing steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Fresh-looking basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera is renowned for one, or sometimes two long-term lava lakes that have been active since at least 1967, or possibly since 1906. Recent fissure eruptions have occurred on the N flank.

Information Contacts: European Space Agency (ESA), Copernicus (URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus; Robert Simon, Sr., Data Visualization Engineer, Planet Labs Inc. (URL: http://www.planet.com/) [Images used under https://creativecommons.org/licenses/by-sa/4.0/]; Cultur Volcan, Journal d'un volcanophile (URL: https://laculturevolcan.blogspot.com); Toucan Photo (URL: www.toucan.photo); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Indonesia's Sinabung volcano has been highly active since its first confirmed Holocene eruption during August and September 2010; ash plumes initially rose up to 2 km above the summit, and falling ash and tephra caused fatalities and thousands of evacuations (BGVN 35:07). It remained quiet after the initial eruption until 15 September 2013, when a new eruptive phase began that has continued uninterrupted through February 2018. Ash plumes rising several kilometers, avalanche blocks falling several kilometers down the flanks, and deadly pyroclastic flows travelling more than 4 km have all been documented repeatedly during the last several years. Details of events during October 2017-March 2018, including the largest explosion to date on 19 February 2018, are covered in this report. Information is provided by, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), referred to by some agencies as CVGHM or the Indonesian Center of Volcanology and Geological Hazard Mitigation, the Darwin Volcanic Ash Advisory Centre (VAAC), and the Badan Nacional Penanggulangan Bencana (National Disaster Management Authority, BNPB). Additional information comes from satellite instruments and local observers.

When activity began in 2010, and again when eruptions resumed in 2013, many news accounts included statements that Sinabung had last been active 400 years ago, or even saying specifically that the last eruption was in 1600 CE. Those claims appear to have been caused by a misunderstanding related to the boundary time that Indonesian volcanologists use to categorize volcanoes. Those volcanoes with historical activity, defined as being about 400 years ago (corresponding to the beginning of the Dutch East India Company era), are in the "Type A" group. Those in the "Type B" group, including Sinabung prior to 2010, have not had reported activity in more than 400 years. Using charcoal associated with the most recent pyroclastic flow, Hendrasto et al. (2012) determined that the last previous eruptive activity was 1200 years before present using carbon dating techniques, or 740-880 CE (at 1 sigma).

Although activity remained high from October 2017 through March 2018, a gradual decline in the overall eruptive activity from the beginning of 2017 was apparent. The number of explosions per month generally declined, with no explosions reported during March 2018, for the first time since August 2013 (figure 45). The thermal anomaly record was similar; periods of high heat flow persisted through mid-November 2017, followed by a gradual reduction in the amount of thermal activity, although the intensity remained consistent, according to the MIROVA project (figure 46). Much of the heat flow was attributed to the dome growth at the summit; the dome was destroyed in the large explosion of 19 February 2018.

Figure 45. The number of explosions per month at Sinabung as reported by PVMBG from January 2017-March 2018. Only partial data was reported for 18-31 January 2018, and no explosions were observed during March 2018.

Figure 46. Thermal anomaly data at Sinabung from satellite-based MODIS instruments, plotted on a Log Radiative Power scale, persisted through the end of 2017 and then decreased in frequency through the end of February 2018. Much of the heat flow was attributed to a dome near the summit which was destroyed in the 19 February 2018 explosion. Graph shows thermal anomalies between 11 May 2017 and 1 April 2018. Courtesy of MIROVA.

Throughout the period from October 2017 through 19 February 2018, steam plumes were constantly rising to heights of 1,000-2,400 m above the summit. Avalanche blocks were ejected daily down the E and S flanks from 500-3,500 m, and multiple pyroclastic flows each month traveled between 1,000 and 4,600 m down the SE flank. Tens of explosions occurred monthly, generating ash plumes that rose from 500 to 5,000 m above the summit. Explosive activity was more intermittent during February than the previous months, until 19 February when the largest explosion to date occurred; it included an ash plume that rose to at least 16.8 km altitude and at least ten pyroclastic flows. In spite of the size of the explosion, no injuries or fatalities were reported as most nearby communities had been evacuated from the ongoing activity. Activity decreased substantially during March 2018; there were no explosions, block avalanches, or pyroclastic flows reported, only steam plumes rising 1,000 m above the summit.

Activity during October 2017-January 2018. During October 2017, steam plume heights reached 1,500 m above the summit. Avalanche blocks traveled down the E and S flanks 500-2,500 m, and eight pyroclastic flows traveled 1,000-4,500 m down the SE and S flanks. Ash plume heights ranged from 500 to 3,600 m above the summit. The Darwin VAAC issued 38 aviation alerts during the month. On 1 October they reported an ash plume drifting both NW at 4.6 km altitude and NE at 3.7 km. The next day, the webcam observed an ash emission that rose to 5.5 km altitude. On 4 October an ash plume was spotted in the webcam rising to 5.8 km altitude and drifting ENE. Later that day it had detached from the volcano and was seen drifting NW in satellite imagery. An ash plume on 5 October rose to 3.9 km altitude and drifted ESE. Two ash emission were reported on 7 October; the first rose to 3 km altitude, the second rose to 4.3 km, they both dissipated quickly. On 8 October, three plumes were reported. The first rose to 4.6 km and drifted WSW, the second rose to 3 km and drifted S and the third rose to 3.4 km and also drifted S. The following day, an ash plume rose to 4.6 km and drifted E. BNPB stated that on 11 October, an event at Sinabung generated an ash plume that rose 1.5 km above the crater and drifted ESE, causing ashfall in several local villages. On 12 October an event produced an ash plume that rose 2 km above the crater and was followed by pyroclastic flows traveling 1.5 and 2 km down the S and ESE flanks, respectively.

PVMBG reported ash plumes rising to 3.7 km on 11, 12, and 13 October 2017. Later on 13 October the Jakarta MWO reported an ash plume at 4.3 km. The next day PVMBG reported an ash plume at 5.5 km altitude. A plume on 15 October rose to 3 km and drifted E. A steam plume on 16 October drifted down the SE flank before drifting SE no 16 October (figure 47). On 17 October, a discrete emission rose a few hundred meters above the summit drifted NE. Later that day, an ash plume was seen in the webcam moving SE at 3.4 km. On 18 October, two ash emissions were reported. The first rose to 3.7 km and drifted E, the second rose to 3.9 km and drifted W. An ash plume rose to 4.6 km altitude on 21 October, and to 3.9 km, drifting S, on 23 October. The next day, three ash plumes were reported; the first rose to 3 km, the second to 4.6 km, and the third to 3.7, all drifting E. After five days of quiet, the webcam observed ash plumes that rose to 4.3 km on 30 October, and to 3.9 km on 31 October. Only two MODVOLC thermal alerts were issued, on 20 and 27 October.

Figure 47. A steam plume drifted down the SW flank of Sinabung before moving SE on 16 October 2017. View is from the SE. Courtesy of PVMBG.

Steam plumes were higher during November 2017, rising 2,400 m above the summit. Block avalanches traveled 500-3,000 m down the E and S flanks most days, and ten pyroclastic flows traveled between 2,000-3,500 m down the ESE and S flanks. The ash plumes rose 700-3,200 m above the summit. The Darwin VAAC issued 41 aviation alerts in November. Near-daily ash plumes were observed mostly in the webcam and occasionally in satellite imagery. They generally rose to 3.4-4.9 km altitude; the most common drift directions were S and SW. A number of times, multiple ash plumes were reported in a single day. On 14 November, four ash plumes were observed. The first rose to 3.7 km, the second and third rose to 4.6 km and drifted S and SSW, the last rose to 3.9 km and also drifted SSW. On 20 November a discrete emission produced an ash plume that rose to 5.5 km altitude and drifted SSW. Three ash plumes were recorded the next day, rising 3.9-4.6 km and drifting in multiple directions under variable winds. An ash plume on 23 November was reported by PVMBG at 6.7 km altitude drifting W, the highest noted for the month. MODVOLC thermal alerts appeared twice on 5 November, once on 14 November, and three times on 17 November.

Activity during December 2017 was similar to the previous two months; steam plumes rose 2,000 m above the summit, block avalanches traveled 500-3,500 m down the E and S flanks numerous times, and nine pyroclastic flows descended the ESE and S flanks distances ranging from 2,000 to 4,600 m. Ash plume heights were from 700-4,000 m above the summit. The Darwin VAAC issued 43 aviation alerts in December 2017. They reported ash plume heights of 3.4-4.9 km altitude on most days. Every day during 10-19 December, ash plumes were reported at altitudes of 4.6-5.5 km drifting SW, E or SE. PVMBG reported ash plumes on 26, 27 and 28 December that rose to 3.9, 5.2, and 5.5 km, respectively. BNPB reported pyroclastic flows on 27 December that traveled 3.5-4.6 km SE, and ashfall was reported in many nearby villages including Sukanalu Village (20 km SE), Tonggal Town, Central Kuta, Gamber (4 km SE), Berastepu (4 km SE), and Jeraya (6 km SE). The highest ash plume of the month rose to 6.4 km altitude on 29 December and drifted E. This was followed by another discrete ash emission the same day that rose to 5.8 km and two plumes the next day that rose to 5.2 km and drifted W. There was only one MODVOLC thermal alert issued on 7 December.

The Darwin VAAC issued 56 aviation alerts for January 2018. Multiple discrete ash emissions were reported on most days. Plume altitudes generally ranged from 3.4 to 5.5 km. A 6.1 km altitude plume was visible in satellite imagery on 18 January (figure 48). The drift directions were highly variable throughout the month. Most plumes dissipated within six hours. Incandescent blocks were reported by PVMBG falling 500-1,500 m down the ESE flank on most days when the summit was visible. They also reported a pyroclastic flow on 27 January that traveled 2,500 m ESE from the summit (figure 49). Three MODVOLC thermal alerts were issued on 6 January, and one on 12 January.

Figure 48. An ash plume rose 3,000 m from the summit of Sinabung on 18 January 2018 in this view looking at the SE flank. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 49. A pyroclastic flow descended 2,500 m down the SE flank of Sinabung on 27 January 2018 while an ash plume also drifts SE in this view of the SE flank. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

Activity during February 2018. During most of February, steam plumes rose only 1,000 m above the summit, and avalanche blocks traveled 500-2,500 m down the ESE and S flanks. Far fewer ash emissions were reported than previous months, but the largest explosive event recorded to date took place on 19 February (figure 50). The Darwin VAAC issued 29 aviation alerts during February 2018. Short-lived ash emissions were reported on 1, 3, 5, 11, and 15 February. The ash plume heights ranged from 3.4-4.6 km altitude, and they drifted S or SW.

Figure 50. A very large ash plume rose to 16.8 km altitude from Sinabung on 19 February 2018. Image is from several tens of kilometers from the volcano a few hours after the eruption. No fatalities were reported. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

The large explosion was first reported by the Darwin VAAC at 0255 UTC on 19 February 2018. It produced an ash plume, which was clearly observed in satellite imagery (figure 51), that quickly rose to at least 16.8 km altitude and began drifting NW (figure 52). It also produced a large SO2 plume that was recorded by satellite instruments (figure 53). Over the next 15 hours the plume dispersed in three different directions at different altitudes. The highest part of the plume drifted NW at 13.7 km and was visible over 300 km from the summit. The lower part of the plume drifted S initially at 6.7 km and gradually lowered to 4.3 km; it was visible 75 km from the summit before dissipating. A middle part of the plume drifted NW at 9.1 km during the middle of the day. Three subsequent minor ash emissions were observed on 20 and 25 February that rose to 3.4 km altitude. There were no VAAC reports issued during March 2018. A MODVOLC thermal alert issued on 11 February was the last for several months.

Figure 51. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite captured this natural-color image of the ash plume at Sinabung at 0410 UTC on 19 February 2018, just a few hours after it began. The ash plume rose over 16 km high and drifted in multiple directions. Courtesy of NASA Earth Observatory.

Figure 52. The large ash plume of 19 February 2018 at Sinabung, viewed here from within a few kilometers of the summit in the first hour or so after the eruption, rose quickly to over 16 km altitude. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 53. Two different Ozone Monitoring Instruments measured the SO2 plume released by Sinabung in the explosion on 19 February 2018. The upper left image was recorded about three hours after the explosions (0616-0621 UTC, 19 February 2018) by the Ozone Mapper Profiler Suite (OMPS) instrument on the Suomi NPP satellite. The upper right image was recorded about 27 hours after the explosion (0619-0802 UTC, 19 February 2018) by the Ozone Monitoring Instrument (OMI) on the Aura satellite, and shows the multi-directional dispersal of the SO2 plume during that time. The lower image uses the data captured at the same time as the upper left image and displays it using different software and detailed background information. The maximum gas concentrations reached 140 Dobson Units. Upper images courtesy of NASA Goddard Space Flight Center, and lower image courtesy of NASA Earth Observatory.

As many as 10 pyroclastic flows were observed during the 19 February explosion, traveling as far as 4.9 km SSE and 3.5 km E (figures 54 and 55). Ash and tephra as large as a few millimeters in diameter fell in areas downwind, including Simpang Empat (7 km SE), the Namanteran district, Pqyung (5 km SSW), Tiganderket (7 km W), Munthe, Kutambaru (20 km NW), Perbaji (4 km SW), and Kutarayat (figure 56 and 57).

Figure 54. A pyroclastic flow traveled several kilometers SSE from Sinabung on 19 February 2018 as tephra fell from the rising ash cloud in this view from several kilometers away to the NE. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 55. The dark gray ash plume rose skyward while the large brown pyroclastic flows traveled SE from Sinabung on 19 February 2018 as viewed from the town of Kutarakyat located 5 km NE of the volcano. Photo by Endro Rusharyanto, courtesy of the Associated Press (AP).

Figure 56. Small tephra fragments fell on the village of Gurukinayan (13 km E) and other villages SE of Sinabung during the eruption of 19 February 2018. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 57. Ash from the eruption at Sinabung on 19 February 2018 covered vegetable plants the following day in the village of Payung (5 km SSW). Photograph by Antara Foto, Ahmad Putra via Reuters.

Villagers were temporarily evacuated from nearby villages, but were able to return a few days later (figure 58). Conditions in five districts were so dark that visibility was reduced to about 5 m. In addition, ashfall was recorded as far away as the town of Lhokseumawe, 260 km N. Magma Indonesia reported that the lava dome that had been growing at the summit for some time was destroyed in the 19 February explosion (figure 59). A PVMBG volcanologist reported the volume of the destroyed lava dome was at least 1.6 million cubic meters.

Figure 58. Villagers from Gurukinayan (13 km E) were evacuated as ash spread over the town from the eruption of Sinabung on 19 February 2018, but they returned to their homes a few days later. Photographer unknown, courtesy of Sutopo Purwo Nugroho, Twitter.

Figure 59. The summit of Sinabung, before (top) and after (bottom) the large explosion of 19 February 2018. The dome size in the upper photo is similar to that shown figure 43 (BGVN 42:12) from September 2017. The lower image was taken within a week after the explosion. Courtesy of MAGMA Indonesia, via Twitter.

Reference: Hendrasto M, Surono, Budianto A, Kristianto, Triastuty H, Haerani N, Basuki A, Suparman Y, Primulyana S, Prambada O, Loeqman A, Indrastuti N, Andreas A S, Rosadi U, Adi S, Iguchi M, Ohkura T, Nakada S, Yoshimoto M, 2012. Evaluation of Volcanic Activity at Sinabung Volcano, After More Than 400 Years of Quiet. Journal of Disaster Research, vol. 7, no. 1, p. 37-44.

Geologic Background. Gunung Sinabung is a Pleistocene-to-Holocene stratovolcano with many lava flows on its flanks. The migration of summit vents along a N-S line gives the summit crater complex an elongated form. The youngest crater of this conical andesitic-to-dacitic edifice is at the southern end of the four overlapping summit craters. The youngest deposit is a SE-flank pyroclastic flow 14C dated by Hendrasto et al. (2012) at 740-880 CE. An unconfirmed eruption was noted in 1881, and solfataric activity was seen at the summit and upper flanks in 1912. No confirmed historical eruptions were recorded prior to explosive eruptions during August-September 2010 that produced ash plumes to 5 km above the summit.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); Sutopo Purwo Nugroho, Head of Information Data and Public Relations Center of BNPB via Twitter (URL: https://twitter.com/Sutopo_PN); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Associated Press (AP), Endro Rusharyanto, Photographer (URL: http://www.ap.org/); Reuters (http://www.reuters.com/).

The first confirmed historical eruption at Kadovar began around mid-day local time on 5 January 2018, according to witnesses. The steeply-sloped island is approximately 1.4 km in diameter and is located about 25 km NNE from the mouth of the Sepik River on the mainland of Papua New Guinea (figure 1). This report covers activity from the beginning of the eruption on 5 January through March 2018. Information about the eruption is provided by the Rabaul Volcano Observatory (RVO), the Darwin Volcanic Ash Advisory Center (VAAC), satellite sources, news reports, and local observers. A possible eruption was witnessed by explorers in 1700; no other activity was reported until an outbreak of thermal activity in 1976 (NSEB 01:14-01:11, SEAN 03:09) and a short period of seismic unrest in 2015, according to RVO.

Figure 1. Kadovar Island is located about 25 km NNE from the mouth of the Sepik River on the mainland of Papua New Guinea. Nearby active volcanoes include Blup Blup (12 km N) and Bam (21 km W); residents of Kadovar were evacuated initially to Blup Blup before being moved to an area near Wewak, the nearest community on the mainland, about 105 km W. The red triangles are Holocene volcanoes, and the blue (cyan) triangles are Pleistocene volcanoes. Base map courtesy of Google Earth.

Ash and steam emissions from Kadovar were first reported on 5 January 2018. After about 24 hours, more than half of the island was covered by volcanic debris. Activity intensified over the next two weeks; RVO identified five distinct vents located at the summit and along the SE coast. Dense ash plumes and steam rose from the summit vents, and a slowly-extruding lava flow emerged from a vent near the shoreline on the SE flank. Persistent steam and intermittent ash plumes were produced from the summit vent through the end of March. The lava flow grew outward from the shore for tens of meters before collapsing in early February, but it reappeared a few days later. By the end of the first week of March 2018 the flow was about 17 m above sea level; its growth rate had slowed, adding only one meter by late March.

The NOAA/CIMSS Volcanic Cloud Monitoring system generated an alert for an ash cloud moving WNW, as imaged by S-NPP VIIRS, at 0330 UTC on 5 January 2018; Himawari-8 imagery subsequently showed that the eruption began around 0220 UTC. The Darwin VAAC reported two discrete ash plumes drifting W at 2.1 km altitude during the day. After local reports of the eruption Samaritan Airlines flew administrators from the Wewak district to investigate, enabling photographs of ash and steam emissions (figure 2).

Figure 2. Steam and ash emerged from a vent near the summit of Kadovar Island and drifted WNW on 5 January 2018. The view is looking NW with the SE flank of Kadovar in the foreground. In the upper photo, the island in the background is Viai Island about 30 km NW. Photo by Ricky Wobar, administrator of the Wewak district. Courtesy of Samaritan Aviation, posted on Facebook on 5 January 2018.

The following day, 6 January 2018, photos from a Samaritan Air flight showed that dark gray ash and steam plumes rising from a crater on the SE side of the summit had intensified (figures 3 and 4). It was estimated that 50 or 60% of the island was covered in volcanic debris, which appeared to be primarily ash along with some pyroclastic flows. According to the International Federation of Red Cross and Red Crescent Societies (IFRC), the entire population of Kadovar, about 600 people who lived on the N side of the island, was relocated to nearby Blup Blup Island which is home to about 800 residents. RVO reported minor ashfall on Kairiru and Mushu islands (115 km WNW), and on mainland Papua New Guinea at Mt. Uru in Yangoru (130 km W), Woginara (140 km W), and the Wewak District (100 km W).

Figure 3. Ash and steam plumes rose from distinct vents on the SE side of the summit at Kadovar. View is to the NE, with Blup Blup volcano located about 12 km in the distance. Photo by Ricky Wobar likely taken on 6 January 2018, published by ABC News on 8 January 2018. Courtesy of ABC News.

Figure 4. Ash and steam emissions intensified from vents at the summit of Kadovar Island on 6 January 2018. Posted on Facebook, 6 January 2018 by Samaritan Aviation.

Also on 6 January 2018, missionary Brandon Buser set out from Wewak to visit Bam by boat. He observed the steam and ash plumes of Kadovar from about 75 km away. About 25 km W of the island, he felt falling ash. From a few hundred meters offshore he witnessed the ash and steam plumes rising from near the summit as he circled the S and E sides of the island (figures 5-8).

Figure 5. Locations of the following photographs of the eruption at Kadovar on 6 January 2018 correspond closely to the purple spots where the boat slowed down on its trip around the island. North is to the top. Numbers indicate approximate locations of the following figures 6-12. Courtesy of Brandon Buser. Base map courtesy of Google Earth.

Figure 6. An ash plume drifted NW from the summit of Kadovar as viewed from a boat a few hundred meters off the SW flank on 6 January 2018. Courtesy of Brandon Buser.

Figure 7. Ash drifted WNW from Kadovar and also covered the vegetation on the SSW flank on 6 January 2018 in this view from a boat a few hundred meters off the SSW flank. Courtesy of Brandon Buser.

Figure 8. Dark ash and white steam both rose from vents at the summit of Kadovar on 6 January 2018. Debris and ashfall killed and denuded the trees on the SE flank, and covered the ground. View is from a boat a few hundred meters off the SE flank. Courtesy of Brandon Buser.

While preparing to head E to Bam, Buser witnessed an explosion that sent large plumes of ash and steam skyward from the SE flank, and a significant cloud of volcanic debris was ejected outward and down the SE flank; large boulders fell into the ocean. Heading rapidly E away from the eruption, he took additional photographs (figures 9-12).

Figure 9. Dark gray ash and white steam billowed up from a vent near the summit of Kadovar on 6 January 2018 at the start of an explosion. The denuded vegetation and bare slopes on the SE flank indicated the extent of the recent activity. The view is from a boat a few hundred meters offshore of the NE flank. Courtesy of Brandon Buser.

Figure 10. An explosion witnessed at Kadovar on 6 January 2018. Steam rose from a vent near the summit (right), dark gray ash billowed up from the SE flank, and brown dust and debris descended the SE flank into the ocean (left) in this view from a few hundred meters off the NE flank. Courtesy of Brandon Buser.

Figure 11. A large explosion at Kadovar witnessed on 6 January 2018. Light gray steam and ash rose from near the summit and drifted NW covering the N half of the island in ash; a large eruption of dark gray ash shot upward from a different vent on the SE flank surrounded by dust and debris that traveled outward at its base. Larger debris caused splashing in the water off the SE flank (left). View is from a few kilometers off the NE flank. Courtesy of Brandon Buser.

Figure 12. The plumes of steam, ash, and debris from the explosion moments earlier at Kadovar on 6 January 2018 rose and began to drift NW covering the island. Blocks landing in the ocean on the SE flank created spray along the shoreline (left). View is from a boat a few kilometers NE of the island. Courtesy of Brandon Buser.

The Darwin VAAC reported on 6 January 2018 that a continuous ash plume was identifiable in satellite imagery moving W and WNW at 2.1 km altitude. By 7 January, the plume could be identified about 220 km WNW in satellite images (figure 13). During their return trip from Bam on 8 January 2018, the missionaries again circled the island and noted that the eruption seemed to be occurring from different vents. The island was covered in ash, and they became covered with wet ash as they traveled under the drifting ash plume. The Darwin VAAC reported the plume drifting WNW extending about 185 km on 8 January. They also noted that the influence of the sea breeze was also spreading minor ash to the SW. Continuous ash emissions were observed by the Darwin VAAC through 11 January, drifting W and NW at 2.1 km altitude.

Figure 13. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite captured the eruption of Kadovar that began two days earlier on 7 January 2018 as a plume of ash and steam that streamed NW from its crater. A second smaller plume, also drifting NW, is visible SE of Kadovar from unrelated activity at nearby Manam, one of Papua New Guinea's most active volcanos. Brown-green plumes visible in the water S of Kadovar near the coast of the mainland, are caused by sediment from the Sepik and Ramu rivers on the mainland. Courtesy of NASA Earth Observatory.

RVO reported a significant escalation in activity during 12-13 January 2018. An explosion during the previous night ejected large incandescent boulders from the fracture on the SE flank. Residents on Blup Blup (15 km N) could see incandescence high on the volcano's flank. During a flyover on 13 January, RVO noted variable steam and gas emissions rising to 1 km above the Main Crater and identified five distinct vents (figure 14). The SE Coastal Vent was very active with dense white steam emissions rising 600 m from the vent (figure 15). A dome of lava was visible at the base of the steam plume, but no incandescence was observed. The Southern Coastal Vent had been vigorously steaming a few days earlier, and RVO interpreted it to be the source of the incandescent blocks in the explosion a few days before.

Figure 14. A sketch map of the five newly identified vents at Kadovar, 14 January 2018, from an RVO overflight the previous day. Courtesy of RVO (VOLCANO INFORMATION BULLETIN- No. 08 14/01/2018).

Figure 15. A vigorous steam plume rose from the SE Coastal Vent at Kadovar on 13 January 2018 while an ash plume rose from Main Crater at the summit. Photo by the office of Allan Bird, Governor of East Sepik Province. Courtesy of RVO (VOLCANO INFORMATION BULLETIN- No. 08 14/01/2018).

Reports of continuous ash emissions at 2.1 km altitude drifting WNW from the Darwin VAAC resumed on 16 January. A brief emission to 3.7 km was also noted that day. Pilot reports on 17 and 18 January indicated that ash was still in the area as high as 3-3.7 km altitude drifting W. The reports of emissions from the Darwin VAAC continued through 24 January. Ash emissions were generally continuous at altitudes from 2.4 to 3 km, although low level emissions of primarily steam and gas were observed on 20 January that included intermittent phases of increased ash content. The plume drift direction was variable, with periods when ash drifted S and SE in addition to the generally prevailing NW and W directions.

During 18-22 January 2018, the Main Crater continued to produce moderate to dark gray ash plumes that rose 500-800 m above the summit, drifting locally S and SE, and a continuous steam plume from the SE Coastal Vent rose as high as 800 m above the island. An incandescent lava flow slowly extruded from the SE Coastal Vent. By the last week of January, the ash plumes were only rising about 100 m above the Main Crater and drifting W; weak incandescence was still observed at night. The white steam plume from the SE Coastal Vent rose closer to 400 m above the island. RVO estimated that the lava flow had risen to about 50 m above sea level and extended 150-200 m out from the coast.

In their report on 2 February 2018, RVO noted that the lava flow continued to grow. A distinct lobe had pushed out from the seaward nose of the flow, by about 20-30 m; it appeared to be channeled by levees which had developed at the flow's sides. At 1830 local time on 1 February, a collapse of the side of the flow facing Blup Blup was observed; it resulted in a plume of gray ash and then vigorous steaming at the collapse site, which also was incandescent at night. The main body of the flow significantly bulged upwards, with a distinct 'valley' visible between the bulge and the island's flank.

RVO reported that on 9 February the lava flow at the SE Coastal Vent had collapsed, causing 5-6 minor tsunamis less than 1 m high that were observed by residents on Blup Blup's E and W coasts. The waves were reported at 1050, before the main collapse of the dome. In a 12 February report, RVO noted that activity from Main Crater consisted of white plumes rising 20 m and drifting a few kilometers SE accompanied by weak nighttime crater incandescence. Activity renewed at the SE Coastal Vent shortly after the collapse of the flow on 9 February 2018; lava re-emerged a few days later, connecting a lava island to the coastline again. Continuous steam emissions from both the Main Crater and the SE Coastal Vent were interrupted by dark ash plumes on 16 and 20-22 February, and occasional explosions were heard by residents on nearby islands. Minor ashfall was reported on Blup Blup on 21 and 22 February.

Eruptive activity continued during March 2018, although at a slower rate. The Main Crater generally produced continuous emissions of white steam and intermittent explosions with dark ash plumes; incandescence was usually visible at night from Blup Blup. According to the Darwin VAAC, a pilot reported an ash plume at 3.9 km altitude drifting SE on 2 March; it was not visible in satellite imagery due to meteoric clouds. The lava flow extruding from the SE Coastal Vent continued to grow, creating a dome that grew from 7-8 m above sea level to 10-17 m above sea level by 8 March. Dark ash emissions from the vent and nighttime incandescence were common. The growth rate slowed later in the month, and only one meter of change was observed between 10 and 20 March.

Satellite data. The MIROVA project recorded thermal anomalies from Kadovar in early January and early March 2018 (figure 16). MODVOLC thermal alerts were issued on three days; 15 and 22 January, and 7 February 2018. During January, small SO₂ plumes were recorded by NASA satellites on four occasions (figure 17).

Figure 16. The MIROVA project thermal anomaly graph for Kadovar from 11 May 2017 through March 2018. The first anomaly in early January 2018 correlates with observations of the first reported explosion. Courtesy of MIROVA.

Figure 17. SO2 plumes from Kadovar were detected several times during January 2018 by the OMI instrument on NASA's Aura satellite. Courtesy of NASA Goddard Space Flight Center.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. Kadovar is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. The village of Gewai is perched on the crater rim. A 365-m-high lava dome forming the high point of the andesitic volcano fills an arcuate landslide scarp that is open to the south, and submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. No certain historical eruptions are known; the latest activity was a period of heightened thermal phenomena in 1976.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea, Contact: steve_saunders@mineral.gov.pg, ima_itikarai@mineral.gov.pg; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NOAA, Cooperative Institute for Meteorological Satellite Studies (CIMSS), Space Science and Engineering Center (SSEC), University of Wisconsin-Madison, 1225 W. Dayton St., Madison, Wisconsin 53706, USA (URL: http://cimss.ssec.wisc.edu/); International Federation of Red Cross and Red Crescent Societies (IFRC) (URL: http://www.ifrc.org/); Samaritan Aviation (URL: http://samaviation.com/, https://www.facebook.com/samaritanaviation/); Brandon Buser (URL: https://ethnos360.org/missionaries/brandon-and-rachel-buser, https://www.facebook.com/brandon.buser.35); ABC News (URL: http://www.abc.net.au/news/2018-01-08/tsunami-warning-for-communities-near-erupting-png-volcano/9311544); Google Earth (URL: https://www.google.com/earth/).

During the first half of 2017, phreatic explosions at Rincón de la Vieja occurred on 23 May, 11-12 June (however, clouds obscured visible observations), 18 and 23 June, and 5 July (BGVN 42:08). This report describes activity from 6 July through December 2017. Information comes from the Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

After a small phreatic explosion on 5 July 2017, there were no further reports of any explosions until 29 September when OVSICORI-UNA reported that at 0848 a small phreatic explosion produced a plume that rose 1 km above the crater rim (figure 27); material also flowed down the S flank.

Figure 27. Webcam image of a phreatic explosion at Rincón de la Vieja on 29 September 2017. Courtesy of OVSICORI-UNA (color adjusted).

According to OVSICORI-UNA, events on 3 October at 0848 and 1445 generated plumes that rose 700 m and 1,500 m, respectively. OVSICORI-UNA also reported that on 9 October at 1048, a small explosion produced a plume that rose 700 m above the crater rim. According to news reports (The Costa Rica Star and CRHoy.com) quoting OVSICORI-UNA, an explosion on 22 October at 0640 generated a steam-and-gas plume that rose about 1 km above the crater. There were no further reports of an explosion through the end of December.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge that was constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of 1916-m-high Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A plinian eruption producing the 0.25 km3 Río Blanca tephra about 3500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); CRHoy.com (URL: http://www.crhoy.com/); The Costa Rica Star (URL: https://news.co.cr/).

A phreatic eruption at Turrialba in January 2010 heralded a series of brief eruptions during subsequent years. Explosions and emissions containing ash increased in 2015 and 2016 (BGVN 42:06). The current report indicates that increased activity continued during 2017. The information below comes from the Observatorio Vulcanologico y Sysmologico de Costa Rica-Universidad Nacional (OVSICORI-UNA) unless otherwise indicated.

Frequent ash emissions, both passive and explosive events, rose the heights of less than 1 km above the crater and were blown downwind, causing ashfall in communities within about 40 km, and a sulfur odor at greater distances. Fumarolic plumes described as consisting of water vapor, aerosols, and magmatic gases were also common from the West Crater. Volcanic seismicity was variable, often corresponding to changes in activity.

Activity during January-June 2017. During the first part of January, no explosions took place. Based on webcam and satellite views, the Washington Volcanic Ash Advisory Center (VAAC) reported that on 22 January, an ash plume rose to an altitude of 4 km and drifted E. The VAAC reported ongoing ash emissions on 27 January.

On 1 February, OVSICORI-UNA reported that since 27 January the seismic network had recorded variable-amplitude, discontinuous tremor indicative of moving pressurized volcanic fluid. Passive emissions of ash were observed during 1-2 February, rising as high as 500 m above the crater. Ashfall was reported in the area of the capital, San Jose (about 37 km WSW), including Desamparados, Calle Blancos, and Tres Ríos (27 km WSW), and a sulfur odor was noted in San Pablo Heredia (35 km W). An explosion at 0900 on 4 February generated an ash plume that rose 300 m and drifted W. Almost continuous ash emissions rose at most 500 above the crater during 4-5 February and drifted WSW (figure 48).

Figure 48. An ash explosion from Turrialba on 4 February 2017 at 1145, taken by an RSN camera at the summit. Courtesy of RSN:UCR-ICE (Resumen de la Actividad Sismica y Eruptiva del Volcan Turrialba, 03 de febrero de 2017).

OVSICORI-UNA reported that at 1610 on 8 February, an ash plume rose 300 m and drifted N. An event at 1531 on 10 February also produced an ash plume, but inclement weather prevented observations. During 11-12 February, variable amplitude tremor was detected, and at night hot blocks ejected from the vent landed in Central Crater. Several events on 13 February (at 0255, 0305, 0415, and 1459) produced ash plumes that rose as high as 1 km and drifted N, NW, and W. Small ejections of incandescent material fell around the active crater during the early morning. On 14 February continuous emissions of gas and steam with low ash content were visible. A strong sulfur odor was reported in San Pablo de Oreamuno (25 km SW). High-amplitude tremor remained constant during 15-16 February and sporadic gas emissions with minor amounts of ash drifted S and E; occasional ballistics were ejected from the crater. During 16-17 February tremor amplitude decreased and sporadic gas emissions with low ash content rose no higher than 300 m and drifted NW and SW. Similar emissions were observed during 20-21 February, drifting NW and NE.

Weak gas emissions during 20-21 March sometimes contained small amounts of ash that rose no higher than 100 m above the crater rim and drifted SW. Volcanic tremor had medium and variable amplitude, and a few low-frequency (LF) earthquakes were recorded. A weak ash emission was visible during 1800-1940 on 25 March. Periods of more intense crater incandescence, from possible Strombolian activity, corresponded to higher tremor amplitude during 0330-0530 on 26 March. Later that day a small plume with minor ash rose 500 m above the crater and drifted S and SE. An event at 0752 on 28 March generated an ash plume that rose 300 m and drifted S.

Ash-and-gas plumes rose 500 m above the crater during 31 March-1 April, and ashfall was reported at the Juan Santamaría airport (48 km W). Ash plumes rose 500 m at 1700 on 2 April, and 200 m at 0601 on 4 April. A passive ash emission occurred on 16 April. An event at 0751 on 17 April generated a plume containing minor amounts of ash that rose 500 m above the crater and drifted SW. On 18 April, a diffuse plume consisting of gas and sometimes ash rose 1 km above the crater and drifted W.

An event at 1700 on 5 May generated a weak ash plume that rose 500 m above the crater and drifted SW. Two short-amplitude events occurred at 1702 and 1820, though it was uncertain if they were associated with an explosion. During 5-7 May volcano-tectonic (VT) and long-period (LP) earthquakes were detected, as well as variable-amplitude tremor. At 1250 on 6 May, an event produced a plume that rose 300 m and drifted W. Passive ash emissions occurred between 1250 and 1730 on 6 May, and at 1000 on 7 May, that rose no higher than 1 km. At 0902 on 9 May an event generated an ash plume that rose 500 m and drifted NW.

An explosion on 10 May was followed by weak and passive ash emissions. Several LP earthquakes were recorded, and inflation continued. Gas measurements indicated a sulfur dioxide flux of 1,000 tonnes/day, and a high carbon dioxide/sulfur dioxide ratio. An event at 0900 on 12 May generated a plume, though poor visibility prevented a height estimate. An event at 0730 on 14 May generated a plume that rose 500 m above the crater rim and drifted N. Low-amplitude tremor was detected during 15-16 May, and a discontinuous ash plume rose no more than 500 m and drifted N and NW.

Ash emissions observed during 17-23 May rose as high as 1 km above the vent. Ashfall was reported in El Tapojo and Juan Viñas (15 km SSE) during 17-18 May, and in Capellades (along with a strong sulfur odor) during 19-20 May. During 23-30 May, tremor amplitude fluctuated from low to high levels, often corresponding to emission characteristics; periods of VT and LP events were also recorded. During 24-26 May several passive ash emissions rose no higher than 500 m above the vent and drifted NW and SW. Frequent and small explosions during 26-27 May generated ash plumes that rose higher than 500 m above the vent and ejected material higher than 200 m and no farther than 100 m towards Central Crater. Small explosions during 27-29 May produced ash plumes that rose 300-500 m. Fumarolic plumes during 30-31 May occasionally contained ash that rose no higher than 300 m above the crater rim and drifted NW.

On 3 June at 1930 an event produced an ash plume that rose 300 m and drifted SW. During 7-13 June, tremor amplitude fluctuated from low to medium levels and periods of small VT events and many small-amplitude LP events were also recorded. Fumarolic plumes rose as high as 1 km above the vent and drifted mainly NW, W, and SW. Gas emissions during 14-15 June sometimes containing ash rose no higher than 300 m above the crater. Events at 0620 and 1405 on 16 June generated ash plumes that rose 500 m and drifted NW, and 200 m and drifted S, respectively. Passive ash emissions during 19-20 June rose as high as 1 km and drifted in multiple directions. During 20-25 June fumarolic plumes rose as high as 1 km above the crater; the gases were strongly incandescent the night of 22-23 June.

Drone observations on 29 June 2017. According to an RSN:UCR-ICE report and meeting abstract (Ruiz and others, 2017), government officials flew a drone over the volcano on 29 June 2017. The observations showed profound changes in the morphology of the active crater since a previous overflight on 30 March 2016. In March 2016, the active crater exhibited internal landslides, an accumulation of materials at the foot of the W wall, and a ring of fumaroles surrounding a small opening that constituted the point of ash emission. The active crater was narrow and had an oblong shape, with a longer axis in the E-W direction.

During the recent overflight, the active crater was deeper and wider, elliptical, with its longest axis in the SW-NE direction, coincident with the preferential direction of explosions. In the N and NE sectors of the crater floor ash and blocks had accumulated. The most significant feature of the crater's central sector was an opening with a major axis of about 50 m across from which incandescent material was observed; the group believed this incandescence originated in the small lava lake from which passive ash emissions or small explosions arise. The authors stated that lava was present on the crater floor, forming a small lava pool (15 x 25 m).

Activity during July-December 2017. During 29 June-11 July seismicity was characterized by low-to-medium amplitude tremor and a small number of low-amplitude VT and LP events. Fumarolic plumes and occasional ash rose as high as 1 km above the West Crater fumaroles. Incandescence from the main crater was recorded at night. Minor ashfall and a sulfur odor was reported in areas of San José including Rancho Redondo, Goicoechea, Moravia, San Pedro Montes de Oca, Guadalupe, and Coronado, and in San Rafael and Barva (Heredia). Parque Nacional Volcán Turrialba staff reported that ash was deposited between La Silvia and La Picada farms. Events at 1325 on 10 July and 1545 on 11 July generated plumes that rose 300 and 500 m above the crater rim, respectively.

Daily explosions over 12-17 July produced gas and ash plumes that rose 200-500 m and generally drifted NW, W, and SW. Multiple events on 15 July caused ashfall in Sabanilla de Montes de Oca (30 km WSW), Ipis (27 km SW), El Carmen de Guadalupe, Purral (26 km WSW), Guadalupe (32 km WSW), and Tibás (35 km WSW). A sulfur dioxide odor was also reported in San José (36 km WSW), Tibás, Guadalupe, Escazú (42 km WSW), and Puriscal (65 km WSW). During 19-24 July fumarolic plumes rose as high as 500 m, and on most nights incandescence emanated from West Crater. The emissions contained ash during 20-22 July; minor ash fell in Coronado (San José) on 20 July, and in Sabanilla de Montes de Oca on 22 July.

Events on 26 July, 9 August (1607), 21 August (1012), 24 August (0715), 28 August (1025), 5 September (0820 and 1550), 11 September (0730), 13 September (0820 and 1555), 14 September (0600), 18 September (0703), 25 September (1112), and 26 September (0910) produced plumes that rose 100-500 m above the crater rim and drifted NW, SW, N, and W.

During 27 September-1 October and on 3 October, daily events generated plumes that rose as high as 1 km above the crater rim and drifted NW, W, SW, and S. On 30 September explosions ejected hot material out of West Crater and minor ashfall was reported in Coronado (San José). On 3 October, ash fell in Santa Cruz (7 km SE), Las Verbenas, Santa Teresita, Calle Vargas, Guayabito, and La Isabel.

Events on 6 October (0815), 9 October (1040), 11 October (0927), and 20 October (0825) produced plumes that rose 50-300 m above the crater rim and drifted NW and N. Events at 1030, 1105, and 1445 on 30 October generated ash plumes that rose 200-500 m above the crater rim and drifted NW, W, and SW. Ashfall was reported in the community of Pacayas (about 12 km SSW).

The Washington VAAC reported that an ash emission was observed in webcam images on 4 November; ash was not identified in satellite images, though weather cloud cover was increasing and may have obscured views. According to OVSICORI-UNA, another ash emission began before 0730 on 13 November and intensified around 0830, generating an ash plume that rose 500 m above the crater rim and drifted SW. A small event at 1319 on 1 December generated a weak ash plume that rose 50 m above the crater rim and drifted SW.

Reference. Ruiz, P., Mora, M., Soto, G.J., Vega, P., Barrantes, R., 2017. Geomorphological mapping using drones into the eruptive summit of Turrialba volcano, Costa Rica. University of Costa Rica. Abstract V23A-0455, AGU Fall meeting of American Geophysical Union, New Orleans, 12 Dec 2017.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Red Sismologica Nacional (RSN) a collaboration between a) the Sección de Sismología, Vulcanología y Exploración Geofísica de la Escuela Centroamericana de Geología de la Universidad de Costa Rica (UCR), and b) the Área de Amenazas y Auscultación Sismológica y Volcánica del Instituto Costarricense de Electricidad (ICE), Costa Rica (URL: http://www.rsn.ucr.ac.cr/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).

Recent activity at Poás has been characterized by intermittent phreatic explosions from the hyperacid lake (figure 118). Explosions were noted in June-August 2016 (BGVN 42:03), but there were no reports explosions since then through March 2017. This report summarizes activity from April 2017 through March 2018. During this period, activity increased substantially during April-August 2017 and thereafter waned. No explosions were reported during 7 November 2017-31 March 2018. Information below was primarily drawn from reports issued by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Figure 118. Landsat imagery of Poás taken 11 April 2016. Courtesy of Digital Globe and Google Earth.

Activity during April 2017. According to OVSICORI-UNA, activity increased substantially at the beginning of 2017, with significant increases in seismicity, steam-and-gas emissions, and surface deformation. Seismicity included numerous long-period (LP) earthquakes, more than 200 daily events between the end of March and the beginning of April, and weak explosions since 30 March. Deformation was characterized by inflation, with a vertical increase of more than 1 cm in a three-month period and an increase of 3 mm horizontally between two sites S and N of the crater separated by 1,570 m.

Gas emissions dramatically shifted toward a more magmatic composition, particularly after 30 March. Sulfur dioxide measurements on 4 April were about an order of magnitude greater than those on 28 March (~180 ± 65 tonnes/day (t/d) vs. ~19 ± 8 t/d), with the dome contributing 25% and the lake 75% of the flow. The increased flow was accompanied by the emergence of new fumaroles that may have contributed to the warming of the lake (which went from 35 to 40°C in just one week). In April, the lake quickly changed from a milky green color to a milky gray color, which suggested that emissions of magmatic gases from vents beneath the lake may have increased. The dome is on the S side of the crater lake and was formed during phreatomagmatic activity between 1953 and 1955; it has been a site of persistent fumarolic degassing for the last 200 years.

OVSICORI-UNA reported that a strong 40-minute phreatic explosion from an area between the lava dome and the hot lake occurred on 12 April 2017, starting at 1830. A plume of steam, altered rocks, sediments, and gases was produced; the height of the column could not be determined due to poor visibility. Ash fell around the crater and in Bajos del Toro (7 km WNW). The water level in the Desague River, with headwaters at the S part of the crater, increased by 2 m. According to news articles (Tico Times, The Costa Rica Star), the National Emergency Commission evacuated residents living near the river. The Poás Volcano National Park closed the next day and has remained closed through March 2018.

On 13 April, at 1546, an eight-minute-long explosion produced a plume that rose 500 m above the crater rim. The event rendered a webcam on the N rim inoperable. Explosions at 0758 (strong) and 1055 on 14 April generated plumes that rose to an undetermined height.

A 10-minute-long event that began at 0810 on 15 April again produced a plume of unknown height. Frequent (2-3 events per hour) small, short-lived, phreatic explosions were recorded by seismographs during 15-16 April. A plume that rose 500 m followed an explosion at 0946 on 16 April. Later that day, at 1350, an event generated a plume that rose 1 km. A news article (The Costa Rica Star) reported that boulders as large as 2 m in diameter fell in an area 30 m away from a tourist trail, breaking a water pipe. Rocks also damaged fences and concrete floors in viewing areas. Small, frequent, and short-lived phreatic explosions continued to be recorded through 18 April. A video posted by a news outlet (The Costa Rica Star) showed an explosion ejecting incandescent material.

According to OVSICORI-UNA, on 20 April a dense steam plume rose from a vent in the newly-forming pyroclastic cone at the site of the old dome in the hot lake. Sulfur dioxide levels increased from 1,000 t/d on 13 April to 2,500 t/d on 20 April. During 20-22 April Strombolian activity ejected tephra that fell around the vent within a 300-m radius. Gas-and-ash plumes rose 200 m above the vent. The Cruz Roja (Red Cross) in Grecia reported ashfall in Alajuela (20 km S), Fraijanes (8 km SE), San Miguel (40 km SSE), Carbonal (8.5 km SSW), Cajón (11 km SSW), San Francisco, San Roque (23 km SSE), and San Juan Norte de Poás (8.5 km S). Explosions at 1316 and 1603 on 22 April produced plumes of unknown height. Several more explosions were recorded that day; an event at 2212 was very intense, ejecting bombs large distances. An event at 1215 on 23 April generated a plume of unknown height.

Figure 119. Photo showing location of the acid lake and dome at Poás during or after April 2017. The dotted line follows the outline of the great lake that covered the entire bottom of the caldera during the first half of the last century. Courtesy of OVSICORI-UNA. Borde de Antiguo lago is "Edge of the Ancient Lake"; Tercio norte: Lago is "north third of the lake"; domo is "dome"; Tercio sur: Playón o Angiguo lago is "South Tercio: Playón or Angiguo lake; Fumarola abril 2017 is "fumarole in April 2017; sector de fumarolas 2005-2006 is "sector of fumaroles 2005-226. Courtesy of OVSICORI-UNA (El Domo y el Lago Caliente en el Volcán Poás: Estructuras Básicas para Comprender las Erupciones Actuales. Nota técnica: 16 de abril de 2017).

Activity during May 2017. OVSICORI-UNA reported that large explosions were seismically recorded at 0621 on 1 May and at 1724 on 6 May, though poor visibility prevented visual confirmation of the events. On 10 May, ash emissions were observed. Gas emissions were measured by an instrument mounted on a drone, revealing a gas plume rich in sulfur dioxide and low in carbon dioxide. Deformation was high, with vertical inflation of 3 cm since February.

During 17-23 May, plumes consisted mainly of gas and steam, sometimes including solid material, that rose no more than 1 km above the vent. During 25-26 May, ashfall was reported in some communities around the volcano. Small phreatic explosions were recorded sporadically during 27-30 May.

Activity during June 2017. An explosion reported by OVSICORI-UNA at 1200 on 2 June generated a plume consisting of steam, gases, and minor amounts of ash that rose 600 m above the crater. Another event recorded at 1353 could not be confirmed visually due to weather conditions. An event at 0858 on 6 June generated a plume that rose 1 km.

During 7-8 June, the webcam recorded strong emissions of steam, magmatic gases, and particulates. A sulfur odor was reported in Alajuela, San Ramon (24 km WSW), and Barva (23 km SSE), and incandescence in the area of the crater was recorded at night. OVSICORI-UNA noted that during 8-9 June, a plume of steam, magmatic gases, and particulates rose from two vents; the lake had evaporated and exposed the vents. A minor sulfur odor was reported on the campus of the Universidad Nacional in Heredia. Explosions at 1610 and 1750 on 11 June generated plumes that rose 300 and 600 m above the crater, respectively. Plumes from the vents rose 1 km during 12-13 June. A sulfur odor was noted in Quesada (26 km ENE), Santa Ana (30 km SSE), San José de Alajuela, and San Juanillo Naranjo.

Gas emissions during 13-15 June rose no higher than 500 m above the crater rim and drifted N. During breaks in weather, observers near the crater on 16 June noted ash emissions rising less than 1 km above the crater rim and drifting N. Ash emissions from events at 1340 on 18 June, and 1100 and 1350 on 20 June, rose less than 1 km.

During 20-25 June, plumes of reddish-colored ash, water vapor, and magmatic gases were recorded rising as high as 500 m above two vents during 20-21 June. Magmatic gases and steam plumes rose as high as 1 km above the vents the rest of the period.

Webcams recorded intense incandescence at night during 28-29 June from the bottom of the crater. A sulfur odor was noted in San Rafael de Poás (12 km SSW) and Vara Blanca (10 km ESE). An event at 1115 on 19 June generated a plume that rose 1 km above the vents. An event at 1450 may have generated a plume, but poor visibility did not allow for confirmation.

Activity during July-December 2017. According to OVSICORI-UNA, frequent, but weak Strombolian activity during 1-4 July ejected incandescent material that fell around vent A (Boca Roja). Plumes of steam, magmatic gases, and particulates rose at most 500 m from the vents.

During 4-9 July, plumes of steam, magmatic gases, and aerosols rose 200-600 m above vents A (Boca Roja) and B (Boca Azufrada). Minor incandescence from the bottom of the crater was observed during 4-5 July, and a strong sulfur odor was reported in some areas of Alajuela and Heredia. During 5-7 July, grayish-red ash emissions rose intermittently from vent A, and on 7 July a loud "jet" sound was noted in Mirador. A strong sulfur odor and minor ashfall was reported in some areas of Alajuela. An event at 1450 on 10 July generated a plume that rose 300 m.

OVSICORI-UNA reported that during 12-17 July, gas plumes rose as high as 1 km above vents A and B and drifted SW and NW. From 19 through 24 July plumes of steam, magmatic gases, and aerosols were emitted from vent A, and plumes of steam, gases, and abundant yellow particles of native sulfur rose from vent B. Plumes rose 300-500 m above the vents and drifted W and SW.

On 1 August an event passively produced a plume that rose 500 m above the crater. Incandescence from the bottom of the crater was recorded at night by the webcams. Sulfur dioxide was emitted at a rate of 1,000-1,500 t/d. Activity on 3 August was similar to that in July, except that plumes rose as high as 1 km above the vents. Gas plumes continued to rise from the vents and drift SW and NW at least through 8 August. OVSICORI-UNA reported additional explosions on 22 August (1517 local), 24 August (0920 and 0930), 29 August (0945), 13 September (0820), and 6 November (0915) that rose 300-600 m above the crater rim.

Seismicity. During May and June, some volcano-tectonic (VT) and LP earthquakes were recorded, and tremor levels generally ranged from low-to-moderate amplitude, although higher tremor levels were sometimes detected during 22-30 May. The tremor amplitude often corresponded to the vigor of emissions of steam, magmatic gases, and material from fumarolic vents. Seismic activity was not identified after 30 June, except for a single report that indicated that during 11-14 August seismographs detected low-amplitude tremor, some VT earthquakes, and high-frequency signals indicating gas emissions.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); National Emergency Commission (CNE) (Comisión Nacional de Prevención de Riesgos y Atención de Emergencias (CNE) (URL: http://www.cne.go.cr); Tico Times (URL: http://www.ticotimes.net/); The Costa Rica Star (URL: https://news.co.cr/).

Ebeko volcano is located on the remote N end of Paramushir Island in the Kuril Islands and contains many craters, lakes, and thermal features. Eruptions and ash plumes were observed at Ebeko in early July 2010 (BGVN 36:07). No additional activity was reported from Ebeko until October 2016, marking the start of the more recent eruptive cycle. New explosive eruptions accompanied by ash fall began on 20 October 2016 through April 2017 (BGVN: 42:08). Explosive eruptions, ash plumes, ash falls were observed and reported at a regular frequency during this reporting period from May through November 2017 (table 4). Eruptions were reported by observations from residents in the town of Severo-Kurilsk, located about 7 km E of Ebeko, by volcanologists and by satellite imagery. The Kamchatkan Volcanic Eruption Response Team (KVERT) is responsible for monitoring Ebeko, and is the primary source of information. The Aviation Color Code (ACC) remained at Orange throughout this reporting period. This color is the second highest level of the four color scale.

Table 4. Summary of activity at Ebeko volcano from May 2017 to November 2017. Aviation Color Code (ACC) is a 4-color scale. Data courtesy of KVERT.

Explosives events, bursts of ash, ashfall, and ash plumes were reported throughout this period, and were quite variable in appearance (figures 12-16). Minor amounts of ash fell in Severo-Kurilsk on 25 April, 2-3, 6-7, 16, and 18 September, and 22 November. Ash plume altitudes during this reporting period ranged from 1.5 to 4 km; with the highest altitude of 4 km recorded on 2 September (table 4).

Figure 12. Ash plume from an explosive event at Ebeko on 15 May 2017. Ash plume altitude reached 2 km. Photo by L. Kotenko, courtesy of Institute of Volcanology and Seismology IVS FEB RAS.

Figure 13. Ash plume from an explosive event at Ebeko on 23 May 2017. Ash plume altitude reached 2 km. Photo by L. Kotenko, courtesy of Institute of Volcanology and Seismology IVS, FEB, RAS.

Figure 14. Ash explosions from Ebeko on 10 August 2017 as seen from Severo-Kurilsk, 7 km E. Photo by V. Rashidov, courtesy of Institute of Volcanology and Seismology IVS FEB RAS.

Figure 15. Ash bursts up to 2 km on 22 August 2017. Photo by T. Kotenk. Courtesy of Institute of Volcanology and Seismology IVS FEB RAS.

Figure 16. Active crater of Ebeko volcano on 13 September 2017. Ash plume altitude reached 2.2 km. Photo by Ivan and Nataliya Cherkashiny. Courtesy of Institute of Volcanology and Seismology IVS FEB RAS.

MIROVA only identified two low-power thermal anomalies in the past year, one in late February 2017 and the other in late March 2017. A weak thermal anomaly was reported by KVERT on 31 July 2017.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Short pulses of intermittent eruptive activity have characterized Piton de la Fournaise, the large basaltic shield volcano on Reunion Island in the western Indian Ocean, for several thousand years. The most recent episode occurred during 31 January-27 February 2017 with an active vent located inside the Enclos caldera on the S flank, about 1 km SE of Château Fort and about 2.5 km ENE of Piton de Bert (BGVN 42:07). The next episode, discussed here, began on 14 July 2017 and lasted for about six weeks. Activity through February 2018 is covered in this report. Information is provided by the Observatoire Volcanologique du Piton de la Fournaise (OVPF) and satellite instruments.

A new fissure eruption began on 14 July 2017 on the S flank inside the caldera about 850 m W of Château Fort and lasted through 28 August. The fissure was initially 450 m long with seven active lava fountains. Within 48 hours the flow had reached its farthest extent, about 2.8 km from the fissure. Activity continued from the southernmost cone of the fissure with three active vents for a few weeks. Surface lava flows diminished, and activity was concentrated in lava tubes flowing SE from the cone with occasional breakouts and ephemeral vents along the flow field. The tremor signal briefly spiked with lava fountains on 16-17 August, and then ceased altogether on 28 August. A brief seismic swarm during 24 August-1 September led OVPF to conclude that magma had moved but did not open a new fissure. Inflation was intermittent through December, and then increased significantly during January before leveling off during February 2018.

Activity during June-July 2017. The brief seismic swarm of 17-18 May 2017 was followed by another brief increase in seismicity during the first few days of June 2017, but no surface eruption was reported. The inflation that occurred during the May event tapered off by early June. The volcano remained quiet until seismicity began increasing on 10 July 2017; this was accompanied by inflation recorded at the GPS stations as well. The observatory (OVPF) noted the beginning of seismic tremors, indicative of a new eruption, around 0050 on 14 July 2017. Webcams revealed that eruptive fissures opened on the S flank of the cone inside the Enclos caldera. A reconnaissance flight conducted later in the morning on 14 July indicated that the eruptive site was located 750 m SE of the Kala-Pele peak and 850 m W of Château Fort, about 2.2 km NE of Piton Bert (Figure 110).

Figure 110. Location of the Piton de la Fournaise eruption that began on 14 July 2017. Courtesy of OVPF/IPGP (Bulletin d'activité du vendredi 14 juillet 2017 à 15h30 Heure locale).

By 0930 that morning, the fissure extended over a total length of approximately 450 m. Seven lava fountains with a maximum height of 30 m were active (figure 111). The fountain farthest downstream began to build a cone with two arms of flowing lava. Satellite measurements indicated an initial flow rate of about 22-30 m³/s at the beginning of the eruption.

Figure 111. A new fissure opened on the S flank of the cone inside the Enclos caldera at Piton de la Fournaise on 14 July 2017. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du vendredi 14 juillet 2017 à 15h30 Heure locale).

The tremor intensity decreased significantly the following day; this was reflected in the decrease in the flow rates and the distribution of activity on the fissure. Only three lava fountains were active on 15 July 2017 near the downstream end of the fissure; they began to form two small cones with lava flows that merged into a single channel (figure 112). The fountains did not exceed 30 m in height. By 1400 on 15 July the flow front was 2.2 km SE from the fissure. Satellite instrument measurements suggested the flow rate had dropped to two m³/s. Sulfur dioxide anomalies were measured by the OMI satellite instrument during 14-16 July (figure 113).

Figure 112. Lava emerged from two vents and merged into a single flow at the eruptive site at Piton de la Fournaise on 15 July 2017 at 1400 local time. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 15 juillet 2017 à 16h30 Heure locale).

Figure 113. Sulfur dioxide anomalies were captured by the OMI instrument on the Aura satellite by NASA on 14 (left) and 16 (right) July 2017 at the beginning of the eruption at Piton de la Fournaise. Courtesy of NASA Goddard Space Flight Center.

Tremors fluctuated over the next few days with changes related to the growth and collapse of various the cones along the fissure. On 18 July, there were six active fountains (figure 114). The flow rate remained approximately 1-3 m³/s. Fountains reached 20 m high on 19 July and a third vent was visible forming on the N side of the main cone. During an overflight on 21 July, OVPF noted that all three vents were active, but lava was only flowing SE from the central one (figure 115). Lava tubes had begun to form downstream of the cone, with numerous breakouts creating small lateral expansion arms.

Figure 114. Six fountains were active along the fissure zone on 18 July 2017 at Piton de la Fournaise. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du mardi 18 juillet 2017 à 16h00 Heure locale).

Figure 115. Lava flowed SE from the central vent of three in the fissure zone at Piton de la Fournaise on 21 July 2017. The magmatic gases are drifting SSE to the upper left of the image. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du vendredi 21 juillet 2017 à 16h30 Heure locale).

OVPF measured the flow dimensions on 22 July as 2.8 km long and 0.6 km wide (figure 116); the flow front had not advanced in the previous seven days. A fourth vent on the N side of the cone was periodically emitting ejecta, and two flows were active; one moving SE towards Château Fort and the other moving towards the SW inside a lava tube. On 24 July OVPF measured the flow rate as 1-4 m³/s, and the total volume of lava to date as 5.3 ± 1.9 million m³. On 25 July 2017, local observers reported that the main vent on the SE flank of the cone was visible, as well as a second vent on the N flank of the growing cone. The main lava channel was clearly visible downstream of the cone with frequent overflows (figure 117), and active flow continued inside the lava tubes.

Figure 116. An outline of the active lava flow at Piton de la Fournainse on 22 July 2017. Base map courtesy of Google Earth. Annotations courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 22 juillet 2017 à 17h00 Heure locale).

Figure 117. The main lava channel flowed SE from the eruptive vent at Piton de la Fournaise on 25 July 2017. Photo copyright by Cité du Volcan/Arthur Vaitilingom). Courtesy of OVPF/IPGP (Bulletin d'activité du mercredi 26 juillet 2017 à 16h00 Heure locale).

By 30 July the flow intensity had decreased to about half of its original flow rate. The cone continued to grow, but no surface lava flows were observed (figure 118). The main vent rarely produced ejecta. Active lava was flowing in tunnels with a few minor breakouts near the cone. The flow front remained 2.8 km from the eruptive vent.

Figure 118. The eruptive vent of Piton de la Fournaise on 30 July 2017 showed no surface flows, but activity continued in lava tunnels. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du dimanche 30 juillet 2017 à 16h00 Heure locale).

Activity during August 2017-February 2018. The intensity of the tremors associated with the eruption continued to taper off into early August to levels below 20% of what they were at the beginning of the eruption, and this corresponded to a decrease in observed activity in the field. During an OVPF overflight on 2 August 2017 no flows or ejecta from the eruptive cone were seen, but a number of surface breakouts from lava tubes were still visible; the nearest to the cone was 520 m to the SE (figure 119). The main vent was completely blocked, but the smaller vent still had visible incandescence and strong degassing (figure 120).

Figure 119. Lava tubes and small breakouts at Piton de la Fournaise on 2 August 2017 (N to the lower right). The breakouts were several hundred meters SE of the main vent. The eroded cone in the upper right is visible in the upper left of figure 115 showing the relative location compared with the main fissure. See also figure 121 for relative location. 1) A hornito formed from overpressure in an underlying lava tube. 2) A 20-m-long flow from a breakout over an active tunnel. 3) Two ephemeral vents had recently opened in the roof of the tunnel just prior to this photo being taken. 4-5-6) The longest breakout flow observed was 220 m long and began at an ephemeral vent located downstream of points 1, 2, and 3. The flow surface was 10 m wide near 4), spreading out and cooling farther downstream (5 and 6). Incandescent lava was still visible near the flow front (6) in two lobes. 7-8) Two other breakout flows from ephemeral vents 520 meters from the main vent were also visible, 50 and 180 m long, respectively. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du mercredi 2 août 2017 à 16h30 Heure locale).

Figure 120. Visible incandescence and strong degassing were apparent from the smaller vent at the eruptive site on 2 August 2017 at Piton de la Fournaise. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du mercredi 2 août 2017 à 16h30 Heure locale).

Estimates of the flow rates during the first week of August were less than 1-2 m³/s, and the total lava volume emitted on the surface was measured at 7.2 +/- 2.3 million m³. A larger breakout from a tunnel on 5 August was visible in the OVPF webcams and fed a surface flow over several hundred meters for several hours. By 6 August 2017 the activity was focused mainly in lava tunnels with a few surface breakouts, although incandescence was visible from the small vent seen in imagery available in Google Earth (figure 121). Small ejecta was observed during 7-9 August from the remaining active small vent on the N flank of the cone (figure 122).

Figure 121. Imagery from Google Earth captured on 6 August 2017 showed incandescence and degassing from the small vent at the S end of the fissure at Piton de la Fournaise (left plume), as well as degassing from surface breakouts along the still active lava tunnels to the SE. Courtesy of Google Earth.

Figure 122. Only the small vent on the N side of the cone was still incandescent at Piton de la Fournaise on 9 August 2017. N is to the upper right. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du mercredi 9 août 2017 à 17h00 Heure locale).

Observations made on 14 August 2017 indicated lava was still active in tunnels as pahoehoe flows were observed about 2 km from the active vent. A brief increase in seismic and surface activity occurred on 16 August. The Piton de Bert webcam captured short-lived lava fountains at the E edge of the eruptive cone. Seismic tremor intensity increased rapidly and then oscillated during 16-17 August. The minor inflation of the cone that had been observed since 1 August ceased by 18 August. Field measurements on 21 August demonstrated a significant decrease in flow activity since 12 August. The volcanic tremor signal was stable at a low level on 25 August; it decreased significantly on 27 August and disappeared altogether about 0300 local time on 28 August 2017, leading OVPF to conclude the eruptive phase had ended.

A number of indications led OVPF to conclude that two migrations of magma that did not reach the surface occurred between 16 August and 1 September. Increased seismicity began on 16 August and was accompanied by a measured increase in SO₂; satellite measurements showed two areas of inflation SE of the active fissure between 7 and 25 August. A seismic swarm in the same area was recorded during 24 and 25 August (figure 123). Overflights by OVPF on 25 August did not identify any new fissures associated with the seismic events and inflation.

Figure 123. A seismic swarm on 24 and 25 August 2017 at Piton de la Fournaise led OVPF to conclude that magma was moving beneath the surface in an area SE of the active fissure zone. Courtesy of and copyright by OVPF/IPGP (Bulletin mensuel du lundi 2 octobre 2017).

After the seismic swarm, the number of daily seismic events decreased to less than one per day by the end of September 2017. OVPF reported minor inflation during the second half of October along with a slight increase in seismicity. Inflation stabilized in November but increased again during January 2018 (figure 124). A gradual increase in shallow seismicity beneath the summit craters was recorded during the second half of February. It was accompanied by an increase in CO₂ concentrations in the soil as well, which rose to some of the highest levels since measurements began in 2015.

Figure 124. Deformation at Piton de la Fornaise from 14 July 2017 to 28 February 2018. The eruption of 14 July- 28 August 2017 is shown in yellow. The y-axis measures the change in length in centimeters of a N-S line crossing the Dolomieu crater between two GPS receivers. The raw data is shown in black and the blue line is the data smoothed over a week. A rise means elongation and therefore swelling of the volcano; conversely, a decrease indicates contraction and therefore deflation of the volcano. Courtesy of and copyright by OVPF/IPGP (Bulletin mensuel du jeudi 1 mars 2018).

Thermal anomaly data. The MIROVA project thermal anomaly record shows both the episodic nature of the activity and the cooling signature of the flows that continued beyond 28 August 2017 when OVPF noted the cessation of tremors associated with eruptive activity (figure 125). The MODVOLC thermal alerts first appeared on 13 July 2017 and continued persistently with multiple daily alerts until 23 August 2017.

Figure 125. MIROVA thermal anomaly data for Piton de la Fournaise for the year ending 5 January 2018. The eruption of February 2017 had very little cooling after the tremors ceased at the end of February, but the July eruption had significant cooling evident for more than two months after the cessation of seismic tremors on 28 August 2017. Courtesy of MIROVA.

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise (OVPF), Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Nicaragua's San Cristóbal volcanic complex has exhibited sporadic eruptive activity dated back to the early 16th century. More consistent modern record keeping has documented short-lived eruptive episodes every year since 1999. Small explosions with intermittent gas-and-ash emissions are typical. Three single-day explosive events were reported in 2015; a series of explosions on 5 March 2015 generated a 500 m high ash plume, 41 explosions on 6 June 2015 ejected ash 200 m above the summit, and the first of two explosions on 12 June 2015 sent an ash plume 2,000 m above the summit. The next eruption did not occur until 22 April 2016 when 11 explosions were recorded, with the largest sending an ash plume 2,000 m above the summit. Activity from July 2016-December 2017 is covered in this report. Information is provided by the Instituto Nicaragüense de Estudios Territoriales (INETER), and the Washington Volcanic Ash Advisory Center (VAAC).

Following little activity during the remainder of 2016 after the 22 April explosions, small explosions with minor ash were reported in February, March, and April 2017. Significant explosions during 18-19 August sent ash plumes over 200 km W and deposited ash in numerous communities. Seismicity was high during October-December 2017, but ash-bearing explosions were only reported on 7 and 11 November.

After the 22 April 2016 explosions, San Cristóbal remained quiet for the remainder of 2016. In the month's they were measured, 45-72 degassing-type seismic events were recorded. During a field visit on 29 November 2016, new landslides around the crater rim, both inside the crater and down the outer flanks, were observed. These were interpreted by INETER scientists as resulting from a major tectonic earthquake that occurred offshore in mid-November that was felt in nearby Chinandega (16 km SW), and not from volcanic activity.

Seismic activity increased slightly in January 2017 with 100 degassing events recorded. INETER reported 15 small ash-and-gas explosions during 18-19 February and 153 degassing events. There were no reports of ashfall in the nearby communities. Only 27 degassing seismic events were reported in March; three small gas explosions with minor ash occurred on 16, 25, and 28 March 2017.

Eight small explosions with gas and minor ash took place during April 2017 on days 13, 15, 16 and 19, but no damage was reported in nearby communities. Very low values of SO₂ (averaging 147 tons/day) were measured at the end of April 2017, far less than values of 854 and 642 measured in September and October 2016. Degassing-type seismic events increased sharply beginning on 20 April, totaling 1,931 events; they remained elevated through 25 April.

Volcano-tectonic (VT) earthquakes increased significantly to 235 recorded events during May, from values in the single digits earlier in the year. Minor fumarolic activity occurred at the S side of the summit crater on 27 May 2017 (figure 33). Two small gas explosions were recorded on 20 and 27 May, but no ash emissions were reported. A significant increase to 2,349 degasification-type earthquakes was reported during June 2017; slightly fewer (1,981) were reported during July.

Figure 33. Minor fumarolic activity was observed at the S side of the summit crater at San Cristóbal during a field visit by INETER on 27 May 2017. Courtesy of INETER (Boletín mensual, Sismos y Volcanes de Nicaragua, Mayo 2017).

Significant explosions early on 18 August 2017 were observed from Chinandega with notable gas and ash emissions (figure 34), and ashfall was deposited around the region (figure 35). Communities affected by the ashfall were located to the W and SW of the volcano and included Belén, La Mora, La Bolsa, El Viejo (18 km WSW), La Grecia, Realejo (25 km SW) and Corinto (30 km SW). Ash plumes rose between 300 and 600 m above the crater rim and drifted W and SW. Additional explosions occurred the next day but had ceased by 20 August.

Figure 34. Explosion and ash plume at San Cristóbal at 1330 on 18 August 2017. Courtesy of INETER (Boletín mensual, Sismos y Volcanes de Nicaragua, Agosto, 2017).

Figure 35. Ash was collected by INETER scientists from the 18 August 2017 explosion at San Cristóbal. Courtesy of INETER (Boletín mensual, Sismos y Volcanes de Nicaragua, Agosto, 2017).

A small plume was noted in satellite imagery by the Washington VAAC on 18 August 2017 moving NW. Later imagery showed gas and ash drifting W at an estimated altitude of 2.1 km. It extended approximately 265 km W of the summit before dissipating. Ground measurements of SO₂ made during 18-20 August showed increases to a peak of 3,519 metric tons per day on 19 August before dropping back to more typical background values below 700 t/d. INETER scientists used GOES and AVHRR satellite images to identify the maximum extent of the ash plume from the eruptive event. The ash cloud covered the area W of San Cristóbal, approximately 2,960 Km², and extended more than 80 km offshore, with a total length of 125 km and a maximum width of 33 km (figure 36). Seismometers recorded 3,880 degassing-type seismic events during August 2017. Seismicity decreased slightly during September 2017 to 2,604 measured events, of which 2,415 were degassing-type, 187 were VT events, and two explosions were recorded on 1 September, but no ashfall was reported.

Figure 36. The extent of the ash plume from the 18-20 August 2017 eruptive episode at San Cristóbal, identified in satellite imagery by INETER scientists. Courtesy of INETER (Boletín mensual, Sismos y Volcanes de Nicaragua, Agosto, 2017).

An order-of-magnitude increase in seismicity occurred during October-December 2017, with the monthly totals of the numbers of events ranging from 17,000-21,000 (figure 37). INETER reported a series of 14 explosions during the evening of 7 November. Ashfall was reported to the W in Los Farallones, San Agustín, La Mora, El Naranjo and the city of Chinandega. The Washington VAAC subsequently reported an ash plume that models suggested rose to 6.7 km and drifted W on 11 November.

Figure 37. Numbers of daily seismic events at San Cristóbal during October-December 2017. Event types include VT (volcano-tectonic), degasification, and tremor. Note scale in each graph as different symbols and colors are used for the same type each month. Total seismic events for October (top) was 17,815, November (middle) was 19,206, and December (bottom) was 20,925. Ash bearing explosions were reported by INETER on 7 November, and the Washington VAAC reported an ash plume on 11 November that possibly rose to 6.7 km altitude and drifted W. Courtesy of INETER (Boletín mensual, Sismos y Volcanes de Nicaragua, Octubre, Noviembre, Diciembre, 2017).

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).

Suwanosejima, an andesitic stratovolcano in Japan's northern Ryukyu Islands, was intermittently active for much of the 20th century, producing ash plumes, Strombolian explosions, and ash deposits. Continuous activity since October 2004 (figure 24) has consisted generally of multiple ash plumes most months rising hundreds of meters above the summit to altitudes between 1 and 3 km, and tens of reported explosions. The rate of activity began increasing during 2014; the frequency of explosions and the height of the plumes have continued to increase through 2017, which is covered in this report. Information is provided primarily by the Japan Meteorological Agency (JMA), and the Tokyo Volcanic Ash Advisory Center (VAAC).

Figure 24. Eruptive history at Suwanosejima from January 2003-December 2017. Black bars represent the height of the emissions in meters above the crater rim, gray volcanoes indicate an explosion, usually accompanied by an ash plume, and the red volcanoes represent large explosions with ash plumes. Courtesy of JMA (Suwanosejima volcanic activity report, December 2017).

Activity at Suwanosejima has been persistent and generally increasing during 2014-2017 (figure 25). During 2017, ash emissions rose from a few hundred to nearly 3 km above the Ontake crater rim. Large explosions were reported 32 times by JMA, including 12 during August. Most explosions sent ash emissions to less than 1,000 m above the crater rim, but the highest ash plume, on 3 August 2017, rose 2.8 km above the crater rim, and was the highest recorded since observations began in 2003. Incandescence was observed at the crater from a thermal camera throughout the year and was witnessed locally many times. Many of the explosions, large and small, were heard in the nearby village. Ashfall was confirmed in the village to the SSW on nine different occasions during the year.

Figure 25. Eruptive history at Suwanosejima for 2014-2017. Black bars represent height of steam, gas, or ash plumes in meters above crater rim, gray arrows or volcanoes represent an explosion, usually accompanied by an ash plume, red arrows or volcanoes represent a large explosion with an ash plume, red bars or orange diamonds indicate incandescence observed in webcams. From top to bottom: Eruptive activity during 2014, 2015, 2016, and 2017. Courtesy of JMA (Suwanosejima volcanic activity reports, December 2014, 2015, 2016, and 2017).

Activity during January-April 2017. There were no large explosions at Suwanosejima during January 2017, but occasional minor ash emissions rose as high as 1,300 m above the Ontake crater rim. Incandescence was visible from the webcam on most clear nights. Ashfall was reported in the village 4 km S on 17 and 26 January. The Tokyo VAAC reported ash emissions four times in January. Ash plumes rose to 1.2 km altitude and drifted SE on 4 January; to 1.8 km and drifted W on 5 January; to 1.2 km and drifted S on 16-17 January; and to 2.1 km and drifted SE on 25 January.

In contrast with January, five large explosions were reported by JMA during February 2017. The first, on 9 February, sent an ash plume to 700 m above the crater rim. An ash emission on 18 February rose to 1,200 m above the rim (figure 26). People in the nearby village reported hearing explosions on 18, 20, 27, and 28 February. The largest explosions occurred during 27-28 February when ejecta was scattered 600 m from the crater rim. The Tokyo VAAC reported ash emissions drifting SE several times: on 9 February at 1.5 km altitude, on 16 and 17 February at 1.8 km, and during 27-28 February at 1.5 km.

Figure 26. An ash emission from Suwanosejima was captured by the 'Campground' webcam on 18 February 2017. Courtesy of JMA (Suwanosejima volcanic activity report, February 2017).

Intermittent ash emissions occurred during March 2017, but no large explosive events were reported. Ejecta was scattered around the edge of the crater on 4 March and an ash plume rose 1,000 m. Small ash plumes were noted rising 900 m on 12 and 15 March; explosions were heard in the village on 14 and 16 March, and ashfall was reported there on 25 March. Incandescence was observed at the summit intermittently throughout the month. During a field survey on 21 and 22 March, JMA noted minor thermal anomalies at the Ontake Crater, the N slope of the Ontake crater, and just above the coastline on the E flank (figure 27). The Tokyo VAAC reported ash emissions three times during March; on 3 March ash plumes rose to 1.5-1.8 km altitude and drifted SE and on both 28 and 31 March they rose to 1.8 km altitude and drifted SE and E.

Figure 27. Thermal anomalies were apparent from the Ontake crater (upper left), the north slope of the crater (upper right), and just above the coastline on the E flank (lower left) in this thermal image of Suwanosejima taken on 22 March 2017 from the NE. Courtesy of JMA (Suwanosejima volcanic activity report, March 2017).

Only minor ash emissions and occasional incandescence was reported during April 2017. Two emission events on 1 April sent ash plumes to 1,200 m above the crater rim. A tremor that lasted nine minutes occurred on 11 April and a small seismic swarm was recorded on 13 April. Small explosions were also reported on 17 and 19 April, with the 19 April event heard at the nearby village; another small explosion was reported on 30 April. There were no reports issued by the Tokyo VAAC.

Activity during May-August 2017. Activity increased slightly during May 2017; two large explosions were recorded by JMA. A small explosion was reported on 1 May, and the highest plume rose to 1,900 m above the crater rim on 10 May during a larger event. Incandescence was observed from the local village on 16 May, and explosions were heard from the village on 16 and 18 May, and again on 28 and 29 May; no ashfall was reported. The Tokyo VAAC reported ash emissions on 7, 8, and 10 May. On 7 May they reported an ash plume located 45 km S at 1 km altitude extending SW. A few hours later ash extended N at 1.5 km. An explosion on 8 May sent an ash plume to 2.1 km where it remained stationary over the volcano for much of the day before dissipating. A higher ash plume was reported on 10 May at 2.7 km altitude drifting E.

Small ash explosions occurred at Ontake Crater on 8 and 21 June 2017, but there were no larger explosive events. Ash plume heights rose to only 600 m above the crater rim, and occasional nighttime incandescence was reported. No reports were issued by the Tokyo VAAC. JMA reported that the highest ash plume during July rose 2.1 km above the summit crater on 17 July, but no large explosions were recorded. Incandescence was observed intermittently throughout the month. A small explosion on 2 July sent an ash plume to 1.9 km above the crater rim. Intermittent ash emissions were noted during 17-19, 22 and 25 July. The Tokyo VAAC reported ash emissions during 2 and 16-18 July. They reported the plumes on 2 July at 1.8-2.4 km altitude, extending N for most of the day. A new explosion on 16 July sent an ash plume to 2.7 km altitude that drifted E. Intermittent ash emissions continued to drift E through 18 July at altitudes ranging from 1.8-2.1 km.

Activity increased substantially during August 2017; JMA reported 12 large explosions, nine of which occurred during the last week. Ashfall was reported in the nearby village on 2 August. The highest plume of the month was reported on 3 August, 2.8 km above the crater rim. Explosions were heard in the village on 3 and 19 August. A small explosion was reported on 12 August. Large explosions occurred on 19, 20, and 24 August in addition to the nine events during the last week. A single MODVOLC thermal alert was reported on 18 August, and the MIROVA system reported thermal anomalies during several days of the last week of the month (figure 28). The Washington VAAC reported ash on 1 August that rose to 2.4 km altitude and drifted SW. A higher plume on 3 August rose to 3.7 km and drifted W. They reported another ash plume that first rose to 3.0 km on 24 August; subsequent emissions that day were drifting NE at 2.1-2.4 km altitude. A new plume on 25 August extended E at 2.4 km. Continuing ash emissions from multiple explosions during 28-31 August rose to 1.2-3.0 km altitude and drifted SE.

Figure 28. Log Radiative Power plot from the MIROVA project for Suwanosejima for 24 May 2017-15 February 2018 shows increased thermal activity during late August 2017, and intermittent pulses of activity from late May-September. Courtesy of MIROVA.

Activity during September-December 2017. Four large explosions were recorded during the first week of September 2017, after which a number of smaller ash emission events were reported. Ashfall was reported four times in the nearby village on 2, 4, 29, and 30 September. The Tokyo VAAC reported explosions on 1, 4, 6, and 29 September. The ash plume from the explosion on 6 September rose to 1.5 km altitude and drifted E; on 29 September, it rose to 2.4 km altitude, also drifting E.

JMA reported four large explosions during October 2017. Two explosions occurred on 11 October; one of the ash plumes rose 1,900 m above the crater rim (figure 29). Explosions were heard in the nearby village on 12 and 31 October, and ashfall was reported on 13 October. During the large explosion of 31 October incandescent ejecta was scattered around the crater rim and the ash plume rose 1,900 m. The Tokyo VAAC reported an explosion with ash on 10 October (UTC) that rose to 2.7 km altitude and remained stationary until dissipating a few hours later. They noted that the explosion on 31 October produced a plume that rose over 1.5 km and drifted NW.

Figure 29. An ash plume from an explosion on 11 October 2017 rises 1.9 km above the Ontake crater of Suwanosejima. Courtesy of JMA (Suwanosejima volcanic activity report, October 2017).

JMA reported five large explosions during November 2017. Incandescent ejecta was seen around the crater rim during the explosion of 1 November, and the plume rose to 2 km above the rim. Loud explosions were heard from the nearby village on 3, 5, 6, 15, and 16 November, and ashfall was reported there on 14, 15, and 20 November. A small explosion was reported on 10 November; intermittent explosions with ash plumes rising 700 m were observed on 20 and 21 November. The Tokyo VAAC reported ash plumes at 1.5 km drifting W on 1 and 5 November, and at 1.8 km altitude drifting NW on 10 November, the last VAAC report issued for 2017.

Only small explosions were reported from Ontake crater during December 2017. The highest plume rose 700 m above the crater rim. Small explosions were heard a number of times in the nearby village on 8-9, 11-13, and 26-30 December. JMA scientists visiting during 8-10 December heard intermittent explosions and witnessed incandescence visible to the naked eye. They also observed ashfall in the village on the morning of 10 December. During a field survey on 14 December, no significant changes were noted from the previous survey in March 2017 (figures 30 and 31).

Figure 30. The summit of Suwanosejima with steam rising from Ontake Crater taken from the W on 14 December 2017. Courtesy of JMA (Suwanosejima volcanic activity report, December 2017).

Figure 31. Steam rises from the Ontake Crater of Suwanosejima viewed from the E on 14 December 2017. Courtesy of JMA (Suwanosejima volcanic activity report, December 2017).

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit of the volcano is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed forming a large debris avalanche and creating the horseshoe-shaped Sakuchi caldera, which extends to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Chikurachki last erupted during April to June 2003 (BGVN 28:07) and subsequently was apparently dormant for nearly two years. On 1 March 2005, observers in Severo-Kurilsk (~ 70 km NE of Chikurachki) saw a gas-and-steam plume rise ~ 400 m above the volcano. On 12 March 2005, MODIS satellite imagery showed an ash plume extending NNW from the volcano and led KVERT to raise the concern color code from Green to Yellow. On 23 March, satellite imagery showed a weak ash plume extending ~ 70 km E. The height of the plume was unknown, and on 25 March the hazard status was raised again from Yellow to Orange. Chikurachki is not monitored with seismic instruments but KVERT has access to satellite data and occasional visual observations of the volcano. Ash from Chikurachki fell on the southern part of Paramushir Island on 29 March. Ash deposits were visible on satellite imagery on 25 and 29 March; on the 29th they extended 19 km SE. Chikurachki remained at concern color code Orange.

During April 2005, weak fumarolic activity occurred at Chikurachki. Ash deposits covered the WNW slope of the volcano. On 7 April, an ash-and-gas plume rose to ~ 500 m above Chikurachki's crater and extended ~ 10 km S. The concern color code remained Orange through 15 April 2005 and was reduced to Yellow when satellite imagery during the week of 20-26 April did not show any thermal anomalies or ash plumes. Since that time there has been no further indication of activity.

In 2005 Gurenkoa and others published a study of glass inclusions and groundmass glasses from Chikurachki explosions in an effort to better understand the relatively rare, highly explosive eruptions of basaltic composition. Such eruptions may be important in terms of atmospheric impact because of the generally much higher solubilities of S in basaltic melts compared with silicic melts. Concentrations of H₂O, major, trace and volatile (S, Cl) elements by EPMA and SIMS from glass inclusions and groundmass glasses of the 1986, 1853, and prehistoric explosive eruptions of basaltic magmas were studied.

Reference. Gurenko, A.A., Belousov, A.B., Trumbull, R.B., and Sobolev, A.V., 2005, Explosive basaltic volcanism of the Chikurachki Volcano (Kurile arc, Russia): Insights on pre-eruptive magmatic conditions and volatile budget revealed from phenocryst-hosted melt inclusions and groundmass glasses: Journal of Volcanology and Geothermal Research, v. 147, p. 203-232. (URL: http://www.sciencedirect.com/)

Geologic Background. Chikurachki, the highest volcano on Paramushir Island in the northern Kuriles, is actually a relatively small cone constructed on a high Pleistocene volcanic edifice. Oxidized basaltic-to-andesitic scoria deposits covering the upper part of the young cone give it a distinctive red color. Frequent basaltic plinian eruptions have occurred during the Holocene. Lava flows from 1781-m-high Chikurachki reached the sea and form capes on the NW coast; several young lava flows also emerge from beneath the scoria blanket on the eastern flank. The Tatarinov group of six volcanic centers is located immediately to the south of Chikurachki, and the Lomonosov cinder cone group, the source of an early Holocene lava flow that reached the saddle between it and Fuss Peak to the west, lies at the southern end of the N-S-trending Chikurachki-Tatarinov complex. In contrast to the frequently active Chikurachki, the Tatarinov volcanoes are extensively modified by erosion and have a more complex structure. Tephrochronology gives evidence of only one eruption in historical time from Tatarinov, although its southern cone contains a sulfur-encrusted crater with fumaroles that were active along the margin of a crater lake until 1959.

Information Contacts: Olga Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Eruptive activity has continued at Colima from July 2005 through February 2006. Explosions that generated ash plumes were common during this period.

The Colima Volcano Observatory reported that ash emission continued at Colima during 29 June 2005 to 5 July 2005 and several plumes rose to 9-10 km altitude. On 30 June, lahars traveled SW down La Lumbre Ravine and SSE down Montegrande Ravine to a maximum length of ~ 10 km. The lahars did not reach populated areas. Due to the presence of new ash on the flanks of the volcano, seasonal heavy rains, and the subsequent threat of lahars forming, Universidad de Colima advised avoiding the ravines of La Lumbre, San Antonio, Monte Grande (in Colima state), and La Arena (in Jalisco state) throughout this interval.

The Washington VAAC reported that the Colima video camera and satellite imagery confirmed an explosive eruption on 5 July at 1821 (figure 80). The Mexico City Meteorological Watch Office (MWO) reported that the resultant ash plume reached an altitude of ~ 9.1 km and drifted NW. Pyroclastic flows accompanying the eruption traveled down the E flank.

Figure 80. A photo of the explosive eruption on Colima on 5 July 2005 taken from the E. Courtesy of CVO.

Several explosions continued during 6-19 July, and small landslides traveled down the volcano's flanks during 8-9 July and 15-18 July. On 21 and 23 July, small ash emissions and lahars occurred. On the 21st during 1750-1830 a lahar traveled SSE down the Monte Grande ravine. Emissions rose to a maximum altitude of 9.1 km on 27 July. During 29 July to 1 August, steam-and-ash emissions occurred at Colima. According to the Washington VAAC, the highest-rising emission reached 6.1 km altitude on 30 July.

On 4 August the Washington VAAC reported that the Mexico City MWO observed a steam plume rising to 7.2 km altitude in imagery seen on the Colima video camera. During15-31 August, small explosions produced low-level ash plumes. The largest events, on 21 and 22 August, produced plumes that drifted W. On 31 August a 45-minute seismic signal associated with a lahar was recorded at the Monte Grande station. The lahar caused no damage.

Throughout the month of September, several small explosions occurred at Colima. On 16 September at 1045 an explosion sent an ash plume to ~ 9.8 km altitude. The local civil defense agency stated in a news report that ash fell on towns NW of the volcano. Prior to the explosion, microseismicity was recorded for several days. Universidad de Colima reported that microseismicity often precedes significant explosions. On 27 September at 0507 an explosion produced a plume to a altitude of ~ 7.6 km altitude. The plume drifted WSW, depositing small amounts of ash in the cities of Colima, Villa de álvarez, and Comala. On 28 September another explosion sent an ash plume to an altitude of ~ 6.1 km altitude and drifted NNW.

Small explosions continued to occur from October through the end of February 2006 (the end of this report), and produced visible ash plumes. Several small explosions during 16-21 November 2005 produced steam-and-ash clouds to low levels above the volcano. Explosions on 12 December 2005 resulted in small amounts of ash deposited in areas SW of the volcano.

Geologic Background. The Colima volcanic complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the 4320 m high point of the complex) on the north and the 3850-m-high historically active Volcán de Colima at the south. A group of cinder cones of late-Pleistocene age is located on the floor of the Colima graben west and east of the Colima complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide caldera, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, and have produced a thick apron of debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions (most recently in 1913) have destroyed the summit and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Observatorio Vulcanológico de la Universidad de Colima, Colima, Col., 28045, México (URL: https://portal.ucol.mx/cueiv/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

Viviane Grandjean wrote of her observations at Erta Ale during 24 December 2005-3 January 2006 in Bulletin No. 57 of the Société de Volcanologie Genève. On 26 December she saw the lava lake through clouds of gas; its surface was calm, with incandescent lava visible through the broken chilled surface. The S pit crater had an estimated diameter of 170 m and vertical walls, and the lava lake was about 80 m in diameter. It seemed to shrink during the next days, one part appearing hardened and forming almost a second terrace. The plates of cooled surface lava were seen moving and converging amidst degassing lava. Lava fountains were periodically visible and generally outlined the borders of the lava lake under the rim.

On 27 December, the walls of the crater were estimated at about 50 m high, with a crater diameter of about 300 m. Members of the group descended into the crater to inspect a series of active hornitos near the N vents. At one end of the line a vent lined with sulfur opened. In the interior cavity of a smaller vent temperatures of about 800°C were measured. Degassing occurred generally in the area. Lava fountaining continued.

The lava lake appeared lower and calmer to observers on 28 December, with a potential second terrace still forming. Some group members descended into the crater again and observed rockfall and continued lava fountaining.

Geologic Background. Erta Ale is an isolated basaltic shield that is the most active volcano in Ethiopia. The broad, 50-km-wide edifice rises more than 600 m from below sea level in the barren Danakil depression. Erta Ale is the namesake and most prominent feature of the Erta Ale Range. The volcano contains a 0.7 x 1.6 km, elliptical summit crater housing steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Fresh-looking basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera is renowned for one, or sometimes two long-term lava lakes that have been active since at least 1967, or possibly since 1906. Recent fissure eruptions have occurred on the N flank.

Information Contacts: Viviane Grandjean, c/o Société Volcanologique Européenne (SVE)-Société Volcanologique de Genève (SVG), Geneva, C.P.1, 1211 Geneva 17, Switzerland (URL: http://www.sveurop.org/).

Galeras was last reported on in BGVN 31:01. During the first weeks of November 2005 seismometers recorded tornillo earthquakes (long-period events with seismic traces that look like screws in profile and are currently thought to be related to pressurized fluid flow at shallow depth). Minor deformation was also recorded at Galeras. The earthquakes were similar to those that occurred before eruptions in 1992-93. On 24 November at 0246 seismic signals indicated the beginning of an eruption. Ash fell in the towns of Fontibon, San Cayetano, Postobon, and in north Pasto. Activity decreased by the next day, so the Alert Level was reduced. Thousands of people were evacuated during the week prior to the eruption. Gas emissions continued through December 2005 and January and February 2006. During 23 January to 6 February, the lava dome in the main crater continued to grow; strong degassing occurred in several sectors of the active cone and around the lava dome. Galeras remained at Alert Level 3 ("changes in the behavior of volcanic activity have been noted") through February 2006.

During the last week of February, seismic stations detected an average of 280 small earthquakes per day. On 26 February a shallow M 4.8 volcano-tectonic earthquake below the volcano was recorded at 1009, followed by 35 smaller earthquakes. SO₂ flux of about 600 metric tons per day was measured during February. Steam and gas rose to ~ 700 m above the volcano.

During 27 February to 6 March an increase in the volume of the lava dome located in the main crater was observed. During March, seismicity at Galeras decreased in comparison to the previous several weeks and deformation was measured at the volcano. Plumes of mainly steam, gas, and small amounts of ash were emitted from the volcano and rose to a maximum height of 1.2 km above the volcano.

Due to an increase in tremor at Galeras beginning on the morning of 28 March 2006, INGEOMINAS raised the Alert Level from 3 to 2 (likely eruption in days or weeks). On 28 March, energetic signals and tremor began and seismic instruments detected very shallow low-energy hybrid signals, similar to ones recorded during 1991-1993 when dome emplacement occurred on the main crater's floor.

The increase in seismic energy ended on 29 March. The number of earthquakes beneath the volcano decreased during 28 March to 3 April (an average of 66 earthquakes was recorded daily), in comparison to the previous week (an average of 89 earthquakes was recorded daily). Steam columns rose up to ~ 500 m above the volcano and the outer layer of the lava dome at the volcano's summit cooled in comparison to previous weeks.

During 5-24 April, decreases were observed in seismicity, deformation, gas emissions, and temperatures. According to INGEOMINAS, most of the explosive eruptions at Galeras in the past 17 years occurred when parameters were at similarly low levels. In addition, the current lava dome has a significantly greater volume than the dome that was destroyed during an eruption in 1992. The volume of magma in the interior of the volcanic system is greater than during 1989-1993. Galeras remained at Alert Level 2.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large horseshoe-shaped caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate historical eruptions since the time of the Spanish conquistadors.

Information Contacts: Diego Gomez Martinez, Observatorio Vulcanológico y Sismológico de Pasto (OVSP), INGEOMINAS, Carrera 31, 1807 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html; Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); El Pais (URL: http://elpais-cali.terra.com.co/paisonline/).

Typical activity continued at Ol Doinyo Lengai from December 2005 through mid-March 2006. Unusual activity, including a large plume and significant lava overflows from the summit crater, occurred during late March and early April. Much of the following information was posted on websites maintained by Fred Belton or Chris Weber, or was contained in email from local sources or visitors relayed by Belton or Celia Nyamweru. None of the reports regarding the unusual March-April activity originated from sources close enough to describe the exact nature of the eruption.

Activity during 20 December 2005-13 March 2006. The local Masai guide William reported an eruption from hornito T49B during a visit on 20 December 2005. When David Bygott climbed the volcano on 22 December the crater was inactive. A recent narrow flow of pahoehoe lava from the NW flank of T49B had flowed across the NW crater rim overflow, and was still warm and making cracking noises. A wide pahoehoe-textured lava flow from T56B had mostly turned white and appeared to be several days to a week old.

On 4 January 2006 Bernhard Donth observed lava escaping from T49B; spatter and little flows went in all directions. One bigger lava flow had reached as far as the NW overflow. A report from Christian Mann of a climb on 10 January only noted degassing from T47. A photo taken that day from the summit showed a white and brown crater with no indication of recent activity. However, Belton noted that during the previous weeks lava had apparently filled up the large open vent of T56B and had flowed from there and possibly other locations onto the NE part of the crater floor.

Chris DeVries and a group of other students from McGill University visited during 25-26 February. Many hornitos were intermittently degassing. T58B was spattering a bit, and magma was heard sloshing around. A small ~ 10-m-long flow had erupted from this vent earlier in the day; it was still very black and hot. T57B had a large opening to its NW, but it did not appear that any recent flows had come from this opening. The base of this cone later ruptured, and the lava inside drained out quickly and violently; the flow proceeded to the E overflow.

Christoph Weber arrived with a film team at the crater on 2 February 2006. The tallest hornito (T49B) reached approximately 2,890 m elevation (measured with GPS), ~ 60 m above the crater floor at the NW overflow (figure 87). No recent eruption had occurred at T49B, but strong noisy degassing took place sometimes. Just E of T49B the hornito T56B had convecting lava deep inside and some days-old lava flows stretched from three different vents at T56B to the E overflow. After the major collapse of T56B in 2004, this hornito (at approximately 2,875 m elevation on 2 February) has nearly grown up again to its former shape and height. Also from T58C and the collapsed T58B hornito some days-old lava flows were found on the eastern slopes passing the old and weathered T37, T37B, and T45 cones.

Figure 87. View of Ol Doinyo Lengai on 6 February 2006, looking NW at the central hornito cluster. Fresh lava flows are black. A person can be seen near the recent lava flow in front of T57B. Courtesy of C. Weber.

The caldera-shaped collapsed T58B had its flat floor at ~ 2,865 m elevation with four active vents inside. Lava convection was close to the surface of T58B and inside the tall T58C. At 1300 on 2 February a sudden increase of activity took place with two lava fountains at T58B lasting only some seconds. At the same time lava spilled from all T58B vents, a T58C flank vent, and a T56B vent. Lava spatter with lava flows inside T58B and up to ~ 150 m towards the E occurred over the following 3 days. On 6 and 7 February, higher activity occurred with lava outflow at T58C. During an observation flight on 13 February, Weber noticed new lava flows from T58B and T56B. Crater rim overflow measurements on 2 February 2006 were unchanged since August 2005 (BGVN 30:10).

Photographs taken by Michael Dalton-Smith from a plane on 13 March 2006 showed many small flows extending in all directions from the central cluster. The flow over the NW rim seemed to be confined to a channel and did not spread out until it was further down the mountain.

Unusual activity starting in late March. David Peterson saw a fairly obvious plume at the top of the mountain (figure 88) on 28 March. A day or two after that he heard reports of lava pouring down the volcano's sides with some residents moving out of Engare Sero as a result. Unconfirmed news reports in The Guardian on 1 April described a scene of "rumbling" noises with lava and ash discharges on 30 March that prompted hundreds to as many as 3,000 local residents to flee the area. Peterson also relayed that his colleague Habibu reported on 1 April that the lava flows had abated. Another friend, Achmed, noted that a river of lava extending from the crater to the base of the volcano was about the "width of a four lane highway" (12 m). An Agence France Presse news report, with quotes from Emmanuel Chausi, a conservation officer with the nearby Ngorongoro Conservation Area Authority (NCAA), claimed that "huge plumes of detritus" were ejected during the nights of both 2 and 3 April, but no lava was reported.

Figure 88. A photograph, undated, but from the time period of the eruption, shows a white plume from Ol Doinyo Lengai. This is probably what started the rumor of a major eruption. Fred Belton saw a similar cloud on 15 July 2004 when lava vaporized a big area of plants on the E rim. Fred Belton received this photograph, taken from Basecamp Tanzania, on 9 April 2006.

Photos received from Dean Polley, taken 1 April, provide additional information about the eruption (figure 89). Based on these aerial photos, Belton's interpretation is that lava on 30 March must have erupted strongly from at or near the central cluster. A deep channel visible down the flank indicates a flow lasting some hours through a channel deepened by thermal erosion. A crater photo from Matt Jones also taken on 1 April (figure 90) confirmed that there had been recent strong activity from the T56B and T58C hornitos. C. Weber relayed that visitors who climbed the volcano later on (with guide Othman Swalehe ) reported a lava channel 5 m wide and 2.5 m deep, starting from the T58C hornito, following the flow field to the SW and then continuing outside the crater at the W overflow where there was a channel 8 m wide and 3 m deep. The collapsed hornito area at T56B and T58B measured about 30 m N-S and 15 m E-W with an active lava lake inside. The tall hornitos T58C (partly collapsed to the SE), T49B, and T57B were mostly not affected by the collapse, and the W part of T56B remained standing.

Figure 89. Aerial photograph of Ol Doinyo Lengai looking approximately ESE showing the summit crater and lava overflows, 1 April 2006. Courtesy of Dean Polley.

Figure 90. Photograph of the Ol Doinyo Lengai crater on 1 April 2006, looking NW at the central hornito cluster. The T58C hornito is completely split, with the south half removed. A significant portion of T56B is also missing. See figure 87 for a comparison with crater morphology on 6 February 2006 and identification of hornitos. Photo by Matt Jones, provided courtesy of F. Belton.

Michael Dalton-Smith flew over on 4 April and saw more recent black flows partially covering the gray flows from 30 March. When Dalton-Smith drove from Seronera to the crater on 4 April, he had a great cloud-free view. Using binoculars it appeared that there was a huge fountain out of one of the hornitos, and all hornitos had black plumes rising from them.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA (URL: http://oldoinyolengai.pbworks.com/); Christoph Weber, Volcano Expeditions International, Muehlweg 11, 74199 Untergruppenbach, Germany (URL: http://www.v-e-i.de/); Bernhard Donth, Waldwiese 5, 66123 Saarbruecken, Germany; Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617, USA; Guardian News, Arusha, Tanzania (URL: http://www.ippmedia.com/); Agence France Presse (URL: http://www.afp.com/).

The twin volcanoes of Lokon and Empung exhibited low levels of activity during 2005. Table 9 is a summary of reported gas emissions and number of volcanic earthquakes during 2005.

Table 9. Summary of activity at Lokon-Empung during 2005, indicating the height and composition of plumes observed and the numbers of earthquakes recorded. Data courtesy of CVGHM.

Geologic Background. The twin volcanoes Lokon and Empung, rising about 800 m above the plain of Tondano, are among the most active volcanoes of Sulawesi. Lokon, the higher of the two peaks (whose summits are only 2 km apart), has a flat, craterless top. The morphologically younger Empung volcano to the NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century, but all subsequent eruptions have originated from Tompaluan, a 150 x 250 m wide double crater situated in the saddle between the two peaks. Historical eruptions have primarily produced small-to-moderate ash plumes that have occasionally damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred. A ridge extending WNW from Lokon includes Tatawiran and Tetempangan peak, 3 km away.

Information Contacts: Dali Ahmad, Hetty Triastuty, Nia Haerani and Suswati, Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Since the previous report in December 2004 (BGVN 29:12) Mayon had remained quiet until 21 February 2006. On that day the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that a minor explosion at 0941 produced an ash plume that rose ~ 500 m above the volcano's crater and drifted SW. Ash was deposited on the upper slopes of the volcano. The ash emission was accompanied by a small explosion-type earthquake, recorded only by seismographs around the volcano.

Prior to the explosion PHIVOLCS had seen an increase in seismicity at the volcano. Between 1545 on 20 February and 0520 on 21 February, there were 147 low-frequency earthquakes recorded, a number considerably above the five or fewer events per day normally detected. Seismicity also indicated some minor rockfalls, which probably resulted from lava blocks detaching from the summit. Steaming was observed. No incandescence was visible at the crater due to clouds obscuring the volcano.

PHIVOLCS reported that about nine earthquakes related to explosive activity took place at Mayon around 23 February. Cloudy conditions prevented visual observations, but the seismic events detected probably signified minor ash explosions. This was supported by reports from local residents who heard rumbling. The seismic network also recorded two low-frequency earthquakes associated with shallow magma movement. The SO₂ flux averaged 1,740 metric tons per day (t/d), similar to values obtained during the last measurements on 28 November 2005. The flux was well above the usual 500 t/d measured at the volcano. Mayon remained at Alert Level 2, with a 6-km-radius Permanent Danger Zone in effect. At this point the possibility of more violent eruptions triggered warnings to tourists and the public in general to remain outside of the danger zone.

Geologic Background. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often devastated populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, Univ. of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

According to a news report, there was a minor eruption at Miyake-jima on 17 February 2006 that consisted of small ash emissions. Residents of the island were warned that there could be gas emissions and mudslides. The Geological Survey of Japan (AIST) website reported that the SO₂ flux at Miyake-jima averaged about 2,000-5,000 tons per day in January 2006 (figure 22). The previous activity took place in November-December 2004, ending on 9 December 2004 when minor eruptions were reported after a two-year lull. As of mid-April 2006 no further activity had been reported.

Figure 22. Sulfur dioxide (SO2) flux monitoring of Miyake-jima by COSPEC V was conducted from 26 August 2000, peaking in early 2000 at values well over 100,000 metric tons per day and dropping off slowly after that. Daily monitoring was performed by the Japanese Meteorological Agency and Geological Survey of Japan.

Geologic Background. The circular, 8-km-wide island of Miyakejima forms a low-angle stratovolcano that rises about 1100 m from the sea floor in the northern Izu Islands about 200 km SSW of Tokyo. The basaltic volcano is truncated by small summit calderas, one of which, 3.5 km wide, was formed during a major eruption about 2500 years ago. Parasitic craters and vents, including maars near the coast and radially oriented fissure vents, dot the flanks of the volcano. Frequent historical eruptions have occurred since 1085 CE at vents ranging from the summit to below sea level, causing much damage on this small populated island. After a three-century-long hiatus ending in 1469, activity has been dominated by flank fissure eruptions sometimes accompanied by minor summit eruptions. A 1.6-km-wide summit caldera was slowly formed by subsidence during an eruption in 2000; by October of that year the crater floor had dropped to only 230 m above sea level.

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); A. Tomiya, Geological Survey of Japan (AIST), 1-1 Higashi, 1-Chome Tsukuba, Ibaraki 305-856, Japan (URL: https://staff.aist.go.jp/a.tomiya/miyakeE.html); Kazahaya Kohei, Geological Survey of Japan (URL: https://staff.aist.go.jp/kazahaya-k/miyakegas/COSPEC.html); Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032, Japan.

Recent volcanism on Montagu Island was discovered based on satellite information (BGVN 30:11). Thanks to a visit from the South African icebreaker MV SA Agulhas, the first photographs of the island are now available, taken from just offshore. The Agulhas is an Antarctic supply and oceanographic research vessel built in the late 1970s; it is affiliated with the South African Department of Environmental Affairs and Tourism, Antarctica and Islands Division. She left Cape Town on 1 December 2005, and her journey was the focus of several reports (e.g., Hunter, 2005). The westerly position of pack ice during the course of this voyage enabled the Agulhas to visit Penguin Bukta, an indentation (bay) in the coastal ice shelf (figure 13).

Figure 13. A map indicating the location of Montagu island with respect to features in the region. The pack ice is mobile and the position shown refers to conditions on 10 January 2006 as mapped by satellite radar (NASA/JAXA). Courtesy of Ian Hunter, South African Weather Service.

The Agulhas departed Penguin Bukta on 8 January to deploy drifting weather buoys and to install an automatic weather station on South Thule island at the extreme S end of the South Sandwich Islands. Besides the usual hazards of Antarctic travel and navigation, the South Sandwich Islands were the scene of some severe undersea earthquakes as the Agulhas entered those waters. This was of concern because such earthquakes can cause significant bathymetric change. The US Geological Survey posted detailed information on two large 2006 earthquakes to the E of the islands. The first, on 2 January, had M 7.3 and, fortunately, a moderately deep focal depth of 46 km.

The ship reached offshore of the remote, uninhabited Montagu island in mid-January 2006 (figures 14 and 15). These pictures were forwarded to the Smithsonian by Ian Hunter who received them from Frikkie Viljoen (the ice navigator), and Dave Hall (the ship's Master) after the Agulhas returned from Antarctica on 19 February 2006.

Figure 14. Lava from Montagu Island eruption entering the sea. The photo was taken on 13 January 2006 from the SA Agulhas while lying to the N of the Island. The geometry of the setting given here is based on the MODIS photo taken on 9 September 2005 (BGVN 30:11) that clearly indicates the lava flow streaming N into the sea. Courtesy of Dave Hall and Frikkie Viljoen, SA Agulhas, and Ian Hunter, South African Weather Service.

Figure 15. Photo taken on 13 January 2006 from the SA Agulhas from N of Montagu Island showing the lava field formed by the recent eruption. Courtesy of Dave Hall and Frikkie Viljoen, SA Agulhas, and Ian Hunter, South African Weather Service.

In an e-mail message to Hunter on the return leg of the voyage (on 16 January), Hall noted the following. "By now you will have heard that we successfully deployed the new weather station at Thule Island and had a good look at the eruption on Montagu. We got to within 1.5 miles [2.4 km] of the lava flow, but it was strangely disappointing. Although it was during the evening it was still full daylight so the lava flow was just the same colour as the surrounding rock, not dramatic at all! The most visible feature was the steam plume as the hot lava entered the sea. The top of the island was covered in cloud but that did part long enough to get a quick sighting of the summit, emitting the smoke and ash cloud."

John Smellie of the British Antarctic Survey reported hearing from a Falklands contact that an RAF flight sent at Christmas 2005 had taken photos and reported the eruption was "over." In addition, there could also be first-hand news from a yacht that was to be in the area during January 2006.

Reference. Hunter, Ian, (12 January) 2006, International Support for the SA Agulhas's mission in Antarctica, in Ports & Ships, Shipping News?reporting from the harbours of South Africa & Southern Africa (URL: http://www.ports.co.za/didyouknow/)

Geologic Background. The largest of the South Sandwich Islands, Montagu consists of a massive shield volcano cut by a 6-km-wide ice-filled summit caldera. The summit of the 10 x 12 km wide island rises about 3000 m from the sea floor between Bristol and Saunders Islands. Around 90% of the island is ice-covered; glaciers extending to the sea typically form vertical ice cliffs. The name Mount Belinda has been applied both to the high point at the southern end of the summit caldera and to the young central cone. Mount Oceanite, an isolated 900-m-high peak with a 270-m-wide summit crater, lies at the SE tip of the island and was the source of lava flows exposed at Mathias Point and Allen Point. There was no record of Holocene or historical eruptive activity until MODIS satellite data, beginning in late 2001, revealed thermal anomalies consistent with lava lake activity that has been persistent since then. Apparent plumes and single anomalous pixels were observed intermittently on AVHRR images during the period March 1995 to February 1998, possibly indicating earlier unconfirmed and more sporadic volcanic activity.

Information Contacts: Ian T. Hunter, South African Weather Service, Private Bag X097, Pretoria 0001, South Africa (URL: http://www.weathersa.co.za/); Department of Environmental Affairs and Tourism, Antarctica and Islands Division, Private Bag X447, Pretoria 0001, South Africa; John Smellie, British Antarctic Survey, Natural Environment Research Council, High Cross, Madingly Road, Cambridge CB3 0ET, United Kingdom (URL: https://www.bas.ac.uk/).

Poás was last reported on in BGVN 28:09, covering the period from September 2001 to December 2002. The focus of activity at Poás during that time was the main crater and its fumaroles, and its low-pH, variably colored lake.

A field team from Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA) visited Poás on 25 January 2006 and found that the level of the volcano's hot acidic crater lake had risen in comparison to the previous month. Sustained rainfall during the previous months caused the water level to rise by ~ 4 m. The area of the lake increased by ~ 20%. Flooding occurred in relatively flat areas to the N, E, and SE. The shoreline extended about 150 m toward the SE. Scattered fumaroles and hot spots at the N base of the lava dome were flooded. Increased steaming was visible from the National Park. The average lake temperature remained at 22°C, with hot spots near the rim reaching up to 80°C. OVSICORI-UNA staff noted that in the past an increase in lake level during a rainy period has been followed by a decrease during the drier months of February to April.

On 24 March 2006 around noon, the first eruptions since 1994 began at Poás. The small, phreatic eruptions originated from the bottom of the volcano's Caliente Lake and dispersed mud, gas, and acid rain toward the S and SW parts of the crater. Witnesses described a sudden emission of water and sediments S of the lake. Roaring was heard in a nearby tourist area and weak earthquakes were felt. The strongest eruption occurred on the night of 24 March, when ejected volcanic material reached 200 m high and acid rain showered park headquarters, located 800 m S of the crater. During 25 March at least 8 eruptions took place. Due to the likelihood of more explosions the local National Emergencies Agency temporarily closed the park.

OVSICORI-UNA staff visited the E side of the volcano on 25 March and confirmed that water, blocks, and sediments from the bottom of the lake had been ejected. Several dozens of impact craters were seen with diameters between 15 and 60 cm, extending E as far as 700 m (figure 80). During 22-27 March, harmonic tremor was recorded. On the 27th, there was a reduction in seismicity and it returned to normal levels. No deformation was measured at the volcano. A news article reported that the area around the volcano was closed to visitors.

Figure 80. Photo of the E side of Poás, annotated with observations made by OVSICORI-UNA staff. Impact craters ranged in size from a few cm to 70 cm; blocks ranged from a few cm to 50 cm and were scattered randomly over the area investigated. Blocks of fine-grained lake sediments were also observed and collected. The material collected was interpreted as pre-existent solid material from the bottom of the lake that has been heavily altered by the action of hot acidic fluids during the last 12 years. Photo courtesy of Eliecer Duarte Gonzalez, OVSICORI-UNA.

Following the eruptions that began on 24 March, seismicity at Poás decreased by 27 March and harmonic tremor that was recorded during the heightened activity ceased.

On 1 April 2006, OVSICORI-UNA staff visited Caliente Lake and its surroundings. During this visit the widening of the lake perimeter was confirmed as well as the emplacement of lake sediments and pre-existent blocks from both the bottom of the lake and its walls. Fracturing of the dome's N wall was also confirmed. The lake temperature was 54°C, with a pH of 0.63. The water was light gray due to the great quantity of suspended sediments. The park surrounding the volcano was reopened on 1 April.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: Eliecer Duarte Gonzalez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica. (URL: http://www.ovsicori.una.ac.cr/); Rafael Barquero, Red Sismiológica Nacional, Sección de Sismología, Vulcanología y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, Aptdo. 560-2300, Curridabat, San José, Costa Rica.

An eruption took place on 17 March 2006 at Raoul Island, killing one person. Brad Scott, New Zealand Institute of Geological and Nuclear Sciences (GNS), reported that on the evening of 12 March 2006 earthquakes began near Raoul Island. More than 200 earthquakes were recorded in the first 24 hours, with many of the larger events felt on the island. Earthquakes continued throughout the week, but the numbers gradually decreased.

An eruption from the Green Lake crater, within the Raoul caldera (figure 2), began at 0821 on 17 March. Other than the precursory seismicity, no water-level or temperature changes were observed, even only 24 hours before the eruption. Based on data from the seismograph on the island, the eruption appears to have continued for up to 30 minutes, although the most intense part of the eruption lasted for only 5 to 10 minutes. Following the eruption, the rate of earthquake activity doubled, but by 23 March the number of earthquakes was reduced to 10-20 per day. No thermal anomalies were detected by the MODIS satellite system during March 2006.

Figure 2. Maps of Raoul Island taken from New Zealand governmental publications issued considerably prior to the 2006 eruption. A) Sketch map of the entire island (from Lloyd and Nathan, 1981). B) A second sketch map showing key areas of volcanism during the past 4,000 years (from Latter and others, 1992). C) A more detailed view of Raoul caldera and the cratered interior of the island, with contour lines at 20 m intervals (from Lloyd and Nathan, 1981). The northern caldera contains three small lakes: Blue Lake (1.17 km2, about 40% overgrown), Green Lake (160,000 m2), and Tui Lake (5,000 m2, drinking water quality). The island's high point is Moumoukai (516 m). Unfortunately, the current report mentions a few other features undisclosed on these maps. Courtesy of GNS.

The 2006 eruption blew over mature trees out to ~ 200 m and deposited dark gray mud and large ballistic blocks. Many of the steep crater margins had post-eruption collapses marked by fresh landslides.

The New Zealand Department of Conservation evacuated five staff members from the island, but one worker, taking water-temperature measurements at Green Lake at the time of the eruption, was killed. Devastation left by the eruption thwarted efforts to find the missing worker (figure 3). A news story reported that the missing man left around 0730 on 17 March to walk to Green Lake. An hour later the volcano erupted.

Figure 3. Photo reportedly taken by the rescue helicopter pilot John Funnell of the area affected by the volcanic eruption on Raoul Island, 17 March 2006. AP Photo; photo credit to John Funnell.

Volcano monitoring of the Raoul crater lakes started after the 1964 eruption, as these lakes responded measurably before that event, consistent with a long-lived hydrothermal system. There are low-temperature (boiling-point) fumaroles in the vicinity of Green Lake and minor seepages of hydrothermal brine from the system (boiling hot springs) along Oneraki Beach, outside of the caldera. The gases have strong hydrothermal signatures (as opposed to proximal magmatic). As such, they do not suggest single-phase vapor transport directly from a magmatic source to the surface, but rather are indicative of the presence of boiling hydrothermal brine at depth. GNS has no quantitative data from Denham Bay (offshore to the W of the island, but scientists from the organization found boiling-point (100° C) steaming ground on the steep crater walls, and gas and water seeps in the sea. Historical observations of volcanic eruptions from this caldera (and Raoul caldera) point to the likely existence of a sizable active system residing there.

Still and video footage taken of the post-eruptive scene on 17 March 2006 showed many new craters and reactivation of 1964 craters. The main steam columns were derived from Crater I, Marker Bay, and Crater XI. Fumarolic activity appeared near the mouth of Crater Gully and the stream that drains from Crater V. The area NW of Bubbling Bay, where there had been a fumarole, contained a crater about 20-30 m across.

In the main body of Green Lake there were two areas of strong upwelling. One occurred near the end of the peninsula S of Crater XII (a promontory that had been explosively removed). Jagged rocks were visible in the lake where it had been 2-4 m deep. There was also a new feature about 200-300 m N of Green Lake's Crater XII (figure 2B); the new feature included a moat near the edge of the crater floor, which contained a vigorously active vent. Green Lake's surface did not appear elevated at the time of the post-eruption 17 March observations.

Sulfur dioxide (SO₂) was detected by satellite about 5 hours after the 17 March eruption (figure 4). SO₂ data was collected by the Aura Ozone Monitoring Instrument (OMI), which is affiliated with the University of Maryland, the US National Aeronautics and Space Administration (NASA), the Royal Netherlands Meteorological Institute (KNMI), and the Finnish Meteorological Institute (FMI). The highest SO₂ values stood over and adjacent to the island and reached as high as two Dobson Units (DU, figure 4). Simon Carn noted that the total mass of SO₂ in figure 4 was ~ 200 tons. Subsequent observations did not detect further SO₂ discharge.

Figure 4. Atmospheric sulfur dioxide (SO2) detected by the Aura/OMI satellite about 5.65 hours after the Raoul Island eruption's onset on 17 March 2006. (The eruption onset was at about 0821 local time and this SO2 observation was at about 1358 local time (0158 UTC).) Image courtesy of Simon Carn, the OMI SO2 group at the University of Maryland, and NASA/KNMI/FMI.

An aerial inspection on 21 March made from a Royal New Zealand Air Force Orion aircraft allowed excellent views of both the Raoul and Denham Bay calderas. Visible steam discharge from the vents had declined significantly owing to a 6-8 m rise in Green Lake's water level and the consequent drowning of most of the active vents. The lake level did not appear to have reached overflow level. Landsliding and collapse also blocked Crater I. Vigorous upwelling and gas discharge was still obvious through Green Lake, which appeared very warm.

There was no evidence of further eruptions since 17 March, nor was there any evidence that activity had occurred from the 1964 craters adjacent to Crater Gully (i.e. craters III, IV, and VI-X). However, many new craters formed at the mouth of Crater Gully where hot bare ground had been present. There was a possible NE-trend through the vents from Crater Gully to NE of Crater XII. In 1964 the craters aligned along three parallel fractures that tended NW. Heightened activity was not confined to the lake.

In Denham Bay GNS scientists observed a weak plume of discolored water approximately coincident with the vent area. There was evidence of hydrothermal seepage along most of the beach (milky discoloration indicating mixing of hydrothermal brine and seawater). There were also discharges in the rocky bay halfway between Hutchison Bluff and the NW end of Denham beach (figure 2A). If these are confirmed as hydrothermal seepages, they represent a significant rise in the surface of the hydrothermal fluids in the system, consistent with that observed in the caldera.

On 23 March 2006, the GNS reported that scientists who flew over noted that the hydrothermal system under the island showed signs of over-pressuring. GNS volcanologist Bruce Christenson stated, "From our aerial observations, it is clear that the heat, gas, and water that are discharging into Green Lake are making this part of the volcano's hydrothermal system unstable." Several new steam vents opened in and around Green Lake during the eruption and some old ones had reactivated. Many of these were drowned as a result of lake-level rise. According to Christenson, "one explanation for the increased hydrothermal activity is that it is being driven by the intrusion of magma at depth."

Steve Sherburn of GNS reported on 24 March on the GeoNet website (the New Zealand GeoNet Project provides real-time monitoring and data collection for rapid response and research into earthquake, volcano, landslide, and tsunami hazards) that over the last few days the level of earthquake activity at or close to Raoul Island had continued to decline to a current level of only 5-10 earthquakes per day, most of which were probably too small to be felt on the island. There is no unequivocal seismic evidence for magma movement (such as the strong volcanic tremor observed before the 1964 eruption). Careful seismic monitoring of Raoul Island will continue.

Brad Scott reported on 3 April 2006 that activity continued to decline in the Green Lake crater area. The most recently available photographs showed the water level continuing to rise slowly in Green Lake, but it had not reached overflow level. Over the last few days the level of earthquake activity at or close to Raoul Island continued to decline and in early April there were only 2-5 earthquakes per day being recorded.

References. Latter, J.H.; Lloyd, E.F.; Smith, I.E.M.; and Nathan, S., 1992, Volcanic hazards in the Kermadec Islands, and at submarine volcanoes between Southern Tonga and New Zealand: Volcanic Hazards Information Series, no. 4 (CD 303), New Zealand Ministry of Civil Defence, 45 p. (Booklet) ISBN 0-477-07472-3 Lloyd, E.F., and Nathan, S., 1981, Geology and tephrochronology of Raoul Island, Kermadec Group, New Zealand: New Zealand Geological Survey Bulletin, no. 95, 105 p. (includes map in back pocket).

Geologic Background. Anvil-shaped Raoul Island is the largest and northernmost of the Kermadec Islands. During the past several thousand years volcanism has been dominated by dacitic explosive eruptions. Two Holocene calderas exist, the older of which cuts the center the island and is about 2.5 x 3.5 km wide. Denham caldera, formed during a major dacitic explosive eruption about 2200 years ago, truncated the W side of the island and is 6.5 x 4 km wide. Its long axis is parallel to the tectonic fabric of the Havre Trough that lies W of the volcanic arc. Historical eruptions during the 19th and 20th centuries have sometimes occurred simultaneously from both calderas, and have consisted of small-to-moderate phreatic eruptions, some of which formed ephemeral islands in Denham caldera. An unnamed submarine cone, one of several located along a fissure on the lower NNE flank, has also erupted during historical time, and satellitic vents are concentrated along two parallel NNE-trending lineaments.

Information Contacts: Brad Scott, Institute of Geological and Nuclear Sciences (GNS), Wairakei Research Centre, 114 Karetoto Road, Taupo, New Zealand (URL: http://www.geonet.org.nz/, http://www.gns.cri.nz/).

According to Simon Carn, volcanic activity at Tinakula appears to have begun on 12 February 2006, with a small explosion followed by degassing. He noted some significant SO₂ emissions on 14 February, as well as small plumes from Ambrym and Aoba. As of 16 February, there was still a small SO₂ signal from Tinakula, but it was no bigger than that from Ambrym or Aoba. Andrew Tupper noted from visible MTSAT (Multi-functional Transport Satellite) images and an Aqua MODIS (Moderate Resolution Imaging Spectroradiometer) screen shot that a plume on 14 February was moving NNE at ~ 10 km/hour and appeared to be not far above summit level; the plume did not register on the IR imagery. MTSAT is a dual-mission satellite for the Japan Ministry of Land, Infrastructure, and Transport and the Japan Meteorological Agency performing an air traffic control and navigation, as well as a meteorological, functions.

On 27 February, Thomas Toba of the Solomon Islands government wrote to Herman Patia of the Rabaul Volcano Observatory, confirming Tinakula activity. Toba contacted authorities from the Temotu Provincial Headquarters who confirmed that there were several small explosions from this volcano around early to middle February 2006.

Satellite thermal-sensor data (using the MODVOLC alert-detection algorithm) revealed a period of thermal anomalies on the uninhabited island of Tinakula during cloud-free intervals in early to mid-February 2006 (table 1). The anomalies were particularly numerous on 11 February. The information was extracted from the MODIS Thermal Alerts website maintained by the Hawai'i Institute of Geophysics and Planetology (HIGP) (see also BGVN 29:06 and 28:01). The satellites used were Aqua and Terra MODIS. Confirmation of the volcanic source of the anomalies was not broadly distributed until late March 2006.

Table 1. MODVOLC thermal anomalies at Tinakula for mid-February through mid-April 2006. Since the start of monitoring by MODIS satellite sensors on 8 May 2001, no thermal anomalies had been measured at Tinakula before 11 February 2006. Courtesy of University of Hawai'i Institute of Geophysics and Planetology MODIS Hotspot Alert website.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. Similar to Stromboli, it has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The satellitic cone of Mendana is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Frequent historical eruptions have originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP), School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, 1680 East-West Road, POST 602, Honolulu, HI 96822 (URL: http://modis.higp.hawaii.edu); Simon Carn, University of Maryland Baltimore County (UMBC), Joint Center for Earth Systems Technology (JCET), Total Ozone Mapping Spectrometer (TOMS) Volcanic Emissions Group, 1000 Hilltop Circle, Baltimore, MD 21250; Andrew Tupper, Darwin Volcanic Ash Advisory Centre, Bureau of Meteorology, Australia (URL: http://www.bom.gov.au/info/vaac/); Thomas Toba, Ministry of Energy, Water, and Minerals Resources, Honiara, Solomon Islands; Herman Patia, Rabaul Volcano Observatory, P.O. Box 386, Rabaul, Papua New Guinea.

Ubinas began erupting ash on 25 March 2006. Since mid-2005 a small increase in fumarolic activity had been seen during visits to the crater by personnel from the Instituto Geofísico del Perú (IGP), UNSA local university, and the Instituto Geologico, Minero y Metalurgico (INGEMMET); it was also reported by local authorities. Increased fumarolic emissions described by INGEMMET were reported on 18 January 2006 by Diario Digital Sur Noticias. Fumaroles started to make strong jet noises, and seismic activity increased, in February 2006. The eruption that began on 25 March, described below, has continued through at least late April.

On 25 March farmers from Querapi village, 4 km from the crater, noted ash deposits on crops. A few millimeters of ash was deposited and quickly removed by rain. The volcano had been mostly cloud-covered during the previous few weeks, but on 27 March residents of Querapi noted a column of ash at 1430. On 30 and 31 March teams from IGP, UNSA, and INGEMMET visited the volcano (figure 2). Although there had been constant snow over the previous days, the summit was completely gray from ashfall. The ash thickness on rocks 2 km NW of the crater was 3 mm, just inside the summit crater there was about 1 cm, and at the inner pit crater edge there was 2 cm. Thick ash surrounded a new 30-m-wide vent in the crater base. This crater was emitting constant ash and gas with larger pulses approximately every 15 minutes. Near the edge of the pit crater were large numbers of flat circular mud discs up to 15 cm in diameter, many with central solid cores. These grew smaller and less frequent with distance. It is thought these are either huge accretionary lapilli, generated in storm clouds above Ubinas, or products of wet eruptions from the new vent. The crater area is dangerous and frequently smothered in ash clouds, so observations remain sketchy.

Figure 2. Photo of Ubinas on 31 March 2006 showing an eruption plume rising from the summit crater. Photo by the Perúvian Civil Defense taken from Moquegua city, provided courtesy of the Associated Press.

Ash emissions through 10 April covered local villages and damaged crops. Clear crop damage was visible around the village of Querapi, with potato and alfalfa leaves and flowers blemished in spots. This is the critical growing time for the crop, and thus any damage is serious for the local farmers. Cattle have been seen suffering from diarrhea.

Short periods of seismic recordings have been made at a site 2,500 m NW of the crater rim. On 20 November 2004 only 16 local events were recorded over 12 hours. In February 2005 there where 96 events over the same time period. Over 12 hours on 27 March 2006 there were 115 events. During this last interval, low-amplitude tremor events lasting 3 minutes on average were recorded, as well as long-period (LP) events. Over the 12 hours of observation the following events were recorded: 62 LP, 18 LP with precursors, 10 volcano-tectonic (VT), five VT with precursors, and 20 tremor events.

Geologic Background. A small, 1.4-km-wide caldera cuts the top of Ubinas, Peru's most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front of Perú. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45 degrees. The steep-walled, 150-m-deep summit caldera contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3700 years ago extend 10 km from the volcano. Widespread plinian pumice-fall deposits include one of Holocene age about 1000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Orlando Macedo, Observatorio de Cayma-Arequipa, Instituto Geofísico del Perú at Arequipa city (IGP-Arequipa), Urb. La Marina B-19, Cayma, Arequipa, Perú; Jersy Marino, Instituto Geologico, Minero y Metalurgico (INGEMMET), Perú; Benjamin van Wyk, Laboratoire Magmas et Volcans (LMV), OPGC, France; Jean-Philippe Métaxian, Laboratire de Geophysique Interne et Tectonophysique-Univ de Savoie, France; Perúvian Civil Defense (URL: http://www.indeci.gob.pe/); Diario Digital Sur Noticias, Tacna, Perú (URL: http://www.surnoticias.com/); Associated Press (URL: http://www.ap.org/).

On 7 September 2005, the Alaska Volcano Observatory (AVO) noted several minor bursts of ash from the volcano during the afternoon. Ash bursts continued to occur through at least 9 September, with ash rising less than 3 km altitude, and with the ash confined to the caldera. Over the following 2 weeks, minor ash emission continued at a rate of 1-5 events per day based on interpretations of seismic data. AVO reported that it was likely that diffuse ash plumes rose to heights less than ~ 3 km and were confined to the summit caldera. Cloudy weather during 16-23 September prohibited web-camera and satellite observations of Veniaminof, but seismic data indicated diminishing activity. On 28 September seismicity had remained at background levels for over a week, and there was no evidence to suggest that minor ash explosions were continuing.

On 4 November 2005, a low-level minor ash emission occurred from the intracaldera cone beginning at 0929. Ash rose a few hundred meters above the cone, drifted E, and dissipated rapidly. Minor ashfall was probably confined to the summit caldera. During the previous 2 weeks, occasional steaming from the intracaldera cone was observed. Very weak seismic tremor and a few small discrete seismic events were recorded at the station closest to the active cone. However, AVO reported that there were no indications from seismic data that a significantly larger eruption was imminent.

On the morning of 3 March 2006 ash again rose a few hundred meters above the intracaldera cone, drifted E, and dissipated rapidly. Ashfall was expected to be minor and confined to the summit caldera. Seismicity was again low and did not indicate that a significantly larger eruption was imminent. Over the week of 5-10 March, seismicity was low but slightly above background.

On the morning of 10 March, AVO received a report from a pilot of low-level ash emission from the intracaldera cone. Clear web-camera views on 9 March showed small diffuse plumes of ash extending a short distance from the intracaldera cone. The Anchorage Volcanic Ash Advisory Center (VAAC) reported a steam/ash plume noted on web-cam and satellite on 13 March 2006 at 0500Z (12 March 2006 at 2000 hours local), moving NNW at 9.2 km/hr and falling to the land surface. Web-cam images on 22 March showed a very diffuse steam-and-ash plume that was confined to the summit caldera, and on 24 March showed a steam-and-ash plume drifting from the summit cone at a height of less than 2.3 km. This level of activity was similar to that on 23 March, but higher than activity on 21 and 22 March, when a very diffuse steam-and-ash plume was confined to the summit caldera.

The flow of seismic data from Veniaminof stopped on the evening of 21 March 2006, and the problem was expected to continue until AVO staff could visit the site to repair the problem. Absent seismic data, the volcano could potentially still be monitored in other ways such as using web-camera and satellite images. Imagery was obscured by cloudy weather after 21 March. On 26 March 2006, a pilot reported a small ash plume rising above the volcano. Low-altitude ash emissions from Veniaminof were visible during 31 March to 7 April. On 6 April, a pilot reported an ash plume at a height of 3 km. AVO stated in its weekly report of 14 April 2006 that the seismicity at Veniaminof remained low but above background. Internet camera and satellite views had been obscured by cloudy weather, and AVO lacked new information about ash clouds or activity.

Geologic Background. Massive Veniaminof volcano, one of the highest and largest volcanoes on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, and Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.avo.alaska.edu/); Charles R. Holliday, Air Force Weather Agency (AFWA), Offutt Air Force Base, NE 68113.

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).