Volcanism at Sheveluch has been ongoing for the past 20 years. Previous activity consisted of pyroclastic flows, explosions, moderate gas-and-steam emissions, and lava dome growth, according to the Kamchatka Volcanic Eruptions Response Team (KVERT). Between May 2018 and mid-December 2018 activity levels were low, with intermittent low-power thermal anomalies and gas-and-steam emissions. Activity increased in the second half of December 2018, remaining high through at least April 2019.

Activity intensified beginning in late December through April 2019, which included increased and more frequent thermal anomalies, according to KVERT and the MIROVA system (figure 50). On 30 December 2018, video data from KVERT showed explosions producing an ash cloud that rose up to 11 km altitude and drifted 244 km WSW and 35 km NE. Eruptive activity included incandescent lava flows and hot avalanches. The ash cloud that drifted WSW resulted in ashfall over Klyuchi Village (50 km SW) and Kozyrevsk (100 km SW).

Figure 50. Thermal anomalies at Sheveluch increased in late December 2018, as seen on this MIROVA Log Radiative Power graph for the year ending 5 April 2019. The elevated thermal activity continued through March 2019. Courtesy of MIROVA.

Beginning in early January and going through April 2019, the lava dome at the northern part of the volcano continued to grow, extruding incandescent, viscous lava blocks (figure 51). Throughout these months, KVERT reported that satellite imagery and video data showed strong fumarolic activity, as well as strong gas-and-steam plumes containing some amount of ash; gas-and-steam plumes rose as high as 7 km. According to the KVERT Daily Reports on 3 and 4 January 2019, a gas-and-steam plume containing ash drifted NE up to about 600 and 400 km, respectively. Gas-and-steam plumes noted in the KVERT Daily Report, Weekly Releases, and Volcano Observatory Notice for Aviation (VONA), drifted 50-263 km in different directions. On 9 November 2018, the KVERT Daily Report recorded an ash plume drifting 461 km E from the volcano and on 26 December 2018, the KVERT Weekly Information Release recorded an ash cloud drifting 300 km NW. The KVERT Weekly Information Release reported that on 10 April 2019 an ash cloud drifted up to 1,300 km SE.

Figure 51. Incandescent avalanches from the lava dome and an ash plume can be seen in this photo of Sheveluch on 22 February 2019. Photo by Yu. Demyanchuk; courtesy of the Institute of Volcanology and Seismology FEB RAS, KVERT.

Thermal anomalies based on MODIS satellite instruments analyzed using the MODVOLC algorithm were frequent beginning on 28 December 2018. In just three days in late December (28-31 December 2018) there were 34 thermal alerts. Hotspots were detected 21-27 days each month between January-April 2019. A majority of these hotspot pixels occurred within the summit crater.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

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/); 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/).

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 phreatic explosion with an ash plume in mid-January 2018 began the latest eruptive episode which included the growth of a lava dome with pyroclastic flows down the flanks and lava fountaining (BGVN 43:04). Activity tapered off during March; occasional ash emissions continued through August 2018. Minor ash emissions and summit incandescence were intermittent from October 2018-April 2019, the period covered in this report. Information is provided primarily by the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Pyroclastic density currents were reported in early November 2018; ash plumes were produced from phreatic events a few times during both November and December 2018. Emissions produced SO₂ anomalies during January-March 2019; a series of events in early March generated several small ash plumes. Satellite images showing a thermal anomaly at the summit were recorded multiple times each month from October 2018-April 2019 (figure 44).

Figure 44. Small but distinct persistent thermal anomalies were recorded in satellite imagery from the summit of Mayon during October 2018-April 2019. Top left: 12 October 2018. Top right: 26 November 2018. Middle left: 11 December 2018. Middle right: 30 January 2019. Bottom left: 14 February 2019. Bottom right: 25 April 2019. All images are using the "Atmospheric penetration" filter (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Very little activity was reported at Mayon during October 2018. Steam plumes rose daily from 250-750 m above the summit before drifting with the prevailing winds and dissipating. Incandescence was observed at the summit most nights during the month, and seismicity remained low with only a few earthquakes reported. Leveling data obtained during 30 August-3 September indicated significant short-term deflation of the volcano relative to 17-24 July 2018. New leveling data obtained on 22-31 October indicated inflation of the SE quadrant and short-term deflation on the N flank relative to the 30 August-3 September data. The volcano remained inflated compared with 2010 baseline data. Electronic tilt data showed pronounced inflation of the mid-slopes beginning 25 June 2018.

Activity increased during November 2018. In addition to steam plumes rising to 750 m and an incandescent glow at the summit most nights, pyroclastic density currents and ash plumes were reported. The seismic monitoring network recorded pyroclastic density currents on 5 and 6 November. On 8 November around noontime, a small, short-lived brownish ash plume, associated with degassing, drifted WSW from the summit. A seismic event on the morning of 11 November was associated with a short-lived fountaining event that produced a brownish-gray ash plume that drifted SW. Another similar plume was reported on the morning of 12 November, also drifting SW before dissipating. Two phreatic events were observed on the morning of 26 November. They produced grayish to grayish-white ash plumes that rose 300-500 m above the summit before drifting SW. The following morning, another event produced a grayish ash plume 500 m above the summit that drifted SW. On 30 November a 1-minute-long ash emission event produced a grayish white plume that also drifted SW.

Steam plume emissions and incandescence at night continued at Mayon during December 2018. The seismic network recorded a four-minute-long event shortly after noon on 9 December that produced a grayish-brown ash plume which drifted W. Precise leveling data obtained on 8-13 December 2018 indicated a slight inflation of the volcano relative to 22-31 October 2018. A 30-second-long ash emission event in the afternoon on 18 December produced a brownish ash plume. Two phreatic events were observed on the morning of 27 December. They produced grayish to grayish-white ash plumes that rose 600 and 200 m above the summit, before drifting SW (figure 45).

Figure 45. Ash plumes rose a few hundred m from the summit of Mayon on 27 December 2018. Courtesy of Twitter users "k i t" (left) and "georgianne" (right).

Very little surface activity except for white steam-laden plumes that crept downslope and drifted NW or SW was noted during January 2019. Incandescence at the summit, visible with the naked eye, became more frequent during February 2019, along with continued steam plumes. Precise leveling data obtained on 25 January-3 February 2019 indicated a slight deflation relative to 8-13 December 2018. However, continuous GPS and electronic tilt data showed inflation of the mid-slopes since June 2018. Small SO₂ plumes were detected by the TROPOMI satellite instrument a few times during January-March 2019 (figure 46).

Figure 46. Emissions of SO2 that exceeded 2 DU (Dobson Units) occurred a few times at Mayon during January-March 2019. Top left: 25 January. Top right: 16 February. Lower left: 4 March. Lower right: 15 March. Courtesy of NASA Goddard Space Flight Center.

Steam plumes rose 250-500 m above the summit and drifted generally W in early March 2019; incandescence continued daily at the summit. Phreatic events occurred on 7 and 8 March, producing ash plumes that rose 500 and 300 m from the summit before drifting SW (figure 47). Three more phreatic events occurred on the afternoon of 12 March; they produced light brown to grayish ash plumes that rose 500, 1,000, and 500 m, respectively, and drifted SW. Six phreatic events occurred throughout the day on 13 March, producing ash plumes that rose 200-700 m above the summit and drifted W. A single explosion the next day produced a 500-m-tall ash plume. The Tokyo VAAC reported an ash plume visible for several hours in satellite imagery drifting W at 3.7 km altitude on 13 March (UTC). An increase in the daily number of rockfall events from 1-2 per day to 5-10 per day was noted during the second half of March. Precise leveling data obtained on 20-26 March 2019 indicated a slight inflation relative to 25 January-3 February 2019.

Figure 47. A small ash emission at Mayon was reported by PHIVOLCS on 8 March 3019; the plume rose 300 m from the summit and drifted SW. Courtesy of PHOVOLCS.

Steam plumes drifted SW or NW throughout April, rising 200-400 m from the summit. Incandescence could be observed at night for the first half of the month. Leveling data obtained during 9-17 April 2019 indicated a slight inflation relative to 20-26 March 2019. Seismicity remained low during the month with only occasional volcanic earthquakes and rockfall events. Lenticular clouds around the summit were observed (figure 48), but these are an unusual meteorological occurrence caused by weather conditions not related to volcanic activity.

Figure 48. A double lenticular cloud surrounded the summit of Mayon early in the morning on 23 April 2019 and was captured by a local observer; it was not related to volcanic activity. Courtesy of Twitter user Ivan.

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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Twitter user "Ivan", Naga City, Philippines (URL: https://twitter.com/ivanxlcsn); Twitter user "k i t", Legazpi City, Philippines (URL: https://twitter.com/jddmgc); Twitter user "georgianne", Costa Leona, Philippines (URL: https://twitter.com/xolovesgia_).

Remote Tinakula lies 100 km NE of the Solomon Trench at the N end of the Santa Cruz Islands, which are part of the country of the Solomon Islands located 400 km to the W. It has been uninhabited since an eruption with lava flows and ash explosions in 1971 when the small population was evacuated (CSLP 87-71). The nearest communities live on Te Motu (Trevanion) Island (about 30 km S), Nupani (40 km N), and the Reef Islands (60 km E); residents occasionally report noises from explosions at Tinakula. Ashfall from larger explosions has historically reached these islands. The most recent eruptive episode was a large ash explosion and substantial SO₂ plume during 21-26 October 2017; satellite imagery suggested that a flow of some type traveled down the scarp on the W flank. Renewed thermal activity that was recognized in satellite imagery beginning in December 2018 continued intermittently through June 2019 and is covered in this report. Satellite imagery and thermal data are the primary sources of information for this volcano. It is occasionally visited by members of the National Disaster Management Office (NDMO) of the Solomon Islands Government, tourists, and research vessels who are able to capture ground-based information.

Satellite images from December 2018 to February 2019 show thermal anomalies at the summit vent. Excellent ship-based photographs of the island on 24-25 January 2019 provided by a crewmember from the R/V Petrel identify numerous volcanic features and show a steam-and-gas plume at the vent. Satellite images from April and May 2019 show thermal anomalies at both the summit vent and along the W flank scarp suggesting flow activity during that time.

A stream of incandescence on the NW flank of Tinakula in a Sentinel 2 satellite image on 24 October 2017 confirmed that some type of high-temperature flow accompanied the explosions and eruptive activity of 21-25 October 2017 (BGVN 43:02). Satellite imagery during most of 2018 recorded steam plumes drifting in several directions from the summit, but no thermal activity (figure 24). There was no further evidence of activity in satellite visible or thermal data until almost exactly one year later when the MIROVA project recorded two thermal alerts in the third week of October 2018 (figure 25). Satellite images from that week were cloudy and did not confirm any surface activity.

Figure 24. Sentinel-2 satellite imagery of Tinakula provides valuable information about activity at this remote volcano in the South Pacific. A large explosion with ash plumes and flows occurred during 21-26 October 2017. Top left: a strong E-W linear thermal anomaly suggesting a flow event from the summit was evident on the NW flank on 24 October 2017. Top right: a small steam plume rose from the summit vent on a cloudless 11 February 2018. Bottom left: a dense steam plume drifted SE from the summit vent on 4 September 2018. Bottom right: clouds and dense steam obscure the summit on 24 October 2018, about the same time that MIROVA reported a thermal anomaly. Top left image uses bands 12, 11, 8A, others use 12, 4, 2. Courtesy of Sentinel Hub Playground.

Figure 25. The MIROVA project recorded the first thermal anomaly in a year from Tinakula during the third week of October 2018. Courtesy of MIROVA.

The first satellite imagery confirming renewed thermal activity appeared on 8 December 2018, around the same time as a small MIROVA anomaly. After that, several images during January and February 2019 confirmed moderately strong thermal activity at the summit (figure 26). Whether the anomalies were the result of active lava effusion or strong incandescent gases from the summit vent is uncertain.

Figure 26. Thermal anomalies at the summit vent of Tinakula were recorded six times between early December 2018 and early February 2019 with Sentinel-2 satellite images. Top row: 8 December 2018 and 2 January 2019. Middle row: 12 (anomaly is just below date) and 27 January 2019. Bottom row: 1 and 6 February 2019. All images are bands 12, 4, 2. Courtesy of Sentinel Hub Playground.

Visual confirmation of activity at Tinakula is rare, but the research vessel R/V Petrel sailed past the volcano on 24 and 25 January 2019 and a crewmember provided detailed images of the W flank and vent area. The summit vent is located at the top of a W facing scarp, and steam is frequently observed rising from the vent (figures 27). Recent flows and volcaniclastic deposits were visible in the ravine on the W flank (figures 28 and 29). Fresh-looking lava was also visible near the summit vent on top of older deposits (figure 30). Eroded volcaniclastic deposits near the base of the scarp on the W flank were visible on top of older veined and layered volcanic rocks (figure 31). Crewmembers on the vessel R/V Petrel could clearly see an incandescent glow from the summit crater at night (figure 32).

Figure 27. A view from the SW of the W flank of Tinakula on 24-25 January 2019. The summit vent is at the top of a W facing scarp, the steam plume drifted E. Used with permission from Paul G Allen's Vulcan Inc.

Figure 28. The W flank of Tinakula as seen from the W on 24-25 January 2019. The steam plume drifted E. Recent flows and volcaniclastic deposits appeared dark in the steep ravine on the W face (left side). Used with permission from Paul G Allen's Vulcan Inc.

Figure 29. Steam and gas rose from the summit vent at Tinakula on 24-25 January 2019. Recent lava deposits are visible in front of the plume and in the ravine on the left (the W flank). Used with permission from Paul G Allen's Vulcan Inc.

Figure 30. The edge of the summit vent of Tinakula on 24-25 January 2019 had recent lava on older deposits; steam and gas is rising from the vent in the background. Used with permission from Paul G Allen's Vulcan Inc.

Figure 31. The W flank of Tinakula on 24-25 January 2019. Eroded volcaniclastic deposits overlie older veined and layered volcanic rocks. Used with permission from Paul G Allen's Vulcan Inc.

Figure 32. Incandescence was clearly visible from the summit vent at Tinakula on 24-25 January 2019. Used with permission from Paul G Allen's Vulcan Inc.

During April and May 2019, both the MIROVA project and MODVOLC measured a number of thermal anomalies (figure 33) using MODIS satellite data. MODVOLC alerts were issued on 4 and 20 April, and 11, 18, and 27 May. Sentinel-2 satellite images during the period confirmed that a flow on the W flank was a likely source of the thermal energy in addition to the summit vent (figure 34). Thermal anomalies appeared again at the end of June in MIROVA data, but no satellite images showed anomalies at that time.

Figure 33. The number and intensity of MIROVA thermal anomalies increased at Tinakula during April and May 2019. After a short pause, they returned at the end of June. Courtesy of MIROVA.

Figure 34. Sentinel-2 satellite images captured thermal anomalies at the summit and on the W flank of Tinakula during April and May 2019 suggesting the presence of an incandescent flow down the W scarp. Top row: 7 and 22 April 2019 (bands 12, 8, 4). Bottom row: 27 April and 12 May 2019 (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

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: 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Vulcan Inc. (URL: https://www.vulcan.com/), additional details about the R/V Petrel (URL: https://www.paulallen.com/).

Short pulses of intermittent eruptive activity have characterized Piton de la Fournaise, the large basaltic shield volcano on La Réunion Island in the western Indian Ocean, for several thousand years. For the last 20 years, frequent effusive basaltic eruptions have occurred on average twice per year. The activity is characterized by lava fountains and lava flows, and occasional explosive eruptions that shower blocks over the summit area and produce ash plumes. Almost all of the recent activity has occurred within the Enclos Fouqué caldera, although past eruptions in 1977, 1986, and 1998 have occurred at vents outside of the caldera. Four separate eruptive episodes were reported during 2018; from 3-4 April, 27 April-1 June, 13 July, and 15 September-1 November (BGVN 43:12, 43:09). Two episodes from 2019 during February-March and June are covered in this report, with information provided primarily by the Observatoire Volcanologique du Piton de la Fournaise (OVPF) as well as satellite instruments.

Piton de la Fournaise experienced two eruptions during November 2018-June 2019. The first lasted from 18 February to 10 March 2019, and the second episode was 11-13 June. The episode in February-March started consisted of multiple fissures opening on the E flank of the Dolomieu crater on 18 February with lava flows that traveled several hundred meters. After a brief pause, one new fissure opened nearby on 19 February and produced up to 3 million m³ of lava in a little over four days. Although the flow rate then declined, the eruption continued until 10 March. During the last three days, 7-10 March, two new fissures opened nearby and produced large volumes of lava, bringing the total eruptive volume to about 14.5 million m³. After little activity during April and May, a small eruption occurred on the SSE outer slope of Dolomieu crater that lasted for about 48 hours on 11-13 June; multiple small flows traveled about 1,000 m down the steep flank before ceasing. The MIROVA thermal anomaly graph of log radiative power clearly showed the abruptness of the beginning and ends of the last three eruptive episodes at Piton de la Fournaise from August 2018 through June 2019 (figure 165).

Figure 165. The MIROVA graph of thermal energy from Piton de la Fournaise from 30 July 2018 through June 2019 shows the last three eruptive episodes at the volcano. From 15 September through 1 November 2018 fissures and flows were active on the SW flank of Dolomieu crater near Rivals crater (BGVN 43:12). Fissures opened on the E flank of the crater on 18 February 2019, and after a brief pause resumed on 19 February at the foot of Piton Madoré. Lava flows remained active until 10 March 2019. A short episode of lava effusion occurred on 11-12 June 2019 on the SSE outer slope of Dolomieu crater. Courtesy of MIROVA.

Activity during November 2018-March 2019. Following the end of the 15 September-1 November 2018 eruption, seismic activity immediately below the summit remained low (with only 20 shallow and two deep earthquakes during November). The inflationary signal recorded since the beginning of September stopped, and the OVPF deformation networks did not record any significant deformation. There were 35 shallow earthquakes (0-2 km depth) below the summit crater during December, and one deep earthquake. Only 12 shallow earthquakes and one deep earthquake (greater than 2 km below the surface) were reported in January.

OVPF reported an increase in CO₂ concentrations beginning in December 2018, and noted the beginning of inflation on 13 February 2019. A seismic swarm of 379 earthquakes accompanied by minor but rapid deformation (less than 1 cm) was reported on 16 February 2019. A new seismic swarm of 208 earthquakes began early on 18 February with a much larger ground deformation (10 cm of elongation of the summit zone). A volcanic tremor indicative of the arrival of magma near the surface began at 0948 that morning. Webcams indicated that eruptive fissures had opened in the NE part of the Enclos Fouqué caldera. The onset of the eruption was marked by a sudden drop in CO₂ flux which then stabilized. The eruptive sites were confirmed visually around 1130. Three fissures with actively flowing lava opened on the E flank of Dolomieu Crater; the fountains of lava were less than 30 m high. The front of the longest flow had reached 1,900 m elevation after one hour. The eruption lasted a little over 12 hours and was over by 2200 that evening; it covered about 150-200 m of the hiking trail to the summit.

Seismicity remained high after the event ended, and at 1500 on 19 February 2019 another seismic swarm of 511 deep earthquakes located under the E flank at about 2.5 km depth occurred. It was not accompanied by a significant amount of deformation. At 1710 tremor signals appeared on the observatory seismographs and the first gas plumes and lava ejection were observed at 1750 and 1912, respectively. During an overflight the next day (20 February), OVPF team members observed the new eruptive site at an elevation of 1,800 m at the foot of Piton Madoré. One fissure and one fountain were active at 0620 on 20 February and the flow front was at 1,300 m elevation (figure 166). During the night of 20-21 February the flow front crossed over the "Grandes Pentes" area in the eastern half of the Enclos Fouque (figure 167).

Figure 166. The eruption which began on 19 February 2019 on the E flank of Dolomieu crater at Piton de la Fournaise produced a lava fountain and flow which traveled down at least 500 m of elevation by the next morning when this photo was taken at 0620 local time. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du mercredi 20 février 2019 à 11h00, Heure locale).

Figure 167. The active fissure at Piton de la Fournaise was producing lava fountains and an active flow during the evening of 20 February 2019. Overnight the flow crossed over the "Grandes Pentes" area of the caldera. Photo courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du jeudi 21 février 2019 à 14H00, Heure locale).

OVPF reported on 22 February 2019 that 22 shallow earthquakes had been reported since the eruption began on 19 February. Surface flow rates estimated from satellite data, via the HOTVOLC system (OPGC - University of Auvergne), were between 2.5 and 15 m³/s. The quantity of lava emitted between 19 and 22 February was between 1 and 3 million m³. OVPF observed the growth of an eruptive cone that was filled with a small lava lake producing ejecta during a morning overflight on 22 February. A channelized flow moved downstream from the cone and split into two lobes about 1 km from (and 200 m below) the cone (figure 168). The split in the flow occurred near the Guyanin crater. The N flowing lobe, about 50 m wide, had an actively flowing front located at 1,320 m elevation; the incandescent flow was travelling over a recent flow (likely from the previous night). The S-flowing lobe spread to 200 m wide and split into two tongues 300 m SE of Guyanin crater.

Figure 168. During an overflight on the morning of 22 February 2019 scientists from OVPF observed a growing spatter cone with a small lava lake at Piton de la Fournaise. A channelized flow moved downstream from the fissure and split into two flows. Photo courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du vendredi 22 février 2019 à 13h30, Heure locale).

Incandescent ejecta from the cone was captured in a webcam image overnight on 22-23 February 2019 (figure 169). The rate of advance of the flow slowed significantly by 24 February, but the intensity of the eruptive tremor remained relatively constant. Mapping of the lava flow on 28 February carried out by the OI2 platform (OPGC - University Clermont Auvergne) from satellite data confirmed the slow progress of the flow after 24 February (300 m in 5 days) (figure 170). The flow front was located at 1,200 m elevation, and only the N arm was active; the lava had traveled about 2.2 km from the vent by 28 February.

Figure 169. Incandescent ejecta from the eruptive cone at Piton de la Fournaise was captured in the webcam in the early hours of 23 February 2019. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 23 février 2019 à 13h30, Heure locale).

Figure 170. Contours of the lava flows at Piton de la Fournaise from 18-28 February 2019 were determined from satellite data by the OI2 platform (Université Clermont Auvergne), dated 18 (red) and 19 (blue) February (top image); 20 (green), 21 (red), 22 (blue), 27 (turquoise), and 28 (pink) February (bottom image). Courtesy of and copyright by OVPF/IPGP. Top: Bulletin d'activité du vendredi 22 février 2019 à 13h30 (Heure locale); bottom: Bulletin d'activité du jeudi 28 février 2019 à 16h30 (Heure locale).

Between 28 February and 1 March 2019 a third lobe of lava appeared flowing NE from the vent on the N side of the new flow area; it split into two lobes sometime on 1 March. Very little new lava was recorded on the other lobes. By 4 March the flow rate estimated by satellite data was about 7.5 m³/s. During a site visit on the morning of 5 March OVPF scientists sampled the N lobe of the flow and bombs and tephra near the cone, and acquired infrared and visible images. They noted the continued growth of the cone which still had an open vent at the summit and a base 100 m in diameter. It was 25 m high with a 50-m-wide eruptive vent at the top (figure 171). High-temperature gas emissions and strong Strombolian activity issued from the vent. Steam emissions were present around the base of the cone, suggesting the presence of lava tunnels. A single lobe of lava flowed N from the cone.

Figure 171. The eruptive cone at Piton de la Fournaise on 5 March 2019 had a 100-m-diameter base, 25 m of vertical height, and 50-m-wide vent at the summit. Courtesy of and copyright by OVPF/IPGP, (Bulletin d'activité du mardi 5 mars 2019 à 17h30, Heure locale).

A new fissure that opened about 150 m from the main vent on the NW flank of Piton Madoré was first observed on the morning of 6 March (figure 172); OVPF concluded that it had opened late on 5 March. A small cone was forming and a new flow traveled N from the main eruptive site. At least six new emission points were noted the following morning (7 March) around the Piton Madoré. Poor weather prevented confirmation by aerial reconnaissance that day, but in a site visit on 8 March OVPF scientists determined that the new fissure from 5 March remained active; a small cone about 10 m high had two flow lobes on the W and N sides (figure 173). A fissure that opened on 7 March was located 300 m S of the 19 February vent and oriented E-W. It was very active on the morning of 8 March with two 50-m-high lava fountains (figure 174). Samples collected by OVPF indicated that the vents of 5 and 7 March produced lava of different compositions.

Figure 172. A new fissure that opened about 150 m from the main vent on the NW flank of Piton Madoré at Piton de la Fournaise was first observed on the morning of 6 March 2019; OVPF concluded that it had opened late on 5 March. A small cone was forming on the flank of an old one and a new flow traveled N from the main eruptive site. Courtesy of OVPF/IPGP, copyright by Helicopter Coral (Bulletin d'activité du jeudi 7 mars 2019 à 15h00 Heure locale).

Figure 173. The 5 March 2019 fissure at Piton de la Fournaise on the NW flank of Piton Madoré still had two active flow lobes emerging from it and heading N and W on 8 March 2019. Courtesy of and copyright by OVPF/IPGP (Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, March 2019).

Figure 174. A fissure that opened on 7 March 2019 at Piton de la Fournaise was located 300 m S of the 19 February vent and oriented E-W. It was very active on the morning of 8 March 2019 with two 50-m-high lava fountains. Courtesy of and copyright by OVPF/IPGP (Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, March 2019).

There was a strong increase in the eruptive tremor intensity on 7 March, related to the opening of the two new fissures on 5 and 7 March (figure 175). As a result, the surface flow estimates made from satellite data increased significantly to high values greater than 50 m³/s, with the average values on 7-8 March of around 20-25 m³/s. The increased flow rates resulted in the flows traveling much greater distances. By the morning of 9 March the active flow had reached 650-700 m above sea level. The flow front had traveled about 1 km in 24 hours. Strong seismicity had been increasing under the summit zone for the previous 48 hours. After a phase of very strong surface activity observed overnight on 9-10 March that included lava fountains 50-100 m high (figure 176), surface activity ceased around 0630 on 10 March, and seismic activity decreased significantly. OVPF noted that sudden increases in seismicity and flow rates near the end of an eruption have occurred at about half of the eruptions at Piton de la Fournaise in recent years. Lava volumes emitted on the surface between 18 February and 10 March 2019 were estimated at about 14.5 million m³ (figure 177).

Figure 175. An infrared view of the eruptive site on the E flank of Dolomieu crater at Piton de la Fournaise on 8 March 2019 clearly showed the original fissure from 19 February (bottom right of center), the fissure on Piton Madore that opened on 5 March (right) and the fissures that opened on 7 March (upper, right of center). The combined activity produced significant thermal and seismic activity at the volcano. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du vendredi 8 mars 2019 à 17h00, Heure locale).

Figure 176. Lava fountains 50-100 m high were the result of very strong surface activity observed overnight on 9-10 March 2019 at Piton de la Fournaise. Surface activity ceased around 0630 on 10 March, and seismic activity decreased significantly. Photo taken on 9 March 2019 around midnight from the RN2. Courtesy of OVPF/IPGP, copyright by A. Finizola LGSR/IPGP (Bulletin d'activité du dimanche 10 mars 2019 à 19h30 Heure locale).

Figure 177. A sudden increase in the flow rate at the end of the 18 February-10 March 2019 eruption at Piton de la Fournaise was recorded by researchers at the Université Clermont Auvergne. OVPF noted this was typical of about half of the eruptions at Piton de la Fournaise. Courtesy of OVPF/IPGP, copyright by HOTVOLC, Université Clermont Auvergne (OVPF Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, March 2019).

Significant SO₂ plumes were captured by the TROPOMI instrument on the Sentinel 5-P satellite throughout the 18 February-10 March eruption (figure 178). After the surface eruption ceased, shallow seismicity continued at a lower rate of about 12 earthquakes per day. The end of the eruption (7-10 March) was accompanied by a marked deflation, interpreted by OVPF as the rapid emptying of the magma reservoir. Following the end of the eruption, inflation resumed for the rest of March but then ceased. Seismicity continued at a lower level during April with an average of six shallow earthquakes per day.

Figure 178. Multiple days of high DU value SO2 plumes were recorded by the TROPOMI instrument on the Sentinel 5-P satellite during the 18 February-10 March 2019 eruption at Piton de la Fournaise. Top row: during 18, 21, and 22 February SO2 plumes drifted SE. Middle row: during 23, 24, and 25 February the wind direction changed from SE through S to SW and left a curling trail of SO2. Bottom row: 5, 7, and 8 March showed an increase in SO2 emissions that corresponded with increased seismicity and lava flow output before the eruption ceased.

Activity during May-June 2019. OVPF reported slight inflation near the summit beginning in early May, and an increase in CO₂ concentration in the soil near Plaine des Cafres and Plaine des Palmistes. Strong shallow seismicity reappeared on 27 May 2019 and recurred on 30 and 31 May. Two small seismic swarms were measured on 31 May in the early morning. A new seismic swarm beginning at 0603 on 11 June accompanied by rapid deformation suggested a new eruption was imminent. A tremor near the summit area was first noted at 0635 local time; the webcams indicated a plume of gas, but poor visibility prevented evidence of fresh lava. Around 0930 that morning OVPF confirmed that five fissures had opened on the outer SSE slope of Dolomieu crater at elevations ranging from 2480 to 2025 m (figure 179). The flow fronts were not visible due to weather. Lava fountains under 30 m in height and lava flows were present in the three lowest fissures. The flows traveled rapidly down the steep flank of the crater (figure 180).

Figure 179. Around 0930 on the morning of 11 June 2019 OVPF confirmed that five fissures had opened on the outer SSE slope of Dolomieu crater at Piton de la Fournaise at elevations ranging from 2480 to 2025 m. Courtesy of and copyright by OVPF-IPGP and Imazpress (Bulletin d'activité du mardi 11 juin 2019 à 11h00).

Figure 180. Thermal imaging of the 11-12 June 2019 eruptive site at Piton de la Fournaise showed multiple streams of lava traveling rapidly down the steep flank from several fissures on 11 June 2019. Courtesy of and copyright by OVPF-IPGP (Bulletin d'activité du mardi 11 juin 2019 à 11h00).

The intensity of the eruptive tremor decreased throughout the day, and by 1530 only the lowest elevation fissure was still active (figure 181). The next afternoon (12 June) images in the OVPF webcam located in Piton des Cascades indicated the flow front was at about 1,200-1,300 m elevation. Seismographs indicated that the eruption stopped around 1200 on 13 June. Poor weather obscured visibility of the flow activity. Seismic activity decreased following the eruption, but appeared to increase again beginning on 21 June, with 10 events detected on 30 June. SO₂ plumes were recorded in satellite data on 11 and 12 June 2019.

Figure 181. The intensity of the eruptive activity at Piton de la Fournaise on 11 June 2019 decreased throughout the day, and by 1530 only the lowest elevation fissure was still active. Courtesy of and copyright by OVPF-IPGP (Bulletin d'activité du mardi 11 juin 2019 à 17h45 Heure locale).

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, 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); 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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Heard Island, in the Southern Indian Ocean, includes the large Big Ben stratovolcano and the smaller, apparently inactive, Mt. Dixon. Because of the island's remoteness, satellites are the primary monitoring tool. Big Ben has been active intermittently since 1910, and was active during October 2017-September 2018 (BGVN 43:10). Activity continued during October 2018-March 2019.

Satellite photos using Sentinel Hub showed hotspots every month between October 2018 and March 2019. Because the area was frequently covered by a heavy cloud layer, most of the hotspot signals were partially obscured. Though thermal anomalies are usually seen at summit vents, on 18 October 2018 an anomaly was present about 300 m down the E flank. Similarly, on 1 January 2019, a weak anomaly beginning about 200 m down the NW flank was about 300 m long (figure 40).

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected three hotspots, two in October and one in early November 2018, all of low radiative power. There were no MODVOLC alert pixels during this period.

Figure 40. Sentinel-2 L1C image of Heard Island's Big Ben volcano on 1 January 2019 one summit hotspot and an elongated thermal anomaly to the NW. Scale bar (bottom right) is 200 m. The photo was taken in atmospheric penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon volcano lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben volcano because of its extensive ice cover. The historically active Mawson Peak forms the island's 2745-m high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported in historical time at this isolated volcano, but observations are infrequent and additional activity may have occurred.

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/).

The ongoing eruption at Semeru has been characterized by numerous ash explosions and thermal anomalies, but activity apparently diminished in 2018 (BGVN 43:01 and 43:09); this decreased activity continued through at least February 2019. The current report summarizes activity from 24 August 2018 to 28 February 2019.

The Indonesian volcano monitoring agency, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), reported ongoing daily seismicity, dominated by explosion earthquakes and emission-related events from late November through February (figure 35). Ash plumes resulting in aviation advisories by the Darwin Volcanic Ash Advisory Centre (VAAC) were reported on 4, 6-7, and 19 September, and 12 October 2018. The next significant ash plume reported by the VAAC wasn't until 24 February 2019 (table 23).

Figure 35. Seismicity recorded at Semeru during 28 November 2018-26 February 2019. Plot shows explosion earthquakes ('Letusan'), emission-related events ('Hembusan'), felt earthquakes ('Gempa Terasa'), local tectonic events ('Tektonic Lokal'), and distant tectonic events ('Tektonic Jauh'). Courtesy of PVMBG and MAGMA Indonesia.

Table 23. Summary of ash plumes at Semeru during 25 August 2018 through February 2019. The summit is at 3,657 m elevation. Data courtesy of Darwin VAAC.

Thermal anomalies using MODIS satellite instruments processed by the MODVOLC algorithm were only recorded on 26, 28, and 30 August 2018, and 22 and 31 October 2018. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected numerous hotspots within 5 km of the volcano during August and early September, with a significant decrease in frequency through October (figure 36); only a few scattered hotspots were recorded from November 2018 through February 2019.

Figure 36. MIROVA plot of thermal anomalies (Log Radiative Power) at Semeru during July 2018-February 2019. Courtesy of MIROVA.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

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/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); 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/); 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/).

The eruption at Dukono that began in 1933 has showered the area with ash from frequent explosions (BGVN 43:04, 43:12). The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Center for Volcanology and Geological Hazard Mitigation (CVGHM), is responsible for monitoring this volcano.

This long-term pattern of intermittent ash explosions continued during October 2018-March 2019, with ash plumes rising to between 1.5 and 2.7 km altitude, or about 300-1,500 m above the summit (table 19). Although meteorological clouds often obscured views, satellite imagery captured typical ash plumes on 28 September 2018 (figure 10) and 5 February 2019 (figure 11). Instruments aboard NASA satellites (TROPOMI and OMPS) detected high levels of sulfur dioxide near or directly above the volcano on multiple days during January-March 2019. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to remain outside of the 2-km exclusion zone.

Table 19. Monthly summary of reported ash plumes from Dukono for October 2018-March 2019. The direction of drift for the ash plume through each month was highly variable. Data courtesy of the Darwin VAAC and PVMBG.

Figure 10. Satellite image from Sentinel-2 (LC1 natural color) of an ash plume at Dukono on 28 September 2018 with the plume blowing towards the NE. Courtesy of Sentinel Hub Playground.

Figure 11. Satellite image from Sentinel-2 (LC1 natural color) of an ash plume at Dukono on 5 February 2019, with the plume blowing SW. Courtesy of Sentinel Hub Playground.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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/); 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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Intermittent small phreatic explosions from the acid lake of Rincón de la Vieja's active crater has most recently occurred since 2011 (BGVN 42:08, 43:03, and 43:09). This activity continued through at least February 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 18 August 2018 and 28 February 2019. Weather conditions often prevented webcam views and estimates of plume heights. The volcano was in Activity Level 3 throughout the reporting period (volcano erupting, steady state).

According to OVSICORI-UNA, two distinct, 2-minute-long explosions occurred on 31 August 2018 beginning at 0434 and 1305. Several hours after the eruption tremor became continuous but low-frequency long-period (LP) earthquakes ceased. OVSICORI-UNA reported a gas emission late on 7 September. An unconfirmed small phreatic explosion occurred on 11 September at 0634, and another on 17 September at 1014. The seismic record showed continuous background tremor and very sporadic LP earthquakes.

Intermittent background tremor was recorded during the first half of October, along with a few emissions and phreatic explosions. Deformation measurements during October showed a contraction between the N and S of the volcano, with subsidence. On 17 October there was another phreatic explosion, and thereafter tremor disappeared and seismicity decreased. On 23 and 27 October seismic stations signaled additional possible phreatic explosions.

OVSICORI-UNA reported that a series of explosions began at 1945 on 4 November and consisted of at least three 2-minute-long episodes. The next day at 1511 a plume of water vapor and diffuse gas, recorded by a webcam and visible to residents to the N, rose about 100 m above the crater rim and drifted W. On 9 November a 2-minute-long explosion began at 1703. Another explosion on 27 November at 0237 produced a plume of water vapor and gas that rose 600 m above the crater rim and drifted SW. A short 1-minute explosion began at 1054 on 3 December.

Based on OVSICORI-UNA weekly bulletins, activity remained stable in January 2019 with small-amplitude phreatic explosions on 11, 12, and 14 January. More energetic phreatomagmatic explosions on 17 and 20 January produced lahars. Several small-amplitude explosions were detected at the end of the month. During January, a few LPs, no VTs, and intermittent tremor were recorded.

OVSICORI-UNA reported that two small-scale explosions occurred on 1 February, along with possible events at 1906 and 1950 on 5 February and at 0120 on 6 February. An event at 0000 on 6 February was also recorded; the report noted that poor weather conditions prevented visual observations of the crater. On 16 and 17 February strong degassing was observed. No LPs were recorded, but two significant VTs were detected on 17 and 22 February near or under the crater.

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 Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

This report summarizes activity at Turrialba during September 2018-February 2019. During this period there was similar activity as described earlier in 2018 (BGVN 43:09), with occasional ash explosions and numerous, sometimes continuous, periods of gas-and-ash emissions (table 8). Data were provided by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Table 8. Ash emissions at Turrialba, September 2018-February 2019. Cloudy weather sometimes obscured observations. Maximum plume height is above the crater rim. Information courtesy of OVSICORI-UNA.

According to OVSICORI-UNA's annual summary for 2018, a slow decline in activity occurred after the volcano reached its highest emission rate during 2016. Activity during 2018 was consistent with an open system, generating frequent passive ash emissions. The volcano emitted ash on 58% of the days during the year. Some explosions were large enough to eject ballistics more than 400 m around the crater. Typical activity can be seen in a photo from 11 September 2018 (figure 50) and satellite imagery on 7 November 2018 (figure 51).

Figure 50. Photo of an ash explosion at Turrialba taken on 11 September 2018. Courtesy of Red Sismologica Nacional (RSN: UCR-ICE), Universidad de Costa Rica.

Figure 51. Sentinel-2 satellite image of an ash emission from Turrialba on 7 November 2018, taken in natural color (gamma adjusted). Courtesy of Sentinel Hub Playground.

During January into early February 2019, passive ash emissions continued irregularly and with less intensity and duration. Emissions sometimes lacked ash. In their report of 4 February 2019, OVSICORI-UNA indicated that passive ash emissions were weak and slow. For the rest of February, they characterized ash emissions as frequent, but of low intensity.

Seismic activity. On 1 November 2018 OVSICORI-UNA reported that seismicity remained high, and involved low-amplitude banded volcanic tremor along with long-period (LP) and volcano-tectonic (VT) earthquakes. In late January-early February 2019, OVSICORI-UNA reported that seismicity remained relatively stable, although a small increase was associated with the hydrothermal system. VT earthquakes were absent, and tremors had decreased in both energy and duration. The number of low-frequency LP volcanic earthquakes remained stable, although they had decreasing amplitudes. No explosions were documented, and emissions were weak and had short durations and very dilute ash content.

Thermal anomalies. No thermal anomalies were recorded during the reporting period using MODIS satellite instruments processed by MODVOLC algorithm. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected five scattered hotspots during September-October 2018, none during November-December 2018, and two during January-February 2019. All were within 2 km of the volcano and of low radiative power.

Gas measurements. Significant sulfur dioxide levels near the volcano were recorded by NASA's satellite-borne ozone instruments only on 29 September 2018 (both NPP/OMPS and Aura/OMI instruments) and on 11 February 2019 (Sentinel 5P/TROPOMI instrument). OVSICORI-UNA's gas measuring instruments were compromised in September 2018 through January 2019 due to vandalism. In early February, however, they detected hydrogen sulfide for the first time since 2016.

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: https://rsn.ucr.ac.cr/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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://hotspot.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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Costa Rica Star (URL: https://news.co.cr); The Tico Times (URL: https://ticotimes.net).

San Cristóbal has produced occasional weak explosions since 1999, with intermittent gas-and-ash emissions. The only reported explosion during the first half of 2018 was on 22 April, the first since November 2017 (BGVN 43:03). The current report covers activity between 1 August 2018 and 1 May 2019. The volcano is monitored by the Instituto Nicaragüense de Estudios Territoriales (INETER).

According to INETER, a series of explosions occurred on 9 January 2019 that lasted several hours. INETER stated that one explosion occurred at 1643; the Washington VAAC's first advisory stated that an explosion occurred at 1145 (local time). The weak explosions, which occurred after a period of heightened seismic activity, generated an ash plume that reached 200 m above the edge of the crater and drifted W. The Washington VAAC reported volcanic ash plumes on 10-11 January extending about 92 km SW, and on 24-25 January extending about 185 km WSW. A low-energy explosion was detected by the seismic network at 1550 on 4 March 2019. The event produced a gas-and-ash plume that rose 400 m above the crater rim and drifted SW.

Monitoring data reported by INETER (table 6) showed elevated levels of seismicity during October 2018 through January 2019. Sulfur dioxide was also measured at higher levels in January 2019.

Table 6. Monthly sulfur dioxide measurements and seismicity reported at San Cristóbal during August 2018-March 2019. "Most" indicates that type of seismicity was dominant that month. Data courtesy of INETER.

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://webserver2.ineter.gob.ni/vol/dep-vol.html); 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).

The remote Semisopochnoi comprises the uninhabited volcanic island of the same name, ~20 km in diameter, in the Rat Islands group of the western Aleutians (figure 1). Plumes had been reported several times in the 18th and 19th centuries, and most recently observed in April 1987 from Sugarloaf Peak (SEAN 12:04). The volcano is dominated by an 8-km diameter caldera that contains a small lake (Fenner Lake) and a number of post-caldera cones and craters. Monitoring is done by the Alaska Volcano Observatory (AVO) using an on-island seismic network along with satellite observations and lightning sensors. An infrasound array on Adak Island, about 200 km E, may detect explosive emissions with a 13 minute delay if atmospheric conditions permit.

On 16 September 2018 increased seismicity was detected at 0831, prompting AVO to raise the Aviation Color Code (ACC) to Yellow and Volcano Alert Level (VAL) to Advisory. Retrospective analysis of satellite data acquired on 10 September revealed small ash deposits on the N flank of Mount Cerberus, possibly associated with two bursts of tremor recorded on 8 September (figure 5). This new information, coupled with intensifying seismicity and a strong tremor signal recorded at 1249 on 17 September, resulted in AVO raising the ACC to Orange and the VAL to Watch. Seismicity remained elevated on 18 September with nearly constant tremor recorded by local sensors. At the same time, no ash emissions were observed in cloudy satellite images and no eruptive activity was recorded on regional pressure sensors at Adak.

Figure 1. Minor ash deposits can be seen on the south and west flanks of the N cone of Mount Cerberus, Semisopochnoi Island, in this ESA Sentinel-2 image from 1200 on 10 September 2018. Also note probable minor steam emissions obscuring the crater of the N cone. Image courtesy of AVO.

During 19-25 September 2018 seismicity remained elevated, alternating between periods of continuous and intermittent bursts of tremor. Tremor bursts at 1319 on 21 September and at 1034 on 22 September produced airwaves detected on a regional infrasound array on Adak Island; no ash emissions were identified above the low cloud deck in satellite data, and the infrasound detections likely reflected an atmospheric change instead of volcanic activity.

Seismicity remained elevated during 3-9 October 2018, with intermittent bursts of tremor. No volcanic activity was detected in infrasound or satellite data. On 11 October satellite data indicated partial erosion of a tephra cone in the crater of Cerberus's N cone. A crater lake about 90 m in diameter filled the vent. The data also suggested that the vent had not erupted since 1 October. Seismicity remained elevated and above background levels. The next day AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory, noting the recent satellite data results and lack of tremor recorded during the previous week. AVO reported that unrest continued during 11-24 October.

An eruptive event began at 2047 on 25 October 2018, identified based on seismic data; strong volcanic tremor lasted about 20 minutes and was followed by 40 minutes of weak tremor pulses. A weak infrasound signal was detected by instruments on Adak Island. The Aviation Color Code was raised to Orange (the second highest level on a four-color scale) and Volcano Alert Level was raised to Watch (the second highest level on a four-level scale). A dense meteorological cloud deck prevented observations below 3 km, but a diffuse cloud was observed in satellite data rising briefly above the cloud deck, though it was unclear if it was related to eruptive activity. Tremor ended after the event, and seismicity returned to low levels.

Small explosions were detected by the seismic network at 2110 and 2246 on 26 October 2018, and 0057 and 0603 on 27 October. No ash clouds were identified in satellite data, but the volcano was obscured by high meteorological clouds. Additional small explosions were detected in seismic and infrasound data during 28-29 October; no ash clouds were observed in partly-cloudy-to-cloudy satellite images.

AVO reported on 31 October 2018 that unrest continued. Two small explosions were detected, one just before 0400 and the other around 1000. Satellite views were obscured by clouds at the time, and no ash clouds were observed. Unrest continued through 1 November, at which time the satellite link and the seismic line failed. On 21 November the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is 1221-m-high Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked 774-m-high Mount Cerberus volcano was constructed during the Holocene within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side. Other post-caldera volcanoes include the symmetrical 855-m-high Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).

Japan's 24-km-wide Asosan caldera on the island of Kyushu has been active throughout the Holocene. Nakadake has been the most active of 17 central cones within the caldera for 2,000 years. Historical eruptions have been primarily basaltic to basaltic-andesitic ash eruptions, with periodic Strombolian activity, all from Nakadake Crater 1. The most recent major eruptive episode began in late November 2014 and continued through 1 May 2016. Another eruption, with the largest ash plume in 20 years, occurred on 8 October 2016. Asosan remained quiet until renewed activity from Crater 1 began in mid-April 2019; it is covered in this report, through the end of June 2019. The Japan Meteorological Agency (JMA) provides monthly reports of activity; the Tokyo Volcanic Ash Advisory Center (VAAC) issues aviation alerts reporting on possible ash plumes.

Asosan remained quiet during 2017 and 2018 with steam plumes rising a few hundred meters from Crater 1 and low levels of SO₂ emissions; a warm acidic lake was present within the crater. Fumarolic activity from two areas on the S and SW wall of the crater rim generated occasional thermal anomalies in satellite data and incandescence at night. A brief period of increased seismicity was reported in mid-March 2019. An increase in seismic amplitude on 14 April 2019 preceded a small explosion on 16 April; it produced an ash plume which rose 200 m above the crater rim and drifted NW. It was followed by additional small explosions on 19 April. A new explosion on 3 May produced minor ashfall in adjacent communities; ash emissions were reported multiple times during May with plumes reaching 1,400 m above the crater rim. No additional ash emissions were reported in June.

Activity during 2017 and 2018. JMA reported that no eruptions occurred during 2017. Amplitudes of volcanic tremor increased somewhat during March but were generally low for the rest of the year. The earthquake hypocenters were mostly located near the active crater at around sea level. SO2 emissions were slightly less than 1,000 tons per day (t/d) from January through April; for the rest of the year they ranged from 600 to 2,500 t/d. The Alert Level had been lowered from 2 to 1 on 7 February 2017 where it remained throughout the year. Steam plumes generally rose no more than 600 m above the active crater rim (figure 42). JMA noted that from January to June they often observed crater incandescence at night with a high-sensitivity surveillance camera; Sentinel-2 satellite images also captured thermal anomalies a few times (figure 43). The green lake inside the crater persisted throughout the year with water temperatures of 50-60°C. Two fumaroles were present with high-temperature gas emissions on the SW and S crater walls. Temperatures at the S crater wall were over 600°C from February to May; they decreased to 320-560°C during the rest of the year (figure 44). Sulfur deposits were visible around the SW crater wall fumarole during July.

Figure 42. Steam plumes that rose around 600 m above Nakadake Crater 1 at Asosan were typical activity throughout 2017. Images taken with JMA webcam on 9 June (top left), 22 August (top right), 12 November (bottom left), and 20 December (bottom right) 2017. Courtesy of JMA (Aso volcano monthly activity reports, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 43. Sentinel-2 images captured thermal anomalies at the S rim of the green lake at Asosan's Nakadake Crater 1 on 16 February (left) and 27 May 2017 (right). JMA reported that incandescence was occasionally visible during the night from January-June from the same area. Courtesy of Sentinel Hub Playground.

Figure 44. High-temperature gas and steam from fumaroles on the S wall of the Nakadake Crater 1 at Asosan on 24 August (top) and 17 November 2017 (bottom) were persistent all year, with temperatures ranging from 300 to over 600°C. The green lake inside the crater persisted throughout the year as well with water temperatures of 50-60°C. Courtesy of JMA (Aso volcano monthly activity reports, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

The Alert Level did not change at Asosan during 2018, and no eruptions were reported. Sulfur dioxide emissions fluctuated between 400 and 1,800 t/d throughout the year. Steam plumes generally rose less than 500 m above the active crater (figure 45); incandescence was observed at night during May-October and sometimes observed in satellite imagery as thermal anomalies (figure 46). The temperature of the green lake inside the crater ranged from 58 to 75°C throughout the year. The thermal anomaly on the S wall of the crater was consistently in the 300-500°C range, and had a high temperature in April of 580°C; in December the high temperature had risen to 738°C (figure 47). A brief increase in the number of isolated tremors occurred during March, with 1,044 reported on 4 March, exceeding the previous maximum of 1,000 on 27 October 2014. Seismicity also increased briefly during June, with more than 400 events reported each day on 8, 18, and 20 June. The Minami Aso village Yoshioka fumarole zone, located about 5 km W of Nakadake Crater 1, continued to produce modest steam plumes throughout 2017 and 2018 (figure 48).

Figure 45. Typical steam plumes at Asosan during 2018 rose around 500 m above the Nakadake Crater 1. Images are from 4 March (top left), 22 July (top right), 17 August (lower left), and 13 September 2018 (lower right). Courtesy of JMA (Aso volcano monthly activity reports, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 46. Nighttime incandescence was reported by JMA during May-October 2018 from the S rim of Nakadake Crater 1 at Asosan; Sentinel-2 satellite images (bands 12, 4, 2) captured thermal anomalies from the same area numerous times during 2018 including on 16 June (top left), 26 July and 19 September (middle row), and 18 and 23 November (bottom row). JMA photographed incandescence at night on 17 July 2018 at the S fumarole area (top right). Courtesy of Sentinel Hub Playground and JMA (Aso volcano Monthly Report for July 2018).

Figure 47. The "Green Tea Pond" inside Nakadake Crater 1 at Asosan had temperatures that ranged from 58 to 75°C during 2018 (top row, 26 March 2018); the thermal anomaly on the S wall of the crater consistently had temperatures measured in the 300-500°C range and the SW fumarole area had somewhat lower temperatures (bottom row, 22 June 2018). Courtesy of JMA (monthly Asosan reports for March, May, and June 2018).

Figure 48. The Minami Aso village Yoshioka fumarole zone, located about 5 km W of Nakadake Crater 1 at Asosan, continued to produce modest steam plumes throughout 2017 and 2018. It is shown here on 20 December 2017 (top) and 12 March 2018 (bottom). Courtesy of JMA (December 2017 and March 2018 monthly volcano reports).

Activity during 2019. Steam plumes rose to 800 m above the crater rim during January 2019. Overall activity increased slightly during February; SO₂ emissions peaked at 2,200 t/d early in the month; they ranged from 800 to 1,800 t/d for most of the month. The amplitude of volcanic tremor also increased slightly during February. A further increase in tremor amplitude on 11 March 2019 prompted JMA to raise the Alert Level from 1 to 2 the following morning. Volcanic tremor amplitude decreased on 15 March; JMA determined that activity had decreased, and the Alert Level was lowered back to 1 on 29 March 2019. The amount of water in the crater decreased significantly between 27 February and 20 March, exposing part of the crater floor.

The surface temperature of the lake rose during the first part of 2019; it was 78°C in February and 84°C in March. Steam plumes rose to 1,200 m above the crater rim during March and April. SO₂ emissions rose to 4,500 t/d on 12 March but dropped to a lower range of 1,300-2,400 for the rest of the month. Another surge in SO₂ emissions on 12 April 2019 to 3,600 t/d prompted a special report from JMA the following day. SO₂ emissions varied from about 1,700 to 4,100 t/d during the month; values remained high during the second half of the month. JMA noted that the color of the water in the lake inside Nakadake Crater 1 changed from green to gray after 4 April. Fountains of muddy water were periodically observed; they reached 15 m high on 9 April. The temperatures of both the lake (82°C) and around the two fumarole areas (S area about 530°C, SW area about 310°C) remained constant during April and similar to March.

A large increase in the amplitude of volcanic tremor early on 14 April 2019 prompted JMA to raise the Alert Level from 1 to 2 later in the day. The epicenters of the earthquakes were very shallow, located within 1 km beneath the crater. A small eruption occurred at 1828 on 16 April at Nakadake Crater 1; it produced a gray and white plume that rose 200 m above the crater rim and was the first eruption since 8 October 2016 (figure 49). Incandescence was observed inside the crater on 3 and 17 April. The amplitude of seismic tremors decreased on 18 April. Three very small eruptions on 19 April produced ash and steam plumes that rose 500 m above the crater rim. During a site visit that day JMA measured a high-temperature area that produced incandescence from the bottom of the crater at night (figure 50).

Figure 49. The first eruption since October 2016 at Nakadake Crater 1 at Asosan on 16 April 2019 sent an ash plume 200 m above the crater rim (top). Incandescent gas appeared on the crater floor the next day (bottom). Courtesy of JMA (Aso volcano monthly activity reports, April 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 50. Three small explosions on 19 April 2019 at Asosan's Nakadake Crater 1 produced small ash emissions that rose 500 m above the crater rim (top). A strong thermal signal also appeared from the bottom of the crater. Courtesy of JMA (Aso volcano monthly activity reports, April 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

A new eruption began at 1540 on 3 May that lasted until 0620 on 5 May (figure 51). Initially the ash plume rose 600 m above the crater rim, but a few hours later the volume of ash increased, and the plume reached 2 km above the crater rim for a brief period. Incandescence was visible from the webcam. The Tokyo VAAC reported the ash plume at 3 km altitude drifting SE on 3 May. Later in the day it rose to 3.7 km altitude and drifted SW. During a field survey the following day (4 May) JMA reported a steam and ash plume rising from the center of the active crater. The infrared thermal imaging camera recorded the temperature of the plume at about 500°C (figure 52).

Figure 51. An explosion at Asosan's Nakadake Crater 1 on 3 May 2019 produced an ash plume that reached 2 km above the crater rim (top) and incandescence visible from the webcam (bottom). Courtesy of JMA (Aso volcano monthly activity reports, April 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 52. During a site visit on 4 May 2019, staff from JMA witnessed an ash and steam plume rising from the bottom of Nakadake Crater 1 at Asosan (top). The infrared thermal imaging camera recorded the temperature of the plume at about 500°C (bottom). Courtesy of JMA (Aso volcano monthly activity reports, May 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Ash fell on the S flank, and a small amount of ashfall on 4 May was confirmed by evidence on a car windshield in Takamori Town (6 km S), Kumamoto Prefecture (figure 53). Ashfall was also reported in Takamori-machi, Minami Aso village (9 km SW), and part of Yamato-cho (25 km SW), also in the Kumamoto Prefecture. SO₂ emissions were measured as high as 4,000 t/d on 4 May. Additional explosions with ash plumes were reported from Asosan on 9, 12-16, 29, and 31 May; the plumes rose from 200 to 1,400 m above the crater rim but were not visible in satellite imagery. The TROPOMI instrument on the Sentinel-5 satellite captured SO₂ plumes on 3 and 26 May 2019 (figure 54).

Figure 53. Ashfall was reported on 4 May 2019 in Takamori Town, Kumamoto Prefecture, from the eruption at Asosan's Nakadake Crater 1 on 3 May 2019. Courtesy of JMA (Aso volcano monthly activity reports, May 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 54. Plumes of SO2 from Asosan were recorded by the TROPOMI instrument on the Sentinel-5P satellite on 3 (left) and 26 (right) May 2019. Courtesy of NASA Goddard Space Flight Center.

Steam plumes rose to 1,700 m above the crater rim during June 2019 (figure 55). During field visits on 6 and 25 June diffuse ash emissions were observed rising from the center of the active crater, but they did not extend significantly above the crater rim (figure 56). The maximum temperature of the plume was measured at about 340°C with a thermal imaging camera. Almost all of the water in the crater bottom had evaporated since early May; incandescence continued to be observed within the crater at night with the high-resolution webcam (figure 57).

Figure 55. Steam plumes rose to 1,700 m above the crater rim at Asosan's Nakadake Crater 1 on 10 June 2019. Courtesy of JMA (Aso volcano monthly activity reports, June 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 56. Plumes of gas and minor ash were visible at Asosan's Nakadake Crater 1 during site visits by JMA on 6 (left) and 25 (right) June 2019. Courtesy of JMA (Aso volcano monthly activity reports, June 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 57. Incandescent gas was visible from the vent at Asosan's Nakadake Crater 1 on 18 (left) and 25 (right) June 2019. Courtesy of JMA (Aso volcano monthly activity reports, June 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Since 1968, Arenal experienced periods of moderate-to-robust volcanic activity that continued through September 2010, when activity declined (BGVN 35:07 and 36:04). This report discusses events between December 2010 and October 2012, a period of continued relative tranquility.

Although sporadic Strombolian explosions were reported in December 2010, they soon ceased; since then, no explosions had occurred through as late as October 2012. According to the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI), activity was limited to weak gas emissions, primarily through the NE vent in Crater C and through fumaroles in Crater D (figure 113).

Figure 113. A photograph of Arenal's summit taken on 20 February 2011, featuring the volcano's two peaks, both showing weak fumaroles. To the right is crater C, which has been active since 1968; to the left is crater D. Courtesy of Jairo Murillo Solís.

During the reporting period, the pH of rain-water gradually increased near the volcano. According to OVSICORI, the gradual decrease in rainfall acidity was associated with reduced magmatic activity.

According to OVSICORI, 2012 was one of the years of lowest activity for Arenal since 1968. No volcano-tectonic earthquakes, volcanic earthquakes, or tremors were recorded during the year, and no magmatic activity was detected. OVSICORI (citing Muller and others, 2011) reported that the Electronic Distance Measurement (EDM) network on the W flank of Arenal showed some subsidence from 2008 to near the end of 2011, but then the rate of subsidence decreased and no deformation occurred in 2012.

In June 2012, OVSICORI reported that night observations and long-exposure photographs of the summit revealed no incandescence. According to OVSICORI, the lack of incandescence indicated that gas emissions were of low temperature (probably <300°C), allowing water vapor to condense rapidly upon contact with the atmosphere. Hydrothermal activity remained low with only a few diffuse fumaroles rising from the N flank of Crater C (figure 113).

According to OVSICORI, an M_w 7.6 earthquake on 5 September 2012 centered on the Nicoya Peninsula (Costa Rica) caused moderate rock avalanches at Arenal, mainly dislodging unstable blocks on the active crater's N and NW rim. However, no changes were noted either in the hot springs around the volcano or in surficial expressions of volcanism.

A special issue of Journal of Volcanology and Geothermal Research was devoted to Arenal volcano (see Reference subsection below).

References. Marsh, B. (ed.), 2006, Arenal volcano, Costa Rica: Magma genesis and volcanological processes, Journal of Volcanology and Geothermal Research, v. 157, issues 1-3.

Muller, C., del Potro, R., Gottsmann, J., Biggs, J., and Van der Laat, R., 2011, Combined GPS, EDM and triangulation surveys of the rapid down-slope motion of the western flank of Arenal Volcano, Costa Rica, American Geophysical Union, Fall Meeting 2011, abstract ## V53C-2639 (Poster).

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

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/); CostaRica21 (URL: http://www.costarica21.com/).

This report discusses a series of small but punctuated eruptions on 15-17 September 2012 associated with the return of seismicity at Gamalama. Fog obscured visibility but ash fell on inhabited areas. The eruptions were judged similar to those seen 4 December 2011 (BGVN 36:12).

As we noted previously, heavy rains after the 4 December 2011 eruptions led to lahars on 27-28 December that killed four people, injured dozens, and displaced thousands (BGVN 36:12). Photos showed that these lahars had carried many meter-diameter blocks into inhabited areas on the lower flanks. Videos from helicopter flights confirmed that in the upslope region, chutes and drainages had also fed finer ash into the lahars.

According to the Center for Volcanology and Geological Hazard Mitigation (CVGHM), on 24 January 2012, after witnessing an interval of generally reduced seismicity, an absence of significant ash-bearing plumes, and weak steam plumes rising only ~100 m above the summit, they lowered the Alert Level from 3 to 2 (on a scale from 1-4).

As geographic background, Gamalama volcano emerges from the sea to form the near-conical 76 km² Ternate island. The island is situated in the Molucca (Maluku) islands in NE Indonesia about midway between the islands of Borneo and New Guinea (figure 5).

Figure 5. (A) An index map of Indonesia, including the Molucca islands and regional landmarks. Courtesy of U.S. Department of State. (B) A map of the Molucca (Maluku) islands, highlighting Gamalama (Ternate). Courtesy of Indonesia Explore.

Seismicity and eruptions of September 2012. Significant seismicity and other activity at Gamalama remained low from early 2012 until September. During 1-14 September white plumes were sometimes observed rising ~10 m above the crater. When visibility allowed, these plumes were observed from the local obseratory post at Marikuruba and from the W coast of the island, but fog and clouds generally obscured the view.

The telemetered seismograph system (PS-2) recorded deep volcanic earthquakes, shallow volcanic earthquakes, and local tectonic earthquakes, each occurring fewer than five times during 1-14 September. During that same period, there were 63 long-distance tectonic earthquakes and 42 hot air blasts recorded; once they began, signals interpreted as the hot air blasts amounted to 8 occurrences per day. Visual observations and tremor during this time period appeared similar to this volcano's past behavior.

On 15 September 2012 the following seismic events were recorded: 6 long distance tectonic earthquakes, 9 deep volcanic earthquakes, 2 shallow volcanic earthquakes, 14 hot air blasts accompanied by rumbling sounds, and an interval of tremor began with amplitudes reaching 3-4 mm. Six minutes after the tremor, eruption signals occurred with a maximum amplitude of 40 mm. A phreatic explosion produced ash fall and debris fall. Fog obscured the visibility.

On 16 September 2012, CVGHM reported low-amplitude tremor continuing during 0000-1200 (with 1.5-2.5 mm amplitudes). Medium-to-heavy rain fell at the summit around 1200. At 1358 tremor amplitudes increased to 28 mm, followed 17 min later by a "severe eruption."

That eruption drove an ash-laden plume to ~1 km above the crater. The plume drifted S and SE (figure 6A), and 5 min later ash fell at the observation post. The Alert Level was raised to 3 and visitors and residents were warned not to come within 2.5 km of the crater. CVGHM suggested that the eruption vented at the same location as those of December 2011.

Figure 6. (A) Photo of the Gamalama eruption on 16 September 2012 viewed from the NW. The ash plume is immediately blown to the S and SE with almost no vertical development. (B) The 17 September 2012 eruption of Gamalama viewed from the ESE. Both photos courtesy of Associated Press and The Jakarta Globe.

An eruption on 17 September 2012 produced a white-and-gray plume that rose 300 m above the crater and drifted E and SE (figure 6B). Ashfall was reported in the S, SE, and E parts of the island.

Calm prevailed for at least a few weeks after the eruption. Seismicity decreased in early October; on 8 October white plumes rose a mere 10-50 m. The Alert Level was lowered to 2 on 9 October, and the resulting exclusionary zone extended 1.5 km from the crater.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the thorough documentation of Gamalama's historical activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano. Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.go.id/); 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/); The Jakarta Post, Jl. Palmerah Barat 142-143, Jakarta 10270, Indonesia (URL: http://www.thejakartapost.com/); Associated Press (AP) (URL: http://www.apimages.com/); USA Today, 7950 Jones Branch Road, McLean, VA 22102 (URL: http://www.usatoday.com/); BBC News (URL: http://www.bbc.co.uk/); United States Department of State - Bureau of Consular Affairs (URL: http://travel.state.gov/); Indonesia Explore (URL: http://indonesiaexplore.com/).

Our last report on Kasatochi discussed the eruption of 7-8 August 2008 (BGVN 33:07). Since the 2008 eruption, the volcano has remained quiet except for gas emissions. Erosion and deposition of erupted pyroclastic material are rapidly altering slopes and beaches on the island (Scott and others, 2010). This report highlights studies conducted during 2008-2009 of the uninhabited island. Alaska Volcano Observatory (AVO) still monitors Kasatochi (figure 8) indirectly from the Great Sitkin Island seismic network located 42 km away and from satellite imagery. After the 2008 eruption, and the associated almost total biosystem extinction in 2009, Kasatochi Island became a site for monitoring ecosystem succession.

Figure 8. Map showing the location of Kasatochi in the Aleutian Islands. The extent of ash fall from the 7-8 August 2008 eruption is represented by dots with unverified areas indicated by question marks. Courtesy of Alaska Science Center (DeGange, 2010).

The terrestrial and surrounding marine environments of Kasatochi Island examined in June and July of 2009 saw changes in abundance or distribution of the ecosystem when compared to patterns observed on earlier surveys conducted in 1996 through June 2008. The largest direct effect of the eruption to individual animals was probably mortality of young birds. Indirect effects on wildlife consisted of the loss of suitable foraging habitats for species that relied on former terrestrial, intertidal, or nearshore-subtidal habitats and the near-total destruction of all former nesting habitats for most species. Although several species attempted to breed in 2009, all except Steller's sea lions failed due to the lack of suitable breeding sites.

The 7-8 August 2008 eruption. One or more of six remote International Monitoring System (IMS) infrasound arrays (figure 9) detected three well-defined eruption pulses of the 7 August 2008 eruption. The first was an infrasonic very long period (IVLP) acoustic pulse (pulse 1) that began at 21:59:44 UTC on 7 August with a gradual onset and duration of ~123 min and a peak RMS pressure of 0.22 Pa. The acoustic origin time was consistent with that computed for seismic signals (22:01 UTC). Pulse 2 began at 01:34:44 UTC on 8 August with a more impulsive onset, a duration of ~59 min and a peak RMS pressure of 0.46 Pa. Pulse 3 started at 04:20:34 on 8 August with an RMS pressure slightly higher than pulse 1 but lower than pulse 2 and a duration of ~33 min.

Figure 9. Kasatochi's 2008 eruption generated infrasonic signals detected by at least one of these six International Monitoring System (IMS) numbered stations (Fee and others, 2010).

The formerly steep and rugged island which previously had dense low-growing vegetation similar to other Aleutian Islands (figure 10a), became visibly devoid of vegetation after the 7-8 August 2008 eruption (figure 10b). In brief, the island habitat appeared to have been destroyed.

Figure 10. Kasatochi island as viewed before and after the 7-8 August 2008 eruption. (a) Aerial image from 9 July 2008 looking S showed extensive vegetation. (b) Aerial image from 23 October 2008 looking E showed pervasive pyroclastic material mantling the island. By this time, a shallow, gray, acidic lake had reformed in the widened summit crater. Photographs taken by Jerry Morris, Security Aviation; (from Waythomas and others, 2010).

Table 1 compares physical measurements of the island on 9 April 2004 (4 years prior to the 7-8 August 2008 eruption) to those taken on 17 September 2008 (nearly 6 weeks after the eruption). The aerial extent of the island increased by 40% after the eruption, the crater area increased by 25%, and the lake surface area enlarged by 73%. The accumulation of pyroclastic debris (most visible to the right in figure 10b) resulted in the seaward extension of the entire coastline by about 400 m, thus increasing the diameter of the island by about 800 m.

Table 1. Kasatochi Island's physiographic changes resulting from the 7-8 August 2008 eruption. *Data from 18 April 2009 Quickbird image. Reproduced from Waythomas and others (2010).

Post-eruption geology - eruptive deposit studies. Waythomas and others (2010) performed tephra studies in summer 2009 and reported that the bulk of the eruptive products from the 2008 eruption were pyroclastic-flow deposits, produced mainly by phreatomagmatic activity. The eruption lasted ~24 hours and included two initial explosive pulses and pauses over a 6-hr period that produced ash-poor eruption clouds, a 10-hr period of continuous ash-rich emissions initiated by an explosive pulse and punctuated by two others, and a final 8-hr period of nearly continuous ash emission and intermittent phreatic and phreatomagmatic activity. The authors reported that the eruption "...resulted in the accumulation of a uniform cover of medium gray-brown fine ash and pyroclastic-surge deposits over all flanks of the volcano. These deposits are 2-3 m thick and consist of silt, fine sand, and granules that are easily eroded by channelized water flows, and turn to sticky muck when wet." The deposits included a basal muddy tephra from eruptions through the shallow crater lake and accidental lithic debris derived from pre-existing lava flows in the crater. The juvenile material, which accounts for about 20-50% of the volume of the deposits, is pumiceous andesite (58-59% SiO₂).

Surface erosion on the slopes of Kasatochi volcano determined the transfer of sediment to the marine environment and is largely a function of the local hydrologic conditions. Analysis of satellite images and field studies in 2008 and 2009 have shown that within about one year of the 7-8 August 2008 eruption, significant geomorphic changes associated with surface and coastal erosion occurred (figure 11).

Figure 11. Cliffs eroded by wave action on an ENE shoreline of Kasatochi, photographed on 12 June 2009. Courtesy of AVO.

Although technically, sizes of rills and gullies differ, Waythomas and others (2010), using 1 m resolution imagery, could not resolve the size difference; thus they defined both as a narrow, relatively deep, v-shaped or rectangular gully on a hillside formed by flowing water. They observed extensive gully erosion beginning shortly after the eruption and continuing thereafter. Gully erosion removed 300,000 to 600,000 m³ of mostly fine-grained volcanic sediment from the flanks of the volcano, much of which reached the ocean (figure 12).

Figure 12. Images of erosion into pyroclastic deposits from the 7-8 eruption of Kasatochi. (A) The gully pattern that developed on the SW flank (person for scale indicated by arrow). (B) Looking in an E flank gully; maximum gully depth is ~3 m. Courtesy of Waythomas/AVO.

As seen during the summer of 2009 (Scott and others, 2010), the 2008 volcanic deposits that mantle much of the island mainly consisted of decimeter-thick veneers. Veneers greater than 10 m were found locally on middle-to-upper flanks. Broad aprons and fans up to several tens of meters thick were found along much of the lower flanks below former sea cliffs.

Fans originally extended out to 460 m from the former sea cliffs, but by the summer of 2009, fans on the W, N, and E flanks had been truncated to about half that distance or less by coastal erosion. They terminated in active sea cliffs about 15-20 m high. Fans on the S-side of the island either terminated in low cliffs or, more typically, were buried by post-eruption fans of alluvium and debris-flow deposits or by accreting beach sediments that displaced the shoreline an additional 150-250 m seaward.

Post-eruption habitat - vegetation studies. Talbot and others (2010) searched Kasatochi Island for remnant vegetation and signs of re-vegetation at pre-eruption sampling sites. Plants that apparently survived the eruption dominated early plant communities. The most diverse post-eruption community resembled a widespread pre-eruption community. Figure 13 shows a representative plot containing 11 species assigned to bluff ridge vegetation type that inhabited wave-cut cliffs prior to the eruption. Although this ridge vegetation type is nominally species-poor, in this sampling, the mean-species diversity was generally higher than the other post eruption types (Talbot and others, 2010).

Figure 13. A representative plot of 11 species assigned to bluff ridge vegetation type that existed on wave-cut cliffs prior to the 7-8 August 2008 eruption. Photo by Lawrence Walker, UNLV; courtesy of Talbot and others (2010).

Jewett and others (2010) examined the subtidal zone and reported that algal and faunal communities as well as rocky substrates were buried with volcanic deposits from the Kasatochi 2008 eruption. Existing plants were buried and the former stable rocky habitat was buried well into the subtidal zone. The loss of this rocky habitat may constrain kelp recolonization. However, little information is known regarding ocean current directions and velocities that may ultimately help erode soft-sediments and expose the hard rocky substrates necessary for kelp bed recolonization. Higher trophic marine organisms (for example, phytoplankton, the photosynthesizers that provide energy for a vast number of primary consumers, which in turn provide energy for secondary consumers and decomposers) were also affected by the eruption.

Post-eruption habitat - arthropod studies. A 2009 field campaign recorded 17 post-eruption insect species presumed to be non-breeding survivors and 4-9 breeding species. By 2010, 7 of the species seen in 2009 were lost while 18 post-eruption species survived, most of which were breeding (Ridling, 2012). The arthropod, Agyrtidae: Lyrosoma opacum Mannerheim (figure 14) was found to be the only breeding beetle among the 4-9 species found on post-eruption Kasatochi during the 2009 campaign.

Figure 14. During the 2009 field campaign one beetle species remained breeding on Kasatochi (Agyrtidae: Lyrosoma opacum Mannerheim) as seen the on remains of an unidentified bird species. From Ridling (2012).

Post-eruption habitat - avian and mammalian studies. Birds have been studied on Kasatochi by the U.S. Fish and Wildlife Service continually since 1996, providing a critical data base to evaluate ecosystem impact and long-term recovery. The pre-eruption avifauna on Kasatochi was dominated by over 200,000 crested and least auklets. Williams and others (2010) determined that most, if not all, of the auklet nesting habitat was covered by the eruption products (figure 15).

Figure 15. Kasatochi Island auklet rookery seen (a) before the 7-8 August 2012 eruption and (b) after the 2008 eruption, when the auklet hatch had failed completely. Photographs taken by G. Drew, courtesy of Alaska Park Science.

The largest direct effect of the eruption on individual animals was likely the mortality of chicks, with an estimated total 20,000-40,000 young birds lost during and shortly after the August 2008 eruption. Drew and others (2010) found that surviving older least auklets around Kasatochi Island showed little change in densities which ranged from 26 to 34 birds per km². Similar to the least auklet finding, numbers crested auklets were not significantly reduced by the initial explosion. They also returned to attempt breeding in 2009, even though their nesting habitat had been rendered unusable.

Although seven species of birds and mammals attempted to breed in 2009, all but one specie failed due to lack of suitable breeding sites. The one successful breeding specie identified was Steller's sea lions. Williams and others (2010) noted the abundance of sea lions and many seabird species in 2009 was comparable to pre-eruption estimates, suggesting that adult mortality was low for these species. In contrast, shorebirds and passerines, commonly called perching birds, that formerly bred on the island were no longer observed in 2009 and probably perished in the eruption.

Drew and others (2010) also surveyed the marine environment surrounding Kasatochi in June and July of 2009 to document changes, including nutrient abundance, compared to patterns observed in 1996 and 2003. Analysis of SeaWiFS satellite imagery indicated that a large marine chlorophyll-a anomaly may have been the result of ash fertilization during the eruption. Drew and others (2010) found no evidence of continuing marine fertilization from terrestrial runoff 10 months after the eruption.

Post-eruption habitat - volcanic degassing and the landscape. Kasatochi remained quiet except for gas emissions after the 7-8 August 2008 eruption while erosion and deposition have altered the slopes and beaches (figure 16). By April 2009 the level of the crater lake had risen and the lake surface area was 67% larger than it was before the eruption due to an increase in crater diameter (Scott and others, 2010). Fieldwork in summer 2009 determined the locations of various rills and gullies at representative locations on the island. As the gully system on Kasatochi Island began to stabilize and sediment yield declined accordingly, wave action was expected to become the dominant process affecting the landscape (Waythomas and others, 2010).

Figure 16. The prominent cliff-like feature (arrow) seen here in this view from the SW sits well inboard of Kasatochi's present coastline. The cliff was the island's former (pre-eruptive) shoreline. At the post-eruptive coastline, surge deposits are 1-2 m thick, and are much thicker higher up on the flanks. This image, taken 23 August 2008, shows gas emitted from the crater, drifting over the crater rim. Lithic clasts up to 2 m in diameter have been eroded out of the pyroclastic flow deposits by the sea and form a boulder-lag deposit along the coastline. Courtesy of Waythomas/AVO.

Post-eruptive landscape - drainage density. As stated by Waythomas and others (2010), "A fundamental landscape property that describes the degree of dissection by gullies and stream channels is drainage density... Drainage density is the ratio of total channel length to drainage-basin area [km/km²]. Changes in drainage density with time indicate that the threshold for erosion by runoff has been exceeded during individual rainfall events, and that the drainage system has yet to reach a state of quasi-equilibrium where routine rainfall events no longer bring about appreciable changes in drainage density. Time-dependent changes in drainage density also are surrogate measures of erosion because an increase in channel length must reflect channel head processes such as landsliding or gullying... Eventually the rates of gully development will decline and drainage density will approach a steady-state value or perhaps decrease. This is commonly due to the stabilizing effects of vegetation growth... We note that prior to the 2008 eruption of Kasatochi, the flanks of the volcano were covered with a nearly continuous mantle of herbaceous tundra, and no surface streams or drainages were present. Thus, prior to the eruption, the drainage density was very low, if not zero, and over time, we expect that the island will return to this condition."

Based on Waythomas and others (2010) and additional satellite image data from years 2008, 2009, and 2011, Julie Herrick calculated two Kasatochi surface drainage parameters: change in drainage density and change in gully volume. These two calculations used vector images to locate gully lines. These lines were superimposed as vectors on the rasterized (bit digitized) images and then a density analysis was performed. Comparisons of the three years by raster calculations (a form of bit analysis) determined the drainage line density as shown in figure 17A. Spatial analysis determined relative increase, decrease and unchanged surface volumes throughout the island as shown in figure 17B.

Figure 17. Two surface models of drainage trends at Kasatochi developed by Julie Herrick. (A) 3D visualization of 9 March 2011 sedimentation drainage line density in units of km/km2 (see text). Colors represent drainage density as shown in the key (bottom left). Notice that the SE and NE sectors have relatively higher densities. (B) Map (N at top of image) showing volume change of 2008-2011 tephra superimposed on a topographic image (legend at right). The relative net loss of volume areas (blue), are mainly on the island's northerly shorelines. The relatively unchanged areas (gray) are near or on the crater rim. The S shorelines have expanded, as shown by the net gain volume areas (red).

The recovery of habitats at Kasatochi will depend on erosion of the tephra layer blanketing the island to re-expose former breeding habitats as well as anecdotal introduction of various species.

References. DeGange, A.R., Byrd, G.V., Walker, L.R., and Waythomas, C.F., 2010, Introduction-The Impacts of the 2008 Eruption of Kasatochi Volcano on Terrestrial and Marine Ecosystems in the Aleutian Islands, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 245-249.

Drew, G.S., Dragoo, D.E., Renner, M., and Piatt, J.F., 2010, At-sea Observations of Marine Birds and Their Habitats before and after the 2008 Eruption of Kasatochi Volcano, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 306-314.

Fee, D., Steffke A., and Garces, M., 2010, Characterization of the 2008 Kasatochi and Okmok eruptions using remote infrasound arrays, Journal of Geophysical Research, 115, D00L10 (DOI: 10.1029/2009JD013621).

Jewett, S.C., Bodkin, J.L., Chenelot, H., Esslinger, G.G., and Hoberg, M.K., 2010, The nearshore Benthic Community of Kasatochi Island, One Year after the 2008 Eruption, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 315-324.

Neal, C.A., McGimsey, R.G., Dixon, J.P., Cameron, C.E., Nuzhdaev, A.A., and Chibisova, M., 2011, 2008 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory, U.S. Geological Survey Scientific Investigations Report 2010-5243, 94 p.

Ridling, S., 2012, Origins of Post-Eruption Insect Populations on the Volcanic Aleutian Island of Kasatochi (Presentation, URL: www.akentsoc.org/doc/Ridling_S_2012.pptx).

Scott, W.E., Nye, C.J., Waythomas, C.F., and Neal, C.A., 2010, August 2008 Eruption of Kasatochi Volcano, Aleutian Islands, Alaska-Resetting an Island Landscape, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 250-259.

Talbot, S.S., Talbot, S.L., and Walker, L.R., 2010, Post-eruption Legacy Effects and Their Implications for Long-Term Recovery of the Vegetation on Kasatochi Island, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 285-296.

Wang, B., Michaelson, G., Ping, C.L., Plumlee, G., and Hageman, P., 2010, Characterization of Pyroclastic Deposits and Pre-eruptive Soils following the 2008 Eruption of Kasatochi Island Volcano, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 276-284.

Waythomas, C.F., Scott, W.E., and Nye, C.J., 2010, The Geomorphology of an Aleutian Volcano following a Major Eruption: the 7-8 August 2008 Eruption of Kasatochi Volcano, Alaska, and Its Aftermath, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 260-275.

Williams, J.C., Drummond, B.A., and Buxton, R.T., 2010, Initial effects of the August 2008 volcanic eruption on breeding birds and marine mammals at Kasatochi Island, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 306-314.

Geologic Background. Located at the northern end of a shallow submarine ridge trending perpendicular to the Aleutian arc, Kasatochi is small 2.7 x 3.3 km wide island volcano with a dramatic 750-m-wide summit crater lake. The summit of Kasatochi reaches only 314 m above sea level, and the lake surface lies less than about 60 m above the sea. A lava dome is located on the NW flank at about 150 m elevation. The asymmetrical island is steeper on the northern side than the southern, and the volcano's crater lies north of the center of the island. Reports of activity from the heavily eroded Koniuji volcano to the east probably refer to eruptions from Kasatochi. A lava flow may have been emplaced during the first historical eruption in 1760. A major explosive eruption in 2008 produced pyroclastic flows and surges that swept into the sea, extending the island's shoreline.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA; Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA; and Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.avo.alaska.edu/); Julie Herrick, Global Volcanism Program, Smithsonian National Museum of Natural History, Washington, DC 20560.

Our previous report (BGVN 36:08) discussed two eruption episodes: one from 25 October 2010 to March 2011, and another from August 2011 to about 1 October 2011. During the last two weeks of September 2011, the volcano produced persistent volcanic earthquake swarms and thin emissions (BGVN 36:08). This report discusses two visits to the volcano in 2011. Scientists that visited on 8 October 2011 reported degassing and an ongoing seismic swarm then consisting chiefly of M ~1 and smaller earthquakes. During 12-13 November 2011 a photographer noted steady degassing, then observed the start of a 12-hour interval of minor but repeated Stombolian eruptions (see next section).

2011 visits by Øystein Lund Andersen. The photographer and guide Øystein Lund Andersen lives in Jakarta, Indonesia and visits Anak Krakatau often. His website contains photos of the volcano. He shows one photo of a seismograph at CVGHM's Pasauran Observatory recording part of a prolonged swarm of small earthquakes from 8 October 2011. Youtube features a video he took on the same subject.

His visit to Anak Krakatau during 12-13 November 2011 took place during an interval of gas emissions devoid of ash. He stayed up all night to observe Anak Krakatau emit a steady, white, ash-free plume. At dusk on 12 November he noticed that the crater glowed bright red and after a few hours a series of mild Strombolian eruptions occurred in a sequence that lasted 12 hours (figure 29). The time between the eruptions was from 30 seconds to a few minutes. Some of Andersen's photos captured glowing pyroclasts arcing tens of meters above the crater rim (figure 29b, c). Anderson saw ash lava bombs in the plume during these eruptions. He noted that the lava bombs ejected over the crater mainly fell back into the crater. During the night the crater remained almost constantly illuminated by the glowing bombs and the fragments they created when they landed. The eruptions were often accompanied by loud sounds from the volcano.

Figure 29. Three photos of Anak Krakatau associated with mild Stombolian eruptions taken during 12-13 November 2011 amid unusually clear conditions. Provided to Bulletin editors by Øystein Lund Andersen.

Background. See earlier Bulletin reports for maps of the Krakatau complex and of the post-collapse cone that formed an island and now continues as the active vent (Anak Krakatau, Daughter of Krakatau; for example, figure 23 in BGVN 36:08). Krakatau sits ~130 km W of the Indonesian capital, Jakarta. The complex is famous for the devastating caldera-forming eruption in 1883 (Simkin and Fiske, 1983). That eruption injected millions of tons of fine ash, aerosols, and sulfate particles into the atmosphere. That eruption and associated tsunami claimed over 36,000 lives and awakened the world to caldera collapse (Self and Rampino, 1981).

Lockwood and Hazlett (2010) noted that the 1883 eruption "impressed European observers with remarkable, smog-like sunsets and silvery midday skies. This inspired a number of paintings, possibly including the lurid sky in Edvard Munch's famous work The Scream, which he painted in 1893."

According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), between the emergence of Anak Krakatau from the sea surface on 11 June 1927 up to 2011, the volcano had undergone over 100 eruptions. During that period, the volcano's non-eruptive periods lasted between 1 and 6 years. During the past few years, Anak Krakatau underwent several eruptive phases, followed by relatively quiet phases (BGVN 34:05, 34:11, and 36:08).

References. Lockwood, J. and Hazlett, R.W., 2010, Volcanoes: global perspectives. Wiley-Blackwell.

Simkin, T. and Fiske, R.S., 1983, Krakatau, 1883--the volcanic eruption and its effects, Smithsonian Institution Press.

Self, S., Rampino, M.R., 1981, The 1883 eruption of Krakatau, Nature, 294, pp. 699-704.

Geologic Background. The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral Krakatau edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of this ancestral volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption, the 2nd largest in Indonesia during historical time, caused more than 36,000 fatalities, most as a result of devastating tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Øystein Lund Andersen (URL: http://www.oysteinlundandersen.com/).

Ol Doinyo Lengai, located close to the N border of Tanzania (figure 153), is both accessible and monitored closely.

Figure 153. Map of Tanzania showing Ol Doinyo Lengai's proximity to Meru and Kilimanjaro. Courtesy of USGS/CVO.

Frederick Belton's Ol Doinyo Lengai web site has provided many interesting photos from the expeditions that he has taken to the volcano since his initial visit in 1997, including annual visits until 2006, followed by his last expedition in 2008. In addition, Belton has included in his web site observations, photographs, and other graphics provided by many visitors to Ol Doinyo Lengai; these descripions have been the primary source of reports found in the Bulletin. Recently, Belton informed Bulletin editors that he rarely gets any updates on visits to Ol Doinyo Lengai for his web site, but he did receive one in September 2012. Bulletin editors wrote to a number of past contributors to BGVN; some of their comments are included below.

Belton's web site reported that Frank Möckel and Wendy Blank visited Ol Doinyo Lengai's summit area (figure 154) in September 2012. They climbed the volcano during 14-15 September and camped in the S Crater during the nights of 15 and 16 September (figure 155). Figures 155-160 contain some views they captured at the summit of the volcano. Figures 157 and 158 show what appear to be active spatter cones inside the N Crater. According to Belton, the activity looks very typical of the type frequently seen prior to the last explosive eruption in 2007-2008 (BGVN 32:11). Möckel and Blank reported that on the bottom of the active N Crater they saw fresh black natrocarbonatite lava and active vents, and they heard boiling noises from the bottom of the N Crater. They reported a strong smell of hydrogen sulfide (H₂S) everywhere in the area.

Figure 154. Topographic map modified from the Ol Doinyo Lengai quadrangle of 1990, with a contour interval of 20 m. The original map was modified in the N Crater area from observations made on 12 March 2010. Courtesy of Sherrod and others (2010).

Figure 155. Ol Doinyo Lengai's S Crater, seen with tents for scale (center). Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 156. View of Ol Doinyo Lengai's N Crater as seen from the summit. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 157. Ol Doinyo Lengai's active N Crater as seen from an unidentified point on the crater rim. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 158. View looking down into Ol Doinyo Lengai's N Crater at interior spatter cones. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 159. A closer view of Ol Doinyo Lengai's N Crater floor showing several vent openings (black) and an area of fresh spatter covering a region downhill of a vent. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 160. Climbers on the NE rim of Ol Doinyo Lengai's N crater. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Abigail Church, who studied Ol Doinyo Lengai for her PhD dissertation in 1996 and has published several articles on the petrogensis of the natrocarbonatite lavas, now lives in Nairobi, Kenya, and regularly flies over Ol Doinyo Lengai in chartered aircraft. She has flown over and landed on Ol Doinyo Lengai in a helicopter probably 5 times in the last few years. She camped close to the volcano in November 2012 and flew around the summit, however, it was cloudy. On many occasions when she was able to see into the crater, she observed what appeared to be small-scale activity continuing in the base of the deep pit. There are normally 1 or 2 active vents in which one can see very dark material which she assumed was fresh natrocarbonatite lava. She also has good contacts with people in the area and with pilots who fly the routes between Arusha and the Serengeti. On recent flights over Ol Doinyo Lengai, Church has observed that the sides of the central crater within the N Crater are collapsing inwardly, reducing the depth of the crater hole, and that small scale activity in the crater continues. Figures 161-165 show some photographs from 2011 and 2012 of the inside of Ol Doinyo Lengai's N crater.

Figure 161. Ol Doinyo Lengai's N crater floor showing a vent and some spatter. Photo taken 2 December 2011. Courtesy of Phil Mathews and Abigail Church.

Figure 162. A collapse in the floor of Ol Doinyo Lengai's N crater, showing natrocarbonatite lava flows. Photo taken 2 December 2011. Courtesy of Phil Mathews and Abigail Church.

Figure 163. Small active vent in the N Crater floor of Ol Doinyo Lengai. Photo taken 2 December 2011. Courtesy of Phil Mathews and Abigail Church.

Figure 164. A view of Ol Doinyo Lengai's N Crater from over the rim. Photo taken in August 2012. Courtesy of Phil Mathews and Abigail Church.

Figure 165. A recently active vent on Ol Doinyo Lengai's N Crater floor. Photo taken in August 2012. Courtesy of Phil Mathews and Abigail Church.

Hannes Mattsson reported to Bulletin staff that he has 3 PhD projects running on different aspects of Ol Doinyo Lengai volcanism, but he has not been at the volcano for about 1.5 years. He is planning a 6-week field campaign scheduled in mid 2013. He noted that very little current or recent information on Ol Doinyo Lengai is currently available.

Joerg Keller also noted that there are not many recent reports about activity in the crater area. According to reports of Möckel's visit in February 2010 (BGVN 35:05), the access route to and from Ol Doinyo Lengai's summit seems much more difficult to negotiate than before the 2007 eruption. Keller believes that another possible factor leading to less observations of Ol Doinyo Lengai is the change in the crater formations since the eruption of 2007. There seems to be little change since 2008, a stable situation with the new ash cone dominating the entire N crater area completely (see figures in BGVN 32:11 and 33:02, as well as the above photos). The unique crater landscape seen before 2007, with accessible hornitos and lava flows of different ages, and the chance to see active spatter cones, lava pools and flowing lava, was an attraction to visiters. The logistical problems for visiting and climbing the volcano since 2008, incluing safety and political factors, have resulted in greatly diminished numbers of visitors.

Keller reported that Elias Danner, a teenage photographer, filmer, and designer, started Ol Doinyo Lengai photo documentation that shows the ash cone, its deep pit, and, in particular, looks inside the pit with fresh, overlapping lava lobes and vigorously boiling lava pools. Keller received from Danner a video of the boiling lava pools which was so typical and so impressive that he wrote in a recent paper (Keller and Zaitsev, 2012) the following: "The present vertically sided, almost 100 m deep pit crater formed by the 2007-2008 explosive activity is inaccessible. However, since 2008 frequent overflights and reports and photographs by visitors climbing the mountain (Belton, 2012) suggest that new natrocarbonatite effusions are occurring at the bottom of the deep pit. This is indicated from a distance by the typical morphological features of natrocarbonatite appearing as small hornitos and gray pahoehoe flows on the floor of the crater. On 26th June 2011, Elias Danner ... filmed a vigorously boiling and splashing, obviously carbonatitic lava pool at the bottom of the pit, with features very reminiscent of Figs. 5 and 6 in Keller and Krafft (1990)."

MODIS/MODVOLC Satellite Thermal Alerts. Table 26 gives an update of MODVOLC satellite thermal alerts at the Ol Doinyo Lengai summit since a similar update found in BGVN 33:06. It is not uncommon to find thermal alerts down and beyond the sides of the volcano, probably caused by fires. It is possible that fewer thermal alerts are measured by the MODIS satellites because the current deep crater (since the 2007-2008 eruptions) shields some of the hotter areas from the satellite sensors.

Table 26. MODVOLC thermal alerts measured at Ol Doinyo Lengai from 3 April 2008 to December 2012. Courtesy of the Hawai`i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System.

References. Belton, F., 2012, Oldoinyo Lengai, The Mountain of God (URL: www.oldoinyolengai.pbworks.com).

Keller, J., and Krafft, M., 1990, Effusive natrocarbonatite activity of Oldoinyo Lengai, June 1988, Bulletin of Volcanology v. 52, pp. 629-645.

Keller, J., and Zaitsev, A.N., 2012, Geochemistry and petrogenetic significance of natrocarbonatites at Oldoinyo Lengai, Tanzania: Composition of lavas from 1988 to 2007, Lithos, v.148, pp. 45-53.

Sherrod, D., Mollel, K., and Nantatwa, O., 2010, Oldoinyo Lengai: Trip Report, March 12-14, 2010, informal report (URL: http:/Sherrod_OldonyoLengai_March12_20106-1.pdf).

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, University Studies Department, Middle Tennessee State University, Murfreesboro, TN (URL: http://oldoinyolengai.pbworks.com/); Sonja Bosshard, Institute of Geochemistry and Petrology, Swiss Federal Institute of Technology Zürich (ETH Zürich), Zürich, Switzerland; Laura Carmody, Planetary Geoscience Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN; Abigail Church, The Ker & Downey Safari Tradition, P.O. Box 86, Karen 00502, Kenya; Elias Danner, Elias Danner Productions (URL: http://www.mammut-studios.com/); Joerg Keller, Institut für Geowissenschaften/Mineralogie-Geochemie, Universität Freiburg, Albertstrasse 23b, 79104 Freiburg, Germany; Hannes B. Mattsson, Institute of Geochemistry and Petrology, Swiss Federal Institute of Technology Zürich (ETH Zürich), Zürich, Switzerland; Frank Möckel; Celia Nyamweru, St. Lawrence University; David Sherrod, Cascades Volcano Observatory (CVO), U.S. Geological Survey, Vancouver, WA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Christoph Weber, Volcano Expeditions International (VEI), Muehlweg 11, 74199 Untergruppenbach, Germany (URL: http://www.v-e-i.de/); Ben Wilhelmi, commercial pilot (URL: http://benwilhelmi.typepad.com/benwilhelmi/).

Elevated seismicity during January 2011 was discussed in our last report on Cerro Machín volcano (BGVN 36:04). Between September 2010 and January 2011, more than 800 volcano-tectonic (VT) earthquakes were detected per month and local residents reported shaking from these events, particularly during November 2010-February 2011. Here we describe trends in seismicity at Machín from January 2011 to November 2012 and the frequency of seismic swarms. We also include descriptions of monitoring efforts by the Volcanic and Seismological Observatory of Manizales at the Colombia Institute of Geology and Mining (INGEOMINAS) including two field campaigns focused on CO₂ emissions from the crater.

Geophysical monitoring. Since January 2011, INGEOMINAS had been monitoring Cerro Machín with a network that included broadband and short-period seismometers, magnetometers, self potential, and an acoustic monitoring system (acoustic flow detection for early flood warning). The deformation network included electronic and dry tilt (longterm monitoring since 2005), and starting in November 2012, three GPS stations were also operating (figure 4). Electronic-distance measurements (EDM) were conducted in 2012 at seven stations (EDM data was available since 2008). Data from these monitoring efforts were available in the INGEOMINAS online technical reports.

Figure 4. The deformation monitoring network at Cerro Machín in 2012 included three GPS stations and five electronic tilt stations. EDM measurements in September 2011 used three base stations (San Lorenzo, "SLOR;" La Palma, "PALM;" and Anillo, "ANIL") while measurements in October 2012 relied on one base station (San Lorenzo, "SLOR"). Courtesy of INGEOMINAS.

Geochemical monitoring. Geochemical monitoring at Cerro Machín has been conducted within the circular crater and the central dome complex (figures 5 and 6). During 2011-2012, geochemical monitoring included diffuse CO₂ detection, alkaline traps, and radon monitoring from soil emissions (13 stations were online in November 2012) as well as regular testing at fumarolic and hot spring locations.

Figure 5. Geochemical monitoring during 2011-2012 at Cerro Machín included several dozen sampling sites mainly spread across the ~2-km diameter crater. INGEOMINAS released long-term datasets from radon-gas traps, an alkaline trap, and a fumarole in their monthly technical bulletins. Courtesy of INGEOMINAS.

Figure 6. Two aerial views of Cerro Machín were captured during an overflight on 16 November 2011. (Top) In this view of the NE flank of Machín, the crater lake is visible near the left-hand side of the image within a moat-like region surrounding the dome. (Bottom) This view shows an access road along the breached, SW edge of the dome complex (lower center). This view also reveals a glimpse of the crater lake (appears gray) in the distant portion of the moat (right, from center). Courtesy of INGEOMINAS.

In May and September 2012, INGEOMINAS conducted field surveys to measure diffuse carbon dioxide emissions (figure 7). With a mobile LICOR 820 monitoring device, INGEOMINAS technicians traversed the interior crater rim detecting CO₂, air temperature, and pressure. The survey on 28 May determined baseline levels of CO₂ flux at 28 points within the crater. The survey conducted during 19 and 20 September 2012 detected relatively high CO₂ emissions from seven locations along a traverse within the crater. The highest CO₂ fluxes ranged between 739 and 8,077 mol·m^-2·day^-1, and in their technical report, INGEOMINAS noted that future gas monitoring should focus on those sites with peak values.

Figure 7. On 28 May 2012, INGEOMINAS conducted a CO2 campaign within the crater of Cerro Machín. Courtesy of INGEOMINAS.

Seismicity in 2011. Elevated seismicity in late 2010 continued through early 2011 (BGVN 36:04) and local communities reported shaking in January and February 2011 (figure 8). For many months after May 2011, earthquakes per month had declined to below 400 per month. The clear exception to that trend took place during September 2011, a month with over 1,200 earthquakes.

Figure 8. Volcano-tectonic (VT) seismicity at Cerro Machín abruptly increased in September 2010. This histogram shows time on the x-axis and number of earthquakes on the y-axis. Earthquake count per month decreased in January and February 2011 and reached low levels in August. Except for peaks in September and December 2011, the number of earthquakes remained below 400 events per month until November 2012. Courtesy of INGEOMINAS.

Compared with 2010 activity, fewer seismic swarms were detected in 2011 and in the available record for 2012 (table 2). In 2011, swarms tended to cluster beneath the dome complex and in areas ~2 km S and SE. INGEOMINAS frequently noted earthquake epicenters in an area known as Moralito, a location SE of the volcano near the MRAL GPS station (see figure 4). Deeper earthquakes (frequently at depths between 7 and 18 km) were detected in that region and were attributed to displacements along a fault zone.

Table 2. Seismic swarms detected at Machín during 2010-2012. Days were counted and tallied based on whether one or more swarms occurred. For example, during January-February 2010 there were six swarms recorded. Courtesy of INGEOMINAS.

Local residents felt shaking from earthquakes in September 2011 when six occurred with magnitudes greater than 2.5. INGEOMINAS reported that this month had the largest combined free-energy release that year. The largest magnitude event of that group was an M 3.6 volcano-tectonic (VT) earthquake detected at 2013 on 12 September. The average depth of the earthquakes was 4.5 km with some events as deep as 13 km. Epicenters were primarily clustered in the area of Moralito (near the MORA seismic station, see figure 9).

Figure 9. During September 2011, several moderate-sized earthquakes were located in an area SE of Cerro Machín. Seismic stations are labeled and located at purple squares. Note that the summit of Machín sits ~4 km NW of the clustered earthquakes, near the CIMA seismic station. Courtesy of INGEOMINAS.

In December 2011, INGEOMINAS reported that rockfall-type seismic signals were detected within the area. A total of 19 signatures were counted on 11 December; some events had durations up to 73 seconds. The largest earthquake that month was an M 2.32 that occurred at 0542 on 1 December.

Seismicity from January to November 2012. Rockfall-type signatures were also recorded in January 2012. These events occurred on 10 January at 1556 and lasted up to 64 seconds. As frequently observed during previous months, VT earthquakes tended to occur beneath the dome, S, and SE in the area of Moralito.

From January to August 2012, seismic swarms occurred intermittently (table 2). Elevated seismicity occurred during April 2012 and was felt by local residents. During this time period, the largest earthquake was an M 2.8 VT detected on 11 April at 0655. In April, VT earthquakes clustered ~1 km S of the dome complex and were ~4 km deep.

During May-August 2012, earthquakes were rarely clustered and occurred at a wide range of depths (0-16 km). In August, several earthquakes were located ~8 km SE of the CIMA station at depths between 12-15 km. The largest earthquake that month was an M 1.45 detected at 2026 on 9 August.

During September-October, seismic swarms occurred on six days (table 2). Local residents in the municipalities of Cajamarca and Ibagué (locations appear in figure 2 of BGVN 36:04) as well as the nearby departments of Quindio, Risaralda, and Caldas reported shaking due to these earthquakes (locations of these districts appear in the regional map of figure 5 in this report). These events were clustered beneath the dome complex at depths between 2 and 5 km. In October, however, relatively large earthquakes were detected in an area ~8 km SE of the dome at depths around 13 km. The largest earthquakes were on 9 September (M 3.6) and on 7 October (M 4.6) prompting INGEOMINAS staff to visit residences and investigate the impact of the events (figure 10). The M 4.6 earthquake was one of several located SE of the dome (near the TAPI seismic station, see figure 9).

Figure 10. A visit to areas around Machín by INGEOMINAS staff in order to evaluate the possible damage from seismic unrest that was detected on 7 October 2012. Courtesy of INGEOMINAS.

In November, INGEOMINAS reported that VT earthquakes continued to occur beneath the dome although at a reduced rate compared to October. Earthquakes tended to occur 2-5 km beneath the dome, and deeper events were detected to the SE at depths between 9 and 15 km. The largest earthquake detected was an M 2.8 on 20 November at 1754. This earthquake was located at a depth of 2.75 km and was ~2 km SW of the dome complex.

Geologic Background. The small Cerro Machín stratovolcano lies at the southern end of the Ruiz-Tolima massif about 20 km WNW of the city of Ibagué. A 3-km-wide caldera is breached to the south and contains three forested dacitic lava domes. Voluminous pyroclastic flows traveled up to 40 km away during eruptions in the mid-to-late Holocene, perhaps associated with formation of the caldera. Late-Holocene eruptions produced dacitic block-and-ash flows that traveled through the breach in the caldera rim to the west and south. The latest known eruption of took place about 800 years ago.

Information Contacts: Instituto Colombiano de Geologia y Mineria (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Manizales, Manizales, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

During April 2010-June 2012 the Japan Meteorological Agency (JMA) maintained the hazard status for Miyake-jima at Alert Level 2, where it had stood since 31 March 2008. Our last report (BGVN 34:06) mentioned a minor eruption at Miyake-jima on 1 April 2009 which produced an ash plume that rose ~600 m above the crater. Since that time, activity was relatively low with up to four minor eruptions occurring during April-July 2010, as reported by the Tokyo Volcanic Ash Advisory Center (VAAC) based on information from JMA.

Eruptions occurred on 11 April, 4 July (two possible eruptions during the early morning), and 21 July 2010; the 21 July eruption was the only eruption for which the Tokyo VAAC issued an altitude and drift direction for the plume (~1.2 km altitude with E drift; table 5). The eruptions were characterized by gas and steam emissions lacking significant ash content (e.g. figure 23).

Table 5. Summary of detailed activity reports for Miyakejima during April 2010-June 2012; '--' indicates data not reported. Courtesy of JMA and Tokyo VAAC.

Figure 23. A S-looking photograph of Miyake-jima's crater taken from a flight on 17 March 2011 showing an apparent small gas-and-steam emission. Miyake Jima Airport is located along the coast, just out of view to the E. Courtesy of Flickr user R. Forrest.

JMA reported low levels of seismicity centered just beneath the crater during the reporting interval. Occasional episodes of volcanic tremor occurred, but were not correlated with other data indicating emissions or eruptions (table 5). Sources in Miyakemura village reported that high SO₂ concentrations were occasionally detected in some inhabited flank areas.

GPS data revealed contraction in some parts of the edifice, a process that, although diminishing, had continued since 2000. Over the same time period, long-term extension of the baseline along the N-S section of Miyake-jima had been observed since 2006, indicating inflation in deeper portions of the volcano.

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), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).

Our most recent report on Tangkubanparahu (also known as Tangkuban Perahu) described increased seismicity during April 2005, consisting primarily of volcanic earthquakes and tremor (BGVN 30:12). This report describes elevated seismicity during August-September 2012. The Center of Volcanology and Geological Hazard Mitigation (CVGHM) notes that at least three magmatic eruptions and four phreatic eruptions had occurred at Ratu Crater, the most active vent, during 1829-1994. Ratu Crater is about 30 km N of Bandung in W Java. Figure 1 indicates the general location of the volcano.

Figure 1. Sketch maps of Indonesia and W Java indicating the location of Tangkubanparahu (Tangkuban Perahu). Courtesy of Kartadinata and others (2002).

The next report we received on Tangkubanparahu described activity starting in August 2012. According to CVGHM, the frequency of earthquakes and tremor increased on both 13 and 23 August. Around this time, hot blasts of sulfuric gases, white in color, rose from Ratu Crater to heights of 50-400 m above the crater's floor. CVGHM reported that the temperature of emissions from Ratu Crater on 24 August was 246°C, compared to a measurement of 111°C on 18 August. On 23 August, the Alert Level was raised to 2 (on a scale of 1-4), and visitors and residents were prohibited within a 1.5-km radius of the active crater.

Seismic activity declined on 23 August; shallow volcanic earthquakes continued to be recorded but were less frequent through 21 September (table 2 provides data through 20 September). Hypocenters of volcanic tremors during this period were located beneath an area W of Ratu Crater at depths of 4-12 km. Soil temperatures at Ratu Crater were 30.5°C on 26 August, then were 35°C on 30 August, but then gradually declined during 31 August-21 September to ~34°C.

Table 2. Type and occurrence of earthquakes at Tangkubanparahu between 24 August and 20 September 2012. Courtesy of CVGHM.

Between 5-11 September, sulfur dioxide gas emissions were elevated in an area NW of the crater associated with the plume, but in the latter part of September 2012 concentrations averaged4in Ratu Crater increased from 0.11 in December 2011 to ~4 on 24 August 2012 and remained at that level on 11 September 2012, which suggested to CVGHM that hot fluid was rising to the surface.

Based on seismicity, visual observations, deformation data, gas measurements, and soil and crater lake water temperatures, the Alert Level was lowered to 1 on 21 September 2012.

The eruptive history of Tangkubanparahu was described by Kartadinata and others (2002).

Reference. Kartadinata, M., Okuno, M., Nakamura, T., and Kobayashi, T., 2002, Eruptive history of Tangkuban Perahu volcano, West Java, Indonesia: A preliminary report, Journal of Geography, v. 111, issue 3, p. 404-409.

Geologic Background. Tangkubanparahu (also known as Tangkuban Perahu) is a broad shield-like stratovolcano overlooking Indonesia's former capital city of Bandung. The volcano was constructed within the 6 x 8 km Pleistocene Sunda caldera, which formed about 190,000 years ago. The volcano's low profile is the subject of legends referring to the mountain of the "upturned boat." The rim of Sunda caldera forms a prominent ridge on the western side; elsewhere the caldera rim is largely buried by deposits of Tangkubanparahu volcano. The dominantly small phreatic historical eruptions recorded since the 19th century have originated from several nested craters within an elliptical 1 x 1.5 km summit depression.

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

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/).