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/).

John Seach previously reported his observations of the Ambrym caldera made during a visit in December 2002 (BGVN 27:12). This report contains his observations of the caldera during a 7-11 September 2003 visit and flyovers on 6 and 13 September. The level of activity during September 2003, with visible lava in six vents, was higher than that during his previous visit.

Observations of Benbow. During the 6 September flyover, two white plumes were rising 200 m above the crater rim and drifting NW. On the evening of 7 September, orange glows were seen from the caldera edge (3 km SE). A strong glow originated N of the crater and the central crater pit produced a less intense fluctuating glow. During the 13 September flyover, both pits continued to emit white and light-brown plumes to 200 m above the rim.

Observations of Mbogon Niri Mbwelesu. Large white vapor emissions from the collapse pit formed mushroom-shaped clouds on 6 September that drifted W and attained a height of 300 m. A visit to the S rim on 7 September showed a weak orange glow and copious gas emissions. On 8 September, observations from the N rim showed the pit full of swirling brown and white vapor. The NW wall was stained with yellow and red deposits, and pungent sulfurous gases were being emitted. Loud, rhythmic degassing sounds were heard every few seconds. The bottom of the pit was visible on 10 September, allowing views of two glowing red holes 150 m below the rim separated by a small wall a few meters wide. The two vents degassed simultaneously, but the E vent emitted larger amounts of brown ash.

Observations of Niri Mbwelesu. During the 6 September overflight, the pit of Niri Mbwelesu crater was filled with white vapor. The crater was climbed on 8 September and observations from the S rim showed the crater still filled with vapor; no sounds were heard. During that evening, an orange glow was observed. Excellent visibility on 10 September enabled sighting of a 10-m-diameter, crusted lava pond. Red lava was visible through surface cracks, and lava spatter rose 10 m above them at infrequent intervals.

Loud cannon-like explosions about every 20 minutes shook the ground and were accompanied by the sounds of cracking rock. During the evening, glowing projectiles were ejected into the air, although none fell outside the crater. Loud, roaring degassing noises like a jet engine at take-off were also heard. The roar would gain intensity over 30 seconds, cease for 15 seconds and then re-start. During periods of intense roaring, red lava was observed through cracks in the crusted surface.

Both types of intense degassing were accompanied by gentle emissions of brown vapor. A pit, 6 m in diameter, located N of the crusted pond in the crater wall, emitted brown ash. Fumaroles were high on the N inner crater wall. Brown ash was emitted from the S crater floor.

Observations of Mbwelesu. Mbwelesu crater was observed for 3 hours during mid-day on 8 September from a position on the SW rim. At times, the crater was filled with vapor, but observation of the lake surface was only possible about 60% of the time. The lava lake showed remarkable similarities in location, size, and dynamics compared to December 2002. The 50-m-diameter lava lake was contained inside a circular funnel-shaped pit 100-120 m in diameter. Violent agitation of the surface occurred most of the time. Lava splashed onto the pit walls and drained back vertically 25 m into the pit.

Large 10-m-diameter gas bubbles burst in the SE half of the lava lake with up to eight bubbles visible at the same time. Jets of lava were ejected every few seconds, created by wave intersections from the bursting bubbles. During periods of low activity, lasting tens of seconds, lava drained back into the middle of the pit. Surface crusting occurred after as little as one minute during quiet periods. Subsequently, the crust was broken up by a resumption of degassing from the SW side of the pit. On several occasions, up to 80% of the lava lake surface was covered by darker crust.

Acid rain was experienced on the edge of the crater and observers felt minor burning on the face. White, light-brown, and blue-tinged vapors smelling of sulfur were emitted from the crater.

Mbwelesu was scaled again on 10 September and observations of the lava lake (figure 10) were made over eight hours. The crater was clear, enabling detailed observations. At times 80% of the lake surface was deformed by bubbling. The SE portion of the pit contained the most degassing. Violent explosions regularly sprayed orange lava mixed with black crust in all directions. At one point the whole lake surface rotated clockwise and lava drained back into the middle of the pit. This whirlpool was followed by an avalanche on the W side of the pit that threw black material into the lake. A second pit with a diameter of 75 m NE of the lava lake was separated by an unstable 10-m-wide wall from which numerous avalanches occurred during the day; red lava spatter was ejected once.

Figure 10. Lava lake inside Mbwelesu crater at Ambrym on 10 September 2003. Surface crusting and degassing are clear, note new crater at top of photo. Courtesy of John Seach.

An afternoon flyover on 13 September enabled excellent views of the active lava lake. The smaller pit NE of the lava lake contained a small lava pond with a diameter of ~ 8-10 m.

Observations of Marum. Two areas of fumarolic activity were seen at the edge of the 1953 crater (between Marum and Mbwelesu). Brown ash was being emitted from the ground at these locations.

Geologic Background. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides arc. A thick, almost exclusively pyroclastic sequence, initially dacitic, then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major plinian eruption with dacitic pyroclastic flows about 1900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the caldera floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: John Seach, PO Box 4025, Port Vila, Vanuatu (URL: http://www.volcanolive.com/).

The first recorded historical eruption at Anatahan, which began on 10 May 2003, continued through that month with nearly continuous ash plumes (BGVN 28:04 and 28:05). Two strong explosions on 14 June removed much of a small lava dome that had been extruded in the crater; dark ash plumes were last reported on 16 June, after which time seismicity decreased significantly (BGVN 28:06). Only steaming without ash emissions was reported by scientists doing fieldwork immediately afterwards (BGVN 28:07) and on overflights in July. Volcanic tremor and other seismicity reported by the Commonwealth of the Northern Mariana Islands (CNMI) Emergency Management Office (EMO) persisted into early August at a relatively low level. This report covers observed activity from 4 August to 5 October 2003.

Seismicity was low throughout the report period and no apparent eruption signals or potential precursory events occurred. Tremor and seismic energy release were at low levels. During 2-6 August, small long-period (LP) events occurred regularly. At the end of that interval, the number of small LP events increased to several hundred in 24 hours, compared to a couple dozen per day earlier in the swarm, but the overall energy release increase was not significant. No LP events were reported again until mid-September. On 5 September, tremor and seismic energy release were reported to be at their lowest levels since early July.

Overflights of the volcano were made by USGS and EMO personnel on 30 August and 8, 9, and 11 September. Observations on these days revealed no ash emissions, and the feeble plume was dominated by steam and lesser amounts of volcanic gases, mainly SO₂. Sporadic emissions sometimes rose above the crater rim. The E crater floor was covered by dirty, sediment-laden, steaming water, and an active geothermal system had mud pots, mini-geysers, and steam jets. Steaming water and sulfurous gases were emitted from the crater walls and floor. Observations during an 18 September overflight were similar to those earlier in the month, although the crater floor appeared to be covered by muddy water instead of a shallow lake. A distinct odor of SO₂ and blue fume were noted during a helicopter inspection of the E crater lake on 27 September. On 29 September, geysering was seen and the odor of H₂S was present in addition to SO₂.

By 12 September USGS and EMO had reestablished the original, pre-eruption Anatahan seismic station (ANAT) on the SW caldera rim. On 15 September, several, small-amplitude, LP events lasting up to 15 seconds were visible on the ANAT records with dominant frequencies of 4-5 Hz. Some of the larger events had a short burst of 6-7 Hz energy about 2.5 seconds after the onset. The largest events were barely above background at the E Anatahan station (ANA2) and may have been occurring undetected for the past several weeks. The LP events at the ANAT station continued over the next two days at a rate of several per hour.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: Juan Takai Camacho and Ramon Chong, Commonwealth of the Northern Mariana Islands Emergency Management Office, P.O. Box 10007, Saipan, MP 96950 USA (URL: http://www.cnmihsem.gov.mp/); Frank Trusdell, U.S. Geological Survey, Hawaiian Volcano Observatory (HVO), PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/nmi/activity/).

On 5 September the Observatorio Vulcanologico y Sismologico de Costa Rica (OVSICORI-UNA) reported that a new sequence of pyroclastic flows started at 1055 that day (figure 98). At least eight signals related to the collapses were recorded within the next two hours by seismographs at the observatory. Material shed from high-elevation accumulations of lava generated the pyroclastic flows, which descended the N and NE flanks down to 800 m elevation; accompanying ash drifted W and NW. No injuries or deaths occurred, and the main effects were limited to within the National Park boundaries. Patches of vegetation at the flow terminations caught on fire. Similar flows have occurred in recent years (e.g. May 1998, August 2000, and March 2001) affecting the summit and upper areas of the active cone C. No explosive eruptions or extraordinary seismic activity were associated with these latest pyroclastic flows.

Figure 98. Photograph of a pyroclastic flow descending the NE flank of Arenal, 5 September 2003. Courtesy of OVSICORI-UNA.

Unreported observations from 2002. At the time of the last summary report about Arenal (BGVN 28:08), information from January, February, and April 2002 was not available; those OVSICORI-UNA reports have since been located. Both seismic and volcanic activity were low during those months, without significant pyroclastic flows or energetic eruptions. Pyroclastic flows from other months that had been described in that and other reports all originated from failures along the margins of lava flows, rather than stemming from explosive eruptive processes.

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: E. Fernández, E. Duarte, E. Malavassi, R. Sáenz, V. Barboza, R. Van der Laat, T. Marino, E. Hernández, and F. Chavarría, Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

Geologist Raul Mora, along with Carlos Ramirez and Maritta Alvarado, visited Barva volcano during December 2002 and investigated the Barva and Copey crater lakes. Located in a small crater, the Barva crater lake (figure 1) was very clear; at 5 m from the shore the water had a temperature of 11-12°C with a pH of 4-5. Water in the Copey lake was amber colored and very cloudy, with a temperature at 0.5 m depth of 12.2°C and a pH of 5. Near-surface black lapilli deposits were found that were more than a meter thick near the Barva lake, but became more irregular in thickness around the Copey lake.

Figure 1. Photograph of the Barva crater lake, December 2002. The lake has an area of 9,000 m2 and a depth of ~ 7.7 m. Courtesy of Raul Mora.

Geologic Background. The central and least known of three massive volcanoes towering over the capital city of San José, Volcán Barva (Barba) is a complex volcano with multiple summit and flank vents. Its three principal summits visible from the Central Valley give it the common local name of Las Tres Marías. The voluminous andesitic-to-dacitic Tiribí Tuff, exposed in the Central Valley, was erupted about 322,000 years ago from the Barva summit caldera. Four pyroclastic cones are constructed within the 2 x 3 km caldera at the central and NW part of the summit. The SW peak contains four cones, one of which has a crater lake. Satellitic cones are found on the N and S flanks, and lava flows blanket the S side. The Los Angeles flow, one of the most recent, descends nearly to the city of Heredia. A large Plinian eruption occurred during the early Holocene. Eruptions were reported in 1760 or 1766, 1776? (also a mudflow), and 1867, but later visits to the summit did not provide evidence of eruptions during historical time.

Information Contacts: Raul Mora Amador, Red Sismologica Nacional, Laboratorio de Sismologia, Vulcanologia y Exploracion Geofisica, Universidad de Costa Rica, Apartado 214 (2060) UCR, San Jose, Costa Rica (URL: http://rsn.ucr.ac.cr/).

Reports from March 2002 through September 2003 were provided by Instituto Nicaraguense de Estudios Territoriales (INETER). Activity has been generally constant from 2001 through 2003, with tremor and very low magnitude earthquakes, usually detected by the station on the N side of the volcano (CONN). Throughout the summary period, there were occasionally technical difficulties at the Mombacho station, so no activity was registered on those days. Periods of noticeably high seismicity occurred between June and October 2002, in April 2003, and during June-August 2003 (table 2).

Table 2. Monthly count of earthquakes registered at Concepción, February 2002-September 2003. Courtesy of INETER.

Seismicity between April 2002 and February 2003. In April 2002 there were 1,433 microearthquakes detected, a significant increase over the total of 33 recorded during February-March; the majority of the seismicity was recorded on 5, 9, and 10 April. The majority of activity was classified as long-period (LP) events with frequencies between 1 and 4 Hz; some events related to rock fracturing had frequencies between 8 and 10 Hz. Activity in May was similar, with low-magnitude earthquakes and tremor. However, due to problems with CONN, only 346 earthquakes were detected. On the day of the highest activity, 19 May, 76 microearthquakes were recorded. One earthquake, only recorded at CONN, occurred on 28 May with an S-P time difference of 0.8 seconds, suggesting the hypocenter was at ~ 6.4 km depth.

June-August activity was consistent with previous months. June recorded 865 microearthquakes, while July recorded 1,229 events, mostly early in the month. CONN registered 1,219 earthquakes in August. Seismicity was heaviest on 29 and 30 August, with 116 and 139 earthquakes, respectively. The earthquakes were classified as mainly LP. On 4 August an earthquake of M 2.7 occurred ~ 15 km S of the volcano at a depth of 12.5 km. On 14 August another seismic station (URBN) was installed around Concepción, this one in the community of Urbaite, on the S flank.

In September activity levels were again generally stable. Reception problems continued but by 2 September the signal was reestablished. There were 1,250 earthquakes recorded, the majority at the end of the month, with highs of 149 on 26 September and 152 on 27 September. In October, technical problems prevented recordings until after 21 October. However, in those ten days 1,031 microearthquakes registered, with 161 and 172 on 28 and 31 October, respectively. Both CONN and URBN detected lahars on the N flank on 28 and 31 October, during a time of moderate rainfall. Activity declined in November, although 784 earthquakes were still recorded. Activity was highest on 1 and 2 November, with 115 and 129 earthquakes respectively.

Activity declined further in December, with 389 microearthquakes, although no recordings were obtained on five days due to technical problems. Similar to the past several months, activity was classified as generally LP or degassing events. Only 179 microearthquakes were recorded in January (data was not received on four days). In February, only 108 microearthquakes were detected. All events ranged between 1.5 and 3.5 Hz frequency and were classified as LP or degassing events.

Seismicity between March and June 2003. Beginning in March 2003 and continuing through April and May, activity increased to unusual levels. Between 2 and 18 March CONN registered a series of 31 earthquakes with considerable amplitude; they were not felt by residents in the area. Because the stations at Urbaite (URBN) and Maderas (MADN) were not working, only CONN recorded the activity. However, the difference in arrival times between the S and P waves indicated a depth of 15-16 km. The seismic signals began at low frequencies, followed by an increase in the spectral frequency content.

On 19 March the volcano entered a new period of increased activity. By the end of March more than 700 events were registered by the seismic station. Although during the first week of April very few earthquakes were recorded, by 11 April the station began to register a series of earthquakes of considerable amplitude, similar to the series in March. More than 1,400 events were recorded, mainly LP events. Only 476 events were recorded in May, also mainly LP events. A total of 1,298 events were recorded in June.

Seismicity between July and September 2003. Unusual seismic activity, including harmonic tremor that began at the end of June, continued in July. Starting 1 July, CONN began to register a series of LP events accompanied by low-frequency harmonic tremor and a saturated seismic signal like the one that occurred in March. Harmonic tremor occurred throughout July, with episodes of 7 minutes on 2 July, 45 minutes on 4 July, and about 60 minutes on 13 July. Long-period earthquakes and harmonic tremor increased between 23 July and the end of the month.

A total of 43 earthquakes with saturated amplitudes were registered only by CONN in July, but it was not possible to determine locations or magnitudes. The time difference in the S-P arrivals implied hypocenters 15-16 km beneath the volcano. They lasted a little over a minute and had a combination of high and low frequencies. The earthquakes with saturated signals had frequencies of 2-4 Hz; some were accompanied by a low-energy high-frequency signal. The majority of these events (7) occurred on 15 and 16 July, and had ceased by 23 July. Taking the spectral content into account, these appear to be LP events; however, it is not very common for LP events to begin with low frequencies followed by high. No data were recorded on 18, 21, and 22 July due to technical problems at Mombacho, but a total of more than 1,100 earthquakes were recorded by seismic stations.

With 1,586 earthquakes registered, seismicity was unusually high in August. Harmonic tremor also increased. Starting 1 August, CONN began to register a series of LP earthquakes accompanied by low-frequency harmonic tremor and earthquakes with saturated signals, as in previous months. Frequency ranged from 1 to 2.5 Hz, with occasionally higher values. On 16 August tremors were registered that lasted for four minutes; on 22 August, after two days with no tremor and few earthquakes, there was more unusual activity consisting of seven hours of intermittent tremor episodes.

Seismicity continued in September with 828 total events, the majority on 12 and 13 September. Seismic tremor was present throughout September, with frequency levels similar to those of the previous months.

Geologic Background. Volcán Concepción is one of Nicaragua's highest and most active volcanoes. The symmetrical basaltic-to-dacitic stratovolcano forms the NW half of the dumbbell-shaped island of Ometepe in Lake Nicaragua and is connected to neighboring Madera volcano by a narrow isthmus. A steep-walled summit crater is 250 m deep and has a higher western rim. N-S-trending fractures on the flanks have produced chains of spatter cones, cinder cones, lava domes, and maars located on the NW, NE, SE, and southern sides extending in some cases down to Lake Nicaragua. Concepción was constructed above a basement of lake sediments, and the modern cone grew above a largely buried caldera, a small remnant of which forms a break in slope about halfway up the N flank. Frequent explosive eruptions during the past half century have increased the height of the summit significantly above that shown on current topographic maps and have kept the upper part of the volcano unvegetated.

Information Contacts: Emilio Talavera, Instituto Nicaraguense de Estudios Territoriales (INETER), Dirección General de Geofísica, Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/ geofisica).

The Volcanological Survey of Indonesia (VSI) activity report for the week of 4-10 August 2003 noted, for the Sileri crater in the Dieng volcano complex, one shallow volcanic earthquake, a white gas plume rising 25-60 m, and water temperature of 83°C. The hazard status was set at Alert Level 2 (on a scale of 1-4).

Geologic Background. The Dieng plateau in the highlands of central Java is renowned both for the variety of its volcanic scenery and as a sacred area housing Java's oldest Hindu temples, dating back to the 9th century CE. The Dieng volcanic complex consists of two or more stratovolcanoes and more than 20 small craters and cones of Pleistocene-to-Holocene age over a 6 x 14 km area. Prahu stratovolcano was truncated by a large Pleistocene caldera, which was subsequently filled by a series of dissected to youthful cones, lava domes, and craters, many containing lakes. Lava flows cover much of the plateau, but have not occurred in historical time, when activity has been restricted to minor phreatic eruptions. Toxic gas emissions are a hazard at several craters and have caused fatalities. The abundant thermal features and high heat flow make Dieng a major geothermal prospect.

Information Contacts: Dali Ahmad and Nia Haerani, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Volcanological Survey of Indonesia (VSI) reports for June and July 2003 noted volcanic activity and ash emissions from Dukono. VSI reported an ash explosion commencing on 7 June, with ashfall in the Galela area (~ 7 km from the summit) on 9 June (BGVN 28:06). Explosive events had decreased by 9 June, but as of 10 June the plume was still visible on satellite imagery. No additional activity was reported through the end of June.

Ash explosions were again reported by VSI during 9-23 July, with a maximum plume height of 1,000 m in clear weather on 22 July (BGVN 28:06). No Dukono activity was reported in the report for 21-27 July. Ash explosions were reported again during 28 July-3 August, with a white-gray column, under weak pressure, rising 15-75 m. Some explosions produced dark-gray ash columns reaching 95-450 m high. On 27 and 28 July some blasting sounds were heard in the Galela area and continuous blasting sounds were heard on 25, 26, and 29 July. Minor ash fell around the crater, and ash drifted E, SE, and NE.

Ash explosions continued during 18-31 August, producing a gray ash plume 75 m high and an ash column that rose 200-250 m accompanied by booming sounds. VSI reported that ash explosions during the 1-28 September period produced a gray ash plume 50-200 m high. When there was no explosive activity, white-gray ash emissions were observed rising 50 m from the crater. The hazard status has remained at Alert Level 2 (on a scale of 1-4) since early June.

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: Dali Ahmad and Nia Haerani, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

A seismic crisis started at 2225 on 30 September 2003 beneath the SW corner of Dolomieu crater ~ 2 km below the summit. At 2330 eruption tremor appeared and was localized beneath the SSW flank of Piton de la Fournaise. A straight 400-m-long fissure opened at 2,350 m elevation. The eruption tremor reached a maximum at 0100 on 1 October and declined after 0200, disappearing completely at 1300.

Since March 2003, the extensometer network and GPS measurements had indicated inflation of Piton de la Fournaise. A new eruption that began on 30 May within Dolomieu crater proceeded in multiple phases through 7 July, followed by new activity through 27 August (BGVN 28:05, 28:06, and 28:08).

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: Thomas Staudacher, Observatoire Volcanologique du Piton de la Fournaise Institut de Physique du Globe de Paris, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/observatoire-volcanologique-piton-de-fournaise).

An eruptive event on 31 July 2003 at Gamalama produced ashfall and pyroclastic flows (BGVN 28:07). The Volcanological Survey of Indonesia (VSI) report for the week of 28 July-3 August noted that the hazard status was downgraded to Alert Level 3 on 2 August. A white gas plume was reported as rising 10-50 m above the summit and the seismograph record was dominated by emission events.

Volcanic activity was low during 18-31 August, with white gas emissions and several small ash explosions. White-gray ash plumes emitted from the crater reached 100 m high. Night glow was seen just above the crater rim. Recorded emission and tectonic earthquakes averaged four events per day. Reduced activity continued during 1-28 September 2003, again with white gas emission and small ash explosions that occurred several times. Seismicity was dominated by tectonic and emission events (table 1). The hazard status since 18 August has been at Alert Level 2 (on a scale of 1-4).

Table 1. Seismicity at Gamalama during 1-28 September 2003. Courtesy of VSI.

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: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Ash explosions have been frequent at Karangetang during 2003 (BGVN 28:05 and 28:07). A red glow at night and lava avalanches were reported during 9-15 June (BGVN 28:07). Although detailed observations were not provided by the Volcanological Survey of Indonesia (VSI) for the next two weeks, the hazard status remained at Alert Level 2 (on a scale of 1-4).

VSI weekly reports from 30 June through 3 August indicated that white gas plumes from the S crater typically rose 350-500 m above the crater rim, night glow often extended 25 m above the crater, and white gas plumes from the N crater rose as high as 350 m. Seismic data showed that lava avalanches and shallow volcanic earthquakes in early July were significantly reduced compared to the first half of June (table 8).

Table 8. Seismicity at Karangetang during 2 June-28 September 2003. VSI did not issue reports for Karangetang during weeks not included in the table; a dash indicates no data reported. Courtesy of VSI.

During 18-20 July there were ash-producing explosions and lava avalanches. On 21-22 July an ash explosion produced a 150-m-high ash column and a glowing lava avalanche flowed 350 m toward the Beha river. During the week of 28 July-3 August another glowing lava avalanche flowed 1,500 m toward the Beha river and 350 m toward the Batang river. On 29 July volcanic tremor was recorded with a maximum amplitude of 0.5-2 mm.

Karangetang was not included in August reports, but the report for 1-28 September noted white gas emissions from the S crater rising 150-350 m and red glow at night reaching 25 m over the crater, with the N crater exhibiting white gas emissions to 50-150 m above the crater. There were no lava avalanches during this period. The Alert Level remained at 2.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi island. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: Dali Ahmad, Hetty Triastuty, and Nia Haerani, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

During 2003, lava from Kilauea continued to flow down the S flanks and into the ocean at several points. The Mother's Day flow, which began erupting from Pu`u `O`o on 12 May 2003, remained active. Seismicity generally persisted at normal (background) levels. A recent report from the U.S. Geological Survey, edited by Heliker, Swanson, and Takahashi (2003) described the nearly uninterupted Pu`u `O`o-Kupaianaha eruption that started 3 January 1983 and continues today.

Lava flows. Lava entered the sea mainly at the Highcastle ocean entry during 11-17 June and surface lava flows were visible on the coastal flat and Pulama pali during June and July 2003. However, no lava flowed into the sea during the later half of July and into early August.

Deflation that began on 8 August amounted to ~ 1.8 µrad at the Uwekahuna (UWEV) tiltmeter and ~ 4 µrad at the Pu`u `O`o tiltmeter, both located near the Kilauea summit (figure 159). The deflation was accompanied by a drop in the level of lava in a lava tube, as seen by field workers at midday. Inflation began later that day at 1928, and in ~ 3.5 hours ~ 3.5 µrad of inflation was recorded at Uwekahuna and ~6 µrad at Pu`u `O`o.

Figure 159. Map of selected deformation stations at Kilauea, 2003. Courtesy of HVO.

A lava breakout occurred on 9 August between 0200 and 0300, ~ 1.3 km SE of the center of the Pu`u `O`o cone. A very large sheet flow emerged from the up-tube side of a rootless shield formed on 21 January. Observers saw a lava stream up to 40 m wide. By 0600 the flow covered ~ 5.2 hectares (0.052 km²).

Later in August and into September, surface lava flows were visible on Kilauea's coastal flat, in some areas flowing to within 500 m of the sea. On 2 October lava began to flow westward after filling West Gap Pit on the W flank of Pu`u `O`o cone. Fairly vigorous spattering was visible in the pit, but decreased to only sporadic bursts later in the day. The flow appeared to have stopped by 4 October when no glow was observed coming from the pit.

Lava flows have erupted from 1983 through 10 October 2003 from Pu`u `O`o and Kupaianaha. The area of recent lava flows on the W side of the flow-field has been designated the Mother's Day flow, which began erupting on 12 May 2002 and continues to the present (figure 160). Through September and into early October, lava was moving along the E and W sides of the Mother's Day flow. The E-side lava (mentioned previously as the 9 August breakout) came from the 9 August rootless shield, itself fed by the main Mother's Day tube from Pu`u `O`o. The W-side lava, known as the Kohola arm of the Mother's Day flow, branched off the tube system below the rootless shield. In early October, the E-side flow stopped moving, the W-side flow died back to a trickle, and the rootless shield gained prominence. By 16 October, however, the shield had partly collapsed, leaving several drained perched ponds. Upstream from the shield, many hornitos and small flows formed over the Mother's Day tube.

Figure 160. Map sequence showing Mother's Day lava flows that began on 12 May 2002 (darkest shade) from the Pu`u `O`o cone at Kilauea as of 21 May 2002, 25 November 2002, 16 May 2003, and 10 October 2003. Modified from original maps created by the USGS Hawaiian Volcano Observatory.

Geophysical activity. During the second half of June and into August 2003, seismicity at the summit was at moderate-to-high levels, with many small, low-frequency earthquakes occurring at shallow depths beneath the summit caldera at a rate of about 1-2 per minute. Little or no volcanic tremor accompanied the swarm at the caldera, however. Volcanic tremor at Pu`u `O`o remained at moderate-to-high levels, as is the norm. A quasi-cyclic tilt pattern ended at Kilauea's summit and Pu`u `O`o on 13 June after lasting about a week. Small periods of inflation and deflation occurred during July and into August.

During the deflation on 8 August, there was an increase in small, low-frequency earthquakes and changes in their frequency content. Some larger events occurred at depths of a few kilometers, as during the previous several weeks. A magnitude 5.0 earthquake 10 km beneath Kilauea's central S flank on 26 August at 2024 was the largest since 2 April 2000, which occurred in almost exactly the same spot. No significant damage was done, no cracks or rockfalls were seen, and there was no change in the eruption. Generally, following that event and into September, summit seismicity continued at moderate levels with 1-2 small low-frequency earthquakes per minute occurring at shallow depths beneath the summit caldera. There were some larger events at depths of a few kilometers.

At about 1500 on 20 September 2003, first Uwekahuna and then Pu'u O'o started to deflate. Pu'u O'o lost ~ 1.5 µrad during the deflation, and Uwekahuna lost ~ 0.9 µrad. The deflation ended with a sharp inflation in the early morning on 21 September, which lasted until early on 22 September, when the tilt flattened.

Reference. Heliker, C., Swanson, D.A., and Takahashi, T.J. (eds), 2003, The Pu`u `O`o-Kupaianaha eruption of Kilauea Volcano, Hawaii: The first 20 years: U.S. Geological Survey Professional Paper 1676, Denver, CO.

Geologic Background. Kilauea, which overlaps the E flank of the massive Mauna Loa shield volcano, has been Hawaii's most active volcano during historical time. Eruptions are prominent in Polynesian legends; written documentation extending back to only 1820 records frequent summit and flank lava flow eruptions that were interspersed with periods of long-term lava lake activity that lasted until 1924 at Halemaumau crater, within the summit caldera. The 3 x 5 km caldera was formed in several stages about 1500 years ago and during the 18th century; eruptions have also originated from the lengthy East and SW rift zones, which extend to the sea on both sides of the volcano. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1100 years old; 70% of the volcano's surface is younger than 600 years. A long-term eruption from the East rift zone that began in 1983 has produced lava flows covering more than 100 km2, destroying nearly 200 houses and adding new coastline to the island.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

The Rabaul Volcanological Observatory reported that Lamington remained quiet over the period 25 June-9 October 2003. Vapor emissions were difficult to observe because of the distance to the observation point, but on a few clear days very small volumes of thin white vapor were seen in the summit area. The report also noted that high-frequency volcano-tectonic-like earthquakes began in early July at a rate of up to five events per day and continued into early October. This is the first time since the seismic station was re-established in 1997 that these types of earthquakes have been recorded in significant numbers over a short period of time.

Geologic Background. Lamington is an andesitic stratovolcano with a 1.3-km-wide breached summit crater containing a lava dome. Prior to its renowned devastating eruption in 1951, the forested peak had not been recognized as a volcano. Mount Lamington rises above the coastal plain north of the Owen Stanley Range. A summit complex of lava domes and crater remnants tops a low-angle base of volcaniclastic deposits dissected by radial valleys. A prominent broad "avalanche valley" extends northward from the breached crater. Ash layers from two early Holocene eruptions have been identified. After a long quiescent period, the volcano suddenly became active in 1951, producing a powerful explosive eruption during which devastating pyroclastic flows and surges swept all sides of the volcano, killing nearly 3000 people. The eruption concluded with growth of a 560-m-high lava dome in the summit crater.

Information Contacts: Ima Itikarai, Rabaul Volcanological Observatory, P.O. Box 386, Rabaul, Papua New Guinea.

Recent activity at Manam has consisted of white vapor emissions from both the Main and Southern craters, and low seismicity (BGVN 28:03). The Rabaul Volcanological Observatory reported that the two vents in the Main crater gently released weak, thin white vapor during 7-12 May, with occasional white-gray emissions on 11 May. Fine ashfall resulting from occasional emissions of thin white gray ash plumes from Main crater was reported on the NW side of the island on 17-19 and 23 May. No audible noise or glow was reported. Southern crater continued to gently release small amounts of thin white vapor. The volcano was quiet over the period 25-30 June, with both craters gently releasing occasional thin white vapor emissions and low seismicity.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical 1807-m-high basaltic-andesitic stratovolcano to its lower flanks. These "avalanche valleys" channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most historical eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent historical eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: Ima Itikarai, Rabaul Volcanological Observatory, P.O. Box 386, Rabaul, Papua New Guinea.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported on 18 September 2003 that earthquake activity at Mayon had been within background levels (< 5 events/day) since 14 August with no volcanic earthquakes over the previous five days and moderate volcanic gas outputs. However, the sulfur dioxide (SO₂) flux at 1,237 metric tons per day (t/d) was above baseline levels, having increased from 829 t/d since 5 September. In view of increased SO₂ gas emissions, and recent significant earthquake occurrences, PHIVOLCS set the hazard status at Alert Level 1 (on a scale of 0-5).

For the period 29 September-5 October, 16 low-frequency volcanic earthquakes (19.0 mm amplitude), five high-frequency volcanic earthquakes (26.0 mm amplitude), and four high-frequency short-duration volcanic earthquakes (2.5 mm amplitude) were recorded, accompanied by weak to moderate steaming and no visible crater glow. During 6-12 October, 29 low-frequency volcanic earthquakes (14.0 mm amplitude), four high-frequency volcanic earthquakes (6.2 mm amplitude), and two high-frequency short duration volcanic earthquakes (2.0 mm amplitude) were recorded, with moderate steaming and faint crater glow.

PHIVOLCS reported on 9 October that a faint glow had been seen by telescope at the inner E portion of the summit crater between 2330 on 8 October and 0048 on 9 October, and again between 1630 and 1650 on 9 October. Low-frequency volcanic earthquakes occurred four and six times, respectively, during 8 and 9 October. Steam emission remained moderate, with visible plumes barely rising above the crater rim. Mayon's SO₂ flux on 9 October rose to 2,386 t/d from 1,616 t/d on 1 October.

On 11 October PHIVOLCS noted persistent and significant incandescence inside the summit crater, apparently from lava in the E portion of the volcano's conduit. Seismicity over the previous 24 hours was relatively low (three low-frequency volcanic earthquakes). The Alert Level was raised to 2, signifying instability that may lead to ash explosions or a magmatic eruption.

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

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

Instituto Nicaraguense de Estudios Territoriales (INETER) reports from March 2002 through September 2003 indicate that seismicity has generally been low. Occasional visits to the summit of Momotombo (figure 10) are made to sample gases and take temperature measurements.

Figure 10. Photograph of Momotombo (unknown date) showing the E flank and the 1905 lava flows. Note that a small steam plume is rising from the crater fumaroles. Lake Managua is in the background. Courtesy of INETER.

The first visit during this time period was on 13 April 2002. Temperature measurements in the crater fumaroles showed little variation from previous measurements, except for fumarole 14, which showed an increase from 434 to 583°C. There were no visits in May; seismic monitoring recorded only one earthquake.

Seismicity increased during the early part of June, with a seismic cluster from 1 to 11 June SW of Momotombo consisting of more than 120 earthquakes. Thirty of these earthquakes occurred on 9 June. An event on 8 June was felt at the geothermal plant W of the volcano. The majority of these events were volcano-tectonic earthquakes with frequencies between 15 and 20 Hz. The unusual tornillos (screw-type events) have continued to occur at Momotombo, usually lasting 2-5 seconds with a dominant frequency of 5 Hz.

Only 16 earthquakes were recorded in July, four of them on 12 July; none were located. Tornillos continued with a frequency of 7.5 Hz in both July and August. Seismicity increased in August with a small seismic cluster and 176 registered earthquakes, mainly volcano-tectonic. The majority of the activity took place on 1 and 2 August, including one event felt by staff at the geothermal plant. Seismicity dropped dramatically in September, October, and November, with 7 and 12 volcano-tectonic events in September and October, respectively, and none in November. Visits were made on 19, 20, 21, and 22 November for gas sampling and temperature measurements. Temperatures were measured in 12 fumaroles and around the seismic stations at the base of the volcano. The highest temperatures were found at fumaroles 3, 4, 5, 8, and 9, with the maximum temperature of 768°C at fumarole 9. Temperatures at the three fumaroles around the seismic station were 89.9°C, 99.1°C, and 90.2°C.

Seismicity increased again in December 2002 and January 2003. A seismic cluster of 88 events was recorded during 24-25 December. Locations determined for 18 of the events put them all very close to the volcano. In January 55 tectonic earthquakes were registered. After January, seismicity dropped considerably. No earthquakes were registered in February, and only one was recorded in March.

Site visits in February included walking around the crater; no morphological changes were observed. The visit also included gas sampling and temperature measurements. Fumaroles 8 and 9 measured 759°C and 762°C, respectively; more monitoring on 8 and 27 March showed that temperatures were staying relatively constant. No visits were made in April, May, or June, but seismic monitoring continued. Although only one volcano-tectonic earthquake registered in April, tornillos continued, with frequencies above 12 Hz. There were 35 volcano-tectonic events in May, including a three-hour-long cluster on 30 May. Six seismic events registered in June.

A visit was made to the volcano on 12 July 2003; temperatures were similar to the previous months, ranging from 243°C at fumarole 13 to 737°C at fumarole 9. Two earthquakes registered in August; seismicity stayed low through September.

Geologic Background. Momotombo is a young stratovolcano that rises prominently above the NW shore of Lake Managua, forming one of Nicaragua's most familiar landmarks. Momotombo began growing about 4500 years ago at the SE end of the Marrabios Range and consists of a somma from an older edifice that is surmounted by a symmetrical younger cone with a 150 x 250 m wide summit crater. Young lava flows extend down the NW flank into the 4-km-wide Monte Galán caldera. The youthful cone of Momotombito forms an island offshore in Lake Managua. Momotombo has a long record of Strombolian eruptions, punctuated by occasional stronger explosive activity. The latest eruption, in 1905, produced a lava flow that traveled from the summit to the lower NE base. A small black plume was seen above the crater after a 10 April 1996 earthquake, but later observations noted no significant changes in the crater. A major geothermal field is located on the south flank.

Information Contacts: Martha Navarro, Emilio Talavera, and Virginia Tenorio, Instituto Nicaraguense de Estudios Territoriales (INETER), Dirección General de Geofísica, Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

According to the National Weather Service, strong winds in the Katmai area on 21 September 2003 picked up old, loose volcanic ash and carried it E. Reports of minor ashfall were reported from Kodiak Island, ~ 100 km from Katmai. This phenomenon was not the result of volcanic activity and no eruption occurred.

Andrea Steffke of the Geophysical Institute, University of Alaska Fairbanks, reported a relatively large ash cloud observed in satellite images coming from the Katmai area on 21 September 2003. The cloud was first seen in satellite imagery (AVHRR, GOES, and MODIS) extending ~ 69 km to the SE. The maximum temperature difference observed was -1.46°C. Dave Schneider of the Alaska Volcano Observatory reported on 22 September 2003 that at its greatest extent the cloud was detectable for ~ 400 km. It was initially observed by an overflying (high-altitude) jet, and subsequently identified in split-window images from AVHRR, MODIS, and GOES satellites. Additional pilot reports placed the cloud top at ~ 2.1 km altitude.

The Katmai Group of volcanoes are seismically monitored by AVO, so it was possible to quickly confirm that an eruption had not taken place. SIGMETS were issued by the Alaska Aviation Weather Unit (AAWU) for this event and an AVO Information Release was distributed that indicated that this cloud of re-suspended ash was potentially hazardous to aircraft. This event is unusual in its intensity and extent of transport. The Katmai region is characterized by frequent high winds that can be strong enough to re-suspend large (several centimeters in size) pumice fragments, yet these events typically don't produce large, extensive airborne ash clouds.

Geologic Background. Novarupta, the least topographically prominent volcano in the Katmai area, was formed during a major eruption in 1912. This eruption was the world's largest during the 20th century and produced a voluminous rhyolitic airfall tephra and the renowned Valley of Ten Thousand Smokes (VTTS) ash flow. At the end of the eruption a small, 65-m-high, 400-m-wide lava dome grew to an elevation of 841 m within the source vent of the VTTS ashflow, a 2-km-wide area of subsidence NW of Trident volcano. The NE side of the Falling Mountain lava dome of the Trident volcanic cluster, as well as Broken Mountain and Baked Mountain, was removed by collapse of the Novarupta depression, which is marked by radial and scalloped arcuate fractures. Much larger collapse took place at Katmai volcano, 10 km to the east, where a 3 x 4 km wide caldera formed in response to magma reservoir drainage toward Novarupta.

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

The last eruption at Nyamuragira occurred during 25 July-27 September 2002 (BGVN 27:07, 27:10, and 28:01). Tectonic and magmatic seismicity continued through June 2003, but there has been no confirmed eruptive activity. This report covers activity from early July to the beginning of August 2003. Seismicity generally consisted of long-period (LP) earthquakes on the NE side of the volcano. In addition, earthquake swarms were occasionally observed.

Between 6 and 12 July, seismicity was dominated by LP earthquakes NE of the volcano and SE along the fracture zone between Nyamuragira and Nyiragongo. Two large swarms occurred on 7 and 8 July, with 161 LP earthquakes and 10 short-period earthquakes. The earthquakes at Nyamuragira have been deep, between 15 and 20 km.

During 13-19 July 2003, LP earthquakes NE of the volcano again dominated seismicity. Compared to the previous week, activity was low, with no swarms and only one high-frequency earthquake. The following week, between 20 and 26 July, LP earthquakes continued in the NE and to a lesser extent along the SE fracture zone. Between 19 and 21 July new sequences of earthquakes occurred, with LP events followed by short-period earthquakes, coupled with high-amplitude tremor episodes.

Between 27 July and 2 August, LP earthquakes continued to dominate seismicity NE of the volcano as well as along the SE fracture zone. Seismicity increased from the previous week, with sequences of LP earthquakes coupled with volcanic tremor episodes between 28 and 31 July. Average seismicity doubled to 200 earthquakes with hypocenters between 3 and 20 km deep.

Geologic Background. Africa's most active volcano, Nyamuragira, is a massive high-potassium basaltic shield about 25 km N of Lake Kivu. Also known as Nyamulagira, it has generated extensive lava flows that cover 1500 km2 of the western branch of the East African Rift. The broad low-angle shield volcano contrasts dramatically with the adjacent steep-sided Nyiragongo to the SW. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Historical eruptions have occurred within the summit caldera, as well as from the numerous fissures and cinder cones on the flanks. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.

Information Contacts: Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

New reports of activity at Nyiragongo include observations from visits on 12-13 July and 14-15 August 2003. Seismicity was low during the report period, but tremor related to the lava lake continued to characterize volcanic activity. Staff at the Goma observatory have kept the hazard status for Nyiragongo at Yellow (Vigilance).

During 6-12 July two long-period earthquakes were detected. Four tectonic earthquakes registered to the S and beneath Lake Kivu; none of these were felt by area residents. Fracture measurements at Monigi, Mugara, and the Nyiragongo hut did not show any significant change from previous measurements, but at Lemera fracture spacing increased from 7.537 to 7.550 m, and there was an extension of 8 mm at Shaheru. Also during the visit, Pele's hair as long as 10-15 cm was observed between Shaheru and the crater; gas plumes were noted in the S, SW, and W, along with large scoriae. Crater observations indicated the possible formation of a third platform at 650 m depth. Two small vents formed NE of the main lava lake and there was significant degassing along the S base of the internal wall.

Between 13 and 19 July, seismic activity remained low, with four long-period earthquakes beneath the NE flank. No earthquakes were felt and only seven tectonic earthquakes were recorded to the S and beneath Lake Kivu. Volcanic tremor persisted, indicating activity in the lava lake. Fracture spacing measurements were taken at Shaheru and the Nyiragongo hut, but without noticeable changes (14.778 m at Shaheru 1, 29.602 m at Shaheru 2, and 0.942 m at Nyiragongo hut). Observations of fumarole openings had been reported by residents in the Mutwanga district. Also on 18 July investigations at Kiziba revealed a recent tongue of lava infiltrating older lava layers, found in a hole dug as a septic tank.

Volcanic tremors continued between 20 July and 2 August; no earthquakes were reported. Fracture measurements at Busholoza and Kabutembo did not indicate significant changes; temperature and deformation measurements at the top of Nyiragongo, the Nyiragongo hut, Shaheru, Mugara, and Monigi also did not reveal any notable changes. However, local CH4 (methane) was present at concentrations of 35.5%.

Between 1 and 3 August the lava lake appeared very active, with lava fountains up to 10 m high, projecting large but light scoriae into the atmosphere. Pele's hair was observed at Shaheru (2,200 m elevation) and heat radiating from the lake could be felt at the observation camp on the edge of the crater. Because of the considerable projection of volcanic products, pilots were advised to avoid the area.

Following a magnitude 5.2 earthquake in the Virunga region on 5 August, scientists from the Goma observatory visited Nyiragongo on 14-15 August. Measurements included deformation and gas geochemistry in fractures, and the lava lake was monitored. No significant deformation was observed at cracks on the S side of Nyiragongo. Gas measurements at Shaheru showed that local CO₂ concentrations had increased by 1.7%, while methane there had doubled. At the top of Nyiragongo, however, measurements on 15 August were half those on 14 August. Late on 14 August a "swirl" of air caused gas to fill the crater, and ~ 2 hours later scientists as well as residents west of Virunga felt an earthquake. Another earthquake was felt in Kibati and at the crater on 15 August.

The lava lake appeared calm on 14 August, and two small vents were visible; only one was visible the next day. The lava lake was measured to be 260 m in diameter, nearly the same as on 2 August. Also during the visit scientists installed a scorimeter: Two hours worth of scoria, weighing 236.2 g per square meter, were sampled.

Geologic Background. One of Africa's most notable volcanoes, Nyiragongo contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes of a stratovolcano contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

This report concerns Poás during the interval September 2001 through December 2002. It draws on both a set of extensive half-year reports from UCR-ICE (Mora, 2001a, b; 2002) and monthly OVSICORI-UNA reports (available on the web, and sometimes prepared with co-authors Orlando Vaselli and Franco Tassi). OVSICORI-UNA reports were absent for November and December 2001.

Poás was non-eruptive during the reporting interval. The key focus of activity remains the main crater and its fumaroles, and its low-pH, variably colored lake. That lake is sometimes called Laguna Caliente or el Poás, but more frequently in past issues of the Bulletin simply described with terms like the active lake, lake in the active crater, hot lake, etc. During the reporting interval the active lake repeatedly changed pH, color, and temperature. As in the past, Laguna Caliente contained some thermally active zones, sometimes displaying up-welling water, bubbles, and zones of native sulfur. Lake Botos lies in a crater S of the active one. It remained inactive.

The origin and terminology for the main crater's dome or pyroclastic cone remains controversial; both terms are used in this report, congruent with those favored by the authors of summarized reports and included photos. Whatever its name or origin, this feature supports especially active fumaroles, and is frequently masked by steam.

Observers at the crater noted acoustical noise from vigorous degassing. Again, as typical, monthly reports consistently mentioned variable secondary fumarolic activity and occasional mass-wasting along the crater walls. Seismicity, including tremor, continued and is mentioned below, but it will be discussed more comprehensively in a later report.

UCR-ICE observations. Mora (2001b and 2002) included an overview photo of Poás (figure 74). Those reports also included numerous other photos of fumaroles and mass wasting, most of which are not shown here. Some pronounced arcuate cracks associated with mass wasting along the NE side of the lake were thought possibly related to changes in lake level and pore pressure (figure 75). A shot of the steaming dome appears as figure 76.

Figure 74. A vertical or sub-vertical aerial photo taken of the summit at Poás, with N toward the bottom left. Numbers on the photo refer to locations named on the key. As an approximate scale, the lake is ~ 200-300 m in diameter. This was taken from figures in Mora (2001a and b, and 2002) that had several other photos around the margin. Construction lines originally across this photo have been removed here, with some resulting loss and local misrepresentation of what must have been present on the original photo. Courtesy of UCR-ICE (after Mora 2001b and 2002).

Figure 75. Laguna Caliente, the hot lake at Poás (lower right) lies within a crater bounded by unstable cliffs. This photo shows part of the lake's NE margin. The person in this scene stands on a substantial though eroding terrace and inspects arcuate cracks (circumferential faults) in unstable material along the crater rim. Some of these cracks reached 40 cm wide. Landslide deposits from failures along this and other cliff faces were mentioned frequently in reports. Courtesy of UCR-ICE (from Mora 2001a).

Figure 76. The N face of the dome (or pyroclastic cone) at Poás rises from the lake and supports strong fumaroles. This photo was taken looking S. Scientists partially visible atop the dome were walking to fumaroles where they measured gas temperatures and pH. Courtesy of UCR-ICE (from Mora, 2001).

Mora (2001a, b and 2002) collected and presented considerable data on Laguna Caliente, and we include several available plots. Lake temperature and pH during 2001-2 appears as figure 77; precipitation and lake level for most of 2002, as figure 78.

Figure 77. For Laguna Caliente at Poás, plots showing temperature and pH versus month during (top) 2001 and (bottom) 2002. The various scales are unequal. The two-year peak temperature measured 41.5°C in September 2002. The lowest pH measured ~ 0 during March-October 2001 and during January, July, and August 2002. (After Mora, 2001b and 2002).

Figure 78. For Laguna Caliente at Poás, a plot showing precipitation and lake-surface level versus month during March-December 2002. The location where the precipitation measurements were taken was unstated. Values shown on the plot are in millimeters (After Mora, 2002).

Mora (2002) reported March-December 2002 precipitation ranging from 33 to 607 mm per month (figure 78). The lake's variable surface heights during March-December 2002 deviated from an established (arbitrary) datum (zero point), from which heights ranged from ~ 400 mm below the datum to ~ 100 mm above it. During this interval the lake's high stand occurred in December; it then covered the border of the lowest N terrace. The lowest stand for the interval occurred during May. During this time interval the variables of precipitation and lake height appeared to lack consistent correlation.

OVSICORI-UNA observations. During late 2001 and through 2002, low-frequency earthquakes continued to dominate the record, with OVSICORI-UNA reporting ~ 500 events per day on 8 September, but more typically 100-300 events per day. In addition during this interval instruments typically recorded several hours of tremor per month. During some months of the reporting interval, medium- and high-frequency earthquakes continued to occur in conjunction with new fumaroles appearing in the active crater.

The OVSICORI-UNA report discussing September 2002 noted that tremor rose slightly, prevailing for ~ 5 hours on each of several days. Long-period earthquakes numbered more than 100 per day, and typically 300-450 per day. Medium-frequency earthquakes occurred much less often, their numbers approaching ~ 20 per day on several days, and more typically fewer than 10 per day.

During the last half of 2002 the lake's water temperature rose above 30°C, attaining 39°C during September-December 2002. Lowered air temperatures in late 2002, particularly in November 2002, led to condensate forming over the lake's surface and rising to accumulate in larger, optically dense clouds (figure 79).

Figure 79. Conspicuous condensate hung over the active crater lake at Poás during late 2002. The condensate stemmed from warm lake temperatures (~ 39°C) combined with cooler ambient air temperatures. At the time of this photo (November 2002) the lake was light green in color. Courtesy of OVSICORI-UNA.

References. Mora, R., 2002, Informe anual de la actividad de la Cordillera Volcánica Central, 2002, Costa Rica (proofed and revised by Alvarado, G., Fernández, M., Mora, M., Paniagua S., and Ramírez, C.): Universidad de Costa Rica, Red Sismológica Nacional, UCR-ICE, Sección de Sismología, Vulcanologíay Exploración Geofísica (published June 2003 as mini-CD Rom with PDF files).

Mora, R., 2001a, Informe semestral de la actividad de la Cordillera Volcánica Central, Enero-Junio 2001, Costa Rica: Universidad de Costa Rica, Red Sismológica Nacional, UCR-ICE, Sección de Sismología, Vulcanologíay Exploración Geofísica (published November 2001 as mini-CD Rom with PDF files).

Mora, R., 2001b, Informe semestral de la actividad de la Cordillera Volcánica Central, Julio-Diciembre 2001, Costa Rica (proofed and revised by Alvarado, G., Fernández, M., Montero, W., and Ramírez, C.): Universidad de Costa Rica, Red Sismológica Nacional, UCR-ICE, Sección de Sismología, Vulcanologíay Exploración Geofísica (published 6 May 2001 as mini-CD Rom with PDF files).

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

Information Contacts: R. Mora (Amador), C. Ramírez, and M. Fernández, Universidad de Costa Rica, Laboratorio de Sismologia, Vulcanología y Exploración Geofisica, Aptdo. 560-2300, Curridabat, San José, Costa Rica; E. Fernández, E. Duarte, E. Malavassi, R. Sáenz, V. Barboza, R. Van der Laat, T. Marino, E. Hernández, and F. Chavarría, Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI-UNA); Jorge Barquero and Wendy Sáenz, Laboratorio de Química de la Atmósfera (LAQAT), Depto. de Química, Universidad Nacional, Heredia, Costa Rica; María Martínez (at both affiliations above); Orlando Vaselli and Franco Tassi, Department of Earth Sciences, University of Florence, Via La Pira 4, 50121 Florence, Italy.

Reports from the Rabaul Volcanological Observatory (RVO) over the period 20 March-9 October show that ash eruptions from the Tavurvur cone at Rabaul are continuing. Activity has been nearly continuous since the major September 1994 eruption (BGVN 19:08).

Eruptions during 20 March-6 April were characterized by discrete, slow, convoluted ash plumes occurring at long irregular intervals rising slowly to several hundred meters over the summit. The ash plumes were mainly light to pale gray, blowing to the SE. Seismicity was generally low, with low- to intermediate-frequency events of 1-5 minute duration associated with the ash emissions, and greater energy expended over the first 10 seconds of the more forceful eruptions. Ground deformation fluctuated without showing any real trends.

Short forceful and slow sub-continuous discrete ash emissions were reported for 7-29 April. Light to pale gray ash-laden plumes rose as high as 1,500 m over the summit, blowing NW and SE on variable winds, with ash accumulation in Rabaul Town to the NW. Seismicity was generally low and reflected the eruptive activity. Most activity involved low-frequency, low-amplitude short- to long-duration sub-continuous volcanic tremors. Some high-frequency earthquakes were recorded NE of Rabaul Town. Deformation measurements showed minor inflation.

Steady ash eruptions continued during 7-12 May. While the ash content in individual plumes was fairly low, the accumulation of ash on the ground became quite significant within 5 km of the volcano. Seismicity was generally low (low-frequency earthquakes with durations of several minutes), reflecting summit activity. This increased to moderate seismicity over 10-12 May. Short-term ground-deformation measurements were ambiguous; long-term trends showed minor inflation.

There was a noticeable decline in ash eruptions and seismicity during 19-30 June, from one every few minutes to less than one per hour and then complete cessation on 29 June. Very occasional low roaring noises were heard early in the period. Tavurvur released only variable amounts of thin white vapor through 9 August. It began to erupt again on 10 August, with slow convoluted emissions of mainly white to pale-gray ash at irregular intervals blowing to the NW, including over the Rabaul Town area. Discrete moderate to large explosions began to occur on 25 August (1-3 per day). Occasional low rumbling noises were heard. Seismic activity was low and there were no significant ground movements.

From 29 August to 11 September the level of eruptive activity was low to moderate, characterized by convoluted ash clouds at short irregular intervals. Moderate explosions (3-6 per day) produced thick columns of pale gray to dark ash clouds rising 2-4 km above the summit. The prevailing SE winds resulted in ashfall to the NW, including in the Rabaul Town area. Seismic activity was low, with some high-frequency earthquakes NE of Rabaul Caldera and no significant ground-deformation movements.

The level of eruptive activity was generally low during 12-25 September (figure 38), with light to pale gray ash clouds rising 500-1,500 m above the summit and light downwind ashfall in the early part of the reporting period. Over 22-25 September the ash cloud emissions became light gray, with high water vapor content. Low to moderate rumbling noises were heard, but seismic activity was low and ground deformation movements were not significant.

Figure 38. Photograph showing a plume from the Tavurvur cone at Rabaul (left background) taken from the Rabaul Volcanological Observatory, with Rabaul Town and Harbor in the foreground, 17 September 2003. Courtesy of William Kiene, UCLA.

Eruptive activity continued at a low level from 26 September to 9 October, with light to pale gray emissions (containing some ash but mostly water vapor) rising 500-1,500 m. The emissions occurred at long, irregular intervals, and many were accompanied by low roaring and rumbling noises. Very fine ash was blown mainly to the N and NW. Seismic activity was low, with no high-frequency earthquakes inside the caldera or NE of the caldera. Ground-deformation measurements showed a long-term inflationary trend between May and September, but the magnitude of change was small.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the 688-m-high asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: Ima Itikarai and Steve Saunders, Rabaul Volcanological Observatory, P.O. Box 386, Rabaul, Papua New Guinea; William Kiene, UCLA, 405 Hilgard Avenue, Box 951361, Los Angeles, CA 90095-1361.

Volcanic activity at Semeru between 30 June and 28 September remained at high levels. Except for the middle two weeks of July, ash explosions were reported several times every week, producing white-gray plumes that rose 400-500 m above the summit. Recorded seismic data (table 13) reflected this continued activity, with between 447 and 804 explosion events weekly (~ 88 per day on average over this 90-day period). Avalanche events, tremor, tectonic, deep-volcanic, shallow-volcanic, and flood-related seismicity were also recorded. A pilot report from Qantas noted a plume to twice the height of the volcano (~ 7.2 km altitude) on 9 September that was drifting S. The hazard status remained at Alert Level 2 throughout the report period.

Table 13. Seismicity at Semeru, 30 June-28 September 2003. Courtesy of VSI.

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: Dali Ahmad and Nia Haerani, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 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/).

Volcanic seismicity at Tandikat increased significantly following a felt event (MM III) on 20 January 2002 (table 1). Deep-volcanic earthquakes totaled 149 during the week of 20-26 January, a period when 174 tectonic events were also recorded. Both types of earthquakes decreased significantly the next week, and gradually declined further over the following two weeks. The weekly report for 27 January-2 February noted that visual observations were not possible due to thick fog. The hazard status was set at Alert Level 2 (on a scale of 1-4) on 25 January 2002 and remained at that level through 16 February.

Geologic Background. Tandikat and its twin volcano to the NNE, Singgalang, lie across the Bukittinggi plain from Marapi volcano. Volcanic activity has migrated to the SSW from the higher Singgalang, and only Tandikat has had historical activity. The summit of Tandikat has a partially eroded 1.2-km-wide crater containing a large central cone capped by a 360-m-wide crater with a small crater lake. The only three reported historical eruptions, in the late 19th and early 20th centuries, produced only mild explosive activity.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

The Volcanological Survey of Indonesia (VSI) reported deep volcanic and A-type earthquakes at Tongkoko (also known as Tangkoko) over the period 7 October-24 November 2002 and more deep-volcanic events during 23 December 2002-19 January 2003 (table 1). The earthquakes, which began in May 2002, were recorded following relocation of an observatory post to Wainenet village in the Bitung area. The temperature at Batu Angus hot spring on 10 October 2002 was 70-73°C. While no visible activity has been observed, the hazard status was raised to Alert Level 2 (on a scale of 1-4) on 24 October 2002 as a result of the increased seismicity. The last recorded activity at Tongkoko consisted of flank lava flows and lava dome extrusion in 1880.

Table 1. Earthquakes recorded at Tongkoko, 7 October 2002-19 January 2003. In addition, one shallow volcanic event was recorded during 13-19 January 2003, and single B-type earthquakes each occurred during 21-27 October and 4-10 November 2002. Courtesy of VSI.

Geologic Background. The eastern peninsula at the far NE end of Sulawesi near the city of Bitung is occupied by a volcanic complex consisting of two major edifices within a nature reserve. To the north is Tangkoko (also known as Tongkoko), with a large caldera (~3 x 1.5 km) elongated towards the SE from the highest rim point; the rim at the opposite end is more than 400 m lower. Eruptions occurred from the summit crater in the 17th century and in 1801, when the caldera also reportedly contained a cone surrounded by a lake. About 1.5 km down the outer E flank is the Batuangus (or Batu Angus) lava dome, formed in 1801, along with an adjacent vent (Baru Batuangus) that has been the source of all subsequent eruptions. The higher twin-peaked Duasudara (also Dua Suadara) stratovolcano is about 4.5 km SW of the Tangkoko summit. A NE-facing open crater appears to have a hummocky debris flow that reaches the base of the Tangkoko edifice.

Information Contacts: Dali Ahmad, Hetty Triastuty, Nia Haerani, and Suswati, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Variable amounts of emergent vapor and minor debris flows at Ulawun were reported during January-March 2003 (BGVN 28:03). Rabaul Volcanological Observatory (RVO) reports, covering much of the period 14 April-5 October 2003, indicated the volcano remained quiet over this time, without emissions from the N-valley vent.

The main summit crater continued to release weak to moderate volumes of white (occasionally white-gray) vapor during 14-29 April, 7-27 May, and 11-18 June. Seismicity was low except for an episode of volcanic tremor between 15 and 19 April. Gas effervescence was reported close offshore of Ulamona Jetty in the second half of April. A slight increase in seismicity was noted between 18 and 23 May.

The period 25 June-22 July was quiet, with no audible noise or night-time glow, and weak to moderate volumes of vapor from the main summit crater. The Volcanic Ash Advisory Center in Darwin reported these plumes as being visible on weather satellite imagery. The plumes appeared white-gray on occasions and were unusually strong bluish white gray over the last three days of the period. Volcanic seismicity was low, with several strongly felt tectonic earthquakes on the night of 3-4 July. A large regional earthquake centered 45 km N of Rabaul affected the area on 16 July, leading to a large tiltmeter offset, which slowly recovered over the following days.

Reports for the period 12 September-5 October indicated that the main summit continued to release weak to moderate volumes of white vapor, with occasional white-gray emissions. Seismicity was low with no significant ground movements.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the north coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: Ima Itikarai, Rabaul Volcanological Observatory, P.O. Box 386, Rabaul, Papua New Guinea; 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 eruption at Pago that began in August 2002 continued during early 2003 with lava effusion through at least 28 February and vapor emissions (BGVN 28:03). The Rabaul Volcanological Observatory (RVO) reports that activity at Pago continued, but remained low, from 14 April through 9 October 2003.

The line of vents on the NW slope of Pago continued to release small amounts of thin white vapor over the whole of the period. Occasional weak audible booming noises were heard (eg. on 20 April) and roaring noises were heard on 24 April, 6 May, and 22 May. Very small traces of blue vapor were seen coming from the lower vents on 8 May.

An aerial inspection on 22 May showed that lava effusion from the NW vent had ceased since the February inspection; there were no indications of fresh lava near the vent, no movement of the N and S lobes, and no change in the height of lava against the caldera wall. It also revealed a new fumarolic area to the E.

Monitoring instruments were restored on 19 May. Leveling measurements showed a few centimeters of inflation compared to December 2002. This was considered by RVO to be very significant when compared to previous measurements, but may have been due to nearby roadwork.

Less than 20 volcano-tectonic earthquakes per day were recorded during 25-30 June. A local tectonic earthquake on 9 August seemed to lead to an increase in energy release and event numbers at one seismic station, but it may have been an instrumentation problem. An airborne spectrophotometer revealed only trace amounts of SO₂ in early August. Between two and seven volcano-tectonic earthquakes per day were reported in the 26 September-9 October period.

Geologic Background. The 5.5 x 7.5 km Witori caldera on the northern coast of central New Britain contains the young historically active cone of Pago. The Buru caldera cuts the SW flank of Witori volcano. The gently sloping outer flanks of Witori volcano consist primarily of dacitic pyroclastic-flow and airfall deposits produced during a series of five major explosive eruptions from about 5600 to 1200 years ago, many of which may have been associated with caldera formation. The post-caldera Pago cone may have formed less than 350 years ago. Pago has grown to a height above that of the Witori caldera rim, and a series of ten dacitic lava flows from it covers much of the caldera floor. The youngest of these was erupted during 2002-2003 from vents extending from the summit nearly to the NW caldera wall.

Information Contacts: Ima Itikarai, Rabaul Volcanological Observatory, P.O. Box 386, Rabaul, Papua New Guinea.

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/).