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/).

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/).

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 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/).

The Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo is part of the western branch of the East African Rift System. Nyamuragira (or Nyamulagira), a high-potassium basaltic shield volcano on the W edge of VVP, includes a lava field that covers over 1,100 km² and contains more than 100 flank cones in addition to a large central crater (see figure 63, BGVN 42:06). A lava lake that had been active for many years emptied from the central crater in 1938. Numerous flank eruptions were observed after that time, the most recent during November 2011-March 2012 on the NE flank. This was followed by a period of degassing with unusually SO₂-rich plumes from April 2012 through April 2014 (BGVN 42:06). The lava lake reappeared during July 2014-April 2016 and November 2016-May 2017, producing a strong thermal signature. After a year of quiet, a new lava lake appeared in April 2018, reported below (through May 2019) with information provided by the Observatoire Volcanologique de Goma (OVG), MONUSCO (the United Nations Organization working in the area), and satellite data and imagery from multiple sources.

Fresh lava reappeared inside the summit crater in mid-April 2018 from a lava lake and adjacent spatter cone. Satellite imagery and very limited ground-based observations suggested that intermittent pulses of activity from both sources produced significant lava flows within the summit crater through April 2019 when the strength of the thermal signal declined significantly. Images from May 2019 showed a smaller but persistent thermal anomaly within the crater.

Activity from October 2017-May 2019. Indications of thermal activity tapered off in May 2017 (BGVN 42:11). On 20 October 2017 OVG released a communication stating that a brief episode of unspecified activity occurred on 17 and 18 October, but the volcano returned to lower activity levels on 20 October. There was no evidence of thermal activity during the month. The volcano remained quiet with no reports of thermal activity until April 2018 (figure 73).

Figure 73. Sentinel-2 satellite images (bands 12, 4, 2) indicated no thermal activity at Nyamuragira on 19 November (top left), 14 December 2017 (top right) and 18 January 2018 (bottom). However, Nyiragongo (about 13 km SE) had an active lava lake with a gas plume drifting SW on 18 January 2018 (bottom right). Courtesy of Sentinel Hub Playground.

OVG reported the new lava emissions beginning on 14 April 2018 as appearing from both the lava lake and a small adjacent spatter cone (figure 74). The first satellite image showing thermal activity at the summit appeared on 18 April 2018 (figure 75) and coincided with the abrupt beginning of strong MIROVA thermal signals (figure 76). MODVOLC thermal alerts also first appeared on 18 April 2018. An image of the active crater taken on 9 May 2018 showed the lake filled with fresh lava and two adjacent incandescent spatter cones (figure 77).

Figure 74. Fresh lava reappeared at Nyamuragira's crater during April 2018 from the lava lake (left) and the adjacent small spatter cone (right). Courtesy of OVG (Republique Democratique du Congo, Ministere de la Recherche Scientifique, Observatoire Volcanologique de Goma, Direction Generale Goma, Rapport Avril 2018).

Figure 75. The first satellite image (bands 12, 4, 2) indicating renewed thermal activity at the Nyamuragira crater appeared on 18 April 2018; the signal remained strong a few weeks later on 3 May 2018. Courtesy of Sentinel Hub Playground.

Figure 76. A strong thermal signal appeared in the MIROVA graph of Log Radiative Power on 18 April 2018 for Nyamuragira, indicating a return of the lava lake at the summit crater. Courtesy of MIROVA.

Figure 77. Fresh lava filled the lake inside the crater at Nyamuragira on 9 May 2018. Two spatter cones were incandescent with gas emissions. Courtesy of OVG (Republique Democratique du Congo, Ministere de la Recherche Scientifique, Observatoire Volcanologique de Goma, Direction Generale Goma, Rapport Mai 2018).

Satellite images confirmed that ongoing activity from the lava lake remained strong during June -September 2018 (figure 78). A mission to Nyamuragira was carried out by helicopter provided by MONUSCO on 20 July 2018; lava lake activity was observed along with gas emissions from the small spatter cone (figure 79). OVG reported increased volcanic seismicity during 1-3 and 10-17 September 2018, and also during October, located in the crater area, mostly at depths of 0-5 km.

Figure 78. Sentinel-2 satellite images (bands 12, 4, 2) confirmed that ongoing activity from the lava lake at Nyamuragira remained strong during June-September 2018, likely covering the crater floor with a significant amount of fresh lava. Image are from 12 June (top left), 7 July (top right), 17 July (middle left), 22 July (middle right), 11 August (bottom left), and 20 September (bottom right). Courtesy of Sentinel Hub Playground.

Figure 79. The crater at Nyamuragira on 20 July 2018 had an active lava lake and adjacent incandescent spatter cone with gas emissions. Courtesy of OVG (Republique Democratique du Congo, Ministere de la Recherche Scientifique, Observatoire Volcanologique de Goma, Direction Generale Goma, Rapport Juillet 2018).

Personnel from OVG and MONUSCO (United Nations Organization Stabilization Mission in DR Congo) made site visits on 11 October and 2 November 2018 and concluded that the level of the active lava lake had increased during that time (figure 80). On 2 November OVG measured the height from the base of the active cone to the W rim of the crater as 58 m (figure 81).

Figure 80. OVG scientists reported a rise in the lake level between site visits to the Nyamuragira crater on 11 October (top) and 2 November 2018 (bottom). Top image courtesy of MONUSCO and Culture Vulcan, bottom image courtesy of OVG (Republique Democratique du Congo, Ministere de la Recherche Scientifique, Observatoire Volcanologique de Goma, Direction Generale Goma, Rapport Octobre 2018).

Figure 81. On 2 November 2018 scientists from OVG measured the height from the base of the active cone to the W rim of the crater as 58 m. Courtesy of OVG (Republique Democratique du Congo, Ministere de la Recherche Scientifique, Observatoire Volcanologique de Goma, Direction Generale Goma, Rapport Octobre 2018).

Seismicity remained high during November 2018 but decreased significantly during December. Instrument and access issues in January 2019 prevented accurate assessment of seismicity for the month. The lava lake remained active with periodic surges of thermal activity during November 2018-March 2019 (figure 82). Multiple images show incandescence in multiple places within the crater, suggesting significant fresh overflowing lava.

Figure 82. The active lava lake at Nyamuragira produced strong thermal signals from November 2018 through March 2019 that were recorded in Sentinel-2 satellite images (bands 12, 4, 2). Several images suggest fresh lava cooling around the rim of the crater in addition to the active lake. A relatively cloud-free day on 19 November 2018 (top left) revealed no clear thermal signal, but a strong signal was recorded on 29 November (top right) despite significant cloud cover. Images from 13 and 28 January 2019 (second row) both showed evidence of incandescent lava in multiple places within the crater. The thermal signal was smaller and focused on the center of the crater on 12 and 27 February 2019 (third row). Images taken on 9 and 19 March 2019 clearly showed incandescent material at the center of the crater and around the rim (bottom row). Courtesy of Sentinel Hub Playground.

On 12 April 2019 a Ukrainian Aviation Unit supported by MONUSCO provided support for scientists visiting the crater for observations and seismic analysis. Satellite data confirmed ongoing thermal activity into May, although the strength of the signal appeared to decrease (figure 83). MODVOLC thermal alerts ceased after 8 April, and the MIROVA thermal data also confirmed a decrease in the strength of the thermal signal during April 2019 (figure 84).

Figure 83. Sentinel-2 satellite data (bands 12, 4, 2) confirmed ongoing thermal activity at Nyamuragira into May 2019. The thermal anomalies on 18 April (left) and 3 May (right) 2019 were smaller than those recorded during previous months. Courtesy of Sentinel Hub Playground.

Figure 84. The MIROVA graph of thermal activity (log radiative power) at Nyamuragira from 16 July 2018 through April 2019 showed near-constant levels of high activity through April 2019 when it declined. This corresponded well with satellite and ground-based observations. Courtesy of MIROVA.

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: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Katcho Karume, Director; Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/); MONUSCO, United Nations Organization Stabilization Mission in the DR Congo (URL: https://monusco.unmissions.org/en/, Twitter: @MONUSCO); Cultur Volcan, Journal d'un volcanophile (URL: https://laculturevolcan.blogspot.com), Twitter: @CultureVolcan).

The 16-km-wide Tengger Caldera in East Java, Indonesia is a massive volcanic complex with numerous overlapping stratovolcanos (figure 11). Mount Bromo is a pyroclastic cone that lies within the large Sandsea Caldera at the northern end of the complex (figure 12) and has erupted more than 20 times during each of the last two centuries. It is part of the Bromo Tengger Semeru National Park (also a UNESCO Biosphere Reserve) and is frequently visited by tourists. The last eruption from November 2015 to November 2016 produced hundreds of ash plumes that rose as high as 4 km altitude; some of them drifted for hundreds of kilometers before dissipating and briefly disrupted air traffic. Only steam and gas plumes were observed at Mount Bromo from December 2016 to February 2018 when a new series of explosions with ash plumes began; they are covered in this report with information provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC). Copyrighted ground and drone-based images from Øystein Lund Andersen have been used with permission of the photographer.

Figure 11. The Tengger Caldera viewed from the north Mount Bromo issuing steam in the foreground and Semeru volcano in the background on 30 September 2018. Courtesy of Øystein Lund Andersen, used with permission.

Figure 12. Aerial view of the Bromo Cone in Tengger Caldera seen from the west on 30 September 2018. Courtesy of Øystein Lund Andersen, used with permission.

PVMBG lowered the Alert Level at Bromo on 21 October 2016 from III to II near the end of an eruptive episode lasting nearly a year. The last VAAC report was issued on 12 November 2016 (BGVN 41:12) noting that the last ash emission had been observed the previous day drifting NW at 3 km altitude. Throughout 2017 and 2018 Bromo remained at Alert Level II, with no unusual activity described by PVMBG. During 1-2 September 2018, a wildfire in the Bromo Tengger Semeru National Park burned 65 hectares of savannah (figure 13); the fire produced 12 MODVOLC thermal alerts around the Tengger Caldera rim. No reports of increased volcanic activity were issued by PVMBG during the period.

Figure 13. A wall of fire in the Bromo Tengger Semeru National Park savanna during 1-2 September 2018 produced thermal alerts that were not related to volcanic activity at the Bromo Cone in Tengger Caldera. Image courtesy of the park authority, reported by Mongabay. MODVOLC thermal alerts courtesy of Hawai'i Institute of Geophysics and Planetology (HIGP).

After slightly more than two years of little activity other than gas and steam plumes, ash emissions resumed from the Bromo Cone on 18 February 2019. After a brief pause, a new explosion on 10 March marked the beginning of a series of near-daily ash emissions that lasted for the rest of March, producing ash plumes that rose to altitudes ranging from 3.0 to 5.2 km and drifted in many different directions. A new series of ash emissions began on 6 April, rising to 3 km and also drifting in multiple directions. Ash emission density decreased during the month; plumes were only rising a few hundred meters above the summit by the end of April and consisted of mostly steam and moderate amounts of ash.

Activity during February-April 2019. PVMBG reported that at 0600 on 18 February 2019 an eruption at Tengger Caldera's Bromo Cone generated a dense white-and-brown ash plume that rose 600 m and drifted WSW. The plume was not visible in satellite imagery, according to the Darwin VAAC. The Alert Level remained at 2 (on a scale of 1-4). After a few weeks of quiet a new explosion on 10 March (local time) produced a white, brown, and gray ash plume that rose 600 m above the summit; the plume was visible in satellite imagery extending SW. Increased tremor amplitude was also reported on 10 March. A new emission the next morning produced similar ash plumes that drifted S, SW, and W at 3 km altitude. On the morning of 12 March (local time) a continuous ash plume was observed in satellite imagery at 3.4 km altitude drifting SW. The plume drifted counterclockwise towards the S, E, and NE throughout the day and continued to drift NE and SE on 13 March. The altitude of the plume was reported at 4.3 km later that day based on a pilot report.

Continuous brown, gray, and black ash emissions were reported by PVMBG during 14-19 March at altitudes ranging from 3 to 3.9 km; they drifted generally NE to NW. Ashfall was noted around the crater and downwind a short distance. The Darwin VAAC reported continuous ash emissions to 5.2 km altitude drifting SE on 20 March. It was initially reported by a pilot and partially discernable in satellite imagery before dissipating. Ongoing ash emissions of variable densities and colors ranging from white to black were intermittently visible in satellite imagery and confirmed in webcam and ground reports at around 3.0 km altitude during 21-25 March (figures 14-17). Ashfall impacted the closest villages to Bromo, including Cemara Lawang (30 km NW), which was covered by a thin layer of ash. A few trees in the area were toppled over by the weight of the ash. The plume altitude increased slightly on 26 March to 3.7-3.9 km, drifting N and NE. The higher altitude plume dissipated early on 28 March, but ash emissions continued at 3.0 km for the rest of the day.

Figure 14. Ash drifted NNE from the Bromo Cone in Tengger Caldera on 23 March 2019. Courtesy of Øystein Lund Andersen (drone image), used with permission.

Figure 15. Ash drifted N from the Bromo Cone in Tengger Caldera on 23 March 2019. The Batok Cone is on the right, Segera Wedi is behind Bromo, and Semeru is in the far background. Courtesy of Øystein Lund Andersen, used with permission.

Figure 16. A few trees toppled from ashfall in the vicinity of the Bromo Cone in Tengger Caldera on 24 March 2019. Courtesy of Øystein Lund Andersen, used with permission.

Figure 17. Ash plumes from the Bromo Cone in Tengger Caldera on 24 March 2019 caused ashfall in communities as far as 30 km away. View is from the floor of the Sandsea Caldera. Courtesy of Øystein Lund Andersen, used with permission.

After just a few days of quiet, new ash emissions rising to 3.0 km altitude and drifting SE were reported by both PVMBG (from the webcam) and the Darwin VAAC on 6 April 2019. By the next day the continuous ash emissions were drifting N, then E during 8-10 April, and S during 11 and 12 April. A new emission seen in the webcam was reported by the Darwin VAAC on 15 April (UTC) that rose to 3.0 km and drifted W. Ash plumes were intermittently visible in either webcam or satellite imagery until 17 April rising 500-1,000 m above the crater; from 19-25 April only steam plumes were reported rising 300-500 m above the summit. A minor ash emission was reported from the webcam on 26 April that rose to 3.0 km altitude and drifted NE for a few hours before dissipating. PVMBG reported medium density white to gray ash plumes that rose 400-600 m above the crater for the remainder of the month.

Geologic Background. The 16-km-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru volcano. The massive volcanic complex dates back to about 820,000 years ago and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The Ngadisari caldera at the NE end of the complex formed about 150,000 years ago and is now drained through the Sapikerep valley. The most recent of the calderas is the 9 x 10 km wide Sandsea caldera at the SW end of the complex, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, one of Java's most active and most frequently visited volcanoes.

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/); 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/); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Mongabay, URL: https://news.mongabay.com/2018/09/fires-tear-through-east-java-park-threatening-leopard-habitat/.

Our last Bulletin report (BGVN 31:12) on Aoba (Ambae) described the destruction of vegetation by acidic gas emissions and the breach of the islet lake during 2006. This report discusses comparative quiescence into late 2009 when degassing escalated (substantial gas plumes were seen) and the hazard status rose. The volcano has remained quiet into mid-2011.

The Vanuatu region lies ~2,200 km N off the New Zealand coast and ~2,100 km NE off the coast of Australia (figure 31). A 1999 census suggested ~9,400 people resided on Ambae. Cronin and others (2004) describe the residents as "dispersed amongst more than 276 small extended family settlements and villages (Wallez 2000). Settlements are mostly restricted to the lower island slopes within 4 km of the coast. The highest population densities occur at the NE and SE ends of the island."

Figure 31. (A) Major islands of the Republic of Vanuatu. (B) Aoba (Ambae) Island, showing locations of settlements, main stream channels and roads. Cronin and others (2004) discuss the communities of Lolovange and Lolowai (ellipses). Taken from Cronin and others (2004) after work by Wallez (2000).

The Vanuatu Geohazards Observatory (VGO) noted increases in activity from Aoba (Ambae) starting in December 2009.This began when local villagers near the volcano reported seeing a plume over the island. In December 2009 the Vanuatu Volcanic Alert Level (VVAL) was raised to Level 1. The scale ranges from 0 to 4: 0 represents normal low-level activity and 4 represents a large eruption and island wide danger. The reported source of activity is a recent cone located in the crater lake, Voui (BGVN 30:11 and 30:12).

The VGO went on to note that "An expatriate pilot based on Gaua, also witnessed a plume on Ambae on Tuesday 6th April on his way back to Gaua from Santo. Aerial pictures that were taken by two Geohazards staff on 11 April 2010 also confirmed gas emissions that were more concentrated than normal... [which] reaffirms the [Ozone Monitoring Instrument or OMI] satellite image of gas emissions above. Another observation made on Ambae is the presence of sulphur-hydromagmatic activity on the SE part of the second crater of Ambae enclosing Manaro Lakua indicated by what seemed like two fumarolic zones.... There was also some discoloration of the water in Manaro Lakua near the 'fumaroles' with some areas near the shore [colored] brown, and some areas [colored] pale blue—a sign of the incorporation of sulphur dioxide. It was also reported that while flying above the area, strong sulphur dioxide gas could be smelt even at 5,000 feet [~1.5 km altitude] on 11 April."

The VGO also noted that the OMI satellite pictures depicted fluctuating gas emissions during this period. The image for 11 April 2010 indicated elevated SO₂ and gave the integrated concentration-pathlength as 15 kilotons. On this day, VGO had noted SO₂ fluxes over 3,000 tons/day.

References. Cronin, SJ, Gaylord, DR, Charley, D., Alloway, BV, Wallez, S, and Esau, JW, 2004, Participatory methods of incorporating scientific with traditional knowledge for volcanic hazard management on Ambae Island, Vanuatu, Bulletin of Volcanology, v. 66, pp.652-668, Springer-Verlag.

Wallez S, 2000, Socio-economic survey of the impact of the volcanic hazards for Ambae Island: geo-hazards mitigation program section. Department of Geology, Mines and Water Resources, Port Vila, Vanuatu. p 39.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Vanuatu Geohazards Observatory (VGO) (URL: http://www.vmgd.gov.vu/vmgd/); Ozone Monitoring Instrument (OMI), Sulfur Dioxide Group), Joint Center for Earth Systems Technology, University of Maryland Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD 21250, USA (URL: https://so2.gsfc.nasa.gov/).

In our last report on Ambrym (BGVN: 3411), we described the frequent thermal anomalies from the volcano's active lava lakes during October 2008-September 2009. Satellite imagery in 2009 and 2010 suggested ongoing visible plumes and thermal alerts consistent with active lava lakes. Several satellite images of Vanuatu appear below (figures 21-22).

Figure 21. A hazy layer of vog (volcanic fog) overlies Malekula and a few other islands of the Vanuatu archipelago in this natural-color satellite image from 6 October 2009. The source of the vog is Ambrym, a volcano (and island of the same name) in the SE (lower right) corner of this scene. The haze extends over the Coral Sea several hundred kilometers to the NW. Ambrym emits SO2, the gas responsible for the formation of vog, intermittently. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Aqua satellite acquired this natural-color image. NASA image by Jeff Schmaltz, MODIS Rapid Response, NASA Goddard Space Flight Center (the Rapid Response Team provides twice-daily images of this region). Caption by Robert Simmon.

Figure 22. Diffuse plumes rise from Gaua volcano (top) and Ambrym volcano (bottom) in the Vanuatu archipelago. Both Gaua and Ambrym are located in the New Hebrides island arc, where the Pacific plate is subducting beneath the Australian plate. This natural-color image was acquired on 2 August 2010, by the Moderate Resolution Imaging Spectrometer (MODIS) aboard NASA's Terra satellite. NASA image by Jeff Schmaltz ; caption by Robert Simmon. Courtesy of NASA Earth Observatory.

Based on observations by aircraft pilots, analyses of satellite imagery, and information from the Vanuatu Geohazards Observatory (VGO), the Wellington Volcanic Ash Advisory Center (VAAC) reported that on 8 and 10 August 2010 ash and steam plumes from Ambrym volcano rose to an altitude 6.1 km and drifted W and NW.

Ambrym is a major source of SO₂ in the Vanuatu Republic. The VGO web site shows daily Vanuatu volcanic sulfur dioxide (SO₂) fluxes through a partnership with GNS Science Institute (Taupo, New Zealand) using OMI satellite images.

Figure 23 shows a 22 May 2011 satellite image of Ambrym. Similar images were acquired by NASA satellites on 28 March 2011 and on 7 June 2011. On figure 23, a blue-tinged volcanic plume emissions extends from Ambrym to the W. The plume contains vog, a mix of gases and aerosols that is formed when SO₂ and other volcanic gases react with sunlight, oxygen, and moisture.

Figure 23. A natural-color image showing Ambrym and its W-blowing plume. The image was acquired by MODIS (the Moderate Resolution Imaging Spectroradiometer aboard the Terra satellite) on the morning of 22 May 2011. NASA image by Jeff Schmaltz, MODIS Rapid Response Team, NASA-GSFC. Courtesy of NASA Earth Observatory.

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: Vanuatu Geohazards Observatory, Department of Geology, Mines and Water Resources of Vanuatu (URL: http://www.vmgd.gov.vu/vmgd/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); MODIS/MODVOLC Thermal Alerts System, Hawai'i Institute of Geophysics and Planetology (HIGP), 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/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, URL: http://vaac.metservice.com/).

Occasional small ash eruptions occurred at Cleveland during 2009 through early June 2010 (BGVN 35:06). Mild restless behavior continued at least into mid-September 2010 but it was uncertain whether ash had been emitted.

Table 3 compiles key observations and alerts for Cleveland volcano during mid-June through 31 March 2011. The Alaska Volcano Observatory (AVO) reported that thermal anomalies were sometimes visible and sometimes absent on satellite imagery. One or two ash plumes may have also been emitted. Accordingly, these observations caused authorities to raise and lower the Volcano Alert Level and Aviation Color Code (table 3). Volcano seismicity was absent because Cleveland lacks a real-time seismic network. The thermal anomalies and possible plumes could both could stem from steam emissions (see AVO statement at bottom of this report).

Table 3. Reports of activity at Cleveland based on satellite imagery during 10 June 2010 through 31 March 2011. Also shown are Volcano Alert Level and Aviation Color Code fluctuations based on that activity. Courtesy AVO.

On 12 September 2010, a possible ash plume was visible in satellite imagery; it rose to an estimated altitude of 7.6 km and drifted E. A 14 September image showed a dense white plume issuing from Cleveland (figure 9).

Figure 9. Image of a small, dense, compact white plume issuing from Cleveland at 1431 on 14 Sept 2010, captured by the GeoEye IKONOS satellite. In color versions of the image, red highlights areas of vegetation detected by the near-infrared channel. Photographed/created by Rick Wessels. Image processed by AVO/USGS; copyright 2010 by GeoEye. Courtesy AVO.

On 31 March 2011, AVO lowered the Volcano Alert Level and the Aviation Color Code to Unassigned, noting that no eruptive activity had been confirmed during the previous few months. No significant thermal anomalies or ash deposits on snow were observed in satellite imagery.

In its 31 March 2011 report, AVO stated that "Cleveland experiences frequent episodes of low-level unrest; the summit crater at Cleveland often emits visible plumes of water vapor and possibly small quantities of volcanic gas. Heat associated with this process can produce occasional weak thermal anomalies detected by satellite; however, these do not always indicate eruptive activity has occurred or is imminent."

AVO also stated, in an earlier report, that low-level ash emissions at Cleveland occur frequently and also do not necessarily mean that a larger eruption is imminent.

Geologic Background. The beautifully symmetrical Mount Cleveland stratovolcano is situated at the western end of the uninhabited Chuginadak Island. It lies SE across Carlisle Pass strait from Carlisle volcano and NE across Chuginadak Pass strait from Herbert volcano. Joined to the rest of Chuginadak Island by a low isthmus, Cleveland is the highest of the Islands of the Four Mountains group and is one of the most active of the Aleutian Islands. The native name, Chuginadak, refers to the Aleut goddess of fire, who was thought to reside on the volcano. Numerous large lava flows descend the steep-sided flanks. It is possible that some 18th-to-19th century eruptions attributed to Carlisle should be ascribed to Cleveland (Miller et al., 1998). In 1944 Cleveland produced the only known fatality from an Aleutian eruption. Recent eruptions have been characterized by short-lived explosive ash emissions, at times accompanied by lava fountaining and lava flows down the flanks.

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

A new eruptive fissure opened on W flank at ~2,800 m elevation on 13 May 2008 (BGVN 33:05)). Effusive eruptions there continued until 4 July 2009. There was some degassing at some of the summit craters degassed, while others were quiet. Figure 137 presents a map made in 2009 showing summit craters and the eruptive fissure. The following account was compiled from reports of the Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania (INGV-CT) surrounding events from 16 July 2008 through 10 November 2009.

Figure 137. Schematic map of the eruptive fissure at Etna that opened on 13 May 2008, as updated on 22 May 2009, showing the lava field that emanated from it. In colored versions of this Bulletin, the fissure is a thin blue line, the lava field appears in yellow, and red arrows show some of the near-source flow directions. Summit crater abbreviations (SEC, NEC, and BN) defined in text. From the 25-31 May 2009 report by INGV-CT's Marco Neri (with reference to report WKRVG20090526).

2008. On 15 July 2008 INGV-CT scientists inspected the summit craters at 2,800 m and found degassing from the Northeast Crater (NEC) and to a lesser degree from the Bocca Nuova (BN) crater BN-1. Eruptions issued from Vent 2 of the active NW-trending fissure located E of the summit craters. The activity consisted mainly of weak Strombolian and diffuse ash emissions.

During 15 and 17 July lava flows occurred in the Valle del Bove. During 11-17 August, there was less intense activity and reduced emissions. During 18-24 August NEC and, to lesser degree, one of two craters at the BN degassed. The other summit craters, many obstructed with eroded debris, degassed from the walls and fumaroles along fissures. Although the Southeast Crater (SEC) appeared obstructed with debris, it emitted both diffuse and occasionally intense fumarolic emissions from its walls and crater floor. During 25-31 August the eruptive activity at the fissure at 2,800 m elevation showed little change, with only weak degassing.

A lull in activity ended in mid-October. On 13-20 October, observers saw increased degassing on the NW flanks, including at NEC and BN-1. During 27 October-2 November, the NEC continued with intense degassing. The other summit craters, all obstructed by detritus, showed degassing diffused from the walls and localized fumarolic fields along fissures. The SEC showed diffused and occasionally intense fumarolic activity from its walls and the crater floor.

During 17-23 November, the fissure at 2,800 m continued to show modest effusive activity, producing a small lava flow along the high part of the western wall of the Valle del Bove. On 19 November the lava flow front had reached the elevation of ~2,500-2,600 m.

During 1-7 December the degassing at summit craters was particularly intense at NEC, while at SEC, fumarolic degassing was observed along the flanks of the cone and the crater rim. Observations on 5 December showed small sporadic ash emissions at the upper portion of the eruptive fissure at 2,800 m. Images recorded on 5 December near Mt. Zoccolaro revealed two lava flows trending parallel to the eruptive fissure to the W of the Valle del Bove. Between 29 December 2008 and 19 January 2009 weak degassing continued.

2009. During 19-25 January lava flows from the fissure at 2,800 m fanned out at elevations between ~2,600 m and ~2,450 m. During the same week, the SO₂ flux increased. During 26 January-2 February effusive activity at the eruptive vents along the W rim of the Valle del Bove continued in the lava field that has been active since May 2008. During 16-22 March eruptive activity continued along the high flanks of the volcano. At times observers saw intense degassing at the NEC and BN (figure 137).

During 6-12 April the level of activity remained constant and unchanged from the preceding time period.

During 18-31 May and 29 June-5 July 2009 the level of activity remained substantially unchanged, although in the earlier interval there were at least three lava flows, the foremost of which reached ~2,400 m elevation. The SO₂ fluxes increased and on 27 and 28 May became particularly elevated, to 8,000 and 6,000 metric tons per day (t/d), respectively. For the later interval, the SO₂ fluxes often remained more modest, ~2,900 t/d, with a maximum of ~3,500 t/d recorded 30 June. On 1, 3 and 5 July instruments measured higher peaks, to 7,000 t/d.

Although the explosive eruptive phases ceased in early July, ongoing degassing continued. Throughout August, the activity level remained unchanged, although roaring sounds emerged at SEC. Activity during 28 September-4 October showed little variance, but elevated SO₂ fluxes became elevated, with average values ranging between 1,500 and 4,500 t/d, with a peak on 4 October 2009 at 8,000 t/d.

A 10 November message from INGV's Sonia Calvari explained that the effusive fissure eruption that began on 13 May 2008 ended 4 July 2009. There was thereafter an absence of significant explosive activity at the summit craters for a few months before deep explosive activity resumed once again at SEC on 6 November. The INGV monitoring web cameras detected pulsating red glowing from SEC's eastern floor, venting within the depression that cuts its E flank. However, as late as 10 November, no ejecta were found on the summit's snow cover.

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania (INGV-CT), Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

Our previous report (BGVN 34:12) covered explosive eruptions at Galeras on 30 September 2009, 20 November 2009, and 2 January 2010 (included in table 11). This report discusses the recent eruption in August 2010 and intense degassing and seismic events between January 2011 and May 2011.

Table 11. Minimum volumes of erupted material from Galeras calculated for events from August 2004 to August 2010. Number of eruptions, Date, Local Time (hour and minute), and Calculated Minimum Volume in cubic meters. Adapted from INGEOMINAS (2010a).

The Instituto Colombiano de Geología y Minería (INGEOMINAS) use an alert scale from I-IV, a Level I being the highest. They declared a Level I alert during an eruption in August 2010 and a Level II during unrest in late January 2011. To date, Level III status has been maintained since 8 February 2011.

Eruption of 25 August 2010. On 20 August 2010, after several days of increased gas emissions, an earthquake swarm began. According to INGEOMINAS, seismicity remained high during 21-22 August. Five volcano-tectonic earthquakes were felt by local residents and caused windows to vibrate. The events were located within a 300-900 m radius of the crater, at depths of less than 2 km. The largest event was M 4.3.

On 23 August a M 4.6 earthquake struck E of Galeras at a depth of 2 km. The Alert Level was raised to II (Orange; "probable eruption in terms of days or weeks"). SO₂ emissions peaked at 304 tons/day on 23-24 August (table 12).

Table 12. Emissions of SO2 (metric tons/day) from Galeras during August 2010 measured by ScanDOAS and MovilDOAS (supported by Proyecto NOVAC). Low (under 500); Moderate (500-1,000); High (1,000-3,000); Very High (over 3,000). Courtesy of INGEOMINAS (2010a).

An eruption began at 0400 on 25 August, prompting INGEOMINAS to raise the Alert Level to I (Red; "imminent eruption or in progress"). Meteorological cloud cover initially prevented visual observations of the summit, although an eruption plume was seen among the clouds and thermal anomalies were detected by an infrared camera. At ~0700, an overflight of the flanks was videorecorded which documented a low-altitude gray plume distinctive from the atmospheric clouds (INGEOMINAS, 2010c). With a thermal camera, the Colombian Air Force documented that hot material fell from the secondary crater, El Paisita (INGEOMINAS, 2010b). Ashfall was reported to the NW, as far away as 30 km and quantified as over 37,033 m³ of material (table 11). Ash was reported in Samaniego, Linares, Ancuya, Sandoná, and Consacá. Observers in Pasto (~ 10 km E) reported that gas-and-ash plumes rose 300 m above the crater.

Seismicity associated with the 25 August eruption continued for a period of about 12 hours and gradually declined in the afternoon. INGEOMINAS lowered the Alert Level to II. According to news articles, at least 7,000 residents were ordered by government officials to evacuate, although few left their homes. During 26-31 August, at least 12 earthquakes, M 2-4, struck within a 2 km radius from the crater, at depths not more than 3 km. Gas plumes drifted NW, then S.

Seismic and thermal activity in 2011 through May. Overflights conducted by INGEOMINAS on 7 January 2011 collected multiple infrared and visible photopairs, one set of which appears as figure 113. The maximum temperature of the central crater reached 287.2°C. Temperatures of the rim and flank fumaroles were also recorded. A calculation was made for an area of three fumaroles close to the rim yielding a maximum temperature of 31.2°C.

Figure 113. A set of visible (left) and infrared (right) photo of both the crater and some flank fumaroles at Galeras taken during an INGEOMINAS overflight on 7 January 2011. The flank location is not disclosed. Taken from INGEOMINAS (2011b).

According to INGEOMINAS, on 25 January an emerging seismic pattern from Galeras, characterized by "tornillo-type" earthquakes, was similar to patterns detected before past eruptions. Tornillos are "monochromatic [narrow range of frequencies] long-period seismic events of a few minutes duration with long codas of constantly decreasing amplitude" (Morrissey and Mastin, 2000). The waveform of a tornillo is illustrated and described in more depth in BGVN27:05. The staff noted a strong sulfur gas odor and observed emissions that drifted N from various areas of the crater. Based on changes in seismicity and observed gas emissions, INGEOMINAS raised the Alert Level to II.

On 27 January 2011, scientists again observed emissions from various areas of the crater during an overflight (figure 114) and there was a slight increase in the number of vents. Gas plumes drifted NW and thermal imagery showed clearly-defined fumaroles. Imagery measured the maximum temperature of the central vent around 300°C (reported as 294.7°C).

Figure 114. A stillshot from video footage of degassing at Galeras taken during an INGEOMINAS overflight on 27 January 2011. Gas from the crater and fumaroles was blown in a diffuse plume moving approximately W. Look direction is W. Taken from INGEOMINAS (2011b).

On the morning of 30 January, tornillos ceased.

In early February 2011, seismic levels continued to fluctuate. On 6 February an overflight revealed that gas emissions had increased in comparison to the previous week, forming plumes that drifted NW; however INGEOMINAS lowered the Alert Level to III.

INGEOMINAS reported gas-and-steam emissions on 31 March and 1 April with low ash content. On 1 April, a M 2.3 earthquake occurred 3 km E of the crater at a depth of 6 km and was felt by nearby residents. During an overflight on 2 April, scientists noted a sulfur gas odor and observed that gas emissions rose from multiple areas of the active cone. During 30 March-5 April, SO₂ gas values were between 50 and 2,000 tons per day, the latter value considered high for Galeras.

As of 7 April 2011 there was a decrease in transient seismic signals. Within the first week of the month there were three tornillo events with oscillations around 7.5 Hz. After 7 April tornillos were no longer recorded and seismicity was dominated by an increase of events interpreted as the result of fluid motion within the volcanic system and gas emissions. The April 2011 INGEOMINAS monthly report concluded that hypocenters of earthquakes clustered in three distinct zones (figure 115).

Figure 115. A map and cross sections plotting 206 epicenters and hypocenters of volcano-tectonic and hybrid earthquakes that occurred during 1-30 April 2011 at Galeras. In the N-S section (right) and the E-W section (bottom), each line represents 1.7 km of depth with respect to the summit (elevation, ~4,200 m). Magnitudes of seismic events are indicated by circle size. Colors indicate depths, which ranged from under 1 km to over 10 km. Courtesy of INGEOMINAS (2011c).

One zone was a shallow (under1 km) area focused on the crater in the SE sector. A second source was located to the W of the crater with depths up to 2.5 km (with respect to the summit) and a deeper source was identified between 5 and 7.5 km to the E of the crater. The most distant events (up to 8 km) were dispersed with depths around 11 km. The largest of these, M 2.4, occurred at 0454 on 1 April.

Between 13 April-17 May 2011, steam rose up to 1.2 km in altitude and the values of SO₂ ranged from low to high with emission values reaching up to 1,600 tons per day. Residents in the city of San Juan de Pasto, just to the E, reported the foul odor of sulfur gases, mainly H₂S. On 18 April, an onsite INGEOMINAS team noted a strong odor of sulfur gases and emission from both the main crater and secondary craters and fumarolic fields. That same day a M 1.9 earthquake occurred 6 km SW of Galeras at a depth of ~7 km.

According to INGEOMINAS, favorable weather conditions during 11, 15-10, and 22 May allowed observers to note plumes with heights up to 700 m. On 15-16 May, heavy rains produced lahars that swept down Galeras'slopes carrying rocks, soil, and plant material into and down drainages.

With support from the Colombian Air Force, overflights of Galeras were conducted on 18, 20 and 22 May. Various rates of gas emissions were observed, mostly from vents, secondary craters, and cracks on the slopes of the active cone. Thermal anomalies were detected in various areas, with an average value of 170°C at the bottom of the main crater and 205°C in the secondary crater "The Paisita" N of the active cone.

References. INGEOMINAS, Instituto Colombiano de Geología y Minería, 2010a, Pasto Observatory Work Group: Monthly Report on Galeras and the Volcanoes of Doña Juana, Cumbal, and Azufral, August 2010 (URL: http://intranet.ingeominas.gov.co/pasto/images/1/1e/Boletin_mensual_de_actividad_de_los_volcanes_del_sur_agosto_2010.pdf).

INGEOMINAS, Instituto Colombiano de Geología y Minería, 2010b, Review of Activity from Galeras 24 Aug.-30 Aug., 2010, (URL: http://intranet.ingeominas.gov.co/pasto/Imagen:Resumen_actividad_galeras_ago_24_ago_30_2010.pdf).

INGEOMINAS, Instituto Colombiano de Geología y Minería, 2010c, Sobrevuelo volcán Galeras 8/25/2010, (URL: http://intranet.ingeominas.gov.co/pasto/Videos_2010).

INGEOMINAS, Instituto Colombiano de Geología y Minería, 2011a, Thermal Images 2011, 1/7/2011,

(URL: http://intranet.ingeominas.gov.co/pasto/Imágenes_térmicas_2011).

INGEOMINAS, Instituto Colombiano de Geología y Minería, 2011b, Sobrevuelo volcán Galeras 1/27/2011, (URL: http://intranet.ingeominas.gov.co/pasto/Videos_2011).

INGEOMINAS, Instituto Colombiano de Geología y Minería, 2011c, Pasto Observatory Work Group: Monthly Report on Galeras and the Volcanoes of Doña Juana, Cumbal, and Azufral, April 2011 (URL: http://intranet.ingeominas.gov.co/pasto/images/8/8b/Boletin_mensual_de_actividad_de_los_volcanes_del_sur_abril_2011.pdf).

Morrissey, M.M., and Mastin, L.G., 2000, Vulcanian Eruptions, in Sigurdsson, H., ed., Encyclopedia of Volcanoes: San Diego, California, Academic Press, p. 463-475.

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

Information Contacts: Instituto Colombiano de Geología y Minería (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Popayán, Popayán, Colombia.

The Vanuatu Geohazards Observatory (VGO) report of December 2010 noted seismicity activity and gas emissions during the period from September 2010 through 21 December 2010. This follows the more substantial emissions reported through 19 June 2010. The Ambrym report figure in BGVN 36:05 showing a 2 August 2010 satellite image of the region, includes a plume from Gaua visible for at least 80 km.

The geologic map of Vanuatu (figure 19), formerly called the New Hebrides islands, is centered ~2,200 km N off the New Zealand coast and ~2,100 km NE off the coast of Australia (figure 19). Gaua is sometimes referred to as residing on the island of Santa Maria. This island is also sometimes labeled Gaua, and volcano's topographic high is sometimes called Mont-Geret. The map locates the archepelago's major islands, volcanoes, igneous and metamorphic rocks. Most of the Pliocene and Quaternary islands have been formed by volcanic growth, with uplift in a few cases. Those islands containing older Tertiary rocks resulted from differential elevation of fault bounded blocks (Mitchel and Warden, 1971). Map revised from one on the VGO web site.

Figure 19. Geologic map of Vanuatu. Gaua island is labeled with its alternate name, Santa Maria (Gaua volcano is also called Mont-Geret). The volcano Ambae is situated on Aoba island; the volcano Ambrym, on Ambrym island; and the volcano Yasur, on Tanna island. The key shows basic igneous and metamorphic rock units. Those islands containing older Tertiary rocks have resulted from differential elevation of fault bounded blocks (Mitchel and Warden, 1971). Map revised from one on the VGO web site.

Based on VGO information, the Wellington VAAC reported that on 7 and 16-19 June 2010 an ash plume from Gaua rose to an altitude of ~3 km. On 19 June the plume drifted more than 90 km W but later plume dispersal and emissions were obscured on satellite imagery.

Late 2010 observations on Gaua indicated renewed growth of the vegetation near the volcano's vent and on the island's leeward W side. That area had suffered damage during April-May 2010 due to gas emissions (BGVN 35:05). These observations suggested diminished emissions from the volcano.

Since September 2010, seismic monitoring showed decreasing numbers of counts of volcano-related earthquakes (figure 20). The Alert Level of Gaua volcano was lowered to Level 1 in December 2010. No satellite thermal alerts were measured by MODVOLC during 6 April 2010 through late July 2011.

Figure 20. Gaua volcanic tremor counts recorded for the year 2010. REG adjacent to large pulse traces identify regional earthquakes and/or earthquakes close to Gaua unrelated to volcanic activity. Our previous report (BGVN 35:05) presented a similar plot through 22 April 2010. Courtesy of VGO.

Reference. Mitchel, AH and Warden, AJ, 1971, Geological evolution of the New Hebrides island arc, Journal of Geological Soc. of London, October 1971, 127, p. 501-529 (DOI: 10.1144/gsjgs.127.5.0501)

Geologic Background. The roughly 20-km-diameter Gaua Island, also known as Santa Maria, consists of a basaltic-to-andesitic stratovolcano with an 6 x 9 km wide summit caldera. Small parasitic vents near the caldera rim fed Pleistocene lava flows that reached the coast on several sides of the island; several littoral cones were formed where these lava flows reached the sea. Quiet collapse that formed the roughly 700-m-deep caldera was followed by extensive ash eruptions. Construction of the historically active cone of Mount Garat (Gharat) and other small cinder cones in the SW part of the caldera has left a crescent-shaped caldera lake. The symmetrical, flat-topped Mount Garat cone is topped by three pit craters. The onset of eruptive activity from a vent high on the SE flank in 1962 ended a long period of dormancy.

Information Contacts: Vanuatu Geohazards Observatory (VGO), Department of Geology, Mines and Water Resources (DGMWR), Vanuatu (URL: http://www.vmgd.gov.vu/vmgd/); MODIS/MODVOLC thermal alerts satellite system, Hawai'i Institute of Geophysics and Planetology (HIGP) 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/).

A VEI 4 (Volcanic Explosivity Index) eruption began at Merapi volcano on 26 October 2010. Within the last 100 years, this volcano had not produced such large-magnitude explosions (Surono and others, in review; Andreastuti and others, 2011). The eruption and secondary events affected areas in all directions around the volcano; pyroclastic flows reached 4 km to the N, 11.5 km to the W, 7 km to the E, and ~15 km to the S, and explosive bombs reached 4 km from the summit in all directions (Jousset, 2010). These events included explosive central vent eruptions that caused significant changes in the summit morphology (figure 48) and according to Act Forum Indonesia, triggered evacuations of communities within a 20 km radius of the summit. In BGVN 36:1/2 we reported on preliminary damage assessments that included significant fatalities and damaged infrastructure.

Figure 48. A photo comparison of Merapi's morphological changes on 23 March 2010 and 16 March 2011. Viewed from the S flank, these photos focus on the summit dome. Non-juvenile material was excavated during two episodes of explosive activity. During 26-29 October ~1.4 x 106 m3 was excavated from the crater and from 4-5 November ~10 x 106 m3 was removed (Surono and others, in review). Photo courtesy of the Volcano Technical Research Center (Balai Penyelidikan dan Pengembangan Teknologi Kegunungapian, "BPPK") from their report covering 14-20 March 2011.

The explosive events of 2010 represent a break in Merapi's iconic style of activity (Surono and others, in review). "Merapian" is a term often assigned to volcanic events characterized by hot pyroclastic block flows generated during the collapse of growing viscous lava domes (Schmincke, 2004). Standard eruptive activity at Merapi includes "continuous degassing and extrusion of andesitic lava domes whose collapses generate block avalanches and gravitational pyroclastic flows" (Allard and others, 2011).

At least 17 VEI = 2 events have occurred since the catastrophic 15 April 1872 eruption (Siebert and others, 2010). While explosive activity is characteristic of past behavior, assessments of data from 2010 confirm that the 26 October eruptive sequence did not begin with lava extrusion (typical of past eruptions). Instead, intense explosions initiated activity that lasted for ~5 weeks (Surono and others, in review).

During the Merapi special session at the EGU General Assembly held in April 2011, Andreastuti and others (2011) concluded that "the rate of magma extrusion [during the peak of Merapi's 2010 activity] was as much as 17-to 21-times higher [than] the 2006 eruption and the distance of pyroclastic flows in the same drainage (Gendol River) reached 15 km in 2010 and only 7 km in 2006."

This assessment and others (e.g. Alder and others, 2011) linked the highly explosive eruptions of October-November 2010 to elevated and variable gas emissions.

On 4 December 2010, after 40 days of maintaining the highest alert, the Indonesian Center of Volcanology and Geological Hazard Mitigation (CVGHM) downgraded the hazard Alert Level from 4 to 3 ("Awas," Red Alert to "Siaga," Watch). The Alert Level was reduced again on 19 January 2011 from 3 to 2 (to "Waspada," Advisory). The Alert Level remained at Level 2 into June 2011.

In this report we review the recovery efforts, Merapi's intermittent activity, and the long-term lahar crisis from March to June 2011. We also include a review of intervals of gas geochemistry data recorded prior to the 26 October 2010 disaster that recently became available.

Recovery efforts. Since October 2010, of the ~300,000 people evacuated, 11,000 were still displaced as of January 2011 (Jakarta Post and IRIN). Authorities had set up nine camps within the city of Yogyakarta and ~70 camps were located farther away within Central Java. On 2 May 2011 the head of Badan Nasional Penanggulangan Bencana (BNPB), Indonesia's National Disaster Management Agency, reported: "With almost all the displaced having moved to temporary shelters, our focus now is how to rebuild communities affected by the disaster" (IRIN, 2011).

In May 2011 the Indonesian government sought international aid (including the International Red Cross and United Nations) and international non-governmental organizations were working in Indonesia for relief efforts. The Jakarta Post reported on 12 May 2011 that Australia had agreed to help Indonesia establish a Disaster Relief Center for disaster management training; the location will be in Sentul, West Java and will serve members of the Association of Southeast Asian Nations (ten countries currently belong to ASEAN). BNPB had called upon the World Bank to begin a Risk Transfer scheme, allowing the local government to focus aid specifically on reconstruction programs.

According to reports from the Jakarta Globe in April 2011, the recent disaster and long history of volcanism at Merapi prompted the Indonesian government to implement an extensive recovery plan for the Yogyakarta province. They prioritized the development of spatial planning maps, expansion of the Merapi National Park, large-scale reforestation (approximately 1,300 hectares), and allocation of 1.35 trillion Rupiah ($155 million) to improve housing, infrastructure, social efforts, and economic stimulation plans. New mapping in the province will reassign land-use and designate relocation sites for former residents. In general, residential areas lying within 10 km of the summit will remain off limits (Sayudi and others, 2010). The Jakarta Post noted these maps also highlight where reforestation will occur. Impacts were substantial to Merapi National Park which lost up to 2,800 hectares out of 6,410 hectares of forest due to the recent eruptions. The Volcano Technical Research Center (BPPTK) reassessed zones in the Sleman region, the area hardest hit by volcanic activity, and will release a map indicating hazard zones. "[These maps] will show which areas are safe, unsafe and suitable for habitation," stated Sleman administration spokeswoman Endah Sri Widiastuti (Jakarta Post).

A controversial location within the 10 km exclusion zone is the village of Kinahrejo, the former home of spiritual leader Mbah Maridjan, called the guardian of Merapi. Working with a team of 17 respected community members, he preserved traditional ceremonies and local culture for Merapi residents. Pyroclastic flows covered the village on 26 October 2010, taking the life of the guardian and other inhabitants who did not evacuate.

The new guardian is Mbah Maridjian's son, Asihono (his new name: Mas Lurah Suraksosihono). During Merapi's disastrous eruptions of October and November, Asihono cooperated with the local government and agencies including the Volcanology and Geological Disaster Mitigation Agency (PVMBG) and BPPTK. On 4 April 2011 Sultan Hamengku Buwono X elected Asihono from a group of eight candidates. In an interview with Jakarta Globe on 5 April 2011, the new guardian explained: "I'm not just going to take a cultural approach based on the dreams or guidance from the spirits, but I will also coordinate with the authorities to protect human life and the environment on Mount Merapi and anticipate the fall of victims to future eruptions."

New dome growth. Seismicity was variable and intermittent explosions were observed at Merapi at least every month through June 2011 since the main eruptive events of October and November 2010. This activity kept local residents vigilant and caused some alarm when incandescence suddenly appeared on Merapi's summit on 25 March and 13 April (figure 49). On these two occasions, a bright glow on the crater's E side was recorded on closed circuit television (CCTV).

Figure 49. Bright incandescence visible on the E side of Merapi's crater was observed at 1940 on 25 March 2011. Courtesy of Volcano Technical Research Center (BPPTK Activity Report 21-27 March 2011).

The point of incandescence was a location of concentrated degassing. In the aftermath of the eruption in 2010, fumaroles became well established and BPPTK intends to resume gas monitoring. They reported that a new dome was growing in the crater: "The final phase is usually marked by eruption of lava dome growth. However, we won't lower the [alert] status as long as the condition of Merapi is still volatile," reported Subandriyo of BPPTK on 11 April 2011 (Kompas News). Since 19 January 2011, the Alert Level was at 2, Advisory.

Gas monitoring. From ultraviolet correlation spectrometer (COSPEC) measurements, BPPTK reported continuous SO₂ emissions for both 1992 through early 2009 (BPPTK, 2011b) and January 2005-January 2010 (figure 50). Other data resulted from sampling with Giggenbach bottles; a method of condensate retrieval requiring evacuated alkaline-solution-filled bottles (Williams-Jones and Rymer, 2000). Gas species such as CO₂, SO₂, H₂S, and HCL were analyzed during June 2003-June 2010 (figure 51).

Figure 50. Merapi SO2 fluxes measured from January 2005 to roughly April 2009 using COSPEC (sulfur dioxide in metric tons per day). The curve shown displays an undisclosed averaging function on the data. Modified from BPPTK, 2011a.

Figure 51. Gas sampling at Merapi's Woro Crater (a location map is posted in BGVN 32:02) conducted during June 2003-January 2011. Results from Giggenbach bottle collection and lab analysis for gas species are plotted on log scales. Right-hand vertical axis corresponds to upper (blue) data and trendlines. Left-hand vertical axis corresponds to the lower (black) data trendlines and data. Original concentration units were undisclosed (but Bulletin editors hope to clarify these units in later discussions on Merapi). Modified from BPPTK, 2011a.

SO₂ ranged from ~75 metric tons/day (t/d) to ~285 t/d and appeared to peak mid-year in 2005 and 2006 (figure 50). A sudden decrease of 50 t/d in January 2007 preceded an increasing trend that ended in mid-2008. These fluxes also had fewer sustained peaks around March 2008 and declined until the available record ends around March 2009.

The SO₂ peak of ~200 t/d generally correlated with the 2005 mid-year episode of elevated seismicity that prompted the BPPTK (at that time called the Directorate of Volcanology and Geological Hazard Mitigation, "DVGHM") to raise the Alert Level from Normal to Advisory (from 1 to 2). However, there were no additional reports of plumes or increased dome activity then (BGVN 32:02).

In 2006, the Alert Level was raised to the highest level on 13 May due to intense dome growth and earthquake activity (BGVN 31:05), a time when SO₂ reached ~225 t/d.

According to information recorded in Bulletin reports, the abrupt decrease of SO₂ in late 2006-early 2007 did not appear to correlate with significant volcanism in that time interval. The gradual increasing-and-decreasing trend in SO₂ flux from 2007 until the end of the record was marked by rare ash plumes (e.g. 19 March 2007, 9 Aug 2007, and 19 May 2008), and modest dome growth (BGVN 32:02). Bulletin reports also noted incandescence and ashfall had continued during 23 May-29 May 2007. MODVOLC thermal anomalies became rare after 5 September 2006 (BGVN 33:10).

Intermittent activity during 18 April-1 May 2011. Unrest at Merapi since the 2010 crisis was characterized by intermittent increases in seismicity as observed from 18 to 24 April 2011 (figure 52). Over the course of that week, rockfall signals doubled from the previous observation period and 39 multiphase events were recorded.

Figure 52. Histograms from September 2010 to June 2011 summarize the number of seismic events from four categories is summarized for Merapi. Events shown are rockfalls; MP, multiphase (shallow source, dominant frequency ~1.5 Hz); VA, deep volcano-tectonic earthquakes, 2.5-5 km below the summit; VB, shallow volcano-tectonic earthquakes, less than ~1.5 km below the summit. Tremor episodes were infrequent and thus excluded. Courtesy of BPPTK (Activity Report 6-12 June 2011). (A figure in BGVN 36:1/2 presented representative samples of these various waveforms.)

BPPTK also reported that ground deformation was variable throughout this time period as EDM (Electronic Distance Meter) measurements were recorded across the summit. Measurements made on 18 April 2011 compared with those recorded on 25 April 2011 from the monitoring post of Selo showed the following changes: a difference in distance amounting to +8 mm (R1) and a change in movement amounting to 0.1 mm per day.

Measurements carried out on 18 April 2011 compared to those of 24 April 2011 from Jrakah monitoring post indicated the following changes: a difference in distance amounting to -4 mm (R1) with a change in movement amounting to 0.5 mm per day, and a difference in distance of +6 mm (R2) with a movement of 0.7 mm per day.

Plumes of ash and gas reached an altitude of ~800 m on 24 and 25 April. Communities near Merapi's flanks reported ashfall on 29 April, 30 April, and 1 May 2011. (BPPTK Activity Report 25 April-1 May 2011).

Ongoing hazards. The recent weekly report by BPPTK (20 March to 12 June 2011), described plumes of gas and ash that occurred regularly. As measured from above the summit, the average height of these plumes was ~500 m; a maximum height of 900 m was recorded on 20 April. The tallest plume was accompanied by a ramping up of earthquakes and the regular occurrence of lahars, some hot enough to steam while racing through river drainages (figure 53).

Figure 53. On 21 March 2011 and 14 April 2011 steaming lahars descended Merapi's flanks. This photo was taken on 14 April 2011from a CCTV camera installed in Pencar village, less than 20 km S of Merapi in the Desa Bimomartani region. Note Merapi volcano in upper right-hand area of the photo. Courtesy of BPPTK (Activity Report discussing 11-17 April 2011).

A large amount of volcanic ash fell from Merapi's explosive eruptions in 2010; this has aggravated slope stability and led to increased lahar hazards. In an interview on 11 April 2011 for Kompas News, Subandriyo, the Head of the BPPTK explained that "only about 30 percent" of the material that fell on Merapi's flanks has been remobilized by erosion. "Therefore, the threat of [lahars] will occur two to three years ahead."

As of June 2011, 15 major lahars had occurred since November 2010. The worst occurred on 23 January 2011 along the eroded banks of the Putih river. The major highway between Magelang and Yogyakarta was cut off when a 60 m wide section of blacktop was torn away by torrential mudflows. As a result, hundreds of homes within 12 different villages near the river were inundated forcing 5,000 people to flee. There were three fatalities.

Major infrastructure was also affected; 52 levees were damaged and 14 bridges were destroyed. Intense lahar damage was also reported along the SE rivers: Blongkeng, Batang, Progo, Code, and Gendol.

References. Allard, P., Métrich, N., and Sabroux, J.-C., 2011, Volatile and magma supply to standard eruptive 549 activity at Merapi volcano, Indonesia. EGU General Assembly 2011, Geophysical 550 Research Abstracts 13, EGU 2011-13522 (2011).

Andreastuti, S., Costa, F., Pallister, J.,Sumarti., S., Subandini, S., Heriwaseso, A., Kurniadi, Y. , Petrology and pre-eruptive conditions of the 2010 Merapi magma. EGU General Assembly 2011, Geophysical 550 Research Abstracts 13, EGU2011-5150 (2011).

BPPTK, Volcano Technical Research Center, 2011a, Geochemistry of Merapi. (URL: http://www.merapi.bgl.esdm.go.id/aktivitas_merapi.php?page=aktivitas-merapi&subpage=geokimia)BPPTK, Volcano Technical Research Center, 2011b, Monitoring of Geochemical and Temperature of Merapi. (URL: http://www.merapi.bgl.esdm.go.id/pages.php?page=geokimia-dan-suhu)

Schmincke, H.-U, 2004, Volcanism, Berlin:Springer, 324 pp.

Jousset, P., 12/6/10, Centennial Eruption at Merapi volcano: October/November 2010, MIAVITA, European Commission. (URL: http://miavita.brgm.fr/Documents/MIAVITA-Merapi-eruption.pdf)

Sayudi, D.S., Nurnaning, A., Juliani, DJ., Muzani, M.; 2010, "Peta Kawasan Rawan Bencana Gunungapi Merapi, Jawa Tengah Dan Daerah Istimewa Yogyakarta 2010," (The map of the Rawan Bencana Gunungapi Merapi Region, Central Java: Yogyakarta Special District 2010), Volcano Technical Research Center (Balai Penyelidikan dan Pengembangan Teknologi Kegunungapian, "BPPTK"). (URL: http://www.merapi.bgl.esdm.go.id/peta/2011/04/KRBGMerapi2010FINALcopyright_78a74b.jpg)

Siebert L., Simkin T., and Kimberly P., 2010, Volcanoes of the World, 3rd edition, University of California Press, Berkeley, 558 p.

Surono, Jousset, P., Pallister, J., Boichu, M., Buongiorno, M.F., Budisantoso, A., Costa, F., Andreastuti, S., Prata, F., Schneider, D., Clarisse, L., Humaida, H., Sumarti, S., Bignami, C., Griswold, J., Carn, S., Oppenheimer, C., (in review), 100-year explosive eruption of Java's Merapi volcano, Journal of Volcanology and Geothermal Research.

Williams-Jones, G. and Rymer, H., 2000, Hazards of Volcanic Gases, in Sigurdsson, H., ed., Encyclopedia of Volcanoes: San Diego, California, Academic Press, p. 997-1004.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequently growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent eruptive activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities during historical time.

Information Contacts: Volcano Technical Research Center (Balai Penyelidikan dan Pengembangan Teknologi Kegunungapian, "BPPTK") (URL: http://www.merapi.bgl.esdm.go.id/index.php); Badan Nasional Penanggulangan Bencana (BNPB- Indonesian National Disaster Management Agency) (URL: http://dibi.bnpb.go.id); Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); IRIN News (URL: http://www.IRINnews.org); Jakarta Globe (URL: http://www.thejakartaglobe.com); The Jakarta Post (URL: http://www.thejakartapost.com); KompasNews, Jakarta, Indonesia (URL: http://www.Kompas.com); Mitigate and Assess risk from Volcanic Impact on Terrain and human Activities project (MIAVITA) (URL: http://miavita.brgm.fr/default.aspx); Act Forum Indonesia (URL: http://www.actalliance.org/); Relief Web (URL: https://reliefweb.int/).

This report discusses Ulawun's ongoing mild seismicity, variably colored, though often white plumes, and other observations during May 2010 to late May 2011. The bulk of the reporting came from the Rabaul Volcano Observatory (RVO) with some information on plumes from the Darwin Volcanic Ash Advisory Center (VAAC) and others. Seismicity at Ulawun has generally been low since 2007, with occasional modest increases and steam emission (BGVN 33:03, 34:10, and 35:02).

Activity during 2010. RVO reported that, according to Real-time Seismic Amplitude Measurements (RSAM), seismic activity increased on 18 May 2010. According to RVO, white vapor emitted during 1-20 May 2010 became thicker during 22-28 May. On 22 and 25 May, some plumes were partially gray. According to the Darwin VAAC, plumes on 22-28 May reached an altitude of 3 km and extended as far as 70 NM in variable directions.

People on the S and SE sides of the island heard "low jetting" noises during 24-25 May 2010. Weak and fluctuating incandescence was seen from the S at night during 28-29 May. Emissions became gray in color during 29-31 May, and on 30 May very fine ashfall was reported in areas to the SSW, S, and SSE. On 1 and 2 June only white vapor emissions were noted. RVO recommended a Stage 1 Alert as a result of increasing seismicity and occasional gray plumes, incandescence, and audible noises. According to a news report (Radio Australia), up to 10,000 residents live adjacent to Ulawun.

According to RVO, during 2-12, 16-19, and 23-25 June 2010 residents heard occasional low roaring or rumbling noises daily on the ESE, SE, S, and NW flanks. During 2-19 and 23-26 June, white to gray-brown plumes rose to ~3 km altitude (figure 15). The Darwin VAAC noted that between 3-6 June, the plumes extended up to 315 km W. On 16-17 and 19-20 June, white and gray plumes rose 1 km above the summit. Very fine ash particles fell in Ulamona (~10 km NW) on 3 and 8 June, and then fell daily during 9-19 and 23-25 June on the NW, W, and SW flanks. Throughout June, fluctuating incandescence from the summit crater was seen at night from the S, SW, N, and SE flanks. RVO reported that on 18 and 19 June, seismicity increased to a high level and was dominated by volcanic tremor. Seismicity declined to moderate levels on 20 June and, based on RSAM values, declined further on 26 June 2010.

Figure 15. Natural-color image of a small white plume venting from Ulawun's summit crater and blowing W on 10 June 2010. Image was taken by the Advanced Land Imager on NASA's Earth Observing-1 (EO-1) satellite. Green vegetation predominates outboard of 1-2 km from the summit. On the upper slopes, bare (unvegetated) volcanic rocks prevail, appearing as charcoal-brown streaks. The plume's pale color suggests that, of the visible components in the plume, steam rather than ash predominates. NASA Earth Observatory image created by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team and the United States Geological Survey. Original (but now slightly revised) caption crafted by Michon Scott.

According to RVO, white-to-gray plumes rose less than 500 m from Ulawun during 27 June-9 July 2010, and fine ash fell in areas to the SW, W, and NW. The Darwin VAAC reported that during 1-5 July, ash plumes drifted 55-195 km at an altitude of 3 km. On 28 June and during 5-6 July the volcano omitted occasional roaring noises. A slight increase in seismicity (above moderate levels) took place during 5-8 July.

RVO reported diffuse gray plumes that rose 200-500 m above Ulawun during 16-21 July 2010. Plumes were white to light-brown during 21-29 July. During 6-24 August, white and gray-to-brown plumes rose no more than 300 m above Ulawun, and fine ash fell on the NW and W flanks. Tremor continued, but overall seismicity declined slightly. RSAM values remained at a moderate level.

Based on analyses of satellite imagery and information from RVO, the Darwin VAAC reported that on 26 November 2010 an ash plume from Ulawun rose to an altitude of 3.7 km and drifted 55 km NE.

Several videos of Ulawun's plumes as posted on the web in 2010 showed them as white in color (Sabretoothed69, 2010). Other brief video by the same author take viewers to the Ulawun seismic station and its drum recorder, and to witness aspects of local culture such as villagers dancing.

Activity during 2011. According to RVO, the mild activity that began in May 2010 continued during 1 January-28 February 2011. The activity was characterized by brown-to-gray ash plumes that rose less than 500 m and produced fine ashfall to the SE. Sulfur-dioxide plumes drifted SE on 5 and 31 January. During 23-26 February, gray ash plumes occasionally drifted NE, SW, and NW.

RVO reported that during 1-9 May 2011, diffuse white plumes rose from Ulawun and low to modest RSAM values occurred (70-100 units). During 9-10 May, RSAM values distinctly increased, fluctuated, and peaked at 1,300 units before declining back to 100 units. During this time, local residents heard booming.

During 10, 13-14, 17, and 19-27 May, RVO reported gray-to-brown ash plumes rose above Ulawun's summit crater. On 17 May, emissions became briefly forceful and booming noises were reported. Light ashfall deposited between Ubili and Ulamona to the NW and Voluvolu to the NE, as well as on the NW and W flanks. Weak, fluctuating incandescence was observed on 22 May.

Reference. Sabretoothed69, 2010 (uploaded on 7 November 2010), YouTube (URL: http://www.youtube.com/watch?v=UnCSeky3Mes, uploaded by sabretoothed69)

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: Rabaul Volcano Observatory (RVO), PO 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/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Radio Australia (URL: http://www.radioAustralia.net.au/pacbeat/).

On 12 May 2011, Yasur's crater (figure 42), which has undergone near-continuous eruption for over 200 (possibly 800) years, emitted persistent strong explosions that could be heard and felt by nearby residents. Satellite images (OMI and MODIS) and seismic data collected from the volcano's monitoring station also confirmed strong degassing and stronger than typical explosive activity since the beginning of May 2011. The volcano sits on Tanna Island in the island nation called the Republic of Vanuatu (formerly New Hebrides; ~2,200 km N off of New Zealand's coast and ~2,100 km NE off of Australia's coast). As seen on the map in the Gaua report in this issue (BGVN 36:05), Tanna island lies near the S end of the Republic. Figure 42 presents information about Vanuatu's tectonic setting and Yasur volcano's location and shape.

Figure 42. (a) Map showing the Vanuatu arc in the SW Pacific, the position of the 6-7 km deep Vanuatu trench, convergence rates (indicated by arrows in cm/yr), and the location of Tanna volcanic island, which rises from a 1-km-deep plateau (redrawn from Pelletier and others, 1998 and Calmant and others, 2003). (b) Schematic map of Tanna island with the locations of the main volcanic centers (redrawn from Carney and MacFarlane, 1979). In its long dimension (N-S), Tanna island stretches ~40 km; its width is ~19 km. (c) Map of southeastern Tanna showing the Yasur crater and Yenkahe horst, bounded by the Siwi ring fracture (redrawn from Nairn and others, 1988 and Allen, 2005). From Métrich and others (2011).

An assessment conducted by the Vanuatu Geohazards Observatory (VGO) during 30 May-3 June 2011 found Yasur's crater in a state of high activity with strong explosions and bomb emissions from all of the three active vents. Fresh volcanic bombs fell around the crater rim, and some reached ~500 m S to the parking area, landing there about once per minute. Residents heard and viewed explosions from their villages.

Observations and assessments made by VGO during 11-12 June 2011 indicated a decrease in eruptive vigor and a return to more typical conditions. Explosions became both slightly weaker and less frequent. Constant Strombolian activity with occasional ejections of lava bombs still occured around the volcano.

On 7-8 July 2011 the VGO reported that Yasur volcano was at a high level of activity with strong degassing and ash emissions from all three active vents. The ash falls were mostly over the W part of the island. Fresh volcanic bombs had fallen around the crater rim. Some explosions could be heard and viewed from the villages, a pattern locals had noticed since the beginning of the year.

Hazard terminology and levels. The operative hazards scale for Yasur spans from 0 to 4, with larger values indicating greater hazards. It is called the VVAL (Vanuatu Volcano Alert Level).

Level 2 is defined as "Moderate eruptions, danger close to the volcano vent, within parts of Volcanic Hazards Map Red Zone" (see map in BGVN 35:04).

Level 3 is defined as "Large eruption, danger in specific areas within parts of Volcanic Hazards Map Red and Yellow Zones."

In the early phases of the upsurge in vigor, the VVAL for Yasur remained at Level 2 with the note that the risk area for volcanic projectiles remained in areas near the volcano crater and vicinity.

Associated with the assessed late-May to early June behavior, the VVAL stepped up to Level 3. A zone surrounding the summit became strictly prohibited (see visitor's map, BGVN 35:04).

Associated with the 11-12 June 2011 observations of decreasing vigor, the VVAL dropped to Level 2.

Background. Métrich and others (2011) point out that Siwi caldera is a volcanic complex containing both persistent eruptive activity of basaltic-trachyandesite composition (Yasur volcano) and rapid block resurgence (Yenkahe horst). They note that available data suggested that Yasur volcano releases, on average, over 134 x 10³ tons/day of H₂O and 680 tons/day of SO₂. Measurements also indicated other gas fluxes: 840 tons/day of CO₂, 165 tons/day of HCl, and 23 tons/day of HF.

References. Allen, S.R., 2005, Complex spatter- and pumice-rich pyroclastic deposits from an andesitic caldera forming eruption: The Siwi pyroclastic sequence, Tanna, Vanuatu, Bulletin of Volcanology, v. 67, pp. 27-41.

Calmant, S., Pelletier, B., Lebellegard, P., Bevis, M., Taylor, F.W., and Phillips, D.A., 2003, New insights on the tectonics along the New Hebrides subduction zone based on GPS results, Journal of Geophysical Research, v. 108, no. B6, pp. 2319-2339.

Carnay, JN., and MacFarlane, A, 1979, Geology of Tanna, Aneityum, Futuna and Aniva, New Hebrides Geological Survey Report 1979, pp. 5-29.

Métrich, N., Allard, P., Aiuppa, A., Bani, P., Bertagnini, A., Shinohara, H., Parello, F., Di Muro, A., Garaebiti, E., Belhadj, O., and Massare, D., 2011, Magma and Volatile Supply to Post-collapse Volcanism and Block Resurgence in Siwi Caldera (Tanna Island, Vanuatu Arc), Journal of Petrology, v. 52, no. 6, pp. 1077-1105; DOI: 10.1093/petrology/egr019.

Nairn, I.A., Scott, B.J., and Giggenbach, W.F., Yasur volcanic investigations, Vanuatu September 1988, New Zealand Geological Survey Report 1988, pp.1-74.

Pelletier, B., Calmant, S., and Pillet, R., 1998, Current tectonic of the Tonga-New Hebrides region, Earth and Planetary Science Letters, v. 164, pp. 263-276.

Geologic Background. Yasur, the best-known and most frequently visited of the Vanuatu volcanoes, has been in more-or-less continuous Strombolian and Vulcanian activity since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island, this mostly unvegetated pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide, horseshoe-shaped caldera associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

Information Contacts: Vanuatu Geohazards Observatory, Department of Geology, Mines and Water Resources of Vanuatu (URL: http://www.geohazards.gov.vu); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); MODIS/MODVOLC Thermal Alerts System, Hawai'i Institute of Geophysics and Planetology (HIGP), 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/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://vaac.metservice.com/).

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/).