Sangeang Api is located in the eastern Sunda-Banda Arc in Indonesia, forming a small island in the Flores Strait, north of the eastern side of West Nusa Tenggara. It has been frequently active in recent times with documented eruptions spanning back to 1512. The edifice has two peaks – the active Doro Api cone and the inactive Doro Mantori within an older caldera (figure 37). The current activity is focused at the summit of the cone within a horseshoe-shaped crater at the summit of Doro Api. This bulletin summarizes activity during May 2019 through January 2020 and is based on Darwin Volcanic Ash Advisory Center (VAAC) reports, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, or CVGHM) MAGMA Indonesia Volcano Observatory Notice for Aviation (VONA) reports, and various satellite data.

Figure 37. A PlanetScope satellite image of Sangeang Api with the active Doro Api and the inactive Doro Mantori cones indicated, and the channel SE of the active area that contains recent lava flows and other deposits. December 2019 monthly mosaic copyright of Planet Labs 2019.

Thermal anomalies were visible in Sentinel-2 satellite thermal images on 4 and 5 May with some ash and gas emission visible; bright pixels from the summit of the active cone extended to the SE towards the end of the month, indicating an active lava flow (figure 38). Multiple small emissions with increasing ash content reached 1.2-2.1 km altitude on 17 June. The emissions drifted W and WNW, and a thermal anomaly was also visible. On the 27th ash plumes rose to 2.1 km and drifted NW and the thermal anomaly persisted. One ash plume reached 2.4 km and drifted NW on the 29th, and steam emissions were ongoing. Satellite images showed two active lava flows in June, an upper and a lower flow, with several lobes descending the same channel and with lateral levees visible in satellite imagery (figure 39). The lava extrusion appeared to have ceased by late June with lower temperatures detected in Sentinel-2 thermal data.

Figure 38. Sentinel-2 satellite thermal images of Sangeang Api on 20 May and 9 June 2019 show an active lava flow from the summit, traveling to the SE. False color (urban) image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 39. PlanetScope satellite images of Sangeang Api show new lava flows during June and July, with white arrows indicating the flow fronts. Copyright Planet Labs 2019.

During 4-5 July the Darwin VAAC reported ash plumes reaching 2.1-2.3 km altitude and drifting SW and W. Activity continued during 6-9 July with plumes up to 4.6 km drifting N, NW, and SW. Thermal anomalies were noted on the 4th and 8th. Plumes rose to 2.1-3 km during 10-16th, and to a maximum altitude of 4.6 km during 17-18 and 20-22. Similar activity was reported during 24-30 July with plumes reaching 2.4-3 km and dispersing NW, W, and SW. The upper lava flow had increased in length since 15 June (see figure 39).

During 31 July through 3 September ash plumes continued to reach 2.4-3 km altitude and disperse in multiple directions. Similar activity was reported throughout September. Thermal anomalies also persisted through July-September, with evidence of hot avalanches in Sentinel-2 thermal satellite imagery on 23 August, and 9, 12, 22, and 27 September. Thermal anomalies suggested hot avalanches or lava flows during October (figure 40). During 26-28 October short-lived ash plumes were reported to 2.1-2.7 km above sea level and dissipated to the NW, WNW, and W. Short-lived explosions produced ash plumes up to 2.7-3.5 km altitude were noted during 30-31 October and 3-4 November 2019.

Figure 40. Sentinel-2 satellite thermal images of Sangeang Api on 7 and 22 October 2019 show an area of elevated temperatures trending from the summit of the active cone down the SE flank. False color (urban) image rendering (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Discrete explosions produced ash plumes up to 2.7-3.5 km altitude during 3-4 November, and during the 6-12th the Darwin VAAC reported short-lived ash emissions reaching 3 km altitude. Thermal anomalies were visible in satellite images during 6-8 November. A VONA was released on 14 November for an ash plume that reached about 2 km altitude and dispersed to the west. During 14-19 November the Darwin VAAC reported short-lived ash plumes reaching 2.4 km that drifted NW and W. Additional ash plumes were observed reaching a maximum altitude of 2.4 km during 20-26 November. Thermal anomalies were detected during the 18-19th, and on the 27th.

Ash plumes were recorded reaching 2.4 km during 4-5, 7-9, 11-13, and 17-19 December, and up to 3 km during 25-28 December. There were no reports of activity in early to mid-January 2020 until the Darwin VAAC reported ash reaching 3 km on 23 January. A webcam image on 15 January showed a gas plume originating from the summit. Several fires were visible on the flanks during May 2019 through January 2020, and this is seen in the MIROVA log thermal plot with the thermal anomalies greater than 5 km away from the crater (figure 41).

Figure 41. MIROVA log plot of radiative power indicates the persistent activity at Sangeang Api during April 2019 through March 2020. There was a slight decline in September-October 2019 and again in February 2020. Courtesy of MIROVA.

Geologic Background. Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large trachybasaltic-to-tranchyandesitic volcanic cones, Doro Api and Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent historical eruptions have been recorded since 1512, most of them during in the 20th century.

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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/).

Shishaldin is located near the center of Unimak Island in Alaska and has been frequently active in recent times. Activity includes steam plumes, ash plumes, lava flows, lava fountaining, pyroclastic flows, and lahars. The current eruption phase began on 23 July 2019 and through September included lava fountaining, explosions, and a lava lake in the summit crater. Continuing activity during October 2019 through January 2020 is described in this report based largely on Alaska Volcano Observatory (AVO) reports, photographs, and satellite data.

Minor steam emissions were observed on 30 September 2019, but no activity was observed through the following week. Activity at that time was slightly above background levels with the Volcano Alert Level at Advisory and the Aviation Color Code at Yellow (figure 17). In the first few days of October weak tremor continued but no eruptive activity was observed. Weakly elevated temperatures were noted in clear satellite images during 4-9 October and weak tremor continued. Elevated temperatures were recorded again on the 14th with low-level tremor.

Figure 17. Alaska Volcano Observatory hazard status definitions for Aviation Color Codes and Volcanic Activity Alert Levels used for Shishaldin and other volcanoes in Alaska. Courtesy of AVO.

New lava extrusion was observed on 13 October, prompting AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch. Elevated surface temperatures were detected by satellite during the 13th and 17-20th, and a steam plume was observed on the 19th. A change from small explosions to continuous tremor that morning suggested a change in eruptive behavior. Low-level Strombolian activity was observed during 21-22 October, accompanied by a persistent steam plume. Lava had filled the crater by the 23rd and began to overflow at two places. One lava flow to the north reached a distance of 200 m on the 24th and melted snow to form a 2.9-km-long lahar down the N flank. The second smaller lava flow resulted in a 1-km-long lahar down the NE flank. Additional snowmelt was produced by spatter accumulating around the crater rim. By 25 October the northern flow reached 800 m, there was minor explosive activity with periodic lava fountaining, and lahar deposits reached 3 km to the NW with shorter lahars to the N and E (figure 18). Trace amounts of ashfall extended at least 8.5 km SE. There was a pause in activity on the 29th, but beginning at 1839 on the 31st seismic and infrasound monitoring detected multiple small explosions.

Figure 18. PlanetScope satellite images of Shishaldin on 3 and 29 October 2019 show the summit crater and N flank before and after emplacement of lava flows, lahars, and ashfall. Copyright PlanetLabs 2019.

Elevated activity continued through November with multiple lava flows on the northern flanks (figure 19). By 1 November the two lava flows had stalled after extending 1.8 km down the NW flank. Lahars had reached at least 4 km NW and trace amounts of ash were deposited on the north flank. Elevated seismicity on 2 November indicated that lava was likely flowing beyond the summit crater, supported by a local pilot observation. The next day an active lava flow moved 400 m down the NW flank while a smaller flow was active SE of the summit. Minor explosive activity and/or lava fountaining at the summit was indicated by incandescence during the night. Small explosions were recorded in seismic and infrasound data. On 5 November the longer lava flow had developed two lobes, reaching 1 km in length. The lahars had also increased in length, reaching 2 km on the N and S flanks. Incandescence continued and hot spatter was accumulating around the summit vent. Activity continued, other than a 10-hour pause on 4-5 November, and another pause on the 7th. The lava flow length had reached 1.3 km on the 8th and lahar deposits reached 5 km.

Figure 19. Sentinel-2 thermal satellite images show multiple lava flows (orange) on the upper northern flanks of Shishaldin between 1 November and 1 December 2019. Blue is snow and ice in these images, and partial cloud cover is visible in all of them. Sentinel-2 Urban rendering (bands 21, 11, 4) courtesy of Sentinel Hub Playground.

After variable levels of activity for a few days, there was a significant increase on 10-11 November with lava fountaining through the evening and night. This was accompanied by minor to moderate ash emissions up to around 3.7 km altitude and drifting northwards, and a significant increase in seismicity. Activity decreased again during the 11-12th while minor steam and ash emissions continued. On 14 November minor ash plumes were visible on the flanks, likely caused by the collapse of accumulated spatter. By 15 November a large network of debris flows consisting of snowmelt and fresh deposits extended 5.5 km NE and the collapse of spatter mounds continued. Ashfall from ash plumes reaching as high as 3.7 km altitude produced thin deposits to the NE, S, and SE. Activity paused during the 17-18th and resumed again on the 19th; intermittent clear views showed either a lava flow or lahar descending the SE flank. Activity sharply declined at 0340 on the 20th.

Seismicity began increasing again on 24 November and small explosions were detected on the 23rd. A small collapse of spatter that had accumulated at the summit occurred at 2330 on the 24th, producing a pyroclastic flow that reached 3 km in length down the NW flank. A new lava flow had also reached several hundred meters down the same flank. Variable but elevated activity continued over 27 November into early December, with a 1.5-km-long lava flow observed in satellite imagery acquired on the 1st. On 5 December minor steam or ash emissions were observed at the summit and on the north flank, and Strombolian explosions were detected. Activity from that day produced fresh ash deposits on the northern side of the volcano and a new lava flow extended 1.4 km down the NW flank. Three small explosions were detected on the 11th.

At 0710 on 12 December a 3-minute-long explosion produced an ash plume up to 6-7.6 km altitude that dispersed predominantly towards the W to NW and three lightning strokes were detected. Ash samples were collected on the SE flank by AVO field crews on 20 December and analysis showed variable crystal contents in a glassy matrix (figure 20). A new ash deposit was emplaced out to 10 km SE, and a 3.5-km-long pyroclastic flow had been emplaced to the north, containing blocks as large as 3 m in diameter. The pyroclastic flow was likely a result from collapse of the summit spatter cone and lava flows. A new narrow lava flow had reached 3 km to the NW and lahars continued out to the northern coast of Unimak island (figure 21). The incandescent lava flow was visible from Cold Bay on the evening of the 12th and a thick steam plume continued through the next day.

Figure 20. An example of a volcanic ash grain that was erupted at Shishaldin on 12 December 2019 and collected on the SE flank by the Alaska Volcano Observatory staff. This Scanning Electron Microscope images shows the different crystals represented by different colors: dark gray crystals are plagioclase, the light gray crystals are olivine, and the white ones are Fe-Ti oxides. The groundmass in this grain is nearly completely crystallized. Courtesy of AVO.

Figure 21. A WorldView-2 satellite image of Shishaldin with the summit vent and eruption deposits on 12 December 2019. The tephra deposit extends around 10 km SE, a new lava flow reaching 3 km NW with lahars continuing to the N coast of Unimak island. Pyroclastic flow deposits reach 3.5 km to the N and contain blocks as large as 3 m. Courtesy of Hannah Dietterich, AVO.

A new lava flow was reported by a pilot on the night of 16 December. Thermal satellite data showed that this flow reached 2 km to the NW. High-resolution radar satellite images over the 15-17th showed that the lava flow had advanced out to 2.5 km and had developed levees along the margins (figure 22). The lava channel was 5-15 m wide and was originating from a crater at the base of the summit scoria cone, which had been rebuilt since the collapse the previous week. Minor ash emissions drifted to the south on the 19tt and 20th (figure 23).

Figure 22. TerraSAR-X radar satellite images of Shishaldin on 15 and 17 December 2019 show the new lava flow on the NW flank and growth of a scoria cone at the summit. The lava flow had reached around 2.5 km at this point and was 5-15 m wide with levees visible along the flow margins. Pyroclastic flow deposits from a scoria cone collapse event on 12 December are on the N flank. Figure courtesy of Simon Plank (German Aerospace Center, DLR) and Hannah Dietterich (AVO).

Figure 23. Geologist Janet Schaefer (AVO/DGGS) collects ash samples within ice and snow on the southern flanks of Shishaldin on 20 December 2019. A weak ash plume is rising from the summit crater. Photo courtesy of Wyatt Mayo, AVO.

On 21 December a new lava flow commenced, traveling down the northern slope and accompanied by minor ash emissions. Continued lava extrusion was indicated by thermal data on the 25th and two lava flows reaching 1.5 km and 100 m were observed in satellite data on the 26th, as well as ash deposits on the upper flanks (figure 24). Weak explosions were detected by the regional infrasound network the following day. A satellite image acquired on the 30th showed a thick steam plume obscuring the summit and snow cover on the flanks indicating a pause in ash emissions.

Figure 24. This 26 December 2019 WorldView-2 satellite image with a close-up of the Shishaldin summit area to the right shows a lava flow extending nearly 1.5 km down the NW flank and a smaller 100-m-long lava flow to the NE. Volcanic ash was deposited around the summit, coating snow and ice. Courtesy of Matt Loewen, AVO.

In early January satellite data indicated slow lava extrusion or cooling lava flows (or both) near the summit. On the morning of the 3rd an ash plume rose to 6-7 km altitude and drifted 120 km E to SE, producing minor amounts of volcanic lightning. Elevated surface temperatures the previous week indicated continued lava extrusion. A satellite image acquired on 3 January showed lava flows extending to 1.6 km NW, pyroclastic flows moving 2.6 km down the western and southern flanks, and ashfall on the flanks (figure 25).

Figure 25. This WorldView-2 multispectral satellite image of Shishaldin, acquired on 3 January 2019, shows the lava flows reaching 1.6 km down the NW flank and an ash plume erupting from the summit dispersing to the SE. Ash deposits cover snow on the flanks. Courtesy of Hannah Dietterich, AVO.

On 7 January the most sustained explosive episode for this eruption period occurred. An ash plume rose to 7 km altitude at 0500 and drifted east to northeast then intensified reaching 7.6 km altitude with increased ash content, prompting an increase of the Aviation Color Code to Red and Volcano Alert Level to Warning. The plume traveled over 200 km to the E to NE (figure 26). Lava flows were produced on the northern flanks and trace amounts of ashfall was reported in communities to the NE, resulting in several flight cancellations. Thermal satellite images showed active lava flows extruding from the summit vent (figure 27). Seismicity significantly decreased around 1200 and the alert levels were lowered to Orange and Watch that evening. Through the following week no notable eruptive activity occurred. An intermittent steam plume was observed in webcam views.

Figure 26. This Landsat 8 satellite image shows a detached ash plume drifts to the NE from an explosive eruption at Shishaldin on 7 January 2020. Courtesy of Chris Waythomas, AVO.

Figure 27. This 7 January 2019 Sentinel-2 thermal satellite image shows several lava flows on the NE and NW flanks of Shishaldin, as well as a steam plume from the summit dispersing to the NE. Blue is snow and ice in this false color image (bands 12, 11, 4). Courtesy of Sentinel-Hub playground.

Eruptive activity resumed on 18 January with lava flows traveling 2 km down the NE flank accompanied by a weak plume with possible ash content dispersing to the SW (figure 28). A steam plume was produced at the front of the lava flow and lahar deposits continued to the north (figures 29 to 32). Activity intensified from 0030 on the 19th, generating a more ash-rich plume that extended over 150 km E and SE and reached up to 6 km altitude; activity increased again at around 1500 with ash emissions reaching 9 km altitude. AVO increased the alert levels to Red/Warning. Lava flows traveled down the NE and N flanks producing meltwater lahars, accompanied by elevated seismicity (figures 33). Activity continued through the day and trace amounts of ashfall were reported in False Pass (figure 34). Activity declined to small explosions over the next few days and the alert levels were lowered to Orange/watch shortly after midnight. The next morning weak steam emissions were observed at the summit and there was a thin ash deposit across the entire area. Satellite data acquired on 23 January showed pyroclastic flow deposits and cooling lava flows on the northern flank, and meltwater reaching the northern coast (figure 35).

Figure 28. This Worldview-3 multispectral near-infrared satellite image acquired on 18 January 2020 shows a lava flow down the NE flank of Shishaldin. A steam plume rises from the end of the flow and lahar deposits from snowmelt travel further north. Courtesy of Matt Loewen, AVO.

Figure 29. Steam plumes from the summit of Shishaldin and from the lava flow down the NE flank on 18 January 2020. Lahar deposits extend from the lava flow front and towards the north. Photo courtesy of Matt Brekke, via AVO.

Figure 30. A lava flow traveling down the NE flank of Shishaldin on 18 January 2020, seen from Cold Bay. Photo courtesy of Aaron Merculief, via AVO.

Figure 31. Two plumes rise from Shishaldin on 18 January 2020, one from the summit crater and the other from the lava flow descending the NE Flank. Photos courtesy of Woodsen Saunders, via AVO.

Figure 32. A low-altitude plume from Shishaldin on the evening of 18 January 2020, seen from King Cove. Photo courtesy of Savannah Yatchmeneff, via AVO.

Figure 33. This WorldView-2 near-infrared satellite image shows a lava flow reaching 1.8 km down the N flank and lahar deposits filling drainages out to the Bering Sea coast (not shown here) on 19 January 2020. Ash deposits coat snow to the NE and E. Courtesy of Matt Loewen, AVO.

Figure 34. An ash plume (top) and gas-and-steam plumes (bottom) at Shishaldin on 19 January 2020. Courtesy of Matt Brekke, via AVO.

Figure 35. A Landsat 8 thermal satellite image (band 11) acquired on 23 January 2019 showing hot lava flows and pyroclastic flow deposits on the flanks of Shishaldin and the meltwater flow path to the Bering Sea. Figure courtesy of Christ Waythomas, AVO.

Activity remained low in late January with some ash resuspension (due to winds) near the summit and continued elevated temperatures. Seismicity remained above background levels. Infrasound data indicated minor explosive activity during 22-23 January and small steam plumes were visible on 22, 23, and 26 January. MIROVA thermal data showed the rapid reduction in activity following activity in late-January (figure 36).

Figure 36. MIROVA thermal data showing increased activity at Shishaldin during August-September, and an even higher thermal output during late-October 2019 to late January 2020. Courtesy of MIROVA.

Geologic Background. The beautifully symmetrical Shishaldin is the highest and one of the most active volcanoes of the Aleutian Islands. The glacier-covered volcano is the westernmost of three large stratovolcanoes along an E-W line in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." A steam plume often rises from its small summit crater. Constructed atop an older glacially dissected volcano, it is largely basaltic in composition. Remnants of an older ancestral volcano are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is blanketed by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century.

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/); Simon Plank, German Aerospace Center (DLR) German Remote Sensing Data Center, Geo-Risks and Civil Security, Oberpfaffenhofen, 82234 Weßling (URL: https://www.dlr.de/eoc/en/desktopdefault.aspx/tabid-5242/8788_read-28554/sortby-lastname/); 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/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, which lies on the NW flank of the cone of the large stratovolcano referred to as Volcán Viejo. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and the first half of 2019. This report covers continuing activity from June-December 2019 when ongoing explosive events produced ash plumes, lava, and pyroclastic flows. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Nevados de Chillán was relatively quiet during June 2019, generating only a small number of explosions with ash plumes. This activity continued during July; some events produced incandescent ejecta around the crater. By August a distinct increase in activity was noticeable; ash plumes were larger and more frequent, and incandescent ejecta rose hundreds of meters above the summit a number of times. Frequent explosions were typical during September; the first of several blocky lava flows emerged from the crater mid-month. Inflation that began in mid-July continued with several centimeters of both horizontal and vertical displacement. By October, pyroclastic flows often accompanied the explosive events in addition to the ash plumes, and multiple vents opened within the crater. Three more lava flows had appeared by mid-November; explosions continued at a high rate. Activity remained high at the beginning of December but dropped abruptly mid-month. MODVOLC measured three thermal alerts in September, two in October, seven in November, and six in December. This period of increased thermal activity closely matches the thermal anomaly data reported by the MIROVA project (figure 37), which included an increase at the end of August 2019 that lasted through mid-December before stopping abruptly. Several lava flows and frequent explosions with incandescent ejecta and pyroclastic flows were reported throughout the period of increased thermal activity.

Figure 37. MIROVA thermal anomaly data for Nevados de Chillán from 3 February through December 2019 show low activity during June-August and increasing activity from August through mid-December. This correlates with ground and satellite observations of lava flows, incandescent explosions, ash plumes, and pyroclastic flows during the period of increased thermal activity. Courtesy of MIROVA.

Activity during June-August 2019. Nevados de Chillán remained relatively quiet during June 2019 with a few explosions of ash. At the active Nicanor crater, located on the E flank of the Volcán Nuevo dome, predominantly white steam plumes were observed daily in the nearby webcams. The growth rate of the dome inside the crater was reported by SERNAGEOMIN as continuing at about 260 m³/day. They noted an explosion on 3 June; the Buenos Aires VAAC reported a puff of ash seen from the webcam drifting SE at 3.7 km altitude (figure 38). The webcam indicated sporadic weak emissions continuing that day and the next. Minor explosions were also reported on 7-8 June and included incandescence observed at night and ejecta deposited around the crater rim. The Buenos Aires VAAC reported a narrow ash plume drifting ENE in multispectral imagery under clear skies late on 7 June. The webcams showed sporadic emissions of ash at 3.4 km altitude on 19 June that dissipated rapidly.

Figure 38. Explosions at Nevados de Chillán on 3 (left) and 20 (right) June 2019 produced ash plumes that quickly dissipated in the strong winds. Courtesy of the SERNAGEOMIN Portezuelo webcam, Pehuenia Online (left) and Eco Bio Bio La Red Informativa (right).

Minor pulsating explosive activity continued during July 2019 with multiple occurrences of ash emissions. Ash emissions rose to 3.7 km altitude on 4 July and were seen in the SERNAGEOMIN webcam; the VAAC reported an emission on 8 July that rose to 4.3 km altitude and drifted SE. Monitoring stations near the complex recorded an explosive event early on 9 July; incandescence with gases and ejecta were deposited around the crater and an ash plume rose to 3.9 km and drifted SE. Small ash plumes from sporadic puffs on 12 July rose to 4.6 km altitude. An explosive event on 14 July also produced incandescent ejecta around the crater along with weak sporadic ash emissions. Single ash emissions on 18 (figure 39) and 22 July at 3.7 km altitude drifted ESE from summit before dissipating; another emission on 26 July was reported at 4.3 km altitude.

Figure 39. Local news sources reported ash emissions at Nevados de Chillán on 18 July 2019. Courtesy of INF0SCHILE (left) and Radio Ñuble (right).

A distinct increase in the intensity and frequency of explosive activity was recorded during August 2019. SERNAGEOMIN noted ash emissions and explosions during 3-4 August in addition to the persistent steam plumes above the Nicanor crater (figure 40). The Buenos Aires VAAC reported a single puff on 3 August that was seen in the webcam rising to 3.9 km altitude and dissipating quickly. The next day a pilot reported an ash plume estimated at 5.5 km altitude drifting E. It was later detected in satellite imagery; the webcam revealed continuous emission of steam and gas with intermittent puffs of ash. SERNAGEOMIN issued a special report (REAV) on 6 August noting the increase in size and frequency of explosions, some of which produced dense ash plumes that rose 1.6 km above the crater along with incandescent ejecta. They also reported that satellite imagery indicated a 1.5-km-long lahar that traveled down the NNE flank as a result of the interaction of the explosive ejecta with the snowfall near the summit.

Figure 40. Climbers captured video of a significant explosion at Nevados de Chillán on 4 August 2019. Courtesy of CHV Noticias.

Beginning on 9-10 August 2019, and continuing throughout the month, SERNAGEOMIN observed explosive nighttime activity with incandescent ejecta scattered around the crater rim along with moderate levels of seismicity each day. A diffuse ash plume was detected in satellite imagery by the VAAC on 9 August drifting NW at 4.9 km altitude. SERNAGEOMIN issued a new warning on 12-13 August that the recent increase in activity since the end of July suggested the injection of a new magmatic body that could lead to larger explosive events with pyroclastic and lava flows. They reported pyroclastic ejecta from multiple explosions on 13 August rising 765 and 735 m above the crater. Drone images taken between 4 and 12 August showed the destruction of the summit dome from multiple explosions with the Nicanor Crater (figure 41). The VAAC reported sporadic pulses of volcanic ash drifting N during 12-14 August, visible in satellite imagery estimated at 4.3 km altitude. By 17-18 August, they noted constant steam emissions interspersed with gray plumes during explosive activity.

Figure 41. Drone images taken at Nevados de Chillán between 4 and 12 August 2019 showed destruction of the dome caused by multiple explosions at the summit crater. Courtesy of Movisis.org Internacional.

An increase in seismicity, especially VT events, during 21-22 August 2019 resulted in multiple special REAV reports from SERNAGEOMIN. They noted on 21 August that an explosion produced gas emissions and pyroclastic material that rose 1,400 m above the crater; the next day material rose 450 m. That night, in addition to incandescent ejecta around the crater, they reported small high-temperature flows on the N flank which extended to the NNE flank a few days later. The VAAC reported pulses of ash plumes moving SE on 22 August at 4.3 km altitude. A faint ash cloud was visible in satellite imagery on 29 August drifting E at 3.7 km altitude (figure 42). The cloud was dissipating rapidly as it moved away from the summit. Sporadic ash emissions from intermittent explosions continued moving ESE then N and NE; they were reported daily through 5 September. They continued to rise in altitude to 3.9 km on 30 August, 4.3 km on 1 September, and 4.6 km on 3 September.

Figure 42. Incandescence at the summit of Nevados de Chillán and ashfall covering snow to the E was captured in Sentinel-2 satellite imagery on 29 August 2019. Courtesy of Copernicus EMS.

Activity during September-October 2019. Frequent explosions from Nicanor crater continued during September 2019, producing numerous ash plumes and small high-temperature flows along the NNE flank. A webcam detected a small lateral vent on the NNE flank about 50 m from the crater rim emitting gas and particulates on 2-3 September. Multiple explosions during 3-5 September were associated with gas and ash emissions and incandescent ejecta deposited around the crater rim (figure 43). The network of GNSS stations recording deformation of the volcanic complex confirmed on 3-4 September that inflation, which had been recorded since mid-July 2019, was continuing at a rate of about 1 cm/month. Blocks of incandescent ejecta from numerous explosions were observed rolling down the N flank on 6-7 September and the E flank the following night.

Figure 43. Activity at Nevados de Chillán on 3 September 2019 included ash and steam explosions (left) and incandescent ejecta at the summit (right). Courtesy of Carlos Bustos and SERNAGEOMIN webcams.

SERNAGEOMIN reported on 9-10 September that satellite imagery revealed a new surface deposit about 130 m long trending NNE from the center of crater. They reported an increase in the level of seismicity from moderate to high on 10-11 September and observed incandescent ejecta at the summit during several explosions (figure 44). During a flyover on 12 September scientists confirmed the presence of a new blocky lava flow emerging from Nicanor Crater and moving down the NNE flank of Nuevo volcano. The flow was about 600 m long, 100 m wide, and 5 m thick with a blocky surface and incandescent lava at the base within the active crater. Measurements with a thermal camera indicated a temperature around 800°C within the active crater, and greater than 100°C on the surface of the flow. Frequent high-energy explosions that day produced incandescent ejecta that could be seen from Las Trancas and Shangri-La (figure 45). Ashfall 0.5 cm thick was reported 2 km from the volcano to the SW. The flow was visible from the webcam located N of Nicanor on 16-17 September. Satellite imagery indicated the flow was about 550 m long and moving at a rate of about 21 m/day.

Figure 44. A blocky lava flow moved down the NNE flank of Nevados de Chillán on 11 September (left); incandescent ejecta covered the summit area the next night (right). Courtesy of EarthQuakesTime (left), Red Geocientifica de Chile (right) and SERNAGEOMIN Webcams.

Figure 45. The SERNAGEOMIN Portezuelo webcam revealed the blocky lava flow, incandescent ejecta and ash emissions at Nevados de Chillán on 12 September 2019. Courtesy of American Earthquakes (left), PatoArias (right), and SERNAGEOMIN.

During 18-22 September 2019 multiple special reports of seismicity were released each day with incandescent ejecta, gas, and particulate emissions often observed at the summit crater; the lava flow remained active. On 24 September ashfall was reported about 15 km NW in communities including Las Trancas; small pyroclastic flows were observed the following day. Horizontal inflation of 2.4 cm was reported on 25 September, and vertical inflation was measured at 3.4 cm since mid-July. SERNAGEOMIN noted that while the frequency of explosions had increased, the energy released had decreased. Morphological changes in Nicanor crater suggested that it was growing at its SW edge and eroding the adjacent Arrau crater; the NE edge of the crater was unstable.

Plumes of steam and ash continued along with the explosions for the remainder of the month. During the night, incandescent ejecta was observed, and the low-velocity lava flow continued to move. Multiple VAAC reports were issued virtually every day of September. Pulses of ash were moving SE at 4.3 km altitude on 7-8 September. For most of the rest of the month sporadic emissions with minor amounts of ash were observed in either the webcam or satellite images at an altitude of 3.7 km, occasionally rising to 4.3 km. They drifted generally SE but varied somewhat with the changing winds. Continuous ash emissions were observed during 24-25 September that rose as high as 4.9 km altitude and drifted E, clearly visible in satellite imagery. After that, the altitude dropped back to 3.7 km and the plume was only faintly and intermittently visible in satellite imagery.

Low-altitude gray ash plumes were observed rising from Nicanor crater almost every day that weather permitted during October 2019. Incandescent ejecta was frequently observed at night. Beginning on 6-7 October, SERNAGEOM reported pyroclastic flows traveling short distances from the crater most days. They traveled 1.13 km down the NNE flank, 0.42 km down the NNW flank and 0.88 km down the SW flank. The blocky lava flow on the NNE flank was no longer active (figure 46). During 9-12 October, multiple special reports of increased seismic activity (REAVs) were issued each day. Inflation continued throughout the month. On 10 October the total horizontal deformation (since mid-July) was 3 cm, with a rate of movement a little over 1 cm/month; the total vertical displacement was 4.5 cm, with a rate of 1.93 cm/month during the previous 30 days.

In a special report issued on 11 October, SERNAGEOMIN mentioned that analysis of satellite imagery indicated a new emission center within the Nicanor crater adjacent to the dome vent active since December 2017 and to the lava flow of September. The new center was oval shaped with an E-W dimension of 60 m and a N-S dimension of 55 m, located about 90 m SE of the old, still active center, and was the site of the explosive activity reported since 30 September.

Figure 46. Drone footage posted 10 October 2019 from Nevados de Chillán shows steam emissions from the Nicanor crater and a blocky lava flow down NNE flank. The snow-covered cone in background is Volcan Baños. Courtesy of Volcanologia Chile and copyright by Nicolas Luengo V.

On 16 October a new blocky flow was observed on the NE flank of the Nicanor Crater; it was about 70 m long, moving about 30 m/day. By 21 October it had reached 130 m in length, and its rate of advance had slowed significantly. Beginning on 25 October seismicity decreased noticeably and much less surface activity was observed at the crater. Explosions at the end of the month produced steam plumes, gas emissions and minor pulsating ash emissions.

The Buenos Aires VAAC reported a puff of ash at 4.9 km altitude on 1 October moving SE. Continuous emission of steam and gas with sporadic puffs of ash that rose to around 3.7-4.3 km altitude were typical every day after that until 25 October usually drifting S or E; they were most often visible in the webcams, and occasionally visible in satellite imagery when weather conditions permitted. A diffuse plume of ash was detected on 16 October drifting SE at 4.6 km altitude. The VAAC reported incandescence visible at the summit in webcam images on 22 October; a significant daytime explosion on 24 October produced a large incandescent ash cloud (figure 47). The next day the VAAC detected weak pulses of ash plumes in satellite images extending E from the summit for 130 km. Intermittent ash emissions were reported drifting SE at 3.7-4.3 km each day from 29-31 October.

Figure 47. A large incandescent ash plume at Nevados de Chillán on 24 October 2019 sent ejecta around the summit (left); a dense ash plume was produced during an explosion on 30 October 2019 (right). Courtesy of Cristian Farian (left) and SERNAGEOMIN (right); both images taken from the SERNAGEOMIN webcams.

Activity during November-December 2019. Moderate seismicity continued during November 2019 with recurrent episodes of pulsating gas and ash emissions. Incandescent ejecta was visible many nights that the weather conditions were favorable (figure 48). In the Daily Report (RAV) issued on 6 November, SERNAGEOMIN noted that the original 700-m-long blocky lava flow on the NNE flank active during September had been partly covered by another flow, about 350 m long. They also reported that pyroclastic density currents were observed in the area immediately around the crater extending in several directions. They extended 850 m down the SW flank, 670 m down the NW flank, 1,680 m down the N flank, and 440 m to the NNE.

Changes in the crater area indicated a growth of the SW edge of the Nicanor Crater, continuing to erode the Arrau crater, with the constant emission of gas, ash, and incandescent ejecta that produced plumes up to 1.8 km high. SERNAGEOMIN also observed activity from a vent at the NE edge of the crater that included gas emission and ejecta, but no lava flow. The fourth lava flow observed in recent months (L4) was identified on the NNE slope on 13 November adjacent to the earlier flows; it was about 70 m long and slowly advancing. By 19 November L4 consisted of two lobes and extended about 90 m from the edge of the Nicanor crater advancing at an average rate of 0.4 m/hour. The vent producing L4 was located about 60 m SSE of the vent that produced the earlier flows (L1, L2, and L3). By 28 November the flow had reached a length of 165 m and was no longer advancing. A series of explosions reported on 25-27 and 30 November produced ejecta that rose 800, 1,000, 1,300, and 700 m above the crater.

Figure 48. Incandescent ejecta at Nevados de Chillán was clearly visible at night on 3 November 2019. Courtesy of Claudio Kanisius.

Ash emissions were reported by the Buenos Aires VAAC during most of November, usually visible from the webcams, but often also seen in satellite imagery. The plumes generally reached 3.7-4.6 km altitude and drifted SSE. They usually occurred as continuous emission of steam and gas accompanied by sporadic pulses of ash but were sometimes continuous ash for several hours. They were visible about 100 km E of the summit on 2 November, and over 200 km SE the following day. A narrow plume of ash was seen in visual satellite imagery extending 50 km E of the summit on 9 November. Intermittent incandescence at the summit was seen from the webcam on 18 November. Pulses of ash were detected in satellite imagery extending 125 km SE on 22 November. Strong puffs of ash briefly rose to 4.9 km altitude and drifted NE on 26 November (figure 49); incandescence during the nighttime was visible in the webcam on 28 November.

Figure 49. An explosion on 26 November 2019 at Nevados de Chillán produced a dense ash plume and small pyroclastic flows down the flank. Courtesy of Volcanes de Chile and the SERNAGEOMIN Portezuelo webcam.

Pulsating emissions of gas and ejecta continued into December 2019. Five explosions were reported on 1 December that produced gas plumes which rose 300-800 m above the crater. Three more explosions occurred on 3 December sending gas plumes 500-1,000 m high. SERNAGEOMIN reported on 4 December that explosive activity was observed from four vents within the Nicanor crater. This activity triggered new pyroclastic flows that extended 1,100 m E and 400 m S. By 5 December the total vertical inflation reported since July was 8 cm. A large explosion on 5 December sent material 1.6 km above the summit and pyroclastic flows down the flanks (figure 50). The webcams at Andarivel and Portezuelo showed a pyroclastic flow moving 400 m W, a direction not previously observed; this was followed by additional pyroclastic flows to the N and E.

Figure 50. A large explosion at Nevados de Chillán on 5 December 2019 produced an ash plume that rose 1.6 km above the summit and sent pyroclastic flows down the flanks. Courtesy of SERNAGEOMIN.

On 9 December SERNAGEOMIN noted that the increase to four active vents was causing erosion on the S and SE edges of the crater making the most affected areas to the SW, S, SE and E of the crater. Major explosions reported that day produced pyroclastic flows that descended down the E and ESE flanks and particulate emissions that rose 1 km. The SW flank near the crater was also affected by ejecta and pyroclastic debris carried by the wind. The most extensive pyroclastic flows travelled down the E flank for the next several days; explosions on 10 December sent material 1.2 km high. Three explosions were noted on 11 December; the first sent incandescence close to 200 m high, and the second produced a column of particulate material 1.2 km high. The first of two explosions on 12 December sent material 1.8 km above the crater and pyroclastic flows down the flanks (figure 51). Although explosions were reported on 13 and 14 December, cloudy skies prevented observations of the summit.

Figure 51. A large explosion at Nevados de Chillán on 12 December 2019 produced an ash plume that rose 1.8 km above the summit and sent pyroclastic flows down the flanks. Courtesy of Volcanes de Chile and SERNAGEOMIN.

Intermittent ash emissions were reported by the Buenos Aires VAAC during 1-13 December 2019. They rose to 3.7-4.3 km and drifted generally E. Pulses of ash were detected at 4.9 km altitude moving S in satellite imagery on 9 December. The last reported ash emission for December was on the afternoon of 12 December; puffs of ash could be seen in satellite imagery moving E at 4.6 km altitude. A decrease in particulate emissions and explosions was reported beginning on 14 December, and no further explosions were recorded by infrasound devices after 15 December. The deposits from the earlier pyroclastic flows had reached 600 m E and 300 m W of the crater. Seismic activity was recorded as low instead of moderate beginning on 25 December. A total horizontal inflation of about 6 cm since July was measured at the end of December.

Geologic Background. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The largest stratovolcano, dominantly andesitic, Cerro Blanco (Volcán Nevado), is located at the NW end of the group. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The new Volcán Nuevo lava-dome complex formed between 1906 and 1945 between the two volcanoes and grew to exceed Volcán Viejo in elevation. The Volcán Arrau dome complex was constructed SE of Volcán Nuevo between 1973 and 1986 and eventually exceeded its height.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/), Twitter: @Sernageomin; 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/); Cristian Farias Vega, Departamento de Obras Civiles y Geología, Universidad Católica de Temuco, Vilcún, Región de La Araucanía, Chile (URL: https://twitter.com/cfariasvega/status/1187471827255226370); Copernicus Emergency Management Service (Copernicus EMS), Joint Research Centre, European Union (URL: https://emergency.copernicus.eu/, https://twitter.com/CopernicusEMS/status/1168156474817818624); Volcanes de Chile, Proyectos de la Fundación Volcanes de Chile, Chile (URL: https://www.volcanesdechile.net/, https://twitter.com/volcanesdechile/status/1199496839491395585); Pehuenia Online, Pehuenia, Argentina (URL: http://pehueniaonline.com.ar/, https://twitter.com/PehueniaOnline/status/1135703309824745472); Eco Bio Bio La Red Informativa, Bio Bio Region, Chile (URL: http://emergenciasbiobio.blogspot.com/, https://twitter.com/Eco_BioBio_II/status/1141734238590574593); INF0SCHILE (URL: https://twitter.com/INF0SCHILE/status/1151849611482599425); Radio Ñuble AM y FM, Chillán, Chile (URL: http://radionuble.cl/linea/, lhttps://twitter.com/RadioNuble/status/1151858189299781632); CHV Noticias, Santiago, Chile (URL: https://www.chvnoticias.cl/, https://twitter.com/CHVNoticias/status/1159263718015819777); Movisis.org Internacional, Manabi, Ecuador (URL: https://movisis.org/, https://twitter.com/MOVISISEC/status/1160778823031558144); Carlos Bustos (URL: https://twitter.com/cbusca1970/status/1168932243873644548); EarthQuakesTime (URL: https://twitter.com/EarthQuakesTime/status/1171654504841908229); Red Geocientifica de Chile (URL: https://twitter.com/RedGeoChile/status/1171972482875703296); American Earthquakes (URL: https://twitter.com/earthquakevt/status/1172271139760091136); PatoArias, Talca, Chile (URL: https://twitter.com/patoarias/status/1172287142191665153); Volcanologia Chile, (URL: http://www.volcanochile.com/joomla30/, https://twitter.com/volcanologiachl/status/1182707451554078720); Claudio Kanisius (URL: https://twitter.com/ClaudioKanisius/status/1191182878346031104).

The large Asosan caldera reaches around 23 km long in the N-S direction and contains a complex of 17 cones, of which Nakadake is the most active (figure 58). A recent increase in activity prompted an alert level increase from 1 to 2 on 14 April 2019. The Nakadake crater is the site of current activity (figure 59) and contains several smaller craters, with the No. 1 crater being the main source of activity during July-December 2019. The activity during this period is summarized here based on reports by the Japan Meteorological Agency and satellite data.

Figure 58. Asosan is a group of cones and craters within a larger caldera system. January 2010 Monthly Mosaic images copyright Planet Labs 2019.

Figure 59. Hot gas emissions from the Nakadake No. 1 crater on 25 June 2019 reached around 340°C. Courtesy of the Japan Meteorological Agency (July 2019 monthly report).

Small explosions were observed at the No. 1 vent on the 4, 5, 9, 13-16, and 26 July. There was an increase in thermal energy detected near the vent leading to a larger event on the 26th (figures 60 and 61), which produced an ash plume up to 1.6 km above the crater rim and continuing from 0757 to around 1300 with a lower plume height of 400 m after 0900. Light ashfall was reported downwind. Elevated activity was noted during 28-29 July, and an ash plume was seen in webcam footage on the 30th. Incandescence was visible in light-sensitive cameras during 4-17 and after the 26th. A field survey on 5 July measured 1,300 tons of sulfur dioxide (SO₂) per day. This had increased to 2,300 tons per day by the 12th, 2,500 on the 24th, and 2,400 by the 25th. A sulfur dioxide plume was detected in Sentinel-5P/TROPOMI satellite data acquired on 28 July (figure 62).

Figure 60. Thermal images taken at Asosan on 26 July 2019 show the increasing temperature of emissions leading to an explosion. Courtesy of the Japan Meteorological Agency (July 2019 monthly report).

Figure 61. An eruption from the Nakadake crater at Asosan on 26 July 2019. Courtesy of the Japan Meteorological Agency (July 2019 monthly report).

Figure 62. A sulfur dioxide plume was detected from Asosan (to the left) on 28 July 2019. The larger plume (red) to the right is not believed to be associated with volcanism in this area. NASA Sentinel-5P/TROPOMI satellite image courtesy of the NASA Goddard Space Flight Center.

The increased eruptive activity that began on 5 July continued to 16 August. There were 24 eruptions recorded throughout the month, with eruptions occurring on 18-23, 25, and 29-31 August. An ash plume at 2100 on 4 August reached 1.5 km above the crater rim. Detected SO₂ increased to extremely high levels from late July to early August with 5,200 tons per day recorded on 9 August, but which then reduced to 2,000 tons per day. Ashfall occurred out to around 7 km NW on the 10th (figure 63). Activity continued to increase at the Nakadake No. 1 crater, producing incandescence. High-temperature gas plumes were detected at the No. 2 crater.

Figure 63. Ashfall from Asosan on 10 August 2019 near Otohime, Aso city, which is about 7 km NW of the Nakadake No. 1 crater that produced the ash plume. The ashfall was thick enough that the white line in the parking lot was mostly obscured (lower photo). Courtesy of the Japan Meteorological Agency (August 2019 monthly report).

Thermal activity continued to increase, and incandescence was observed at the No. 1 crater throughout September. There were 24 eruptions recorded throughout August. Light ashfall occurred out to around 8 km NE on the 3rd and ash plumes reached 1.6 km above the crater rim during 10-13, and again during 25-30 (figures 64 and 65). During the later dates ashfall was reported to the NE and NW. The SO₂ levels were back down to 1,600 tons per day by 11 September and increased to 2,600 tons per day by the 26th.

Figure 64. Ash plumes at Asosan on 29 September 2019. Courtesy of Volcanoverse.

Figure 65. Activity at Asosan in late September 2019. Left: incandescence and a gas plume at the Nakadake No. 1 crater on the 28th. Right: an eruption produced an ash plume at 0839 on the 30th. Aso Volcano Museum surveillance camera image (left) and Kusasenri surveillance camera image (right) courtesy of the Japan Meteorological Agency (September 2019 monthly report).

Similar elevated activity continued through October with ash plumes reaching 1.3 km above the crater and periodic ashfall reported at the Kumamoto Regional Meteorological Observatory, and out to 4 km S to SW on the 19th and 29th. Temperatures up to 580°C were recorded at the No. 1 crater on 23 October and incandescence was occasionally visible at night through the month (figure 66). Gas surveys detected 2,800 tons per day of SO₂ on 7 October, which had increased to 4,000 tons per day by the 11th.

Figure 66. Drone images of the Asosan Nakadake crater area on 23 October 2019. The colored boxes show the same vents and the photographs on the left correlate to the thermal images on the right. The yellow box is around the No. 1 crater, with temperature measurements reaching 580°C. The emissions in the red box reached 50°C, and up to 100°C on the southwest crater wall (blue box). Courtesy of the Japan Meteorological Agency (October 2019 monthly report).

Ash plume emission continued through November (figure 67 and 68). Plumes reached 1.5 to 2.4 km above sea level during 13-18 November and ashfall occurred downwind, with a maximum of 1.4 km above the crater rim for the month. Ashfall was reported near Aso City Hall on the 27th. Incandescence was observed until 6 November. During the first half of October sulfur dioxide emissions were slightly lower than the previous month, with measurements detecting under 3,000 tons per day. In the second half of the month emissions increased to 2,000 to 6,300 tons per day. This was accompanied by an increase in volcanic tremor.

Figure 67. Examples of ash plumes at Asosan on 2, 8, 9, and 11 November 2019. The plume on 2 November reached 1.3 km above the crater rim. Kusasenri surveillance camera images courtesy of the Japan Meteorological Agency.

Figure 68. Ash emissions from the Nakadake crater at Asosan on 15 and 17 November 2019. The continuous ash emission is weak and is being dispersed by the wind. Copyright Mizumoto, used with permission.

Throughout December activity remained elevated with ash plumes reaching 1.1 km above the Nakadake No. 1 crater and producing ashfall. The maximum gas plume height was 1.8 km above the crater. A total of 23 eruptions were recorded, and incandescence at the crater was observed through the month. Sulfur dioxide emissions continued to increase with 5,800 tons per day recorded on the 27th, and 7,400 tons per day recorded on the 31st.

Overall, eruptive activity has continued intermittently since 26 July and SO₂ emissions have increased through the year. Incandescence was seen at the crater since 2 October and this is consistent with an increase in thermal energy detected by the MIROVA algorithm around that time (figure 69).

Figure 69. Thermal anomalies were low through 2019 with a notable increase around October to November. Log radiative power plot courtesy of MIROVA.

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), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); 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/); Planet Labs, Inc. (URL: https://www.planet.com/); Mizumoto, Kumamoto, Kyushu, Japan (Twitter: https://twitter.com/hepomodeler); Volcanoverse (URL: https://www.youtube.com/channel/UCi3T_esus8Sr9I-3W5teVQQ).

Remote Tinakula lies 100 km NE of the Solomon Trench at the N end of the Santa Cruz Islands, which are part of the South Pacific 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. A large ash explosion during 21-26 October 2017 was a short-lived event; renewed thermal activity was detected beginning in December 2018 and intermittently throughout 2019. This report covers the ongoing activity from July-December 2019. Since ground-based observations are rarely available, satellite thermal and visual data are the primary sources of information.

MIROVA thermal anomaly data indicated intermittent but ongoing thermal activity at Tinakula during July-December 2019 (figure 35). It was characterized by pulses of multiple alerts of varying intensities for several days followed by no activity for a few weeks.

Figure 35. The MIROVA project plot of Radiative Power at Tinakula from 2 March 2019 through the end of the year indicated repeated pulses of thermal energy each month except for August 2019. It was characterized by pulses of multiple alerts for several days followed by no activity for a few weeks. Courtesy of MIROVA.

Observations using Sentinel-2 satellite imagery were often prevented by clouds during July, but two MODVOLC thermal alerts on 2 July 2019 corresponded to MIROVA thermal activity on that date. No thermal anomalies were reported by MIROVA during August 2019, but Sentinel-2 satellite images showed dense steam plumes drifting away from the summit on four separate dates (figure 36). Two distinct thermal anomalies appeared in infrared imagery on 9 September, and a dense steam plume drifted about 10 km NW on 14 September (figure 37).

Figure 36. Sentinel-2 satellite imagery for Tinakula recorded ongoing steam emissions on multiple days during August 2019 including 10 August (left) and 20 August (right). The island is about 3 km in diameter. Left image is natural color rendering with bands 4,3,2, right image is atmospheric penetration with bands 12, 11, and 8a. Courtesy of Sentinel Hub Playground.

Figure 37. A bright thermal anomaly at the summit and a weaker one on the nearby upper W flank of Tinakula on 9 September 2019 (left) indicated ongoing eruptive activity in Sentinel-2 satellite imagery. While no thermal anomalies were visible on 14 September (right), a dense steam plume originating from the summit drifted more than 10 km NW. Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

During October 2019 steam emissions were captured in four clear satellite images; a weak thermal anomaly was present on the W flank on 9 October (figure 38). MODVOLC recorded a single thermal alert on 9 November. Stronger thermal anomalies appeared twice during November in satellite images. On 13 November a strong anomaly was present at the summit in Sentinel-2 imagery; it was accompanied by a dense steam plume drifting NE from the hotspot. On 28 November two thermal anomalies appeared part way down the upper NW flank (figure 39). Thermal imagery on 3 December suggested that a weak anomaly remained on the NW flank in a similar location; a dense steam plume rose above the summit, drifting slightly SW on 18 December (figure 40). A thermal anomaly at the summit on 28 December was accompanied by a dense steam plume and corresponded to multiple MIROVA thermal anomalies at the end of December.

Figure 38. A weak thermal anomaly was recorded on the upper W flank of Tinakula on 9 October 2019 in Sentinel-2 satellite imagery (left). Dense steam drifted about 10 km NW from the summit on 29 October (right). Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Figure 39. On 13 November 2019 a strong anomaly was present at the summit of Tinakula in Sentinel-2 imagery; it was accompanied by a dense steam plume drifting NE from the hotspot (left). On 28 November two thermal anomalies appeared part way down the upper NW flank (right). Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Figure 40. Thermal imagery on 3 December 2019 from Tinakula suggested that a weak anomaly remained in a similar location to one of the earlier anomalies on the NW flank (left); a dense steam plume rose above the summit, drifting slightly SW on 18 December (center). A thermal anomaly at the summit on 28 December was accompanied by a dense steam plume (right) and corresponded to multiple MIROVA thermal anomalies at the end of December. Atmospheric penetration rendering (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).

Heightened continuing activity at Ibu since March 2018 has been dominated by frequent ash explosions with weak ash plumes, and numerous thermal anomalies reflecting one or more weak lava flows (BGVN 43:05, 43:12, and 44:07). This report summarizes activity through December 2019, and is based on data from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellites.

Typical ash plumes during the reporting period of July-December 2019 rose 800 m above the crater, with the highest reported to 1.4 km in early October (table 5). They were usually noted a few times each month. According to MAGMA Indonesia, explosive activity caused the Aviation Color Code to be raised to ORANGE (second highest of four) on 14, 22, and 31 August, 4 and 30 September, and 15 and 20 October.

Table 5. Ash plumes and other volcanic activity reported at Ibu during December 2018-December 2019. Plume heights are reported above the crater rim. Data courtesy of PVMBG and Darwin VAAC.

Thermal anomalies were sometimes noted by PVMBG, and were also frequently obvious in infrared satellite imagery suggesting lava flows and multiple active vents, as seen on 22 November 2019 (figure 19). Thermal anomalies using MODIS satellite instruments processed by the MODVOLC algorithm were recorded 2-4 days every month from July to December 2019. In contrast, the MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected numerous hotspots on most days (figure 20).

Figure 19. Example of thermal activity in the Ibu crater on 22 November 2019, along with a plume drifting SE. One or more vents in the crater are producing small lava flows, an observation common throughout the reporting period. Sentinel-2 false color (urban) images (bands 12, 11, 4), courtesy of Sentinel Hub Playground.

Figure 20. Thermal anomalies recorded at Ibu by the MIROVA system using MODIS infrared satellite data for the year 2019. Courtesy of MIROVA.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, contained several small crater lakes through much of historical time. The outer crater, 1.2 km wide, is breached on the north side, creating a steep-walled valley. A large parasitic cone is located ENE of the summit. A smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. Only a few eruptions have been recorded in historical time, the first a small explosive eruption from the summit crater in 1911. An eruption producing a lava dome that eventually covered much of the floor of the inner summit crater began in December 1998.

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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Lateiki (Metis Shoal) is one of several submarine and island volcanoes on the W side of the Tonga trench in the South Pacific. It has produced ephemeral islands multiple times since the first confirmed activity in the mid-19th century. Two eruptions, in 1967 and 1979, produced islands that survived for a few months before eroding beneath the surface. An eruption in 1995 produced a larger island that persisted, possibly until a new eruption in mid-October 2019 destroyed it and built a new short-lived island. Information was provided by the Ministry of Lands, Survey and Natural Resources of the Government of the Kingdom of Tonga, and from satellite information and news sources.

Review of eruptions during 1967-1995. The first reported 20th century eruption at this location was observed by sailors beginning on 12 December 1967 (CSLP 02-67); incandescent ejecta rose several hundred meters into the air and "steam and smoke" rose at least 1,000 m from the ocean surface. The eruption created a small island that was reported to be a few tens of meters high, and a few thousand meters in length and width. Eruptive activity appeared to end in early January 1968, and the island quickly eroded beneath the surface by the end of February (figure 6). When observed in April 1968 the island was gone, with only plumes of yellowish water in the area of the former island.

Figure 6. Waves break over Lateiki on 19 February 1968, more than a month after the end of a submarine eruption that began in December 1967 and produced a short-lived island. Photo by Charles Lundquist, 1968 (Smithsonian Astrophysical Observatory).

A large steam plume and ejecta were observed on 19 June 1979, along with a "growing area of tephra" around the site with a diameter of 16 km by the end of June (SEAN 04:06). Geologists visited the site in mid-July and at that time the island was about 300 m long, 120 m wide, and 15 m high, composed of tephra ranging in size from ash to large bombs (SEAN 04:07); ash emissions were still occurring from the E side of the island. It was determined that the new island was located about 1 km E of the 1967-68 island. By early October 1979 the island had nearly disappeared beneath the ocean surface.

A new eruption was first observed on 6 June 1995. A new island appeared above the waves as a growing lava dome on 12 June (BGVN 20:06). Numerous ash plumes rose hundreds of meters and dissipated downwind. By late June an elliptical dome, about 300 x 250 m in size and 50 m high, had stopped growing. The new island it formed was composed of hardened lava and not the tuff cones of earlier islands (figure 7) according to visitors to the island; pumice was not observed. An overflight of the area in December 2006 showed that an island was still present (figure 8), possibly from the June 1995 eruption. Sentinel-2 satellite imagery confirming the presence of Lateiki Island and discolored water was clearly recorded multiple times between 2015 and 2019. This suggests that the island created in 1995 could have lasted for more than 20 years (figure 9).

Figure 7. An aerial view during the 1995 eruption of Lateiki forming a lava dome. Courtesy of the Government of the Kingdom of Tonga.

Figure 8. Lateiki Island as seen on 7 December 2006; possibly part of the island that formed in 1995. Courtesy of the Government of the Kingdom of Tonga and the Royal New Zealand Air Force.

Figure 9. Sentinel-2 satellite imagery confirmed the existence of an island present from 2015 through 2019 with little changes to its shape. This suggests that the island created in 1995 could have lasted for more than 20 years. Courtesy of Sentinel Hub Playground.

New eruption in October 2019. The Kingdom of Tonga reported a new eruption at Lateiki on 13 October 2019, first noted by a ship at 0800 on 14 October. NASA satellite imagery confirmed the eruption taking place that day (figure 10). The following morning a pilot from Real Tonga Airlines photographed the steam plume and reported a plume height of 4.6-5.2 km altitude (figure 11). The Wellington VAAC issued an aviation advisory report noting the pilot's observation of steam, but no ash plume was visible in satellite imagery. They issued a second report on 22 October of a similar steam plume reported by a pilot at 3.7 km altitude. The MODVOLC thermal alert system recorded three thermal alerts from Lateiki, one each on 18, 20, and 22 October 2019.

Figure 10. NASA's Worldview Aqua/MODIS satellite imagery taken on 14 October 2019 over the Ha'apai and Vava'u region of Tonga showing the new eruption at Lateiki. Neiafu, Vava'u, is at the top right and Tofua and Kao islands are at the bottom left. The inset shows a closeup of Late Island at the top right and a white steam plume rising from Lateiki. Courtesy of the Government of the Kingdom of Tonga and NASA Worldview.

Figure 11. Real Tonga Airline's Captain Samuela Folaumoetu'I photographed a large steam plume rising from Lateiki on the morning of 15 October 2019. Courtesy of the Government of the Kingdom of Tonga.

The first satellite image of the eruption on 15 October 2019 showed activity over a large area, much bigger than the preexisting island that was visible on 10 October (figure 12). Although the eruption produced a steam plume that drifted several tens of kilometers SW and strong incandescent activity, no ash plume was visible, similar to reports of dense steam with little ash during the 1968 and 1979 eruptions (figure 13). Strong incandescence and a dense steam plume were still present on 20 October (figure 14).

Figure 12. The first satellite image of the eruption of Lateiki on 15 October 2019 showed activity over a large area, much bigger than the preexisting island that was visible on 10 October (inset). The two images are the same scale; the island was about 100 m in diameter before the eruption. Image uses Natural Color Rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 13. The steam plume from Lateiki on 15 October 2019 drifted more than 20 km SE from the volcano. A strong thermal anomaly from incandescent activity was present in the atmospheric penetration rendering (bands 12, 11, 8a) closeup of the same image (inset). Courtesy of Sentinel Hub Playground.

Figure 14. A dense plume of steam drifted NW from Lateiki on 20 October 2019, and a strong thermal signal (inset) indicated ongoing explosive activity. Courtesy of Annamaria Luongo and Sentinel Hub Playground.

A clear satellite image on 30 October 2019 revealed an island estimated to be about 100 m wide and 400 m long, according to geologist Taaniela Kula of the Tonga Geological Service of the Ministry of Lands, Survey and Natural Resources as reported by a local news source (Matangitonga). There was no obvious fumarolic steam activity from the surface, but a plume of greenish brown seawater swirled away from the island towards the NE (figure 15). In a comparison of the location of the old Lateiki island with the new one in satellite images, it was clear that the new island was located as far as 250 m to the NW (figure 16) on 30 October. Over the course of the next few weeks, the island's size decreased significantly; by 19 November, it was perhaps one-quarter the size it had been at the end of October. Lateiki Island continued to diminish during December 2019 and January 2020, and by mid-month only traces of discolored sea water were visible beneath the waves over the eruption site (figure 17).

Figure 15. The new Lateiki Island was clearly visible on 30 October 2019 (top left), as was greenish-blue discoloration in the surrounding waters. It was estimated to be about 100 m wide and 400 m long that day. Its size decreased significantly over subsequent weeks; ten days later (top right) it was about half the size and two weeks later, on 14 November 2019 (bottom left), it was about one-third its original size. By 19 November (bottom right) only a fraction of the island remained. Greenish discolored water continued to be visible around the volcano. Courtesy of Sentinel Hub Playground.

Figure 16. The location of the new Lateiki Island (Metis Shoal), shown here on 30 October 2019 in red, was a few hundred meters to the NW of the old position recorded on 5 September 2019 (in white). Courtesy of Annamaria Luongo and Sentinel Hub Playground.

Figure 17. Lateiki Island disappeared beneath the waves in early January 2020, though plumes of discolored water continued to be observed later in the month. Courtesy of Sentinel Hub Playground.

Geologic Background. Lateiki, previously known as Metis Shoal, is a submarine volcano midway between the islands of Kao and Late that has produced a series of ephemeral islands since the first confirmed activity in the mid-19th century. An island, perhaps not in eruption, was reported in 1781 and subsequently eroded away. During periods of inactivity following 20th-century eruptions, waves have been observed to break on rocky reefs or sandy banks with depths of 10 m or less. Dacitic tuff cones formed during the first 20th-century eruptions in 1967 and 1979 were soon eroded beneath the ocean surface. An eruption in 1995 produced an island with a diameter of 280 m and a height of 43 m following growth of a lava dome above the surface.

Information Contacts: Government of the Kingdom of Tonga, PO Box 5, Nuku'alofa, Tonga (URL: http://www.gov.to/ ); Royal New Zealand Air Force (URL: http://www.airforce.mil.nz/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); 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); Annamaria Luongo, Brussels, Belgium (Twitter: @annamaria_84, URL: https://twitter.com/annamaria_84 ); Taaniela Kula, Tonga Geological Service, Ministry of Lands, Survey and Natural Resources; Matangi Tonga Online (URL: https://matangitonga.to/2019/11/06/eruption-lateiki).

Sakurajima is a highly active stratovolcano situated in the Aira caldera in southern Kyushu, Japan. Common volcanism for this recent eruptive episode since March 2017 includes frequent explosions, ash plumes, and scattered ejecta. Much of this activity has been focused in the Minamidake crater since 1955; the Showa crater on the E flank has had intermittent activity since 2006. This report updates activity during July through December 2019 with the primary source information from monthly reports by the Japan Meteorological Agency (JMA) and various satellite data.

During July to December 2019, explosive eruptions and ash plumes were reported multiple times per week by JMA. November was the most active, with 137 eruptive events, seven of which were explosive while August was the least active with no eruptive events recorded (table 22). Ash plumes rose between 800 m to 5.5 km above the crater rim during this reporting period. Large blocks of incandescent ejecta traveled as far as 1.7 km from the Minamidake crater during explosions in September through December. The Kagoshima Regional Meteorological Observatory (11 km WSW) reported monthly amounts of ashfall during each month, with a high of 143 g/m² during October. Occasionally at night throughout this reporting period, crater incandescence was observed with a highly sensitive surveillance camera. All explosive activity originated from the Minamidake crater; the adjacent Showa crater produced mild thermal anomalies and gas-and-steam plumes.

Table 22. Monthly summary of eruptive events recorded at Sakurajima's Minamidake crater in the Aira caldera, July through December 2019. The number of events that were explosive in nature are in parentheses. No events were recorded at the Showa crater during this time. Ashfall is measured at the Kagoshima Local Meteorological Observatory (KLMO), 10 km W of Showa crater. Data courtesy of JMA (July to December 2019 monthly reports).

An explosion that occurred at 1044 on 4 July 2019 produced an ash plume that rose up to 3.2 km above the Minamidake crater rim and ejected material 1.1 km from the vent. Field surveys conducted on 17 and 23 July measured SO₂ emissions that were 1,200-1,800 tons/day. Additional explosions between 19-22 July generated smaller plumes that rose to 1.5 km above the crater and ejected material 1.1 km away. On 28 July explosions at 1725 and 1754 produced ash plumes 3.5-3.8 km above the crater rim, which resulted in ashfall in areas N and E of Sakurajima (figure 86), including Kirishima City (20 km NE), Kagoshima Prefecture (30 km SE), Yusui Town (40 km N), and parts of the Kumamoto Prefecture (140 km NE).

Figure 86. Photo of the Sakurajima explosion at 1725 on 28 July 2019 resulting in an ash plume rising 3.8 km above the crater (left). An on-site field survey on 29 July observed ashfall on roads and vegetation on the N side of the island (right). Photo by Moto Higashi-gun (left), courtesy of JMA (July 2019 report).

The month of August 2019 showed the least activity and consisted of mainly small eruptive events occurring up to 800 m above the crater; summit incandescence was observed with a highly sensitive surveillance camera. SO₂ emissions were measured on 8 and 13 August with 1,000-2,000 tons/day, which was slightly greater than the previous month. An extensometer at the Arimura Observation Tunnel and an inclinometer at the Amida River recorded slight inflation on 29 August, but continuous GNSS (Global Navigation Satellite System) observations showed no significant changes.

In September 2019 there were 32 eruptive events recorded, of which 11 were explosions, more than the previous two months. Seismicity also increased during this month. An extensometer and inclinometer recorded inflation at the Minamidake crater on 9 September, which stopped after the eruptive events. On 16 September, an eruption at 0746 produced an ash plume that rose 2.8 km above the crater rim and drifted SW; a series of eruptive events followed from 0830-1110 (figure 87). Explosions on 18 and 20 September produced ash plumes that rose 3.4 km above the crater rim and ejecting material as far as 1.7 km from the summit crater on the 18th and 700 m on the 20th. Field surveys measured an increased amount of SO₂ emissions ranging from 1,100 to 2,300 tons/day during September.

Figure 87. Webcam image of an ash plume rising 2.8 km from the Minamidake crater at Sakurajima on 16 September 2019. Courtesy of Weathernews Inc.

Seismicity, SO₂ emissions, and the number of eruptions continued to increase in October 2019, 41 of which were explosive. Field surveys conducted on 1, 11, and 15 October reported that SO₂ emissions were 2,000-2,800 tons/day. An explosion at 0050 on 12 October produced an ash plume that traveled 1.7 km from the Minamidake crater. Explosions between 16 and 19 October produced an ash plume that rose up to 3 km above the crater rim (figure 88). The Japan Maritime Self-Defense Force 1st Air group observed gas-and-steam plumes rising from both the Minamidake and Showa craters on 25 October. The inflation reported from 16 September began to slow in late October.

Figure 88. Photos taken from the E side of Sakurajima showing gas-and-steam emissions with some amount of ash rising from the volcano on 16 October 2019 after an explosion around 1200 that day (top). At night, summit incandescence is observed (bottom). Courtesy of Bradley Pitcher, Vanderbilt University.

November 2019 was the most active month during this reporting period with increased seismicity, SO₂ emissions, and 137 eruptive events, 77 of which were explosive. GNSS observations indicated that inflation began to slow during this month. On 8 November, an explosion at 1724 produced an ash plume up to a maximum of 5.5 km above the crater rim and drifted E. This explosion ejected large blocks as far as 500-800 m away from the crater (figure 89). The last time plumes rose above 5 km from the vents occurred on 26 July 2016 at the Showa crater and on 7 October 2000 at the Minamidake crater. Field surveys on 8, 21, and 29 November measured increased SO₂ emissions ranging from 2,600 to 3,600 tons/day. Eruptions between 13-19 November produced ash plumes that rose up to 3.6 km above the crater and ejected large blocks up 1.7 km away. An onsite survey on 29 November used infrared thermal imaging equipment to observe incandescence and geothermal areas near the Showa crater and the SE flank of Minamidake (figure 90).

Figure 89. Photos of an ash plume rising 5.5 km above Sakurajima on 8 November 2019 and drifting E. Photo by Moto Higashi-gun (top left), courtesy of JMA (November 2019 report) and the Geoscientific Network of Chile.

Figure 90. Webcam image of nighttime incandescence and gas-and-steam emissions with some amount of ash at Sakurajima on 29 November 2019. Courtesy of JMA (November 2019 report).

Volcanism, which included seismicity, SO₂ emissions, and eruptive events, decreased during December 2019. Explosions during 4-10 December produced ash plumes that rose up to 2.6 km above the crater rim and ejected material up to 1.7 km away. Field surveys conducted on 6, 16, and 23 December measured SO₂ emissions around 1,000-3,000 tons/day. On 24 December, an explosion produced an ash plume that rose to 3.3 km above the crater rim, this high for this month.

Sentinel-2 natural color satellite imagery showed dense ash plumes in late August 2019, early November, and through December (figure 91). These plumes drifted in different directions and rose to a maximum 5.5 km above the crater rim on 8 November.

Figure 91. Natural color Sentinel-2 satellite images of Sakurajima within the Aira caldera from late August through December 2019 showed dense ash plumes rising from the Minamidake crater. Courtesy of Sentinel Hub Playground.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent thermal anomalies beginning in mid-August to early September 2019 after a nearly two-month hiatus (figure 92). Activity increased by early November and continued through December. Three Sentinel-2 thermal satellite images between late July and early October showed distinct thermal hotspots within the Minamidake crater, in addition to faint gas-and-steam emissions in July and September (figure 93).

Figure 92. Thermal anomalies at Sakurajima during January-December 2019 as recorded by the MIROVA system (Log Radiative Power) started up in mid-August to early September after a two-month break and continued through December. Courtesy of MIROVA.

Figure 93. Sentinel-2 thermal satellite images showing small thermal anomalies and gas-and-steam emissions (left and middle) at Sakurajima within the Minamidake crater between late July and early October 2019. All images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Weathernews Inc. (Twitter: @wni_jp, https://twitter.com/wni_jp, URL: https://weathernews.jp/s/topics/201608/210085/, photo posted at https://twitter.com/wni_jp/status/1173382407216652289); Bradley Pitcher, Vanderbilt University, Nashville. TN, USA (URL: https://bradpitcher.weebly.com/, Twitter: @TieDyeSciGuy, photo posted at https://twitter.com/TieDyeSciGuy/status/1185191225101471744); Geoscientific Network of Chile (Twitter: @RedGeoChile, https://twitter.com/RedGeoChile, Facebook: https://www.facebook.com/RedGeoChile/, photo posted at https://twitter.com/RedGeoChile/status/1192921768186515456).

Suwanosejima, located south of Japan in the northern Ryukyu Islands, is an active andesitic stratovolcano that has had continuous activity since October 2004, typically producing ash plumes and Strombolian explosions. Much of this activity is focused within the Otake crater. This report updates information during July through December 2019 using monthly reports from the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and various satellite data.

White gas-and-steam plumes rose from Suwanosejima on 26 July 2019, 30-31 August, 1-6, 10, and 20-27 September, reaching a maximum altitude of 2.4 km on 10 September, according to Tokyo VAAC advisories. Intermittent gray-white plumes were observed rising from the summit during October through December (figure 40).

Figure 40. Surveillance camera images of white gas-and-steam emissions rising from Suwanosejima on 10 December 2019 (left) and up to 1.8 km above the crater rim on 28 December (right). At night, summit incandescence was also observed on 10 December. Courtesy of JMA.

An explosion that occurred at 2331 on 1 August 2019 ejected material 400 m from the crater while other eruptions on 3-6 and 26 August produced ash plumes that rose up to a maximum altitude of 2.1 km and drifted generally NW according to the Tokyo VAAC report. JMA reported eruptions and summit incandescence in September accompanied by white gas-and-steam plumes, but no explosions were noted. Eruptions on 19 and 29 October produced ash plumes that rose 300 and 800 m above the crater rim, resulting in ashfall in Toshima (4 km SW), according to the Toshima Village Office, Suwanosejima Branch Office. Another eruption on 30 October produced a similar gray-white plume rising 800 m above the crater rim but did not result in ashfall. Similar activity continued in November with eruptions on 5-7 and 13-15 November producing grayish-white plumes rising 900 m and 1.5 km above the crater rim and frequent crater incandescence. Ashfall was reported in Toshima Village on 19 and 20 November; the 20 November eruption ejected material 200 m from the Otake crater.

Field surveys on 14 and 18 December using an infrared thermal imaging system to the E of Suwanose Island showed hotspots around the Otake crater, on the N slope of the crater, and on the upper part of the E coastline. GNSS (Global Navigation Satellite Systems) observations on 15 and 17 December showed a slight change in the baseline length. After 2122 on 25-26 and 31 December, 23 eruptions, nine of which were explosive were reported, producing gray-white plumes that rose 800-1,800 m above the crater rim and ejected material up to 600 m from the Otake crater. JMA reported volcanic tremors occurred intermittently throughout this reporting period.

Incandescence at the summit crater was occasionally visible at night during July through December 2019, as recorded by webcam images and reported by JMA (figure 41). MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed weak thermal anomalies that occurred dominantly in November with little to no activity recorded between July and October (figure 42). Two Sentinel-2 thermal satellite images in early November and late December showed thermal hotspots within the summit crater (figure 43).

Figure 41. Surveillance camera image of summit incandescence at Suwanosejima on 31 October 2019. Courtesy of JMA.

Figure 42. Weak thermal anomalies at Suwanosejima during January-December 2019 as recorded by the MIROVA system (Log Radiative Power) dominantly occurred in mid-March, late May to mid-June, and November, with two hotspots detected in late September and late December. Courtesy of MIROVA.

Figure 43. Sentinel-2 thermal satellite images showing small thermal anomalies (bright yellow-orange) within the Otake crater at Suwanosejima on 8 November 2019 (left) and faintly on 23 December 2019 behind clouds (right). Both images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Barren Island is a remote stratovolcano located east of India in the Andaman Islands. Its most recent eruptive episode began in September 2018 and has included lava flows, explosions, ash plumes, and lava fountaining (BGVN 44:02). This report updates information from February 2019 through January 2020 using various satellite data as a primary source of information.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent thermal anomalies within 5 km of the summit from mid-February 2019 through January 2020 (figure 41). There was a period of relatively low to no discernible activity between May to September 2019. The MODVOLC algorithm for MODIS thermal anomalies in comparison with Sentinel-2 thermal satellite imagery and Suomi NPP/VIIRS sensor data, registered elevated temperatures during late February 2019, early March, sparsely in April, late October, sparsely in November, early December, and intermittently in January 2020 (figure 42). Sentinel-2 thermal satellite imagery shows these thermal hotspots differing in strength from late February to late January 2020 (figure 43). The thermal anomalies in these satellite images are occasionally accompanied by ash plumes (25 February 2019, 23 October 2019, and 21 January 2020) and gas-and-steam emissions (26 April 2019).

Figure 41. Intermittent thermal anomalies at Barren Island for 20 February 2019 through January 2020 occurred dominantly between late March to late April 2019 and late September 2019 through January 2020. Courtesy of MIROVA.

Figure 42. Timeline summary of observed activity at Barren Island from February 2019 through January 2020. For Sentinel-2, MODVOLC, and VIIRS data, the dates indicated are when thermal anomalies were detected. White areas indicated no activity was observed, which may also be due to meteoric clouds. Data courtesy of Darwin VAAC, Sentinel Hub Playground, HIGP, and NASA Worldview using the "Fire and Thermal Anomalies" layer.

Figure 43. Sentinel-2 thermal images show ash plumes, gas-and-steam emissions, and thermal anomalies (bright yellow-orange) at Barren Island during February 2019-January 2020. The strongest thermal signature was observed on 23 October while the weakest one is observed on 26 January. Sentinel-2 False color (bands 12, 11, 4) images courtesy of Sentinel Hub Playground.

The Darwin Volcanic Ash Advisory Center (VAAC) reported ash plumes rising from the summit on 7, 14, and 16 March 2019. The maximum altitude of the ash plume occurred on 7 March, rising 1.8 km altitude, drifting W and NW and 1.2 km altitude, drifting E and ESE, based on observations from Himawari-8. The VAAC reports for 14 and 16 March reported the ash plumes rising 0.9 km and 1.2 km altitude, respectively drifting W and W.

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast.

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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

Whakaari/White Island has been New Zealand's most active volcano since 1976. Located 48 km offshore, the volcano is a popular tourism destination with tours leaving the town of Whakatane with approximately 17,500 people visiting the island in 2018. Ten lives were lost in 1914 when part of the crater wall collapsed, impacting sulfur miners. More recently, a brief explosion at 1411 on 9 December 2019 produced an ash plume and pyroclastic surge that impacted the entire crater area. With 47 people on the island at the time, the death toll stood at 21 on 3 February 2019. At that time more patients were still in hospitals within New Zealand or their home countries.

The island is the summit of a large underwater volcano, with around 70% of the edifice below the ocean and rising around 900 m above sea level (figure 70). A broad crater opens to the ocean to the SE, with steep crater walls and an active Main Crater area to the NW rear of the crater floor (figure 71). Although the island is privately owned, GeoNet continuously monitors activity both remotely and with visits to the volcano. This Bulletin covers activity from May 2017 through December 2019 and is based on reports by GeoNet, the New Zealand Civil Defence Bay of Plenty Emergency Management Group, satellite data, and footage taken by visitors to the island.

Figure 70. The top of the Whakaari/White Island edifice forms the island in the Bay of Plenty area, New Zealand, while 70% of the volcano is below sea level. Courtesy of GeoNet.

Figure 71. This photo from 2004 shows the Main Crater area of Whakaari/White Island with the vent area indicated. The crater is an amphitheater shape with the crater floor distance between the vent and the ocean entry being about 700 m. The sediment plume begins at the area where tour boats dock at the island. Photo by Karen Britten, graphic by Danielle Charlton at University of Auckland; courtesy of GeoNet (11 December 2019 report).

Nearly continuous activity occurred from December 1975 to September 2000, including the formation of collapse and explosion craters producing ash emissions and explosions that impacted all of the Main Crater area. More recently, it has been in a state of elevated unrest since 2011. Renewed activity commenced with an explosive eruption on 5 August 2012 that was followed by the extrusion of a lava dome and ongoing phreatic explosions and minor ash emissions through March 2013. An ash cone was seen on 4 March 2013, and over the next few months the crater lake reformed. Further significant explosions took place on 20 August and 4, 8, and 11 October 2013. A landslide occurred in November 2015 with material descending into the lake. More recent activity on 27 April 2016 produced a short-lived eruption that deposited material across the crater floor and walls. A short period of ash emission later that year, on 13 September 2016, originated from a vent on the recent lava dome. Explosive eruptions occur with little to no warning.

Since 19 September 2016 the Volcanic Alert Level (VAL) was set to 1 (minor volcanic unrest) (figure 72). During early 2017 background activity in the crater continued, including active fumaroles emitting volcanic gases and steam from the active geothermal system, boiling springs, volcanic tremor, and deformation. By April 2017 a new crater lake had begun to form, the first since the April 2016 explosion when the lake floor was excavated an additional 13 m. Before this, there were areas where water ponded in depressions within the Main Crater but no stable lake.

Figure 72. The New Zealand Volcanic Alert Level system up to date in February 2020. Courtesy of GeoNet.

Activity from mid-2017 through 2018. In July-August 2017 GeoNet scientists carried out the first fieldwork at the crater area since late 2015 to sample the new crater lake and gas emissions. The crater lake was significantly cooler than the past lakes at 20°C, compared to 30-70°C that was typical previously. Chemical analysis of water samples collected in July showed the lowest concentrations of most "volcanic elements" in the lake for the past 10-15 years due to the reduced volcanic gases entering the lake. The acidity remained similar to that of battery acid. Gas emissions from the 2012 dome were 114°C, which were over 450°C in 2012 and 330°C in 2016. Fumarole 0 also had a reduced temperature of 152°C, reduced from over 190°C in late 2016 (figure 73). The observations and measurements indicated a decline in unrest. Further visits in December 2017 noted relatively low-level unrest including 149°C gas emissions from fumarole 0, a small crater lake, and loud gas vents nearby (figures 74 and 75). By 27 November the lake had risen to 10 m below overflow. Analysis of water samples led to an estimate of 75% of the lake water resulting from condensing steam vents below the lake and the rest from rainfall.

Figure 73. A GeoNet scientists conducting field work near Fumarole 0, an accessible gas vent on Whakaari/White Island in August 2017. Courtesy of GeoNet (23 August 2017 report).

Figure 74. GeoNet scientists sample gas emissions from vents on the 2012 Whakaari/White Island dome. The red circle in the left image indicates the location of the scientists. Courtesy of GeoNet (23 August 2017 report).

Figure 75. Active fumaroles and vents in the Main Crater of Whakaari/White Island including Fumarole 0 (top left). The crater lake formed in mid-2017 and gas emissions rise from surrounding vents (right). Courtesy of GeoNet (22 December 2017 report).

Routine fieldwork by GeoNet monitoring teams in early March 2018 showed continued low-level unrest and no apparent changes after a recent nearby earthquake swarm. The most notable change was the increase in the crater lake size, likely a response from recent high rainfall (figure 76). The water remained a relatively cool 27°C. Temperatures continued to decline at the 2012 dome vent (128°C) and Fumarole 0 (138°C). Spring and stream flow had also declined. Deformation was observed towards the Active Crater of 2-5 mm per month and seismicity remained low. The increase in lake level drowned gas vents along the lake shore resulting in geyser-like activity (figure 77). GeoNet warned that a new eruption could occur at any time, often without any useful warning.

In mid-April 2018 visitors reported loud sounds from the crater area as a result of the rising lake level drowning vents on the 2012 dome (in the western side of the crater) and resulting in steam-driven activity. There was no notable change in volcanic activity. The sounds stopped by July 2018 as the geothermal system adjusted to the rising water, up to 17 m below overfill and filling at a rate of about 2,000 m³ per day, rising towards more active vents (figure 78). A gas monitoring flight taken on 12 September showed a steaming lake surrounded by active fumaroles along the crater wall (figure 79).

Figure 76. The increase in the Whakaari/White Island crater lake size in early March 2018 with gas plumes rising from vents on the other side. Courtesy of GeoNet (19 March 2018 report).

Figure 77. The increasing crater lake level at Whakaari/White Island produced geyser-like activity on the lake shore in March 2018. Courtesy of Brad Scott, GeoNet.

Figure 78. Stills taken from a drone video of the Whakaari/White Island Main Crater lake and active vents producing gas emissions. Courtesy of GeoNet.

Figure 79. Photos taken during a gas monitoring flight with GNS Science at Whakaari/White Island show gas and steam emissions, and a steaming crater lake on 12 September 2018. Note the people for scale on the lower-right crater rim in the bottom photograph. Copyright of Ben Clarke, University of Leicester, used with permission.

Activity during April to early December 2019. A GeoNet volcanic alert bulletin in April 2019 reported that steady low-level unrest continued. The level of the lake had been declining since late January and was back down to 13 m below overflow (figure 80). The water temperature had increased to over 60°C due to the fumarole activity below the lake. Fumarole 0 remained steady at around 120-130°C. During May-June a seismic swarm was reported offshore, unrelated to volcanic activity but increasing the risk of landslides within the crater due to the shallow locations.

Figure 80. Planet Labs satellite images from March 2018 to April 2019 show fluctuations in the Whakaari/White Island crater lake level. Image copyright 2019 Planet Labs, Inc.

On 26 June the VAL was raised to level 2 (moderate to heightened volcanic unrest) due to increased SO₂ flux rising to historically high levels. An overflight that day detected 1,886 tons/day, nearly three times the previous values of May 2019, the highest recorded value since 2013, and the second highest since measurements began in 2003. The VAL was subsequently lowered on 1 July due to a reduction in detected SO₂ emissions of 880 tons/day on 28 June and 693 tons/day on 29 June.

GeoNet reported on 26 September that there was an increase in steam-driven activity within the active crater over the past three weeks. This included small geyser-like explosions of mud and steam with material reaching about 10 m above the lake. This was not attributed to an increase in volcanic activity, but to the crater lake level rising since early August.

On 30 October an increase in background activity was reported. An increasing trend in SO₂ gas emissions and volcanic tremor had been ongoing for several months and had reached the highest levels since 2016. This indicated to GeoNet that Whakaari/White Island might be entering a period where eruptive activity was more likely. There were no significant changes in other monitoring parameters at this time and fumarole activity continued (figure 81).

Figure 81. A webcam image taken at 1030 on 30 October 2019 from the crater rim shows the Whakaari/White Island crater lake to the right of the amphitheater-shaped crater and gas-and-steam plumes from active fumaroles. Courtesy of GeoNet.

On 18 November the VAL was raised to level 2 and the Aviation Colour Code was raised to Yellow due to further increase in SO₂ emissions and volcanic tremor. Other monitoring parameters showed no significant changes. On 25 November GeoNet reported that moderate volcanic unrest continued but with no new changes. Gas emissions remained high and gas-driven ejecta regularly jetting material a few meters into the air above fumaroles in the crater lake (figure 82).

Figure 82. A webcam image from the Whakaari/White Island crater rim shows gas-driven ejecta rising above a fumarole within the crater lake on 22 November 2019. Courtesy of GeoNet.

GeoNet reported on 3 December that moderate volcanic unrest continued, with increased but variable explosive gas and steam-driven jetting, with stronger events ejecting mud 20-30 m into the air and depositing mud around the vent area. Gas emissions and volcanic tremor remained elevated and occasional gas smells were reported on the North Island mainland depending on wind direction. The crater lake water level remained unchanged. Monitoring parameters were similar to those observed in 2011-2016 and remained within the expected range for moderate volcanic unrest.

Eruption on 9 December 2019. A short-lived eruption occurred at 1411 on 9 December 2019, generating a steam-and-ash plume to 3.6 km and covering the entire crater floor area with ash. Video taken by tourists on a nearby boat showed an eruption plume composed of a white steam-rich portion, and a black ash-rich ejecta (figure 83). A pyroclastic surge moved laterally across the crater floor and up the inner crater walls. Photos taken soon after the eruption showed sulfur-rich deposits across the crater floor and crater walls, and a helicopter that had been damaged and blown off the landing pad (figure 84). This activity caused the VAL to be raised to 4 (moderate volcanic eruption) and the Aviation Colour Code being raised to Orange.

Figure 83. The beginning of the Whakaari/White Island 9 December 2019 eruption viewed from a boat that left the island about 20-30 minutes prior. Top: the steam-rich eruption plume rising above the volcano and a pyroclastic surge beginning to rise over the crater rim. Bottom: the expanded steam-and-ash plume of the pyroclastic surge that flowed over the crater floor to the ocean. Copyright of Michael Schade, used with permission.

Figure 84. This photo of Whakaari/White Island taken after the 9 December 2019 eruption at around 1424 shows ash and sediment coating the crater floor and walls. The helicopter in this image was blown off the landing pad and damaged during the eruption. Copyright of Michael Schade, used with permission.

A steam plume was visible in a webcam image taken at 1430 from Whakatane, 21 minutes after the explosion (figure 85). Subsequent explosions occurred at 1630 and 1749. Search-and-Rescue teams reached the island after the eruption and noted a very strong sulfur smell that was experienced through respirators. They experienced severe stinging of any exposed skin that came in contact with the gas, and were left with sensitive skin and eyes, and sore throats. Later in the afternoon the gas-and-steam plume continued and a sediment plume was dispersing from the island (figure 86). The VAL was lowered to level 3 (minor volcanic eruption) at 1625 that day; the Aviation Colour Code remained at Orange.

Figure 85. A view of Whakaari/White Island from Whakatane in the North Island of New Zealand. Left: there is no plume visible at 1410 on 9 December 2019, one minute before the eruption. Right: A gas-and-steam plume is visible 21 minutes after the eruption. Courtesy of GeoNet.

Figure 86. A gas-and-steam plume rises from Whakaari/White Island on the afternoon of 9 December 2019 as rescue teams visit the island. A sediment plume in the ocean is dispersing from the island. Courtesy of Auckland Rescue Helicopter Trust.

During or immediately after the eruption an unstable portion of the SW inner crater wall, composed of 1914 landslide material, collapsed and was identified in satellite radar imagery acquired after the eruption. The material slid into the crater lake area and left a 12-m-high scarp. Movement in this area continued into early January.

Activity from late 2019 into early 2020. A significant increase in volcanic tremor began at around 0400 on 11 December (figure 87). The increase was accompanied by vigorous steaming and ejections of mud in several of the new vents. By the afternoon the tremor was at the highest level seen since the 2016 eruption, and monitoring data indicated that shallow magma was driving the increased unrest.

Figure 87. This RSAM (Real-Time Seismic Amplitude) time series plot represents the energy produced at Whakaari/White Island from 11 November to 11 December 2019 with the Volcanic Activity Levels and the 9 December eruption indicated. The plot shows the sharp increase in seismic energy during 11 December. Courtesy of GeoNet (11 December 2019 report).

The VAL was lowered to 2 on the morning of 12 December to reflect moderate to heightened unrest as no further explosive activity had occurred since the event on the 9th. Volcanic tremor was occurring at very high levels by the time a bulletin was released at 1025 that day. Gas emissions increased since 10 January, steam and mud jetting continued, and the situation was interpreted to be highly volatile. The Aviation Colour Code remained at Orange. Risk assessment maps released that day show the high-risk areas as monitoring parameters continued to show an increased likelihood of another eruption (figure 88).

Figure 88. Risk assessment maps of Whakaari/White Island show the increase in high-risk areas from 2 December to 12 December 2019. Courtesy of GeoNet (12 December 2019 report).

The volcanic activity bulletin for 13 December reported that volcanic tremor remained high, but had declined overnight. Vigorous steam and mud jetting continuing at the vent area. Brief ash emission was observed in the evening with ashfall restricted to the vent area. The 14 January bulletin reported that volcanic tremor had declined significantly over night, and nighttime webcam images showed a glow in the vent area due to high heat flow.

Aerial observations on 14 and 15 December revealed steam and gas emissions continuing from at least three open vents within a 100 m² area (figure 89). One vent near the back of the crater area was emitting transparent, high-temperature gas that indicated that magma was near the surface, and produced a glow registered by low-light cameras (figure 90). The gas emissions had a blue tinge that indicated high SO₂ content. The area that once contained the crater lake, 16 m below overflow before the eruption, was filled with debris and small isolated ponds mostly from rainfall, with different colors due to the water reacting with the eruption deposits. The gas-and-steam plume was white near the volcano but changed to a gray-brown color as it cooled and moved downwind due to the gas content (figure 91). On 15 December the tremor remained at low levels (figure 92).

Figure 89. The Main Crater area of Whakaari/White Island showing the active vent area and gas-and-steam emissions on 15 December 2019. Gas emissions were high within the circled area. Before the eruption a few days earlier this area was partially filled by the crater lake. Courtesy of GeoNet (15 December 2019 report).

Figure 90. A low-light nighttime camera at Whakaari/White Island imaged "a glow" at a vent within the active crater area on 13 December 2019. This glow is due to high-temperature gas emissions and light from external sources like the moon. Courtesy of GeoNet (15 December 2019 report).

Figure 91. A gas-and-steam plume at Whakaari/White Island on 15 December 2019 is white near the crater and changes to a grey-brown color downwind due to the gas content. Courtesy of GeoNet (15 December 2019 report).

Figure 92. The Whakaari/White Island seismic drum plot showing the difference in activity from 12 December (top) to 15 December (bottom). Courtesy of GeoNet (15 December 2019 report).

On 19 December tremor remained low (figure 93) and gas and steam emission continued. Overflight observations confirmed open vents with one producing temperatures over 650°C (figure 94). SO₂ emissions remained high at around 15 kg/s, slightly lower than the 20 kg/s detected on 12 December. Small amounts of ash were produced on 23 and 26 December due to material entering the vents during erosion.

Figure 93. This RSAM (Real-Time Seismic Amplitude) time series plot represents the energy produced at Whakaari/White Island from 1 November to mid-December 2019. The Volcanic Alert Levels and the 9 December eruption are indicated. Courtesy of GeoNet.

Figure 94. A photograph and thermal infrared image of the Whakaari/White Island crater area on 19 December 2019. The thermal imaging registered temperatures up to 650°C at a vent emitting steam and gas. Courtesy of GeoNet.

The Aviation Colour Code was reduced to Yellow on 6 January 2020 and the VAL remained at 2. Strong gas and steam emissions continued from the vent area through early January and the glow persisted in nighttime webcam images. Short-lived episodes of volcanic tremor were recorded between 8-10 January and were accompanied by minor explosions. A 15 January bulletin reported that the temperature at the vent area remained very hot, up to 440°C, and SO₂ emissions were within normal post-eruption levels.

High temperatures were detected within the vent area in Sentinel-2 thermal data on 6 and 16 January (figure 95). Lava extrusion was confirmed within the 9 December vents on 20 January. Airborne SO₂ measurements on that day recorded continued high levels and the vent temperature was over 400°C. Observations on 4 February showed that no new lava extrusion had occurred, and gas fluxes were lower than two weeks ago, but still elevated. The temperatures measured in the crater were 550-570°C and no further changes to the area were observed.

Figure 95. Sentinel-2 thermal infrared satellite images show elevated temperatures in the 9 December 2019 vent area on Whakaari/White Island. False color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: New Zealand GeoNet Project, a collaboration between the Earthquake Commission and GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.geonet.org.nz/); GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Bay of Plenty Emergency Management Group Civil Defense, New Zealand (URL: http://www.bopcivildefence.govt.nz/); Auckland Rescue Helicopter Trust, Auckland, New Zealand (URL: https://www.rescuehelicopter.org.nz/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/); Ben Clarke, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom (URL: https://le.ac.uk/geology, Twitter: https://twitter.com/PyroclasticBen); Michael Schade, San Francisco, USA (URL: https://twitter.com/sch).

Kadovar is an island volcano north of Papua New Guinea and northwest of Manam. The first confirmed historical activity began in January 2018 and resulted in the evacuation of residents from the island. Eruptive activity through 2018 changed the morphology of the SE side of the island and activity continued through 2019 (figure 36). This report summarizes activity from May through December 2019 and is based largely on various satellite data, tourist reports, and Darwin Volcanic Ash Advisory Center (VAAC) reports.

Figure 36. The morphological changes to Kadovar from 2017 to June 2019. Top: the vegetated island has a horseshoe-shaped crater that opens towards the SE; the population of the island was around 600 people at this time. Middle: by May 2018 the eruption was well underway with an active summit crater and an active dome off the east flank. Much of the vegetation has been killed and ashfall covers a lot of the island. Bottom: the bay below the SE flank has filled in with volcanic debris. The E-flank coastal dome is no longer active, but activity continues at the summit. PlanetScope satellite images copyright Planet Labs 2019.

Since this eruptive episode began a large part of the island has been deforested and has undergone erosion (figure 37). Activity in early 2019 included regular gas and steam emissions, ash plumes, and thermal anomalies at the summit (BGVN 44:05). On 15 May an ash plume originated from two vents at the summit area and dispersed to the east. A MODVOLC thermal alert was also issued on this day, and again on 17 May. Elevated temperatures were detected in Sentinel-2 thermal satellite data on 20, 21, and 30 May (figure 38), with accompanying gas-and-steam plumes dispersing to the NNW and NW. On 30 May the area of elevated temperature extended to the SE shoreline, indicating an avalanche of hot material reaching the water.

Figure 37. The southern flank of Kadovar seen here on 13 November 2019 had been deforested by eruptive activity and erosion had produced gullies down the flanks. Copyrighted photo by Chrissie Goldrick, used with permission.

Figure 38. Sentinel-2 thermal satellite images show elevated temperatures at the summit area, and down to the coast in the top image. Gas-and-steam plumes are visible dispersing towards the NW. Sentinel-2 false color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel-Hub Playground.

Throughout June cloud-free Sentinel-2 thermal satellite images showed elevated temperatures at the summit area and extending down the upper SE flank (figure 38). Gas-and-steam plumes were persistent in every Sentinel-2 and NASA Suomi NPP / VIIRS (Visible Infrared Imaging Radiometer Suite) image. MODVOLC thermal alerts were issued on 4 and 9 June. Similar activity continued through July with gas-and-steam emissions visible in every cloud-free satellite image. Thermal anomalies appeared weaker in late-July but remained at the summit area. An ash plume was imaged on 17 July by Landsat 8 with a gas-and-ash plume dispersing to the west (figure 39). Thermal anomalies continued through August with a MODVOLC thermal alert issued on the 14th. Gas emissions also continued and a Volcano Observatory Notice for Aviation (VONA) was issued on the 19th reporting an ash plume to an altitude of 1.5 km and drifting NW.

Figure 39. An ash plume rising above Kadovar and a gas plume dispersing to the NW on 17 July 2019. Truecolor pansharpened Landsat 8 satellite image courtesy of Sentinel Hub Playground.

An elongate area extending from the summit area to the E-flank coastal dome appears lighter in color in a 7 September Sentinel-2 natural color satellite image, and as a higher temperature area in the correlating thermal bands, indicating a hot avalanche deposit. These observations along with the previous avalanche, persistent elevated summit temperatures, and persistent gas and steam emissions from varying vent locations (figure 40) suggests that the summit dome has remained active through 2019.

Figure 40. Sentinel-2 visible and thermal satellite images acquired on 7 September 2019 show fresh deposits down the east flank of Kadovar. They appear as a lighter colored area in visible, and show as a hot area (orange) in thermal data. Sentinel-2 natural color (bands 4, 3, 2) and false color (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel-Hub Playground.

Thermal anomalies and emissions continued through to the end of 2019 (figure 41). A tour group witnessed an explosion producing an ash plume at around 1800 on 13 November (figure 42). While the ash plume erupted near-vertically above the island, a more diffuse gas plume rose from multiple vents on the summit dome and dispersed at a lower altitude.

Figure 41. The summit area of Kadovar emitting gas-and-steam plumes in August, September, and November 2019. The plumes are persistent in satellite images throughout May through December and there is variation in the number and locations of the source vents. PlanetScope satellite images copyright Planet Labs 2019.

Figure 42. An ash plume and a lower gas plume rise during an eruption of Kadovar on 13 November 2019. The summit lava dome is visibly degassing to produce the white gas plume. Copyrighted photos by Chrissie Goldrick, used with permission.

While gas plumes were visible throughout May-December 2019 (figure 43), SO₂ plumes were difficult to detect in NASA SO₂ images due to the activity of nearby Manam volcano. The MIROVA thermal detection system shows continued elevated temperatures through to early December, with an increase during May-June (figure 44). Sentinel-2 thermal images showed elevated temperatures through to the end of December but at a lower intensity than previous months.

Figure 43. This photo of the southeast side Kadovar on 13 November 2019 shows a persistent low-level gas plume blowing towards the left and a more vigorous plume is visible near the crater. This is an example of the persistent plume visible in satellite imagery throughout July-December 2019. Copyrighted photo by Chrissie Goldrick, used with permission.

Figure 44. The MIROVA plot of radiative power at Kadovar shows thermal anomalies throughout 2019 with some variations in frequency. Note that while the black lines indicate that the thermal anomalies are greater than 5 km from the vent, the designated summit location is inaccurate so these are actually a the summit crater and on the E flank. Courtesy of MIROVA.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: 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/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Worldview (URL: https://worldview.earthdata.nasa.gov); Chrissie Goldrick, Australian Geographic, Level 7, 54 Park Street, Sydney, NSW 2000, Australia (URL: https://www.australiangeographic.com.au/).

Explosive activity continued . . . with September's 21 explosions . . . . The month's highest ash plume rose 2,000 m . . . and a total of 108 g/m² of ash . . . was deposited [at KLMO]. No swarms of volcanic earthquakes were recorded. Similar activity continued through early October, with 13 explosions occurring by the 13th.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: JMA.

In September, continuous gas and lava emission were accompanied by sporadic block and ash ejections from Crater C (300 m W of the summit). Blocks landed as much as 1.2 km away (at 850 m elevation), and impact craters on the W flank reached 50 cm in diameter. Explosion columns rose to 1 km height and were carried by wind predominantly to the NW, W, and SW. Fumarolic activity was visible in the summit crater. Vegetation on the NE, E, and SE flanks continued to be affected by acid rain. Resulting erosion problems included small debris flows, especially on the NE flank (Quebrada Guillermina), E flank (Quebrada Calle de Arenas), and SW flank (Río Agua Caliente).

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

Information Contacts: J. Barquero, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.

The eruption was apparently continuing on 17 October, when a NOAA 11 satellite image (at 1354) showed a plume extending ~150 km WNW from a point source at the volcano.

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast.

Information Contacts: W. Gould, NOAA.

Strombolian activity in Northeast Crater had decreased by mid-July, and was no longer apparent on 23 August. Strombolian activity was observed in Southeast Crater in mid-July. All four summit craters were in a state of almost continuous mild degassing through August.

The following is from a report on activity 10-14 July, by J.P. Kloster in LAVE Bulletin no. 33, p. 4.

A network of curvilinear fissures, up to 2 m wide, covered the N, E, and SE parts of Southeast Crater. In the NW part of the crater, a vent 15 m in diameter emitted puffs of gas roughly every 3 seconds. Every half hour, a more violent explosion ejected lava fragments to 200 m above the vent, covering the walls of the crater and occasionally sending projectiles outside of the crater. The largest projectiles were estimated at around 5 kg, and were fluid enough to deform on impact. Two vents emitted slightly incandescent plumes at night. Each explosion was preceded by ~12 seconds of increased incandescence at the non-exploding vent, and corresponding intensification of glow at the exploding vent. On one occasion, a roughly 100-kg block of lava was ejected to 50 m height.

In the S part of Northeast Crater, a long-persistent vent, 5 m in diameter, emitted puffs of gas. At night, the emission was incandescent to 30 m height, with small lava fragments ejected during the most violent explosions. No projectiles fell beyond the crater rim.

About 70 m below Bocca Nuova crater's S rim, gas was strongly emitted from an E-W fissure, several meters wide and 12 m long, that probably formed in October 1989 (14:10). Explosions were heard emanating from the fissure area, roughly every 4-5 minutes. Night glow was visible at the fissure and at the vents of two coalesced scoria cones in the S part of the crater floor.

Very little activity occurred in La Voragine crater. One 20-m-diameter vent, on a small cinder cone, degassed quietly to several meters, and had night glow. Snow on the cone was largely covered by scoriae, suggesting recent activity.

No Strombolian activity or lava emission were observed during a 23 August visit to the summit by J. Dehn and B. Behncke. The vent in Northeast Crater had strong gas emission, accompanied by loud roaring noises, but no solid material was ejected. A dense continuous gas column was rising from Southeast Crater.

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: J. Kloster, LAVE; J. Dehn and B. Behncke, GEOMAR, Kiel.

A greenish water discoloration was observed during a 30 September overflight by the JMSA. No discoloration was present during a similar overflight on 21 August.

Geologic Background. Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the pyramidal island of Minami-Ioto. Water discoloration is frequently observed from the volcano, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto ("New Sulfur Island") in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

Information Contacts: JMA.

Increased tephra emission, seismicity, and deformation culminated in the extrusion of a summit-crater lava dome in mid-October.

Activity continued to increase in September, with the highest number of events, the greatest released energy, and largest reduced displacement of long-period earthquakes since the start of monitoring in February 1989 (figure 44), surpassing the previous highs recorded in August. Tremor episodes and long-period events were associated with explosions, continuous ash emission, and occasional block ejections to 200 m from the vent. The SO₂ flux was relatively low, <=650 t/d, except on 20-25 September when values of 960-1,045 t/d were recorded. Low SO₂ fluxes were also recorded during August (<=370 t/d). The electronic tiltmeter [at Crater Station] became saturated, after recording 231 µrad of inflation during the first 2 weeks of August. Two km E of the crater, the Peladitos electronic tiltmeter showed 46.2 µrad of inflation at 242° azimuth (figure 45), compared to 3.7 µrad at 183° azimuth during the first 2 weeks of August. A change in the inclination of the radial component was recorded 9-13 September.

Figure 44. Daily number of earthquakes at Galeras, February 1989-September 1991. Courtesy of INGEOMINAS.

Figure 45. Radial component of deformation measured 2 km E of the crater ("Peladitos" electronic tiltmeter) at Galeras, September 1991. Courtesy of INGEOMINAS.

Fieldwork in mid-October revealed a lava dome in the main crater [but see 16:10]. [The dome was 30 m thick and 80-100 m in diameter by the 17th]. Incandescence was visible in cracks on the dome's surface, but bad weather prevented detailed observation.

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: INGEOMINAS-OVP; S. Williams and M. Calvache, Arizona State Univ.

In the first new activity since the paroxysmal event of 12-15 August, a small eruption occurred several days before 6 October, emitting ash that was carried 8 km SE. The probable source was the new vent in the SW part of the caldera, which showed numerous fumaroles, especially along the S wall, during a helicopter overflight on 14 September. At about 2200 on 11 October, a mudflow, estimated to be 2-3 times larger than the 8 August mudflow (16:7), traveled 20 km NW down the Huemules valley, and SSW down the Cupquelán valley to reach the coast at Bahía Erasmo (45 km from the volcano). During the evening a brief ashfall was reported 60 km SSE (at Bahía Murta), and a strong degassing event occurred within the caldera. Sulfurous fumes were very intense in the Huemules valley, where some inhabitants became sick, vomiting and losing consciousness. On 12 October, a sulfur odor was detected in Cochrane (150 km S) and at Coyhaique (75 km NE), but it had disappeared by 13 October. Several small earthquake swarms were recorded between 0400 and 1130 on 12 October.

A 16 October helicopter overflight of the mudflow deposits showed them to include ash, snow (ice), and mud; five dead animals were observed together with some others trapped in the mud. Bad weather conditions hindered direct observation of the caldera in October, and as of the 21st it was not certain whether the mudflow was caused by fumarolic activity (which apparently triggered a mudflow in 1973) or new lava emission (as in 1971 and on 8 August 1991). Seismicity fluctuated below ~ 10 recorded earthquakes/day through mid-October (figure 6), typical of levels since a rapid decrease following the paroxysmal eruption 12-15 August.

Figure 6. Daily number of earthquakes at Hudson, 17 August-17 October 1991. No data exist for 1-11 September or 1-10 October. A swarm recorded between 0400 and 1130 on 12 October is not included. Data for 11 October are for 1200-midnight; data for 17 October are for midnight-1020. August-September data are from Héctor Massone; October data are from Mario Villouta. All data were collected at a seismic station 50 km NE of the volcano near Lago Elizalde.

The following, from the Geological Team of the Santa Cruz Volcanic Emergency Commission, describes the impact of the Hudson eruption in Argentina.

The eruption's first effect in Argentina was reported on 9 August at 1600, when an Aerolineas Argentinas aircraft flew through a dense SO₂-rich cloud at ~ 10.5 km altitude, near 40°S, 69°W. NOAA 10 and 11 satellite images of the eruption's basaltic phase on 8-9 August showed a narrow 550-km-long plume extending NE across the central area of Chubut Province (Argentina). The 12-15 August paroxysmal trachyandesitic eruptive phase produced a dense plume of gas and ash that was transported ESE by the jet stream at nearly 200 km/hour, at ~ 8 km altitude near the caldera and rising progressively to more than 12 km at the Chile-Argentina border. This dense, elongate plume traveled in the lower stratosphere (12-16 km) without great dispersion, spanning a horizontal angle of only 15° during 12-15 August. As a consequence, tephra fall was concentrated in a narrow fan-shaped sector of Patagonia, extending to Puerto Deseado (580 km ESE) and San Julián (550 km SE) on the Atlantic coast. By the end of the eruption, 100,000 km² of Santa Cruz Province, Argentina, had been covered by an ash blanket 0.1-10 cm thick, and a 6.5-km-thick stratus cloud of suspended particles remained.

Deposits. On 16 August, shortly after tephra fall had effectively ceased, a strong wind began to blow (surface winds of up to 110 km/hour are common in Patagonia), lasting for a week. When the winds stopped, four multidisciplinary groups of the Santa Cruz Volcanic Emergency Commission were able to enter the affected area, employing specially dust-protected trucks. A survey was performed between 25 August and 1 September along access roads; the main technical problem in making isopach and volumetric estimations was correcting for remobilization of the primary tephra-fall deposits by the violent winds.

Sampling was performed, on individual layers or on the whole deposit, with an attempt to avoid remobilized materials. Isopach curves differ from the usual pattern, in which thickness rapidly decreases with distance from the vent. Instead, some maxima were found 500 km from the volcano, close to the Atlantic coast. This is probably a consequence of a special air-circulation condition: low-density pumice fragments transported by a powerful jet stream. In Santa Cruz Province, ash deposits originally ranged from 0.1 to 11 cm thick, but accumulations up to 150 cm thick were observed on the protected, downwind sides of obstacles in the zones affected by remobilization. The volume of the accumulated ash deposits was estimated at 2.8 km³, equivalent to ~ 2.3 x 1012 kg of ash deposited over a 100,000 km² area.

Nature of the materials. Investigation of the internal structure of the deposits revealed strong grading with a sharp transition between sand-sized and silt-sized layers. Bimodality is also reflected in the grain-size distribution, with one mode for sand and one mode for silt, and a remarkably low content of very fine sand. The layers indicated at least 5 strong pulses of eruptive activity.

The proportion of fine material clearly increases with distance to the eruptive center. Following this trend, the maximum pumice-fragment size diminished from around 3 mm near Lago Buenos Aires (a 110-km lake, 65 km from the volcano at its closest point) to ~ 0.3 mm along the Atlantic coast.

Density measured on 39 pumice samples ranged from 0.61 to 1.05 g/cm³. Coarse floating pumice, occasionally forming rafts, was observed in Lago Buenos Aires.

Mineralogical composition was determined for the very fine sand fraction (0.062-0.125 mm), which consists of vitric particles (87%) and crystals (13%) with rare lithics. The vitric particles are composed of pumice (~70%) and cuspate to platy shards (30%). Most pumice particles have a fibrous texture with tubular vesicles of subparallel orientation and longitude/diameter ratios larger than 10/1. Crystals are fresh, euhedral, commonly broken, and often have glass rinds. The main components are feldspar, olivine, ortho- and clinopyroxene, and magnetite. Biotite, tridymite and unidentified iron/manganese oxides are less abundant.

Regional impact. The consequences of the eruption on Argentine territory, together with the difficult meteorological conditions that usually affect this region, produced a serious and persistent situation. The tephra fall hindered transportation and caused the collapse of some house roofs due to the weight of the ash accumulations. Ashfall and/or the later remobilization by winds polluted the atmosphere, causing respiratory ailments, damage by abrasion to engines and mechanical devices, and other problems. Low visibility provoked numerous accidents and aviation inconveniences. Problems persisted in mid-October.

The eruption has caused many difficulties for livestock (principally sheep and cattle) and wildlife, including: thirst, because the great water absorption capacity of the fine pumice particles dried every source of water in this already desert area; hunger, because the tephra deposit covered the grass, preventing ingestion or obliging the animals to swallow a large proportion of ash; blindness, due to the abrasive action of the glass shards and minerals, continuously blown by the wind, on the cornea; and immobilization due to increase in weight by accumulation of ash into the wool fleece, impeding movement. As a consequence, 50% of the sheep and cattle had died within 30 days of the end of the Hudson eruption. [see also 16:10]

Vegetation in this desert region (<150 mm/year precipitation) was initially covered by a thin layer of ash that retained moisture. However, the strong winds removed this protective cover, damaging pastures by the impact of the particles and by completely burying them under dune formations.

Other phenomena related to the Hudson eruption were:

Earthquakes. In Bajo Caracoles, 200 km ESE of Hudson, shocks of medium intensity destroyed some adobe buildings. People from these rural areas reported loud noises and ground vibrations.

Acid rain. The odor of sulfurous gas was noticeable everywhere in the area affected by ashfalls, but acid rain was only reported in Gobernador Gregores, 360 km ESE of the volcano, where H₂SO₄ produced light face and hand burns, and damaged roof paint.

Atmospheric contamination. After the tephra fall stopped (August 15), remobilization of fine ash occurred each time the W wind blew at speeds higher than 35 km/hour, causing the formation of huge dust clouds. The population was warned to employ protective dust masks to prevent respiration of the fine glass shards in these clouds (<5 mm, in average concentrations of 300 p/cm³ with maximum of 1,500 p/cm³). This last phenomenon has had the largest impact on the rural and urban population, not counting the economic losses suffered.

Geologic Background. The ice-filled, 10-km-wide caldera of the remote Cerro Hudson volcano was not recognized until its first 20th-century eruption in 1971. It is the southernmost volcano in the Chilean Andes related to subduction of the Nazca plate beneath the South American plate. The massive volcano covers an area of 300 km2. The compound caldera is drained through a breach on its NW rim, which has been the source of mudflows down the Río de Los Huemeles. Two cinder cones occur N of the volcano and others occupy the SW and SE flanks. This volcano has been the source of several major Holocene explosive eruptions. An eruption about 6700 years ago was one of the largest known in the southern Andes during the Holocene; another eruption about 3600 years ago also produced more than 10 km3 of tephra. An eruption in 1991 was Chile's second largest of the 20th century and formed a new 800-m-wide crater in the SW portion of the caldera.

Information Contacts: H. Corbella, R. Scasso, P. Rial, M. Palacios, M. Lucero, P. Tiberi, and D. Perez, Geological Team of the Santa Cruz Volcanic Emergency Commission, Argentina; J. Naranjo, SERNAGEOMIN ; M. Villouta, Univ de la Frontera; H. Moreno, Univ de Chile; H. Massone, Univ de Chile; G. Villaroes Pinilla, ONEMI, Santiago; R. Osorio Araneda, Coyhaique, Chile; N. Banks, USGS.

Crater fumarolic activity continued in September, with the most intense emissions in the N and NW sections, where distinct gas vents were visible. Gas temperatures were 90°C and condensates had a pH of 3.5. One fumarole produced a roaring sound audible at the summit rim overlook. The level of the turquoise-green crater lake continued to rise; numerous fumaroles were drowned but continued emitting subaqueously. The average lake temperature was 37°C (compared to 35°C in July), with a temperature of 90°C measured near active fumaroles in the N part of the lake. Weaker fumarolic activity occurred in the SE, E, and NE parts of the lake, and on the crater's E slope. In September, 74 earthquakes (M 1-2) were recorded, with depths <8 km.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: J. Barquero, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.

Frequent breakouts . . . fed numerous surface lava flows between ~450 m (1,500 ft) altitude and the coast during September. Some lobes advanced rapidly downslope, but upon reaching the more level area near the coast they generally inflated, oozed small viscous breakouts, then stagnated. New flows covered previously untouched portions of Royal Gardens subdivision, and started a 1-day brush fire in a nearby section of Hawaii Volcanoes National Park.

The numerous breakouts and surface flows apparently reduced lava supply to the ocean entries, since all were inactive by late September. Lava had ceased to reach the sea by 3 September at the eastern of the two main ocean entries (Paradise). Two kilometers to the SW (at Poupou), lava had been flowing into the ocean at two points. A 1-2-m tumulus developed atop Poupou's E lava bench on 4 September, and extruded small, very viscous flows through cracks before all activity stopped at the E entry point 2 days later. Lava continued to pour into the ocean at the W Poupou entry point for the next 2 weeks. On 19 September, a very viscous pahoehoe flow broke out from its main feeder tube <500 m inland, in an area of inflated and fractured ground, remaining active for a few days before stagnating. Lava stopped flowing into the sea on 20 September, when small collapses of the bench front exposed hot rock to seawater, creating short-lived steam plumes. Entry of lava into the sea had not resumed as of early October, but surface flows from breakouts between 30 and 300 m elevation spread over a broad area.

Little surface activity was evident at Kupaianaha vent, source of the lava for the main tube system, although a skylight in the neck of its frozen lava pond was open on 7 September, and heavy fuming and a glowing crack were visible on the uprift edge of the pond on the 17th. Three km uprift, an 80 x 50 m lava pond covered ~15% of the floor of Pu`u `O`o crater during most of September, and frequent overflows sent lava across the crater bottom, ~36 m below the 1986 spillway. During 1.5 hours of observations from the crater rim on 17 September, vigorous upwelling of lava occurred on the downrift side of the pond while continuous spattering on the pond's N side added to its 2-3-m-high spatter rampart. Two episodes of strong degassing were characterized by vigorous upwelling and spattering followed by roughly 2-3 m of rapid drainback. Pond level was high and the entire crater floor was typically covered by overflows before drainback episodes. Seismic instruments registered tremor during the degassing episodes, which were sometimes precipitated by collapses from the crater walls. On 28 September, lava in the pond withdrew to >15 m below the crater floor. After at least 5 hours of inactivity, a small vent opened on the crater floor and a cascade of lava poured into the drained pond, filling it to within a meter of the crater floor by the next day. The vent remained open and intermittently active, sending lava across the crater floor and into the pond. The vent continued to feed periodic overflows the following week, and the crater floor was covered with lava during observations on 4 October.

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

Information Contacts: T. Mattox, HVO.

"Moderate to strong activity persisted at Langila throughout September. Ash-laden vapor emissions were released almost continuously from Crater 2, accompanied by weak to loud rumbling sounds. A weak glow was visible on most nights, except 21-25 September. Activity at Crater 3 consisted of occasional Vulcanian explosions, at rates of 30-80/day. The activity was stronger 23-26 September and a steady night glow was visible 25-26 September.

"The amplitude of discontinuous tremor declined markedly in late August, while cumulative daily tremor duration dropped to <=100 minutes from levels that had been as high as 200 minutes/day since mid-June."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower eastern flank of the extinct Talawe volcano. Talawe is the highest volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila volcano was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the north and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit of Langila. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: P. de Saint-Ours and C. McKee, RVO.

The eruption was first observed on 25 September at about 2100 from a ship ~ 75 km S of Marchena [but see 16:10], and glow remained visible through the night. A large black and white eruption cloud was reported the next day, but no glow was evident during cloudy weather that night from a nearby island. During an overflight around midday on 27 September, a dark plume was visible above low weather clouds.

When David Day and others arrived at the island's W coast on 28 September at about 2230, lava was flowing into the sea along a front ~1.5 km wide. Incandescence was evident at about 10 sites over a roughly 3 x 3 km area, but lava fountaining had apparently stopped. The next day, small quantities of Pele's hair were found on the beach near the fresh flows, along with substantial numbers of dead fish and other marine organisms. Scuba divers found glassy breccia near the shore, a zone of aa rubble extending seaward for several hundred meters, and, with gradual increase in slope from 25 to 35 m depth, a small lava flow that included pillow structures. A 30 September summit climb revealed new lava covering much of the caldera's SW floor, suggesting that a circumferential fissure several kilometers long had been active on the W to SW rim, supplying lava to both the caldera floor and the outer flank.

Geologic Background. The low shield volcano forming Marchena Island contains one of the largest calderas of the Galápagos Islands. The 6 x 7 km caldera and its outer flanks have been largely buried by a cluster of pyroclastic cones and associated lava flows. Its first historical eruption occurred in 1991. Other young lava flows, some of which may be only a few thousand, or even a few hundred years old, filled the caldera and flowed down its outer forested flanks, in some cases to the sea.

Information Contacts: D. Day, Isla Santa Cruz.

An eruption . . . apparently began on 20 or 22 September . . . . On 24 September, the Nimbus-7 satellite's TOMS showed an SO₂ enhancement, just above detection limits, centered immediately over the volcano. A small amount of SO₂ may have been present the previous day, but increased SO2 was not evident 20-22 or 25 September. No additional reports of activity were received until the crew of a British Airways flight observed an eruption cloud rising to ~7.5 km altitude on 15 October at 0555. In a report received 18 October, observers in Goma, Zaire (roughly 30 km S of the summit) described lava flows to the NW and SE [but see 17:1] and an eruption cloud rising to ~3.5 km from a site at 1.28°S, 29.22°E (on the NNE flank) [see slightly revised coordinates in 17:1]. South African Airways reported a plume to ~7.5 km altitude, probably on 20 October.

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: ICAO; S. Doiron, GSFC; H-L. Hody, GEOVAR, Kigali, Rwanda.

A strong explosive phase occurred on 22 August, after 3 weeks of relative quiet that followed strong explosive activity from 6 June to 1 August. Volcanologists visiting the summit observed an increase in fumarolic activity beginning at 1400, and the start of small explosions 20 minutes later. The frequency and sound of the explosions increased, and by 1425, lapilli and blocks were rising above the rim of the crater, before falling back into the interior. Between 1,500 and 1630 the explosions increased in size, feeding a continuous lava fountain 100-200 m high. The eruption entered its most vigorous period at 1700, with a black plume 1,000 m high, and explosions heard to 1 km from the crater. Activity declined 1710-1730, as lava fountaining was replaced by strong black plume emission that gradually ceased.

The summit was visited again on 20 September, two days after strong explosions were observed from Guatemala City (45 km N). Fresh bombs, flattened on impact, with crystal-rich interiors, covered the E part of MacKenney Cone. The explosions on 18 September coincided with the last in a 24-hour series of earthquakes that killed at least 25 people. The largest shock [M 6.1 at 5 km depth] was centered about [30 km NNW] of the volcano.

Geologic Background. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

Information Contacts: Franck Pothé, Sallanches, France; Sección de Vulcanología, INSIVUMEH; USGS NEIC.

Although no explosions have occurred in the caldera since 2 September (16:8), small secondary explosions continued through early October, at pyroclastic-flow deposits from the paroxysmal June eruption. Seismicity was at low levels, and a decrease in the number of mudflows reflected decreased rainfall.

16-22 September. Several secondary explosions were observed at pyroclastic-flow deposits in the Sacobia valley (NE flank). Ash was periodically reported falling at Clark Air Base (20 km E) and Angeles (25 km E), with 5 mm deposited on the morning of 18 September. No steam or ash emissions were recorded from the summit caldera.

Clusters of high-frequency earthquakes persisted near the caldera (figure 22), but noticeably fewer events have been recorded since June, especially at shallow depths (figure 23). A slight increase in seismicity was associated with a M 4.5 earthquake at 0151 on 17 September. The shock was felt at intensity V (modified Rossi-Forel scale) at Clark Air Base, with aftershocks of intensities I-IV. Occasional low-frequency, small-amplitude harmonic tremor of uncertain origin was recorded 21-22 September. Explosion-type low-frequency signals were periodically observed during and after heavy rainfall.

Figure 22. Epicenters of 619 earthquakes recorded near Pinatubo, 27 May-22 September 1991; 515 had magnitudes less than 1, 104 were of M 1-2. Contours are diagrammatic. Courtesy of PHIVOLCS.

Figure 23. Depths of 619 earthquakes at Pinatubo, 27 June-22 September 1991. Courtesy of PHIVOLCS.

A lake that had formed in the Mapanuepe River (SW flank), following damming by pyroclastic-flow deposits, continued to grow until heavy rain caused it to overflow on 21 September. The dam was quickly incised and partially breached, leading to flooding in San Narciso (~35 km SW) on 22 September.

23-29 September. No ashfall occurred at Clark Air Base, although several secondary explosions were observed. The number of high-frequency earthquakes (M < 3.6) decreased slightly from the previous week. Three shocks were felt (intensity I-II) on 28 September. Minimal rains occurred during the week, and no mudflows were detected. The caldera lake was turquoise-blue and steamed along its S wall during a 23 September helicopter overflight. Fresh landslides were noted on the steep-sided caldera walls.

30 September-6 October. Secondary explosions produced plumes to 4,500 m high, and ash fell on Angeles 3 October. SO₂ flux from the caldera, measured by COSPEC, was only 18 t/d. Geologists noted that this flux represented SO₂ rising from the caldera lake, and subaerial fumaroles; an unknown quantity of SO₂ from fumaroles below the lake may be going into solution. The lake had a temperature of 40°C and a pH of 6.0 on 8 October.

Investigations of hydrologic changes caused by the thick pyroclastic-flow deposits and voluminous debris-flow deposits were not yet able to resolve whether the Abacan River (E flank) would be captured by the Sacobia River. Numerous catchment basins and dams were under construction in an attempt to constrain the Abacan River.

High-frequency seismicity decreased slightly, with a daily average of 63 recorded earthquakes, compared to 71 the preceding week. Events were centered at 2-10 km depths, shallower than the preceding week (5-15 km depths), and were distinctly clustered E of the caldera. Most of the earthquakes had magnitudes less than 2, although some reached M 3, and many were felt at Clark Air Base. Two mudflows were detected during the week.

Casualties and damages. The United Nations Development Programme reported on 15 October that the human death toll had reached 722. Of these, 358 deaths were attributed to disease, 281 to the initial eruption, and 83 to mudflows. An additional 184 people were injured, and 23 reported missing. More than 108,000 houses were partially or totally damaged, more than 100,000 people were still being fed at 143 evacuation centers, and an additional 70,000 were being fed near affected villages.

Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Information Contacts: R. Punongbayan, PHIVOLCS; J. Tomblin, UN Disaster Relief Organization, Switzerland.

The level of crater lake water, bright yellow and 71°C, decreased in September, despite continuation of the rainy season. Phreatic explosions in the central part of the lake ejected water to a height of 2 m. Steam and gas rose nearly continuously, creating small plumes that were carried W and SW by wind. A strong sulfur smell and respiratory difficulties were reported by park rangers on 22-23 September. The gas plume rose vertically during calm conditions the morning of 23 September, and was visible >10 km away from the Central Valley.

A daily average of 262 low-frequency (<2.5 Hz) earthquakes was recorded in September, with the highest levels of activity on the 21st-22nd. Low- and medium-frequency tremor episodes were recorded, with maximum durations of 2 hours.

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

Information Contacts: J. Barquero, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.

The number of seismic events and the amount of released energy were both at low levels in September. Only a few short pulses of very low-energy tremor were recorded. Deformation showed no significant changes. The monthly average SO₂ flux was low, declining to ~925 t/d from 1,135 t/d in August.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

Activity was moderate from August through mid-September, with occasional explosions from craters C1 and C3 (about 1-4 hourly/crater). During 1 August to 25 September, the average number of explosion shocks/hour gradually increased to the "normal" value of 6; (figure 20). Relatively low levels of activity lasted from mid-July to mid-August. The number of instrument-saturating events was low and concentrated on a few days during the second week in August, while average tremor amplitude remained relatively stable until it declined in late August (figure 21).

Figure 20. Average number of explosion shocks/hour at Stromboli, 1 August-25 September 1991. The mean value for the period is shown by the dotted line. Courtesy of M. Riuscetti.

Figure 21. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude in volts (upper curve) at Stromboli, 1 August-25 September 1991. Courtesy of M. Riuscetti.

The following report from Jonathan Dehn and Boris Behncke, on 28-29 August activity, supplements information in 16:8. "During a visit to Stromboli's summit and craters on 28-29 August, the volcano, much less active than during visits in March-April 1990 and November 1990, was in a state similar to that observed in mid-September 1989. Eruptions came from only two vents, one at the NE end of C1 (figure 22), and the other in the center of C3. The former erupted frequently during the evening of 28 August producing fountains up to 150 m high. Such eruptions lasted at least 10 seconds and began without any noticeable premonitory signs. Bombs and spatter fell over much of the C1 area and into C2. After midnight on 29 August, explosions from this vent became much less frequent, averaging 1-2/hour. The other active vent produced ~2-4 explosions/hour but much lower fountains, and ejecta mostly fell back into C3. These explosions were often preceded by an increasingly bright glow from the interior of C3.

Figure 22. Sketch of Stromboli's crater terrace as seen from Pizzo sopra la Fossa, 28-29 August 1991. Features (such as cone at vent 3 in C1) are not to scale. Courtesy of J. Dehn and B. Behncke.

"During the night, four glowing but not erupting vents (1 and 2 in C2, and 1 and 2 in C3) could be observed, at the same positions as in late 1990. Two more vents (2 and 3) were visible within or near C1. Vent 2 appeared to be a larger pit created by collapse of the former vents 1 and 2. A symmetrical cone ~10 m high had been built around C1's vent 3, feeding a dense gas plume.

"C3 could be approached around daybreak (0530-0630) on 29 August, and observations of the crater's interior and its eruptive activity were made during a 45-minute period. The SW half of the crater was occupied by a pit ~50 m deep. This pit contained at least three active vents, of which two were open and displaying continuous glow and gas emission. Minor spattering from these vents occurred at times, often heralding eruptions from a third vent, in the center of the pit's floor, that was buried under a cover of bombs and scoriae. Eruptions from this vent fragmented the old overlying material, forming diffuse ash plumes followed by the ejection of fresh glowing spatter. All of the ejecta from C3 fell back into the pit and onto its walls, and slumping of material from the pit's walls commonly covered the vent within seconds after each eruption.

"Vents 1 and 2 (in C3) were also observed at close range. They contained small active lava ponds, or molten sulfur, and displayed pulsating gas emission. The ponds were located within a cavern, but no obvious vertical conduit was seen. The rim of C3 was covered with a thin coating of fine tephra and fresh Pele's hair, but no ejecta fell in this area during the stay on the crater rim. From 0630 until about 1100 (29 August), the end of the summit visit, eruptions were separated by intervals ranging from 10 to 40 minutes."

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5,000 years ago due to a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: J. Dehn and B. Behncke, GEOMAR, Kiel; M. Riuscetti, Univ di Udine.

Increased seismicity has been observed at Ulawun, but surface activity remained limited to release of small to moderate volumes of white vapor until late September. Recorded seismicity since mid-August has consisted of numerous to nearly continuous, very small, low-frequency events. Larger events began to occur in mid-September. Periods of stronger seismic activity were occasionally separated by intervals of little seismicity, producing a banded appearance on seismograms. Similar seismicity has been recorded previously at Ulawun, and at Karkar prior to its 1979 eruption. Gas emission on 29 and 30 September was stronger, and brown plumes were reported.

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 N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 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: P. de Saint-Ours and C. McKee, RVO.

Lava dome growth continued in Jigoku-ato crater through early October, generating frequent pyroclastic flows by partial dome collapse. A seismic swarm began beneath the crater on 6 September, peaking 7-9 September and again on 15-16 September. Seismicity was similar to that of mid-August when dome 3 (formerly called the central dome, or C-dome) began to grow, although September activity was more vigorous.

Extruded lava pushed the crater wall NE, leading to a collapse of the crater wall and the adjoining dome 3 at 1854 on 15 September. Red-colored ash plumes were observed intermittently earlier in the day, becoming stronger and more frequent by the start of a 2-hour increase in the number and size of pyroclastic flows at 1644. The activity culminated in a major pyroclastic flow that generated an 11-minute seismic signal, the longest since 8 June, composed of 6 successive events including one that lasted 4 minutes beginning at 1857. The pyroclastic flow traveled ENE down the Oshiga Valley, then followed it SE into the Mizunashi River valley, reaching 5.5 km from the dome (figure 33), the same distance as the 8 June flow. An ash cloud surge detached from the flow at the confluence of the Oshiga with the Mizunashi River, continuing SE into the Onokoba area of Fukae city. A car previously burned by June pyroclastic flows was thrown about 60 m by the surge. Although it appeared that the surge rapidly lost force when it detached from the rest of the pyroclastic flow, its temperature remained high. [A total of 218] houses were burned, but the area had already been evacuated and there were no injuries.

Figure 33. Map showing the distribution of the 15 September 1991 pyroclastic-flow deposits at Unzen. Courtesy of Setsuya Nakada.

The average speed of the pyroclastic flow was estimated at 95 km/hour, using the interval between the start of the flow's seismic signal and its cessation when the flow destroyed the telemeter cable. The volume of collapsed dome lavas and pre-existing summit rock was estimated to be 3.5 x 10⁶ m³, of which 2.5 x 10⁶ m³ was believed to be from the dome. Using aerial photos, the Ministry of Construction estimated the volume of 15 September pyroclastic-flow deposits at 4 x 10⁶ m³, indicating that the main flow was the largest since the start of activity in May. Flow deposits were about 5 m thick, and although the interior was still hot 3 weeks after deposition, no gas-pipe structures were observed. Lava blocks in the deposits were massive, without the breadcrust or cauliflower surfaces seen in the 8 June pyroclastic-flow deposits. The biotite hornblende dacite composition of the blocks was the same as that of lava fragments from the 3 and 8 June flows and the pumice ejected from Jigoku-ato crater on 11 June. Peripheral surge deposits consisted of a lower coarse sand layer, and an upper reddish ash layer.

A total of 448 earthquakes was recorded on 16 September, the most in one day since July 1990, but seismicity rapidly declined, reaching former levels by the following morning. Dome 4 began to grow in the horseshoe-shaped depression (about 300 m wide and 400 m long) formed by the collapse of dome 3 (figure 34). By 20 September, it had grown to about 1.6 x 10⁶ m³, indicating an extrusion rate of about 3.5 x 10⁵ m³/day. The new dome was 350 m long, 250 m wide, and >150 m high on 10 October. Following the pattern of dome 2, the length of dome 4 stopped increasing at the end of September, as continuing collapse of its distal end generated small-scale block-and-ash flows. The upper reaches of the Oshiga valley were filled with debris shed from the domes, and large pyroclastic flows were rare.

Figure 34. Sketch map showing the growth of domes 3 and 4 at Unzen, 13 August-28 September 1991. Numbered outlines show boundaries of dome 3 (upper) and dome 4 (lower) on indicated dates. Dome 4 was extruded within the horseshoe-shaped depression, opening NE, formed on 15 September 1991. Courtesy of Setsuya Nakada.

A total of 521 seismically determined pyroclastic-flow events and 2102 earthquakes were recorded in September, increases from 292 and 559, respectively, in August. There were no felt shocks in either September or early October.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: JMA; S. Nakada, Kyushu Univ.

Several explosions have been documented from May 91 vent since the 28-29 August fieldwork. The first occurred on 29 August at 2132, shortly after geologists left the island. A second, observed by R. Fleming on 5 September at 1149, produced a 2-km eruption column. Blocks larger than 1 m across fell 250 m SE of the vent and floating scoriae washed onto the shore at Crater Bay, roughly 800 m SE. A large eruption column that rose to 4-5 km altitude on 16 September at 1630, accompanied by intermittent block ejection, was observed from a nearby boat and from ~50 km away on the North Island coast (at Whakatane). Emission of ash continued overnight, with ash falling on a boat off the SE tip of the island, but tephra emission declined the next morning. Another eruptive episode was reported on 18 September at 1625, lasting 5-6 minutes and feeding a 2-km column.

When geologists returned to the island on 9 October, lithic blocks occupied scattered impact craters in a zone extending roughly 400-750 m SE from May 91 vent. Closer to the vent, blocks and craters became more common and fragments of slaggy scoria began to appear. Most of the scoriae were highly vesicular, but bombs of denser scoria, characterized by a smaller range of vesicle sizes (most <5 mm), were also found. Bombs reached 0.8 m in size and the largest observed lithic block was 1.3 m across. Most of the ballistic ejecta appeared to have fallen on the S side of the main crater floor, where it seemed to be directed by the vent configuration. Between 5 and 10 mm of fine gray ash was found at sites roughly 250 and 400 m SE of the vent, thickening toward the rim of the 1978/91 Crater complex.

Activity at May 91 vent on 9 October was similar to but less intense than that observed in late August, with emission of pink fume accompanied by loud roaring and occasional sharp detonations. The noises suggested that the top of the magma column was deeper in the conduit than at the end of August. Shock waves were visible about once a minute, less often than in late August, although conditions were less suitable for their observation. Low-pressure white gas emission from TV1 vent, roughly 50 m SE of May 91 vent, was similar to that of August, although its N wall had apparently migrated outwards 10-20 m by collapse. Voluminous gas emission continued from Noisy Nellie fumarole, on the main crater floor NE of the 1978/91 Crater complex.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: I. Nairn and B. Scott, DSIR Geology & Geophysics, Rotorua.

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/).