The remote island of Semisopochnoi in the western Aleutians is dominated by a caldera measuring 8 km in diameter that contains a small lake (Fenner Lake) and a number of post-caldera cones and craters. A small (100 m diameter) crater lake in the N cone of Semisopochnoi's Cerberus three-cone cluster has persisted since January 2019. An eruption at Sugarloaf Peak in 1987 included an ash plume (SEAN 12:04). Activity during September-October 2018 included increased seismicity and small explosions (BGVN 44:02). The primary source of information for this reporting period of July-August 2019 comes from the Alaska Volcano Observatory (AVO), when there were two low-level eruptions.

Seismicity rose above background levels on 5 July 2019. AVO reported that data from local seismic and infrasound sensors likely detected a small explosion on 16 July. A strong tremor on 17 July generated airwaves that were detected on an infrasound array 260 km E on Adak Island. In addition to this, a small plume extended 18 km WSW from the Cerberus vent, but no ash signals were detected in satellite data. Seismicity decreased abruptly on 18 July after a short-lived eruption. Seismicity increased slightly on 23 July and remained elevated through August.

On 24 July 2019 AVO reported that satellite data showed that the crater lake was gone and a new, shallow inner crater measuring 80 m in diameter had formed on the crater floor, though no lava was identified. Satellite imagery indicated that the crater of the Cerberus N cone had been replaced by a smooth, featureless area of either tephra or water at a level several meters below the previous floor. Satellite imagery detected faint steam plumes rising to 5-10 km altitude and minor SO₂ emissions on 27 July. Satellite data showed a steam plume rising from Semisopochnoi on 18 August and SO2 emissions on 21-22 August. Ground-coupled airwaves identified in seismic data on 23-24 August was indicative of additional explosive activity.

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

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

Krakatau volcano in the Sunda Strait between Java and Sumatra, Indonesia experienced a major caldera collapse around 535 CE; it formed a 7-km-wide caldera ringed by three islands. Remnants of this volcano joined to create the pre-1883 Krakatau Island which collapsed during the major 1883 eruption. Anak Krakatau (Child of Krakatau), constructed beginning in late 1927 within the 1883 caldera (BGVN 44:03, figure 56), was the site of over 40 smaller episodes until 22 December 2018 when a large explosion and tsunami destroyed most of the 338-m-high edifice (BGVN 44:03). Subsequent activity from February-July 2019 is covered in this report with information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG). Aviation reports are provided by the Darwin Volcanic Ash Advisory Center (VAAC), and photographs from several social media sources.

The cyclical nature of the growth and destruction of Krakatau was made apparent again in the explosive events of 22 December 2018-6 January 2019, when much of the island of Anak Krakatau was destroyed in a series of events that included a deadly tsunami from a flank collapse, a Vulcanian explosion, and several days of Surtseyan phreatomagmatic activity (figure 83) (Gouhier and Paris, 2019). Due to the location of the volcano in the middle of Sunda Strait, surrounded by coastal communities, damage from the tsunami was once again significant; over 400 fatalities and 30,000 injuries were reported along with damage to thousands of homes, businesses, and boats (figure 84) (BGVN 44:03). After a small explosion on 8 January 2019, the volcano remained quiet until 14 February when a new seismic event was recorded. Intermittent explosions increased in frequency and continued through July 2019; images of Surtseyan explosions with ejecta and steam rising a few hundred meters were occasionally captured by authorities patrolling the Krakatau Islands Nature Preserve and Marine Nature Reserve (KPHK), and by a newly installed webcam.

Figure 83. The dramatic morphologic changes of Anak Krakatau before and after the explosive events of 22 December 2019-6 January 2019 were apparent in these Planet Labs, Inc. images published by the BBC. Left: Planet Lab's Dove satellite captured this clear image of the 338-m-high cone with a summit crater on 17 December 2018. Center: The skies cleared enough on 30 December to reveal the new crater in place of the former cone after the explosions and tsunami of 22-23 December, and multiple subsequent explosions. Right: Surtseyan explosions continued daily through 6 January; Planet Labs captured this event on 2 January 2019. Courtesy of BBC and Planet Labs, Inc.

Figure 84. The location of Anak Krakatau in the middle of Sunda Strait surrounded by populated coastal communities (left) places great risk on those communities from explosive events and tsunamis at the volcano, such as what occurred during the 22 December 2018-6 January 2019 destruction of Anak Krakatau. The village of Tanjung in South Lampung (right) was especially hard hit. Map courtesy of BBC News, and photo courtesy of Daily Mail.

Three explosions were reported on 14, 18, and 23 February. No ash plume was observed on 14 February. The event on 18 February produced a dense gray ash plume that rose 720 m and drifted SSW. On 23 February the plume was white and rose 500 m, drifting ENE. During most days, no emissions were observed; occasional plumes of steam rose 50-100 m above the crater. Authorities visited the island on 15 February and observed the new crater lake and ash-covered flank of the remnant cone (figure 85 and 86).

Figure 85. The denuded slope and new crater at Anak Krakatau on 15 February 2019. Bright orange discoloration of the water on the W side of the volcano is from recent iron-rich discharge. The new summit was measured at 155 m high. Verlaten Island is in the background. Courtesy of Sutopo Purwo Nugroho, BNPB.

Figure 86. The new crater at Anak Krakatau on 15 February 2019. Fumarolic activity is visible in the narrow strip between the crater and the bay; bright orange discoloration of the water on the W side of the volcano is from recent iron-rich discharge. Courtesy of Sutopo Purwo Nugroho, BNPB.

Activity increased during March 2019 with 14 seismic events recorded. Four events on 14 March were reported, with durations ranging from 30 seconds to 4 minutes; neither ash nor steam plumes were reported from these events. Events on 16, 17, and 18 March produced N-drifting white steam plumes that were reported at altitudes of 1.2 km, 650 m, and 350 m, respectively (figure 87). Multiple additional explosions were reported on 24, 30, and 31 March; dense white plumes drifted NE on 30 and 31 March. Nearby rangers for the KPHK who witnessed the explosions on 30 March reported material rising 500-1,000 m above the crater (figure 88). The duration of the seismic events associated with the explosions ranged from 40 seconds to 5 minutes during the second half of March. PVMBG lowered the Alert Level from III to II on 25 March, noting that while explosions continued, the intensity and frequency had decreased; none of the explosions were heard at the Pasauran-Banten (SE) or Kalianda-Lampung (NE) stations that were each about 50 km away.

Figure 87. An eruption at Krakatau on 18 March 2019 produced a steam plume that rose several hundred meters, barely visible from a community across the strait. Courtesy of Oystein Anderson and PVMBG.

Figure 88. White steam and dark ejecta were observed at Anak Krakatau during an explosion on 30 March 2019 by the local patrol team from BKSDA Bengkulu-Ministry of LHK, which manages the Krakatau Islands Nature Preserve and Marine Nature Reserve. Courtesy of Krakatau Islands KPHK.

Although the number of reported seismic events increased significantly during April and May 2019, with 22 VONA's issued during April and 41 during May, only a single event had witnessed evidence of ejecta on 3 April (figure 89). The KPHK patrol that monitors conditions on the islands observed the first plant life returning on Sertung Island (5 km W of Anak Krakatau) on 5 April 2019, emerging through the several centimeters of fresh ash from the explosions and tsunami in late December and early January (figure 90). A 200-m-high steam plume was observed on 14 April, and plumes drifted NE and E on 27 and 29 April.

Figure 89. Rangers for KPHK photographed a Surtseyan explosion with tephra and steam at Anak Krakatau on 3 April 2019. Courtesy of Krakatau Islands KPHK.

Figure 90. A new plant on nearby Sertung Island emerges on 5 April 2019 through several centimeters of fresh ash from the Anak Krakatau explosions of December 2018 and January 2019. Courtesy of Krakatau Islands KPHK.

Members of an expedition to the island on 4 May 2019 photographed the still-steaming lake inside the new crater and the eroding ash-covered slopes (figure 91). Only the explosions on 10 and 17 May produced visible steam plumes that month, to 300-350 m high. By 15 May 2019 a new station had been installed at Anak Krakatau by PVMBG (figure 92). Four separate seismic events were recorded that day. Fog covered the island on a daily basis, and very few visible steam plumes were reported throughout April and May. The durations of the explosion events ranged from 30 seconds to 13 minutes (on 10 May); most of the events lasted for 1-2 minutes.

Figure 91. Members of an expedition photographed the water-filled crater and ash-laden slopes of Anak Krakatau on 4 May 2019. Top image is looking S with Rakata island in the background, bottom image is looking W from the flank of the cone remnant. Photo by Galih Jati, courtesy of Volcano Discovery.

Figure 92. By 15 May 2019 a new seismic station had been installed at Anak Krakatau by PVMBG. Four separate seismic events were recorded on 15 May 2019. Courtesy of Krakatau Islands KPHK.

Nine explosive events were reported during June 2019, but none produced visible steam or ash plumes until 25 June when a PVMBG webcam placed on Anak Krakatau captured a video of a Surtseyan event that lasted for about one minute. Dark gray ejecta shot tens of meters into the air over the lake, accompanied by billowing steam plumes which soon engulfed the webcam (figure 93). The other explosive events during March-July were likely similar, but frequent fog and the short-lived nature of the events made visual evidence scarce from webcams located 50 km away. During July there were 21 VONAs issued reporting similar seismic events that lasted from 30 seconds to 5 minutes; no plumes or sounds were seen or heard.

Figure 93. Dark gray ejecta and billowing steam plumes were captured by a newly installed PVMBG webcam during an explosion at Anak Krakatau on 25 June 2019. The water-laden ash rose tens of meters and scattered ejecta around the island. See Information Contacts for a link to the video. Courtesy of Devy Kamil Syahbana and PVMBG.

Satellite imagery provided solid evidence that activity at Anak Krakatau during February-July 2019 included underwater venting. Dark orange submarine plumes were visible drifting away from the SW flank of the volcano near the new crater multiple times each month (figure 94). The patterns of the plumes varied in size and intensity, suggesting repeated injections of material into the water. The thermal activity showed a marked decline from the period prior to the large explosions and tsunami on 22-23 December 2018. Very little thermal activity was reported during January-March 2019, it increased moderately during April-July 2019 (figure 95).

Figure 94. Dark orange plumes were visible in the seawater around Anak Krakatau during February-July 2019, strongly suggesting submarine discharges from the volcano. Top left: On 2 February 2019 the plume was discharging to the SW and visible in the water for nearly 10 km. Top center and right: on 29 March and 3 April the brightest areas of discharge were off the immediate SW flank; the plumes were drifting both NW and SE around the island. By 28 May (bottom left) the discharge was concentrated close to the SW flank with multiple underwater plumes suggesting several emission points. The only satellite image evidence suggesting a subaerial eruption appeared on 9 June (bottom center) when a dense steam plume rising and possible ejecta in the crater were visible. By 27 July (bottom right), discharge was still visible from the underwater vents on the SW flank, and the gradual filling in of the embayment on the W flank, when compared with the 2 February image, was clear. The island is about 2 km in diameter. Sentinel-2 satellite images with natural color rendering (bands 4,3,2) courtesy of Sentinel Hub Playground.

Figure 95. Thermal activity dropped abruptly at Anak Krakatau after the major flank collapse, explosions, and tsunami on 22-23 December 2018; it remained quiet through March and increased modestly during April-July 2019. Courtesy of MIROVA.

References: Gouhier, M, and Paris, R, 2019, SO₂ and tephra emissions during the December 22, 2018 Anak Krakatau flank-collapse eruption, Volcanica 2(2): 91-103. doi: 10.30909/vol.02.02.91103.

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

Information Contacts: 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/); Krakatau Islands KPHK, Conservation Area Region III Lampung, BKSDA Bengkulu-Ministry of LHK, (URL: https://www.instagram.com/krakatau_ca_cal); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); BBC News, (URL: https://www.bbc.com, article at https://www.bbc.com/news/science-environment-46743362); Planet Labs Inc. (URL: http://www.planet.com/); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN, image at https://twitter.com/Sutopo_PN/status/1101007655290589185/photo/1); Øystein Lund Andersen? (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com, image at https://twitter.com/OysteinLAnderse/status/1107479025126039552/photo/1); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/), images at https://www.volcanodiscovery.com/krakatau/news/80657/Krakatau-volcano-Indonesia-activity-update-and-field-report-increasing-unrest.html; Devy Kamil Syahbana, Volcanologist, Bandung, Indonesia, (URL: https://twitter.com/_elangtimur, video at https://twitter.com/_elangtimur/status/1143372011177033728); The Daily Mail (URL: https://www.dailymail.co.uk, article at https://www.dailymail.co.uk/sciencetech/article-6910895/FORTY-volcanoes-world-potential-Anak-Krakatoa-eruptions.html) published 11 April 2019.

The Mount Bromo pyroclastic cone within the Tengger Caldera erupts frequently, typically producing gas-and-steam plumes, ash plumes, and explosions (BGVN 44:05). Information compiled for the reporting period of May-July 2019 is from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

The eruptive activity at Tengger Caldera that began in mid-February continued through late July 2019, including white-and-brown ash plumes, ash emissions, and tremors. During the months of May through June 2019, white plumes rose between 50 to 600 m above the summit. Satellite imagery captured a small gas-and-steam plume from Bromo on 5 June (figure 18). Low-frequency tremors were recorded by a seismograph from May through July 2019.

Figure 18. Sentinel-2 satellite image showing a small gas-and-steam plume rising from the Bromo cone (center) in the Tengger Caldera on 5 June 2019. Thermal (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

According to PVMBG and a Volcano Observatory Notice for Aviation (VONA), an ash eruption occurred on 19 July 2019; however, no ash column was observed due to weather conditions. A seismograph recorded five earthquakes and three shallow volcanic tremors the same day. In addition, rainfall triggered a lahar on the SW flank of Bromo.

On 28 July the Darwin VAAC reported that ash plumes originating from Bromo rose to a maximum altitude of about 3.9 km and drifted NW from the summit, based on webcam images and pilot reports. PVMBG reported that lower altitude ash plumes (2.4 km) on the same day were also recorded by webcam images, satellite imagery (Himawari-8), and weather models.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

Large areas of floating pumice, termed rafts, were encountered by sailors in the northern Tonga region approximately 80 km NW of Vava'u starting around 9 August 2019; the pumice reached the western islands of Fiji by 9 October (figure 7). Pumice rafts are floating masses of individual clasts ranging from millimeters to meters in diameter. The pumice clasts form when silicic magma is degassing, forming bubbles as it rises to the surface, which then rapidly cools to form solid rock. The isolated vesicles formed by the bubbles provide buoyancy to the rock and in turn, the entire pumice raft. These rafts are spread and carried by currents across the ocean; rafts originating in the Tonga area can eventually reach Australia. This report summarizes the pumice raft eruption from early August 2019 using witness accounts and satellite images (acquisition dates are given in UTC). Pending further research, the presumed source is the unnamed Tongan seamount (volcano number 243091) about 45 km NW of Vava'u, the origin of an earlier pumice raft produced during an eruption in 2001.

Figure 7. The path of the pumice from the unnamed Tongan seamount from 9 August to 9 October 2019 based on eye-witness accounts and satellite data discussed below, as well as additional Aqua/MODIS satellite images from NASA Worldview. Blue Marble MODIS/NASA Earth Observatory base map courtesy of NASA Worldview.

The first sighting of pumice was around 1430 on 9 August NW of Vava'u in Tonga (18° 22.068' S, 174° 50.800' W), when Shannon Lenz and Tom Whitehead on board SV Finely Finished initially encountered isolated rocks and smaller streaks of pumice clasts. The area covered by rock increasing to a raft with an estimated thickness of at least 15 cm that extended to the horizon in different directions, and which took 6-8 hours to cross (figure 8). There was no sulfur smell and the sound was described as a "cement mixer, especially below deck." There was also no plume or incandescence observed. Their video, posted to YouTube on 17 August, showed a thin surface layer of cohesive interconnected irregular streaks of pumice with the ocean surface still visible between them. Later footage showed a continuous, undulating mass of pumice entirely covering the ocean surface. Larger clasts are visible scattered throughout the raft. The pumice raft was visible in satellite imagery on this day NW of Late Island (figure 9). By 11 August the raft had evolved into a largely linear feature with smaller rafts to the SW (figure 10). Approximately four hours later, about 15 km to the WSW, Rachel Mackie encountered the pumice. Initially the pumice was "ribbons several hundred meters long and up to 20m wide. It was quite fine and like a slick across the surface of the water." By 2130 they were surrounded by the pumice, and around 25 km away the smell of sulfur was noted.

Figure 8. The pumice raft from the unnamed Tongan seamount on 9 August 2019 taken by Shannon Lenz and Tom Whitehead on board SV Finely Finished. The photos show the pumice raft extending to the horizon in different directions. Scattered larger clasts protrude from the relatively smooth surface that entirely obscures the ocean surface. Courtesy of Shannon Lenz and Tom Whitehead via noonsite.

Figure 9. The pumice raft from the unnamed Tongan seamount on 9 August 2019 (UTC) can be seen NW of Late Island of Tonga in this Aqua/MODIS satellite image. The dashed white line encompasses the visible pumice. The location of the pumice in this image is shown in figure 7. Courtesy of NASA WorldView.

Figure 10. The Sentinel-2 satellite first imaged the pumice from the unnamed Tongan seamount on 11 August 2019 (UTC). This image indicates the pumice distribution with the main raft towards the W and the easternmost area of pumice approximately 45 km away. The eastern tip of the pumice area is located approximately 30 km WNW of Lake islands in Tonga. The location of the pumice in this image is shown in figure 7. Natural color (bands 4, 3, 2) Sentinel-2 satellite image courtesy of Sentinel Hub Playground.

Michael and Larissa Hoult aboard the catamaran ROAM encountered the raft on 15 August (figure 11). They initially saw isolated clasts ranging from marble to tennis ball size (15-70 mm) at 18° 46′S, 174° 55'W. At around 0700 UTC (1900 local time) they noted the smell of sulfur at 18° 55′S, 175° 21′W, and by 0800 UTC they were immersed in the raft with visible clasts ranging from marble to basketball (25 cm) sizes. At this point the raft was entirely obscuring the ocean surface. On 16 and 21 August the pumice continued to disperse and drift NW (figures 12 and 13). On 20 August Scott Bryan calculated an average drift rate of around 13 km/day, with the pumice on this date about 164 km W of the unnamed seamount.

Figure 11. Images of pumice from the unnamed Tongan seamount encountered by Michael and Larissa Hoult aboard the catamaran Roam on 15 August. Left: Larissa takes photographs with scale of pumice clasts; top right: a closeup of a pumice clast showing the vesicle network preserving the degassing structures of the magma; bottom left: Michael holding several larger pumice clasts. The location of their encounter with the pumice is shown in figure 7. Courtesy of SailSurfROAM.

Figure 12. The pumice from the unnamed Tongan seamount (volcano number 243091) on 16 August 2019 UTC. The location of the pumice in this image is shown in figure 7. Natural color (bands 4, 3, 2) Sentinel-2 satellite image courtesy of Sentinel Hub Playground.

Figure 13. On 21 August 2019 (UTC) the pumice from the unnamed Tongan seamount (volcano number 243091) had drifted at least 120 km WNW of Late Island in Tonga. The location of the pumice in this image is shown in figure 7. Natural color (bands 4, 3, 2) Sentinel-2 satellite image courtesy of Sentinel Hub Playground.

An online article published by Brad Scott at GeoNet on 9 September reported the preliminary size of the raft to be 60 km2, significantly smaller than the 2012 Havre seamount pumice raft that was 400 km2. Satellite identification of pumice-covered areas by GNS scientists showed the material moving SSW through 14 August (figure 14).

Figure 14. A compilation of mapped pumice raft extents from 9 August (red line) through to 14 August (dark blue) from Suomi NPP, Terra, Aqua, and Sentinel-2 satellite images. The progression of the pumice raft is towards the SW. Courtesy of Salman Ashraf, GNS Science.

On 5 September the Maritime Safety Authority of Fiji (MSAF) issued a notice to mariners stating that the pumice was sighted in the vicinity of Lakeba, Oneata, and Aiwa Islands and was moving to the W. On 6 September a Planet Labs satellite image shows pumice encompassing the Fijian island of Lakeba over 450 km W of the Tongan islands (figure 15). The pumice entered the lagoon within the barrier reef and drifted around the island to continue towards the W. The pumice was imaged by the Landsat 8 satellite on 26 September as it moved through the Fijian islands, approximately 760 km away from its source (figure 16). The pumice is segmented into numerous smaller rafts of varying sizes that stretch over at least 140 km. On 12 September the Fiji Sun reported that the pumice had reached some of the Lau islands and was thick enough near the shore for people to stand on it.

Figure 15. Planet Labs satellite images show Lakeba Island to the E of the larger Viti Levu Island in the Fiji archipelago. The top image shows the island on 7 July 2019 prior to the pumice raft from the unnamed Tongan seamount. The bottom image shows pumice on the sea surface almost entirely encompassing the island on 6 September. The location of the pumice in this image is shown in figure 7. Courtesy of Planet Labs.

Figure 16. Landsat 8 satellite images show the visible extent of the unnamed seamount pumice on 26 September 2019 (UTC), up to approximately 760 km from the Tongan islands. The pumice seen here extends over a distance of 140 km. The top image shows the locations of the other three images in the white boxes, with a, b, and c indicating the locations. White arrows point to examples of the light brown pumice rafts in these images, seen through light cloud cover. The island in the lower right is Koro Island, the island to the lower left is Viti Levu, and the island to the top right is Vanua Levu. The location of the pumice in this image is shown in figure 7. Landsat 8 true color-pansharpened satellite images courtesy of Sentinel Hub.

Pumice had reached the Yasawa islands in western Fiji by 29 September and was beginning to fill the eastern bays (figure 17). By 9 October bays had been filled out to 500-600 m from the shore, and pumice had also passed through the islands to continue towards the W (figure 18). At this point the pumice beyond the islands had broken up into linear segments that continued towards the NW.

Figure 17. These Sentinel-2 satellite images show the pumice from the unnamed Tongan seamount drifting towards the Yasawa islands of Fiji. The 24 September 2019 (UTC) image shows the beaches without the pumice, the 29 September image shows pumice drifting westward towards the islands, and the 9 October image shows the bays partly filled with pumice out to a maximum of 500-600 m from the shore. These islands are approximately 850 km from the Tongan islands. The Yasawa islands coastline impacted by the pumice shown in these images stretches approximately 48 km. The location of the pumice in this image is shown in figure 7. Sentinel-2 natural color (bands 4, 3, 2) satellite images courtesy of Sentinel Hub.

Figure 18. This Sentinel-2 satellite image acquired on 9 October 2019 (UTC) shows expanses of pumice from the unnamed Tongan seamount that passed through the Yasawa islands of Fiji and was continuing NWW, seen in the center of the image. The location of the pumice in this image is shown in figure 7. Sentinel-2 natural color (bands 4, 3, 2) satellite images courtesy of Sentinel Hub.

Geologic Background. A submarine volcano along the Tofua volcanic arc was first observed in September 2001. The newly discovered volcano lies NW of the island of Vava'u about 35 km S of Fonualei and 60 km NE of Late volcano. The site of the eruption is along a NNE-SSW-trending submarine plateau with an approximate bathymetric depth of 300 m. T-phase waves were recorded on 27-28 September 2001, and on the 27th local fishermen observed an ash-rich eruption column that rose above the sea surface. No eruptive activity was reported after the 28th, but water discoloration was documented during the following month. In early November rafts and strandings of dacitic pumice were reported along the coast of Kadavu and Viti Levu in the Fiji Islands. The depth of the summit of the submarine cone following the eruption determined to be 40 m during a 2007 survey; the crater of the 2001 eruption was breached to the E.

Information Contacts: GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Salman Ashraf, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/, https://www.geonet.org.nz/news/8RnSKdhaWOEABBIh0bHDj); Brad Scott, 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/, https://www.geonet.org.nz/news/8RnSKdhaWOEABBIh0bHDj); Scott Bryan, School of Earth, Environmental & Biological Sciences, Science and Engineering Faculty, Queensland University of Technology, R Block Level 2, 204, Gardens Point (URL: https://staff.qut.edu.au/staff/scott.bryan); Shannon Lenz and Tom Whitehead, SV Finely Finished (URL: https://www.noonsite.com/news/south-pacific-tonga-to-fiji-navigation-alert-dangerous-slick-of-volcanic-rubble/, YouTube: https://www.youtube.com/watch?v=PEsHLSFFQhQ); Michael and Larissa Hoult, Sail Surf ROAM (URL: https://www.facebook.com/sailsurfroam/); Rachel Mackie, OLIVE (URL: http://www.oliveocean.com/, https://www.facebook.com/rachel.mackie.718); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/); Fiji Sun (URL: https://fijisun.com.fj/2019/09/12/pumice-menace-hits-parts-of-lau-group/).

The current eruptive period of Popocatépetl began on 9 January 2005 and it has since been producing frequent explosions accompanied by ash plumes, gas emissions, and ballistic ejecta that can impact several kilometers away from the crater, as well as dome growth and destruction. This activity continued through March-August 2019 with an increase in volcano alert level during 28 March-6 May. This report summarizes activity during this period and is based on information from Centro Nacional de Prevención de Desastres (CENAPRED), Universidad Nacional Autónoma de México (UNAM), and various webcam and remote sensing data.

An overflight on 28 February confirmed that dome 82, which was first observed on 14 February, was still present and was 200 m in diameter. During March there were 3,291 observed low-intensity emissions, and 33 larger explosions that produced ash plumes to a maximum height of 5 km, accompanied by near-continuous emission of water vapor and volcanic gases. Explosions ejected blocks that fell on the flanks out to 1.2-2 km on 1, 10, 13, 17, 26, 27, and 29 March. The events on the 17th and 27th resulted in vegetation fires. Frequent sulfur dioxide (SO₂) plumes were detected by TropOMI (figure 130). An overflight on 7 March showed intense degassing and an ash plume at 1142, preventing visibility into the crater (figure 131). On 13 March Strombolian activity was observed for approximately 15 minutes at 0500, accompanied by incandescent ejecta that deposited mainly on the ESE flank.

An overflight on 15 March was taken by CENAPRED and UNAM personnel to observe changes to the crater after explosions on the 13th and 14th. They reported that dome 82 had been destroyed and the crater maintained its previous dimensions of 300 m in diameter and 130 m deep. An explosion on the 27th ejected incandescent rocks out to 2 km from the crater and produced a 3-km-high ash plume that dispersed to the NE. Ashfall was reported in Santa Cruz, Atlixco, San Pedro, San Andrés, Santa Isabel Cholula, San Pedro Benito Juárez, and in the municipalities of Puebla, Hueyapan, Tetela del Volcán, and Morelos.

On 28 March an explosion at 0650 generated a 2.5-km-high ash plume and ejecta out to 1 km from the crater, and a 130-minute-long event produced gas and ah plumes (figure 132). On this day the volcano alert level was increased from Yellow Phase 2 to Yellow Phase 3. On the 29th an ash plume rose to 3 km and was accompanied by ejecta that reached 2 km away from the crater. Later that day a 20-minute-long event produced ash and gas. During a surveillance flight on 30 March a view into the crater showed no dome present, and the crater size had increased to 350 m in width and 250-300 m in depth after recent explosions (figure 131). On this day Strombolian activity was also observed lasting for 14 minutes, producing an ash plume to 800 m and ejecta out to 300 m from the crater. Incandescence at the crater was often seen during nighttime throughout the month.

Figure 130. Significant SO2 plumes at Popocatépetl detected by the TROPOMI instrument on the Sentinel-5P satellite during 3-11 March 2019. SO2 plumes are frequently observed and these images show examples of plume drift directions on 3 March 2019 (top left), 6 March 2019 (top right), 7 March 2019 (bottom left), and 11 March 2019 (bottom right). Date, time, and measurements are provided at the top of each image. Courtesy of NASA Goddard Flight Center.

Figure 131. Activity at Popocatépetl and views of the crater during surveillance flights in March 2019. The top images show an ash plume (left) and a gas-and-steam plume (right) on 7 March. On 30 March (bottom left and right) no lava dome was observed in the crater, which was measured to be 350 m in diameter and 250-300 m deep. Courtesy of CENAPRED and Geophysics Institute of UNAM.

Figure 132. Explosive activity at Popocatépetl on 28 March 2019 producing ash plumes (top and bottom left) and ejecting incandescent ejecta out to 2 km from the crater at 1948. Courtesy of Carlos Sanchez/AFP (top), CENAPRED (bottom left and right), and Webcams de Mexico (bottom left).

There was a decrease in events during the next two months with 1,119 recorded low-intensity emissions and no larger ash explosions throughout April, followed by 1,210 low-intensity emissions and seven larger ash explosions through May (figure 133). Water vapor and volcanic gas emissions were frequently observed through this time and incandescence was observed some nights. A surveillance overflight on 26 April noted no new dome within the crater. On 6 May the alert level was lowered back to Yellow Phase 2. Another overflight on 9 May showed no change in the crater. An explosion at 1910 on 22 May produced an ash plume to 3.5 km above the crater with ashfall reported in Ozumba, Temamatla, Atlautla, Cocotitlán, Ayapango, Ecatzingo, Tenango del Aire and Tepetlixpa.

Figure 133. Graph showing the number of daily ash explosions and low-intensity emissions at Popocatépetl during March-August 2019. There was a decrease in the number of events during April and March, with an increase from March onwards. Data courtesy of CENAPRED.

Through the month of June there were 2,820 low-intensity emissions and 21 larger ash explosions recorded. Gas emissions were observed throughout the month. Two explosions on 3 June produced ash plumes up to 3.5 and 2.8 km, with ejecta out to 2 km S during the first explosion. On 11 June an explosion produced an ash plume to 1 km above the crater and ballistic ejecta out to 1 km E. Observers on a surveillance overflight on the 12th reported no changes within the crater

Explosions with estimated plume heights of 5 km occurred on the 14th and 15th, with the latter producing ashfall in the municipalities of San Pablo del Monte, Tenancingo, Papantla, San Cosme Mazatencocho, San Luis Teolocholco, Acuamanala, Nativitas, Tepetitla, Santa Apolonia Teacalco, Santa Isabel Tetlatlahuaca, and Huamantla, in the state of Tlaxcala, as well as in Nealtican, San Nicolás de los Ranchos, Calpan, San Pedro Cholula, Juan C. Bonilla, Coronango, Atoyatempan, and Coatzingo, in the state of Puebla.

On 17 June an explosion produced an ash plume that reached 8 km above the crater and dispersed towards the SW. An ash plume rising 2.5 km high was accompanied by incandescent ejecta impacting a short distance from the crater on the 21st, and another ash plume reached 2.5 km on the 22nd. Explosions on 26, 29, and 30 June resulted in ash plumes reaching 1.5 km above the crater and ballistic ejecta impacting on the flanks out to 1 km.

For the month of July there was an increased total of 5,637 recorded low-intensity emissions, and 173 larger ash explosions (figure 134). On 8 July an explosion produced ballistic ejecta out to 1.5 km and an ash plume up to 1 km above the crater. An ash plume up to 2.6 km was produced on the 12th. On 19 July a surveillance overflight observed a new dome (dome 83) with a diameter of 70 m and a thickness of 15 m (figure 135). Explosions on 20 July produced ashfall, and minor explosions that ejected incandescent ballistics onto the slopes. An event on the 24th produced an ash plume that reached 1.2 km, and ash plumes the following day reached 1 km. An overflight on 27 July confirmed that these explosions destroyed dome 83, and the crater dimensions remained the same (figure 136). The following day, ash plumes reached up to 1.6 km above the crater, and up to 2 km on the 29th. Minor ashfall was reported in the municipality of Ozumba on 30 June.

Figure 134. Examples of ash plumes at Popocatépetl on 1 July (top left), 18 July (top right and bottom left), and 30 July (bottom right) 2019. In the night time image taken on 18 July hot rocks are visible on the flank. Webcam images courtesy of CENAPRED and Webcams de Mexico.

Figure 135. A surveillance overflight at Popocatépetl on 19 July 2019 confirmed a new dome, dome number 83, with a width of 70 m and a thickness of 15 m. Courtesy of CENAPRED and Geophysics Institute of UNAM.

Figure 136. Photos of the summit crater of Popocatépetl taken during a surveillance flight on 27 July 2019 confirmed that the 83rd lava dome was destroyed by recent explosions and the crater maintained the same dimensions as previously measured. Courtesy of CENAPRED and Geophysics Institute of UNAM.

Throughout August the number of recorded events was higher than previous months, with 5,091 low-intensity emissions and 204 larger ash explosions (figure 137). Two explosions generated ash plumes and incandescent ejecta on 2 August, the first with a plume up to 1.5 km with ejecta impacting the slopes, and the second with an 800 m plume and ejecta landing back in the crater. Ashfall from the events was reported in in the municipalities of Tenango del Aire, Ayapango and Amecameca. On the 14th ashfall was reported in Juchitepec, Ayapango, and Ozumba. Explosions on 16 August produced ash plumes up to 2 km that dispersed to the WSW. Over the following two days ash plumes reached 1.2 km and resulted in ashfall in Cuernavaca, Tepoztlán, Tlalnepantla, Morelos, Ozumba, and Ecatzingo. Over 30-31 August ash plumes reached between 1-2 km above the crater and ashfall was reported in Amecameca, Atlautla, Ozumba, and Tlalmanalco. Incandescence was sometimes observed at the crater through the month.

Figure 137. Ash plumes at Popocatépetl on 7 August (top) and 26 August 2019 (bottom). Courtesy of CENAPRED and Webcams de Mexico.

The MODVOLC algorithm for MODIS thermal anomalies registered thermal alerts through this period, with 22 in March, three in May, five in July, and one in August. The MIROVA system showed that the frequency of thermal anomalies at Popocatépetl was higher in March, sporadic in April and May, low in June, and had increased again in July and August (figure 138). Elevated temperatures were frequently visible in Sentinel-2 thermal satellite data when clouds and plumes were not covering the crater (figure 139).

Figure 138. Thermal activity at Popocatépetl detected by the MIROVA system showed frequent anomalies in March, intermittent anomalies through April-May, low activity in June, and an increase in July-August 2019. Courtesy of MIROVA.

Figure 139. Sentinel-2 thermal satellite images frequently showed elevated temperatures in the crater of Popocatépetl during March-August 2019, as seen in this representative image from 7 May 2019. Sentinel2- atmospheric penetration (bands 12, 11, 8A) scene courtesy of Sentinel Hub Playground.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/); Universidad Nacional Autónoma de México (UNAM), University City, 04510 Mexico City, Mexico (URL: https://www.unam.mx/); 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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://SO2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Webcams de Mexico (URL: http://www.webcamsdemexico.com/); Agence France-Presse (URL: http://www.afp.com/).

The ongoing eruption at Semeru weakened in intensity during 2018, with occasional ash plumes and thermal anomalies (BGVN 44:04); this reduced but ongoing level of activity continued through August 2019. The volcano is monitored by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC). The current report summarizes activity from 1 March to 31 August 2019. The Alert Level remained at 2 (on a scale from 1-4); the public was warned to stay 1 km away from the active crater and 4 km away on the SSE flank.

Based on analysis of satellite images, the Darwin VAAC reported that ash plumes rose to an altitude of 4-4.3 km on 19 April, 20 June, 10 July, and 13 July, drifting in various directions. In addition, PVMBG reported that at 0830 on 26 June an explosion produced an ash plume that rose around 600 m above the summit and drifted SW. A news article (Tempo.com) dated 12 August cited PVMBG as stating that the volcano had erupted 17 times since 8 August.

During March-August 2019 thermal anomalies were detected with the MODIS satellite instruments analyzed using the MODVOLC algorithm only on 5 July and 22 August. No explosions were recorded on those two days. Scattered thermal anomalies within 5 km of the volcano were detected by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system, also based on analysis of MODIS data: one at the end of March and 3-6 hotspots over the following months, almost all of low radiative power. Satellite imagery intermittently showed thermal activity in the Jonggring-Seloko crater (figure 37), sometimes with material moving down the SE-flank ravine.

Figure 37. Sentinel-2 satellite images showing the persistent elevated thermal anomaly in the Jonggring-Seloko crater of Semeru were common through August 2019, as seen in this view on 20 July. Hot material could sometimes be identified in the SE-flank ravine. Atmospheric penetration rendering (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Tempo.com (URL: https://www.tempo.com/).

Historical observations of eruptive activity from the glacier-covered Mount Michael stratovolcano on Saunders Island in the South Sandwich Islands were not recorded until the early 19th century at this remote site in the southernmost Atlantic Ocean, and remain extremely rare. With the advent of satellite observation technology, indications of more frequent eruptive activity have become apparent. Vapor emission is frequently reported from the summit crater, and AVHRR and MODIS satellite imagery has revealed evidence for lava lake activity in the summit crater (Lachlan-Cope and others, 2001). Limited thermal anomaly data and satellite imagery indicated at least intermittent activity during May 2000-November 2013, and from November 2014 through April 2018 (Gray and others, 2019). Ongoing observations, including photographs from two site visits in February and May 2019 suggest continued activity at the summit during most months through May 2019, the period covered in this report. Information, in addition to on-site photographs, comes from MIROVA thermal anomaly data, NASA SO₂ instruments, and Sentinel-2 and Landsat satellite imagery.

Near-constant cloud coverage for much of the year makes satellite data intermittent and creates difficulty in interpreting the ongoing nature of the activity. Gray and others (2019) concluded recently after a detailed study of shortwave and infrared satellite images that there was continued evidence for the previously identified lava lake on Mount Michael since January 1989. MIROVA thermal anomaly data suggest intermittent pulses of thermal energy in September, November, and December 2018, and April 2019 (figure 17). Satellite imagery confirmed some type of activity, either a dense steam plume, evidence of ash, or a thermal anomaly, each month during December 2018-March 2019. Sulfur dioxide anomalies were recorded in January, February, and March 2019. Photographic evidence of fresh ash was captured in February 2019, and images from May 2019 showed dense steam rising from the summit crater.

Figure 17. MIROVA thermal anomaly data from 19 September 2018 through June 2019 showed sporadic, low-level pulses of thermal energy in late September, November, and December 2018, and April 2019. Courtesy of MIROVA.

After satellite imagery and thermal anomaly data in late September 2018 showed evidence for eruptive activity (BGVN 43:10, figure 16), a single thermal anomaly in MIROVA data was recorded in mid-November 2018 (figure 17). A rare, clear Sentinel-2 image on 2 December revealed a dense steam plume over the active summit crater; the steam obscured the presence of any possible thermal anomalies beneath (figure 18).

Figure 18. Sentinel-2 images of Mount Michael on Saunders Island on 2 December 2018 revealed a dense steam plume over the summit crater that was difficult to distinguish from the surrounding snow in Natural Color rendering (bands 4,3,2) (left), but was clearly visible in Atmospheric Penetration rendering (bands 12,11, 8a) (right). Courtesy of Sentinel Hub Playground.

Clear evidence of recent activity appeared on 1 January 2019 with both a thermal anomaly at the summit crater and a streak of ash on the snow (figure 19). Steam was also present within the summit crater. A distinct SO₂ anomaly appeared in data from the TROPOMI instrument on 14 January (figure 20).

Figure 19. A thermal anomaly and dense steam were recorded at the summit of Mount Michael on Saunders Island on 1 January 2019 in Sentinel-2 Satellite imagery with Atmospheric Penetration rendering (bands 12, 11, 8a) (left). The same image shown with Natural Color rendering (bands 4,3,2) (right) shows a recent streak of brown particulates drifting SE from the summit crater. Courtesy of Sentinel Hub Playground.

Figure 20. A distinct SO2 plume was recorded drifting NW from Saunders Island by the TROPOMI instrument on the Sentinel 5-P satellite on 14 January 2019. Courtesy of NASA Goddard Space Flight Center.

Multiple sources of satellite data and sea-based visual observation confirmed activity during February 2019. SO₂ emissions were recorded with the TROPOMI instrument on 10, 11, 15, and 16 February (figure 21). A Landsat image from 10 February showed a dense steam plume drifting NW from the summit crater, with the dark rim of the summit crater well exposed (figure 22). Sentinel-2 images in natural color and atmospheric penetration renderings identified a dense steam plume drifting S and a thermal anomaly within the summit crater on 15 February (figure 23). An expedition to the South Sandwich Islands between 15 February and 8 March 2019 sponsored by the UK government sailed by Saunders in late February and observed a stream of ash on the NNE flank beneath the cloud cover (figure 24).

Figure 21. Faint but distinct SO2 plumes were recorded drifting away from Saunders Island in various directions on 10, 11, 15, and 16 February 2019. Courtesy of NASA Goddard Space Flight Center.

Figure 22. The dark summit crater of Mount Michael on Saunders Island was visible in Landsat imagery on 10 February 2019. A dense steam plume drifted NW and cast a dark shadow on the underlying cloud cover. Courtesy of Sentinel Hub Playground.

Figure 23. At the summit of Mount Michael on Saunders Island, Sentinel-2 images in Natural Color (bands 4,3,2) (left) and Atmospheric Penetration (bands 12, 11, 8a) (right) renderings identified a dense steam plume drifting S and a thermal anomaly within the summit crater on 15 February 2019. Courtesy of Sentinel Hub Playground.

Figure 24. Recent ash covered the NNE flank of Mount Michael on Saunders Island in late February 2019 when observed by an expedition to the South Sandwich Islands sponsored by the UK government. Courtesy of Chris Darby.

Faint SO₂ emissions were recorded twice during March 2019 (figure 25), and a dense steam plume near the summit crater was visible in Landsat imagery on 23 March (figure 26). Two thermal anomalies were captured in the MIROVA data during April 2019 (figure 17).

Figure 25. Faint SO2 plumes were recorded on 1 and 11 March 2019 emerging from Saunders Island. Courtesy of NASA Goddard Space Flight Center.

Figure 26. A dense steam plume drifted E from the summit crater of Mount Michael at Saunders Island on 25 March 2019. Landsat-8 image courtesy of Sentinel Hub Playground.

A volcano-related research project "SSIVOLC" explored the South Sandwich Islands volcanoes during 15 April-31 May 2019. A major aim of SSIVOLC was to collect photogrammetric data of the glacier-covered Mount Michael (Derrien and others, 2019). A number of still images were acquired on 17 and 22 May 2019 showing various features of the island (figures 27-30). The researchers visually observed brief, recurrent, and very weak glow at the summit of Mount Michael after dark on 17 May, which they interpreted as reflecting light from an active lava lake within the summit crater.

Figure 27. The steep slopes of an older eroded crater on the E end of Saunders island in the 'Ashen Hills' shows layers of volcanic deposits dipping away from the open half crater. In the background, steam and gas flow out of the summit crater of Mount Michael and drift down the far slope. Drone image PA-IS-03 taken during 17-22 May 2019, courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

Figure 28. A dense steam plume drifts away from the summit of Mount Michael on Saunders Island in this drone image taken during 17-22 May 2019. The older summit crater is to the left of the dark patch in the middle of the summit. North is to the right. Image SU-3 courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

Figure 29. This close-up image of the summit of Mount Michael on Saunders Island shows steam plumes billowing from the summit crater, and large crevasses in the glacier covered flank, taken during 17-22 May 2019. The old crater is to the left. Image TL-SU-1 courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

Figure 30. A dense plume of steam rises from the summit crater of Mount Michael on Saunders Island and drifts over mounds of frozen material during 17-22 May 2019. The older crater is to the left, and part of the Ashen Hills is in the foreground. Image TL-SU-2 courtesy of Derrien and others (2019) used under Creative Commons Attribution 4.0 International (CC-BY 4.0) License.

References: Lachlan-Cope T, Smellie J L, Ladkin R, 2001. Discovery of a recurrent lava lake on Saunders Island (South Sandwich Islands) using AVHRR imagery. J. Volcanol. Geotherm. Res., 112: 105-116.

Gray D M, Burton-Johnson A, Fretwell P T, 2019. Evidence for a lava lake on Mt. Michael volcano, Saunders Island (South Sandwich Islands) from Landsat, Sentinel-2 and ASTER satellite imagery. J. Volcanol. Geotherm. Res., 379:60-71. https://doi.org/10.1016/j.volgeores.2019.05.002.

Derrien A, Richter N, Meschede M, Walter T, 2019. Optical DSLR camera- and UAV footage of the remote Mount Michael Volcano, Saunders Island (South Sandwich Islands), acquired in May 2019. GFZ Data Services. http://doi.org/10.5880/GFZ.2.1..2019.003

Geologic Background. Saunders Island is a volcanic structure consisting of a large central edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young constructional Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of parasitic cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weatehr conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

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/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Chris Darby (URL: https://twitter.com/ChrisDDarby, image at https://twitter.com/ChrisDDarby/status/1100686838568812544).

Pacaya is one of the most active volcanoes in Guatemala, with activity largely consisting of frequent lava flows and Strombolian activity at the Mackenney crater. This report summarizes continued activity during February through July 2019 based on reports by Guatemala's Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) and Sistema de la Coordinadora Nacional para la Reducción de Desastres (CONRED), visiting scientists, and satellite data.

At the beginning of February activity included Strombolian explosions ejecting material up to 5 to 30 m above the Mackenney crater and a degassing plume up to 300 m. Multiple lava flows were observed throughout the month on the N, NW, and W flanks, reaching 350 m from the crater and resulting in avalanches from the flow fronts. Strombolian activity continued with sporadic to continuous explosions ejecting material 5-75 m above the Mackenney crater. Degassing produced plumes up to 300 m above the crater, and incandescence from the crater and lava flows were seen at night. Daniel Sturgess of Bristol University observed activity on the 24th, noting a 70-m-long lava flow with individual blocks from the front of the flow rolling down the flanks (figure 108). He reported that mild Strombolian explosions occurred every 10-20 minutes and ejected blocks, up to approximately 4 m in diameter, as high as 5-30 m above the crater and towards the northern flank.

Figure 108. An active lava flow on the NW flank of Pacaya on 24 February 2019 with incandescence visible in lower light conditions. Courtesy of Daniel Sturgess, University of Bristol.

Similar activity continued through March with multiple lava flows reaching a maximum of 200 m N and NW, and avalanches descending from the flow fronts. Ongoing Strombolian explosions expelled material up to 75 m above the Mackenney crater. Degassing produced a white-blue plume to a maximum of 900 m above the crater (figure 109) and incandescence was noted some nights.

Figure 109. A degassing plume at Pacaya reaching 350 m above the crater and dispersing to the S on 19 March 2019. Courtesy of CONRED.

During April lava flows continued on the N and NW flanks, reaching a maximum length of 300 m, with avalanches forming from the flow fronts. Degassing formed plumes up to 600 m above the crater that dispersed with various wind directions. Strombolian activity continued with explosions ejecting material up to 40 m above the crater. On the 2nd and 3rd weak rumbles were heard at distances of 4-5 km. Similar activity continued through May with lava flows reaching 300 m to the N, degassing producing plumes up to 600 m above the crater, and Strombolian explosions ejecting material up to 15 m above the crater.

Lava flows continued out to 300 m in length to the N and NW during June (figures 110 and 111). Strombolian activity ejected material up to 30 m above the crater and degassing resulted in plumes that reached 300 m. During July there were multiple active lava flows that reached a maximum of 300 m in length on the N and NW flanks (figure 112). Avalanches generated by the collapse of material at the front of the lava flows were accompanied by explosions ejecting material up to 30 m above the crater.

Figure 110. An active lava flow on Pacaya on 9 June 2019 with incandescent blocks rolling down the flank from the flow front. Courtesy of Paul Wallace, University of Liverpool.

Figure 111. Activity at Pacaya on 22 June 2019 with a degassing plume dispersed to the W and a 300-m-long lava flow. Photos by Miguel Morales, courtesy of CONRED.

Figure 112. Two lava flows were active to the N and NW at Pacaya on 20 July 2019. Photos courtesy of CONRED.

During February through July multiple lava flows and crater activity were detected in Sentinel-2 satellite thermal images (figures 113 and 114) and relatively constant thermal energy was detected by the MIROVA system with a slight decrease in the energy and frequency of anomalies during June (figure 115). The thermal anomalies detected by the MODVOLC system for each month from February through July spanned 6-30, with six during June and 30 during April.

Figure 113. Sentinel-2 thermal satellite images of Pacaya show lava flows to the N and NW during February through April 2019. There was a reduction in visible activity in early March. False color (urban) satellite images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 114. Sentinel-2 thermal satellite images of Pacaya showing lava flow and hot avalanche activity during June and July 2019. False color (urban) satellite images (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Figure 115. MIROVA log radiative power plot of MODIS thermal infrared at Pacaya during October 2018 through July 2019. Detected thermal energy is relatively stable with a decrease through June and subsequent increase during July. Courtesy of MIROVA.

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: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Coordinadora Nacional para la Reducción de Desastres (CONRED), Av. Hincapié 21-72, Zona 13, Guatemala City, Guatemala (URL: http://conred.gob.gt/www/index.php); 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/); Daniel Sturgess, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, United Kingdom (URL: http://www.bristol.ac.uk/earthsciences/); Paul Wallace, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, United Kingdom (URL: https://www.liverpool.ac.uk/environmental-sciences/staff/paul-wallace/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Frequent historical eruptions at Volcán de Colima date back to the 16th century and include explosive activity, lava flows, and large debris avalanches. The most recent eruptive episode began in January 2013 and continued through March 2017. Previous reports have covered activity involving ash plumes with extensive ashfall, lava flows, lahars, and pyroclastic flows (BGVN 41:01 and 42:08). In late April 2019, increased seismicity related to volcanic activity began again. This report covers activity through July 2019. The primary source of information was the Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC).

On 11 May 2019, CUEIV-UdC reported an explosion that was recorded by several monitoring stations. A thermal camera located south of Colima captured thermal anomalies associated with the explosion as well as intermittent degassing, which mainly consisted of water vapor (figure 131). A report from the Unidad Estatal de Protección Civil de Colima (UEPCC), and seismic and infrasound network data from CUEIV-UdC, recorded about 60 high-frequency events, 16 landslides, and 14 low-magnitude explosions occurring on the NE side of the crater during 11-24 May. Drone imagery showed fumarolic activity occurring on the inner wall of this crater on 22 May (figure 132).

Figure 131. Gas emissions from Colima during the 11 May 2019 eruption as seen from the Naranjal station. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 17 mayo 2019 no 121).

Figure 132. A drone photo showing fumarolic activity occurring within the NE wall of the crater at Colima on 22 May 2019. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 24 mayo 2019 no 122).

Small explosions and gas-and-steam emissions continued intermittently through mid-July 2019 concentrated on the NE side of the crater. An overflight on 9 July 2019 revealed that subsidence from the consistent activity slightly increased the diameter of the vent; other areas within the crater also showed evidence of subsidence and some collapsed material on the outer W wall (figure 133). During the weeks of 19 and 26 July 2019, monitoring cameras and seismic data recorded eight lahars.

Figure 133. A drone photo of the crater at Colima on 8 July 2019 shows continuing fumarolic activity and evidence of a collapsed wall on the W exterior side. Courtesy of CUEIV-UdC (Boletin Seminal de la Actividad del Volcan de Colima 12 julio 2019 no 129).

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

Information Contacts: Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC), Colima, Col. 28045, Mexico; Centro Universitario de Estudios Vulcanologicos y Facultad de Ciencias de la Universidad de Colima, Avenida Universidad 333, Colima, Col. 28045, Mexico (URL: http://portal.ucol.mx/cueiv/); Unidad Estatal de Protección Civil, Colima, Roberto Esperón No. 1170 Col. de los Trabajadores, C.P. 28020, Mexico (URL: http://www.proteccioncivil.col.gob.mx/).

Masaya, in Nicaragua, contains a lava lake found in the Santiago Crater which has remained active since its return in December 2015 (BGVN 41:08). In addition to this lava lake, previous volcanism included explosive eruptions, lava flows, and gas emissions. Activity generally decreased during March-July 2019, including the number and frequency of thermal anomalies, lava lake levels, and gas emissions. The primary source of information for this report comes from the Instituto Nicareguense de Estudios Territoriales (INETER).

On 21 July 2019 a small explosion in the Santiago Crater resulted in some gas emissions and an ash cloud drifting WNW. In addition to the active lava lake (figure 77), monthly reports from INETER noted that thermal activity and gas emissions (figure 78) were decreasing.

Figure 77. Active lava lake visible in the Santiago Crater at Masaya on 27 June 2019. Photo by Sheila DeForest (Creative Commons BY-SA license).

Figure 78. Gas emissions coming from the Santiago Crater at Masaya on 29 June 2019. Photo by Sheila DeForest (Creative Commons BY-SA license).

On 15 May and 22 July 2019, INETER scientists used a FLIR SC620 thermal infrared camera to measure temperatures of fumaroles on the Santiago Crater. In May 2019 the temperature of fumaroles had decreased by 48°C since the previous month. Between May and July 2019 fumarole temperatures continued to decline; temperatures ranged from 90° to 136°C (figure 79). Compared to May 2019 these temperatures are 3°C lower. INETER reports that the level of the lava lake has been slowly dropping during this reporting period.

Figure 79. FLIR (forward-looking infrared) and visible images of the Santiago Crater at Masaya showing fumarole temperatures ranging from 90° to 136°C. The scale in the center shows the range of temperatures in the FLIR image. Courtesy of INETER (March 2019 report).

According to MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS satellite instruments, frequent thermal anomalies were recorded from mid-March through early May 2019, with little to no activity from mid-May to July 2019 (figure 80). Sentinel-2 thermal images show high temperatures in the active lava lake on 10 March 2019 (figure 81). Thermal energy detected by the MODVOLC algorithm showed 14 hotspot pixels with the most number of hotspots (7) occurring in March 2019.

Figure 80. Thermal anomalies were relatively constant at Masaya from early September 2018 through early May 2019 and then abruptly decreased until mid-June 2019 as recorded by MIROVA. Courtesy of MIROVA.

Figure 81. Sentinel-2 thermal satellite image showing a detected heat signature from the active lava lake at Masaya on 10 March 2019. The lava lake is visible (bright yellow-orange). Approximate diameter of the crater containing the lava lake is 500 m. Thermal (urban) satellite image (bands 12, 11, 4) courtesy of Sentinel Hub Playground.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The twin volcanoes of Nindirí and Masaya, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals cause health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); 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); Sheila DeForest (URL: https://www.facebook.com/sheila.deforest).

The acid lake of Rincón de la Vieja's active crater has generated intermittent weak phreatic explosions regularly since 2011, continuing during the past year through at least August 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 4 March and 2 September 2019. Clouds often prevented webcam and satellite views. The current report describes activity from March through July 2019.

OVSICORI-UNA reported that weak events occurred on 19 March at 1851 and on 29 March 2019 at 2043. A two-minute-long phreatic explosion on 1 April at 0802 produced a plume that rose 600 m above the crater rim. Continuous emissions were visible during 3-4 April, rising 200 m above the crater rim. On 3 April, at 1437, a small explosion was detected. An explosion on 10 April at 0617 produced a gas-and-steam plume that rose 1 km above the crater rim and drifted SE. On 12 April at 0643, a plume rose 500 m. Another event took place at 0700 on 13 April, although poor weather conditions prevented visual observations. On 14 April, OVSICORI-UNA noted that aerial photographs showed a milky-gray acid lake at a relatively low water level with convection cells of several tens meters of diameter in the center and eastern parts of the lake.

According to an OVSICORI-UNA bulletin, a small phreatic explosion occurred on 1 May. Another explosion on 11 May at 0720 produced a white gas-and-steam plume that rose 600 m above the crater rim. Phreatic explosions were recorded on 14 May at 1703 and on 17 May at 0357, though dense fog prevented visual confirmation of both events with webcams. On 15 May a local observer noted a diffuse plume of steam and gas, material rising from the crater, and photographed milky-gray deposits on the N part of the crater rim ejected from the event the day before. A major explosion occurred on 24 May.

OVSICORI-UNA recorded a significant phreatic 10-minute-long explosion that began on 11 June at 0343, but plumes were not visible due to weather conditions. No further phreatic events were reported in July.

Seismic activity was very low during the reporting period, and there was no significant deformation. Short tremors were frequent toward the end of April, but were only periodic in May and June; tremor almost disappeared in July. A few long-period earthquakes were recorded, and volcano-tectonic earthquakes were even less frequent.

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

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

Sakurajima rises from Kagoshima Bay, which fills the Aira Caldera near the southern tip of Japan's Kyushu Island. Frequent explosive and occasional effusive activity has been ongoing for centuries. The Minamidake summit cone has been the location of persistent activity since 1955; the Showa crater on its E flank has also been intermittently active since 2006. Numerous explosions and ash-bearing emissions have been occurring each month at either Minamidake or Showa crater since the latest eruptive episode began in late March 2017. This report covers ongoing activity from January through June 2019; the Japan Meteorological Agency (JMA) provides regular reports on activity, and the Tokyo VAAC (Volcanic Ash Advisory Center) issues tens of reports each month about the frequent ash plumes.

From January to June 2019, ash plumes and explosions were usually reported multiple times each week. The quietest month was June with only five eruptive events; the most active was March with 29 (table 21). Ash plumes rose from a few hundred meters to 3,500 m above the summit during the period. Large blocks of incandescent ejecta traveled as far as 1,700 m from the Minamidake crater during explosions in February and April. All the activity originated in the Minamidake crater; the adjacent Showa crater only had a mild thermal anomaly and fumarole throughout the period. Satellite imagery identified thermal anomalies inside the Minamidake crater several times each month.

Table 21. Monthly summary of eruptive events recorded at Sakurajima's Minamidake crater in Aira caldera, January-June 2019. The number of events that were explosive in nature are in parentheses. No events were recorded at the Showa crater during this time. Data courtesy of JMA (January to June 2019 monthly reports).

There were eight eruptive events reported by JMA during January 2019 at the Minamidake summit crater of Sakurajima. They occurred on 3, 6, 7, 9, 17, and 19 January (figure 76). Ash plume heights ranged from 600 to 2,100 m above the summit. The largest explosion, on 9 January, generated an ash plume that rose 2,100 m above the summit crater and drifted E. In addition, incandescent ejecta was sent 800-1,100 m from the summit. Incandescence was visible at the summit on most clear nights. During an overflight on 18 January no significant changes were noted at the crater (figure 77). Infrared thermal imaging done on 29 January indicated a weak thermal anomaly in the vicinity of the Showa crater on the SE side of Minamidake crater. The Kagoshima Regional Meteorological Observatory (KRMO) (11 km WSW) recorded ashfall there during four days of the month. Satellite imagery indicated thermal anomalies inside Minamidake on 7 and 27 January (figure 77).

Figure 76. Incandescent ejecta and ash emissions characterized activity from Sakurajima volcano at Aira during January 2019. Left: A webcam image showed incandescent ejecta on the flanks on 9 January 2019, courtesy of JMA (Explanation of volcanic activity in Sakurajima, January 2019). Right: An ash plume rose hundreds of meters above the summit, likely also on 9 January, posted on 10 January 2019, courtesy of Mike Day.

Figure 77. The summit of Sakurajima consists of the larger Minamidake crater and the smaller Showa crater on the E flank. Left: The Minamidake crater at the summit of Sakurajima volcano at Aira on 18 January 2019 seen in an overflight courtesy of JMA (Explanation of volcanic activity in Sakurajima, March 2019). Right: Two areas of thermal anomaly were visible in Sentinel-2 satellite imagery on 27 January 2019. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

Activity increased during February 2019, with 15 eruptive events reported on days 1, 3, 7, 8, 10, 13, 14, 17, 22, 24, and 27. Ash plume heights ranged from 600-2,300 m above the summit, and ejecta was reported 300 to 1,700 m from the crater in various events (figure 78). KRMO reported two days of ashfall during February. Satellite imagery identified thermal anomalies at the crater on 6 and 26 February, and ash plumes on 21 and 26 February (figure 79).

Figure 78. An explosion from Sakurajima at Aira on 7 February 2019 sent ejecta up to 1,700 m from the Minamidake summit crater. Courtesy of JMA (Explanation of volcanic activity in Sakurajima, February 2019).

Figure 79. Thermal anomalies and ash emissions were captured in Sentinel-2 satellite imagery on 6, 21, and 26 February 2019 originating from Sakurajima volcano at Aira. Top: Thermal anomalies within the summit crater were visible underneath steam and ash plumes on 6 and 26 February (closeup of bottom right photo). Bottom: Ash emissions on 21 and 26 February drifted SE from the volcano. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

The number of eruptive events continued to increase during March 2019; there were 29 events reported on numerous days (figures 80 and 81). An explosion on 14 March produced an ash plume that rose 3,500 m above the summit and drifted E. It also produced ejecta that landed 800-1,100 m from the crater. During an overflight on 26 March a fumarole was the only activity in Showa crater. KRMO reported 14 days of ashfall during the month. Satellite imagery identified an ash plume on 13 March and a thermal anomaly on 18 March (figure 82).

Figure 80. A large ash emission from Sakurajima volcano at Aira was photographed by a tourist on the W flank and posted on 1 March 2019. Courtesy of Kratü.

Figure 81. An ash plume from Sakurajima volcano at Aira on 18 March 2019 produced enough ashfall to disrupt the trains in the nearby city of Kagoshima according to the photographer. Image taken from about 20 km away. Courtesy of Tim Board.

Figure 82. An ash plume drifted SE from the summit of Sakurajima volcano at Aira on 13 March (left) and a thermal anomaly was visible inside the Minamidake crater on 18 March 2019 (right). "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

A decline in activity to only ten eruptive events on days 7, 13, 17, 22, and 25 was reported by JMA for April 2019. An explosion on 7 April sent ejecta up to 1,700 m from the crater. Another explosion on 13 April produced an ash plume that rose 2,200 m above the summit. Most of the eruptive events at Sakurajima last for less than 30 minutes; on 22 April two events lasted for almost an hour each producing ash plumes that rose 1,400 m above the summit. Ashfall at KRMO was reported during seven days in April. Two distinct thermal anomalies were visible inside the Minamidake crater on both 12 and 27 April (figure 83).

Figure 83. Two thermal anomalies were present inside Minamidake crater at the summit of Sakurajima volcano at Aira on 12 (left) and 27 (right) April 2019. "Geology" rendering (bands 12, 4, and 2) courtesy of Sentinel Hub Playground.

There were 15 eruptive events during May 2019. An event that lasted for two hours on 12 May produced an ash plume that rose 2,900 m from the summit and drifted NE (figure 84). The Meteorological Observatory reported 14 days with ashfall during the month. Two thermal anomalies were present in satellite imagery in the Minamidake crater on both 17 and 22 May.

Figure 84. An ash plume rose 2,900 m above the summit of Sakurajima at Aira on 12 May 2019 (left); incandescent ejecta went 1,300 m from the summit crater on 13 May. Courtesy of JMA (Explanation of volcanic activity in Sakurajima, May 2019).

During June 2019 five eruptive events were reported, on 11, 13, and 24 June; the event on 11 June lasted for almost two hours, sent ash 2,200 m above the summit, and produced ejecta that landed up to 1,100 m from the crater (figure 85). Five days of ashfall were reported by KRMO.

Figure 85. A large ash plume on 11 June 2019 rose 2,200 m above the summit of Sakurajima volcano at Aira. Courtesy of Aone Wakatsuki.

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), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Mike Day, Minnesota, Twitter (URL: https://twitter.com/MikeDaySMM, photo at https://twitter.com/MikeDaySMM/status/1083489400451989505/photo/1); Kratü, Twitter (URL: https://twitter.com/TalesOfKratue, photo at https://twitter.com/TalesOfKratue/status/1101469595414589441/photo/1); Tim Board, Japan, Twitter (URL: https://twitter.com/Hawkworld_, photo at https://twitter.com/Hawkworld_/status/1107789108754038789); Aone Wakatsuke, Twitter (URL: https://twitter.com/AoneWakatsuki, photo at https://twitter.com/AoneWakatsuki/status/1138420031258210305/photo/3).

During 1995, the geophysical system described below registered Sakura-jima's 126th explosion on 23 August. During 23-25 August, 28 explosions were recorded. The total through August of 153 explosions is relatively small compared to 1960, 1974, 1983, and 1985, years when over 400 explosions took place. During August no measurable ash fell at Kagoshima Local Meteorological Observatory, 10 km W of the crater. On the other hand, ash-bearing explosions were of sufficient size to send a dense ash cloud NW of the volcano that dropped ash in N Kyushu and closed a highway for an unspecified duration. The highest plume of the month vented on 30 August and rose to 3 km above the crater rim. Station B (2.3 km NE of Minami-dake crater) registered 671 earthquakes and 378 tremor events.

Geophysical determination of explosions. The monthly tally of "explosions" (sometimes also called "explosive eruptions") at Sakura-jima has a geophysical definition, with its origins closely linked to aircraft safety. The volcano sits ~25 km from the busy Kagoshima International Airport and generates frequent Vulcanian explosive eruptions (BGVN 19:11). A video camera monitors the volcano and a real-time image is transmitted to air traffic control. In order to alert aviation dispatchers and pilots of potential hazards regardless of the time of day or the weather, scientists devised a system to rapidly classify the volcano's seismic and acoustic signals (Onodera and Kamo, 1994). This geophysical system has been linked to the Japan Airlines office at Kagoshima Airport since March 1991.

When the amplitudes of incoming seismic signals rise above an established threshold (table 12) their dominant frequency is computed. The above-threshold signals also have an associated air-shock wave that is received at an "infrasonic" microphone with a 0.02-100 Hz detection range. For reference, the low-frequency range of the human ear stops at around 16 Hz. Once the infrasonic air-shock wave is received, the system measures its amplitude and computes its spectrum. The combination of seismic and air-shock amplitudes and spectra allow the events to be classified into "non-eruption," "eruption," or "explosion." categories (table 2).

Table 12. (above) Definition of "explosion" at Sakura-jima, a geophysical characterization of explosive, bomb- and ash-bearing eruptions. (below) Recognized volcanic earthquakes at Sakura-jima, showing the maximum values registered for explosion earthquakes and corresponding air-shock waves (after Onodera and Kamo, 1994).

Although passing typhoons can trigger inappropriate warnings or false alarms, and small-magnitude eruptions may be missed, the number of explosions correlates well with the measured deposition of fresh volcanic ash. The system has been effective at reducing aviation risks. A future goal is to use "explosion" category to automatically trigger the calculation of volcanic ash diffusion based on meteorological data. This program would thus automatically estimate the likely trajectory of ash discharged from the volcano.

References. Onodera, S., Iguchi, M., and Ishihara, K., 1994, Recent advances in Japan, volcano monitoring system of Japan Airlines at Kagoshima Airport: 9th Annual International Oceanic Airspace Conference, 9 November 1994.

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: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Saburo Onodera, Director of Meteorology, Flight Operations, Japan Airlines, 3-3-2 Haneda Airport, Tokyo 144, Japan; Kosuke Kamo, Masato Iguchi, and Kazuhiro Ishihara, Sakurajima Volcano Observatory (SVO), Disaster Prevention Research Institute, Kyoto University, Sakurajima-cho, Kagoshima 89114, Japan.

Seismicity increased after 15 August. On 18 August, 127 events were recorded 2.3 km from Ponmachineshiri Crater on Me-Akan volcano. The total number of earthquakes in August was 363. High seismicity was previously reported in March-April 1993.

Geologic Background. Akan is a 13 x 24 km caldera located immediately SW of Kussharo caldera. The elongated, irregular outline of the caldera rim reflects its incremental formation during major explosive eruptions from the early to mid-Pleistocene. Growth of four post-caldera stratovolcanoes, three at the SW end of the caldera and the other at the NE side, has restricted the size of the caldera lake. Conical Oakandake was frequently active during the Holocene. The 1-km-wide Nakamachineshiri crater of Meakandake was formed during a major pumice-and-scoria eruption about 13,500 years ago. Within the Akan volcanic complex, only the Meakandake group, east of Lake Akan, has been historically active, producing mild phreatic eruptions since the beginning of the 19th century. Meakandake is composed of nine overlapping cones. The main cone of Meakandake proper has a triple crater at its summit. Historical eruptions at Meakandake have consisted of minor phreatic explosions, but four major magmatic eruptions including pyroclastic flows have occurred during the Holocene.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

A pyroclastic explosion on the morning of 3 March 1995 generated a vapor-and-ash column ~3 km high (BGVN 20:02). Preliminary analysis of the resulting deposit did not reveal any juvenile material. On the morning of 5 March, a vapor plume rose ~500 m. It is possible that vapor plumes were emitted over several days, but were not observed at other times because of the thick clouds that usually hide the summit area. The center of activity on 3 March was between two small islands in Lake Voui (figures 4 and 5). Because of poor weather conditions, ORSTOM scientists were unable to observe the lake at close range until 13 March. Aerial photos taken on 20 March (figure 6) show the thermal contrast between Lake Manaro Lakua, formed by the accumulation of water in a low-lying area of the caldera, and Lake Voui, which fills the active crater. Convection cells, ~300-400 m in diameter, could be discerned within Lake Voui.

Figure 4. Sketch of the Aoba summit area, 3 March 1995, showing a very thick dense plume rising from Lake Voui. Based on images taken during an overflight by Vanair pilot Capt. Norman Samson; courtesy of ORSTOM.

Figure 5. Map of the Aoba summit area (after an IGN map) showing the lakes and landing site of the helicopter on 27 June 1995. Courtesy of ORSTOM.

Figure 6. Photograph of the Aoba summit looking approximately SE, showing the steaming Lake Voui in the active crater (foreground) and Lake Manaro Lakua (background), on 20 March 1995. Convection cells ~300-400 m in diameter can be seen in Lake Voui. Courtesy of ORSTOM.

A drop in the level of Lake Voui that began on 6 March (BGVN 20:02) was visible in photographs taken on 20 March. During another overflight on 6 April, the level of the crater lake had dropped by ~2 m. By the time of a 27 June landing on the NW island in Lake Voui (figure 5), the lake level had dropped ~5 m below the maximum, as determined by recent vegetation. Water temperatures measured around the most accessible parts of the island averaged 38-40°C, with highs of 63-67°C. The strongly acidic (pH 2.3) emerald-green lake was mostly obscured by clouds, but vapor emissions were visible between the island and the NW edge of the crater. A small island seen on 6 April in the N part of the lake had enlarged noticeably because of the drop in water level. The topography of the islands is steep towards the center of the lake and gentle towards crater edge. All of the trees on the island were dead, but other vegetation was beginning to reappear. Some blocks of dried mud (40-50 cm in diameter) ejected during the phreatic explosion at the beginning of March were still visible. Sulfur deposits were noted, and gas bubbles were coming from numerous fissures at the edge of the island.

A bathymetric survey of Lake Voui has never been done, but ORSTOM estimates that it has a volume of 50 million cubic meters. Although activity has declined in recent months, ORSTOM will maintain the current low-level alert status until approximately the end of November.

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

Information Contacts: M. Lardy, D. Douglas, P. Wiart, and K. Kalkaua, Centre ORSTOM, Port Vila, Vanuatu, and Bureau des Desastres Nationaux, P.M.B. 014, Port Vila, Vanuatu; M. Regnier and S. Temakon, ORSTOM et Departement des Mines et de la Geologie et des Ressources en Eaux, Port Vila, Vanuatu; Chief N. Tahi, Village de Nambangahake (Ndui-Ndui) Aoba, Vanuatu; C. Robin and M. Monzier, Centre ORSTOM, Quito, EcuadorJ-P.Eissen, Centre ORSTOM de Brest, France; J-P. Metaxian, Universite de Savoie.

In August, Crater C continued its regular emission of gases, lava, and sporadic Strombolian eruptions; in addition, there were occasional lava avalanches. Lava that began to be emitted in May 1995 followed a course toward the SW and flowed down as far as 720 m elevation. Lava emitted a month earlier branched into two arms at ~1,300 m elevation. Explosions sent columns to 1 km above the active vent. These columns were typically blown to the NW, W, and SW; however, some minor local ashfalls also took place on the N and NE as well.

During August, OVSICORI-UNA reported comparative highs in both the number of seismic events (925) and the duration of tremor (348 hours). No seismic data are currently available for July 1995. Comparing the available record starting in January 1994, greater numbers of seismic events took place only in February and March 1994 (956 and 1,011 events) and in June 1995 (1,027 events). For the same interval, tremor prevailed longer only in May and June 1995 (419 and 402 hours, respectively).

A May explosion of >1.2 minutes duration and a plot of the available 1994-95 seismic record were both discussed in BGVN 20:04. The volcano's first chronicled eruption occurred in 1968 and many basaltic andesite discharges have followed since then.

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

Information Contacts: E. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

The bottom of Naka-dake Crater 1 remained covered with a pool of hot water throughout August. The central part of the lake was gray, changing to grayish white or green near the margins. Mud and water ejections were frequently observed; the highest rose 10 m. Isolated tremors increased late in the month (recorded 800 m W of the crater). Isolated tremor events totalled 2,613 during August, and five earthquakes were detected. Tremor events continued increasing in early September; by the 10th there had been >2,000 counted.

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: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

Although one of Melanesia's youngest and most active volcanoes, reports on Bagana are infrequent. Bagana sits along Bougainville's axial highlands ~140 km NW of the 16 August, M 7.8 earthquake's epicenter. Following the earthquake, a report from the Buka Passage Administration Office described a change in activity. Specifically, the report mentioned "black thick clouds coming out of the volcano" and stated that "lava had fallen along the SW coastline." The report included a request for RVO to carry out an inspection. As a result, Chris McKee (RVO) flew around the volcano on 22 August.

McKee noted moderate-to-strong vapor emissions forming a 15-20 km long plume. While the plume looked white near Bagana's summit, downwind a prevailing brown color suggested high SO₂ concentrations in the emissions. No ash was noted in the plume. The summit, a region occupied by a blocky lava dome, fed the plume from numerous sources. There was no appreciable force in the emissions and the plume rose only slightly above the summit.

A lava flow that has been active for several years was observed on the volcano's SW flank. In late August the flow's front was estimated to have reached 580 m elevation. In overview, conditions at the volcano appeared stable. There was no indication of recent explosive activity.

Although reports documenting activity have been absent since early 1991, reported Bagana eruptions in the interval 1972-87 described activity that included long-term lava effusion and slow dome growth, coupled with moderate explosive activity ending with dome destruction. After destruction there was a return to dome growth and lava flows.

Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

Information Contacts: Patrice de Saint-Ours and Ben Talai, RVO.

Aerial inspection was carried out on 22 August, after the 16 August, M 7.8 earthquake that struck 100 km to the W. The inspection revealed profuse white vapor coming from large-output fumaroles in the main fumarole field of the stratovolcano's summit amphitheater. In contrast, emissions at Crater B were moderate and from diffused sources.

Recent landslides were noted in two of the summit craters. The more extensive slides were on the W wall of Crater B. These landslides were thought to have been caused by shaking during the 16 August earthquake.

In general, the visible activity at Balbi appeared to be similar to that observed during previous inspections in the late 1980's. However, emissions may have been more voluminous in 1995.

Balbi marks the highest point on Bougainville Island, forming a summit composed of coalesced cones and lava domes and hosting a large solfatera field. Interviews with local inhabitants suggested that Balbi's last eruption took the lives of a number of people in about 1800-1850.

Geologic Background. The large Balbi stratovolcano forms the highest point on Bougainville Island. The summit of the complex andesitic volcano is part of a large number of coalesced cones and lava domes. Five well-preserved craters occupy a NW-SE-trending ridge north of the summit cone, which also contains a crater. Three large valleys with steep headwalls dissect the flanks. The age of the most recent eruption is not known precisely. An oral tradition of a major eruption during the 19th century is now thought to be in error, but could refer to minor eruptive activity from this relatively youthful-looking volcano. Fumaroles are located within 600-m-wide Crater B and on its W flank.

Information Contacts: Patrice de Saint-Ours and Ben Talai, RVO.

Strombolian activity resumed at Bocca Nuova vent on 30 July and in Northeast Crater on 2 August. Etna's last magmatic activity within its summit craters stopped 3 years and 7 months earlier, two days after the beginning of the 1991-93 flank lava flow eruption (BGVN 16:12). During that 15-month-long eruption and the following 28 months, the four summit craters exhibited continuous steam emission with frequent non-juvenile ash puffs, several collapses, and some strong phreatic explosions from Northeast Crater and Bocca Nuova. No morphological changes were observed in August at either Voragine or Southeast craters, where gas and steam emissions continued as in previous months.

On 30 July the first red spatters were observed inside Bocca Nuova, but bad weather prevented an evaluation of the intensity of this new magmatic activity. Observations the next day revealed that the vent was located in a new pit crater (20-30 m wide and ~50 m deep) on the N part of the crater floor. That part of the crater floor collapsed in June 1994, and probably dropped again in June 1995 when some phreatic explosions occurred (BGVN 20:06). The vent was a few meters across, and magma was sometimes visible during pulsed degassing episodes frequently interrupted by mild Strombolian explosions. The most energetic events were lava jets lasting 15-20 seconds that threw large spatters 120-130 m above the vent to the crater rim. The activity climaxed on 2 and 3 August when lava jets frequently rose up to a few tens of meters above the crater rim. Strombolian activity stopped abruptly on the night of 4 August, leaving a thick tongue of lava on the pit floor. During the Strombolian phase no spatter fell beyond the crater rim; most fell close to the vent inside the inner pit. In the days following 4 August, several ash emissions were observed at Bocca Nuova, which gradually resumed its quiet degassing. No further activity at Bocca Nuova was observed through the end of August.

Until 2 August, no lava emissions had been observed from Northeast Crater since September 1986 (SEAN 11:09); only scoria was ejected on 13 May 1991 (BGVN 16:07). Strombolian explosions during 2-3 August issued from a small vent in the lowest part of the crater, ~150 m below the crater rim. Almost continuous spatter ejections never reached the crater rim. During 3 August the activity gradually changed to puffs of black ash that continued in the following days. After ash emissions decreased, three incandescent degassing points on the crater floor were seen for several days. On 18 August, Strombolian activity resumed and during the night some incandescent bombs rose above the crater rim. Ash emission the following days prevented observations inside the crater, but no blasts were heard. On 29 August, another 1-day phase of Strombolian activity was followed the next day by ash emissions that marked the end of this eruptive episode.

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: Mauro Coltelli, CNR Istituto Internazionale di Vulcanologia, Piazza Roma 2, 95123 Catania, Italy.

Godfrey Merlen was granted permission by the Galapagos National Park Service to make a post-eruption visit to Fernandina. The recent eruption ceased on about 8 April 1995 (BGVN 20:05). Merlen looked at the eruptions impact on plants and animals, and viewed the newly formed lava fields and cone in the absence of the acrid gases and heat present during the eruption.

Merlin arrived at Cape Hammond on 26 July (figure 3) and climbed for 2 hours along a well-known route to the kipuka adjacent the cone. From this point, he approached the cone itself. He noted Jasminocereus cactus, which form a distinctive part of the flora in the area, and, which were partially scalded within several hundred meters of the lava. Surprisingly, they were heavy with fruit.

Figure 3. Sketch map showing newly recognized lava flows and the location of kipukas, including the one termed "Iguana Hill" (IH). Scale is approximate.

The cone was approached from the SW. Large blocks of new lava formed the base of the cone, and below them the old lavas were totally covered with a scoria layer. Poking through this layer were a number of Opuntia cactus. Many had been badly burnt by the heat, but they had undergone strong regrowth, and some had up to 9 or 10 new pads. A few flowers were also present

He ascended the cone (figure 3) easily, due to its firm surface composed of congealed spatter. From the rim he observed that the lava lake had drained, leaving a reddish rubble in the bottom of the crater. A visible entrance to a lava tunnel on the crater's W side probably served as a lava exit route. Circumferential fissures had developed in many areas around the rim, leading to inward collapses. On the N side of the cone's rim, hot spots disclosed by shimmering, heated air indicated that they were still too hot to approach closely.

From the rim one could look upslope and see the earliest flows from the eruption (figure 3). Though previously obscured by gases, it now seems clear that the flow farthest to the N was of significant extent, even though previously unseen. It had traveled a considerable distance past the cone and then turned N, filling in a low area well down toward the coast. Later, lava from the cone butted up against this flow, making a continuous field of new lava.

In descending from the cone's N side towards the "Iguana Hill" kipuka (figure 3), he crossed over the fresh new aa lava, but there were also some smooth patches and many small lava tubes on the surface. The track of the main lava tube could be followed by noting the white encrustations on the rocks. On approach to these encrustations extreme heat was felt. He assumed that a short distance below the surface there were partially liquid lavas that were still degassing. Away from these encrustations the surface of the lava was quite cool.

Although Iguana Hill was wreathed in acidic volcanic gases for many weeks during the eruption, Land Iguanas trapped there survived and four adults were seen. This hardly represents all the iguanas, as the dense scrub vegetation impeded investigation. Many of the plants on the Iguana Hill kipuka were putting out leaves. Zanthoxylum, Croton, and Cordia were all in full leaf, the former were a particularly noticeable bright green.

Blue "smoke" was still visible a little to the NE of Iguana Hill. There was also a little smoke in the low area behind the shoreline. The coast itself was volcanically quiet. Heavy southerly swells broke along a long, black beach that stood in front of the near-vertical sea cliff. This eruption changed conditions at the Cape Hammond landing little, if at all. Flightless cormorants were building nests and some had eggs. The pupping season for the fur seals and sea lion had begun.

A perspective sketch (figure 4) from a point several kilometers offshore shows that the lava flow that started high on the shoulder of the volcano lined up with the westernmost string of cones, including one cone on the coast. However, the new cone, the vent for much of this recently erupted lava, lies off this line to the S.

Figure 4. Sketch of Fernandina drawn from a point several kilometers offshore looking NE. The sketch shows the alignment of cones and some of the upper lava flows.

Geologic Background. Fernandina, the most active of Galápagos volcanoes and the one closest to the Galápagos mantle plume, is a basaltic shield volcano with a deep 5 x 6.5 km summit caldera. The volcano displays the classic "overturned soup bowl" profile of Galápagos shield volcanoes. Its caldera is elongated in a NW-SE direction and formed during several episodes of collapse. Circumferential fissures surround the caldera and were instrumental in growth of the volcano. Reporting has been poor in this uninhabited western end of the archipelago, and even a 1981 eruption was not witnessed at the time. In 1968 the caldera floor dropped 350 m following a major explosive eruption. Subsequent eruptions, mostly from vents located on or near the caldera boundary faults, have produced lava flows inside the caldera as well as those in 1995 that reached the coast from a SW-flank vent. Collapse of a nearly 1 km3 section of the east caldera wall during an eruption in 1988 produced a debris-avalanche deposit that covered much of the caldera floor and absorbed the caldera lake.

Information Contacts: Godfrey Merlen, skipper of motor vessel "Ratty," Fundacion Charles Darwin Para Las Islas Galapagos, Estacion Cientifica Charles Darwin, Ecuador.

An offshore micro-earthquake swarm began around 0700 on 11 September near Kawanazaki (figure 14). The number of events increased from 61 on 11 September to 125 the next day before decreasing to only 12 on 13 September. The largest event was M 2.6 at 1059 on the 12th. Volume strain meters detected contraction of 10^-7. The last earthquake swarm was detected between 27 February and 9 March 1994; it consisted of 287 events, including a M 3.8 earthquake.

Figure 14. Epicenter map of events at Izu-Tobu during 11-13 September 1995. Different symbols indicate hypocenter depths: circles, 0-5 km; triangles, 5-10 km; squares, 10-15 km. Courtesy of JMA.

Geologic Background. The Izu-Tobu volcano group (Higashi-Izu volcano group) is scattered over a broad, plateau-like area of more than 400 km2 on the E side of the Izu Peninsula. Construction of several stratovolcanoes continued throughout much of the Pleistocene and overlapped with growth of smaller monogenetic volcanoes beginning about 300,000 years ago. About 70 subaerial monogenetic volcanoes formed during the last 140,000 years, and chemically similar submarine cones are located offshore. These volcanoes are located on a basement of late-Tertiary volcanic rocks and related sediments and on the flanks of three Quaternary stratovolcanoes: Amagi, Tenshi, and Usami. Some eruptive vents are controlled by fissure systems trending NW-SE or NE-SW. Thirteen eruptive episodes have been documented during the past 32,000 years. Kawagodaira maar produced pyroclastic flows during the largest Holocene eruption about 3000 years ago. The latest eruption occurred in 1989, when a small submarine crater was formed NE of Ito City.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

Clouds obscured the volcano during much of the second half of July, preventing ground and satellite observations. Observers on Adak got a brief view on 14 July and reported light steaming from the summit. On 20 July an AVO geologist on Adak viewed Kanaga for several hours. As has been reported intermittently by U.S. Fish and Wildlife Service (FWS) observers since last fall, fumaroles high on the E and SE flank steamed vigorously and a hazy plume of steam and possibly volcanic gas emanated from the summit crater and drifted a few kilometers downwind. In contrast to other mountain peaks of similar elevation, most of Kanaga was dark and snow-free. One snow patch just below the summit was mantled by debris. It is not known if material mantling the cone is the result of possible activity in late June or merely wind-reworking of material deposited during the extended 1994 eruption. Clouds again obscured Kanaga through the first half of August, but FWS personnel on Adak observed minor steaming from the summit crater during 12-25 August.

Geologic Background. Symmetrical Kanaga stratovolcano is situated within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. The volume of subaerial dacitic tuff is smaller than would typically be associated with caldera collapse, and deposits of a massive submarine debris avalanche associated with edifice collapse extend nearly 30 km to the NNW. Several fresh lava flows from historical or late prehistorical time descend the flanks of Kanaga, in some cases to the sea. Historical eruptions, most of which are poorly documented, have been recorded since 1763. Kanaga is also noted petrologically for ultramafic inclusions within an outcrop of alkaline basalt SW of the volcano. Fumarolic activity occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA, 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.

During 25-30 August there was an increase in seismicity near Shinmoe-dake; 126 earthquakes on the 26th were recorded 1.7 km SW. The maximum seismic amplitude of 3.2 µm occurred on the 30th. In total, 463 events were recorded in August.

Geologic Background. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Information Contacts: JMA.

During August, intermittent Vulcanian explosions interspersed with weaker ash-and-vapor emissions continued at Crater 2. Larger explosions rose several hundred meters above the crater rim and resulted in ashfalls on the downwind (N-NW) side of the volcano. Observers heard low detonations to deep rumblings; on 4, 5, and 17 August they saw weak, steady crater glow. No activity was seen from Crater 3. The seismographs remained inoperative in August.

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: Patrice de Saint-Ours and Ben Talai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

An aerial inspection took place after the 16 August, M 7.8 earthquake 200 km to the NW. Weak-to-moderate, white vapor emissions were observed from the main fumarole field in a valley on the N flank of the summit lava dome. Sublimate deposits in the valley appeared to be less extensive than when last inspected in 1989. The lake at Loloru's summit appeared normal. There was no discoloration of lake water and the level of water appeared to be unchanged.

Geologic Background. Loloru, the SE-most volcano on Bougainville Island, is the source of a broad ignimbrite apron that covers much of the southern part of the island. The summit consists of two nested calderas, and a forested andesitic lava dome that restricts a crescent-shaped lake to the eastern side of the younger caldera. The smooth flanks of the pyroclastic shield are dissected by radiating deep valleys. A pristine lava flow occurs on the SE flank. Loloru is constructed within the 10 x 15 km Pleistocene Laluai caldera. The topographically higher Taroka group of volcanoes to the NW and the Takuan group to the north also were constructed within the caldera and served to deflect the bulk of Loloru ignimbrites to the south. The most recent of several major Holocene explosive eruptions at Loloru took place about 3000 years ago.

Information Contacts: Patrice de Saint-Ours and Ben Talai, RVO.

During August, Manam's summit craters were covered with atmospheric clouds, but when visible, they were simply emitting white vapor at low-to-moderate rates. There were no audible sounds from either crater and no sightings of crater glow. Tiltmeters (installed 4 km SW of the summit) registered little or no ground deformation.

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

Information Contacts: Patrice de Saint-Ours and Ben Talai, RVO.

On the evening of 23 August, staff at the Lignon Hill Observatory in Legazpi observed moderate to intense glow from the crater of Mayon. Moderate steam emissions rising ~300 m above the summit preceded the observation of glow. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) increased the Alert Level to 2, indicating that volcanic activity had increased slightly. PHIVOLCS also recommended strict compliance with the 6-km-radius permanent danger zone, an area restricted to regular human activity, especially below the Bonga channel on the SE flank. The PHIVOLCS Quick Response Team was dispatched to augment monitoring staff at the Lignon Hill and Mayon Resthouse observatories.

The summit crater continued to exhibit glow on 25 August, with varying intensity, and there was moderate steam emission. COSPEC measurements of SO₂ levels in the steam plume were ~630 metric tons/day (t/d), well above the 100-200 t/d measured during quiet periods. No unusual seismicity was detected. The last sighting of crater glow was on 2 September, although it was not until 12 September that SO₂ measurements by COSPEC decreased to near background.

As of mid-September, the dominant seismicity consisted of occasional high-frequency volcanic earthquakes (<5 events/day); they were large enough to be located, and occurred within the E and N parts of the edifice. Observations of the crater area disclosed that some multi-phase events were due to large lava blocks detaching from the vent. The vent is open to the SE as a result of the 1993 explosions. A preliminary investigation of a water well in Malilipot (ENE of the summit) on 25 August revealed a slight decrease in water level, also an indicator of volcanic unrest. However, further measurements of water wells on the SE and S margins of lahar fans around Mayon, where most wells are located, did not show measurable or significant changes. PHIVOLCS therefore concluded that little ground deformation was taking place.

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

Information Contacts: Ernesto G. Corpuz, Chief of Volcano Monitoring Division, Philippine Institute of Volcanology and Seismology (PHIVOLCS), 5th & 6th Floors Hizon Building, 29 Quezon Avenue, Quezon City, Philippines.

The Kudriavy cone at Medvezhia has been the subject of great interest since Korzhinsky and others (1994) discovered a pure rhenium sulfide mineral in its high-temperature (535°C) fumaroles. Given the concentration of Re found in gas samples (2-10 ppb), the occurrence of ReS₂ or Re₂S₃ (exact form still uncertain) requires enrichment of Re by eight orders of magnitude. During a 21 August-5 September visit, Stan Williams, Tobias Fisher, and Russian colleagues made COSPEC measurements of SO₂ flux. Gas samples were also collected from crater fumaroles.

The COSPEC was operated from the base camp (150 m elevation) 2.7 km SE of the elongate ENE-WSW summit (990 m elevation), while the wind velocity was measured at the summit using a hand-held anemometer. The first measurements were made on 28 August in vertically oriented stationary mode with the wind blowing the wide gas plume directly over the camp. Wind velocity was measured at 3 m/s and the plume was estimated to be rising only 50 m above the summit before being blown downwind. The flux was found to be 100 +- 20 metric tons/day (t/d) for the three measurements possible before the wind shifted to the SW, making any measurements essentially oblique to the plume axis, and therefore of great uncertainty. An occasional strong odor of H₂S was detected at the camp during measurements.

During a crater visits on 29 and 31 August, and during a helicopter flight downwind in the plume, there was always a strong odor of H₂S. Under clear skies on 31 August the wind carried the plume SW, allowing stationary mode vertical and horizontal measurements. One specific goal of the research was to quantify the output of the isolated fumarole fields with different temperatures, which was possible for part of the day. Wind velocity was measured at 3 m/s and the SO₂ flux was calculated to be only 30 +- 10 t/d.

Kudriavy has been consistently degassing passively at high temperatures since at least 1961, when it was first visited by Russian scientists; annual fieldwork began in 1989. No change in activity was noted during this visit. At the low levels at which Kudriavy was found to be degassing, a realistic SO₂ norm may be 75 +- 50 t/d, with the oscillations potentially reflecting meteoric conditions. These low-pressure fumaroles, some with temperatures up to 950°C, produced hissing to roaring to deafening levels of noise. There was more noise and higher flux than at Momotombo (Nicaragua), where 950°C fumaroles were studied through the early to mid-1980s by U.S. and Russian collaboration.

Reference. Korzhinsky, M.A., Tkachenko, S.I., Shmulovich, K.I., Taran, Y.A., and Steinberg, G.S., 1994, Discovery of a pure rhenium mineral at Kudriavy volcano: Nature, v. 369, p. 51-52.

Geologic Background. The Moyorodake volcanic complex (also known as Medvezhia) occupies the NE end of Iturup (Etorofu) Island. Two overlapping calderas, 14 x 18 and 10 x 12 km in diameter, were formed during the Pleistocene. The caldera floor contains several lava domes, cinder cones and associated lava fields, and a small lake. Four small closely spaced stratovolcanoes were constructed along an E-W line on the eastern side of the complex. The easternmost and highest, Medvezhii, lies outside the western caldera, along the Pacific coast. Srednii, Tukap, and Kudriavy (Moyorodake) volcanoes lie immediately to the west. Historically active Moyorodake is younger than 2000 years; it and Tukap remain fumarolically active. The westernmost of the post-caldera cones, Menshoi Brat, is a large lava dome with flank scoria cones, one of which has produced a series of young lava flows up to 4.5 km long that reached Slavnoe Lake. Eruptions have been documented since the 18th century, although lava flows from cinder cones on the flanks of Menshoi Brat were also probably erupted within the past few centuries.

Information Contacts: Stanley N. Williams and Tobias P. Fischer, Geology Dept., Arizona State University, Tempe AZ 85287, USA; Kirill I. Shmulovich and Mikhail A. Korzhinsky, Inst. of Experimental Mineralogy, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia; Genrikh S. Steinberg, Inst. of Volcanology & Geodynamic ANSRF, 693008 Yuzhno-Sakhalinsk, Box 18, Russia.

Small volcanic tremors were recorded in late August from a station 1.5 km SE of the summit.

Geologic Background. The massive Ontakesan stratovolcano, the second highest volcano in Japan, lies at the southern end of the Northern Japan Alps. Ascending this volcano is one of the major objects of religious pilgrimage in central Japan. It is constructed within a largely buried 4 x 5 km caldera and occupies the southern end of the Norikura volcanic zone, which extends northward to Yakedake volcano. The older volcanic complex consisted of at least four major stratovolcanoes constructed from about 680,000 to about 420,000 years ago, after which Ontakesan was inactive for more than 300,000 years. The broad, elongated summit of the younger edifice is cut by a series of small explosion craters along a NNE-trending line. Several phreatic eruptions post-date the roughly 7300-year-old Akahoya tephra from Kikai caldera. The first historical eruption took place in 1979 from fissures near the summit. A non-eruptive landslide in 1984 produced a debris avalanche and lahar that swept down valleys south and east of the volcano. Very minor phreatic activity caused a dusting of ash near the summit in 1991 and 2007. A significant phreatic explosion in September 2014, when a large number of hikers were at or near the summit, resulted in many fatalities.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

During August, the level of the sky blue lake within the N crater climbed 1.5 m with respect to its position in June. The lake's temperature was 37.5°C. Fumaroles on the W lake terrace generated gas columns <50 m high; those on the NW lakeshore continued their constant bubbling. The gases escaping from the pyroclastic cone had an 89°C temperature. Fumaroles on the S and SW crater walls had 94°C and 96°C temperatures and produced columns reaching over 50 m high. When the wind blew towards the S, Rangers at the park entry station smelled sulfur.

Poás was very active in terms of both moderate- to low-frequency earthquakes and tremor. There were 5,651 seismic events in August, predominantly low-frequency (table 6). Both medium- and high-frequency events prevailed when seismicity peaked on 25 August with 312 events.

August can be compared to the 17 other months where data were available during 1994-95 (table 6). The number of low-frequency events was fourth largest in August 1995. The largest value for the 1994-1995 interval was 7,119 events (March 1994). Tremor in August 1995 took place for 9 hours; this compares with high values seen in April 1995 (11 hours) and June 1994 (307 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: E. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA).

Although Rabaul remained quiet in August, the region was subjected to a strong tectonic earthquake (Ms 7.8) followed by associated aftershocks. The earthquake struck on 16 August centered ~260 km SE of Rabaul. In Rabaul the earthquake struck at 2027 with a modified Mercalli (MM) intensity of V, causing some minor landslides and the collapse of a few buildings.

The earthquake triggered noteworthy responses at the caldera. On 16 August there were 24 high-frequency events, and, as late as 23 August, 3-9 additional events/day; in July there were 7 high-frequency earthquakes (M <1). On 22 August, one event was felt by residents. With an approximate magnitude of ML 2.8, this was the first felt caldera earthquake since the early phase of Rabaul's September 1994 eruption.

These high-frequency earthquakes occurred mostly in the NE part of the caldera (Namanula Hill area). In contrast, throughout the period of heightened seismicity the number of low-frequency earthquakes remained low, peaking on 30-31 August at 10 and 26 events, respectively. For comparison, during July low-frequency events occurred 11 times. The August low-frequency earthquakes appeared to originate from within or just outside the N caldera area.

During the first half of the month ground deformation remained below the measurable level. Following the Ms 7.8 earthquake, electronic and water-tube tiltmeters recorded offsets as large as 90 µrad. The offsets suggested subsidence in the central part of the caldera with the outer rim remaining stationary. As of 15 September, adequate elevation data were unavailable to confirm the pattern of offset seen in the deformation data.

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

Information Contacts: Patrice de Saint-Ours and Ben Talai, RVO.

Eruptive activity . . . began with a phreatic explosion on 18 July that caused ashfall around the island (20:6). Formation of a second vent in the summit crater on 28 July and increased steam-and-ash emissions prompted evacuations from communities near the summit (20:7). Activity was variable but generally low in early August, with small mudflows and continued steaming. Vent 1 reactivated on 11 August, and some earthquakes were centered beneath St. George's Hill, 3 km WNW of the summit. Relatively heavy cloud cover and bad weather prevented observations on many days in late August.

Moderate emissions continued from both vents through 19 August (20:7). An ash-and-steam eruption from Vent 1 at 1220 on 19 August was similar in size to the two previous events that caused ashfall in Plymouth. The eruption lasted for ~35 minutes, during which a 10-minute-long phase of more vigorous activity deposited ~1 mm of ash in areas NW of the vent. Ashfall along the road between Lees and Gages has caused reduced traction; police were advising motorists to drive slower on the slippery surface.

Another small phreatic episode at 1657 on 20 August produced minor ashfall. Since the disappearance of continuous tremor on 19 August, seismicity consisted of low-intensity spasmodic tremor and occasional small volcano-tectonic (VT) earthquakes attributed to very shallow activities under the summit area.

Formation of Vent 3 on 22 August. Seismicity generally increased in frequency and amplitude until the largest single phreatic eruption episode to date at 0803 on 21 August produced a column to a height of ~2 km. Ash blown down the W flank engulfed Plymouth within eight minutes, caused darkness for ~25 minutes, and deposited an estimated 2 mm of ash. An analysis of the new ash showed that it consisted only of old altered material. Other reported phenomenon were projectiles in the Long Ground area and a density current in Gages. Over 70 locatable VT earthquakes were recorded between noon on 20 August through noon the next day; most of these events occurred before the 21 August eruption and were very shallow.

Following a contingency plan, the volcano observatory was relocated to the Vue Pointe Hotel in Old Towne (~ 4.5 km N of Plymouth) at approximately 1600 on the 21st; by 1730 all of the seismic instruments were back on-line. Seismicity was less frequent and vigorous through noon on 22 August, with only four eruption signals of smaller intensity and duration than the previous episodes. Over 5,000 residents were evacuated from the capital city of Plymouth to camps on the N part of the island. Day use was permitted, but restricted. Government offices were also relocated.

Six phreatic eruptions occurred between noon on 22 August and noon the next day. The largest, at 1551 on 22 August, produced ashfall ~3-3.5 km to the SW. This event was followed by three long-period (LP) earthquakes (events associated with the movement of pressurized fluids). When the crater area was visited on the afternoon of 22 August it was discovered that a new vent (Vent 3) had opened along the inside of English's Crater rim. Only 24 earthquakes were located during 21-23 August, all very shallow. Seismicity was comparable or slightly lower during the next twenty-four hours. There were six episodes of increased gas venting, some of which were followed by small VT events. Fifteen earthquakes, including two LP events, were located beneath Soufriere Hills at depths ranging from near-surface to ~5 km.

Aerial reconnaissance on the afternoon of 24 August showed a NNW-SSE trending line of several small explosion craters in the summit crater, with Vent 1 at the N end and Vent 3 at the S; Vent 2 was offset to the SE. The rate of steam emission from Vent 2 was very low, while slightly more steam was being emitted from Vent 3. No gas emission was observed from Vent 1. Seismicity continued to be relatively low, with seven earthquakes distributed beneath St. George's Hill and Soufriere Hills at depths of 0-4 km. Three episodes of increased gas venting and 30 minutes of broadband tremor also occurred. A small mudflow originating from Gages Upper Soufriere during heavy rains on the afternoon of 24 August flowed through Fort Ghaut. Seismicity remained low until a swarm of VT earthquakes that lasted from 2157 on 25 August until 0230 the next day. Located earthquakes consisted of 22 VT events at depths of 0-5 km beneath the N flank. Five episodes of gas venting during 25-26 August had repose intervals of ~4 hours.

Formation of Vent 4 on 27 August. On 27 August there was one episode of broadband tremor that lasted ~20 minutes and a small mudflow in Fort Ghaut that began around 0820. From 26 to 28 August, fourteen small VT earthquakes were located beneath Soufriere Hills and St. Georges Hill at depths of <5 km. During this same period there were twelve episodes of increased gas venting. One episode at 1443 on 26 August ejected ash that could be seen from the Vue Pointe Hotel; other emissions caused light ashfall in the Tar River area. Observations on the morning of 28 August confirmed the presence of a fourth vent that had probably opened the day before. Located on the NNE flank of Castle Peak dome, it was vigorously emitting steam and ash through mid-day on 28 August; emissions from the other vents were low. Eight VT earthquakes were located beneath Soufriere Hills at depths of 0-4 km. Five episodes of increased gas venting occurred.

Vent 4 was still emitting mainly steam at a reduced rate on 29-30 August. Another nine episodes of increased gas venting were detected, and five small shallow earthquakes were located beneath and to the N of the Soufriere Hills during 29-30 August. Unusually good visibility allowed Castle Peak dome to be inspected at around 0900 on 30 August. Steam emissions from all vents were low and there was no ash. The main vent system (a linear chain of vents extending from Vent 1 on the NW margin of the dome SE to the S margin of the dome) had enlarged since 24 August. Mud or muddy water was locally present in the bottom of the main vent system. Several pools of standing water were located atop Castle Peak, and the moat pond on the NW side of the dome still existed. A recent mudflow from the W side of the dome southward down the Tar River had buried Vent 2, on the S side of the dome.

In terms of earthquake activity, the 24-hour period beginning at 1400 on 30 August was probably the most active since the 21 August phreatic eruption. Thirty-four shallow earthquakes were located WNW of Soufriere Hills. A few earthquakes were also located beneath Windy Hill (3.5 km NNW) and in the area between Windy Hill and Soufriere Hills. Seismicity decreased the next day, when only ten shallow earthquakes were located WNW of Soufriere Hills; two were also located beneath Windy Hill. In addition, four LP earthquakes occurred at shallow depths beneath the NW edge of Soufriere Hills. During these two days, thirteen episodes of increased gas venting were detected, but steam and ash emissions from all vents remained low.

Deformation and SO₂ measurements. A review of the Brodrick's dry-tilt data completed on 23 August indicated that some deformation of the volcano may have occurred between January and 9 August, confirming that magma may be at a shallow depth (as suggested by the earthquake data). Tiltmeter readings in late August were generally within background noise levels; no tilt related to volcanism was observed. EDM reflectors were deployed on 30 August in Gages Upper Soufriere and on Castle Peak dome.

COSPEC gas measurements taken on the afternoon of 20 August indicated that the rate of SO₂ emission was just above the detection level, ~50 metric tons/day (t/d). Additional measurements taken during favorable conditions on the next afternoon and morning of 22 August did not detect any SO₂. This lack of SO₂ was thought to be a result either of the system running out of gases or a sealing off of the fluid access path to the surface. A COSPEC flight on the afternoon of 23 August detected a slight trace of SO₂ (~ 40 t/d) while a flight the next morning showed none. The flux rate on the morning of 26 August was ~50 t/d, and on 28 August was ~85 t/d. Further COSPEC measurements on 29, 30, and 31 August showed no detectable SO₂.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: VDAP, USGS; Seismic Research Unit, UWI; Montserrat EOC.

The only significant tremor variations for the period 11 June-15 September 1995 were between 12 and 27 August, when intensity slowly increased (figure 45). The following four days were characterized by a rapid return of the intensity to the range observed throughout the first half of September. Noteworthy is the lack of a great number of saturating events compared to March-May 1995, including the 5 March explosion (BGVN 20:04 and 20:05). This means that explosive activity was either less energetic or shallower, with more energy released towards the air than into seismic waves. The total number of recorded events showed a rise and subsequent decrease from mid-June to mid-July, with two minima of 50-60 events/day and a maximum of almost 200 events/day recorded on 26 June. Another rise starting in mid-July reached values of 260-280 events on 28 August and 3 September, corresponding to the period of decreased tremor.

Figure 45. Seismicity detected at Stromboli, 11 June-15 September 1995. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 µ/s (instrument saturation level). The line shows daily tremor energy computed by averaging hourly 60-second samples. The seismic station is located 300 m from the craters at 800 m elevation. Courtesy of Roberto Carniel.

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: Roberto Carniel, Dipartimento di Georisorse e Territorio, via Cotonificio 114, I-33100 Udine, Italy.

A volcanic tremor event and 20 seismic events were observed on 18 August at Station A, 4.5 km NNW of Crater 62-2. On 23 August another 19 events were recorded. In total, 77 earthquakes were detected in August. This period of increased seismicity began on 9 July.

Geologic Background. Tokachidake volcano consists of a group of dominantly andesitic stratovolcanoes and lava domes arranged on a NE-SW line above a plateau of welded Pleistocene tuffs in central Hokkaido. Numerous explosion craters and cinder cones are located on the upper flanks of the small stratovolcanoes, with the youngest Holocene centers located at the NW end of the chain. Frequent historical eruptions, consisting mostly of mild-to-moderate phreatic explosions, have been recorded since the mid-19th century. Two larger eruptions occurred in 1926 and 1962. Partial cone collapse of the western flank during the 1926 eruption produced a disastrous debris avalanche and mudflow.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

Clouds frequently obscured the volcano in July and August, preventing ground and satellite observations. Observers in Perryville (~30 km S of Veniaminof) got good views on 13 and 27 July, and reported light steaming on both days. On 28 July a weak hot spot centered on Veniaminof was noted on an AVHRR image. Perryville residents reported clear skies but no evidence of activity on the morning of 4 August, and AVHRR satellite images on 4 and 5 August showed no hot spot. Perryville residents saw a small steam plume on 9 August, a small steam plume and "smoke" during the week of 12-18 August, and a small steam plume again during 19-25 August. AVHRR satellite images on 14 and 21 August showed no hot spots. Another AVHRR image from late on 31 August showed a possible steam plume ~50 km long blowing NW of Veniaminof. The hot spot was ~15°C warmer than the surrounding features (probably ice and snow near the summit).

Geologic Background. Veniaminof, on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3,700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA, 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.

During October 1993, a telemetered surveillance station registered variations in the seismicity at Yasur. The station is located 2 km from the crater on the ashflow plain (figure 3). Seismicity (detected at five stations) generally increased from October 1993 to January 1994, corresponding to renewed eruptive activity. Very strong activity from January 1994 to January 1995 (comparable to 1976-77) ejected bombs in a radius of ~400 m from the crater rim (figure 4). Ashfall measured at the surveillance station totaled ~12 cm during January-October 1994. Seismicity remained high throughout 1994, then declined after January 1995.

Figure 3. Photograph of an explosion at Yasur on 17 March 1994. The ARGOS-linked monitoring station is in the foreground. View is approximately to the W. Photo by M. Lardy, courtesy of ORSTOM.

Figure 4. Sketch map of Yasur showing the area of volcanic bomb fallout during 1994. Courtesy of ORSTOM.

Because Yasur is very accessible, it has been promoted as a tourist destination, resulting in a greater number of visitors and greater risk of accidents. In mid-January 1994 and early February 1995, when activity began to decline, there were two accidents with three victims. In one incident, two visitors on the crater rim died when they were struck by a 15 kg bomb ejected from the vent. Although there is little danger when the volcano is having regular explosions at intervals of a few minutes to tens of minutes, the local authorities want to better inform visitors of the constant danger. In January 1994, ORSTOM began publishing a series of booklets to inform the general public of the volcanic risks. Risks discussed include bombs falling near the crater, modifications of the crater topography (raising of the floor, migration of the vent, etc.). In addition, the 200-m distance between the observation site and the active vents, and the very frequent bomb ejections at speeds of 100-300 m/second that have rendered approaching the crater rim dangerous.

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

Information Contacts: M. Lardy and D. Charley, Centre ORSTOM, Port Vila, Vanuatu, and Department des Mines et de la Geologie et des Ressources en Eaux; J. Tabbagh, Centre de Teleobservation Informatise des Volcans, Garchy, France; J-P. Eissen, Centre ORSTOM de Brest, France; C. Robin and M. Monzier, Centre ORSTOM, Quito, Ecuador.

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/).