Remote Heard Island in the southern Indian Ocean is home to the snow-covered Big Ben stratovolcano, which has had confirmed intermittent activity since 1910. The nearest continental landmass, Antarctica, lies over 1,000 km S. Visual confirmation of lava flows on Heard are rare; thermal anomalies and hotspots detected by satellite-based instruments provide the most reliable information about eruptive activity. Thermal alerts reappeared in September 2012 after a four-year hiatus (BGVN 38:01), and have been intermittent since that time. Information comes from instruments on the European Space Agency's (ESA) Sentinel-2 satellite and MODVOLC and MIROVA thermal anomaly data from other satellite instruments. This report reviews evidence for eruptive activity from November 2017 through September 2018.

Satellite observations indicated intermittent hot spots at the summit through 12 December 2017. A few observations in January and February 2018 suggested steam plumes at the summit, but no significant thermal activity. An infrared pixel indicative of renewed thermal activity appeared again on 7 March, and similar observations were made at least twice each month in April and May. Activity increased significantly during June and remained elevated through September 2018 with multiple days of hotspot observations in satellite data each of those months, including images that indicated lava flowing in different directions from Mawson Peak. MODVOLC and MIROVA data also indicated increased thermal activity during June-September 2018.

Activity during October-December 2017. MIROVA thermal anomalies recorded during October 2017 indicated ongoing thermal activity at Heard (figure 32). This was confirmed by Sentinel-2 satellite imagery that revealed hotpots at the summit on ten different days in October (3, 6, 8, 13, 16, 21, 23, 26, 28, and 31), and included images suggesting lava flows descending from the summit in different directions on different days (figure 33).

Figure 32. MODVOLC thermal alerts indicated significant thermal activity at Heard during October 2017 that tapered off during November. Intermittent signals appeared in December 2017, March, and April 2018, and a strong signal returned in June 2018 that continued through September. Courtesy of MIROVA.

Figure 33. Sentinel-2 images of Heard Island's Big Ben volcano during October 2017 showed strong evidence of active effusive activity. a) 3 October 2017: at least three hot spots were visible through cloud cover at the summit and W of Mawson Peak, suggesting active lava flows. b) 6 October 2017: a small hot spot is visible at the peak with a small steam plume, and a larger hotspot to the NW suggested a still active lava flow. c) 16 October 2017: a small hotspot at the summit and larger hotspots W of the summit were indicative of ongoing flow activity. d) 23 October 2017: a steam plume drifted SE from a small summit hotspot and a larger hotspot to the W suggested a lava lake or active flow. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

The MODVOLC thermal alert data showed no further alerts for the year after 22 October 2017, and the MIROVA system anomalies tapered off in mid-November 2017. The Sentinel-2 satellite imagery, however, continued to record intermittent hotspots at and around Mawson Peak, the summit of Big Ben volcano, into December 2017 (figure 34). Hotspots were visible during six days in November (7, 15, 20, 25, 27, and 30) and three days during December (5, 7, and 12).

Figure 34. Sentinel-2 images of Heard Island's Big Ben volcano showed reduced but ongoing thermal activity during November and December 2017. a) 7 November 2017: a steam plume drifts NE from a hotspot at Mawson Peak. b and c) 15 November and 12 December 2017: a small hotspot is distinct at the summit. d) 20 December 2017: a steam plume drifts east from the peak, but no clear hotspot is visible. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Activity during January-May 2018. The satellite images during January and February 2018 were indicative of steam plumes at the summit, but distinct thermal signals reappeared on 7 and 12 March 2018 (figure 35). In spite of extensive cloud cover, the Sentinel-2 imagery also captured thermal signals twice each month in April (4 and 14) and May (9 and 14) (figure 36).

Figure 35. Sentinel-2 images of Heard Island's Big Ben volcano showed only steam plumes at the summit during January and February, but hotspots reappeared in March 2018. a) 4 January 2018: a steam plume drifts SE from the summit under clear skies. b) 8 February 2018: a steam plume drifts SE from the summit adjacent to a large cloud on the N side of the volcano. c) 7 March 2018: the first hotspot in about three months is visible at the summit. d) 12 March 2018: a distinct hotspot is visible at Mawson Peak. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Figure 36. Sentinel-2 images of Heard Island's Big Ben volcano showed intermittent low-level thermal activity during April and May 2018. a) 4 April 2018: a small hotspot is visible at the summit through a hazy atmosphere. b) 9 May 2018: a distinct hotspot glows from the summit beneath cloud cover. Sentinel-2 images with Atmospheric Penetration view(bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Activity during June-September 2018. Thermal signals increased significantly in the satellite data during June 2018. The sizes of the thermal anomalies were bigger, and they were visible at least nine days of the month (3, 5, 8, 10, 15, 18, 23, 25, and 30). Five substantial thermal signals appeared during July (3, 10, 15, 18, and 28); images on 23 June and 3 July distinctly show a lava flow trending NE from the summit (figure 37). MODVOLC thermal alerts appeared in June 2018 on three days (2, 26, and 27) and on four days during July (7, 8, 9, 10) indicating increased activity during this time. The MIROVA thermal signals also showed a substantial increase in early June that peaked in mid-July and remained steady through September 2018 (figure 32).

Figure 37. Sentinel-2 images of Heard Island's Big Ben volcano showed significantly increased thermal activity during June and July 2018. a) 8 June 2018: a substantial hotspot is visible through the cloud cover at the summit of Big Ben. b) 10 June 2018: the darker red hotspot at Mawson Peak was significantly larger than it was earlier in the year. c) 23 June 2018: the first multi-point hotspot since 31 October shows a distinct glow trending NE from the summit. d) 3 July 2018: a trail of hotspots defines a lava flow curving NNE from Mawson Peak. e) 18 July 2018: a second significant hotpot is visible a few hundred meters NE of the summit hotspot indicating a still active flow. f) 28 July 2018: the summit hotspot continued to glow brightly at the end of July, but no second hotspot was visible. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Six images in August (2, 7, 9, 22, 27, 29) showed evidence of active lava at the summit, and suggested flows both NE and SE from the summit that were long enough to cause multiple hotspots (figure 38). During September and early October 2018 the satellite images continued to show multiple hotspots that indicated flow activity tens of meters SE from the summit multiple days of each month (figure 39).

Figure 38. Sentinel-2 images of Heard Island's Big Ben volcano showed lava flow activity in two different directions from the summit during August 2018. a) 2 August 2018: lava flows NE from Mawson Peak while a steam plume drifts E from the summit. b) 9 August 2018: a second hotspot NE of the summit hotspot indicates continued flow activity in the same area observed on 2 August. c and d) 27 and 29 August 2018: a different secondary hotspot appeared SSE from the summit indicating a distinct flow event from the one recorded earlier in August. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

Figure 39. Sentinel-2 images of Heard Island's Big Ben volcano in September and October 2018 showed hotspots indicating active flows SE of the summit on multiple days. a) 3 September 2018: a small hotspot at the summit and a larger hotspot SE of the summit indicated continued flow activity. b) 3 October 2018: a small steam plume drifted east from a small hotspot at the summit and a larger pair of hotspots to the SE indicated continued effusive activity. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of ESA Sentinel Hub Playground.

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

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Krakatau volcano in the Sunda Strait between Java and Sumatra, Indonesia experienced a major caldera collapse, likely in 535 CE, that formed a 7-km-wide caldera ringed by three islands (see inset figure 23, BGVN 36:08). Remnants of this volcano coalesced to create the pre-1883 Krakatau Island which collapsed during the 1883 eruption. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed within the 1883 caldera has been the site of frequent eruptions since 1927. The most recent event was a brief episode of Strombolian activity, ash plumes, and a lava flow during the second half of February 2017. Activity resumed in late June 2018 and continued through early October, the period covered in this report. Information is provided primarily 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 came from several social media sources and professional photographers.

After the brief event during February 2017, Anak Krakatau remained quiet for about 15 months. PVMBG kept the Alert Level at II, noting no significant changes until mid-June 2018. Increased seismicity on 18 June was followed by explosions with ash plumes beginning on 21 June. Intermittent ash emissions were accompanied by Strombolian activity with large blocks of incandescent ejecta that traveled down the flanks to the ocean throughout July. Explosions were reported as short bursts of seismic activity, repeating multiple times in a day, and producing dense black ash plumes that rose a few hundred meters from the summit. Similar activity continued throughout August, with the addition of a lava flow visible on the S flank that reached the ocean during 4-5 August. Generally increased activity in September resulted in the highest ash plumes of the period, up to 4.9 km altitude on 8 September; high-intensity explosions were heard tens of kilometers away during 9-10 September. PVMBG reported significantly increased numbers of daily explosions during the second half of the month. The thermal signature recorded in satellite data also increased during September, and a large SO2 plume was recorded in satellite data on 23 September.

Activity during June-July 2018. PVMBG noted an increase in seismic activity beginning on 18 June 2018. Foggy conditions hampered visual observations during 19-20 June, but on 21 June gray plumes were observed rising 100-200 m above the summit (figure 41). Two ash plumes were reported on 25 June; the first rose to about 1 km altitude and drifted N, and the second rose to 600 m altitude and drifted S (figure 42).

Figure 41. Anak Krakatau began a new eruptive episode on 21 June 2018 with an ash plume that rose 200 m above the summit. Photo by undisclosed source, courtesy of Øystein Lund Andersen‏.

Figure 42. The first of two ash plumes rose to about 1 km altitude and drifted N from Anak Krakatau on 25 June 2018; the first events after about 18 months of no activity were reported on 21 June. Courtesy of PVMBG (Eruption Information on Mt. Anak Krakatau, June 25, 2018).

Incandescence was observed at the summit during 1-2 July 2018, and two ash emissions were reported in VONA's (Volcano Observatory Notice for Aviation) on 3 July. PVMBG reported that during 4-5 July there were four additional ash-producing events, each lasting between 30 and 41 seconds. The last three of these events produced ash plumes that rose 300-500 m above the crater rim and drifted N and NW. The Darwin VAAC reported essentially continuous ash emissions during 3-9 July drifting generally W and SW at about 1.2 km altitude (figure 43). They were intermittently visible in satellite imagery when not obscured by meteoric clouds.

Figure 43. A dense gray ash plume rose several hundred meters above Anak Krakatau on 7 July 2018 (local time) while large volcanic bombs traveled down the flanks. Photo by Sam Hidayat, courtesy of Øystein Lund Andersen‏.

Ash plumes were again observed by the Darwin VAAC in satellite imagery beginning on 13 July 2018 at 1.2 km altitude drifting NW. They were essentially continuous until they gradually decreased and dissipated early on 17 July, rising to 1.2-1.5 km altitude and drifting W, clearly visible in satellite imagery several times during the period. Satellite imagery revealed hotspots several times during July; they ranged from small pixels at the summit (9 July) to clear flow activity down the SE flank on multiple days (12, 19, and 24 July) (figure 44). In the VONA's reported by PVMBG during 15-17 July, they noted intermittent explosions that lasted around 30-90 seconds each. PVMBG reported a black ash plume 500 m above the summit drifting N during the afternoon of 16 July. The Darwin VAAC continued to report ash emission to 1.2-1.5 km altitude during 18-19 July, moving in several different directions; Strombolian activity sent incandescent ejecta in all directions on 19 July (figure 45). During 25-26 July the Darwin VAAC noted continuous minor ash emissions drifting SW at 1.2 km altitude, and a hotspot visible in infrared imagery.

Figure 44. Sentinel-2 satellite imagery clearly documented the repeated thermal activity at Anak Krakatau throughout July 2018. a) 9 July 2018: a small hotspot was visible at the summit and an ash plume drifted NW. b) 12 July 2018: a much larger hotspot showed a distinct flow down the SE flank. c) 19 July 2018: even under partly cloudy skies, incandescent ejecta is visible on the S flank. d) 24 July 2018: incandescent lava had almost reached the SE coast. Sentinel-2 images with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

Figure 45. Strombolian activity sent incandescent ejecta down all the flanks and into the sea at Anak Krakatau on 19 July 2018, as seen from the island of Rakata (5 km SE). Courtesy of Reuters / Stringer.

Activity during August-early October 2018. A series of at least nine explosions took place on 2 August 2018 between 1333 and 1757 local time. They ranged from 13 to 64 seconds long, and produced ash plumes that drifted N. The Darwin VAAC reported minor ash observed in imagery at around 2 km altitude for much of the day. In a special report, PVMBG noted a black ash plume 500 m above sea level drifting N at 1757 local time. Continued explosive activity was reported by local observers during the early nighttime hours of 3 August (figure 46).

Figure 46. A dark ash plume rose 100-200 m from Anak Krakatau during the early morning hours of 3 August 2018, and incandescent ejecta rolled down the flanks. Tens of explosions were heard in Serang (80 km E) and Lampung (80 km N). Courtesy of Sutopo Purwo Nugroho.

The Darwin VAAC reported continuous ash emissions rising to 1.8 km altitude and drifting E on 5 August, clearly visible in satellite imagery, along with a strong hotspot. The ash plume drifted SE then S the next day before dissipating. PVMBG reported incandescence visible during the nights of 5-15 August. Photographer Øystein Lund Andersen visited Krakatau during 4-6 August 2018 and recorded Strombolian activity, lava bomb ejecta, and a lava flow entering the ocean (figures 47-50).

Figure 47. Strombolian explosions sent incandescent ejecta skyward, and blocks of debris down the flanks of Anak Krakatau on 5 August 2018 as captured in this drone photograph. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Figure 48. Large volcanic bombs flew out from the summit vent of Anak Krakatau while a dark gray plume of ash rose a few hundred meters on 5 August 2018 in this drone photograph. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Figure 49. A blocky lava flow traveled down the S flank of Anak Krakatau on 5 August 2018 in this closeup image taken by a drone. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Figure 50. Views of Anak Krakatau from the SE showed Strombolian activity and incandescent lava (upper photo) and steam from the lava flowing into the ocean and dark ash emissions from the summit (lower photo) on 5 August 2018. Copyrighted photo by Øystein Lund Andersen‏, used with permission.

Emissions were reported intermittently drifting W on 11, 14, and 16 August at 1.2-1.5 km altitude. Video of explosions on 12 August with large bombs and dark ash plumes were captured by photographer James Reynolds (Earth Uncut TV). PVMBG reported black ash plumes drifting N at 500 m above the summit on 17 and 18 August after explosions that lasted 1-2 minutes each. The Darwin VAAC also reported ash plumes rising to 1.2 km altitude on 17-18 drifting NE. VONA's were issued during 22-23 August reporting at least three explosions that lasted 30-40 seconds and produced ash plumes that drifted N and NE. The Darwin VAAC reported the plume on 22 August as originating from a vent below the summit. PVMBG noted that a dark plume on 23 August drifted NE at about 700 m above the summit. During 27-30 August, the Darwin VAAC reported ash plumes intermittently visible in satellite imagery extending SW at 1.2-1.5 km altitude.

Ash plumes drifting N and NW were visible in satellite imagery during 3-4 September at 1.2-1.5 km altitude. The Darwin VAAC reported an ash plume moving NW and W at 4.9 km altitude on 8 September, the highest plume noted for the report period. The following day, the plume height had dropped to 1.5 km altitude, and was clearly observed drifting W in satellite imagery. A hotspot was reported on 12 September. During the night of 9-10 September PVMBG reported bursts of incandescent material rising 100-200 m above the peak, with explosions that rattled windows at the Anak Krakatau PGA Post, located 42 km from the volcano. Ash plumes continued to be observed through 13 September. The Darwin VAAC reported continuous ash emissions to 1.8 km altitude drifting W and NW on 16-17 September (figure 51). The ash plume was no longer visible on 18 September, but a hotspot remained discernable in satellite data through 20 September.

Figure 51. On 16 September 2018 a dark ash plume rose several hundred meters above Anka Krakatau as incandescent lava flowing down the SE flank to the sea created steam plumes. Courtesy of Thibaud Plaquet.

PVMBG reported incandescence at the summit and gray and black ash plumes on 20 September that rose 500 m above the summit. A low-level ash emission was reported drifting S on 21 September and confirmed in the webcam. Four VONA's were issued that day, reporting explosions at 0221, 0827, 2241, and 2248, lasting from 72-115 seconds each. PVMBG subsequently reported observing 44 explosions with black ash plumes rising 100-600 m above the summit, and incandescence at night on 21 September. Ash emissions continued on 22 September at 1.5 km altitude, with a secondary explosion rising to 2.4 km altitude drifting W. The plume height was based on and infrared temperature measurement of 12 degrees C. Later in the day, an additional plume was observed in satellite imagery at 3.7 km altitude drifting N. PVMBG reported observations of 56 explosions, with 200-300 m high (above the summit) black ash plumes and incandescence at night on 22 September. Observations from nearby Rakata Island on 22 September indicated that tephra from incandescent explosions of the previous night mostly fell on the flanks, but some reached the sea. A lava flow on the SSE flank had also reached the ocean (figure 52).

Figure 52. Activity at Krakatau during 22-23 September 2018 included substantial Strombolian explosions, a dark ash plume, lava flows, and large volcanic bombs traveling nearly to the ocean. Photo courtesy of Malmo Travel.

By 23 September 2018, a single plume was observed at 2.1 km altitude drifting WNW. A glow at the summit was visible in the webcam that day, and a hotspot was seen in satellite imagery the next day as observations of an ash plume drifting W at 2.1 km continued. A significant SO2 plume was captured in satellite data on 23 September (figure 53).

Figure 53. A significant SO2 plume dispersed NW of Krakatau (lower right corner) on 23 September 2018 after a surge in activity was observed the previous two days. Courtesy of NASA Goddard Space Flight Center.

On 24 September, PVMBG reported black ash plumes rising 1,000 m above the summit, incandescence at the summit, and lava flowing 300 m down the S flank observed in the webcam during the night. An ash plume was observed by the Darwin VAAC drifting WSW and then W on 25-26 September at 2.1 km altitude, lowering slightly to 1.8 km the following day, and to 1.2 km on 28 September. Continuous ash emissions were observed through 29 September. A new emission was reported on 30 September drifting SW at 1.8 km altitude. Ash emissions were observed daily by the Darwin VAAC from the 1st to at least 5 October at 2.1 km altitude drifting W. A large hotspot near the summit was noted on 3 October. The thermal activity at Anak Krakatau from late June into early October 2018, as recorded in infrared satellite data by the MIROVA project, confirmed the visual observations of increased activity that included Strombolian explosions, lava flows, ash plumes, and incandescent ejecta witnessed by ground observers during the period (figure 54).

Figure 54. The MIROVA project graph of thermal activity at Krakatau from 12 February through early October 2018 showed the increasing thermal signature that appeared in late June at the onset of renewed explosive activity, the first since February 2017. Courtesy of MIROVA.

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 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 volcanoes, and left only a remnant of Rakata volcano. 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN); Øystein Lund Andersen? (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Reuters Latam (Twitter: @ReutersLatam, URL: http://www.reuters.com/); James Reynolds, Earth Uncut TV (Twitter: @EarthUncutTV, URL: https://www.earthuncut.tv/, Video: https://www.youtube.com/watch?v=UD3SLWtuPZs); Thibaud Plaquet (Instagram: tibomvm, URL: https://www.instagram.com/tibomvm/); Malmo Travel (Instagram: malmo.travel, URL: https://www.instagram.com/malmo.travel/).

Historical observations of eruptive activity on ice-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. With the advent of satellite observation technology, indications of more frequent eruptive activity have become apparent. The last confirmed eruption evidenced by MODVOLC thermal alerts was during August-October 2015 (BGVN 41:02). Limited thermal anomaly data and satellite imagery since then have indicated intermittent activity through September 2018. Information for this report comes from MODVOLC and MIROVA thermal anomaly data and Sentinel-2, Landsat, and NASA Terra satellite imagery.

Evidence for thermal activity at Mount Michael tapered off in MIROVA data from October 2015 through January 2016. MODVOLC thermal alerts reappeared on 28 September 2016 and recurred intermittently through 6 January 2017. Low-level MIROVA thermal signals appeared in June and September-November 2017. During January-September 2018, evidence for some type of thermal or eruptive activity was recorded from either MODVOLC, MIROVA, or satellite imagery each month except for May and June.

Although MODVOLC thermal alerts at Mount Michael ended on 8 October 2015, the MIROVA radiative power data showed intermittent pulses of decreasing energy into early January 2016 (figure 10, BGVN 41:02). At a high-latitude, frequently cloud-covered site such as Saunders Island, this could be indicative of continued eruptive activity. A white plume in low resolution NASA's Terra satellite data was spotted drifting away from Saunders in April 2016, but no thermal activity was reported. The only high-confidence data available from April 2016 through May 2017 is from the MODVOLC thermal alert system, which recorded two thermal alerts on 28 September 2016, one the next day, one on 30 October, and eight alerts on four days in November. Activity continued into January 2017 with one alert on 17 December 2016, and six alerts on 2 and 6 January 2017 (figure 11).

Figure 11. Seventeen MODVOLC thermal alerts between 28 September 2016 and 6 January 2017 were the best evidence available for eruptive activity on Saunders Island from April 2016 through May 2017. Courtesy of MODVOLC.

A low-level log radiative power MIROVA signal appeared in early June 2017; two more signals appeared in September 2017, one in early October and one in late November (figure 12). Additional signals plotted as more than 5 km from the source may or may not reflect activity from the volcano. Steam plumes were visible in NASA Terra satellite images drifting away from the island in August, October, and December 2017, but no thermal signatures were captured.

Figure 12. The MIROVA log radiative power graph for Mount Michael on Saunders Island from 25 May-30 December 2017 showed intermittent heat sources that indicated possible eruptive activity each month except July and December. Location uncertainty makes the distinction between greater and less than 5 km summit distance unclear.

More sources of evidence for activity became available in 2018 with the addition of the Sentinel-2 satellite data during the months of February-April and September. Multiple thermal signals appeared from MIROVA in January 2018 (figure 13), and the first Sentinel-2 satellite image showed a distinct hotspot at the summit on 10 February (figure 14).

Figure 13. MIROVA thermal data for January-September 2018 indicated intermittent thermal anomaly signals in January, March, April, and July-September (top). A Sentinel-2 image with a hotspot was captured on 23 September, the same day as the MIROVA thermal signal (bottom). Courtesy of MIROVA.

Figure 14. A Sentinel-2 image of Saunders Island on 10 February 2018 revealed a distinct hotspot and small steam plume rising from the summit crater of Mount Michael. Sentinel-2 image with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

A MODVOLC thermal alert appeared on 26 March 2019 followed by a significant hotspot signal in Sentinel-2 imagery on 29 March (figure 15). The hotspot was still present along with a substantial steam plume on 3 April 2018. Sentinel-2 imagery on 11 April revealed a large steam plume and cloud cover, but no hotspot.

Figure 15. Hotspots in Sentinel-2 imagery on 29 March and 3 April 2018 indicated eruptive activity at Mount Michael on Saunders Island. Sentinel-2 image with Atmospheric Penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

MIROVA thermal signals appeared in mid-July and mid-August 2018 (figure 13) but little satellite imagery was available to confirm any thermal activity. The next clear signal of eruptive activity was evident in a Sentinel-2 image as a hotspot at the summit on 23 September. A small MIROVA signal was recorded the same day (figure 13, bottom). A few days later, on 28 September 2018, a Landsat 8 image showed a clear streak of dark-gray ash trending NW from the summit of Mount Michael (figure 16).

Figure 16. Satellite imagery confirmed eruptive activity at Mount Michael on Saunders Island in late September 2018. Top: a hotpot in a Sentinel-2 image on 23 September coincided with a MIROVA thermal signal (see figure 13); Bottom: A Landsat 8 image on 28 September has a distinct dark gray streak trending NW from the summit indicating a fresh ash deposit. The lighter gray area SW of the summit is likely a shadow. Sentinel-2 image with Atmospheric Penetration view, (bands 12, 11, and 8A), Landsat 8 image with pansharpened image processing, both courtesy of Sentinel Hub Playground.

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). Vapor emission is frequently reported from the summit crater. Recent AVHRR and MODIS satellite imagery has revealed evidence for lava lake activity in the summit crater.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground, (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

The most recent eruptive period at Karymsky, on the Kamchatka Peninsula of Russia, began on 28 April 2018, with thermal anomalies, gas-and-steam emissions, and ash plumes observed through July 2018. The current report discusses activity through September 2018 (table 11). This report was compiled using information from the Kamchatka Volcanic Eruptions Response Team (KVERT).

KVERT reported ongoing thermal anomalies and intermittent ash plumes over Karymsky during August and September 2018 (table 11). Ash plumes drifted 50 km SE on 7 August, and 40 km S on 25 August. Stronger activity during 10-11 September consisted of continuous dense ash emissions along with explosions that sent plumes 5-6 km high which drifted 860 km NE. Incandescence photographed the next night was attributed to fumarolic activity (figure 41). Ash plumes were identified drifting 365 km E on 22-23 September. The last thermal anomaly was identified in satellite images on 28 September, and an ash plume was last visible on 30 September.

Table 11. Ash plumes and thermal anomalies at Karymsky, 1 August-30 September 2018. Clouds often obscured the volcano. Data compiled from KVERT reports.

Figure 41. Incandescence, attributed to fumarolic activity, was visible above the crater of Karymsky on 12 September 2018. Photo by D. Melnikov; courtesy of Institute of Volcanology and Seismology (IVS FEB RAS, KVERT).

Figure 42. Sentinel-2 satellite imagery of Karymsky on 30 September 2018 showing a diffuse plume and thermal anomaly in the crater. Top: Natural color view (bands 4, 3, 2). Bottom: Short-wave Infrared view (bands 12, 8A, 4). Courtesy of Sentinel Hub Playground.

Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were last observed on 31 July 2018. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected one hotspot in early August (moderate power), and two hotspots in late September (low power).

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://hotspot.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Nishinoshima is an active volcano in the Ogasawara Arc, about 1,000 km S of Tokyo, Japan. After 40 years of dormancy, activity increased in November 2013 and has since formed an island. The eruption has continued with subaerial activity that largely consists of lava flows and small gas-and-ash plumes. This report covers November 2017 through July 2018, and summarizes activity noted in reports issued by the Japan Meteorological Agency, and images and footage taken by the Japan Coast Guard (JCG).

No eruptive activity at Nishinoshima had been noted since mid-August 2017, when lava was last entering the ocean. Activity recommenced on 12 July and a 200-m-long lava flow was confirmed on 13 July. The lava flow was accompanied by explosive activity that ejected blocks and bombs out to 500 m from the vent, plumes and water discoloration (figures 60, 61, and 62). An aerial survey by the JCG on 30 July showed that activity had ceased and the lava flow had reached 700 m in length, terminating 100 m from the ocean.

Figure 60. Aerial photo of Nishinoshima taken on 18 July 2018. The photo shows the active lava flow emanating from the vent along with a gas plume, and water discoloration. A closer view of the lava flow is given in figure 61. The island is approximately 1.9 x 1.9 km in size. Courtesy of the Japan Coast Guard.

Figure 61. A view of the active Nishinoshima vent and 200-m-long lava flow on 13 July 2018. The vent is also producing a dilute ash plume from the eastern side of the cone. Courtesy of the Japan Coast Guard.

Figure 62. Screenshot from a thermal infrared video of the active Nishinoshima vent taken on 13 July 2018. The video shows explosions ejecting incandescent material onto the flanks of the cone and the active lava flow. Courtesy of the Japan Coast Guard.

After the July activity, Nishinoshima again entered a phase of quiescence with activity limited to fumaroles around the vent. Himawari-8 satellite observations noted no increased thermal output following the July 2018 eruption. Thermal anomalies detected by satellite-based MODIS instruments were identified by the MODVOLC system from during 12-21 July 2018.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Another eruption that began offshore in 2013 completely covered the previous exposed surface and enlarged the island again. Water discoloration has been observed on several occasions since. The island is the summit of a massive submarine volcano that has prominent satellitic peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the sea surface 9 km SSE.

Information Contacts: Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www.kaiho.mlit.go.jp/info/kouhou/h29/index.html, http://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo18-e1.htm); Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/).

Mayon is a frequently active volcano in the Philippines that produces ash plumes, lava flows, pyroclastic flows, and lahars. In early 2018, eruptive activity included lava fountaining that reached 700 m above the summit, and lava flows that traveled down the flanks and collapsed to produce pyroclastic flows (figure 39). Lava fountaining and lava flows decreased then ceased towards late March. Lava effusion was last detected on 18 March 2018, and the last pyroclastic flow for this eruptive episode occurred on 27 March 2018 (see BVGN 43:04). The hazard status for was lowered from alert level 4 to 3 (on a scale of 0 to 5) on 6 March 2018 due to decreased seismicity and degassing; the level was lowered again to 2 on 29 March. This report summarizes the activity during April through September 2018 and is based on daily bulletins issued by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and satellite data.

Figure 39. Sentinel-2 thermal satellite images showing the lava flow activity at Mayon during January through March 2018. Three lava flow lobes flowed down the Mi-isi, Bonga-Buyuan, and Basud channels, and are shown in bright orange/red in these images. These are false color images created using bands 12, 11, 4, courtesy of Sentinel Hub Playground.

The hazard status remained on Alert level 2 (increasing unrest) throughout the reporting period. Activity was minimal with low seismicity (zero to four per day) and a total of 19 rockfall events throughout the entire period. White to light-brown plumes that reached a maximum of 1 km above the crater were observed almost every day from April through September (figure 40). Two short-lived light brown plumes were noted on 27 and 28 August and both reached 200 m above the crater.

Figure 40. An emission of white steam-and-gas at Mayon and a dilute brown plume that reached 200 m above the crater was seen on 24 May 2018. Courtesy of PHIVOLCS.

On the days that sulfur dioxide was measured, the amount ranged from 436 to 2,800 tons per day (figure 41). Mayon remains inflated relative to 2010 baselines but the edifice has experienced deflation since 20 February, a period of inflation from 2-14 April, and slight inflation of the mid-slopes beginning 5 May, which then became more pronounced beginning 25 June. No other notable inflation or deflation was described throughout the reporting period.

Figure 41. Measurements of sulfur dioxide output at Mayon during 1 April-30 September 2018. Data courtesy of PHIVOLCS.

Incandescence at the summit was observed almost every night (when weather permitted) from April through to the end of September 2018, and this elevated crater temperature is also seen in satellite thermal imagery (figure 42). Thermal satellite data showed a slight increase in output during April through to June, although not as high as the earlier 2018 activity, with a decline in thermal output starting in July (figure 43).

Figure 42. Sentinel-2 thermal satellite image showing an elevated thermal signature in the crater of Mayon and a steam-and-gas plume on 15 May 2018. Similar indications of activity in the crater were frequently imaged on cloud-free days from April through September. This is a false color image created using bands 12, 11, 4, courtesy of Sentinel Hub Playground.

Figure 43. Log radiative power MIROVA plot of MODIS thermal data for the year ending 11 October 2018 at Mayon. An elevated period of activity reflecting the lava flows in January through March is notable, followed by a second period of lower intensity activity during May into June, then a prolonged period of reduced activity through to the end of the reporting period; the August anomaly was not at the volcano. Courtesy of MIROVA.

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

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The first confirmed eruption of Kadovar began on 5 January 2018 with dense ash plumes and steam and a lava flow. The eruption continued through February and then slowed during March (BGVN 43:04). This report describes notices of ash plumes from the Darwin Volcanic Ash Advisory Centre (VAAC) and satellite images during April through 1 October 2018.

According to the Darwin VAAC a pilot observed an ash plume rising to an altitude of 1.2 km on 10 June. The ash plume was not identified in satellite data. Another ash plume identified by a pilot and in satellite images rose to an altitude of 1.8 km on 20 June and drifted W. An ash plume was visible in satellite images on 28 September drifting SE at an altitude of 2.1 km. On 1 October an ash plume rose to 2.7 km and drifted W.

Infrared satellite data from Sentinel-2 showed hot spots in the summit crater and at the Coastal Vent along the W shoreline on 10, 15, and 25 April 2018; plumes of brown discolored water were streaming from the western side of the island (figure 18). Similar activity was frequently seen during clear weather in the following months. A steam plume was also often rising from the crater. The Coastal Vent cone was still hot on 8 August, but no infrared anomaly was seen in imagery from 28 August through September.

Figure 18. Sentinel-2 natural color satellite image of Kadovar on 10 April 2018. The island is about 1.5 km in diameter. Steam can be seen rising from the summit and the Coastal Vent just off the western shore; both locations show thermal anomalies in infrared imagery. Discolored water plumes extend NE from the island. Courtesy of Sentinel Hub Playground.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. Kadovar is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. The village of Gewai is perched on the crater rim. A 365-m-high lava dome forming the high point of the andesitic volcano fills an arcuate landslide scarp that is open to the south, and submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. No certain historical eruptions are known; the latest activity was a period of heightened thermal phenomena in 1976.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Gas-and-steam emissions were previously reported at Ketoi (figure 1) in January, July, and August 2013 (BGVN 40:09). Intense fumarolic activity originating from the same area, the N slope of Pallas Peak, was reported in 1981, 1987, and 1989. Based on a report from the Sakhalin Volcanic Eruption Response Team (SVERT) using Himawari-8 imagery, the Tokyo VAAC reported an ash plume on 21 September 2018 which drifted to the NE; however, evidence of the plume could not be confirmed by the VAAC from satellite imagery. The original VONA (Volcano Observatory Notice for Aviation) issued by SVERT noted a volcanic cloud without a specific mention of ash, but also remarked that thermal anomalies had been observed on 17 and 20 September.

Figure 1. Natural color Sentinel-2 satellite image of Ketoi on 18 September 2018. A large freshwater lake can be seen SW of the Pallas Peak andesitic cone, which also hosts a crater lake. Lava flows originating from the younger cone extend primarily N to SW, and a white fumarolic area is immediately NE of the crater. The island is approximately 10 km in diameter. Courtesy of Sentinel Hub Playground.

Geologic Background. The circular, 10-km-wide Ketoi island, which rises across the 19-km-wide Diana Strait from Simushir Island, hosts of one of the most complex volcanic structures of the Kuril Islands. The rim of a 5-km-wide Pleistocene caldera is exposed only on the NE side. A younger 1172-m-high stratovolcano forming the NW part of the island is cut by a horst-and-graben structure containing two solfatara fields. A 1.5-km-wide freshwater lake fills an explosion crater in the center of the island. Pallas Peak, a large andesitic cone in the NE part of the caldera, is truncated by a 550-m-wide crater containing a brilliantly colored turquoise crater lake. Lava flows from Pallas Peak overtop the caldera rim and descend nearly 5 km to the SE coast. The first historical eruption of Pallas Peak, during 1843-46, was its largest.

Information Contacts: Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Semeru volcano is the tallest volcano in Java (figure 33) and one of the most active in Indonesia. The Mahameru summit area contains the active Jonggring-Seloko vent where activity consists of dome growth and regular ash plumes, along with pyroclastic flows, avalanches, and lava flows that travel down the SE-flank ravine. The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) Volcano Alert level for Semeru throughout the report period is II (on a scale of I-IV). The last Volcano Observatory Notice for Aviation (VONA) was issued on 9 January 2017, and the status has not changed during the reporting period. This report summarizes the activity from January to 24 August 2018 and is based on Volcano Ash Advisory Center (VAAC) ash advisories and satellite data.

Figure 32. View looking NW at the quiet Mahameru summit area of Semeru on 24 August 2018 taken by a webcam courtesy of MAGMA Indonesia via Ø.L. Andersen's Twitter feed.

While there were no observatory activity reports issued, the Darwin VAAC issued reports for five events that produced ash plumes to altitudes ranging 3.4 to 4.9 km (table 22). MIROVA (Middle InfraRed Observation of Volcanic Activity) thermal data indicate near-consistent low-level thermal activity at Semeru after a period of no detected thermal anomalies in late January through early February. This supports the elevated thermal energy detected by Sentinel-2 satellite data at the Jonggring-Seloko vent and along the SE-flank ravine (figure 34). The MODVOLC algorithm detected 16 high-temperature hotspots through the reporting period, six in January, two in March, three in April, one in July, and two in August through to the 24th.

Table 22. Summary of ash plumes (altitude and drift direction) and pyroclastic flows at Semeru, January to 24 August 2018. The summit is at 3,657 m elevation. Data courtesy of Darwin VAAC report.

Figure 33. MIROVA plot of Log Radiative Power showing the relative thermal energy at Semeru ending September 2018. The detected thermal activity is more intense before mid-January 2018 when there was a gap in detected data before regular low-level activity resumed. Courtesy of MIROVA.

Figure 34. Sentinel-2 false color thermal satellite images showing the persistent elevated thermal anomaly in the Jonggring-Seloko crater of Semeru from January through to 24 August 2018. Hot material can sometimes be identified in the SE-flank ravine. The larger central image is annotated with the major morphological features. False color (urban) images (bands 12, 11, 4) 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/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Øystein Lund Andersen? (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com).

Sinabung volcano is located in the Karo regency of North Sumatra, Indonesia. The current eruptive episode commenced in late 2013, after phreatic activity in 2010, producing ash plumes, lava domes and flows, and pyroclastic flows that caused evacuation and relocation of nearby communities. This report covers activity from April through early July, and is based on information provided by MAGMA Indonesia, the Darwin Volcanic Ash Advisory Center (VAAC), the Center for Volcanology and Geological Hazard Mitigation (CVGHM, also known as PVMBG), satellite data, and field photographs. Sinabung has been on Alert Level IV, the highest hazard status, since 2 June 2015.

The eruption has built a pyroclastic flow and lava fan to the SE (figure 60). This activity continued into 2018, with the last significant ash plume reported on 22 June (table 8). However, minor ash emissions continued at least through 5 July 2018.

Figure 60. Satellite images showing Sinabung before and after the eruption with the newly-developed fan of pyroclastic flow, volcanic ash, and lava flow deposits. Top: Landsat-8 true color satellite image (pan-sharpened) acquired on 7 June 2013 before the eruption began. Bottom: Sentinel-2 natural color satellite image acquired on 16 July 2018, after the eruption ended. Courtesy of Sentinel Hub Playground.

Table 8. Summary of ash plumes (altitude and drift direction) and pyroclastic flows at Sinabung, April-June 2018. The summit is at 2,460 m elevation. Data courtesy of Darwin VAAC reports, MAGMA Indonesia VAAC reports, and CVGHM volcanic activity reports.

An eruption on 6 April 2018 at 1607 local time produced an ash plume that reached about 7.5 km above the summit. The eruption also produced pyroclastic flows that traveled about 3.5 km from the summit down the SE slope (figure 61). The eruption resulted in the closure of a nearby airport and ashfall affected hundreds of hectares of agricultural land. Two more notable ash plumes were reported on 12 and 19 April, to altitudes of about 2.7 and 5.5 km, respectively. A pyroclastic flow was reported during the 12 April eruption. Smaller ash and gas emissions occurred through the month.

Figure 61. Eruption of Sinabung on 6 April 2018 at 1600 local time that produced an ash plume that reached over 5 km above the summit, and pyroclastic flows that reached about 3.5 km down the SE flank. Courtesy of Agence France-Presse via Straits Times.

Two ash plumes were recorded on 19 and 20 May, rising to about 3.2 and 5 km altitude, respectively. Throughout June small diffuse gas-and-ash plumes continued (figures 62 and 63). The last activity reported by the agencies was on the 15 and 22 June, when ash plumes reached 3 and 3.5 km altitude (figure 64). Activity after 22 June was limited to seismicity and ash, gas, and steam plumes to several hundred meters above the summit (figure 65). Although an elevated thermal signature was detected in Sentinel-2 satellite data on 30 August 2018, there were no reports of renewed activity.

Figure 62. View of the Sinabung summit vent area during ash venting on 20 June 2018. This view from the SW shows the perched remains of the lava dome and collapse scar. Photo courtesy of Brett Carr, Lamont-Doherty Earth Observatory.

Figure 63. Relatively consistent ash venting at Sinabung on 20 June 2018. This view shows the pyroclastic flow fan and the 2014 lava flow in the lower center of the photo. Drone photo courtesy of Brett Carr, Lamont-Doherty Earth Observatory.

Figure 64. Small ash plume rising from Sinabung at 2106 on 22 June 2018. The ash plume reached about 1 km above the crater. Courtesy of BNPB (color adjusted).

Figure 65. Minor ash venting at Sinabung on 5 July 2018. Photo courtesy of Brett Carr, Lamont-Doherty Earth Observatory.

Geologic Background. Gunung Sinabung is a Pleistocene-to-Holocene stratovolcano with many lava flows on its flanks. The migration of summit vents along a N-S line gives the summit crater complex an elongated form. The youngest crater of this conical andesitic-to-dacitic edifice is at the southern end of the four overlapping summit craters. The youngest deposit is a SE-flank pyroclastic flow 14C dated by Hendrasto et al. (2012) at 740-880 CE. An unconfirmed eruption was noted in 1881, and solfataric activity was seen at the summit and upper flanks in 1912. No confirmed historical eruptions were recorded prior to explosive eruptions during August-September 2010 that produced ash plumes to 5 km above the summit.

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/); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/, Twitter: https://twitter.com/BNPB_Indonesia ); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/, Twitter: https://twitter.com/id_magma); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Brett Carr, Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY (URL: https://www.ldeo.columbia.edu/user/bcarr); Agence France-Presse (URL: http://www.afp.com/); Straits Times (URL: https://www.straitstimes.com).

The Telica volcano complex, which consists of several cones and craters, has had intermittent eruptions since the Spanish conquest, with emissions of gas and ash. According to The Instituto Nicaragüense de Estudios Territoriales (INETER), the volcano is monitored in real time by a permanent seismic station near the crater. It is also visited several times per year for visual observations, to measure sulfur dioxide emissions, and measure temperatures in the crater and fumaroles near the seismic station. A gas-and-ash explosion occurred in early May 2016 (BGVN 42:01). This report covers activity from September 2016 through June 2018.

INETER reported that local residents heard a small gas explosion on 10 September 2017, and warned the public to stay at least 2 km away from the crater. No ash emissions were reported related to this event.

According to INETER and the Sistema Nacional para la Prevención, Mitigación y Atención de Desastres (SINAPRED), an eruption began at 0708 on 21 June 2018. Explosions produced an ash plume that rose 500 m above the crater and drifted E, S, and SW. Ejected tephra was deposited within a 1-km-radius of the volcano, and ashfall was reported in nearby areas, including La Joya, Las Marías (7 km NNW), Pozo Viejo (10 km NNW), Ojo de Agua, San Lucas (11 km NNW), Las Higueras, Las Grietas (12 km NNW), and Posoltega (16 km WSW).

Another explosion on 15 August 2018 was reported by SINAPRED that generated an ash plume to 200 m above the crater rim.

Seismicity. INETER monthly reports indicated that during September through December 2016, between 3,500 and 3,900 monthly seismic events took place, with the majority having hybrid signatures. During 2017, the monthly number of seismic events ranged from 40,584 (September) to 105,555 (November), of which 50-90% were hybrid events, 9-10% long-period events (but 23 percent in January), and 0-35% multiple events. A few scattered volcanic-tectonic events occurred, and tremor was usually low. Seismic data for January and March consisted of percentages of different earthquake types similar to those during 2017.

About 5% of the monthly seismic signals between April 2017 and January 2018 were doublets, or paired earthquakes with two predominant frequencies. INETER did not mention doublets in their March 2018 report, and did not include seismic data in their February or April 2018 reports.

Sulfur dioxide measurements. According to INETER, during fieldwork on 8 and 11 May 2017 the sulfur dioxide level was measured at 368 ± 194 metric tons/day. This value was lower than those in November 2015 with an average of 765 ± 94 tons/day. On 28 February and 1 March 2018, measurements using the Mobile-DOAS technique found levels greater than 426 tons/day and a minimum value of 152 tons/day, with an average of 260 tons/day, higher than the value measured in September 2017 with 183 tons/day. On 16 and 19 April 2018, the minimum and maximum values were 229 and 567 tons/day, with an average of 353 tons/day.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://webserver2.ineter.gob.ni/vol/dep-vol.html); Sistema Nacional para la Prevencion, Mitigacion y Atencion de Desastres (SINAPRED), Edificio SINAPRED, Rotonda Comandante Hugo Chávez 50 metros al Norte, frente a la Avenida Bolívar, Managua, Nicaragua (URL: http://www.sinapred.gob.ni/).

The Rincón de la Vieja volcano complex has generated intermittent phreatic explosions since 2011; during 2017, weak phreatic explosions occurred during May, June, July, September, and October (BGVN 42:08 and 43:03). This activity continued through August 2018. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

According to OVSICORI-UNA, at 1758 on 9 January 2018, an explosion produced a plume that rose 1 km above the crater rim. On 12 January, OVSICORI-UNA reported some small phreatic explosions. The webcam detected weak explosions again in mid-February. Another weak explosion on 22 February confirmed the presence of a crater lake.

During the first week of March OVSICORI-UNA reported weak phreatic explosions of low amplitude that were only be detected by the webcam (figure 28), and not by seismic instruments. During the week of 5-11 March there were 2-4 weak phreatic explosions occurred per day, along with strong tremor on the 10th. Small eruptions were seen on unspecified days the week of 12-18 March.

Figure 28. Webcam image of a phreatic explosion at Rincon de la Vieja on 3 March 2018. Courtesy of OVSICORI-UNA.

No phreatic activity was reported during the second half of March through June, though on 20 May a seismic swarm of about 30 earthquakes was recorded. After a tremor on 3 July, a possible weak phreatic explosion occurred on 4 July at 0044, followed by a pulse of tremor. On 28 July, at 1828, a small explosion followed by tremor was recorded.

On 3 August OVSICORI-UNA reported that two weak explosions occurred at dawn. On 14 August, another weak explosion began at 1828 and lasted three minutes. Foggy conditions prevented webcam views and an estimate of a plume height. Other weak explosions were recorded on 17 August at 1407 and 2015.

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/).

Eighteen explosions occurred . . . in July . . ., bringing the yearly total to 171. Ejecta from an explosion at 1057 on 5 August struck the windshield of a Boeing 737 airliner 13 minutes later as it flew at an altitude of 1.2 km, 10 km N of the volcano. A crack 50 cm long formed in the outer surface of the windshield, but the plane (domestic flight ANK 793) landed its 122 passengers and five crew safely. Dense weather clouds had prevented the pilot from seeing the eruption plume. This was the first incident of in-flight damage since 24 June 1986, and the 12th near the volcano since 1975. A car windshield a few kilometers from the crater was cracked by ejecta from another explosion (at 1249) the same day. These were the third and fourth cases of explosion-related damage in 1991.

On 23 July, the month's highest ash cloud rose 2,500 m. Prevailing wind directions prevented ash from being deposited at [KLMO]. Earthquake swarms, not unusual for Sakura-jima, were recorded on 1, 2, 9, 15, 18, 21, and 22 July.

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

Information Contacts: JMA.

"Three anomalous 'boiling' areas with large bubbles and burned vegetation were observed at Lake Vui on 13 July, by P. Fogarty (Chief Pilot of VANAIR). This was the first time he had observed such a phenomenon, and he noted that the vegetation had still been green in May. An aerial survey of the two summit calderas was carried out (during a VANAIR flight) on 24 July. At that time, no strong degassing was visible, but 3 areas of discolored water (each several tens of meters in diameter) were noticeable in the crater lake. Burned vegetation was observed up to the crater rim, 120 m above the water. On 26 July, microseismicity in the caldera was very weak and without any volcanic characteristics.

"Although continuous weak solfataric activity occurs beneath Lake Vui (Warden, 1970), an anomalously strong SO₂ degassing is believed to have occurred between May and July. This event was unnoticed by island residents, but since Aoba has been quiet for 300 years, vigilance for this kind of phenomenon must be improved. The existence of a summit caldera lake, numerous lahar deposits, and thick layers of ash (vesiculated and accretionary lapilli) demonstrate the hazards that would accompany renewed activity. Thus, as a precaution, a seismological station was installed in July on the SW flank of the volcano.

Reference. Warden, A.J., 1970, Evolution of Aoba caldera volcano, New Hebrides: BV, v. 34, p. 107-140.

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: C. Robin and M. Monzier, ORSTOM, Nouméa, New Caledonia; M. Lardy and C. Douglas, ORSTOM, Vanuatu; C. Mortimer, Dept. of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.

"Aerial surveys on 13 and 24 July (VANAIR flights) showed puffs of gas and ash rising several hundred meters above Mbuelesu crater, and weak degassing from Benbow crater. Mbuelesu's lava lake, ~100 m in diameter and very deep in the crater, was still present. Activity has remained more or less constant since 1990, and no new lava flows have been observed since 1989."

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

Information Contacts: C. Robin and M. Monzier, ORSTOM, New Caledonia; M. Lardy and C. Douglas, ORSTOM, Vanuatu; C. Mortimer, Dept of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.

Strombolian activity, lava effusion, and seismicity all increased in July . . . . The number of volcanic earthquakes rose to a maximum of 59 recorded events/day on 11 July (figure 39). Seismometers recorded intermittent, vigorous tremor episodes, several hours long (6-hour average duration), especially at the beginning of the month.

Figure 39. Daily number of earthquakes at Arenal, July 1991. Courtesy of the Instituto Costarricense de Electricidad.

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: R. Barquero and Guillermo Alvarado, ICE.

Block lava continued to advance down the main cone's SW flank, generating small avalanches from the flow front and levees. Avalanches have also occurred from the summit area, similar to those that preceded the partial collapse of the newly extruded dome on 16 April. A new lobe was observed in the W part of the summit area on 28 July. Poor weather has severely limited observations of the summit, so the date of the new lobe's extrusion is not known.

On 3 August at about 0600, a NW-flank seismic station (EZV4) recorded the beginning of signals that formed a distinctive wave package with a periodicity of about 15-20 seconds. By 5 August at 1200, the amplitude of these signals had nearly doubled and the periodicity had dropped to 10 seconds. The next day at about 0100, seismicity decreased to nearly background levels, but at 0900 sustained harmonic tremor was registered by EZV4 and other nearby stations (EZV3, 5, and 6); heavy rain during the second week in July had damaged the seismic station about 1 km NE of the summit (EZV7, at Volcancito), and poor weather has prevented it from being re-established. Harmonic tremor continued until 8 August at about 0600. During the increased seismicity, the plume was vigorous and a dense white color.

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: Francisco Núñez-Cornú, Julián Flores, F. Alejandro Nava, R. Saucedo, G.A. Reyes-Dávila, Ariel Ramírez-Vázquez, J. Hernández, A. Cortés, and Hector Tamez, CICT, Universidad de Colima; Z. Jiménez and S. de la Cruz-Reyna, UNAM.

Strong degassing continued .. during fieldwork in June and July. Strombolian activity was reported at a vent in the NE part of Southeast Crater. Small explosions occurred almost continuously, with more powerful blasts ejecting material to the level of the crater rim occurring every 10-15 minutes (in July). Meanwhile, a vent in the center of the crater gently degassed. In June, occasional emissions of small (<20 cm) sublimate-covered lithic blocks and scoria occurred from a 20 x 10 m pit in Northeast Crater. Lava was visible within the vent, which continued to glow through July. The vent widened internally, giving the appearance of a large chamber inclined in the direction of La Voragine. The elliptical vent at La Voragine crater (reopened prior to a 24 May visit; 16:05) showed incandescence in July, but not in June. Degassing continued from numerous fumaroles within the crater. The floor of Bocca Nuova crater was hidden by large quantities of gas in June, but in July two scoria cones were seen gently emitting vapor. At night, a strongly degassing vent on the SE side of the crater emitted tongues of incandescent gas at 15-minute intervals. A fumarole (56°C) was observed on the October 1989 fracture where it crossed the Canalone Della Montagnola at an altitude of ~ 2,200 m.

The following is from Steve Saunders. "A resurvey, in July, of an EDM network (67 lines) on the upper S flank showed a shortening of the majority of the lines (56), suggesting that minor deflation had occurred since the previous survey in July 1990. At that time, length increases along most lines were interpreted as resulting from minor inflation of the upper flanks since November 1989."

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: H. Gaudru, EVS, Switzerland; T. De St. Cyr, Fontaines St. Martin, France; S. Saunders, West London Institute of Higher Education; W. McGuire, Cheltenham and Glouster College of Higher Education.

A short eruption occurred on 19-20 July, following a slight increase in seismicity that began 24 June (figure 28), and immediately preceded by a shallow microearthquake swarm. Almost 80 earthquakes (M <1.5), located beneath the S flank of the summit cone at depths of <1 km, were recorded from 0256 to 0350 on 19 June. At 0350, the appearance of tremor signaled the start of lava outflow.

Figure 28. Daily number of earthquakes (top), measured tilt at Dolomieu station 100 m S of the crater (middle), and difference of magnetic field from the reference station 3.5 km W of the fissure (bottom) at Piton de la Fournaise, 30 May-19 July 1991. Courtesy of J. Toutain.

EDM (sampled every 5 minutes) and radial tilt measurements (every minute) at a station (DOLO) ~200 m from the eruptive fissure (figure 29) showed relatively slow inflation beginning at 0310 (figure 30), believed associated with the beginning of intrusion from the magma reservoir. At 0340, radial tilt began to increase rapidly (up to 54 µrad/min), while EDM indicated a rapid decrease in the distance between the rims of the two summit craters. Inflation led to southward tilting (mean azimuth, 175°) of the DOLO station area. Rapid deflation began at 0350, corresponding with the start of tremor, and lasted until 0434. Deflation occurred at maximum rates of 48 µrad/min, causing DOLO to tilt roughly N (azimuth ~10°).

Figure 29. Sketch map showing the summit area of Piton de la Fournaise and the 19 July 1991 lava flows. Courtesy of J.P. Toutain.

Figure 30. Deformation at Piton de la Fournaise, 0140-0500 on 19 July 1991. Top: EDM, sampled every 5 minutes at Dolomieu. Middle: tilt measurements, sampled every minute at Dolomieu and Soufriere; bold lines=radial component, normal lines=tangential component. Bottom: measured strain, sampled every minute at Dolomieu; Z=vertical, X and Y= horizontal components. Arrow indicates start of eruption. Stations are shown in Figure 33. Courtesy of J. Toutain.

The magnetic field near the eruptive vents (station 6) showed a clear decreasing trend beginning on 16 June (figure 28). On 19 July, a rapid magnetic field increase was measured, corresponding with the onset of the eruption.

Lava was emitted from two vents along an eruptive fissure, one inside and one outside of the summit (Dolomieu) crater (figure 29). Lava fountains, 30 m high, were observed during the morning of the 19th and flow velocity was estimated at 3-4 m/sec that afternoon. Lava flowed E through the Grandes Pentes area, covering ~ 1 x 10⁶ m², with a total volume estimated at 5 x 10⁶ m³. The eruption lasted until about 2000 on 20 July.

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: J. Toutain and P. Taochy, OVPDLF; P. Bachelery, Univ de la Réunion; J-L. Cheminée, P. Blum, A. Hirn, J. LePine, and J. Zlotnicki, IPGP; F. Garner and I. Appora, Univ Paris VI.

Seismicity and emissions began to increase at the end of July, leading to the evacuation of 11 people working on the summit . . . in early August. Released seismic energy (see figure 52) and reduced displacement (figure 42) of long-period earthquakes reached the highest values since the start of monitoring in February 1989. Amplitudes and durations for long-period events showed slow increases, as well. Tremor was recorded in low-frequency bands and modulated packs, with small variations in amplitude and period.

Figure 42. Daily reduced displacement of long-period earthquakes at Galeras, July-August 1991. Courtesy of INGEOMINAS.

Long-period events, shallow in origin and often associated with gas-and-ash emissions, increased to >100/day by mid-August. The number of gas-and-ash emissions increased correspondingly. Plume heights reached 2 km and ash was deposited to 8 km N and NW. Head-sized blocks, hot to the touch, were periodically ejected onto the crater rim.

Inflation, continuing since September 1990, increased dramatically during the first half of August, when 265.8 µrad tangential and -180.6 µrad radial deformation were measured (figure 43) 0.9 km E of the crater ("Crater" electronic tiltmeter). The resultant inflation vector was 321.35 µrad with an azimuth of 115.81°.

Figure 43. Tangential (top curve) and radial (bottom curve) deformation at the Crater electronic tiltmeter at Galeras, January-August 1991. Courtesy of INGEOMINAS.

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

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

"An increase in fumarolic activity was noted by VANAIR pilots since April. On 13 July, a detailed aerial survey was conducted over the island during a VANAIR flight. Strong continuous degassing was observed, forming a dense white plume from the SE crater of Mt. Gharat cone. The NW slopes of the cone were largely denuded of vegetation, and the area of the caldera affected by the prevailing SE trade winds had burned vegetation. Due to this increasing activity, we plan to install a seismological station to monitor the volcano as soon as possible.

"Gaua is a composite volcano with a large (8 x 6 km) central caldera occupied by Lake Letas (428 m elev). Mt. Gharat (797 m elev) is an active basaltic cone located near the center of this caldera. Only solfataric activity was recorded from 1868 to 1962 (Mallick and Ash, 1975). Beginning in 1962, central crater explosions with frequent associated ash columns were reported nearly every year until 1977. Information on activity from 1977 to 1990 is scarce, but the volcano was probably quiet, with only minor steam emissions from the SE crater." [BVE reported strong gas emission in mid-1980, a black plume on 9 July 1981, and a brown plume with tephra on 18 April 1982.]

Reference. Mallick, D.I.J., and Ash, R.P., 1975, Geology of the southern Banks Islands: New Hebrides Geological Survey Regional Report, 33 p.

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

Information Contacts: C. Robin and M. Monzier, ORSTOM, Nouméa, New Caledonia; M. Lardy and C. Douglas, ORSTOM, Vanuatu; C. Mortimer, Dept. of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.

On 12 August, the volcano entered a paroxysmal phase, after four days of lesser explosive activity. Tephra was ejected to 16-18 km height, falling up to 1,000 km SE on the Falkland Islands, and with estimates of >1 km³ deposited in Argentina [but see 16:8]. Ash leacheate analyses showed unusually high levels of fluorine. The SO₂-rich plume produced by the eruption was rapidly transported around the world, returning to Chile within 7 days. Airline pilots reported sighting the plume as it passed near Melbourne, Australia (roughly 15,000 km from the volcano).

Initial strong explosive activity, 8-10 August. The following quoted material is from José A. Naranjo. "Just 20 years after the previous activity, Hudson started a new eruption on 8 August at 1820. Local inhabitants who were evacuated from the Huemules River (to the W) reported small precursory seismic activity 3-4 hours before the first explosion. The eruption started with a phreato-magmatic explosion that produced a column almost 7-10 km high. Immediately following the initial explosion, a dense, ash-laden column (light brown-greyish in color) formed, reaching ~12 km. Intense lightning discharged from the mushroom-shaped cloud. Activity steadily decreased through 11 August, when direct observation of the summit showed that the 8 August eruption vent was on the W side of the caldera (10 x 7 km; figure 1). The caldera floor was covered by glacial ice estimated to be at least 40 m thick, and having a volume of about 2.5 km³. In addition, a flank valley, extending 10 km NW from the summit to Huemules valley, is filled with a tongue of ice from the main summit glacier. This terminates at the Huemules Valley, which extends onward ~35 km W to the coast.

Figure 1. Sketch map of the summit area of Hudson, 11 August 1991. Courtesy of José Naranjo.

"Prevailing winds during clear weather carried the column NNE (figure 2) over Puerto Chacabuco (50 km away), where 5-7 mm of ash was deposited. At Puerto Aisén (~ 65 km NNE), ash accumulations reached 5 mm in 16 hours. Lava was observed beneath glacial ice near the vent, flowing down to Ventisquero ('glacial tongue') Huemules. Between 3 and 4 hours after the main explosion, a jökullhaup flowed down the Huemules valley to the coast. A 2-m-thick deposit of ash- to lapilli-sized sand and 0.2-5-m-diameter ice blocks was randomly dispersed near the delta. These ice blocks probably floated in the mudflow." The press reported that the flow increased the river width from 80 m to 170 m.

Figure 2. Map showing the location of Hudson and the direction of ash dispersal on 8-9 and 12-15 August 1991. Courtesy of José Naranjo.

Late on 9 August, a NOTAM reported the plume at 11-12 km altitude. Although the eruption remained nearly continuous, intensity declined. By 10 August, Ladeco (Chilean Airlines) pilots reported the plume at ~ 6 km altitude.

"Eleven people were evacuated from along the Huemules River on 11 August. Direct observations at 1250 showed an explosion from a new vent (Crater 2), about 2.5 km SSE of the first vent (Crater 1; figure 1). The new white-and-black explosion cloud was smaller and spread laterally, developing black, cold pyroclastic-ice flows around the vent, similar to the original. White-grey columns, reaching 3 km height, were observed up to the last direct observation at 1630 on 11 August.

Paroxysmal activity, 12-15 August. "A second, larger eruption started at about 1200 on 12 August. Bad weather prevented aerial observation, but heavy ashfall was reported at Río Murta (60 km SSE) at 1245, and 7 minutes later at Río Tranquilo, 20 km farther S. The ashfall was accompanied by intense lightning, and a sulfur odor. At 1300, ashfall was reported at Puerto Guadal (105 km S). The eruption was directly observed on a commercial flight at 1430. The dense, brown-grey cauliflower-shaped cloud, carried SE, was visible from 4 km altitude, but clearly reached >10 km, with more than a 5-km thickness. One explosion was observed rising at a rate of 1.9 km/min. Observations ended at 1440.

"Since 12 August the eruption has continued without variation, and the plume has been carried SE. On 13 August at 1415, a black ash-laden column was reported from a commercial airplane at >10 km altitude. Pumice fall was since reported beginning 14 August, and coarse lapilli up to 5 cm in diameter fell 55 km SE."

Although weather clouds obscurred the eruption plume to visible and infrared satellite images on the 12th and much of the 13th, preliminary data from the Nimbus-7 satellite (TOMS) indicated 250,000 metric tons of SO₂, within a disconnected section of the eruption cloud near the Falkland Islands at about 1100 on the 13th. Beginning at about 2000, a continuous, narrow, eruption plume was visible on AVHRR (NOAA 9 and 11) and GOES satellite images, gradually extending 1200 km SE, beyond the Falkland Islands, at ~12 km altitude. The plume became disconnected from the volcano at about 1200 on 14 August, by which time, Naranjo reported, the eruptive column reached a stable altitude of 16 km. TOMS data from 1100 on the 14th revealed a segment of SO₂-rich plume (probably the same as on the 13th) near South Georgia Island (2,600 km ESE of the volcano), and a second, smaller segment over the Falkland Islands. No other SO₂-rich plume was visible.

Intense seismic activity was felt on 14 August at 1630, 60 km SSE, where 3-cm-diameter pumice was falling. A continuous eruption began again at about 2000, when satellite images (GOES and NOAA 9 and 11) showed that the plume was carried SE at 185 km/hr (100 knots) at stratospheric altitudes of 17-18 km (figure 3). Seismicity increased, with felt earthquakes at Coyhaique (80 km NE) beginning at 2200, and a series of five large earthquakes (M>5) detected near Hudson by the WWSSN beginning at 2238 (table 1). Early on the 15th, the plume extended 1,500 km SE, past the Falkland Islands, where it divided into two components, one travelling E, the other S, both quickly becoming diffuse. At its widest point (the Falkland Islands), the plume was 370 km wide. Infrared satellite imagery showed the plume before it disconnected from the volcano at 1130. TOMS data from 1100 on the 15th (figure 4) showed the plume already disconnected from the volcano, and containing roughly twice as much SO₂ as on the 13th (missing data prevented more accurate determinations). No additional emissions have been reported as of 23 August.

Figure 3. Infrared image from the NOAA 10 polar orbiting weather satellite on 15 August 1991 at about 0800, showing the ash plume extending SE from Hudson. Temperature estimates suggest that the plume is at aboout 17-18 km altitude. Courtesy of G. Stephens.

Table 1. Earthquakes near Hudson recorded by the Worldwide Standardized Seismic Net on 14-15 August 1991. Original, very preliminary data are replaced by information from the National Earthquake Information Center's Preliminary Determination of Epicenters.

Eruption plume migration. The eruption plume of 14-15 August was rapidly carried E by the "Roaring Forties" winds as shown by TOMS data (figure 4), reaching Australia (15,000 km E) on 20 August. There the following report was compiled from airline information by Alfred Prata:

Figure 4. Preliminary data from the TOMS on the Nimbus-7 satellite showing a polar view of an eruption cloud from Hudson on 20 August 1991 at about 1100 (local time). Each dot represents SO2 values above 10 milliatmosphere-cm (100 ppm-m), within an area 50 km across. The prominent concentration of SO2 to the left represents the cloud's position 24 hours after that to the right, but both are 20 August because they straddle the International Date Line. Envelopes surrounding the cloud's position at approximately 1100 (local time) on 15, 16, and 18 August have been added to illustrate its passage around the globe. Courtesy of Scott Doiron.

"On 20 August, Australian Airlines flight FL418 (Airbus) from Melbourne to Sydney reported an encounter with a strange hazy cloud 260 km NE of Melbourne at about 0230. The haze was faint grey, much like the material often trapped under a temperature inversion, and had a brownish-orange tinge. The haze appeared uniform (not wispy) and there was no evidence of any trace of debris. Associated with this was a strong smell of sulfurous gas which entered the aircraft and was noticed by the crew and passengers. The return flight departed Sydney at about 0400 and encountered the same haze in roughly the same place at 0445. The aircraft was in the haze for 5-10 minutes (75-150 km) and did not change their flight level (FL330, ~10 km altitude). A NOTAM was issued for the period of the evening of the 20th through the morning of the 22nd." The cloud was also reported by pilots from Qantas and Ansett, as late as 2000 on the 20th.

The Atmospheric Research Division of CSIRO were able to discriminate the plume, ~ 500 km long and 100 km wide, on an AVHRR image by ratioing bands 4 and 5. TOMS data showed the plume continuing its eastward path, reaching Chile on 21 August.

Deposits and post-eruptive activity. Intense fumarolic activity continued from a 2-km fissure (oriented N20°E) on the WNW caldera margin during a 23 August overflight. Weaker fumarolic activity was observed on the interior slopes of the 500-m-diameter Crater 1, located 400 m E of the fissure (figure 1). The fissure and Crater 1 were the site of activity 8-10 August.

A black flow (probably lava), with shades of reddish-brown, extended about 3.5 km from the extreme N end of the fissure, onto Ventisquero Huemules. The flow was 50-300 m wide, with several broader sections, and covered recent scoria (8-10 August) in places. Several weak vapor/gas emissions were visible. Scoriaceous pyroclastic flow deposits containing large quantities of ice and snow extended from the fissure toward the interior of the caldera, and in part, over Ventisquero Huemules toward the NW, and Huemules Valley.

Products of the 8-10 August activity were basaltic in composition. Ash samples (ranging to 0.1 mm in size) from Puerto Aisén contained abundant magnetite, pyroxene, plagioclase, and black glass shards. Silica contents of the ash were determined to be 50.98% (at Sernageomin Laboratory).

At Crater 2, believed to be the site of activity on 12-15 August, intense degassing occurred at 3 fumaroles along the S margin. Concentric cracks were visible in the thick ice surrounding the 800-m-wide Crater 2. Pumice from 12-15 August activity differed in composition from the earlier erupted material. Whole rock analyses (from Peter Bitschene) indicated a trachyandesitic composition, with ~ 60% SiO₂ and 8-9% alkalies. The distal fallout ash was >98% vitric with predominant pumice and platy shards, and some entrained blocky basaltic shards.

Bitschene estimated that more than 1 km³ of tephra was deposited in Argentina's Santa Cruz province [but see 16:8]. Lakes near the volcano were highly turbid and had layers of floating pumice along their E shores. Increased sediment load resulted in the acceleration of delta growth in Lago Buenos Aires (SE; also called Lago General Carrera), and silting up of the mouth of Río Ibáñez near Puerto Ingeniero Ibáñez (75 km SE) creating a flood risk.

Roughly 50-60,000 sheep and cattle are located within the airfall zone. Extremely high values of fluorine (225 ppm water extractable) were obtained from the ash analyzed 4 days after the eruption. Alberto Villa (INTA, Univ de Chile) reported that grass samples collected at the same site had 280 ppm fluorine (on a dry basis). [but see 16:9-10]

Reference. Stern, C.R., 1991, Mid-Holocene Tephra on Tierro del Fuego (54°S) Derived from the Hudson Volcano (46°S): Evidence for a Large Explosive Eruption; Revista Geológica de Chile, v. 18, no. 2, in press.

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

Information Contacts: J. Naranjo, SERNAGEOMIN; H. Moreno, Univ de Chile; G. Fuentealba and P. Riffo, Univ de La Frontera; P. Bitschene, Patagonia Volcanism Project, Argentina; N. Banks, USGS; SAB, NOAA; G. Stephens, NOAA/NESDIS; S. Doiron, GSFC; B. Presgrave, NEIC; C. Stern, Univ of Colorado, Boulder; A.J. Prata, CSIRO, Australia; ICAO; Radio Nacional de Chile; AP.

In July, the turquoise-green crater lake continued to rise, eventually covering 2/3 of the crater floor, including several fumaroles that formed during early-mid June. Sulfur deposits had been observed at some of these fumaroles. On 17 July, the lake was 150 x 100 m, with a maximum depth of 2 m. Water temperatures increased with proximity to the bubbling springs (90°C), mud pots, and roaring fumaroles, ranging from 35°C to 55°C (compared to 30-48°C in late June). The lake had pH of 3.7.

Seismicity remained at high levels in July, but was decreased in comparison to late May-June (16:5-6). July's highest seismicity occurred on the 4th, when 75 earthquakes were recorded (seismic station IRZ2, 5 km WSW, Univ Nacional network; figure 3), 34 of which occurred in a NW-SE trend. The 4 July earthquakes (M 1.5-2.7) were centered 0.6-10 km from the crater at <10 km depth. Tremor episodes and B-type earthquakes continued to be recorded in July.

Figure 3. Daily number of earthquakes at Irazú, July 1991. Courtesy of Universidad Nacional.

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

Information Contacts: R. Barquero, Guillermo Alvarado, and Alain Creussot, ICE; Mario Fernández and Hector Flores, Sección de Sismología y Vulcanología, Univ de Costa Rica; J. Barquero, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.

An eruption built a small temporary island . . . first observed on 4 May, but its location was initially uncertain. However, more precise navigational data from the chief pilot of Western Pacific Air Services placed the activity at 9.00°S, 157.97°E, roughly 3 km NE of Kavachi's summit.

Activity apparently had not changed when, during an overflight on 5 June, [John] Monroe observed a vigorously active lava fountain roughly 25 m high and a plume that rose >2,500 m. The island's dimensions were estimated at 150-200 m long and ~50 m high. Carl Rossiter reported that divers ~45 km NE of Kavachi (at Kicha Island) felt powerful explosions while underwater on 7-8 and 12-13 June. Individual explosions occurred a few seconds apart in groups of 12-20. Explosion groups generally lasted a total of 1-2 minutes, were typically preceded and followed by rumbling, and were separated by roughly 30 minutes of quiet. No explosions were felt at other dive sites, where islands were between the observers and Kavachi.

The eruption weakened in mid-June, and the island disappeared beneath the ocean surface later in the month. No additional activity has been reported.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is one of the most active submarine volcanoes in the SW Pacific, located in the Solomon Islands south of Vangunu Island about 30 km N of the site of subduction of the Indo-Australian plate beneath the Pacific plate. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

Information Contacts: R. Addison and A. Papabatu, Ministry of Natural Resources, Honiara; J. Monroe, San Jose, USA; C. Rossiter, See and Sea Travel Service, San Francisco, USA.

The . . . eruption continued through July, as lava from Kupaianaha vent flowed into the sea. The surface of Kupaianaha's lava pond remained frozen, while lava was still active at the bottom of Pu`u `O`o crater. Nearly simultaneous earthquake swarms occurred in the summit areas of Kilauea and its larger neighbor Mauna Loa.

Eruptive activity. Lava from Kupaianaha was confined to tubes as it advanced down the upper slopes, where skylights at ~650 m (2,150-2,140 ft) elevation revealed an average velocity of ~1 m/s. Active surface flows were intermittently observed in a steeper area near 350 m (1,100 ft) elevation, and additional large surface flows emerged from the tube system between there and the coast through July. One large flow, active since June, advanced on top of the main (Wahaula) tube's E branch (figure 79). Its terminus was near 40 m (140 ft) elevation on 9 July. Although the flow front was wide with many active lobes, it did not reach the coast. Numerous small breakouts were active behind its front. Another flow emerged from a tube near 180 m (600 ft) elevation, moved downslope above the tube's W branch, and reached the coastal plain on 14 July. Two fluid pahoehoe lobes were advancing toward the coast on 16 July, moving past a kipuka at 35 m (120 ft) elevation. By the end of the month, the active flow front was > 400 m wide, and small breakouts from the flow were burning vegetation in Royal Gardens subdivision.

Despite the extensive surface activity, lava continued to pour into the sea from tubes at two main entries. The tube's W branch fed two active sites (at the Poupou entry). The littoral cone at the W Poupou site continued to erode, but erosion slowed toward the end of July as a bench growing outward below the littoral cone absorbed most of the waves' force. A cycle of bench erosion and rebuilding occurred repeatedly at the E Poupou site. Undercutting by wave action removed meter-sized blocks from the cliff face, and the resulting rapid collapse and erosion generated increased spatter activity, initiating construction of a new lower bench. At the entry fed by the E branch of the tube (Paradise), a prominent mid-bench scarp was noted on 4 July. Spatter was found draped over the scarp but none was evident on the lower portion of the bench, suggesting that the lower bench grew after the collapse episode. However, no seismic evidence of collapse was noted. The lower bench grew to within 1 m of the upper bench by 26 July. By the end of the month, the lava entry point shifted from the middle to the E side of the bench. Its W side began eroding and soon developed a cliff facing the ocean.

Seismicity. Continuous volcanic tremor persisted through July at the seismic stations nearest the eruption site and near the W ocean entry. Tremor amplitudes were generally low, although occasional brief bursts of higher amplitude tremor were recorded.

Earthquake activity beneath the summit appeared to have changed slightly since mid-late June. Shallow activity (0-5 km depth) had decreased, especially from the first 3 months of 1991. Daily visual scans of analog records since mid-June suggest that the dominant frequency content of shallow harmonic events had also changed, from 3-5 Hz to 1-3 Hz. The number of deeper (5-13 km) harmonic events fluctuated through July. Between 3 and 6 July, there were swarms of both shallow and deeper long-period events, then activity declined before a second, less intense swarm of intermediate-depth long-period events occurred on 11 July. This was followed first by an increase in shallower long-period activity, then a swarm of several hundred short-period microearthquakes on 13 July between 1400 and 2300, ~2 hours after the onset of a swarm under neighboring Mauna Loa. Almost all were too small for precise location. The 13 July seismicity was not associated with obvious eruptive changes, but geophysicists believe that it may indicate changes in magmatic activity or the state of stress beneath the summit.

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

Information Contacts: T. Moulds and P. Okubo, HVO.

"Kuwae is a mainly submarine caldera (~10x5 km) that, according to C14 ages, Tongan folklore, and reconnaissance fieldwork (Garanger, 1972; Crawford, 1988), is probably very young (~1,500 A.D.). The caldera is located between Epi, Laika, and Tongoa islands in the central part of Vanuatu. During the ORSTOM-CALIS cruise in May 1991, detailed bathymetric and magnetic surveys of the collapse structure were made, and data are presently under analysis. August fieldwork was carried out on Tongoa and Laika Islands in order to study caldera eruption products, their composition, and their age. Several ignimbrite units, including non-welded ash and pumice flow deposits, and thick, complex sequences of poorly-welded to densely-welded tuffs, have been discovered. C14 ages will be determined for charcoal samples from these deposits.

"During the last century, the caldera's active Karua volcanic cone has emerged at least six times, in 1897, [1901], . . . 1948, [1949], 1959, and 1971. Each period of activity was accompanied by explosions. The ephemeral island reached a maximum size of 100 m tall and 1.5 km in diameter in 1949. On 6 August, during a visit by speedboat, the submerged summit area was 50-70 m large at 2-3 m depth. No fumarolic activity was observed despite a strong sulfur smell." [Turbulence and discolored sea water were observed in 1971-74 and 1977.]

References. Crawford, A.J., 1988, Circum-Pacific Council for Energy and Mineral Resources: Earth Science Series, v. 8.

Garanger, J., 1972, Publication de la Société Océanistes, no. 30.

Geologic Background. The largely submarine Kuwae caldera occupies the area between Epi and Tongoa islands. The 6 x 12 km caldera contains two basins that cut the NW end of Tongoa Island and the flank of the late-Pleistocene or Holocene Tavani Ruru volcano on the SE tip of Epi Island. Native legends and radiocarbon dates from pyroclastic-flow deposits have been correlated with a 1452 CE ice-core peak thought to be associated with collapse of Kuwae caldera; however, others considered the deposits to be of smaller-scale eruptions and the ice-core peak to be associated with another unknown major South Pacific eruption. The submarine volcano Karua lies near the northern rim of Kuwae caldera and is one of the most active volcanoes of Vanuatu. It has formed several ephemeral islands since it was first observed in eruption during 1897.

Information Contacts: C. Robin and M. Monzier, ORSTOM, New Caledonia; M. Lardy and C. Douglas, ORSTOM,Vanuatu; C. Mortimer, Dept of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.

"Activity of both craters remained moderately strong in July, as in June. Crater 3, which had resumed activity in mid-May, released white-to-grey vapor and ash clouds, and light ashfall occurred towards the NE of the volcano on the 6th and 8th. Occasional weak to loud explosions were heard throughout the month. Weak to bright red glow was observed on the 8th, 9th, 13th, and throughout the last week of the month.

"Activity at Crater 2 was characterized by the emission of moderate to thick pale grey ash clouds. Occasional loud to low explosions, some of which were accompanied by light ashfall, were heard during the second and last week of the month. Steady, weak night glow was visible throughout the second week and on the 22nd and 23rd.

"Seismicity remained high throughout the month, with the occurrence of explosion earthquakes and tremor. The daily number of Vulcanian explosions recorded by the summit station (LAN) reached a maximum of 40-60 between the 21st and 26th. Tremor, hardly noticeable in May, occurred almost daily in June-July (up to 100-200 minutes/day). Two types were recognized: high-frequency, discontinuous tremor periods, lasting 1-2 minutes; and lower-frequency harmonic tremor, continuous for periods of several (up to 10) minutes. The tremor became strong enough to be recorded at both the summit station (LAN) and the 9-km-distant CGA station."

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: C. McKee, RVO.

Press releases reported increased activity, with small eruptions occurring around 19 July. One eruption reportedly ejected incandescent material 100 m high, dropping hot ash (smelling of sulfur) onto nearby areas and causing residents to flee. At 1645 on 29 July, a 300-m-high ash cloud extending ~35 km W was reported by pilots on Qantas flight A61. By the week of 14-19 August the volcano was no longer exploding, and gas emissions, 50-100 m high, appeared to be decreasing.

Geologic Background. The Lewotobi "husband and wife" twin volcano (also known as Lewetobi) in eastern Flores Island is composed of the Lewotobi Lakilaki and Lewotobi Perempuan stratovolcanoes. Their summits are less than 2 km apart along a NW-SE line. The conical Lakilaki has been frequently active during the 19th and 20th centuries, while the taller and broader Perempuan has erupted only twice in historical time. Small lava domes have grown during the 20th century in both of the crescentic summit craters, which are open to the north. A prominent flank cone, Iliwokar, occurs on the E flank of Perampuan.

Information Contacts: W. Modjo, VSI; ICAO; UPI.

"The volcano was totally quiet during overflights (VANAIR) on 4 September 1990, and 13 and 24 July 1991. . . . As with Gaua, the scarcity of information from 1977 to 1989 prevents a precise description of its activity. Nevertheless, it seems that no major event occurred during this period."

[The Bulletin of Volcanic Eruptions (BVE) reports lava flows in November 1978, ash eruptions and lava flows February-March 1979, a black eruption column on 2 July 1979, minor ash emissions on 12 September 1979, vigorous ash eruptions in April and July 1980, and an eruption cloud and lava flow on 18-20 August 1980.]

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: C. Robin and M. Monzier, ORSTOM, New Caledonia; M. Lardy and C. Douglas, ORSTOM,Vanuatu; C. Mortimer, Dept of Geology, Mines, and Rural Water Supply,Vanuatu; J. Eissen, ORSTOM, France.

"Activity . . . increased slightly in July, as shown by more voluminous vapour and ash emissions, stronger sounds, and the resumption of night glow over Main Crater. Emissions from Main Crater consisted of weak to moderate white-grey ash and vapour accompanied by thin blue vapour from 22 to 25 July. Occasional deep roaring noises were heard on the 4th-6th. A weak fluctuating night glow was visible 23-25 July for the first time since April. Southern Crater emitted thin to thick grey-brown ash clouds, occasionally rising to ~400-500 m above the crater rim. Booming and deep roaring noises were heard on most days throughout the month, but no night glow was observed. Seismicity was at a moderate level and tiltmeter measurements showed no change."

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: C. McKee, RVO.

Surface deformation measurements indicate gradual reinflation of Mauna Loa's summit since its 1984 eruption. Earthquake counts have fluctuated, but have apparently increased since late 1990.

Two bursts of intermediate-depth volcanic tremor, beginning at about 1200 on 13 July, preceded a swarm of long-period earthquakes that continued for ~14 hours. Activity peaked between 2300 on 13 July and 0100 the next morning. As the long-period events gradually declined, shallow microearthquake activity increased, and continued for about 6 hours. All of the events were too small for precise location.

The 13 July activity began ~2 hours before an earthquake swarm under the summit of Kilauea. Seismicity at Mauna Loa has apparently returned to average background levels since mid-July.

Geologic Background. Massive Mauna Loa shield volcano rises almost 9 km above the sea floor to form the world's largest active volcano. Flank eruptions are predominately from the lengthy NE and SW rift zones, and the summit is cut by the Mokuaweoweo caldera, which sits within an older and larger 6 x 8 km caldera. Two of the youngest large debris avalanches documented in Hawaii traveled nearly 100 km from Mauna Loa; the second of the Alika avalanches was emplaced about 105,000 years ago (Moore et al. 1989). Almost 90% of the surface of the basaltic shield volcano is covered by lavas less than 4000 years old (Lockwood and Lipman, 1987). During a 750-year eruptive period beginning about 1500 years ago, a series of voluminous overflows from a summit lava lake covered about one fourth of the volcano's surface. The ensuing 750-year period, from shortly after the formation of Mokuaweoweo caldera until the present, saw an additional quarter of the volcano covered with lava flows predominately from summit and NW rift zone vents.

Information Contacts: P. Okubo, HVO.

Seismicity decreased in July, with 94 earthquakes and two tremor episodes recorded . . . (figure 10). Summit vents continued emitting white steam plumes but these rose weakly to ~ 100 m . . . .

Figure 10. Daily number of earthquakes January-15 August 1991.

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: JMA.

Fourteen eruptions occurred during the most recent phase of strong explosive activity, 6 June-1 August, with the strongest and most destructive activity occurring 27-31 July. Activity was at low levels as of 15 August.

The following report from Philippe Rocher describes activity through mid-June.

"During the first half of 1991, activity was continuous and relatively quiet, with several small eruptions and lava flows from the main crater. This last cycle of activity began in November 1990. The continuous ejection of material built a cone that reached 400-500 m height. Although seismicity showed no significant changes in May, occassional pulses of increased surface activity occurred. On 11-15 May, explosion counts ranged from 1,170 to 1,730/day and a new lava flow was emitted. The cone reached 500 m high and lava traveled down the SE slope.

"On 6 June, explosive activity increased again, with explosions every 10-40 seconds and ash reaching 100-500 m heights. The next pulse occurred on 11 June. On the following day, strong explosions sent material to 500 m height and triggered avalanches that destroyed the summit of the cone. Lava flowed down the SW slope. Ash emissions to 500 m height and short lava flows characterized the next increase, lasting 4.5 hours on 14 June. On 16 June, a 10-hour episode of strong explosions ejected a black plume to 600 m height and caused avalanches that traveled to the foot of the volcano. Between the different eruptions, strong degassing continued, accompanied by B-type earthquakes and small, low-amplitude (about 1 mm) tremor episodes."

The following is from Eddy Sánchez.

"The most explosive and destructive activity during the current phase of activity began at 0100 on 27 July. Strombolian activity destroyed the main crater, and ejected ash and lapilli to the SW, principally affecting Caracol, Rodeo, and Patrocinio, the same towns affected by the eruption on 25 January 1987. Activity decreased at 0230." The press reported that three people were injured and 2,000 left homeless.

"Intense activity resumed at 1330-2230 on 30 July, with four cycles of moderate explosions, each cycle lasting 1.5 hours. Similar activity occurred the next day, when columns of fine ash and gas rose 400-1,000 m above MacKenney Crater. The last strong episode of Strombolian activity began at 0230 on 1 August, when ash clouds reached 700-1,000 m heights, with pulses and pauses of 30-60 minutes, and blocks (>=5 m in diameter) were ejected onto the flanks of the volcano.

"Local agriculture was significantly damaged by airfall from this recent phase of explosive activity. Corn and bean fields were destroyed, as well as part of the coffee crop. Airfall thicknesses ranged from 0.5 to 26 cm, with up to 5 cm in Rodeo and 15 cm in Santa Lucía Cotzumalguapa (figure 8). The ash was deposited as far as 55 km WSW (Pueblo Nuevo Tiquisate).

Figure 8. Isopach map of airfall deposits from activity on 27-31 July 1991 at Pacaya. Base Map is a portion of Guatemala 1:250,000 sheet (ND 15-8, Dirección General de Cartografía, Guatemala City, Guatemala). Contour interval, 100 m. Courtesy of E. Sánchez.

"During the last eruption, on 1 August, INSIVUMEH recommended to emergency agencies that the approximately 1,500 residents of Caracol, Rodeo, and Patrocinio be evacuated, due to the hazard of a new violent eruption. The next day, seismic and eruptive activity decreased considerably, allowing the evacuated people to return home. Activity continued to decrease quickly, with 40 B-type microearthquakes (frequency, 4-5 Hz, and amplitude, 2.0-2.5 mm) recorded daily on 7 August. Activity as of 15 August was considered at low levels."

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: E. Sánchez, INSIVUMEH; Philippe Rocher, L.A.V.E., France; ACAN network, Panama City, Panama.

Activity declined through the third week of August, although periodic explosions continued to eject material to >15 km height. Heavy rains triggered large mudflows that traveled down all major drainage systems, destroying houses and resulting in numerous casualties. The number of people killed by the eruption, mudflows, and disease (in evacuation camps) now exceeds 500. The stratospheric aerosol cloud produced by the paroxysmal activity on 15-16 June continued to disperse.

Continuing activity, to 20 August. Declining seismicity was interrupted by a M 4.5-5 volcano-tectonic earthquake at 1456 on 26 July and several felt aftershocks. Ash emission continued, often accompanied by tremor during periods of increased plume heights. Two pulses of emissions to >7.5 km at 0136 and 0203, and one to 16.4 km (as determined by radar at Clark Air Base) at 1212 on 27 July, were accompanied by low-amplitude tremor. Aviation officials were notified within 15 minutes of the onset of this more energetic activity. Relatively dry weather continued through early August.

Seismicity continued a gradual downward trend (figure 16), with a decrease in amplitude and number of long-period events, and a decrease in seismic energy released (figure 17). Small upsurges in amplitude (RSAM) corresponded to long-period earthquakes. Ash emissions were rare and did not exceed 8 km height during 8-10 August and had fewer accompanying long-period events. Occasional high-frequency earthquakes were felt at Clark Air Base with intensities up to II. Mudflow signals were seismically recorded on the 10th.

Figure 16. Number of earthquakes per 4 hours (top) and Realtime Seismic Amplitude Measurement (bottom) at Pinatubo, 16 June-11 August 1991. Courtesy of PHIVOLCS.

Figure 17. Accumulated RSAM energy at Pinatubo, 16 June-15 August 1991. Courtesy of PHIVOLCS.

Heavy rain triggered large mudflows on 11 August. The press reported that more than 13,000 people fled their villages, and more than 1,000 houses were destroyed. The Gumain (SE flank) and Sacobia (E flank) Rivers rose an average 1.2 m, and 300 houses were damaged along the Abacan near Mexico (~45 km E of the summit). Five large ash emissions (average height 5 km) occurred on 12 August. United Airlines pilots reported an ash cloud to >15 km altitude at about 1300 on the 12th and to 12 km the following day at 1426.

High ash emissions (maximum plume height about 13 km) and mudflows were reported on 14 August. About 5,000 people evacuated Tabon in the Pampanga region (E flank), as 96 houses were washed away. The press reported debris to 3 m deep. Mudflows on the 18th prompted another large evacuation, with 3,000 fleeing 6 towns in the Pampanga and Tarlac regions (E flank).

On 20 August, the press reported that the largest mudflows since the start of the eruption killed 31 people (primarily in Santa Rita, ~40 km NE), bringing the number of mudflow-related deaths to over 100. Flows 5 m high reportedly traveled down ten rivers, damaging more than 9,000 houses and destroying three bridges. Up to 55,000 people evacuated their homes. Ash clouds rose to 12 km high.

The press reported that by 6 August, more than 46 people (mostly children and infants) had died of various illnesses (primarily diarrhea, measles, and broncho-pneumonia) in evacuation camps. This number had increased to nearly 200 (mostly Aeta tribesmen) by 18 August, and it was reported that almost 1,500 people in the camps were suffering from disease. By 20 August, more than 500 people had died since the start of the eruption according to press reports.

Field geology. Fieldwork and evaluation of the deposits from the paroxysmal activity of 15-16 June continued. A preliminary airfall isopach map was prepared by the PHIVOLCS MGB Lahar Task Force (figure 18), and the volume of material within the 10-cm isopach was estimated to be 0.47 km³. Ash leachates indicated chloride contents to almost 1,000 ppm, and fluoride contents under 10 ppm (table 3). Petrographic analysis of pumice samples revealed the presence of anhydrite micro-phenocrysts scattered in the matrix groundmass (Bernard, and others, 1991). Pyroclastic-flow deposit volumes were estimated to total roughly 7 km³. The following report by Alain Bernard describes one of the pyroclastic-flow deposits.

Figure 18. Preliminary isopach map of 12-16 June 1991 airfall deposits from Pinatubo. Isopachs are centimeters. Prepared by PHIVOLCS MGB Lahar Task Force.

Table 3. Preliminary fluoride and chloride contents in Pinatubo ash leachates, 12 June-4 July 1991. Ash was washed for 12 hours in a 4:1 ratio of water (distilled-deionized water, pH 5.5) to ash. The 12, 15, and 22 June samples were collected by PHIVOLCS and reported "fresh fallen," the other samples were collected shortly after falling, during dry weather. Courtesy of Alain Bernard and PHIVOLCS.

"A pyroclastic-flow deposit emplaced in the Marella River (reaching 15 km SW from the main crater) was visited on 3 July. It was still degassing, with numerous rootless fumaroles present even at low altitude at the end of the deposits. The gases emitted were mostly steam, but minor amounts of SO₂ (and probably H₂S) were present, since incrustations of native sulfur were observed at the mouths of these fumaroles. Strong odors of burned wood (charcoal) were also perceptible in some places, and associated with black-brown deposits at the surface of the pyroclastic-flow deposit resulting from some pyrolysis of wood buried at shallow depth beneath the deposit. Maximum temperatures of the fumarole were close to boiling, 98-99.5°C. The temperature inside of the pyroclastic-flow deposit measured at one location (~10 km from the crater) was 223°C at a depth of 70 cm.

"The surface of the deposit was a hard crust that was very easy to walk on. It looked like some recent pyroclastic-flow deposits observed on Augustine, with rounded pumice clasts (maximum size

"Numerous small cones (maximum diameter about 10 m, up to about 1-2 m high) were also present on the surface of the pyroclastic-flow deposit. These cones resulted from the activity of large steam fumaroles. At the time of the visit, two intermittent fumaroles were active in the upper portion of the deposit (~8 km from the crater) emitting a steam plume 3-4 m high mixed with fine-grained ash. A hot (88°C) stream of muddy water (65 cm wide), with the consistency of a mudflow, was also surging from the ground in the area close to these intermittent fumaroles. A water sample filtered from this stream showed a high chloride content compared to other streams and rivers travelling down the volcano (table 3). Many old tracks of other mudflows were observed on the surface of the pyroclastic flow deposit."

[Additional encounters between aircraft and ash clouds, frequent in the eruption's first days, were reported this month but included above in table 2.]

Reference. Bernard, A., Demaiffe, D., Mattielli, N., and Punongbayan, R.S., 1991, Anhydrite-bearing pumices from Mount Pinatubo: further evidence for the existence of sulphur-rich silicic magmas: Nature, v. 354, p. 139-140.

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

Information Contacts: R. Punongbayan, PHIVOLCS; A. Bernard, Univ Libre de Bruxelles, Belgium; T. Casadevall, USGS Denver; J. Lynch, SAB; Daily Inquirer, Manila, Philippines; AP; UPI; Reuters.

An average of 239 microearthquakes, with a maximum of 485 (3 July), were recorded daily in July (figure 39), at a station 2 km SW of the crater. Of these, 29 were identified as A- and B-type earthquakes. Seismic frequencies ranged from 1.4 to 2.6 Hz. A total of 41 hours of continuous and discrete semi-harmonic tremor episodes were recorded, with durations of up to 6 hours.

Figure 39. Daily number of earthquakes at Poás, July 1991. Courtesy of the Univ Nacional.

The crater lake's average temperature was 63°C. Fumaroles were covered as the lake level continued to rise. Area residents sporadically reported a sulfurous odor.

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

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

A total of 399 microearthquakes were recorded in July (figure 4) at a seismic station (RIN3) 6 km SW of the crater. Six hours of low- and medium-frequency tremor (1.3-3.2 Hz), were recorded in episodes 12 minutes to 3 hours long. Low-frequency earthquakes were also recorded, with durations that reached 175 seconds.

Figure 4. Daily number of earthquakes at Rincón de la Vieja, July 1991. Courtesy of OVSICORI.

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

Seismicity was at very low levels in July, although tremor reached slightly higher levels at the beginning of the month. Deformation measurements showed no significant changes. The SO₂ flux continued to fluctuate, with a monthly average of ~1,220 t/d. Two small ash emissions, restricted to the summit region, were observed during July.

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

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

A swarm of earthquakes, reported on 23-24 July, triggered mudslides that partly buried four villages. In towns within 20 km N of the volcano, the earthquakes caused many houses to collapse, especially in Maca (15 km N) which was almost completely destroyed. The press reported that 20 people were killed, 80 were injured, and 3,000 were left homeless. More than 20 earthquakes/day were reported felt (MM <=V) 75 km SE (in Arequipa). The largest of the shocks (Ms [4.7]), detected at [1444] on 23 July by the WWSSN, was centered [35] km [ENE] from the volcano at shallow depth.

Geologic Background. Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

Information Contacts: NEIC; EFE network, Madrid, Spain; Agence France-Presse; Reuters; UPI; AP.

The volcano was in a moderate explosive phase in May, emitting gray ash clouds 300-500 m high. In June, the number of moderate to strong explosions increased daily, with plumes 400-600 m high, and ashfall on the area surrounding the volcano. Numerous collapses and large avalanches were observed on the SE slope.

Geologic Background. Symmetrical, forest-covered Santa María volcano is one of the most prominent of a chain of large stratovolcanoes that rises dramatically above the Pacific coastal plain of Guatemala. The stratovolcano has a sharp-topped, conical profile that is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four westward-younging vents, the most recent of which is Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Philippe Rocher, L.A.V.E., France.

The number and intensity of explosions has continued to fluctuate in recent months, with the average rate remaining slightly higher since mid-March. During a summit visit on the night of 31 July-1 August, >50 explosions were observed between 2100 and 0600. The strongest ejected incandescent material toward the edge of the summit area. Most of the explosions were from Crater 1, the rest from Crater 3, with only gas emission evident from Crater 2 and from a small cone. On this occasion and during other visits over the past several years, durations of precursory noises appeared linked to explosive vigor, with stronger explosions following noises lasting 3-5 seconds, whereas 1-2-second noises preceded weak explosions [see also 16:08].

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 from about 13,000 to 5000 years ago was followed by formation of the modern edifice. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5000 years ago as a result of the most recent of 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: H. Gaudru, SVE, Switzerland; T. De St. Cyr, Fontaines St. Martin, France.

"During our survey, no change in activity at the major geothermal areas (Frenchman's Solfataras and Hell's Gate) was noted, with respect to descriptions by Aubert de la Rue (1937) and Hochstein (1980). Slightly superheated fumaroles (with sulfur deposits), hot springs, and boiling ponds up to 3 m in diameter occurred over a 300-m strip along the Sulfur River (E flank) between 300 to 400 m elevation. The temperature of the Sulfur River at Hell's Gate remained stable at 50°C.

"Soretimeat . . . is a composite volcano built on an ancient Pleistocene edifice. Ash emissions reported in 1860 and 1965-66 are most likely to have been from hydrothermal explosions (Ash and others, 1980)." ["Flames" were observed during an apparent eruption in 1865 (Atkin, 1868).]

References. Ash, R.P., Carney, J.N., and MacFarlane, A., 1980, Geology of the northern Banks Islands: New Hebrides Geological Survey Regional Report, p. 1-47.

Atkin, J., 1868, On volcanoes in the New Hebrides and Banks Islands: Proceedings of the Geological Society of London, v. 24, p. 305-307.

Aubert de la Rue, E., 1937, La Volcanisme aux Nouvelles Hebrides (Melanesie): BV, v. 2, p. 79-142.

Hochstein, M.P., 1980, Geology of the Northern Banks Islands: New Hebrides Geological Survey Regional Report, p. 47-49.

Geologic Background. Suretamatai volcano forms much of Vanua Lava Island, one of the largest of Vanuatu's Banks Islands. The younger lavas of 921-m-high Suretamatai (also known as Soritimeat) volcano overlie a number of small older stratovolcanoes that form the island. In contrast to other large volcanoes of Vanuatu, the dominantly basaltic-to-andesitic Suretamatai does not contain a youthful summit caldera. A chain of small stratovolcanoes, oriented along a NNE-SSW line, gives the low-angle volcano an irregular profile. The youngest cone, near the northern end of the chain, is the largest and contains a lake of variable depth within its 900-m-wide, 100-m-deep summit crater. Historical activity, beginning during the 19th century, has been restricted to moderate explosive eruptions.

Information Contacts: C. Robin and M. Monzier, ORSTOM, Nouméa, New Caledonia; M. Lardy and C. Douglas, ORSTOM, Vanuatu; C. Mortimer, Dept. of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.

Abnormally high levels of seismicity continued as of mid-August. Up to 5 small high-frequency earthquakes were recorded daily 9-12 August. No earthquakes were felt during this time. The main crater lake temperature remained at 31°C. Close monitoring of the volcano continued.

Geologic Background. Taal is one of the most active volcanoes in the Philippines and has produced some of its most powerful historical eruptions. Though not topographically prominent, its prehistorical eruptions have greatly changed the topography of SW Luzon. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, and several eruptive centers lie submerged beneath the lake. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all historical eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones that has grown about 25% in area during historical time. Powerful pyroclastic flows and surges from historical eruptions have caused many fatalities.

Information Contacts: R. Punongbayan, PHIVOLCS.

The dome in Jigoku-ato crater continued to grow in an easterly direction in July, at a rate of 0.3 x 10⁶ m³/day (figure 26). The magma supply rate remained unchanged in August, but the direction of growth became westerly. By 15 August, the dome was estimated to be 650 x 250 m and 130 m thick. On 19 July it had been 520 x 260 m, with a volume of 5.9 x 10⁶ m³.

Figure 26. Cumulative volumes of magma erupted from Unzen, May-July 1991. Courtesy of S. Nakada.

The number of seismically-detected pyroclastic flows and avalanches from the dome decreased in July (compared to June), showed a gradual increase late July-early August, then decreased suddenly on 12 August to only a few events/day. A total of 326 pyroclastic flows were recorded in July (down from 482 in June), and 155 during 1-15 August. Event durations were shorter than in previous months when flow signals occasionally lasted more than 300 seconds. The longest events lasted 140 seconds in July and 150 seconds in August.

Pyroclastic flows continued to travel as much as 2 km E down the Mizunashi River. None of the flows reached the evacuated areas of Shimabara and Fukae, which remained closed with 12,395 inhabitants relocated. Ash clouds from the larger pyroclastic flows rose 2 km, with ash falling mainly NE on Shimabara. Prevailing winds remained unchanged since May. Continuous ash emission from vents in the crater near the dome occurred in mid-July (16:06), and on 5-6, and 12 August, when the ash cloud rose 1.5 km. Explosive ejections of incandescent blocks to 100 m height were observed from midnight to 0200 on 12 August, presumably from a vent on the W end of the dome that continuously emitted ash throughout the day.

In contrast to the drop in pyroclastic flows on 12 August, the number of summit earthquakes and tremor episodes increased sharply on 11 August. This followed reduced seismic activity in June (230 recorded earthquakes) and July (133), compared to April (1959). More than 460 earthquakes had already been recorded in August by the 15th. Earthquake magnitudes were small and no shocks were felt, nor were changes in ground deformation detected by tiltmeters or EDM lines near the summit. Following the peak on 12 August, seismicity began to decrease. The increase in seismicity may be related to the incandescent ejections on 12 August, the active continuous ash emission, and the westward growth of the dome.

A man died on 8 August from burns suffered on 3 June, bringing the total casualties to 39 dead and three missing.

The following is a report from Setsuya Nakada on dome growth and morphology in June. "Large pyroclastic flows occurred on 3 and 8 June (figure 27), with volumes of about 0.7 x 10⁶, and 1 x 10⁶ m³, respectively. The E half of the lava dome collapsed during the eruption of the 3 June pyroclastic flow, leaving a 150-m-wide horseshoe-shaped depression opening to the E (figure 28). The volume of dome material left behind (referred to as W dome) was about 0.48 x 10⁶ m³. A new lava dome formed within the depression by 8 June, obtaining pre-3 June volumes.

Figure 27. Distribution of the 3 and 8 June 1991 pyroclastic flow deposits at Unzen. From Nakada and Kobayashi (1991).

Figure 28. Growth pattern of the lava dome in Jigoku-ato Crater at Unzen, May-August, 1991. From Nakada and Kobayashi (1991).

"Some of the 8 June pyroclastic flows, which reached 5.5 km beyond the crater, resulted from the direct eruption of magma from the vent. An extensive area of trees was burnt by the accompanying ash clouds. Pyroclastic surge (ash-cloud surge) deposits, such as those in the deposits from 3 June, were not clearly identified. Breadcrust bombs 5 cm in diameter were ejected to 3 km NE of the crater. Half of the W dome and the entire E dome (post-3 June material) were destroyed, widening the horseshoe-shaped depression to 200 m. About 0.15 x 10⁶ m³ of the W dome remained.

"Vulcanian explosions on 11 June ejected breadcrust and cauliflower bombs, up to 46 cm long, to 3 km distance. As a result, a depression 20-30 m in diameter formed within the crater, just above the former Jigoku-ato crater. On 17 June a continuous eruption column was observed rising from the W dome, for the first time since the start of lava extrusion.

"The E dome continued to grow and collapse along its E margin, filling a steep valley on the E slope of Jigoku-ato crater, then growing over the valley-fill deposits, a gentler surface than the original valley floor. The surface of the lava dome had the form of a petal with two lobes. These were created by extrusion near the summit of the E dome. After the middle of June, the lava surface traveled SE at a rate of 40 m/day, but the dome only lengthened a maximum of 10 m/day. By the end of June the horseshoe-shaped depression was filled with dome material, and lava blocks began to overflow NE onto the caldera floor."

Reference. Nakada, S., and Kobayashi, T., 1991, Lava dome and pyroclastic flows of the 1991 eruption at Unzen volcano: Bulletin of the Volcanological Society of Japan, v. 36, in press.

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

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

"Activity remained unchanged during 1990-91, with block and ash emissions and small episodic lava lakes."

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: C. Robin and M. Monzier, ORSTOM, New Caledonia; M. Lardy and C. Douglas, ORSTOM, Vanuatu; C. Mortimer, Dept of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.

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/).