Kavachi is an active submarine volcano in the SW Pacific, located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism has been characterized by phreatomagmatic explosions that ejected steam, ash, and incandescent bombs. The previous report described discolored water plumes extending from a single point during early 2018 and April 2020 (BGVN 45:05); similar activity was recorded for this current reporting period covering May through September 2020 and primarily using satellite data.

Activity at Kavachi is most frequently observed through satellite images and typically consists of discolored submarine plumes. On 2 September 2020 a slight yellow discoloration in the water was observed extending E from a specific point (figure 22). Similar faint plumes continued to be recorded on 5, 7, 12, and 17 September, each of which seemed to be drifting generally E from a point source above the summit where previous activity has occurred. On 7 September the discolored plume was accompanied by white degassing and possibly agitated water on the surface at the origin point (figure 22).

Figure 22. Sentinel-2 satellite images of a discolored plume (light yellow) at Kavachi beginning on 2 September (top left) and continuing through 17 September 2020 (bottom right). The light blue circle on the 7 September image highlights the surface degassing and source of the discolored water plume. The white arrow on the bottom right image is pointing to the faint discolored plume. Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

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. 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: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Krakatau, located in the Sunda Strait between Indonesia’s Java and Sumatra Islands, experienced a major caldera collapse around 535 CE, forming a 7-km-wide caldera ringed by three islands. Presently, the caldera is underwater, except for three surrounding islands (Verlaten, Lang, and Rakata) and the active Anak Krakatau that was constructed within the 1883 caldera and has been the site of frequent eruptions since 1927. On 22 December 2018, a large explosion and flank collapse destroyed most of the 338-m-high island of Anak Krakatau (Child of Krakatau) and generated a deadly tsunami (BGVN 44:03). A larger explosion in December 2019 produced the beginnings of a new cone above the surface of crater lake (BGVN 45:02). The previous report (BGVN 45:06) described activity that included Strombolian explosions, ash plumes, and crater incandescence. This report updates information from June through September 2020 using information primarily from Indonesian Center for Volcanology and Geological Hazard Mitigation, also known as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) and satellite data.

A VONA notice from PVMBG reported that the last eruptive event at Krakatau was reported on 17 April 2020, though the eruptive column was not observed. Activity after that was relatively low through September 2020, primarily intermittent diffuse white gas-and-steam emissions, according to PVMBG. No activity was reported during June-August, except for minor seismicity. During 11-13, 16, and 18 September, the CCTV Lava93 webcam showed intermittent white gas-and-steam emissions rising 25-50 m above the crater.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent hotspots within 5 km of the crater from May through September (figure 113). Some of these thermal hotspots were also detected in Suomi NPP/VIIRS sensor data. Sentinel-2 thermal satellite imagery showed faint thermal anomalies in the crater during June; no thermal activity was detected after June (figure 114).

Figure 113. Intermittent thermal activity at Anak Krakatau from 13 October 2019-September 2020 shown on a MIROVA Low Radiative Power graph. The power of the thermal anomalies decreased after activity in April but continued intermittently through September. Courtesy of MIROVA.

Figure 114. Sentinel-2 thermal satellite images showing a faint thermal anomaly in the crater during 1 (left) and 11 (right) June 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); 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/).

A massive stratovolcano in easternmost Java, Raung has over sixty recorded eruptions dating back to the late 16th Century. Explosions with ash plumes, Strombolian activity, and lava flows from a cinder cone within the 2-km-wide summit crater have been the most common activity. Visual reports of activity have often come from commercial airline flights that pass near the summit; Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) has installed webcams to monitor activity in recent years. An eruption in 2015 produced a large volume of lava within the summit crater and formed a new pyroclastic cone in the same location as the previous one. Confirmation and details of eruptions in 2012, 2013, and 2014-2015 are covered in this report with information provided by PVMBG, the Darwin Volcanic Ash Advisory Center (VAAC), several sources of satellite data, and visitors to the volcano.

Newly available visual and satellite information confirm eruptions at Raung during October 2012-January 2013, June-July 2013, and extend the beginning of the 2015 eruption back to November 2014. The 2015 eruption was the largest in several decades; Strombolian activity was reported for many months and fresh lava flows covered the crater floor. Raung was quiet after the 2015 eruption ended in August of that year until July 2020.

Eruption during October 2012-January 2013. A MODVOLC thermal alert appeared inside the summit crater of Raung on 14 October 2012, followed by another four alerts on 16 October. Multiple daily alerts were reported on many days through 8 November, most within the main crater. Single alerts appeared on 29 November and 1 December 2012 (figure 9). PVMBG raised the Alert Level on 17 October from 1 to 2 due to increased seismicity and raised it further to Level 3 on 22 October. A local news report by Aris Yanto indicted that a minor Strombolian eruption occurred inside the crater on 19 October. Strombolian activity was also observed inside the inner crater on 5 November 2012 by visitors (figure 10); they reported loud rumbling sounds that could be heard up to 15 km from the crater.

Figure 9. Thermal activity at Raung during October and November 2012 included multiple days of multi-pixel anomalies, with almost all activity concentrated within the summit crater. Strombolian activity was observed on 5 November. Image shows all pixels from 23 September-1 December 2012. Courtesy of MODVOLC.

Figure 10. Strombolian activity was observed inside the inner crater of Raung on 5 November 2012 by visitors. They reported loud rumbling sounds that could be heard up to 15 km from the crater. Photo by Galih, courtesy of Volcano Discovery.

The Darwin VAAC issued an advisory of an eruption plume to 9.1 km altitude reported at 0237 UTC on 8 November 2012. In a second advisory about two hours later they noted that an ash plume was not visible in satellite imagery. A press article released by the Center for Volcanology and Geological Hazard Mitigation (PVMBG) indicated that gray ash plumes were observed on 6 January 2013 that rose 300 m above the summit crater rim. Incandescence was observed around the crater and thundering explosions were heard by nearby residents.

Eruption during June-July 2013. Two MODVOLC thermal alerts were measured inside the summit crater on 29 June 2013. A photo taken on 21 July showed minor Strombolian activity at the inner crater (figure 11). A weak SO₂ anomaly was detected in the vicinity of Raung by the OMI instrument on the Aura satellite on 27 July. Thermal alerts were recorded on 29 and 31 July. When Google Earth imageryrom 14 March 2011 created by Maxar Technologies is compared with imagery from 29 July 2013 captured by Landsat/Copernicus, dark tephra is filling the inner crater in the 2013 image; it was not present in 2011 (figure 12).

Figure 11. Strombolian activity was observed inside the inner crater at the summit of Raung on 21 July 2013. Photo by Agus Kurniawan, courtesy of Volcano Discovery.

Figure 12. Satellite imagery from Google Earth showing the eroded pyroclastic cone inside the summit crater of Raung on 14 March 2011 (left) and 29 July 2013 (right). Dark tephra deposits filling the inner crater in the 2013 image were not present in 2011. The crater of the pyroclastic cone is 200 m wide; N is to the top of the images. Courtesy of Google Earth.

Eruption during November 2014-August 2015. Information about this eruption was previously reported (BGVN 41:12), but additional details are provided here. Landsat-8 imagery from 28 October 2014 indicated clear skies and little activity within the summit crater. Local observers reported steam plumes beginning in mid-November (figure 13). MODVOLC thermal alerts within the summit crater were issued on 28 and 30 November, and then 15 alerts were issued on seven days in December. Thermal Landsat-8 imagery from cloudy days on 29 November and 15 December indicated an anomaly over the area of the pyroclastic cone inside the summit crater (figure 14).

Figure 13. Local observers reported steam plumes at Raung beginning in mid-November 2014; this one was photographed on 17 November 2014. Courtesy of Volcano Discovery.

Figure 14. Satellite evidence of new eruptive activity at Raung first appeared on 29 November 2014. The true color-pansharpened Landsat-8 image of Raung from 28 October 2014 (left) shows the summit crater and an eroded pyroclastic cone with its own crater (the inner crater) with no apparent activity. Although dense meteoric clouds on 29 November (center) and 15 December 2014 (right) blocked true color imagery, thermal imagery indicated a thermal anomaly from the center of the pyroclastic cone on both dates. Courtesy of Sentinel Hub Playground.

In January 2015 the MODVOLC system identified 25 thermal anomalies in MODIS data, with a peak of eight alerts on 8 January. Visitors to the summit crater on 6 January witnessed explosions from the inner crater approximately every 40 minutes that produced gas and small amounts of ash and tephra. They reported lava flowing continuously from the inner crater onto the larger crater floor, and incandescent activity was seen at night (figure 15). Landsat-8 images from 16 January showed a strong thermal anomaly covering an area of fresh lava (figure 16).

Figure 15. Visitors to the summit crater of Raung on 6 January 2015 witnessed explosions from the inner crater approximately every 40 minutes that produced abundant gas and small amounts of ash and tephra. Lava was flowing continuously from the inner crater onto the larger crater floor, and incandescent activity was observed at night. Photos by Sofya Klimova, courtesy of Volcano Discovery.

Figure 16. On a clear 16 January 2015, Landsat-8 satellite imagery revealed fresh lava flows NW of the pyroclastic cone within the summit crater at Raung. A strong thermal anomaly matches up with the dark material, suggesting that it flowed NW from within the pyroclastic cone. Left image is true color-pansharpened rendering, right image is thermal rendering. Courtesy of Sentinel Hub Playground.

Satellite images were obscured by meteoric clouds during February 2015, but PVMBG reported gray and brown plumes rising 300 m multiple times and incandescence and rumbling on 14 February. Visitors to the summit crater during the second half of February reported Strombolian activity with lava fountains from the inner crater, at times as frequently as every 15 minutes (figure 17). Loud explosions and rumbling were heard 10-15 km away. MODVOLC thermal alerts stopped on 25 February and did not reappear until late June.

Figure 17. A report issued on 25 February 2015 from visitors to the summit of Ruang noted large Strombolian explosions with incandescent ejecta and lava flowing across the crater floor. The fresh lava on the crater floor covered a noticeably larger area than that shown in early January (figure 15). Photo by Andi, courtesy of Volcano Discovery.

PVMBG raised the Alert Level to 2 in mid-March 2015. Weak thermal anomalies located inside and NW of the pyroclastic cone were present in satellite imagery on 21 March. PVMBG reported gray and brown emissions during March, April, and May rising as high as 300 m above the crater. Landsat imagery from 22 April showed a small emission inside the pyroclastic cone, and on 8 May showed a clearer view of the fresh black lava NW and SW of the pyroclastic cone (figure 18).

Figure 18. Fresh lava was visible in Landsat-8 satellite imagery in April and May 2015 at Raung. A small emission was present inside the pyroclastic cone at the summit of Raung on 22 April 2015 (left). Fresh dark material is also evident in the SW quadrant of the summit crater that was not visible on 16 January 2015. A clear view on 8 May 2015 also shows the extent of the fresh black material around the pyroclastic cone (right). The summit crater is 2 km wide. Courtesy of Sentinel Hub Playground.

Nine MODVOLC thermal alerts appeared inside the summit crater on 21 June 2015 after no alerts since late February, suggesting an increase in activity. The Darwin VAAC issued the first ash advisory for 2015 on 24 June noting an aviation report of recent ash. The following day the Ujung Pandang Meteorological Weather Office (MWO) reported an ash emission drifting W at 3.7 km altitude. The same day, 25 June, Landsat-8 imagery clearly showed a new lava flow on the W side of the crater and a strong thermal anomaly. The thermal data showed a point source of heat widening SW from the center of the crater and a second point source of heat that appeared to be inside the pyroclastic cone. A small ash plume was visible over the cone (figure 19). Strombolian activity and ash plumes were reported by BNPB and PVMBG in the following days. On 26 June the Darwin VAAC noted the hotspot had remained visible in infrared imagery for several days. PVMBG reported an ash emission to 3 km altitude on 29 June.

Figure 19. A new lava flow and strong thermal anomaly appeared inside the summit crater of Raung on 25 June 2015 in Landsat-8 imagery. The new flow was visible on the W side of the crater. The darker area extending SW from the rising ash plume is a shadow. The thermal data showed a point source of heat widening SW from the center of the crater and spreading out in the SW quadrant and a second point source of heat on the flank of the pyroclastic cone. Left image is True color-pansharpened rendering, and right image is thermal rendering. Courtesy of Sentinel Hub Playground.

Activity increased significantly during July 2015 (BGVN 41:12). Ash plumes rose as high as 6.7 km altitude and drifted hundreds of kilometers in multiple directions, forcing multiple shutdowns at airports on Bali and Lombok, as well as Banyuwangi and Jember in East Java. The Darwin VAAC issued 152 ash advisories during the month. Ashfall was reported up to 20 km W during July and 20-40 km SE during early August. Visitors to the summit in early July observed a new pyroclastic cone growing inside the inner crater from incandescent ejecta and dense ash emissions (figure 20). Landsat-8 imagery from 11 July showed a dense ash plume drifting SE, fresh black lava covering the 2-km-wide summit caldera floor, and a very strong thermal anomaly most intense at the center near the pyroclastic cone and cooler around the inner edges of the crater (figure 21). On 12 July, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured a view of an ash-and-gas plume drifting hundreds of kilometers SE from Raung (figure 22).

Figure 20. A new pyroclastic cone was growing inside the inner crater at the summit of Raung when photographed by Aris Yanto in early July 2015. Courtesy of Volcano Discovery.

Figure 21. Landsat-8 imagery of Raung during July 2015 indicated dense ash emissions and a large thermal anomaly caused by fresh lava. On 11 July a dense ash plume drifted SE and a strong thermal anomaly was centered inside the summit crater. The 2-km-wide crater floor was covered with fresh lava (compare with 25 June image in figure 19). Courtesy of Sentinel Hub Playground.

Figure 22. On 12 July 2015 the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured a natural-color view of a plume of ash and volcanic gases drifting hundreds of kilometers SE from Raung. Courtesy of NASA Earth Observatory.

A satellite image on 20 July showed fresh incandescent lava covering the floor of the summit crater and a dense ash plume drifting N from the summit (figure 23). Incandescent ejecta emerged from two vents on the new pyroclastic cone inside the inner crater on 26 July (figure 24). On 27 July a dense ash plume was visible again in satellite imagery drifting NW and the hottest part of the thermal anomaly was in the SE quadrant of the crater (figure 25). Substantial SO₂ plumes were recorded by the OMI instrument on the Aura satellite during July and early August 2015 (figure 26).

Figure 23. A satellite image of the summit of Raung on 20 July 2015 showed fresh, incandescent lava covering the floor of the summit crater and a dense ash plume drifting N from the summit. Thermal activity on the NE flank was likely the result of incandescent ejecta from the crater causing a fire. Image created by DigitalGlobe, captured by WorldView3, courtesy of Volcano Discovery.

Figure 24. Incandescent ejecta emerged from two vents on the new pyroclastic cone growing inside the inner crater of Raung on 26 July 2015. Photo by Vianney Tricou, used with permission, courtesy of Volcano Discovery.

Figure 25. Landsat-8 imagery of Raung during July 2015 indicated dense ash emissions and large thermal anomalies from fresh lava. The 2-km-wide crater floor was fully covered with fresh lava by 11 July. On 27 July the dense ash plume was drifting NW and the highest heat was concentrated in the SE quadrant of the crater. Courtesy of Sentinel Hub Playground.

Figure 26. Substantial plumes of sulfur dioxide from Raung were measured by the OMI instrument on the AURA satellite during July and August 2015. The first plumes were measured in mid-June; they intensified during the second half of July and the first week of August, but had decreased by mid-August. Wind directions were highly variable throughout the period. The date is recorded above each image. Courtesy of NASA Global Sulfur Dioxide Page.

Significant ash emissions continued into early August 2015 with numerous flight cancellations. The Darwin VAAC reported ash plumes rising to 5.2 km altitude and extending as far as 750 km SE during the first two weeks in August (figure 27). Satellite imagery indicated a small ash plume drifting W from the center of the crater on 12 August and weak thermal anomalies along the E and S rim of the floor of the crater (figure 28). The summit crater was covered with fresh lava on 14 August when viewed by visitors, and ash emissions rose a few hundred meters above the crater rim from a vent in the SW side of the pyroclastic cone (figure 29). The visitors observed pulsating ash emissions rising from the SW vent on the large double-crater new cinder cone. The larger vent to the NE was almost entirely inactive except for two small, weakly effusive vents on its inner walls.

Figure 27. A dense ash plume drifted many kilometers S from Raung on 2 August 2015 in this view from nearly 100 km W. Incandescence at the summit indicated ongoing activity from the major 2015 eruption. In the foreground is Lamongan volcano whose last known eruption occurred in 1898. Courtesy of Øystein Lund Andersen, used with permission.

Figure 28. Landsat-8 satellite imagery of Raung indicated a small ash plume drifting W from the center of the crater on 12 August 2015. Courtesy of Sentinel Hub Playground.

Figure 29. The summit crater of Raung on 14 August 2015 was filled with fresh lava from an eruption that began in November 2014. Ash emissions from a vent in the side of the newly grown pyroclastic cone within the crater rose a few hundred meters above the crater rim. Courtesy of Volcano Discovery.

The lengthy sequence of multiple daily VAAC reports that began in late June ended on 16 August 2015 with reports becoming more intermittent and ash plume heights rising to only 3.7-3.9 km altitude. Multiple discontinuous eruptions to 3.9 km altitude were reported on 18 August. The plumes extended about 100 km NW. The last report of an ash plume was from an airline on 22 August noting a low-level plume 50 km NW. Two MODVOLC alerts were issued that day. By 28 August only a very small steam plume was present at the center of the crater; the southern half of the edge of the crater floor still had small thermal anomalies (figure 30). The last single MODVOLC thermal alerts were on 29 August and 7 September. The Alert Level was lowered to 2 on 24 August 2015, and further lowered to 1 on 20 October 2016.

Figure 30. By 28 August 2015 only a very small steam plume was present at the center of the summit crater of Raung, and the southern half of the edge of the crater floor only had weak thermal anomalies from cooling lava. Courtesy of Sentinel Hub Playground.

Geologic Background. Raung, one of Java's most active volcanoes, is a massive stratovolcano in easternmost Java that was constructed SW of the rim of Ijen caldera. The unvegetated summit is truncated by a dramatic steep-walled, 2-km-wide caldera that has been the site of frequent historical eruptions. A prehistoric collapse of Gunung Gadung on the W flank produced a large debris avalanche that traveled 79 km, reaching nearly to the Indian Ocean. Raung contains several centers constructed along a NE-SW line, with Gunung Suket and Gunung Gadung stratovolcanoes being located to the NE and W, respectively.

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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/);Google Earth (URL: https://www.google.com/earth/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/, https://earthobservatory.nasa.gov/images/86213/eruption-of-raung-volcano); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Aris Yanto (URL: https://www.exploredesa.com/2012/11/mount-raung-produce-of-vulcanic-ash-plume-and-continue-eruption/); DigitalGlobe (URL: https://www.maxar.com/, https://twitter.com/Maxar/status/875449111398547457); Øystein Lund Andersen (URL: https://twitter.com/OysteinVolcano/status/1194879946042142726, http://www.oysteinlundandersen.com).

Klyuchevskoy is a frequently active stratovolcano located in northern Kamchatka. Historical eruptions dating back 3,000 years have included more than 100 flank eruptions with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks. The previous report (BGVN 45:06) described ash plumes, nighttime incandescence, and Strombolian activity. Strombolian activity, ash plumes, and a strong lava flow continued. This report updates activity from June through August 2020 using weekly and daily reports from the Kamchatkan Volcanic Eruption Response Team (KVERT), the Tokyo Volcanic Ash Advisory (VAAC), and satellite data.

Moderate explosive-effusive activity continued in June 2020, with Strombolian explosions, frequent gas-and-steam emissions that contained some amount of ash, and an active lava flow. On 1 June a gas-and-steam plume containing some ash extended up to 465 km SE and E. The lava flow descended the SE flank down the Apakhonchich chute (figure 43). Occasionally, phreatic explosions accompanied the lava flow as it interacted with snow. Intermittent ash plumes, reported throughout the month by KVERT using video and satellite data and the Tokyo VAAC using HIMAWARI-8 imagery, rose to 5.5-6.7 km altitude and drifted in different directions up to 34 km from the volcano. On 12 and 30 June ash plumes rose to a maximum altitude of 6.7 km. On 19 June, 28-30 June, and 1-3 July some collapses were detected alongside the lava flow as it continued to advance down the SE flank.

Figure 43. Gray ash plumes (left) and a lava flow descending the Apakhonchich chute on the SE flank, accompanied by a dark ash plume and Strombolian activity (right) were observed at the summit of Klyuchevskoy on 10 June 2020. Courtesy of E. Saphonova, IVS FEB RAS, KVERT.

During 1-3 July moderate Strombolian activity was observed, accompanied by gas-and-steam emissions containing ash and a continuous lava flow traveling down the Apakhonchich chute on the SE flank. On 1 July a Tokyo VAAC advisory reported an ash plume rising to 6 km altitude and extending SE. On 3 July the activity sharply decreased. KVERT reported there was some residual heat leftover from the lava flow and Strombolian activity that continued to cool through at least 13 July; KVERT also reported frequent gas-and-steam emissions, which contained a small amount of ash through 5 July, rising from the summit crater (figure 44). The weekly KVERT report on 16 July stated that the eruption had ended on 3 July 2020.

Figure 44. Fumarolic activity continued in the summit crater of Klyuchevskoy on 7 July 2020. Courtesy of KSRS ME, Russia, KVERT.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows frequent and strong thermal activity within 5 km of the summit crater from March through June followed by a sharp and sudden decline in early July (figures 45). A total of six weak thermal anomalies were detected between July and August. According to the MODVOLC thermal algorithm, a total of 111 thermal alerts were detected at or near the summit crater from 1 June to 1 July, a majority of which were due to the active lava flow on the SE flank and Strombolian explosions in the crater. Sentinel-2 thermal satellite imagery frequently showed the active lava flow descending the SE flank as a strong thermal anomaly, sometimes even through weather clouds (figure 46). These thermal anomalies were also recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data on a MIROVA graph, showing a strong cluster during June to early July, followed by a sharp decrease and then a hiatus in activity (figure 47).

Figure 45. Thermal activity at Klyuchevskoy was frequent and strong during February through June 2020, according to the MIROVA graph (Log Radiative Power). Activity sharply decreased during July through August with six low-power thermal anomalies. Courtesy of MIROVA.

Figure 46. Sentinel-2 thermal satellite images show the strong and persistent lava flow (bright yellow-orange) originating from the summit crater at Klyuchevskoy from 1 June through 1 July 2020. The lava flow was active in the Apakhonchich chute on the SE flank. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 47. Strong clusters of thermal anomalies were detected in the summit at Klyuchevskoy (red dots) during January through June 2020, as recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A). Activity sharply decreased during July through August with few low-power thermal anomalies. Courtesy of MIROVA.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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).

Fuego, located in Guatemala, is a stratovolcano that has been erupting since 2002 with historical eruptions dating back to 1531. Volcanism is characterized by major ashfalls, pyroclastic flows, lava flows, and lahars. The previous report (BGVN 45:04) described recent activity that included multiple ash explosions, block avalanches, and intermittent lava flows. This report updates activity from April through July 2020 that consisted of daily explosions, ash plumes, block avalanches ashfall, intermittent lava flows, and lahars. The primary source of information comes from the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH), the Washington Volcanic Ash Advisory Center (VAAC), and various satellite data.

Summary of activity during April-July 2020. Daily activity throughout April-July 2020 was characterized by multiple hourly explosions, ash plumes that rose to a maximum of 4.9 km altitude, incandescent pulses that reached 600 m above the crater, block avalanches into multiple drainages, and ashfall affecting nearby communities (table 21). The highest rate of explosions occurred on 2 and 3 April and 2 May with up to 16 explosions per hour. White degassing occurred frequently during the reporting period, rising to a maximum altitude of 4.5 km and drifting in multiple directions. Intermittent lava flows were observed each month in the Seca (Santa Teresa) and Ceniza drainages (figure 132); the number of flows decreased in June through July, which is represented in the MIROVA analysis of MODIS satellite data, where the strength and frequency of thermal activity slightly decreased (figure 133). Occasional lahars were detected descending several drainages on the W and SE flanks, sometimes carrying tree branches and large blocks up to 1 m in diameter.

Table 21. Activity summary by month for Fuego with information compiled from INSIVUMEH daily reports.

Figure 132. Sentinel-2 thermal satellite images of Fuego between 9 April 2020 and 13 July 2020 showing lava flows (bright yellow-orange) traveling generally S and W from the summit crater. Some lava flows were accompanied by gas emissions (9 April, 9 May, and 24 May 2020). Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 133. Thermal activity at Fuego was persistent and strong from 16 September through late May 2020, according to the MIROVA graph (Log Radiative Power). From early to mid-June activity seemed to stop briefly before resuming again at a lower rate. Courtesy of MIROVA.

Activity during April-May 2020. Activity in April 2020 consisted of 5-16 explosions per hour, generating ash plumes that rose 4.3-4.9 km altitude and drifted 8-20 km in multiple directions. Ashfall was reported in Morelia (9 km SW), Panimaché I and II (8 km SW), Sangre de Cristo (8 km WSW), Santa Sofía (12 km SW), Finca Palo Verde, San Pedro Yepocapa (8 km NW), Las Cruces Quisache (8 km NW), La Rochela, Ceylan, Osuna (12 km SW). The Washington VAAC issued multiple aviation advisories for a total of six days in April. Intermittent white gas-and-steam emissions reached 4.1-4.5 km altitude drifting in multiple directions. Incandescent ejecta was frequently observed rising 75-400 m above the crater; material ejected up to 600 m above the crater on 11 April. These constant explosions produced block avalanches that traveled down the Taniluyá (SW), Ceniza (SSW), Las Lajas (SE), Trinidad (S), Seca (W), Honda, and Santa Teresa (W) drainages. Effusive activity was reported on 6-13 and 15 April from the summit vent, traveling 150-800 m down the Ceniza drainage, accompanied by block avalanches in the front of the flow up to 1 km. Crater incandescence was also observed.

On 19-20 April a new lava flow descended the Ceniza drainage measuring 200-400 long, generating incandescent block avalanches at the front of the flow that moved up to 1 km. On 22 April lahars descended the Honda, Las Lajas, El Juté (SE), Trinidad, Ceniza, Taniluyá, Mineral, and Seca drainages and tributaries in Guacalate, Achiguate, and Pantaleón. During the evening of 23 April the rate of effusive activity increased; observatory staff observed a second lava flow in the Seca drainage was 170 m long and incandescent blocks from the flow traveled up to 600 m. Two lava flows in the Ceniza (130-400 m) and Seca (150-800 m) drainages continued from 23-28 April and had stopped by 30 April. On 30 April weak and moderate explosions produced ash plumes that rose 4.5-4.7 km altitude drifting S and SE, resulting in fine ashfall in Panimaché I, Morelia, Santa Sofía (figure 134).

Figure 134. Photo of a small ash plume rising from Fuego on 30 April 2020. Photo has been slightly color corrected. Courtesy of William Chigna, CONRED.

During May 2020, the rate of explosion remained similar, with 4-16 explosions per hour, which generated gray ash plumes that rose 4.3-4.9 km altitude and drifted 10-17 km generally W and E. Ashfall was observed in Panimaché I, La Rochela, Ceilán, Morelia, San Andrés Osuna, Finca Palo Verde, Santa Sofía, Seilán, San Pedro Yepocapa, Alotenango (8 km ENE), Ciudad Vieja (13.5 km NE), San Miguel Dueñas (10 km NE), and Antigua Guatemala (18 km NE). The Washington VAAC issued volcanic ash advisory notices on six days in May. White gas-and-steam emissions continued, rising 4-4.5 km altitude drifting in multiple directions. Incandescent ejecta rose 100-400 m above the crater, accompanied by some crater incandescence and block avalanches in the Trinidad, Taniluyá, Ceniza, Las Lajas, Santa Teresa, Seca, and Honda drainages that moved up to 1 km and sometimes reached vegetated areas.

During 8-11 May a new 400 m long lava flow was detected in the Ceniza drainage, accompanied by constant crater incandescence and block avalanches traveling up to 1 km, according to INSIVUMEH. On 8 and 17 May moderate to strong lahars descended the Santa Teresa and Mineral drainages on the W flank and on 21 May they descended the Las Lajas drainage on the E flank and the Ceniza drainage on the SW flank. During 20-24 May a 100-400 m long lava flow was reported in the Ceniza drainage alongside degassing and avalanches moving up to 1 km and during 25-26 May a 150 m long lava flow was reported in the Seca drainage.

Activity during June-July 2020. The rate of explosions in June 2020 decreased slightly to 3-15 per hour, generating gray ash plumes that rose 4.2-4.9 km altitude and drifted 10-26 km in multiple directions (figure 135). As a result, intermittent ashfall was reported in San Pedro Yepocapa, Sangre de Cristo, Panimaché I and II, Morelia, Finca Palo Verde, El Porvenir (8 km ENE), Yucales (12 km SW), Santa Emilia, Santa Sofia, according to INSIVUMEH. VAAC advisories were published on eight days in June. Degassing persisted in the summit crater that rose 4.1-4.5 km altitude extending in different directions. Crater incandescence was observed occasionally, as well as incandescent pulses that rose 100-300 m above the crater. Block avalanches were observed descending the Seca, Taniluyá, Ceniza, Trinidad, Las Lajas, Santa Teresa, and Honda drainages, which could sometimes carry blocks up to 1 km in diameter.

On 2 June at 1050 a weak to moderate lahar was observed in the Las Lajas drainage on the SE flank. On 5 June, more lahars were detected in the Seca and Mineral drainages on the W flanks. A new lava flow was detected on 12 June, traveling 250 m down the Seca drainage on the NW flank, and accompanied by constant summit crater incandescence and gas emissions. The flow continued into 14 June, lengthening up to 300 m long. On 24 June weak and moderate explosions produced ash plumes that rose 4.3-4.7 km altitude drifting W and SW (figure 135). On 29 June at 1300 a weak lahar was reported in the Seca, Santa Teresa, and Mineral drainages on the W flank.

Figure 135. Examples of small ash plumes at Fuego on 15 (left) and 24 (right) June 2020. Courtesy of William Chigna, CONRED.

Daily explosions and ash plumes continued through July 2020, with 1-15 explosions per hour and producing consistent ash plumes 4-4.9 km altitude drifting generally W for 10-24 km. These explosions resulted in block avalanches that descended the Trinidad, Taniluyá, Ceniza, Honda, Las Lajas, Seca, and Santa Teresa drainages. The number of white gas emissions decrease slightly compared to previous months and 4-4.4 km altitude. VAAC advisories were distributed on twenty different days in July. Incandescent ejecta was observed rising 100-350 m above the crater. Occasional ashfall was observed in Panimaché I and II, Morelia, Santa Sofía, Finca Palo Verde, Sangre de Cristo, San Pedro Yepocapa, and El Porvenir, according to INSIVUMEH.

On 4 July in the early morning, a lava flow began in the Seca drainage, which also produced some fine ash particles that drifted W. The lava flow continued into 5 July, measuring 150 m long. On the same day, weak to moderate lahars traveled only 20 m, carrying tree branches and blocks measuring 30 cm to 1 m. On 14, 24, and 29 July more lahars were generated in the Las Lajas drainages on the former date and both the Las Lajas and El Jute drainages on the two latter dates.

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous historical eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); 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); William Chigna, CONRED (URL: https://twitter.com/william_chigna).

Japan’s Nishinoshima volcano, located about 1,000 km S of Tokyo in the Ogasawara Arc, erupted above sea level in November 2013 after 40 years of dormancy. Activity lasted through November 2015 and returned during mid-2017, continuing the growth of the island with ash plumes, ejecta, and lava flows. A short eruptive event in July 2018 produced a small lava flow and vent on the side of the pyroclastic cone. The next eruption of ash plumes, incandescent ejecta, and lava flows began in early December 2019, resulting in significant growth of the island. This report covers the ongoing activity from March-August 2020 when activity decreased. Information is provided primarily from Japan Meteorological Agency (JMA) monthly reports and the Japan Coast Guard (JCG), which makes regular overflights to make observations.

Renewed eruptive activity that began on 5 December 2019 continued during March-August 2020 but appeared to wane by the end of August. Major lava flows covered all sides of the island, with higher levels of activity during late June and early July. Ash emissions increased significantly during June and produced dense black ash plumes that rose up to 6 km altitude in early July. Explosive activity produced lightning and incandescent jets that rose 200 m and large bombs that fell to the base of the pyroclastic cone. Lava flow activity diminished at the end of July. Ash emissions decreased throughout August and appeared to cease after 27 August 2020. The MIROVA plot clearly reflects the high levels of thermal activity between December 2019 and August 2020 (figure 80); this event was reported by JMA as the largest eruption recorded to date. Sulfur dioxide emissions were very high during late June through early August, producing emissions that drifted across much of the western Pacific region.

Figure 80. The MIROVA plot of thermal activity at Nishinoshima from 14 October 2019 through August 2020 indicates the high levels between early December 2019 and late July 2020 that resulted from the eruption of numerous lava flows on all flanks of the pyroclastic cone, significantly enlarging the island. Courtesy of MIROVA.

The Japan Coast Guard (JCG) conducted overflights of Nishinoshima on 9 and 15 March 2020 (figure 81). During both visits they observed eruptive activity from the summit crater, including ash emissions that rose to an altitude of approximately 1,000 m and lava flowing down the N and SE flanks (figure 82). Large ejecta was scattered around the base of the pyroclastic cone. The lava flowing north had reached the coast and was producing vigorous steam as it entered the water on 9 March; whitish gas emissions were visible on the N flank of the cone at the source of the lava flow (figure 83). On 9 March yellow-green discolored water was noted off the NE shore. The lava flow on the SE coast produced a small amount of steam at the ocean entry point and a strong signal in thermal imagery on 15 March (figure 84). Multiple daily MODVOLC thermal alerts were issued during 1-10, 17-24, and 27-30 March. Landsat-8 visual and thermal imagery on 30 March 2020 confirmed that thermal anomalies on the N and SE flanks of the volcano continued.

Figure 81. The Japan Coast Guard conducted an overflight of Nishinoshima on 9 March 2020 and observed ash emissions rising 1,000 m above the summit and lava flowing into the ocean off the N flank of the island. Courtesy of Japan Coast Guard (JCG) and JMA.

Figure 82. Lava flows at Nishinoshima during February and March 2020 were concentrated on the N and SE flanks. The areas in blue indicate topographical changes due to lava flows and pyroclastic deposits from the previous measurement. The growth of the SE-flank flow decreased during March while the N-flank flow rate increased significantly. Left image shows changes between 14 and 28 February and right image shows the differences between 28 February and 13 March. The correlated image analysis uses ALOS-2 / PALSAR-2 and is carried out with the cooperation of JAXA through the activities of the Satellite Analysis Group of the Volcano Eruption Prediction Liaison Committee. The software was developed by the Japan National Research Institute for Earth Science and Disaster Prevention and uses the technical data C1-No 478 of the Geospatial Information Authority of Japan. Courtesy of JAXA and JMA (Volcanic activity commentary material on Nishinoshima, March 2020).

Figure 83. Vigorous steam emissions on the N flank of Nishinoshima on 9 March 2020 were caused by the active flow on the N flank. Whitish steam and gas midway up the flank indicated the outlet of the flow. Ash emissions rose from the summit crater and drifted E. Courtesy of Japan Coast Guard and JMA.

Figure 84. Infrared imagery from 15 March 2020 at Nishinoshima showed the incandescent lava flow on the SE flank (foreground), blocks of ejecta scattered around the summit and flanks of the pyroclastic cone, and the active N-flank flow (left). Courtesy of Japan Coast Guard and JMA.

Ash emissions were not observed at Nishinoshima during JCG overflights on 6, 16, and 19 April 2020, but gas-and-steam emissions were noted from the summit crater, and a yellow discoloration interpreted by JMA to be sulfur precipitation was observed near the top of the pyroclastic cone. The summit crater was larger than during previous visits. Steam plumes seen each of those days on the N and NE coasts suggested active ocean entry of lava flows (figure 85). A lava flow was observed emerging from the E flank of the cone and entering the ocean on the E coast on 19 and 29 April (figure 86). During the overflight on 29 April observers noted lava flowing southward from a vent on the E flank of the pyroclastic cone. A narrow, brown, ash plume was visible on 29 April at the summit crater rising to an altitude of about 1,500 m. Thermal observations indicated continued flow activity throughout the month. Multiple daily MODVOLC thermal alerts were recorded during 2-6, 10-11, 17-23, and 28-30 April. Significant growth of the pyroclastic cone occurred between early February and late April 2020 (figure 87).

Figure 85. Multiple entry points of lava flowed into the ocean producing jets of steam along the N flank of Nishinoshima on 6 April 2020. Courtesy of JCG and JMA.

Figure 86. Lava flowed down the E flank of Nishinoshima from a vent below the summit on 19 April 2020. The ocean entry produced a vigorous steam plume (left). Courtesy of JCG.

Figure 87. The pyroclastic cone at Nishinoshima grew significantly in size between 4 February (left), 9 March (middle), and 19 April 2020 (right). View is to the E. Courtesy of JMA and JCG.

Infrared satellite imagery from 17 May 2020 showed a strong thermal anomaly at the summit and hot spots on the NW flank indicative of flows. Visible imagery confirmed emissions at the summit and steam plumes on the NW flank (figure 88). Gray ash plumes rose to about 1,800 m altitude on 18 May during the only overflight of the month made by the Japan Coast Guard. In addition, white gas emissions rose from around the summit area and large blocks of ejecta were scattered around the base of the pyroclastic cone (figure 89). Steam from ocean-entry lava on the N flank was reduced from previous months, but a new flow moving NW into the ocean was generating a steam plume and a strong thermal signature. Multi-pixel thermal alerts were measured by the MODVOLC system on 1-3, 9-10, 13-15, 18, and 26-30 May. Sulfur dioxide emissions had been weak and intermittent from March through early May 2020 but became more persistent during the second half of May. Although modest in size, the plumes were detectible hundreds of kilometers away from the volcano (figure 90).

Figure 88. Landsat-8 satellite imagery of Nishinoshima from 17 May 2020 confirmed continued eruptive activity. Visible imagery showed emissions at the summit and steam plumes on the NW flank (left) and infrared imagery showed a strong thermal anomaly at the summit and anomalies on the NW flank indicative of lava flows (right). Courtesy of Sentinel Hub Playground.

Figure 89. Lava continued to enter the ocean at Nishinoshima during May 2020. A new lava flow on the NW flank produced a strong steam plume at an ocean entry (left) on 18 May 2020. In addition to a light gray plume of gas and ash, steaming blocks of ejecta were visible on the flanks of the pyroclastic cone. The strong thermal signature of the NW-flank flow in infrared imagery that same day showed multiple new lobes flowing to the ocean (right). Courtesy of JCG and JMA.

Figure 90. Small but distinct SO2 emissions from Nishinoshima were recorded by the TROPOMI instrument on the Sentinel-5P satellite during the second half of May 2020. The plumes drifted tens to hundreds of kilometers away from the volcano in multiple directions as the wind directions changed. Nishinoshima is about 1,000 kilometers S of Tokyo. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity increased significantly during June 2020. Satellite imagery from 2 June revealed two intense thermal anomalies at the summit indicating a new crater, and lava flows active on the NW and NE flanks, all showing gas or steam emissions (figure 91). Dense brown and gray ash emissions were observed rising from the summit crater during JCG overflights on 7 and 15 June (figure 92). Plumes reached at least 1,500 m altitude, and ejecta reached the base of the pyroclastic cone. Between 5 and 19 June the lava flow on the WNW coast slowed significantly, while the flows to the N and E became significantly more active (figure 93). The Tokyo VAAC reported the first ash plume since mid-February on 12 June rose to 2.1 km and drifted NE. On 14 June they reported an ash plume extending E at 2.7 km altitude. Dense emissions continued to drift N and E at 2.1-2.7 km altitude until the last week of the month. The JCG overflight on 19 June observed darker ash emissions than two weeks earlier that drifted at least 180 km NE (figure 94) and incandescent tephra that exploded from the enlarged summit area where three overlapping craters trending E-W had formed.

Figure 91. Landsat-8 satellite imagery on 2 June 2020 confirmed ongoing activity at Nishinoshima. Lava produced ocean-entry steam on the NE coast; a weak plume on the NW coast suggested reduced activity in that area (left). In addition, a dense steam plume drifted E from the summit, while a fainter plume adjacent to it also drifted E. The infrared image (right) indicated two intense anomalies at the summit, and weaker anomalies from lava flows on the NW and NE flanks. Courtesy of Sentinel Hub Playground.

Figure 92. Lava flows at Nishinoshima entered the ocean on the N and NE coasts (left) on 7 June 2020, and dense, gray ash emissions rose to at least 1,500 m altitude. Courtesy of JCG.

Figure 93. The lava flow on the WNW coast of Nishinoshima slowed significantly in early June 2020, while the flows to the N and E covered large areas of those flanks between 5 and 19 June. The areas in blue indicate topographical changes due to lava flows and pyroclastic deposits from the previous measurement. Left image shows the differences between 22 May and 5 June and right image shows changes between 5 and 19 June. The correlated image analysis uses ALOS-2 / PALSAR-2 and is carried out with the cooperation of JAXA through the activities of the Satellite Analysis Group of the Volcano Eruption Prediction Liaison Committee. The software was developed by the National Research Institute for Earth Science and Disaster Prevention and uses the technical data C1-No 478 of the Geospatial Information Authority of Japan. Courtesy of JAXA and JMA (Volcanic activity commentary material on Nishinoshima, June 2020).

Figure 94. Ash emissions and explosive activity at Nishinoshima increased significantly during the second half of June. Dense black ash rose to 2.4 km altitude and drifted at least 180 km to the NE on 19 June 2020. Vigorous white steam plumes rose from the ocean on the E flank where a lava flow entered the ocean. Courtesy of JCG.

The Tokyo VAAC reported ash emissions that rose to 4.6 km altitude and drifted NE on 25 June. For the remainder of the month they rose to 2.7-3.9 km altitude and drifted N and NE. By the time of the JCG overflight on 29 June, the new crater that had opened on the SW flank had merged with the summit crater (figure 95). Dense black ash emissions rose to 3.4 km altitude and drifted NE, lava flowed down the SW flank into the ocean producing violent steam explosions, and incandescent tephra was scattered at least 200 m from the base of the pyroclastic cone from ongoing explosive activity (figure 96). Multiple layers of recent flow activity were visible along the SW coast (figure 97). Yellow-green discolored water encircled the entire island with a width of 1,000 m.

Figure 95. The new crater on the SW flank of Nishinoshima had merged with the summit crater by 29 June 2020. Courtesy of JCG and JMA.

Figure 96. Dense black ash emissions rose to 3.4 km altitude and drifted NE from the summit of Nishinoshima on 29 June 2020. Lava flowed down the SW flank into the ocean producing steam explosions, and incandescent tephra was scattered at least 200 m from the base of the pyroclastic cone from ongoing explosive activity at the summit (inset). Courtesy of JCG.

Figure 97. Different textures of lava flows were visible along the SW flank of Nishinoshima on 29 June 2020. The active flow appeared dark brown and blocky, and produced steam explosions at the ocean entry site (right). Slightly older, brownish-red lava (center) still produced steam along the coastline. Courtesy of JCG.

MODVOLC thermal alerts reached their highest levels of the period during June 2020 with multi-pixel alerts recorded on most days of the month. Sulfur dioxide emissions increased steadily throughout June to the highest levels recorded for Nishinoshima; by the end of the month plumes of SO₂ were drifting thousands of kilometers across the Pacific Ocean and being captured in complex atmospheric circulation currents (figure 98).

Figure 98. Sulfur dioxide emissions at Nishinoshima increased noticeably during the second half of June 2020 as measured by the TROPOMI instrument on the Sentinel-5P satellite. Atmospheric circulation currents produced long-lived plumes that drifted thousands of kilometers from the volcano. Nishinoshima is 1,000 km S of Tokyo. Courtesy of NASA Sulfur Dioxide Monitoring Page.

By early July 2020, satellite data indicated that the NE quadrant of the island was covered with ash, and a large amount of new lava had flowed down the SW flank, creating fans extending into the ocean (figure 99). The Tokyo VAAC reported ash emissions that rose to 3.7-4.9 km altitude and drifted N during 1-6 July. The altitude increased to 6.1 km during 8 and 9 July, and ranged from 4.6-6.1 km during 10-14 July while the drift direction changed to NE. The marine meteorological observation ship "Ryofu Maru" reported on 11 July that dense black ash was continuously erupting from the summit crater and drifting W at 1,700 m altitude or higher. They observed large volcanic blocks scattered around the base of the pyroclastic cone, and ash falling from the drifting plume. During the night of 11 July incandescent lava and volcanic lightning rose to about 200 m above the crater rim (figure 100).

Figure 99. By early July 2020, satellite data from Nishinoshima indicated that the NE quadrant of the island was covered with ash, and a large amount of new lava had flowed down the SW flank creating fans extending into the ocean. The areas in blue indicate topographical changes due to lava flows and pyroclastic deposits from the previous measurement. Left image shows differences between 5 and 19 June and the right image shows changes between 19 June and 3 July that included abundant ashfall on the NE flank. The correlated image analysis uses ALOS-2 / PALSAR-2 and is carried out with the cooperation of JAXA through the activities of the Satellite Analysis Group of the Volcano Eruption Prediction Liaison Committee. The software was developed by the National Research Institute for Earth Science and Disaster Prevention and uses the technical data C1-No 478 of the Geospatial Information Authority of Japan. Courtesy of JAXA and JMA (Volcanic activity commentary material on Nishinoshima, June 2020).

Figure 100. High levels of activity were observed at Nishinoshima by crew members aboard the marine meteorological observation ship "Ryofu Maru” on 11 July 2020. Abundant ash emissions filled the sky and tephra fell out of the ash cloud for several kilometers downwind (left, seen from 6 km NE). Incandescent explosions rose as much as 200 m into the night sky (right, seen from 4 km E). Courtesy of JMA.

During 16-26 July 2020 the Tokyo VAAC reported ash emissions at 3.7-5.2 km altitude that drifted primarily N and NE. The vessel "Keifu Maru" passed Nishinoshima on 20 July and crewmembers observed continuing emissions from the summit of dense, black ash. JCG observed an ash plume rising to at least 2.7 km altitude during their overflight of 20 July. A large dome of fresh lava was visible on the SW flank of the island (figure 101). Lower ash emissions from 2.4-3.7 km altitude were reported by the Tokyo VAAC during 27-29 July, but the altitude increased to 5.5-5.8 km during the last two days of the month. During an overflight on 30 July by the National Research Institute for Earth Science and Disaster Prevention, dark and light gray ash emissions rose to 3.0 km altitude, but no flowing lava or large bombs were observed. They also noted thick deposits of brownish-gray ash on the N side of the island (figure 102).

Figure 101. JCG observed an ash plume at Nishinoshima rising to at least 2.7 km altitude during their overflight of 20 July 2020. A large dome of fresh lava was visible on the SW flank of the island. Courtesy of JCG.

Figure 102. Ash emissions changed from dark to light gray on 30 July 2020 at Nishinoshima as seen during an overflight by the National Research Institute for Earth Science and Disaster Prevention. Thick brownish-gray ash was deposited over the lava on the N side of the island. Courtesy of JMA (Information on volcanic activity in Nishinoshima, July 2020).

JMA reported a sharp decrease in the lava eruption rate during July with thermal anomalies decreasing significantly mid-month. Multiple daily MODVOLC thermal alerts were recorded during the first half of the month but were reduced to two or three per day during the last third of July. Throughout July, SO₂ emissions were the highest recorded in modern times for Nishinoshima. High levels of emissions were measured daily, producing streams with high concentrations of SO₂ that were caught up in rotating wind currents and drifted thousands of kilometers across the Pacific Ocean (figure 103).

Figure 103. Complex atmospheric wind patterns carried the largest SO2 plumes recorded from Nishinoshima thousands of kilometers around the western Pacific Ocean during July 2020. Nishinoshima is about 1,000 km S of Tokyo. Top and bottom left images both show 6 July but at different scales. Courtesy of NASA Sulfur Dioxide Monitoring Page.

Thermal activity was greatly reduced during August 2020. Only one or two MODVOLC alerts were issued on 11, 18, 20, 21, 29, and 30 August, and no fresh lava flows were observed. The Tokyo VAAC reported ash emissions daily from 1-20 August. Plume heights were 4.9-5.8 km altitude during 1-4 August after which they dropped to 3.9 km altitude through 15 August. A brief pulse to 4.6 km altitude was recorded on 16 August, but then they dropped to 3.0 km or lower through the end of the month and became intermittent. The last ash emission was reported at 2.7 km altitude drifting W on 27 August.

No eruptive activity was observed during the Japan Coast Guard overflights on 19 and 23 August. High temperatures were measured on the inner wall of the summit crater on 19 August (figure 104). Steam plumes rose from the summit crater to about 2.5 km altitude during both visits (figure 105). Yellow-green discolored water was present on 23 August around the NW and SW coasts. No lava flows were observed, and infrared cameras did not measure any surface thermal anomalies outside of the crater. Very high levels of SO₂ emissions were measured through 12 August when they began to noticeably decrease (figure 106). By the end of the month, only small amounts of SO₂ were measured in satellite data.

Figure 104. A strong thermal anomaly was still present inside the newly enlarged summit crater at Nishinoshima on 19 August 2020. Courtesy of JCG.

Figure 105. Only steam plumes were observed rising from the summit crater of Nishinoshima during the 23 August 2020 overflight by the Japan Coast Guard. Courtesy of JCG.

Figure 106. Sulfur dioxide emissions remained very high at Nishinoshima until 12 August 2020 when they declined sharply. Circulating air currents carried SO2 thousands of kilometers around the western Pacific region. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

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 Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG), Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo18-e1.htm); 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Japan Aerospace Exploration Agency-Earth Observation Research Center (JAXA-EORC), 7-44-1 Jindaiji Higashi-machi, Chofu-shi, Tokyo 182-8522, Japan (URL: http://www.eorc.jaxa.jp/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Turrialba is a stratovolcano located in Costa Rica that overlooks the city of Cartago. Three well-defined craters occur at the upper SW end of a broad 800 x 2,200 m summit depression that is breached to the NE. Activity described in the previous report primarily included weak ash explosions and minor ash emissions (BGVN 44:11). This reporting period updates information from November 2019-August 2020; volcanism dominantly consists of ash emissions during June-August, based on information from daily and weekly reports by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and satellite data.

Volcanism during November 2019 through mid-June was relatively low, dominated by low SO₂ emissions (100-300 tons/day) and typical low seismic tremors. A single explosion was recorded at 1850 on 7 December 2019, and two gas-and-steam plumes rose 800 m and 300 m above the crater on 25 and 27 December, respectively. An explosion was detected on 29 January 2020 but did not result in any ejecta. An overflight during the week of 10 February measured the depth of the crater (140 m); since the previous measurements made in February 2019 (220 m), the crater has filled with 80 m of debris due to frequent collapses of the NW and SE internal crater walls. Beginning around February and into at least early May 2020 the Sentinel-2 MODIS Thermal Volcanic Activity graph provided by the MIROVA system detected a small cluster of thermal anomalies (figure 52). Some of these anomalies were faintly registered in Sentinel-2 thermal satellite imagery during 10 and 25 April, with a more distinct anomaly occurring on 15 May (figure 53).

Figure 52. A small cluster of thermal anomalies were detected in the summit area of Turrialba (red dots) during February-May 2020 as recorded by the Sentinel-2 MODIS Thermal Volcanic Activity data (bands 12, 11, 8A). Courtesy of MIROVA.

Figure 53. Sentinel-2 thermal satellite imagery detected minor gas-and-steam emissions (left) and a weak thermal anomaly (right) in the summit crater at Turrialba on 11 January and 15 May 2020, respectively. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

On 18 June activity increased, which marked the start of a new eruptive period that produced ash emissions rising 100 m above the crater rim at 1714, 1723, and 1818. The next morning, 19 June, two more events at 1023 and 1039 resulted in ash emissions rising 100 m above the crater. During 23-26 June small ash emissions continued to occur each day, rising no higher than 100 m above the crater. A series of small ash eruptions that rose 100 m above the crater occurred during 28 and 29 June; four events were recorded at 0821, 1348, 1739, and 2303 on 28 June and five more were recorded at 0107, 0232, 0306, 0412, and 0818 on 29 June. The two events at 0107 and 0412 were accompanied by ballistics ejected onto the N wall of the crater, according to OVSICORI-UNA.

Almost daily ash emissions continued during 1-7 July, rising less than 100 m above the crater; no ash emissions were observed on 3 July. On 6 July, gas-and-steam and ash emissions rose hundreds of meters above the crater at 0900, resulting in local ashfall. Passive gas-and-steam emissions with minor amounts of ash were occasionally visible during 9-10 July. On 14 July an eruptive pulse was observed, generating brief incandescence at 2328, which was likely associated with a small ash emission. Dilute ash emissions at 1028 on 16 July preceded an eruption at 1209 that resulted in an ash plume rising 200 m above the crater. Ash emissions of variable densities continued through 20 July rising as high as 200 m above the crater; on 20 July incandescence was observed on the W wall of the crater. An eruptive event at 0946 on 29 July produced an ash plume that rose 200-300 m above the crater rim. During 30-31 July a series of at least ten ash eruptions were detected, rising no higher than 200 m above the crater, each lasting less than ten minutes. Some incandescence was visible on the SW wall of the crater during this time.

On 1 August at 0746 an ash plume rose 500 m above the crater. During 4-5 August a total of 19 minor ash emissions occurred, accompanied by ash plumes that rose no higher than 200 m above the crater. OVSICORI-UNA reported on 21 August that the SW wall of the crater had fractured; some incandescence in the fracture zone had been observed the previous month. Two final eruptions were detected on 22 and 24 August at 1253 and 2023, respectively. The eruption on 24 August resulted in an ash plume that rose to a maximum height of 1 km above the crater.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

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

Etna, located on the island of Sicily, Italy, is a stratovolcano that has had historical eruptions dating back 3,500 years. Its most recent eruptive period began in September 2013 and has continued through July 2020, characterized by Strombolian explosions, lava flows, and ash plumes. Activity has commonly originated from the summit areas, including the Northeast Crater (NEC), the Voragine-Bocca Nuova (or Central) complex (VOR-BN), the Southeast Crater (SEC, formed in 1978), and the New Southeast Crater (NSEC, formed in 2011). The newest crater, referred to as the "cono della sella" (saddle cone), emerged during early 2017 in the area between SEC and NSEC. Volcanism during this reporting period from April through July 2020 includes frequent Strombolian explosions primarily in the Voragine and NSEC craters, ash emissions, some lava effusions, and gas-and-steam emissions. Information primarily comes from weekly reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV).

Summary of activity during April-July 2020. Degassing of variable intensity is typical activity from all summit vents at Etna during the reporting period. Intra-crater Strombolian explosions and ash emissions that rose to a maximum altitude of 5 km on 19 April primarily originated from the Voragine (VOR) and New Southeast Crater (NSEC) craters. At night, summit crater incandescence was occasionally visible in conjunction with explosions and degassing. During 18-19 April small lava flows were observed in the VOR and NSEC craters that descended toward the BN from the VOR Crater and the upper E and S flanks of the NSEC. On 19 April a significant eruptive event began with Strombolian explosions that gradually evolved into lava fountaining activity, ejecting hot material and spatter from the NSEC. Ash plumes that were produced during this event resulted in ashfall to the E of Etna. The flows had stopped by the end of April; activity during May consisted of Strombolian explosions in both the VOR and NSEC craters and intermittent ash plumes rising 4.5 km altitude. On 22 May Strombolian explosions in the NSEC produced multiple ash plumes, which resulted in ashfall to the S. INGV reported that the pit crater at the bottom of BN had widened and was accompanied by degassing. Explosions with intermittent ash emissions continued during June and July and were primarily focused in the VOR and NSEC craters; mild Strombolian activity in the SEC was reported in mid-July.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows multiple episodes of thermal activity throughout the reporting period (figure 296). In early April, the frequency and power of the thermal anomalies began to decrease through mid-June; in July, they had increased in power again but remained less frequent compared to activity in January through March. According to the MODVOLC thermal algorithm, a total of seven alerts were detected in the summit craters during 10 April (1), 17 April (1), 24 April (2), 10 July (1), 13 July (1), and 29 July (1) 2020. These thermal hotspots were typically registered during or after a Strombolian event. Frequent Strombolian activity contributed to distinct SO₂ plumes that drifted in different directions (figure 297).

Figure 296. Multiple episodes of varying thermal activity at Etna from 14 October 2019 through July 2020 were reflected in the MIROVA data (Log Radiative Power). In early April, the frequency and power of the thermal anomalies decreased through mid-June. In July, the thermal anomalies increased in power, but did not increase in frequency. Courtesy of MIROVA.

Figure 297. Distinct SO2 plumes from Etna were detected on multiple days during April to July 2020 due to frequent Strombolian explosions, including, 24 April (top left), 9 May (top right), 25 June (bottom left), and 21 July (bottom right) 2020. Captured by the TROPOMI instrument on the Sentinel 5P satellite, courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during April-May 2020. During April, INGV reported Strombolian explosions that produced some ash emissions and intra-crater effusive activity within the Voragine Crater (VOR) and abundant degassing from the New Southeast Crater (NSEC), Northeast Crater (NEC), and from two vents on the cono della sella (saddle cone) that were sometimes accompanied by a modest amount of ash (figure 298). At night, summit crater incandescence was observed in the cono della salla. The Strombolian activity in the VOR built intra-crater scoria cones while lava flows traveled down the S flank of the largest, main cone. On 18 April effusive activity from the main cone in the VOR Crater traveled 30 m toward the Bocca Nuova (BN) Crater; the pit crater at the bottom of the BN crater had widened compared to previous observations. A brief episode of Strombolian explosions that started around 0830 on 19 April in the NSEC gradually evolved into modest lava fountaining activity by 0915, rising to 3 km altitude and ejecting bombs up to 100 m (figure 299). A large spatter deposit was found 50 m from the vent and 3-4 small lava flows were descending the NSEC crater rim; two of these summit lava flows were observed at 1006, confined to the upper E and S flanks of the cone. Around 1030, one or two vents in the cono della sella produced a gas-and-steam and ash plume that rose 5 km altitude and drifted E, resulting in ashfall on the E flank of Etna in the Valle del Bove, as well as between the towns of Zafferana Etnea (10 km SE) and Linguaglossa (17 km NE). At night, flashes of incandescence were visible at the summit. By 1155, the lava fountaining had gradually slowed, stopping completely around 1300. The NEC continued to produce gas-and-steam emissions with some intra-crater explosive activity. During the week of 20-26 April, Strombolian activity in the VOR intra-crater scoria cone ejected pyroclastic material several hundred meters above the crater rim while the lava flows had significantly decreased, though continued to travel on the E flank of the main cone. Weak, intra-crater Strombolian activity with occasional ash emissions and nightly summit incandescence were observed in the NSEC (figure 300). By 30 April there were no longer any active lava flows; the entire flow field had begun cooling. The mass of the SO₂ emissions varied in April from 5,000-15,000 tons per day.

Figure 298. Photos of Strombolian explosions at Etna in the Voragine Crater (top left), strong degassing at the Northeast Crater (NEC) (top right), and incandescent flashes and Strombolian activity in the New Southeast Crater (NSEC) seen from Tremestieri Etneo (bottom row) on 10 April 2020. Photos by Francesco Ciancitto (top row) and Boris Behncke (bottom row), courtesy of INGV.

Figure 299. Strombolian activity at Etna’s “cono della sella” of the NSEC crater on 19 April 2020 included (a-b) lava fountaining that rose 3 km altitude, ejecting bomb-sized material and a spatter deposit captured by the Montagnola (EMOV) thermal camera. (c-d) An eruptive column and increased white gas-and-steam and ash emissions were captured by the Montagnola (EMOV) visible camera and (e-f) were also seen from Tremestieri Etneo captured by Boris Behncke. Courtesy of INGV (Report 17/2020, ETNA, Bollettino Settimanale, 13/04/2020 – 19/04/2020, data emissione 21/04/2020).

Figure 300. Webcam images showing intra-crater explosive activity at Etna in the Voragine (VOR) and New Southeast Crater (NSEC) on 24 April 2020 captured by the (a-b) Montagnola and (c) Monte Cagliato cameras. At night, summit incandescence was visible and accompanied by strong degassing. Courtesy of INGV (Report 18/2020, ETNA, Bollettino Settimanale, 20/04/2020 – 26/04/2020, data emissione 28/04/2020).

Strombolian explosions produced periodic ash emissions and ejected mild, discontinuous incandescent material in the VOR Crater; the coarse material was deposited onto the S flank of BN (figure 301). Pulsating degassing continued from the summit craters, some of which were accompanied by incandescent flashes at night. The Strombolian activity in the cono della sella occasionally produced reddish ash during 3-4 May. During 5 and 8 May, there was an increase in ash emissions at the NSEC that drifted SSE. A strong explosive event in the VOR Crater located E of the main cone produced a significant amount of ash and ejected coarse material, which included blocks and bombs measuring 15-20 cm, that fell on the W edge of the crater, as well as on the S terrace of the BN Crater (figure 302).

Figure 301. Photos of Strombolian explosions and summit incandescence at Etna on 4 May (left) and during the night of 11-12 May. Photos by Gianni Pennisi (left) and Boris Behncke (right, seen from Tremestieri Etneo). Courtesy of INGV.

Figure 302. A photo on 5 May (left) and thermal image on 8 May (right) of Strombolian explosions at Etna in the Voragine Crater accompanied by a dense, gray ash plume. Photo by Daniele Andronico. Courtesy of INGV (Report 20/2020, ETNA, Bollettino Settimanale, 04/05/2020 – 10/05/2020, data emissione 12/05/2020).

On 10 May degassing continued in the NSEC while Strombolian activity fluctuated in both the VOR and NSEC Craters, ejecting ballistics beyond the crater rim; in the latter, some of the blocks fell back in, accumulated on the edge, and rolled down the slopes (figure 303). During the week of 11-17 May, eruptive activity at the VOR Crater was the lowest observed since early March; there were 4-5 weak, low intensity pulses not accompanied by bombs or ashfall in the VOR Crater. Degassing continued in the BN Crater. The crater of the cono della sella had widened further N following collapses due to the Strombolian activity, which exposed the internal wall.

Figure 303. Map of the summit craters of Etna showing the active vents, the area of cooled lava flows (light green), and the location of the widening pit crater in the Bocca Nuova (BN) Crater (light blue circle) updated on 9 May 2020. The base is modified from a 2014 DEM created by Laboratorio di Aerogeofisica-Sezione Roma 2. Black hatch marks indicate the crater rims: BN = Bocca Nuova, with NW BN-1 and SE BN-2; VOR = Voragine; NEC = North East Crater; SEC = South East Crater; NSEC = New South East Crater. Red circles indicate areas with ash emissions and/or Strombolian activity, yellow circles indicate steam and/or gas emissions only. Courtesy of INGV (Report 29/2020, ETNA, Bollettino Settimanale, 06/07/2020 – 12/07/2020, data emissione 14/07/2020).

On 18 May an ash plume from the NSEC rose 4.5 km altitude and drifted NE. Strombolian explosions on 22 May at the NSEC produced multiple ash plumes that rose 4.5 km altitude and drifted S and SW (figure 304), depositing a thin layer of ash on the S slope, and resulting in ashfall in Catania (27 km S). Explosions from the VOR Crater had ejected a deposit of large clasts (greater than 30 cm) on the NE flank, between the VOR Crater and NEC on 23 May. INGV reported that the pit crater in the BN continued to widen and degassing was observed in the NSEC, VOR Crater, and NEC. During the week of 25-31 May persistent visible flashes of incandescence at night were observed, which suggested there was intra-crater Strombolian activity in the SEC and NSEC. The mass of the SO₂ plumes varied between 5,000-9,000 tons per day.

Figure 304. Photo of repeated Strombolian activity and ash emissions rising from Etna above the New Southeast Crater (NSEC) on 22 May 2020 seen from Zafferana Etnea on the SE flank at 0955 local time. Photo by Boris Behncke, INGV.

Activity during June-July 2020. During June, moderate intra-crater Strombolian activity with intermittent ash emissions continued in the NSEC and occurred more sporadically in the VOR Crater; at night, incandescence of variable intensity was observed at the summit. During the week of 8-14 June, Strombolian explosions in the cono della sella generated some incandescence and rare jets of incandescent material above the crater rim, though no ash emissions were reported. On the morning of 14 June a sequence of ten small explosions in the VOR Crater ejected incandescent material just above the crater rim and produced small ash emissions. On 25 June an overflight showed the developing pit crater in the center of the BN, accompanied by degassing along the S edge of the wall; degassing continued from the NEC, VOR Crater, SEC, and NSEC (figure 305). The mass of the SO₂ plumes measured 5,000-7,000 tons per day, according to INGV.

Figure 305. Aerial photo of Etna from the NE during an overflight on 25 June 2020 by the Catania Coast Guard (2 Nucleo Aereo della Guardia Costiera di Catania) showing degassing of the summit craters. Photo captured from the Aw139 helicopter by Stefano Branca. Courtesy of INGV (Report 27/2020, ETNA, Bollettino Settimanale, 22/06/2020 – 28/06/2020, data emissione 30/06/2020).

Similar modest, intra-crater Strombolian explosions in the NSEC, sporadic explosions in the VOR Crater, and degassing in the BN, VOR Crater, and NEC persisted into July. On 2 July degassing in the NEC was accompanied by weak intra-crater Strombolian activity. Intermittent weak ash emissions and ejecta from the NSEC and VOR Crater were observed during the month. During the week of 6-12 July INGV reported gas-and-steam emissions continued to rise from the vent in the pit crater at the bottom of BN (figure 306). On 11 July mild Strombolian activity, nighttime incandescence, and degassing was visible in the SEC (figure 307). By 15 July there was a modest increase in activity in the NSEC and VOR Craters, generating ash emissions and ejecting material over the crater rims while the other summit craters were dominantly characterized by degassing. On 31 July an explosion in the NSEC produced an ash plume that rose 4.5 km altitude.

Figure 306. Photos of the bottom of the Bocca Nuova (BN) crater at Etna on 8 July 2020 showing the developing pit crater (left) and degassing. Minor ash emissions were visible in the background at the Voragine Crater (right). Both photos by Daniele Andronico. Courtesy of INGV (Report 29/2020, ETNA, Bollettino Settimanale, 06/07/2020 – 12/07/2020, data emissione 14/07/2020).

Figure 307. Mild Strombolian activity and summit incandescence in the “cono della sella” (saddle vent) at the Southeast crater (SEC) of Etna on 11 July 2020, seen from Piano del Vescovo (left) and Piano Vetore (right). Photo by Boris Behncke, INGV.

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: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy.

Ol Doinyo Lengai, located near the southern end of the East African Rift in Tanzania, is a stratovolcano known for its unique low-temperature carbonatitic lava. Frequent eruptions have been recorded since the late 19th century. Activity primarily occurs in the crater offset to the N about 100 m below the summit where hornitos (small cones) and pit craters produce lava flows and spattering. Lava began overflowing various flanks of the crater in 1993. The eruption transitioned to significant explosive activity in September 2007, which formed a new pyroclastic cone inside the crater. Repeated ash emissions reached altitudes greater than 10 km during March 2008. By mid-April 2008 explosive activity had decreased. In September new hornitos with small lava flows formed on the crater floor. The most recent eruptive period began in April 2017 and has been characterized by spattering confined to the crater, effusive activity in the summit crater, and multiple lava flows (BGVN 44:09). Effusive activity continued in the summit crater during this reporting period from September 2019 through August 2020, based on data and images from satellite information.

Throughout September 2019 to August 2020, evidence for repeated small lava flows was recorded in thermal data and satellite imagery. A total of seven low-level pulses of thermal activity were detected within 5 km from the summit in MIROVA data during September 2019 (1), February (2), March (2), and August (2) 2020 (figure 207). Sentinel-2 satellite imagery also provided evidence of multiple lava flows within the summit crater throughout the reporting period. On clear weather days, intermittent thermal anomalies were observed in thermal satellite imagery within the summit crater; new lava flows were detected due to the change in shape, volume, and location of the hotspot (figure 208). During a majority of the reporting period, the thermal anomaly dominantly appeared in the center of the crater, though occasionally it would also migrate to the SE wall, as seen on 3 February, the E wall on 12 July, or the NE wall on 31 August. In Natural Color rendering, fresh lava flows appear black within the crater that quickly cools to a white-brown color. These satellite images showed the migration of new lava flows between February, March, and June (figure 209). The flow on 8 February occurs in the center and along the W wall of the crater; the flow on 9 March is slightly thinner and is observed in the center and along the E wall of the crater; finally, the flow on 17 June is located in the center and along the N wall of the crater.

Figure 207. Seven low-level pulses of thermal activity within 5 km of the summit of Ol Doinyo Lengai were recorded in the MIROVA thermal data between September 2019 to August 2020; one in early September 2019, two in February, two in March, and two in August 2020. Courtesy of MIROVA.

Figure 208. Sentinel-2 thermal satellite images of Ol Doinyo Lengai from November 2019 to August 2020 show intermittent thermal anomalies (bright yellow-orange) within the summit crater. The location of these anomalies occasionally changes, indicating new lava flows. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 209. Sentinel-2 satellite images of new lava flows within the summit crater at Ol Doinyo Lengai during 8 February (left), 9 March (middle), and 17 June (right) 2020. Lava flows appear black in the center of the crater that changes in volume and location from February to June. Images with “Natural Color” (bands 4, 3, 2) rendering; courtesy of Sentinel Hub Playground.

During August, multiple lava flows were detected in Sentinel-2 satellite imagery. On relatively clear days, lava flows were visible in the middle of the summit crater, occasionally branching out to one side of the crater (figure 210). On 6 August, a thin lava flow branched to the E flank, which became thicker by 11 August. On 16 and 21 August, the lava remained mostly in the center of the crater. A large pulse of fresh lava occurred on 31 August, extending to the NW and SE sides of the crater.

Figure 210. Sentinel-2 images of multiple new lava flows at Ol Doinyo Lengai during August 2020. When visible in the first half of August, dark lava is concentrated in the center and E side of the crater; by the end of August the lava flows had reached the NW side of the crater. Images with “Natural Color” (bands 4, 3, 2) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Recent activity at Yasur, which has been erupting since July 1774, includes frequent Strombolian explosions, along with ash and gas plumes from several vents in the summit crater (BGVN 44:02, 45:03). This report summarizes activity during March through August 2020, using information from monthly bulletins of the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and various satellite data. The volcano has remained on Alert Level 2 (major unrest state, on a scale of 0-5), where it has been since 18 October 2016, according to VMGD.

During the current reporting period, VMGD reported that explosive activity continued at an elevated level, with ongoing ash and gas emissions (figure 71). Some of the more intense explosions ejected bombs outside the summit crater. During 2-3, 13, and 17 March, 2-3 April, and 19 July, the Wellington Volcanic Ash Advisory Center (VAAC) identified low-level ash plumes that reached an altitude of 1.5 km and drifted in multiple directions; the ash plume during 2-3 April resulted in ashfall on the SSW part of the island. On 19 May an ash plume rose to a maximum altitude of 2.1 km and drifted SE.

Figure 71. Webcam photos of ash emissions from Yasur on 18 March (left)and gas-and-steam emissions on 2 April (right) 2020. Courtesy of VMGD.

During the reporting period, the MODVOLC thermal algorithm using MODIS satellite data detected a total of 55 thermal hotspots during three days in April, nine days in May, six days in June and August, and four days in July. A maximum of four pixels were recorded on a single day during 26 May, 6 June, and 20 July. The MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data detected numerous hotspots from 16 September 2019 through August 2020, with a slight increase in power and frequency during May (figure 72). Satellite images from Sentinel-2 detected a strong thermal anomaly within the summit crater on 10 May, accompanied by ash and gas emissions (figure 73).

Figure 72. Persistent low to moderate thermal activity at Yasur occurred from the summit area from 16 September 2019 through August 2020, as shown in this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 73. Sentinel-2 images of Yasur on 10 May 2020 showing a strong thermal anomaly from the summit crater (left) and a gas emission that appears to contain some ash (right). The thermal anomaly in the S vent area was stronger than in the N vent, an observation also noted in March and April 2019 (BGVN 44:06). The volcano was usually obscured by clouds during March through August. The left image is in false color (bands 12, 11, 4) rendering, the right image is in natural color (bands 4, 3, 2) rendering; courtesy of Sentinel Hub Playground.

High-resolution satellite sensors commonly recorded moderate sulfur dioxide levels drifting in multiple directions from the volcano. High sulfur dioxide levels were also occasionally observed, especially during March (figure 74).

Figure 74. High-density SO2 emissions streaming from Yasur during 8 (left) and 13 (middle) March and 21 April (right) 2020, were observed using the TROPOMI imaging spectrometer on the Sentinel-5P satellite. The plume drifted W on 8 March and E on both 13 March and 21 April. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

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: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://vaac.metservice.com/index.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/); 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); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Historical eruptions at Chile's Villarrica, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. An intermittently active lava lake at the summit has been the source of Strombolian activity, incandescent ejecta, and thermal anomalies for several decades; the current eruption has been ongoing since December 2014. Continuing activity during February-August 2020 is covered in this report, with information provided by the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN), and Projecto Observación Villarrica Internet (POVI), part of the Fundacion Volcanes de Chile, a private research group that studies volcanoes across Chile. Sentinel satellite imagery also provided valuable data.

Intermittent incandescence was observed at the summit throughout February-August 2020, which was reflected in the MIROVA thermal anomaly data for the period (figure 92). Continuous steam and gas emissions with occasional ash plumes rose 100-520 m above the summit. Every clear satellite image of Villarrica from February -August 2020 showed either a strong thermal anomaly within the summit crater or a dense cloud within the crater that prevented the heat signal from being measured. Sentinel-2 captured on average twelve images of Villarrica each month (figure 93). Larger explosions on 25 July and 7 August produced ejecta and ash emissions.

Figure 92. Thermal anomaly data for Villarrica from 13 October 2019 through August 2020 showed intermittent periods of activity. Incandescence was intermittently reported from the summit and satellite imagery showed a persistent hot spot inside the summit crater throughout the period. Courtesy of MIROVA.

Figure 93. Examples of strong thermal anomalies inside the summit crater of Villarrica each month from March-August 2020 are shown with dates on the image. Sentinel-2 satellite imagery with Atmospheric penetration rendering (bands 12, 11, 8A) showed thermal anomalies at the summit in all clear satellite images during the period. Courtesy of Sentinel Hub Playground.

Primarily white gas emissions rose up to 400 m above the summit during the first half of February 2020 and to 320 m during the second half. Incandescence was observed on clear nights. Incandescent ejecta was captured in the POVI webcam on 7 February (figure 94). Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 5, 8, 10, 13, 18, 20, 23, 25, and 28 February, nine of the eleven days that images were taken; the other days were cloudy.

Figure 94. Incandescent ejecta at the summit of Villarrica was captured in the POVI webcam late on 7 February 2020. Time sequence runs from top to bottom, then left to right. Courtesy of POVI.

Villarrica remained at Alert Level Yellow (on a four-level Green-Yellow-Orange-Red scale) in March 2020. Plumes of gas rose 350 m above the crater during the first half of March. The POVI webcam captured incandescent ejecta on 1 March (figure 95). SERNAGEOMIN reported continuous white emissions and incandescence at night when the weather permitted. During the second half of March emissions rose 300 m above the crater; they were mostly white but occasionally gray and drifted N, S, and SE. Nighttime incandescence could be observed from communities that were tens of kilometers away on multiple occasions (figure 96). Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 1, 3, 4, 6, 9, 11, 14, 16, 19, 26, 29, and 31 March, twelve of the fourteen days images were taken. The other days were cloudy.

Figure 95. Incandescent ejecta rose from the summit of Villarrica in the early morning of 1 March 2020. Courtesy of POVI.

Figure 96. Nighttime incandescence was observed on 24 March 2020 tens of kilometers away from Villarrica. Courtesy of Luis Orlando.

During the first half of April 2020 plumes of gas rose 300 m above the crater, mostly as continuous degassing of steam. Incandescence continued to be seen on clear nights throughout the month. Steam plumes rose 150 m high during the second half of the month. A series of Strombolian explosions on 28-29 April ejected material up to 30 m above the crater rim (figure 97). Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 3, 8, 10, 13, 20, and 30 April, six of the twelve days images were taken; other days were cloudy.

Figure 97. A series of Strombolian explosions on 28-29 April 2020 at Villarrica ejected material up to 30 m above the crater rim. Courtesy of POVI.

Daily plumes of steam rose 160 m above the summit crater during the first half of May 2020; incandescence was visible on clear nights throughout the month. During 5-7 May webcams captured episodes of dark gray emissions with minor ash that, according to SERNAGEOMIN, was related to collapses of the interior crater walls. Plumes rose as high as 360 m above the crater during the second half of May. The continuous degassing was gray and white with periodic ash emissions. Pyroclastic deposits were noted in a radius of 50 m around the crater rim associated with minor explosive activity from the lava lake. The POVI infrared camera captured a strong thermal signal rising from the summit on 29 May (figure 98), although no visual incandescence was reported. Residents of Coñaripe (17 km SSW) could see steam plumes at the snow-covered summit on 31 May (figure 99). Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 5, 13, 20, 23, 25 and 30 May, six of the twelve days images were taken. The other days were cloudy.

Figure 98. The POVI infrared camera captured a strong thermal signal rising from the summit of Villarrica on 29 May 2020; no visual incandescence was noted. Courtesy of POVI.

Figure 99. Residents of Coñaripe (17 km SSW) could see steam plumes at the snow-covered summit of Villarrica on 31 May 2020. Courtesy of Laura Angarita.

For most of the first half of June, white steam emissions rose as high as 480 m above the crater rim. A few times, emissions were gray, attributed to ash emissions from collapses of the inner wall of the crater by SERNAGEOMIN. Incandescence was visible on clear nights throughout the month. Vertical inflation of 1.5 cm was noted during the first half of June. Skies were cloudy for much of the second half of June; webcams only captured images of the summit on 21 and 27 June with 100-m-high steam plumes. Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 4, 7, and 14 June, three of the twelve days images were taken. The other days were cloudy.

Atmospheric clouds prevented most observations of the summit during the first half of July (figure 100); during brief periods it was possible to detect incandescence and emissions rising to 320 m above the crater. Continuous degassing was observed during the second half of July; the highest plume rose to 360 m above the crater on 23 July. On 25 July, monitoring stations in the vicinity of Villarrica registered a large-period (LP) seismic event associated with a moderate explosion at the crater. It was accompanied by a 14.7 Pa infrasound signal measured 1 km away. Meteorological conditions did not permit views of any surface activity that day, but a clear view of the summit on 28 July showed dark tephra on the snow around the summit crater (figure 101). Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 2 and 29 July, two of the twelve days images were taken. The other days were either cloudy or had steam obscuring the summit crater.

Figure 100. Although a multi-layer cap cloud formed over the summit of Villarrica on 15 July 2020, steam emissions could be seen close to the summit drifting down the slope. Cap clouds form when a stable airstream rises to pass over a peak and cools, condensing moisture into clouds. Photograph by Sebastián Campos, courtesy of Geography Fans.

Figure 101. Dark tephra appeared near the summit of Villarrica on 28 July 2020; an explosion had been measured seismically on 25 July but clouds obscured visual observations. Image taken from Coñaripe, courtesy of Laura Angarita.

An explosion on 7 August at 1522 local time (1922 UTC) produced an LP seismic signal and a 10 Pa infrasound signal. Webcams were able to capture an image of the explosion which produced a dense plume of steam and ash that rose 370 m above the summit and drifted SE (figure 102). The highest plumes in the first half of August reached 520 m above the summit on 7 August. Sporadic emissions near the summit level were reported by the Buenos Aires VAAC the following day but were not observed in satellite imagery. When weather permitted during the second half of the month, continuous degassing to 200 m above the crater was visible on the webcams. SERNAGEOMIN participated in a webinar on 20 August 2020 discussing safety at Villarrica and showed an image of the summit crater taken during an overflight on 19 August (figure 103). Sentinel-2 satellite imagery showed bright thermal anomalies at the summit on 6, 21, and 31 August, three of the thirteen days images were taken. The other days were cloudy.

Figure 102. An explosion at Villarrica on 7 August 2020 at 1522 local time (1922 UTC) produced an LP seismic signal and 10 Pa infrasound signal. Webcams were able to capture an image of the explosion which produced a dense plume of steam and ash that rose 370 m above the summit and drifted SE Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, volcan Villarrica, 7 de Agosto de 2020, 16:15 Hora local).

Figure 103. SERNAGEOMIN participated in a webinar on 20 August 2020 discussing safety at Villarrica and showed an image of the summit crater taken during an overflight on 19 August. Courtesy of Turismo Integral.

Geologic Background. Glacier-clad Villarrica, one of Chile's most active volcanoes, rises above the lake and town of the same name. It is the westernmost of three large stratovolcanoes that trend perpendicular to the Andean chain. A 6-km-wide caldera formed during the late Pleistocene. A 2-km-wide caldera that formed about 3500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesitic cone at the NW margin of the Pleistocene caldera. More than 30 scoria cones and fissure vents dot the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Historical eruptions, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); 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); Proyecto Observación Villarrica Internet (POVI), (URL: http://www.povi.cl/, https://twitter.com/povi_cl/status/1237541250825248768); Luis Orlando (URL: https://twitter.com/valepizzas/status/1242657625495539712); Laura Angarita (URL: https://twitter.com/AngaritaV/status/1267275374947377152, https://twitter.com/AngaritaV/status/1288086614422573057); Geography Fans (URL: https://twitter.com/Geografia_Afic/status/1284520850499092480); Turismo Integral (URL: https://turismointegral.net/expertos-entregan-recomendaciones-por-actividad-registrada-en-volcan-villarrica/).

Stromboli, located in northeastern-most part of the Aeolian Islands, is composed of two active summit vents: the Northern (N) Crater and the Central-South (CS) Crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano. The current eruption period began in 1934, continuing to the present with volcanism characterized by consistent Strombolian explosions in both summit craters, ash plumes, pyroclastic flows, and occasional lava flows (BGVN 45:08). This report updates activity consisting of dominantly Strombolian explosions and ash plumes from May to August 2020 with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. Explosion rates ranged from 1-23 events per hour and were of variable intensity, producing material that typically rose from less than 80 to over 300 m above the crater. One ash plume on 19 July rose 1 km above the crater and high energy ballistics were ejected 500 m above the crater during the week of 20-26 July (table 9). Strombolian explosions were often accompanied by gas-and-steam emissions and spattering that has occasionally resulted in material deposited on the slopes of the Sciara del Fuoco. According to INGV, the average SO₂ emissions measured 250-300 tons/day.

Table 9. Summary of activity at Stromboli during May-August 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

Explosive activity was relatively consistent during May 2020 and was mainly produced in 3-4 eruptive vents in the N crater and at least two eruptive vents in the CS crater. As a result of some explosions fallout covered the slopes of the Sciara del Fuoco. Explosion rates varied from 1-17 per hour in the N crater and 1-8 per hour in the CS crater; ejected material rose 80-250 m above the craters.

During June, explosions originated from 2-3 eruptive vents in the N crater and at least 2-3 localized vents in the CS crater. The Strombolian explosions ejected material 80-200 m above the craters, some of which fell back onto the Sciara (figure 182). Explosion rates varied from 5-14 per hour in the N crater and 2-9 per hour in the CS crater. Spattering was typically observed in the CS crater.

Figure 182. An explosion at Stromboli produced gas-and-steam and ash emissions on 18 June 2020 was observed in the CS crater in the Sciara del Fuoco. Courtesy of INGV (Rep. No. 26/2020, Stromboli, Bollettino Settimanale, 15/06/2020 - 21/06/2020, data emissione 23/06/2020).

Ongoing explosive activity continued into July, originating from 2-3 eruptive vents in the N crater and 3-4 eruptive vents in the CS crater. Explosions varied from 3-12 per hour in the N crater and 1-11 per hour in the CS crater; ejected lapilli and bombs rose 80-1,000 m above the craters (figure 183). On 19 July a high-energy explosion between 0500 and 0504 produced an ash plume containing ejecta more than 50 cm that rose to a maximum of 1 km above the crater, with fallout reaching the Pizzo sopra la Fossa and resulting in ashfall on the Sciara and the towns of Liscione and Roccette. During the week of 20-26 July explosions in the E portion of the volcano ejected ballistics 500 m above the crater; the size and shape of these varied between slag bombs to clasts greater than 50 cm.

Figure 183. Webcam (left column) and thermal (right column) images of explosive activity at Stromboli on 29 July (top row) and 2 August (bottom row) 2020 originated from the N and CS craters, producing spatter and ash plumes. Courtesy of INGV (Rep. No. 32/2020, Stromboli, Bollettino Settimanale, 27/07/2020 - 02/08/2020, data emissione 04/08/2020).

Strombolian activity accompanied by discontinuous spattering continued during August. Total daily explosions varied from 3-23 per hour ejecting material that up to 200-250 m above the craters. During the first half of the month the explosions were low-intensity and consisted of fine material. On 13 August the intensity of the explosions increased, producing an ash plume that rose 300 m above the crater drifting SE and resulting in a significant amount of ashfall on the Sciara. During the week of 17-23, explosions in the N1 crater ejected material 200 m above the crater while explosions in the CS crater ejected material 250 m above the crater, predominantly during 22 August in the S2 crater (figure 184).

Figure 184. Images of gas-and-steam and ash plumes rising from the N2 (left), S2 (middle), and CS craters (right) at Stromboli on 22 August 2020. Courtesy of INGV (Rep. No. 35/2020, Stromboli, Bollettino Settimanale, 17/08/2020 - 23/08/2020, data emissione 25/08/2020).

Moderate thermal activity was relatively consistent from October 2019 through mid-April 2020; during May-August thermal activity became less frequent and anomalies were lower in power based on the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 185). Though there were no detected MODVOLC thermal alerts during this reporting period, many thermal hotspots were observed in Sentinel-2 thermal satellite imagery in both summit craters (figure 186).

Figure 185. Low to moderate thermal activity at Stromboli occurred frequently from 16 September to mid-April 2020 as shown in the MIROVA graph (Log Radiative Power). During May-August thermal activity decreased and was less frequent compared to the previous months. Courtesy of MIROVA.

Figure 186. Weak thermal anomalies (bright yellow-orange) at Stromboli were observed in thermal satellite imagery from both of the summit vents throughout May-August 2020. Images with atmospheric penetration (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

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

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

Nineteen explosions occurred . . . in August . . . . Ejecta from an explosion on 5 August at 1057 cracked the windshield of an airliner in flight. A car windshield was cracked by tephra from an explosion at 1249 the same day and another was broken on 20 August at 0851, both on Sakura-jima Island, 3 km from the crater. The month's highest ash cloud rose 4,000 m. A total of 583 g/m² of ash was deposited [at KLMO]; a change in the usual wind direction had carried ash away from this site in July. Typical volcanic earthquake swarms were recorded on 3, 15, 16, and 29 August.

Similar activity continued through mid-September, adding 15 explosions as of the 14th . . . . The highest September ash cloud reached 1,800 m height.

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.

An average of 3 explosions/day was recorded in August . . . . Seismicity also decreased, to a daily average of 20 earthquakes (figure 40). Fumarolic activity continued from the active crater, and lava flows continued to travel down the W and SW flanks of the volcano.

Figure 40. Daily number of earthquakes at Arenal, August 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 G. Soto, ICE; Mario Fernández, Hector Flores, and Sergio Paniagua, Sección de Sismología y Vulcanología, Escuela de Geología, Univ de Costa Rica, San José, Costa Rica.

A plume from the summit area of Welirang . . . was photographed by Space Shuttle astronauts on 13 September at [1535] (photo no. S48-151-064) (figure 1). The dense portion of the apparently ash-poor plume extended roughly 50 km N and more diffuse material continued for another 65 km. The summit area was white and apparently de-vegetated. A plume was observed again on direct video downlink from the spacecraft on [17] September at [1306]. No ground reports were available at press time.

Figure 1. Space Shuttle photograph showing a steam plume from Welirang (just east of the central cloud mass). Also, the lack of vegetation at the peak indicates volcanic activity. Volcanoes on Java form an E-W line of peaks the length of the island; five are in this image. NASA Photo ID: STS048-151-064, 13 September 1991.

Geologic Background. The Arjuno and Welirang volcanoes anchor the SE and NW ends, respectively, of a 6-km-long line of volcanic cones and craters. The Arjuno-Welirang complex overlies two older volcanoes, Gunung Ringgit to the east and Gunung Linting to the south. The summit areas of both volcanoes are unvegetated. Additional pyroclastic cones are located on the north flank of Gunung Welirang and along an E-W line cutting across the southern side of Gunung Arjuno that extends to the lower SE flank. Fumarolic areas with sulfur deposition occur at several locations on Welirang.

Information Contacts: C. Evans and D. Helms, NASA-SSEOP.

Lava production continued from the subsidiary vent on the NE face of the volcanic cone, 80 m below the main crater, during a visit on 26 June. Incandescent material was ejected in a pulsating fountain, to [80] m height, more intensely than during the previous visit on 16 May. Satellite monitoring had indicated a temperature of 1,100°C around the vent on 6 May. A dark plume rose 300-400 m from the crater of a large spatter cone that had formed at the eruptive vent. The main crater remained quiet. The lava flow observed in May had bifurcated, with one branch extending along the NW and W valleys, and a new branch extending S. By 26 June, lava had reached the sea at the boat landing near the NW corner of the island (~1.2 km from the vent); during the 16 May fieldwork, the lava front was still 200 m from shore. Vigorous boiling and thick jets of steam were observed for 100 m along the shore. Studies of water near the shore indicated a considerable decrease in pH, and visibility dropped to <10 cm (Srinivas, 1991). Nearby coral was destroyed.

The following is from a GSI report on lava chemistry and petrography. "Thirteen chemical analyses on samples of recent lava collected on 16 May indentify the rocks as basaltic andesites (table 1). They are porphyritic with phenocrysts of plagioclase (dominant; some grains show labradorite composition), with minor clinopyroxene (augite) and forsteritic olivine, set in a fluidal [intersertal] groundmass of brown glass, plagioclase microlites, and Fe-Ti oxides. The amount of mafic phenocrysts is relatively low. The average ratio between phenocryst and groundmass components is around 0.44. The volumetric composition of the phenocrysts indicates: 72% plagioclase, 17% clinopyroxene, and 11% olivine; while the groundmass consists of 43% plagioclase microlites, 37% glass, and 20% Fe-Ti oxides. The amount of glass in the groundmass is highly variable, exceeding 70% in some sections. There is a complete lack of amphibole grains, in both the phenocrysts and [in] the groundmass."

Table 1. Range and average compositions from 13 chemical analyses of recent lava erupted from Barren Island, collected 16 May 1991. Courtesy of the GSI.

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

Information Contacts: Director General, GSI; S. Acharya, SANE.

"Five high-temperature fumaroles on the SW rim of the summit lava dome have been monitored continuously since May. These fumaroles are ~75 m W of the site of the March-May lava extrusion and occur along fractures radial to the dome. Temperatures were measured at 20-minute intervals and radio-telemetered to the Science Center in the city of Colima. Temperatures at two of the fumaroles have increased at a steady rate between May and August (figure 16). Mean late-August temperatures were 506 and 386°C, increases of 66 and 43°C, respectively, since May. Mean temperatures in three other fumaroles have changed <10°C during the same period. Throughout the sampling period, all fumaroles exhibited marked diurnal temperature variation, on the order of 30-80°C/day. The rainy season, which began in mid-June and has continued through August, has had little effect on fumarole temperatures other than occasional low readings during rainstorms."

Figure 16. May-August 1991 temperatures at two fumaroles on the SW rim of Colima's summit lava dome, about 75 m W of the site of March-May lava extrusion. Measurements, at 20-minute intervals, were radio-telemetered to the Science Center, city of Colima. Courtesy of C. Connor.

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 high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the 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, producing thick debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: C. Connor, FIU, Miami.

People living near the volcano reported that a new eruption began during the night of 8-9 June, after nine years of relative quiet. At the onset of the eruption, residents were awakened by rumblings and a red glow from the volcano, which has since remained active. Ashfalls have occurred regularly in coastal towns 15 km NNW to 15 km ENE of the summit (Galela, Mamoya, and Tobelo). When visited by a geologist on 23-28 June, small to moderate explosions occurred every 4-5 minutes, sometimes accompanied by noise and night glow. Small lahars occurred in rivers draining the volcano.

Space Shuttle astronauts photographed an apparently ash-rich plume extending ~30 km from the summit to slightly beyond the coast on 15 September at 2156 (photos STS048-110-34 & 35). The entire summit area appeared ash-covered.

Figure 1. Photograph of Dukono taken from the Space Shuttle, 2156 on 15 September 1991. The summit area appears to be covered with ash, and the plume extends ~30 km W from the summit. Courtesy of NASA-SSEOP; photo STS048-110-35.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland; C. Evans, NASA-SSEOP.

Seismic activity increased significantly in August, reaching the highest number of events (>150/day), the greatest reduced displacement (>800 cm²), and the highest released energy (~5.0 x 108 ergs; see figure 52) by long-period events since monitoring began in February 1989. Explosions and continuous ash emission from the crater were accompanied by periodic ejections of incandescent blocks up to tens of centimeters in diameter. Incandescence was visible within the crater at dispersed sites. Although weather conditions impeded direct observations, it was possible to confirm that many of the long-period earthquakes and tremor episodes had associated surface activity. SO₂ flux was low, ranging from 7 to ~370 t/d.

Substantial deformation changes were measured by the electronic tiltmeter [at Crater Station], with a resultant vector of 231 µrad of inflation (118° azimuth) in the 2 weeks ending 14 August. Lower levels of deformation, 3.7 µrad at 183° azimuth, were measured [at Peladitos Station].

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.

Two strong explosions were seen from Ternate, 6 km ESE of the summit, on 15 June, ejecting mainly white clouds. A 20 June climb revealed only white vapor filling the summit crater.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the thorough documentation of Gamalama's historical activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano. Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland.

In one of the largest eruptions of the century, Hudson's mid-August paroxysm produced an eruption cloud 18 km high and deposited ash up to 1,000 km SE. Estimates of tephra volume range between 2 and 6 km³; >1 km³ was deposited in Chile, around 2 km³ in Argentina, and 2 km³ may have fallen in the Atlantic Ocean or been lost to the atmosphere. Satellite data showed that the eruption produced a large SO₂-rich cloud, estimated to contain 1.5 megatons of SO₂ on 16 August, that was transported twice around the globe in 2 weeks.

The following is from a report by Norman Banks. "The eruption produced 1 to 2+ km³ (dense rock equivalent) of magma. The initial 8-9 August eruption (beginning about 1820 on 8 August) was from a basaltic (50% SiO₂) dike through a fissure 4 km long, trending through the NW rim of the 10 x 7 km, ice-filled caldera. The basalt erupted both as a lava fountain and phreatomagmatically, producing a tephra column 12 km high, scoria flows that covered 10 km² of the western caldera floor and an unknown area outside of the caldera, a 4-km-long lava flow over the WNW flank's Huemules glacier, long-lived (12 hours) floods down the Río Sorpresa (WSW flank) and Río Huemules valleys, and a rather low-volume tephra-fall deposit N of the volcano. This ash had a moderate level (100-300 ppm dry weight) of soluble fluorine that was quickly reduced to 2-10 ppm by heavy rains during the next 2 weeks. Grass growing through this deposit has a fluorine content of about 30 ppm.

"The andesitic eruption of 12-15 August may have been due to secondary boiling triggered by intrusion of the 8 August basalt, or other basaltic dikes into the andesitic magma body under the caldera; bombs and lapilli of pumiceous andesite (60% SiO₂) mixed with chilled basalt are common in the tephra-fall deposits. The 3-day andesitic eruption produced a strong Plinian column that ejected pyroclastic material into a very strong SE-directed stratospheric wind (185 km/hr) that kept the plume narrow even 700 km from the volcano. Pumiceous ballistic bombs 1 m in diameter were found 10 km from the vent, where tephra-fall deposits were >2.5 m thick. The 10-cm isopach reached just SE of Chile Chico (120 km SE of the vent) and 1-2 cm of ash was deposited at Argentina's coast (figure 5). [As many as 13 distinct layers of ash were deposited in some locations.] Fortunately, pyroclastic flows did not spill onto the outer snow-covered flanks during this episode, and no additional mudflows were reported. Shortly after the 12-15 August eruption, however, secondary water-and-pumice flows formed on the volcano's flanks during daily melting of the snow. Because most of the thick deposits on the steep mountainous terrain SE of the volcano are on and interleaved with snow, downslope movement and associated hydrological problems for the downstream valleys are certain to accelerate as the summer melting and rains begin. The andesitic ash in Chile had low amounts of soluble fluorine (<20 ppm), and grass covered by or growing through the ash deposits has a relatively low fluorine content. Analysis of fine fractions of the Chilean deposits suggest that downwind (Argentinean) fluorine values will not be significantly higher."

Figure 5. Preliminary isopach map of the 12-15 August 1991 tephra-fall deposits from Hudson. Prepared by N. Banks, H. Moreno, H. Corbella, M. Haller, and H. Ostera.

Steam emission, occasionally containing minor quantities of ash, declined rapidly following the eruption's end on 15 August.

Major reworking of ash deposits in Argentina by strong winds led to several false reports of renewed activity at Hudson. Ash was redistributed N to Comodoro Rivadavia (2 mm at 400 km E of Hudson) and was reported S to Río Gallegos (700 km SSE). In early September, GOES satellite images detected ash clouds, probably below 3 km, carried by ground-level winds at 55-65 km/hr; these clouds extended from near the volcano to over the Atlantic ocean. The densest part of the clouds appeared to be ~250 km SE of the volcano, about halfway to the Argentine coast. Poor visibility, down to a few hundred meters, was reported at Puerto Deseado and Puerto San Julián. Argentine officials have expressed concern over the >2 million sheep and 3,000 cattle in the affected region.

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: N. Banks, USGS; H. Moreno, Univ de Chile; J. Naranjo, SERNAGEOMIN; P. Bitschene, Patagonia Volcanism Program, Argentina; P. Maxwell, US Embassy, Buenos Aires; D. Helms, Lockheed, Houston; S. Doiron and G. Bluth, GSFC.

Fumarolic activity continued in August, mainly in a large zone of sulfur and chloride deposition in the N section of the crater, while a new zone of fumarolic activity appeared in the SSE part. The crater lake grew to cover almost the entire floor, >150 m in diameter. Seismicity, abnormally high since late May, continued to decrease in August (figure 4). During the second week in June, a new group of fumaroles appeared in the crater.

Figure 4. Monthly number of earthquakes at Irazú, January-August 1991. Courtesy of the Instituto Costarricense de Electricidad.

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 and G. Soto, ICE; Mario Fernández, Hector Flores, and Sergio Paniagua, Sección de Sismología y Vulcanología, Univ de Costa Rica.

Explosions were clearly visible from the coast (at Ulu Siau) during a visit 2-4 July. A diffuse, red, summit-area glow was continuously observed. Some small earthquakes were felt.

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

Information Contacts: V. Clavel and P. Vetsch, SVG, Switzerland.

The bottom of the summit's Choungou-Chahalé crater, obscured by a cloud of white gas and vapor following the 11 July phreatic eruption, became visible in early August. A new explosion crater (~250 m in diameter) was observed in its SE section. Vigorous degassing occurred through the crater lake and from the wall of the new crater. The following, from Patrick Bachélery, supplements the report in 16:6.

Karthala's 11 July explosion followed an increase in seismicity from 3-5 events/month (June 1988 start of monitoring through early April 1991) to 3-10 events/day in May (figure 2). Earthquakes were centered beneath the crater, mostly at 0-2 km below sea level, with a few 10-20 km below sea level. On 4 May, a swarm of 161 earthquakes (M 0.5-2) was recorded during a 1-hour period beginning at 1609. The number of earthquakes increased to 30-50/day by the end of June, and all were at shallow depths. Deformation measurements showed summit inflation of ~20 µrad during this time; only weak changes in deformation had been measured between the network's installation (May 1987) and June 1989.

Figure 2. Daily number of earthquakes at Karthala, March-June (inset) and May-July 1991. Courtesy of P. Bachélery.

A notable change in seismicity occurred on 30 June at 1645. More than 500 earthquakes (long- and short-period) were recorded that day, as many on the next, and >1,500 daily 2-4 July (figure 2). The short-period events (M 0.5-3.1) were centered in a roughly N-S line below the S part of the summit caldera and the S flank of the volcano (figure 3). Felt shocks caused ~1,000 people to leave the lower part of the S flank.

Figure 3. Epicenter map of short-period earthquakes at Karthala during 30 June-4 July (open squares) and 5-10 July 1991 (filled squares). Courtesy of P. Bachélery.

Seismicity continued to increase from 4 July, with 4,000 earthquakes recorded daily by 10 July. A swarm of nearly continuous seismic events was recorded between 0040 and 0110 the next day. The 4-10 July seismicity was characterized by low-magnitude (mostly M <1, sometimes to M 3.4) short-period events located under the summit at 1-4 km depths, and less numerous deeper earthquakes at 4-8 km depth. Some long-period events with cigar-shaped waveform envelopes were also recorded. The center of seismicity shifted N, resulting in fewer felt shocks in the S part of the island, while several M 3 earthquakes were felt in Moroni (13 km NW of the crater).

Deformation measurements (dry-tilt) the morning of 10 July showed >120 µrad of inflation centered on Choungou-Chahalé and Choungou-Chagnoumeni (figure 4) craters since 28 June. That night, the eruption took place, but no eyewitness accounts are available. Seismicity reached its highest intensity during an 11-hour period that night [but see 16:6], dropping abruptly at 0335 on 11 July to ~100 recorded events/hour. About 1.5 hours later, a strong sulfur odor was detected in Moroni for ~2 hours.

Figure 4. Map showing Karthala's summit region and deposits from the 11 July 1991 explosion. Courtesy of P. Bachélery.

Later visits to the summit revealed that a sizeable phreatic explosion had occurred in Choungou-Chahalé crater. The southern 2/3 of the summit caldera were covered by blocks (up to 10 m³) and ash (figure 4), and the summit vegetation was completely removed from within the limits of the caldera. The crater bottom was hidden by gas and vapor clouds, obscuring the source of a "fountaining" sound heard two weeks after the 11 July explosion. Geologists later believed the sound to have been caused by the forceful arrival of water into the new crater, forming the crater lake.

Seismicity rapidly decreased after the explosion, although several earthquakes of M 3.0-3.5 were recorded through the end of July. In August, 20-40 events/day were recorded, the same level as in June.

Geologic Background. The southernmost and largest of the two shield volcanoes forming Grand Comore Island (also known as Ngazidja Island), Karthala contains a 3 x 4 km summit caldera generated by repeated collapse. Elongated rift zones extend to the NNW and SE from the summit of the Hawaiian-style basaltic shield, which has an asymmetrical profile that is steeper to the S. The lower SE rift zone forms the Massif du Badjini, a peninsula at the SE tip of the island. Historical eruptions have modified the morphology of the compound, irregular summit caldera. More than twenty eruptions have been recorded since the 19th century from the summit caldera and vents on the N and S flanks. Many lava flows have reached the sea on both sides of the island. An 1860 lava flow from the summit caldera traveled ~13 km to the NW, reaching the W coast to the N of the capital city of Moroni.

Information Contacts: P. Bachélery, Univ de la Réunion; D. Ben Ali and J-L. Klein, CNDRS, RFI des Comores; F. Desgrolard, Centre de Recherche Volcanologique, Clermont-Ferrand, France; J-L. Cheminée, J-P. Toutain, and J-C. Delmond, IPGP.

Lava . . . continued to enter the ocean at two main sites through August (figure 79). By the end of the month, numerous breakouts from the tube system had reduced the volume of lava reaching the sea. Flows produced by major breakouts at ~180 and 340 m (600 and 1,100 ft) elevation spread over the W third of the lava field. Most remained on older lava, but a few lobes reached the field's W edge and ignited small brush fires in the remnants of the Royal Gardens subdivision. One flow from the breakout at 180 m reached 20 m elevation in early August.

Since at least January, a small lava pond has been continuously active in the bottom of Pu`u `O`o crater, covering ~20% of the crater floor on its E side. By April, the pond was circular and surrounded by levees. During the evening of 27 August, bright glow was visible over Pu`u `O`o, and a nearby seismometer recorded frequent bursts of higher amplitude tremor lasting 1-3 minutes. Overflights the next morning revealed that the pond had overflowed its levees, covering the entire crater floor with several meters of active lava that had a thin, frequently overturning, crust. Lava periodically drained back to its former level, remaining confined within the original pond until the next overflow. Similar activity continued through the end of the month. Crater depth remained roughly 80 m.

Seismicity in August included the upper East rift zone's third intrusive swarm since December 1990. More than 200 shallow summit microearthquakes were registered between 1100 and 1200 on 21 August. Earthquake counts quickly declined during the next hour, but elevationated levels of seismicity . . . continued through the next day. The largest concentration of events appeared to be centered just SE of the caldera, and very few occurred beyond Hiiaka crater, 4.5 km from the caldera rim. Most of the month's seismicity in the summit/upper east rift area occurred during the swarm.

Earthquake epicenters since December 1990 (figure 80) have been concentrated in several clusters, the largest of which were associated with the period's three intrusive episodes. The three swarms occurred in different portions of what geophysicists infer to be the same shallow (<5 km deep) structure between the summit and the East rift zone, suggesting a significant role for the summit in the current East rift eruption. During the early December swarm earthquakes were located from the summit roughly 6 km downrift (to Pauahi crater). The largest concentration of events was in the SE part of the caldera, perhaps extending a short distance into the rift zone (toward the Chain of Craters). The March activity occurred away from the summit, with the majority of located events between Pauahi and Mauna Ulu, roughly 3 km farther downrift. Following the early December seismicity and a long-period summit swarm late in the month, seismicity increased between the summit and Hiiaka crater. The same segment of the uppermost East rift zone has consistently shown low levels of shallow seismicity throughout Kupaianaha vent's post-1986 eruptive activity. After the March swarm, seismic activity along this rift segment appears to have increased further, and the August swarm was largely confined to this area.

Figure 80. Plot of earthquake epicenters in Kilauea's summit, upper to middle East rift zone, and south flank areas, December 1990-11 September 1991. Some of the larger craters are labeled. The eruption's two currently active vents, Pu`u `O`o and Kupaianaha, are off the map ~3 and 6 km ENE of Napau Crater. Courtesy of HVO.

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

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

"In August, Crater 3 frequently erupted moderate to strong, pale grey-brown ash and vapour clouds accompanied by weak to loud detonations, roaring or rumbling. The eruptions occurred at intervals of several minutes to a few hours. The emission clouds rose as much as 500 m above the crater. Dull to bright red crater glow was observed on the nights of 7-9, 12, and 13 August.

"During an aerial inspection on the 14th, two active vents were observed in a mound of lava filling Crater 3. The vents were ~5-10 m in diameter, 40 m apart and aligned approximately N-S. The N vent was more active and was filled with incandescent lava. The S vent was clogged with dark lava. Both vents released blue vapour. Lava had flowed eastward to form a short (70 m) lobe in the E part of the crater. A longer (~150 m) lobe of lava was present on the NE flank of Cone 3. This lobe was fresh, having a dark surface, and its source appeared to be a tube within the E lobe. The NE-flank flow was first observed on 13 August, and appeared to be inactive then. However, some activity of this flow had been evident the previous night when prolonged incandescence in this area and some movement of incandescent material were observed. Two other very small lava lobes (both inactive) were observed on the NW flank of Cone 3.

"Throughout the month, Crater 2 (roughly 200 m E of Crater 3) almost continuously emitted moderate amounts of pale grey-brown ash and vapour. This activity was accompanied by nearly continuous low roaring sounds. Occasional stronger explosions took place. Dull glow over the crater was observed on the nights of 7-9, 13, 22, 24, and 27 August. A 30-minute period of strong explosive activity on the night of 13 August resulted in a large volume of incandescent lava fragments being ejected onto the NE flank of Cone 2. Incandescent lava-fragment ejections from Crater 2 were also seen on the night of 20 August. A brief aerial view of the interior of Crater 2 on 14 August indicated that it remains funnel-shaped, with several benches. Detailed observation was prevented, however, by emissions of ash and vapour.

"The ash plume from the combined emissions of the craters was usually directed in a sector between NNE and NW. Fine ashfalls were recorded in coastal areas (9 km distant) on 1, 2, 6, and 12 August.

"Seismicity remained at a moderate to high level throughout the month. It appeared that most of the stronger seismicity was associated with events at Crater 3. The daily number of explosion earthquakes recorded by the summit station fluctuated between 20 and 70, with the largest totals of 40-70 events on 16, 25, and 30-31 August. Meanwhile, the remote station (9 km distant) recorded 0-29 events/day. Numerous low-amplitude, short-duration, tremor-like signals were produced by weaker explosions. Several periods of harmonic tremor were recorded but the source was not determined."

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 E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N 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. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

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

Photographs taken . . . by D., M., and T. Peterson on 25 January showed few changes since late 1990. Lava flows of varying ages were evident on the crater floor, with the youngest (F25) extending N toward the crater wall from a hornito on the N flank of . . . T5/T9 (figure 22). Its dark brown color and clearly defined margins indicated that it may have been active during the Petersons' visit. Light gray-brown lava had spread from a source near vent T11, across the former saddle (M1M2) to the S wall of the crater, covering more than half of the floor of the former southern depression. Lava of similar age also covered much of the N part of the main crater.

M. Peterson returned . . . 29-30 March, and reported 10-15 minutes of lava production during the evening of the 30th from 2 or 3 vents on the N side of T5/T9, very close to the source of the freshest flow photographed on 25 January. A number of flows moved away from the vents, the longest advancing ~50 m. Flow widths averaged 1-2 m and thicknesses varied from 10 to 20 cm. Steam and sulfur fumes were issuing from several sources on the crater rim, walls, and floor. Older flows in the N part of the crater were dominantly pahoehoe but some aa lava was also observed. Flows entering the S depression were blocky and ~2/3 m thick.

Figure 22. View SE across the crater floor of Ol Doinyo Lengai, 25 January 1991. A recent flow from vent T5/T9 is shown in black. Prepared by C. Nyamweru from a photo taken by the Peterson group.

Little fresh lava was evident on the dominantly pale gray to white crater floor during a visit by Benoit Wangermez on 6 May. A slightly darker flow covered most of the southern depression, showing that lava had advanced S since January from a source slightly NW of T11. Small flows around the base of T5/T9 (active in late March) did not look very young. One new light-colored zone (at M2) appeared to be a vent, currently inactive, that had formed since March.

When T. Peterson arrived at the crater rim on 28 June at about 1000, lava was flowing W from a new vent (T18) W of T5/T9. Activity had subsided 30 minutes later, and the level of lava in the vent had fallen 5 m. Heat was rising from older vents (T5/T9 and T14), while T11 had partially collapsed and looked like a "sulfur cave." Lava flows on the crater floor ranged from dark (fresh) to almost white.

A group led by Luigi Cantamessa climbed to the summit on 12 July. No effusive activity was evident, but black to grayish flows [were] perhaps 1-2 days old . . . . Fumarolic activity occurred from some small hornitos. Many fissures were seen; one extended E-W, parallel to the former saddle dividing the main and southern craters, and cut across the W rim, but was not visible on the volcano's outer flank.

Eruptive activity was very minor . . . on 9 August between 1000 and 1400. Hot, fresh, dark gray natrocarbonatite lava was found near the H6 vent complex (figure 23). Water poured on the lava boiled violently. The extent of other fresh lava flows was similar to that observed 4 days later (see below). A small hornito on the S side of H6 ejected 2-3-mm droplets of spatter. A frozen, but still fresh lava pool ~4 m in diameter was found ~2 m below the average elevation of M1's crater floor (a group of tourists and a local guide reported that vents H6 and M1 had been active 2 days earlier). Vent T5/T9 emitted hot colorless gas, while T11 exhaled SO₂. Radial fissures on the W flank of the crater produced almost pure (>95%) CO₂, with some SO₂. Holes ~0.5-1 m across on the crater's W rim released hot, humid air with no detectable SO₂ or CO₂. These holes contained a variety of water-loving plants such as moss and algae. Gas compositions were measured with Dräger tubes.

Figure 23. Sketch map of the crater floor of Ol Doinyo Lengai, 13 August 1991. Fresh lava is shown in black. Courtesy of Alain Catté.

Lava production from one vent complex was continuing during a summit climb by Alain Catté and others on 13 August. Irregular, weak, but clearly audible explosions occurred from the 4-5-m-high hornito complex H6, ejecting lava fragments horizontally to 10-15 m from two vents (E1 and E2). Weak effusive activity occurred from a site ([E4]) 5 m below the hornito complex. Young, chocolate-brown flows extended from its base in three directions atop older (>48 hours) whitish flows: ~10 m E; ~40 m NE; and > 100 m N, flowing around other small cones. Production of small flows accompanied vent E1's explosions from the initial observations at 0845 until its activity stopped at about 1000.

When clouds cleared at 1030, a very fluid lava flow 40-50 cm wide was emerging from neighboring vent E2. The flow quickly subdivided into many black lobes ~10 cm wide, with a consistency like lubricating oil. Within a few seconds, these formed channeled pahoehoe flows that turned to aa at their distal ends. Lava also formed tubes that carried it >100 m from the source. No lava temperatures were taken, but it was possible to place one's hand a few centimeters from an active flow, and to touch it after ~2 minutes of cooling. A cascade of lava ~10 cm wide began from a third vent (E3) on the hornito complex at about 1145. Vents E2 and E3 erupted simultaneously and showed parallel fluctuations in activity. Later . . . lava outflow from E2 occurred in a jet 2 m long.

At about noon, lava production resumed from the base of the hornito complex (at [E4]) bubbling out in a manner reminiscent of mud pots. It overflowed after ~45 minutes, gradually building a hornito that grew to 1 m height before activity ceased at about 1330. Above [E4], lava effusion from vent E3 stopped at 1230, emerging from a channel 2 m below in a violent, 3-m jet that reached the base of [E4], beginning to fill the area with lava. The outflow rate increased progressively, and lava had advanced 60 m W by the end of observations at about 1400. Lava production from the H6 complex had roughly quadrupled its size since . . . March.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, St. Lawrence Univ; D. Peterson, M. Peterson, and T. Peterson, Arusha, Tanzania; B. Wangermez, Nairobi, Kenya; L. Cantamessa, Geo-decouverte, Switzerland; P. Vetsch, SVG, Switzerland; T. Dunai, R. Ragettli, K. Schenk-Wenger, and U. Ziegler, ETH Zürich, Switzerland; A. Catté, B. DeMarne, and P. Barois, LAVE.

The press reported that renewed activity on 19 September ejected a plume to ~700 m. Incandescent tephra fell 500 m from the crater, starting fires that burned plantations in seven villages. No casualties were reported. As of the next morning, the eruption was continuing and VSI observers were recording accompanying earthquakes. VSI advised local authorities that residents of nearby villages should remain on alert, but an evacuation was not ordered.

Geologic Background. The twin volcanoes Lokon and Empung, rising about 800 m above the plain of Tondano, are among the most active volcanoes of Sulawesi. Lokon, the higher of the two peaks (whose summits are only 2 km apart), has a flat, craterless top. The morphologically younger Empung volcano to the NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century, but all subsequent eruptions have originated from Tompaluan, a 150 x 250 m wide double crater situated in the saddle between the two peaks. Historical eruptions have primarily produced small-to-moderate ash plumes that have occasionally damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred. A ridge extending WNW from Lokon includes Tatawiran and Tetempangan peak, 3 km away.

Information Contacts: VSI; UPI.

Widely distributed reports of increased activity and up to 20,000 evacuees in mid-September proved false. Heavy cloud cover over the volcano and coincidental tectonic earthquakes prompted claims of an imminent eruption. PHIVOLCS scientists found no signs of activity, although they did locate a previously unknown geothermal area on a remote section of the volcano.

Geologic Background. The Pleistocene-to-Holocene Malindang stratovolcano, located on the western margin of Iligan Bay in north-central Mindanao, contains a small summit caldera. Legends record a large eruption from the 2404-m-high, dominantly basaltic-to-andesitic volcano in the past, although no historical eruptions are known (Salise et al., 1991). Reports of increased activity in 1991 at the time of tectonic earthquakes prompted widespread evacuations, but an eruption did not occur, although a previously unknown geothermal area was discovered.

Information Contacts: D. Sussman, Philippine Geothermal, Inc., Manila; Philippine Daily Inquirer and Manila Times, Manila; Reuters.

"Main Crater produced weak emissions of white vapour with low ash content on 1, 2, and 3 August. Blue vapour was visible on 8, 11, and 12 August and only white vapour during the last week of the month. There were no audible noises and no night glow was seen.

"The emissions from Southern Crater consisted of tenuous white vapour with occasional grey-brown ash clouds resulting in fine ashfalls on parts of the island. Occasional weak deep roaring and rumbling noises were heard 2-14 August and a weak red glow was observed around the crater mouth on the night of 7 August. An aerial inspection was carried out on 13 August. Southern Crater was partly filled with vapour but Main Crater was clear. The floor of Main Crater was occupied by a solid plug or mound of lava, at a level ~20 m below the lower (NE) part of the crater rim. White mofettes were released by numerous fumaroles around the base and lower walls of the crater. The crater floor was mostly covered by debris from the crater walls, but in the central area, the lava plug was visible over an area ~5 m in diameter, and consisted of steaming lava surrounded by small blocks and scoriae ejected during a stronger degassing phase. During the aerial inspection, emissions from Southern Crater were low-energy, thermally buoyant clouds, released fairly regularly at ~15-minute intervals.

"Seismicity was at a moderate level and tilt measurements showed a deflation of ~1.5 µrad since mid-August."

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 basaltic-andesitic stratovolcano to its lower flanks. These 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 observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent 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: B. Talai, C. McKee, and P. de Saint-Ours, RVO.

Marchena . . . started erupting on 25 September. The TOMS instrument aboard the Nimbus-7 satellite passed at about 1100 and sensed no SO₂, but the next pass, at the same time on 26 September, mapped a 300-km plume to the SW with an SO₂ content estimated to be close to 100 kt. High SO₂ values immediately over the volcano indicated that the eruption was still vigorous at that time. On the following day the plume was nearly twice as long, but had almost vanished by the same time on 28 September. Weather satellite images during this period showed low cloud cover, but no conclusive indication of the volcanic plume. . . .

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

Information Contacts: A. Carrasco, Charles Darwin Research Station; S. Doiron, GSFC; SAB.

Activity continued to decline through 15 September, with only three ash/steam emissions since about 25 August. Heavy monsoon rains triggered numerous mudflows and secondary explosions from the 15-16 June pyroclastic-flow deposits. Two large secondary pyroclastic flows occurred, producing associated ash clouds to 15 km height. The press reported continued fatalities from debris/mudflows and disease in evacuation camps, bringing the number of casualties attributed to the eruption to at least 740 by 20 September. Study of the June deposits has resulted in preliminary estimates of 7-11 km³ of material erupted.

5-11 August. Radar at Clark Air Base detected 13 ash/steam emissions rising to 4.5-13.5 km height; plumes were carried NE by the wind. Most RSAM peaks coincided with these emissions. The majority of seismicity was shallow (<=1 km depth), with magnitudes <1. Seven high-frequency earthquakes were felt at Clark Air Base.

12-18 August. Thirteen ash/steam emissions were detected, three with columns >15 km high (maximum 17.5 km). Wind carried the plumes ENE and NE, and ashfall was reported at Clark Air Base on 13 and 16 August. Ejection velocities ranged from about 300-900 m/min, similar to the ejection velocity on 25 June (estimated at about 450 m/min). A large secondary pyroclastic flow occurred sometime on 12-13 August, in the Marunot drainage on the NW flank. The flow was not observed, but satellite imagery was used to identify the deposits and estimate a deposit volume of 31 x 10⁶ m³ (1.25 km² areal coverage). The flow, ~10 km long, created a headwall scarp about 20 m high along a 240° arc in the primary pyroclastic-flow deposit source region. During aerial observations, the still-steaming secondary deposits could be differentiated from those of earlier pyroclastic flows by the absence of rills and dissected morphology.

Seismic energy release decreased notably from the previous week (figure 19), although the number of earthquakes remained about the same (102 recorded events/day compared to 95/day the week before). Several shocks were felt at Clark Air Base. RSAM peaks reflected high-frequency earthquakes generated by mudflows, and occasional long-period signals associated with ash/steam emissions from the caldera. Geologists suggested that small long-period events may also be related to secondary explosions from pyroclastic-flow deposits.

Figure 19. Accumulated RSAM energy at Pinatubo, 28 July-18 August 1991. Courtesy of PHIVOLCS.

19-25 August. Ash/steam emissions averaging ~9.8 km high (maximum 15 km) were detected eight times during the week. Ash was carried E. Some may have originated from secondary explosions at the E flank (Sacobia valley) pyroclastic-flow deposits. Seismicity consisted mostly of high-frequency earthquakes (M < 1.0) centered below the caldera or ~3 km NW, at 0-18 km depths (figure 20). Four events (M 2-4) were felt at Clark Air Base, with intensities to IV (adapted Rossi-Forel scale). RSAM peaks coincided with the larger high-frequency earthquakes, and long-period events were associated with ash/steam emissions.

Figure 20. Epicenters of 648 earthquakes recorded near Pinatubo, 19-25 August 1991. Courtesy of PHIVOLCS.

26 August-1 September. Only two ash/steam emissions were detected; plume heights ranged from about 10 to 16 km. Light ashfall occurred to 40 km SE (San Fernando) during secondary explosions that produced columns to 16 km. Ash related to these events caused poor visibility (300 m) on the highway between San Fernando and Angeles (25 km E of the volcano). The number of felt shocks (M < 4.2) increased to 17, with intensities to V (adapted Rossi-Forel scale). Multiple peaks in RSAM plots were due to mudflows, while single peaks were caused by long-period events associated with the two ash/steam emissions.

2-8 September. One ash/steam emission was detected (2 September), producing a 9-km plume that was carried W (highest portion) and NE (lower portion). Secondary explosions, three of which were recorded as low-amplitude, low-frequency earthquakes, generated ash clouds 2-4.5 km high. Geologists proposed that the heavy ashfall and 15-km-high ash column observed at 1400 on 4 September (figure 21) were from a secondary pyroclastic flow, whose fresh deposits were discovered two days later. The absence of a long-period earthquake coincident with the ash cloud suggested that it had not been generated by caldera explosions. The secondary pyroclastic-flow deposits about 3 km SSW of the caldera (in the upper Marella drainage) were estimated to be 1-2 km wide, and 4-6 km long, with a headwall scarp 15-25 m high. The deposit appeared very recent and seemed water-saturated. It was not known whether the ash cloud was generated purely by convection, or by phreatic explosions resulting from an encounter with water on the river bed. A helicopter overflight of the caldera on 6 September revealed no evidence of activity during the prior several days. Steaming was observed along the margins of the caldera and a bluish lake was present. No evidence of a lava dome was found.

Figure 21. Visible and infrared image from the NOAA 11 polar-orbiting weather satellite on 4 September at about 1445, showing a large, 15-km-high ash cloud above Pinatubo believed to have been generated by a secondary pyroclastic flow. Courtesy of G. Stephens.

Recorded earthquakes averaged 88/day, similar to 89/day the previous week. The majority were of high-frequency, and geologists believed that they were caused by tectonic readjustments. Most of the few low-frequency signals coincided with observed secondary explosions. Seismicity remained shallow (about 38% at less than 2 km depth), centered beneath or NW of the caldera. Long-duration, high-frequency earthquakes corresponding to mudflows created peaks in RSAM plots. A magnitude 5.1 earthquake at 0627 on 5 September, centered ~17 km NNW (15.53°N, 120.31°E) at 10 km depth, was felt at Clark Air Base (intensity RF V).

On 4 September, due to the continued decrease in caldera activity, the volcanic alert was reduced from Level 5 (eruption in progress) to Level 3 (numerous magma-related earthquakes, fumaroles, and gas emission), and the danger zone radius was reduced from 20 to 10 km. The principal remaining hazards and their probable durations were identified (table 4).

Table 4. Principal hazards associated with Pinatubo following the 15-16 June 1991 eruption (as of 4 September 1991). Courtesy of PHIVOLCS.

9-15 September. Although no ash/steam emissions were detected, ash clouds 2-10 km high were produced by secondary explosions. Vigorous steam emission was noted from the S side of the caldera, and the blue crater lake was still present during observations on 10 September. The average number of earthquakes decreased to 54 recorded daily, most centered ~2 km NW or 2 km S of the caldera, at <2 km and 5-10 km depths. The majority of events were M <2. RSAM and accumulated energy both showed decreases corresponding to the drop in seismicity. Multiple RSAM peaks coincided with mudflows, while single peaks were caused by moderate-sized earthquakes.

Debris flows. All of Pinatubo's major drainage systems experienced debris flows, ranging from mudflows to hyperconcentrated flows and floods. Numerous flows also occurred in more distant drainages in which significant quantities of tephra were deposited. To help alleviate hazards and to aid in studying debris-flow processes, rain gauges were installed, observation posts were set up at strategic locations along rivers, and cross sections were monitored at bridges. Timely warnings and evacuations considerably reduced the number of injuries and casualties. High rainfall (to > 30 cm/day) and still-hot pyroclastic-flow deposits generated numerous hot mudflows that deposited as much as several meters of material.

On the SE flank's Pasig-Potrero River, pyroclastic-flow deposits had formed a dam behind which a 1,000 x 600 m lake had formed. The lake drained on 7 September, causing muddy flash floods that reached 1.2 m high in about 5-10 minutes at Bacolar (35 km SE of the volcano). Press reports indicated that 800 homes were destroyed and seven people were confirmed dead. By 15 September, continued flooding and mudflows resulted in the deaths of 12 more people at Bacolar, where 45,000 of the 68,000 residents had fled.

News reports placed the death toll from the eruption, mud flows, and disease at more than 740 by 20 September [see also 16:9]. Of the fatalities in evacuation camps, an estimated 95% were Aeta tribesmen and 75% were children. The Aeta reportedly refused most medical assistance such as vaccinations.

Fieldwork on June eruptive products. Preliminary estimates have been calculated for pyroclastic-flow deposits and airfall tephra from the paroxysmal June eruptive activity. The bulk of the material erupted was found in pyroclastic flow deposits (5-7 km³); several drainage systems included more than 1 km³. An estimated 0.48 km³ of airfall tephra was deposited within the 15-cm isopach (table 5); the total volume of tephra-fall material erupted, including that deposited in the South China Sea or lost to the atmosphere, was believed to be between 2 and 4 km³. The total volume, therefore, is estimated as 7-11 km³ (roughly 3-5 km³ dense rock equivalent).

Table 5. Preliminary volume calculations (±10% error) of June 1991 eruptive products from Pinatubo. Total tephra deposit volume: 0.48 km³. Total pyroclastic-flow volume: 7.0 km³. Courtesy of PHIVOLCS.

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; K. Rodolfo, Pinatubo Lahar Hazards Taskforce, Univ of Illinois; W. Scott, USGS CVO; G. Stephens, NOAA/NESDIS; NEIC; AP; Reuters; UPI.

In August, the crater lake grew to cover all crater fumaroles, while fumarolic activity continued at levels considered "normal" for the volcano. The yearly total of recorded microearthquakes (almost all of low frequency) exceeded 32,500 by the end of the month (figure 40), a decrease from 1990.

Figure 40. Monthly number of earthquakes at Poás, January-August 1991. Courtesy of ICE.

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

Information Contacts: R. Barquero and G. Soto, ICE; M. Fernández, H. Flores, and S. Paniagua, UCR.

The crew of Qantas flight 41 (Sydney-Jakarta) observed a very dense black plume emerging intermittently from a flank vent on 10 September at 1508. The plume was drifting N at ~6 km altitude, well below the aircraft's altitude of nearly 12 km. A voluminous, dense, mostly white plume with small pulses of ash in its center was observed from a commercial flight two days later.

Geologic Background. Raung, one of Java's most active volcanoes, is a massive stratovolcano in easternmost Java that was constructed SW of the rim of Ijen caldera. The unvegetated summit is truncated by a dramatic steep-walled, 2-km-wide caldera that has been the site of frequent historical eruptions. A prehistoric collapse of Gunung Gadung on the W flank produced a large debris avalanche that traveled 79 km, reaching nearly to the Indian Ocean. Raung contains several centers constructed along a NE-SW line, with Gunung Suket and Gunung Gadung stratovolcanoes being located to the NE and W, respectively.

Information Contacts: ICAO; J. Post, SI.

After reports of strong sulfur odors, geologists visited the summit area on 28-30 August. A sulfurous odor was noted at Copelares on the S flank (1,400 m elevation), during the evening of 28 August. An explosion was heard at 0151 the next morning, followed several seconds later by the sound of falling material. Examination of 29 August records from a seismic station 6 km SW of the crater (RIN3) showed that a small earthquake occurred at 0148:47, then a larger earthquake sequence lasting 7.5 minutes began at 0151:40, coinciding with the first audible explosion. As the ascent continued later that morning, traces of fresh ash were observed beginning at about 1,500 m elevation. Large quantities of ash and blocks, ranging from 15 to 75 cm in diameter, were found deposited in the summit area. Impact craters reached 120 cm in diameter and 35 cm deep.

Bad weather obscured the view of the crater floor, but several explosions were heard, and the largest, at 0930, rained very wet ash on the scientists. Near the crater, the smell of sulfur was very strong, making breathing difficult and stinging the eyes. Nearby vegetation was partially or completely dead. Rain collected at Copelares had a pH of 4.1.

On 30 August, scientists visited Ríos Azul and Pénjamo, which flow down the N flank from the crater area. Both rivers were gray-white with suspended sediment, which was also visible, but in lower concentrations, in the Ríos Colorado and Blanco on the S and SE flanks.

[On 6 September, strong fumarolic activity (jet engine noise) was seen in the active crater. During explosive events of May-August 1991 the ejecta was mainly composed of gray mud (sulfide-rich), lithics, and bread-crust bombs (~10% by volume).]

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 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 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 3,500 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 and E. Fernández, OVSICORI; R. Barquero and G. Soto, ICE; Mario Fernández, Héctor Flores, and Sergio Paniagua, Univ. de Costa Rica.

A brief period of strong heating in Crater Lake was accompanied by small volcanic earthquakes and possibly by minor eruptions. Continuously recorded lake temperature data showed a gradual decline to 16°C by mid-June, then little change until a sharp increase began about 1 July. Temperatures reached 24.4°C on the 18th before declining again to 13° by late August. A series of small volcanic earthquakes occurred 5-14 July, none exceeding M 1.8.

Severe winter weather limited observations near the time of the increased activity, although the lake appeared normal on 11 July. When briefly observed on 12 August, evidence of 1-2 m of surging was visible under fresh (about 10 August) snow around the lake margin. More detailed observations during fieldwork 27 and 29 August revealed dirty, ash-covered ice under fresh snow 1-2 m above lake level, and widening of the lake's outlet channel by previous strong outflow or surging. No clear patterns were evident in summit-area deformation data.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake (Te Wai a-moe), is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3,000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P. Otway, DSIR Wairakei.

Seismicity remained at low levels in August, with earthquakes mainly W and N of the crater at 0-5 km depths. Tremor episodes were brief and of low energy. Deformation showed no significant changes. The monthly average SO₂ flux was 1,135 t/d, similar to 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.

During a brief visit on 11 September, vertical explosions occurred hourly, producing plumes to about 1200 m height. The block lava flow erupting from the E summit of Caliente continued to flow down to the Río Nima II.

Geologic Background. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile 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 vents, with activity progressing W towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: W.I. Rose, Michigan Technological Univ.

Explosive activity was restricted to crater C1 (NE part of the summit area; figure 17) during 9 August fieldwork (by F. Iacop, Institute of Earth Sciences, Univ of Udine). C1's central cone ejected hot tephra at ~20-minute intervals, and as a result, it had grown more rapidly than the crater's other two active cones. Glow from two small radial fissures in crater C2 was clearly visible at night. Sustained noisy gas emissions occurred about once an hour. Volcano guides had reported that activity was concentrated in crater C3 (SW part of the summit area), but at its cone 1 only hot vapor emission was occurring, from two vents, on 9 August. Rare explosions, mostly ejecting tephra, took place at bocca 4. The average number of recorded earthquakes remained near the normal value of 6/hour in July, declining below that level in the month's last week (figure 18). Average tremor amplitude also remained relatively constant through the end of July, while large shocks nearly disappeared after a peak on 29 June (figure 19). [see 16:09 for 28-29 August observations].

Figure 17. Active craters at Stromboli as seen from the somma, 6 September 1991. Crosses mark small vents active during the 6 September fieldwork. Courtesy of the Société Volcanologique Européenne.

Figure 18. Average number of explosion events/hour at Stromboli, 22 June-31 July 1991. The mean value for the period is shown. Courtesy of M. Riuscetti.

Figure 19. Number of seismometer-saturating events/day (lower curve) and average daily tremor amplitude (upper curve) at Stromboli, 22 June-31 July 1991. Courtesy of M. Riuscetti.

Moderate activity was observed in early September, with explosive episodes about every 15 minutes at crater C3 and roughly hourly at C1. Activity increased in the 3 hours of observations after 2300 on 6 September, with many moderate to strong explosions from the SW part of C3. Ejections of incandescent bombs and scoria sometimes lasted several minutes. Thick white vapor plumes rose from C2 and a small cone in its center, while blue SO₂-rich plumes emerged from several other vents. Explosions from C1 were vigorous, ejecting glowing fragments and dark brown columns that rose 200 m above the crater. C3's smaller explosive bursts, consisting of tephra-poor incandescent gas jets, were usually preceded by comparatively brief periods of increasing, noisy gas puffs; larger explosions that ejected a higher proportion of tephra followed longer intervals, with fewer or no precursory gas puffs. Geologists attributed this pattern to intermittent closure (by cooling) of the lava-filled conduits to gas-bubble rise from the underlying magma body, allowing higher pressure to build at depth.

Airborne COSPEC measurements by an Italian-French cooperative program in May-July indicated a total SO₂ flux somewhat lower than that measured by the same means in 1980 and 1984 (1,000 ± 200 t/d average; Allard and others, in press), consistent with the current moderate activity. Geologists concluded that combined with microprobe determination of the initial and residual sulfur content of Stromboli's lava, the SO₂ flux data require the degassing of 0.1 km³/year (average) of magma, three orders of magnitude more than the co-erupted volume. Thus, gas output is essentially derived from magma stored within the volcano. To assess the amount of diffuse magmatic degassing through the volcanic pile, other than from the craters, infrared mass spectrometric profiling of CO₂ concentrations in the ground began on 11 September. High CO₂ levels (80-90%), associated with subsurface thermal anomalies, were found to characterize the Pizzo sopra La Fossa crater terrace (at the summit rim, SE of the active craters). Concentrations gradually decreased toward the rim of this former crater, and no CO₂ anomaly was detected in outer areas to the S (down to the Vancori rampart).

Reference. Allard, P., Carbonelle, J., Le Bronec, J., Metrich, N., and Zetwoog, P., Volatile flux and magma degassing budget at Stromboli volcano: Geophysical Research Letters, in review.

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

Information Contacts: M. Riuscetti, Univ di Udine; Patrick Allard, CNRS-CEA, France; J.C. Baubron, BRGM, France; H. Gaudru and Rolf Haubrichs, SVE, Switzerland; Yvonne Miller, Univ de Genève, Switzerland.

Lava extrusion continued at Jigoku-ato crater through mid-September, generating destructive pyroclastic flows that advanced down two valleys. More than 12,000 people remained evacuated and no new casualties were reported.

A summit seismic swarm that began 11 August peaked 12-13 August (figure 29), then gradually declined through the 19th. Incandescent block ejection was seen between 0000 and 0200 on 12 August, followed by continuous ash emission through the day. The number of seismically detected pyroclastic flows from the lava dome decreased suddenly to a few events daily on 12 August. A new lava dome, first recognized from the air on 13 August, emerged W of the former dome, and began to produce pyroclastic flows on 25 August. Pyroclastic flows had previously traveled down the Mizunashi River valley but those from the new dome (C dome; see below) moved ENE down the Oshigatani Valley, which extends N of and parallel to the Mizunashi, then joins it several kilometers downstream. Some of the larger pyroclastic flows from the new dome advanced 3 km down the Oshigatani valley from late August through mid-September, and pyroclastic surges burned vegetation. The mayor of Shimabara city ordered the evacuation of about 500 people from an area (Senbongi) 3.5 km NE of the dome on 31 August. Frequent pyroclastic flows during the afternoon of 3 September included one of about 1 x 10⁵ m³ volume that advanced down the Oshigatani Valley at 1611. The accompanying cloud rose about 1,500 m and ash fell to the N part of Shimabara city. Ashfalls from pyroclastic flow elutriation clouds disrupted traffic around Shimabara city throughout the following day; the cloud from a flow at 1311 was 2,500 m high.

Figure 29. Daily numbers of earthquakes (top), tremor episodes (middle), and pyroclastic-flow events (bottom) recorded at Unzen, 1 May-20 September 1991. Courtesy of JMA.

Another seismic swarm began beneath the crater on 6 September, and a pyroclastic flow that evening at 2121 advanced about 3.5 km down the Oshigatani Valley. Hypocenters and seismic wave characteristics were similar to those of mid-August, although the September swarm was more vigorous.

By 12 September, the lava dome had broken into numerous small blocks. Seismic activity declined through 14 September but increased again on the 15th. Seismometers near the summit began to record larger pyroclastic flows, with longer durations than any since 8 June, on 15 September at 1644 (150 seconds) followed by others at 1759 (120 seconds), 1842 (360 seconds), and the largest at 1854 (670 seconds). The latter moved down the Oshigatani valley, entered the Mizunashi valley, and continued to within 500 m of highway 57, a total of 5.5 km. The main body of the pyroclastic flow turned east into the Mizunashi valley, where it damaged 50 houses in Shimabara city, but the pyroclastic surge continued about 800 m southward, destroying 26 houses and 74 other buildings including those of a primary school (in Onokoba district, Fukae town). All of the affected area had previously been evacuated, so there were no casualties. The largest pyroclastic flow was associated with the collapse of a section of the new lava dome about 250 m wide, 300 m long, and 50 m thick, a volume exceeding 3 x 10⁶ m³. This is about 20% of the total volume of lava domes erupted to date, and 3 times the volume of material removed by the 8 June pyroclastic flow. Two days later, a new lobe had grown to 100 x 200 m and 30 m high (0.3 x 10⁶ m³/day), about twice the June-August extrusion rate (see below).

A total of 292 pyroclastic-flow events was recorded in August, down from 326 in July, but the more frequent episodes toward mid-September raised that month's total to 310 as of the 17th. September earthquake counts had reached 2075 through the 17th, up from 559 in August and 133 in July.

The following, from Setsuya Nakada, describes eruption products through early September.

The size and frequency of pyroclastic flows had decreased until July, and travel distances were almost always <2 km. However, collapse episodes from the E lava dome remained frequent and lava blocks had filled the narrow headwaters of the Mizunashi River, along which the 3 and 8 June pyroclastic flows had descended. As a result, cliffs along the valley disappeared, and valley-fill deposits (talus) became thick enough to act as a cushion to soften the shock of falling blocks. The E dome flowed southeastward on the valley-fill deposits. After the end of June, the horseshoe-shaped depression had filled with dome materials, and lava blocks began to fall northeastward onto the floor of Myoken caldera (figure 30). They filled the E end of the floor with talus, which overflowed the caldera rim at the end of July. Lava blocks then fell down the E and NE flanks as pyroclastic flows and their paths widened northeastward. Some reached the N bank of the Mizunashi River. The E margin of the E dome widened; because the NE slope under the dome was steeper than the SE slope, the northern half of the E dome migrated northeastward, while the southern half did not move and solidified. By the middle of August, the caldera rim NE of the dome had been eroded away by the falling lava blocks.

Figure 30. Sketch map of Unzen's lava dome, 8 September 1991. Courtesy of Setsuya Nakada.

At the beginning of August, the ash-laden plume from the small vent at the northern base of the remnant W dome became stronger, and new lava was extruded on the western part of the E dome. On 5 August, many bubbles were observed coming from an old water-filled crater near the W dome. The small explosions that took place from the W dome on 12 August (see above) enlarged the vent to 20 m across and built a tuff cone around it. The E dome temporarily thickened for a few days prior to the new lava extrusion; the western part of the E dome, just above the former Jigoku-ato Crater, had swelled vertically. By the time new lava appeared 13 August, magma supply into the E dome had stopped, since the E dome did not lengthen and the surface of the dome did not move eastward (figure 31). It was difficult to accurately estimate the change in magma supply rate; talus and pyroclastic flows were deposited over an extensive area with irregular topography, which causes difficulties in calculating volumes of talus plus pyroclastic deposits.

Figure 31. Tracings of photographs from a fixed point about 4.4 km from Unzen's E lava dome, illustrating its growth 10 July-20 August 1991. Courtesy of Setsuya Nakada.

At the end of August, the new dome (central, or C dome) was 375 m long, 275 m wide, and 60-80 m high. The C dome grew eastward and northeastward, keeping a constant thickness. It covered the E dome and talus, plus a part of the old volcanic edifice, which was bulldozed by the growing dome from the former crater wall to the caldera rim. Talus also formed on the E dome. At the end of August, the volume of C dome was about 4 x 10⁶ m³ and the total volume of the domes was about 12 x 10⁶ m³. The resulting dome growth rate is about 0.15 x 10⁶ m³/day for 8 June-28 August.

Lava blocks fell down the E and NE margins of C dome into the Oshigatani Valley, forming pyroclastic flows beginning 25 August. The upstream area of the valley was the source area for lahars on 30 June. The pyroclastic flows traveled a maximum distance of 3 km from the dome, and had associated ash-cloud surge and seared zones like those of 3 and 8 June (figure 32). Flows moving down the Oshigatani Valley changed course southeastward when they encountered a high point dividing the valley and a residential area. Ash-cloud surges climbed the barrier, burning or searing trees, but block-and-ash flows did not. The devastated area was widest for pyroclastic flows that took place within the first week. By mid-September, Oshigatani Valley had been almost filled by pyroclastic-flow deposits.

Figure 32. Map showing the distribution of pyroclastic flows from Unzen as of 9 September 1991. Deposits from lahars, which occurred mainly on 30 June, are omitted. Courtesy of Setsuya Nakada.

Average speeds of pyroclastic flows were estimated using travel distances observed by Ground Self-Defense Force radar and durations of tremor signals. The higher the average speed of a pyroclastic flow, the longer its travel distance: about 100 km/hour for flows reaching 3 km distance and 50 km/hour for flows 1 km long. The average speed of a pyroclastic flow at the end of August was estimated at 93 km/hour using the time lag between the start of the tremor signal and the time when the seismometer was broken by the flow.

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; M. Takahashi, SI; Yomiuri Shinbun, Tokyo.

An increase in fumarolic activity and weak explosions were observed in the crater during August-September. On 26 August, water in a nearby river (Río Carmelito) was cloudy and the river level abnormally high. Four days later, on 30 August, small ash emissions and continuous explosions were observed from 1430 to 1500, followed by a strong explosion at 1506. A weak emission of gray ash and a white gas plume 1 km high were observed on 17 September. Seismicity was at normal levels for the volcano.

Geologic Background. Glacier-clad Villarrica, one of Chile's most active volcanoes, rises above the lake and town of the same name. It is the westernmost of three large stratovolcanoes that trend perpendicular to the Andean chain. A 6-km-wide caldera formed during the late Pleistocene. A 2-km-wide caldera that formed about 3500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesitic cone at the NW margin of the Pleistocene caldera. More than 30 scoria cones and fissure vents dot the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Historical eruptions, documented since 1558, have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: G. Fuentealba and P. Riffo, Univ de la Frontera.

Emission of gas/tephra columns from May 91 vent continued through August. During early-August helicopter overflights, R. Fleming noted flashes and strong low-frequency detonations as a hot, dilute eruption column rose from the vent. Crumbly white lithic blocks and lapilli with rare juvenile scoriae had been deposited nearby. Larger-than-normal plumes were often visible from the North Island coast, roughly 50 km away.

During fieldwork 28-29 August, a convoluting pink-brown column was emitted from May 91 vent. It contained very little ash and no evident incandescent material. Visible shock waves emerged from the vent every few seconds as "flashing arcs," lighting clouds above with a flickering glow like that from a poorly-functioning fluorescent tube. The strongest shock waves were manifested as an instantaneous displacement of the plume at the vent, and could be felt 150 m away. Some could be seen to bounce off the crater walls and travel back through the clouds. The shock waves did not seem to affect the rate of plume emission. The activity was accompanied by dull booming and sloshing noises, and occasional sharp detonations. The sloshing sounds were much like those heard in 1988 at Yasur (Vanuatu), where large gas bubbles were bursting through the surface of an active lava lake. Geologists noted that the activity at May 91 vent was consistent with similar gas-bubble discharge through a liquid magma column.

About 200 mm of coarse and fine ash had been deposited just N of May 91 vent since the previous fieldwork on 27 May. Little new ash was evident elsewhere on the main crater floor, but small (< 0.3 m) lithic blocks and their impact craters were found >200 m SE of the vent and to its W. Scarce, widely scattered scoria bombs, most 0.1-0.2 m across but some reaching 0.3 m, were found on top of the May ash, with only a light ash coating. The bombs seemed most abundant a few hundred meters SE-NE of the vent. They had highly vesiculated interiors of black glass with large pyroxene and plagioclase phenocrysts. Internal vesicles were up to 30 mm across, but decreased rapidly to sub-millimeter size toward the surface.

The pattern of deformation between late May and late August was similar to that of the previous 3 months. Strong subsidence at roughly double the previous rate continued to be centered SE of May 91 vent, while relative inflation persisted ~200 m farther SE. A new zone of inflation was measured E of Noisy Nellie fumarole (NE of May 91 vent). Minor deformation associated with activity at May 91 vent is unlikely to be detected, as the nearest part of the levelling network is 100 m away. Most fumarole temperatures had changed little since May, although values at Noisy Nellie had increased from 240 to 411°C.

The volcano had remained seismically quiet until mid-June, when B-type events became more common, continuing at rates of 2-7/day through the end of the month. Very weak volcanic tremor was sometimes visible on seismic records. A sequence of >45 tectonic earthquakes (to ML 3.7) occurred near White Island 1-2 July. A- and B-type events increased markedly on 7 July, accompanied by a small increase in background volcanic tremor amplitude. E-type eruption earthquakes were recorded on 1, 7, and 11 July. Seismicity had declined by 15 July, but a 3-day swarm of >200 A-type events began on 20 July. Significant volcanic tremor also resumed and continued through mid-August, increasing again 21-28 August. Tremor varied from a nearly pure 1.8 Hz signal to a complex pattern with spectral peaks to 8 Hz. A-type events did not occur daily in August, but often numbered 8-10/day. B-type events were very rare after 24 July. E-type eruption shocks were recorded on 14, 15, 19, 20, 23, 27, 29, and 30 August.

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

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