Krakatau volcano in the Sunda Strait between Indonesia’s Java and Sumatra Islands experienced a major caldera collapse around 535 CE; it formed a 7-km-wide caldera ringed by three islands. Remnants of this volcano joined to create the pre-1883 Krakatau Island which collapsed during the major 1883 eruption. Anak Krakatau (Child of Krakatau), constructed beginning in late 1927 within the 1883 caldera (BGVN 44:03, figure 56), was the site of over 40 eruptive episodes until 22 December 2018 when a large explosion and flank collapse destroyed most of the 338-m-high edifice and generated a deadly tsunami (BGVN 44:03). The near-sea level crater lake inside the remnant of Anak Krakatau was the site of numerous small steam and tephra explosions from February (BGVN 44:08) through November 2019. A larger explosion in December 2019 produced the beginnings of a new cone above the surface of crater lake. Activity from August 2019 through January 2020 is covered in this report with information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG). Aviation reports are provided by the Darwin Volcanic Ash Advisory Center (VAAC), and photographs are from the PVMBG webcam and visitors to the island.

Explosions were reported on more than ten days each month from August to October 2019. They were recorded based on seismicity, but webcam images also showed black tephra and steam being ejected from the crater lake to heights up to 450 m. Activity decreased significantly after the middle of November, although smaller explosions were witnessed by visitors to the island. After a period of relative quiet, a larger series of explosions at the end of December produced ash plumes that rose up to 3 km above the crater; the crater lake was largely filled with tephra after these explosions. Thermal activity persisted throughout the period of August 2019-January 2020. The wattage of Radiative Power increased from August through mid-October, and then decreased through January 2020 (figure 96).

Figure 96. Thermal activity persisted at Anak Krakatau from 20 March 2019-January 2020. The wattage of Radiative Power increased from August through mid-October, and then decreased through January 2020. Courtesy of MIROVA.

Activity during August-November 2019. The new profile of Anak Krakatau rose to about 155 m elevation as of August 2019, almost 100 m less than prior to the December 2018 explosions and flank collapse (figure 97). Smaller explosions continued during August 2019 and were reported by PVMBG in 12 different VONAs (Volcano Observatory Notice to Aviation) on days 1, 3, 6, 17, 19, 22, 23, 25, and 28. Most of the explosions lasted for less than two minutes, according to the seismic data. PVMBG reported steam plumes of 25-50 m height above the sea-level crater on 20 and 21 August. They reported a visible ash cloud on 22 August; it rose to an altitude of 457 m and drifted NNE according to the VONA. In their daily update, they noted that the eruption plume of 250-400 m on 22 August was white, gray, and black. The Darwin VAAC reported that the ash plume was discernable on HIMAWARI-8 satellite imagery for a short period of time. PVMBG noted ten eruptions on 24 August with white, gray, and black ejecta rising 100-300 m. A webcam installed at month’s end provided evidence of diffuse steam plumes rising 25-150 m above the crater during 28-31 August.

Figure 97. Only one tree survived on the once tree-covered spit off the NE end of Sertung Island after the December 2018 tsunami from Anak Krakatau covered it with ash and debris. The elevation of Anak Krakatau (center) was about 155 m on 8 August 2019, almost 100 m less than before the explosions and flank collapse. Panjang Island is on the left, and 746-m-high Rakata, the remnant of the 1883 volcanic island, is behind Anak Krakatau on the right. Courtesy of Amber Madden-Nadeau.

VONAs were issued for explosions on 1-3, 11, 13, 17, 18, 21, 24-27 and 29 September 2019. The explosion on 2 September produced a steam plume that rose 350 m, and dense black ash and ejecta which rose 200 m from the crater and drifted N. Gray and white tephra and steam rose 450 m on 13 and 17 September; ejecta was black and gray and rose 200 m on 21 September (figure 98). During 24-27 and 29 September tephra rose at least 200 m each day; some days it was mostly white with gray, other days it was primarily gray and black. All of the ejecta plumes drifted N. On days without explosions, the webcam recorded steam plumes rising 50-150 m above the crater.

Figure 98. Explosions of steam and dark ejecta were captured by the webcam on Anak Krakatau on 21 (left) and 26 (right) September 2019. Courtesy of MAGMA Indonesia and PVMBG.

Explosions were reported daily during 12-14, 16-20, 25-27, and 29 October (figure 99). PVMBG reported eight explosions on 19 October and seven explosions the next day. Most explosions produced gray and black tephra that rose 200 m from the crater and drifted N. On many of the days an ash plume also rose 350 m from the crater and drifted N. The seismic events that accompanied the explosions varied in duration from 45 to 1,232 seconds (about 20 minutes). The Darwin VAAC reported the 12 October eruption as visible briefly in satellite imagery before dissipating near the volcano. The first of four explosions on 26 October also appeared in visible satellite imagery moving NNW for a short time. The webcam recorded diffuse steam plumes rising 25-150 m above the crater on most days during the month.

Figure 99. A number of explosions at Anak Krakatau were captured by the webcam and visitors near the island during October 2019, shown here on the 12th, 14th, 17th, and 29th. Black and gray ejecta and steam plumes jetted several hundred meters high from the crater lake during the explosions. Webcam images courtesy of PVMBG and MAGMA Indonesia, with 12 October 2019 (top left) via VolcanoYT. Bottom left photo on 17 October courtesy of Christoph Sator.

Five VONAs were issued for explosions during 5-7 November, and one on 13 November 2019. The three explosions on 5 November produced 200-m-high plumes of steam and gray and black ejecta and ash plumes that rose 200, 450, and 550 m respectively; they all drifted N (figure 100). The Darwin VAAC reported ash drifting N in visible imagery for a brief period also. A 350-m-high ash plume accompanied 200-m-high ejecta on 6 November. Tephra rose 150-300 m from the crater during a 43 second explosion on 7 November. The explosion reported by PVMBG on 13 November produced black tephra and white steam 200 m high that drifted N. For the remainder of the month, when not obscured by fog, steam plumes rose daily 25-150 m from the crater.

Figure 100. PVMBG’s KAWAH webcam captured an explosion with steam and dark ejecta from the crater lake at Anak Krakatau on 5 November 2019. Courtesy of PVMBG and MAGMA Indonesia.

A joint expedition with PVMBG and the Earth Observatory of Singapore (EOS) installed geophysical equipment on Anak Krakatau and Rakata during 12 and 13 November 2019 (figure 101). Visitors to the island during 19-23 and 22-24 November recorded the short-lived landscape and continuing small explosions of steam and black tephra from the crater lake (figures 102 and 103).

Figure 101. A joint expedition to Anak Krakatau with PVMBG and the Earth Observatory of Singapore (EOS) installed geophysical equipment on Anak Krakatau and Rakata (background, left) during 12 and 13 November 2019. Images of the crater lake from the same spot (left) in December and January show the changes at the island (figure 108). Monitoring equipment installed near the shore sits over the many layers of ash and tephra that make up the island (right). Courtesy of Anna Perttu.

Figure 102.The crater lake at Anak Krakatau during a 19-23 November 2019 visit was the site of continued explosions with jets of steam and tephra that rose as high as 30 m. Courtesy of Andrey Nikiforov and Volcano Discovery, used with permission.

Figure 103. The landscape of Anak Krakatau recorded the rapidly evolving sequence of volcanic events during November 2019. Fresh ash covered recent lava near the shoreline on 22 November 2019 (top left). Large blocks of gray tephra (composed of other tephra fragments) were surrounded by reddish brown smaller fragments in the area between the crater and the ocean on 23 November 2019 (top right). Explosions of steam and black tephra rose tens of meters from the crater lake on 23 November 2019 (bottom). Courtesy of and copyright by Pascal Blondé.

Activity during December 2019-January 2020. Very little activity was recorded for most of December 2019. The webcam captured daily images of diffuse steam plumes rising 25-50 m above the crater which occasionally rose to 150 m. A new explosion on 28 December produced black and gray ejecta 200 m high that drifted N; the explosion was similar to those reported during August-November. A new series of explosions from 30 December 2019 to 1 January 2020 produced ash plumes which rose significantly higher than the previous explosions, reaching 2.4-3.0 km altitude and drifting S, E, and SE according to PVMBG (figure 104). They were initially visible in satellite imagery and reported drifting SW by the Darwin VAAC. By 31 December meteorological clouds prevented observation of the ash plume but a hotspot remained visible for part of that day.

Figure 104.The KAWAH webcam at Anak Krakatau captured this image of incandescent ejecta exploding from the crater lake on 30 December 2019 near the start of a new sequence of large explosions. Courtesy of PVMBG and Alex Bogár.

The explosions on 30 and 31 December 2019 were captured in satellite imagery (figure 105) and appeared to indicate that the crater lake was largely destroyed and filled with tephra from a new growing cone, according to Simon Carn. This was confirmed in both satellite imagery and ground-based photography in early January (figures 106 and 107).

Figure 105. Satellite imagery of the explosions at Anak Krakatau on 30 and 31 December 2019 showed dense steam rising from the crater (left) and a thermal anomaly visible through moderate cloud cover (right). Left image courtesy of Simon Carn, and copyright by Planet Labs, Inc. Right image uses Atmospheric Penetration rendering (bands 12, 11, and 8a) to show the thermal anomaly at the base of the steam plume, courtesy of Sentinel Hub Playground.

Figure 106. Sentinel-2 images of Anak Krakatau before (left, 21 December 2019) and after (right, 13 January 2020) explosions on 30 and 31 December 2019 show the filling in of the crater lake with new volcanic material. Natural color rendering based on bands 4,3, and 2. Courtesy of Sentinel Hub Playground.

Figure 107. The crater lake at Anak Krakatau changed significantly between the first week of December 2019 (left) and 8 January 2020 (right) after explosions on 30 and 31 December 2019. Compare with figure 101, taken from the same location in mid-November 2019. Left image courtesy of Piotr Smieszek. Right image courtesy of Peter Rendezvous.

Steam plumes rose 50-200 m above the crater during the first week of January 2020. An explosion on 7 January produced dense gray ash that rose 200 m from the crater and drifted E. Steam plume heights varied during the second week, with some plumes reaching 300 m above the crater. Multiple explosions on 15 January produced dense, gray and black ejecta that rose 150 m. Fog obscured the crater for most of the second half of the month; for a brief period, diffuse steam plumes were observed 25-1,000 m above the crater.

General Reference: Perttu A, Caudron C, Assink J D, Metz D, Tailpied D, Perttu B, Hibert C, Nurfiani D, Pilger C, Muzli M, Fee D, Andersen O L, Taisne B, 2020, Reconstruction of the 2018 tsunamigenic flank collapse and eruptive activity at Anak Krakatau based on eyewitness reports, seismo-acoustic and satellite observations, Earth and Planetary Science Letters, 541:116268. https://doi.org/10.1016/j.epsl.2020.116268.

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/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/); Amber Madden-Nadeau, Oxford University (URL: https://www.earth.ox.ac.uk/people/amber-madden-nadeau/, https://twitter.com/AMaddenNadeau/status/1159458288406151169); Anna Perttu, Earth Observatory of Singapore (URL: https://earthobservatory.sg/people/anna-perttu); Simon Carn, Michigan Tech University (URL: https://www.mtu.edu/geo/department/faculty/carn/; https://twitter.com/simoncarn/status/1211793124089044994); VolcanoYT, Indonesia (URL: https://volcanoyt.com/, https://twitter.com/VolcanoYTz/status/1182882409445904386/photo/1; Christoph Sator (URL: https://twitter.com/ChristophSator/status/1184713192670281728/photo/1); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Pascal Blondé, France (URL: https://pascal-blonde.info/portefolio-krakatau/, https://twitter.com/rajo_ameh/status/1199219837265960960); Alex Bogár, Budapest (URL: https://twitter.com/AlexEtna/status/1211396913699991557); Piotr (Piter) Smieszek, Yogyakarta, Java, Indonesia (URL: http://www.lombok.pl/, https://twitter.com/piotr_smieszek/status/1204545970962231296); Peter Rendezvous (URL: https://www.facebook.com/peter.rendezvous ); Wulkany swiata, Poland (URL: http://wulkanyswiata.blogspot.com/, https://twitter.com/Wulkany1/status/1214841708862693376).

Mayotte is a volcanic island in the Comoros archipelago between the eastern coast of Africa and the northern tip of Madagascar. A chain of basaltic volcanism began 10-20 million years ago and migrating W, making up four principal volcanic islands, according to the Institut de Physique du Globe de Paris (IPGP) and Cesca et al. (2020). Before May 2010, only two seismic events had been felt by the nearby community within recent decades. New activity since May 2018 consists of dominantly seismic events and lava effusion. The primary source of information for this report through February 2020 comes from semi-monthly reports from the Réseau de Surveillance Volcanologique et Sismologique de Mayotte (REVOSIMA), a cooperative program between the Institut de Physique du Globe de Paris (IPGP), the Bureau de Recherches Géologiques et Minières (BRGM), and the Observatoire Volcanologique du Piton de la Fournaise (OVPF-IPGP); Lemoine et al. (2019), the Centre National de la Recherche Scientifique (CNRS), and the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER).

Seismicity was the dominant type of activity recorded in association with a new submarine eruption. On 10 May 2018, the first seismic event occurred at 0814, detected by the YTMZ accelerometer from the French RAP Network, according to BRGM and Lemoine et al. (2019). Seismicity continued to increase during 13-15 May 2018, with the strongest recorded event for the Comoros area occurring on 15 May at 1848 and two more events on 20-21 May (figure 1). At the time, no surface effusion were directly observed; however, Global Navigation Satellite System (GNSS) instruments were deployed to monitor any ground motion (Lemoine et al. 2019).

Figure 1. A graph showing the number of daily seismic events greater than M 3.5 occurring offshore of Mayotte from 10 May 2018 through 15 February 2020. Seismicity significantly decreased in July 2018, but continued intermittently through February 2020, with relatively higher seismicity recorded in late August and mid-September 2018. Courtesy of IPGP and REVOSIMA.

Seismicity decreased dramatically after June 2018, with two spikes in August and September (see figure 1). Much of this seismicity occurred offshore 50 km E of Mayotte Island (figure 2). The École Normale Supérieure, the Observatoire Volcanologique du Piton de la Fournaise (OVPF-IPGP), and the REVOSIMA August 2019 bulletin reported that measurements from the GNSS stations and Teria GPS network data indicated eastward surface deformation and subsidence beginning in July 2018. Based on this ground deformation data Lemoine et al. (2019) determined that the eruptive phase began fifty days after the initial seismic events occurred, on 3 July 2018.

Figure 2. Maps of seismic activity offshore near Mayotte during May 2019. Seismic swarms occurred E of Mayotte Island (top) and continued in multiple phases through October 2019. New lava effusions were observed 50 km E of Petite Terre (bottom). Bottom image has been modified with annotations; courtesy of IPGP, BRGM, IFREMER, CNRS, and University of Paris.

Between 2 and 18 May 2019, an oceanographic campaign (MAYOBS 1) discovered a new submarine eruption site 50 km E from the island of Mayotte (figure 2). The director of IPGP, Marc Chaussidon, stated in an interview with Science Magazine that multibeam sonar waves were used to determine the elevation (800 m) and diameter (5 km) of the new submarine cone (figure 3). In addition, this multibeam sonar image showed fluid plumes within the water column rising from the center and flanks of the structure. According to REVOSIMA, these plumes rose to 1 km above the summit of the cone but did not breach the ocean surface. The seafloor image (figure 3) also indicated that as much as 5 km3 of magma erupted onto the seafloor from this new edifice during May 2019, according to Science Magazine.

Figure 3. Seafloor image of the submarine vent offshore of Mayotte created with multibeam sonar from 2 to 18 May 2019. The red line is the outline of the volcanic cone located at approximately 3.5 km depth. The blue-green color rising from the peak of the red outline represents fluid plumes within the water column. Courtesy of IPGP.

On 17 May 2019, a second oceanographic campaign (MAYOBS 2) discovered new lava flows located 5 km S of the new eruptive site. BRGM reported that in June a new lava flow had been identified on the W flank of the cone measuring 150 m thick with an estimated volume of 0.3 km3 (figure 4). According to REVOSIMA, the presence of multiple new lava flows would suggest multiple effusion points. Over a period of 11 months (July 2018-June 2019) the rate of lava effusion was at least 150-200 m3/s; between 18 May to 17 June 2019, 0.2 km3 of lava was produced, and from 17 June to 30 July 2019, 0.3 km3 of lava was produced. The MAYOBS 4 (19 July 2019-4 August 2019) and SHOM (20-21 August 2019) missions revealed a new lava flow formed between 31 July and 20 August to the NW of the eruptive site with a volume of 0.08 km3 and covering 3.25 km2.

Figure 4. Bathymetric map showing the location of the new lava flow on the W flank of the submarine cone offshore to the E of Mayotte Island. The MAYOBS 2 campaign was launched in June 2019 (left) and MAYOBS 4 was launched in late July 2019 (right). Courtesy of BRGM.

During the MAYOBS 4 campaign in late July 2019, scientists dredged the NE flank of the cone for samples and took photographs of the newly erupted lava (figure 5). Two dives found the presence of pillow lavas. When samples were brought up to the surface, they exploded due to the large amount of gas and rapid decompression.

Figure 5. Photographs taken using the submersible interactive camera system (SCAMPI) of newly formed pillow lavas (top) and a vesicular sample (bottom) dredged near the new submarine eruptive site at Mayotte in late July 2019. Courtesy of BRGM.

During April-May 2019 the rate of ground deformation slowed. Deflation was also observed up to 90 km E of Mayotte in late October 2019 and consistently between August 2019 and February 2020. Seismicity continued intermittently through February 2020 offshore E of Mayotte Island, though the number of detected events started to decrease in July 2018 (see figure 1). Though seismicity and deformation continued, the most recent observation of new lava flows occurred during the MAYOBS 4 and SHOM campaigns on 20 August 2019, as reported in REVOSIMA bulletins.

References: Cesca S, Heimann S, Letort J, Razafindrakoto H N T, Dahm T, Cotton F, 2020. Seismic catalogues of the 2018-2019 volcano-seismic crisis offshore Mayotte, Comoro Islands. Nat. Geosci. 13, 87-93. https://doi.org/10.1038/s41561-019-0505-5.

Lemoine A, Bertil D, Roulle A, Briole P, 2019. The volcano-tectonic crisis of 2018 east of Mayotte, Comoros islands. Preprint submitted to EarthArXiv, 28 February 2019. https://doi.org/10.31223/osf.io/d46xj.

Geologic Background. Mayotte, located in the Mozambique Channel between the northern tip of Madagascar and the eastern coast of Africa, consists two main volcanic islands, Grande Terre and Petite Terre, and roughly twenty islets within a barrier-reef lagoon complex (Zinke et al., 2005; Pelleter et al., 2014). Volcanism began roughly 15-10 million years ago (Pelleter et al., 2014; Nougier et al., 1986), and has included basaltic lava flows, nephelinite, tephrite, phonolitic domes, and pyroclastic deposits (Nehlig et al., 2013). Lavas on the NE were active from about 4.7 to 1.4 million years and on the south from about 7.7 to 2.7 million years. Mafic activity resumed on the north from about 2.9 to 1.2 million years and on the south from about 2 to 1.5 million years. Several pumice layers found in cores on the barrier reef-lagoon complex indicate that volcanism likely occurred less than 7,000 years ago (Zinke et al., 2003). More recent activity that began in May 2018 consisted of seismicity and ground deformation occurring offshore E of Mayotte Island (Lemoine et al., 2019). One year later, in May 2019, a new subaqueous edifice and associated lava flows were observed 50 km E of Petite Terre during an oceanographic campaign.

Information Contacts: Réseau de Surveillance Volcanologique et Sismologique de Mayotte (REVOSIMA), a cooperative program of a) Institut de Physique du Globe de Paris (IPGP), b) Bureau de Recherches Géologiques et Minières (BRGM), c) Observatoire Volcanologique du Piton de la Fournaise (OVPF-IPGP); (URL: http://www.ipgp.fr/fr/reseau-de-surveillance-volcanologique-sismologique-de-mayotte); Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); Bureau de Recherches Géologiques et Minières (BRGM), 3 avenue Claude-Guillemin, BP 36009, 45060 Orléans Cedex 2, France (URL: https://www.brgm.fr/); Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), 1625 route de Sainte-Anne, CS 10070, 29280 Plouzané, France (URL: https://wwz.ifremer.fr/); Centre National de la Recherche Scientifique (CNRS), 3 rue Michel-Ange, 75016 Paris, France (URL: http://www.cnrs.fr/); École Normale Supérieure, 45 rue d'Ulm, F-75230 Paris Cedex 05, France (URL: https://www.ens.psl.eu/); Université de Paris, 85 boulevard Saint-Germain, 75006 Paris, France (URL: https://u-paris.fr/en/498-2/); Roland Pease, Science Magazine (URL: https://science.sciencemag.org/, article at https://www.sciencemag.org/news/2019/05/ship-spies-largest-underwater-eruption-ever) published 21 May 2019.

Fernandina is a volcanic island in the Galapagos islands, around 1,000 km W from the coast of mainland Ecuador. It has produced nearly 30 recorded eruptions since 1800, with the most recent events having occurred along radial or circumferential fissures around the summit crater. The most recent previous eruption, starting on 16 June 2018, lasted two days and produced lava flows from a radial fissure on the northern flank. Monitoring and scientific reports come from the Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN).

A report from IG-EPN on 12 January 2020 stated that there had been an increase in seismicity and deformation occurring during the previous weeks. On the day of the report, 11 seismic events had occurred, with the largest magnitude of 4.7 at a depth of 5 km. Shortly before 1810 that day a circumferential fissure formed below the eastern rim of the La Cumbre crater, at about 1.3-1.4 km elevation, and produced lava flows down the flank (figure 39). A rapid-onset seismic swarm reached maximum intensity at 1650 on 12 January (figure 40); a second increase in seismicity indicating the start of the eruption began around 70 minutes later (1800). A hotspot was observed in NOAA / CIMSS data between 1800 and 1810, and a gas plume rising up to 2 km above the fissure dispersed W to NW. The eruption lasted 9 hours, until about 0300 on 13 January.

Figure 39. Lava flows erupting from a circumferential fissure on the eastern flank of Fernandina on 12 January 2020. Photos courtesy of Parque Nacional Galápagos.

Figure 40. Graph showing the Root-Mean-Square (RMS) amplitude of the seismic signals from the FER-1 station at Fernandina on 12-13 January 2020. The graph shows the increase in seismicity leading to the eruption on the 12th (left star), a decrease in the seismicity, and then another increase during the event (right star). Courtesy of S. Hernandez, IG-EPN (Report on 13 January 2020).

A report issued at 1159 local time on 13 January 2020 described a rapid decrease in seismicity, gas emissions, and thermal anomalies, indicating a rapid decline in eruptive activity similar to previous events in 2017 and 2018. An overflight that day confirmed that the eruption had ended, after lava flows had extended around 500 m from the crater and covered an area of 3.8 km² (figures 41 and 42). Seismicity continued on the 14th, with small volcano-tectonic (VT) earthquakes occurring less than 500 m below the surface. Periodic seismicity was recorded through 13-15 January, though there was an increase in seismicity during 17-22 January with deformation also detected (figure 43). No volcanic activity followed, and no additional gas or thermal anomalies were detected.

Figure 41. The lava flow extents at Fernandina of the previous two eruptions (4-7 September 2017 and 16-21 June 2018) and the 12-13 January 2020 eruption as detected by FIRMS thermal anomalies. Thermal data courtesy of NASA; figure prepared by F. Vásconez, IG-EPN (Report on 13 January 2020).

Figure 42. This fissure vent that formed on the E flank of Fernandina on 12 January 2020 produced several lava flows. A weak gas plume was still rising when this photo was taken the next day, but the eruption had ceased. Courtesy of Parque Nacional Galápagos.

Figure 43. Soil displacement map for Fernandina during 10 and 16 January 2020, with the deformation generated by the 12 January eruption shown. Courtesy of IG-EPN (Report on 23 January 2020).

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

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Dirección del Parque Nacional Galápagos (DPNG), Isla Santa Cruz, Galápagos, Ecuador (URL: http://www.galapagos.gob.ec/).

Masaya is a basaltic caldera located in Nicaragua and contains the Nindirí, San Pedro, San Juan, and Santiago craters. The currently active Santiago crater hosts a lava lake, which has remained active since December 2015 (BGVN 41:08). The primary source of information for this August 2019-January 2020 report comes from the Instituto Nicareguense de Estudios Territoriales (INETER) and satellite -based imagery and thermal data.

On 16 August, 13 September, and 11 November 2019, INETER took SO2 measurements by making a transect using a mobile DOAS spectrometer that sampled for gases downwind of the volcano. Average values during these months were 2,095 tons/day, 1,416 tons/day, and 1,037 tons/day, respectively. August had the highest SO2 measurements while those during September and November were more typical values.

Satellite imagery showed a constant thermal anomaly in the Santiago crater at the lava lake during August 2019 through January 2020 (figure 82). According to a news report, ash was expelled from Masaya on 15 October 2019, resulting in minor ashfall in Colonia 4 de Mayo (6 km NW). On 21 November thermal measurements were taken at the fumaroles and near the lava lake using a FLIR SC620 thermal camera (figure 83). The temperature measured 287°C, which was 53° cooler than the last time thermal temperatures were taken in May 2019.

Figure 82. Sentinel-2 thermal satellite imagery showed the consistent presence of an active lava lake within the Santiago crater at Masaya during August 2019 through January 2020. Images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 83. Thermal measurements taken at Masaya on 21 November 2019 with a FLIR SC620 thermal camera that recorded a temperature of 287°C. Courtesy of INETER (Boletin Sismos y Volcanes de Nicaragua, Noviembre, 2019).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent low-power thermal anomalies compared to the higher-power ones before May 2019 (figure 84). The thermal anomalies were detected during August 2019 through January 2020 after a brief hiatus from early may to mid-June.

Figure 84. Thermal anomalies occurred intermittently at Masaya during 21 February 2019 through January 2020. Courtesy of MIROVA.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Historical lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); La Jornada (URL: https://www.lajornadanet.com/, article at https://www.lajornadanet.com/index.php/2019/10/16/volcan-masaya-expulsa-cenizas/#.Xl6f8ahKjct).

Reventador is an andesitic stratovolcano located in the Cordillera Real, Ecuador. Historical eruptions date back to the 16th century, consisting of lava flows and explosive events. The current eruptive activity has been ongoing since 2008 with previous activity including daily explosions with ash emissions, and incandescent block avalanches (BGVN 44:08). This report covers volcanism from August 2019 through January 2020 using information primarily from the Instituto Geofísico (IG-EPN), the Washington Volcano Ash Advisory Center (VAAC), and various infrared satellite data.

During August 2019 to January 2020, IG-EPN reported almost daily explosive eruptions and ash plumes. September had the highest average of explosive eruptions while January 2020 had the lowest (table 11). Ash plumes rose between a maximum of 1.2 to 2.5 km above the crater during this reporting period with the highest plume height recorded in December. The largest amount of SO2 gases produced was during the month of October with 502 tons/day. Frequently at night during this reporting period, crater incandescence was observed and was occasionally accompanied by incandescent block avalanches traveling as far as 900 m downslope from the summit of the volcano.

Table 11. Monthly summary of eruptive events recorded at Reventador from August 2019 through January 2020. Data courtesy of IG-EPN (August to January 2020 daily reports).

During the month of August 2019, between 11 and 45 explosions were recorded every day, frequently accompanied by gas-and-steam and ash emissions (figure 119); plumes rose more than 1 km above the crater on nine days. On 20 August the ash plume rose to a maximum 1.6 km above the crater. Summit incandescence was seen at night beginning on 10 August, continuing frequently throughout the rest of the reporting period. Incandescent block avalanches were reported intermittently beginning that same night through 26 January 2020, ejecting material between 300 to 900 m below the summit and moving on all sides of the volcano.

Figure 119. An ash plume rising from the summit of Reventador on 1 August 2019. Courtesy of Radio La Voz del Santuario.

Throughout most of September 2019 gas-and-steam and ash emissions were observed almost daily, with plumes rising more than 1 km above the crater on 15 days, according to IG-EPN. On 30 September, the ash plume rose to a high of 1.7 km above the crater. Each day, between 18 and 72 explosions were reported, with the latter occurring on 19 September. At night, crater incandescence was commonly observed, sometimes accompanied by incandescent material rolling down every flank.

Elevated seismicity was reported during 8-15 October 2019 and almost daily gas-and-steam and ash emissions were present, ranging up to 1.3 km above the summit. Every day during this month, between 13 and 54 explosions were documented and crater incandescence was commonly observed at night. During November 2019, gas-and-steam and ash emissions rose greater than 1 km above the crater except for 10 days; no emissions were reported on 29 November. Daily explosions ranged up to 42, occasionally accompanied by crater incandescence and incandescent ejecta.

Washington VAAC notices were issued almost daily during December 2019, reporting ash plumes between 4.6 and 6 km altitude throughout the month and drifting in multiple directions. Each day produced 5-52 explosions, many of which were accompanied by incandescent blocks rolling down all sides of the volcano up to 900 m below the summit. IG-EPN reported on 11 December that a gas-and-steam and ash emission column rose to a maximum height of 2.5 km above the crater, drifting SW as was observed by satellite images and reported by the Washington VAAC.

Volcanism in January 2020 was relatively low compared to the other months of this reporting period. Explosions continued on a nearly daily basis early in the month, ranging from 20 to 51. During 5-7 January incandescent material ejected from the summit vent moved as block avalanches downslope and multiple gas-and-steam and ash plumes were produced (figures 120, 121, and 122). After 9 January the number of explosions decreased to 0-16 per day. Ash plumes rose between 4.6 and 5.8 km altitude, according to the Washington VAAC.

Figure 120. Night footage of activity on 5 (top) and 6 (bottom) January 2020 at the summit of Reventador, producing a dense, dark gray ash plume and ejecting incandescent material down multiple sides of the volcano. This activity is not uncommon during this reporting period. Courtesy of Martin Rietze, used with permission.

Figure 121. An explosion at Reventador on 7 January 2020, which produced a dense gray ash plume. Courtesy of Martin Rietze, used with permission.

Figure 122. Night footage of the evolution of an eruption on 7 January 2020 at the summit of Reventador, which produced an ash plume and ejected incandescent material down multiple sides of the volcano. Courtesy of Martin Rietze, used with permission.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed frequent and strong thermal anomalies within 5 km of the summit during 21 February 2019 through January 2020 (figure 123). In comparison, the MODVOLC algorithm reported 24 thermal alerts between August 2019 and January 2020 near the summit. Some thermal anomalies can be seen in Sentinel-2 thermal satellite imagery throughout this reporting period, even with the presence of meteorological clouds (figure 124). These thermal anomalies were accompanied by persistent gas-and-steam and ash plumes.

Figure 123. Thermal anomalies at Reventador persisted during 21 February 2019 through January 2020 as recorded by the MIROVA system (Log Radiative Power). Courtesy of MIROVA.

Figure 124. Sentinel-2 thermal satellite images of Reventador from August 2019 to January 2020 showing a thermal hotspot in the central summit crater summit. In the image on 7 January 2020, the thermal anomaly is accompanied by an ash plume. Courtesy of Sentinel Hub Playground.

Geologic Background. Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic Volcán El Reventador stratovolcano rises to 3562 m above the jungles of the western Amazon basin. A 4-km-wide caldera widely breached to the east was formed by edifice collapse and is partially filled by a young, unvegetated stratovolcano that rises about 1300 m above the caldera floor to a height comparable to the caldera rim. It has been the source of numerous lava flows as well as explosive eruptions that were visible from Quito in historical time. Frequent lahars in this region of heavy rainfall have constructed a debris plain on the eastern floor of the caldera. The largest historical eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Radio La Voz del Santuario (URL: https://www.facebook.com/Radio-La-Voz-del-Santuario-126394484061111/, posted at: https://www.facebook.com/permalink.php?story_fbid=2630739100293291&id=126394484061111); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos).

Pacaya is a highly active basaltic volcano located in Guatemala with volcanism consisting of frequent lava flows and Strombolian explosions originating in the Mackenney crater. The previous report summarizes volcanism that included multiple lava flows, Strombolian activity, avalanches, and gas-and-steam emissions (BGVN 44:08), all of which continue through this reporting period of August 2019 to January 2020. The primary source of information comes from reports by the Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) in Guatemala and various satellite data.

Strombolian explosions occurred consistently throughout this reporting period. During the month of August 2019, explosions ejected material up to 30 m above the Mackenney crater. These explosions deposited material that contributed to the formation of a small cone on the NW flank of the Mackenney crater. White and occasionally blue gas-and-steam plumes rose up to 600 m above the crater drifting S and W. Multiple incandescent lava flows were observed traveling down the N and NW flanks, measuring up to 400 m long. Small to moderate avalanches were generated at the front of the lava flows, including incandescent blocks that measured up to 1 m in diameter. Occasionally incandescence was observed at night from the Mackenney crater.

In September 2019 seismicity was elevated compared to the previous month, registering a maximum of 8,000 RSAM (Realtime Seismic Amplitude Measurement) units. White and occasionally blue gas-and-steam plumes that rose up to 1 km above the crater drifted generally S as far as 3 km from the crater. Strombolian explosions continued, ejecting material up to 100 m above the crater rim. At night and during the early morning, crater incandescence was observed. Incandescent lava flows traveled as much as 600 m down the N and NW flanks toward the Cerro Chino crater (figure 116). On 21 September two lava flows descended the SW flank. Constant avalanches with incandescent blocks measuring 1 m in diameter occurred from the front of many of these lava flows.

Figure 116. Webcam image of Pacaya on 25 September 2019 showing thermal signatures and the point of emission on the NNW flank at night using Landsat 8 (Nocturnal) imagery (left) and a daytime image showing the location of these lava effusions (right) along with gas-and-steam emissions from the active crater. Courtesy of INSIVUMEH.

Weak explosions continued through October 2019, ejecting material up to 75 m above the crater and building a small cone within the crater. White and occasionally blue gas-and-steam plumes rose 400-800 m above the crater, drifting W and NW and extending up to 4 km from the crater during the week of 26 October-1 November. Lava flows measuring up to 250 m long, originating from the Mackenney crater were descending the N and NW flanks (figure 117). Avalanches carrying large blocks 1 m in diameter commonly occurred at the front of these lava flows.

Figure 117. Photo of lava flows traveling down the flanks of Pacaya taken between 28 September 2019 and 4 October. Courtesy of INSIVUMEH (28 September 2019 to 4 October Weekly Report).

Continuing Strombolian explosions in November 2019 ejected material 15-75 m above the crater, which then contributed to the formation of the new cone. White and occasionally blue gas-and-steam plumes rose 100-600 m above the crater drifting in different directions and extending up to 2 km. Multiple lava flows from the Mackenney crater moving down all sides of the volcano continued, measuring 50-700 m long. Avalanches were generated at the front of the lava flows, often moving blocks as large as 1 m in diameter. The number of lava flows decreased during 2-8 November and the following week of 9-15 November no lava flows were observed, according to INSIVUMEH. During the week of 16-22 November, a small collapse occurred in the Mackenney crater and explosive activity increased during 16, 18, and 20 November, reaching RSAM units of 4,500. At night and early morning in late November crater incandescence was visible. On 24 November two lava flows descended the NW flank toward the Cerro Chino crater, measuring 100 m long.

During December 2019, much of the activity remained the same, with Strombolian explosions originating from two emission points in the Mackenney crater ejecting material 75-100 m above the crater; white and occasionally blue gas-and-steam plumes to 100-300 m above the crater drifted up to 1.5 km downwind to the S and SW. Lava flows descended the S and SW flanks reaching 250-600 m long (figure 118). On 29 December seismicity increased, reaching 5,000 RSAM units.

Figure 118. Lava flows moving to the S and SW at Pacaya on 31 December 2019. Courtesy of INSIVUMEH (28 December 2019 to 3 January 2020 Weekly Report).

Consistent Strombolian activity continued into January 2020 ejecting material 25-100 m above the crater. These explosions deposited material inside the Mackenney crater, contributing to the formation of a small cone. White and occasionally blue fumaroles consisting of mostly water vapor were observed drifting in different directions. At night, summit incandescence and lava flows were visible descending the N, NW, and S flanks with the flow on the NW flank traveling toward the Cerro Chino crater.

During August 2019 through January 2020, multiple lava flows and bright thermal anomalies (yellow-orange) within the crater were seen in Sentinel-2 thermal satellite imagery (figures 119 and 120). In addition, constant strong thermal anomalies were detected by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during 21 February 2019 through January 2020 within 5 km of the summit (figure 121). A slight decrease in energy was seen from May to June and August to September. Energy increased again between November and December. According to the MODVOLC algorithm, 37 thermal alerts were recorded during August 2019 through January 2020.

Figure 119. Sentinel-2 thermal satellite images of Pacaya showing thermal activity (bright yellow-orange) during August 2019 to November. All images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 120. Sentinel-2 thermal satellite images of Pacaya showing thermal activity (bright yellow-orange) during December 2019 through January 2020. All images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 121. The MIROVA thermal activity graph (log radiative power) at Pacaya during 21 February 2019 to January 2020 shows strong, frequent thermal anomalies through January with a slight decrease in energy between May 2019 to June 2019 and August 2019 to September 2019. Courtesy of MIROVA.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The 19-km-wide submerged Kikai caldera at the N end of Japan’s Ryukyu Islands was the source of one of the world's largest Holocene eruptions about 6,300 years ago, producing large pyroclastic flows and abundant ashfall. During the last century, however, only intermittent minor ash emissions have characterized activity at Satsuma Iwo Jima island, the larger subaerial fragment of the Kikai caldera; several events have included limited ashfall in communities on nearby islands. The most recent event was a single day of explosions on 4 June 2013 that produced ash plumes and minor ashfall on the flank. A minor episode of increased seismicity and fumarolic activity was reported in late March 2018, but no ash emissions were reported. A new single-day event on 2 November 2019 is described here with information provided by the Japan Meteorological Agency (JMA).

JMA reduced the Alert Level to 1 on 27 April 2018 after a brief increase in seismicity during March 2018 (BGVN 45:05); no significant changes in volcanic activity were observed for the rest of the year. Steam plumes rose from the summit crater to heights around 1,000 m; the highest plume rose 1,800 m. Occasional nighttime incandescence was recorded by high-sensitivity surveillance cameras. SO₂ measurements made during site visits in March, April, and May indicated amounts ranging from 300-1,500 tons per day, similar to values from 2017 (400-1,000 tons per day). Infrared imaging devices indicated thermal anomalies from fumarolic activity persisted on the N and W flanks during the three site visits. A field survey of the SW flank on 25 May 2018 confirmed that the crater edge had dropped several meters into the crater since a similar survey in April 2007. Scientists on a 19 December 2018 overflight had observed fumarolic activity.

There were no changes in activity through October 2019. Weak incandescence at night continued to be periodically recorded with the surveillance cameras (figure 9). A brief eruption on 2 November 2019 at 1735 local time produced a gray-white plume that rose slightly over 1,000 m above the Iodake crater rim (figure 10). As a result, JMA raised the Alert Level from 1 to 2. During an overflight the following day, a steam plume rose a few hundred meters above the summit, but no further activity was observed. No clear traces of volcanic ash or other ejecta were found around the summit (figure 11). Infrared imaging also showed no particular changes from previous measurements. Discolored seawater continued to be observed around the base of the island in several locations.

Figure 9. Incandescence at night on 25 October 2019 was observed at Satsuma Iwo Jima (Kikai) with the Iwanogami webcam. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, October 1st year of Reiwa [2019]).

Figure 10. The Iwanogami webcam captured a brief gray-white ash and steam emission rising above the Iodake crater rim on Satsuma Iwo Jima (Kikai) on 2 November 2019 at 1738 local time. The plume rose slightly over 1,000 m before dissipating. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, October 1st year of Reiwa [2019]).

Figure 11. During an overflight of Satsuma Iwo Jima (Kikai) on 3 November 2019 no traces of ash were seen from the previous day’s explosion; only steam plumes rose a few hundred meters above the summit, and discolored water was present in a few places around the shoreline. Courtesy of JMA (An explanation of volcanic activity at Satsuma Iwo Jima, October 1st year of Reiwa [2019]).

For the remainder of November 2019, steam plumes rose up to 1,300 m above the summit, and nighttime incandescence was occasionally observed in the webcam. Seismic activity remained low and there were no additional changes noted through January 2020.

Geologic Background. Kikai is a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world's largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post-caldera eruptions formed Iodake lava dome and Inamuradake scoria cone, as well as submarine lava domes. Historical eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Tokara-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Tokara-Iojima.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html).

Nevado del Ruiz is a glaciated stratovolcano located in Colombia. It is most known for the eruption on 13 November 1985 that produced an ash plume and pyroclastic flows onto the glacier, triggering a lahar and killing approximately 25,000 people in the towns of Armero (46 km W) and Chinchiná (34 km E). Since the September 1985-July 1991 eruption, volcanism has occurred dominantly at the Arenas crater, with eruptive periods during February 2012-July 2013 and November 2014-May 2017 (BGVN 42:06 and 44:12). The previous eruption included ash and gas-and-steam plumes, ashfall, and thermal anomalies through May 2017, after which no clear observations of ongoing activity were available until an ash plume was seen in satellite and webcam images on 18 December 2017. This report provides data and observations from January 2018 through December 2019 using information primarily from reports by the Servicio Geologico Colombiano and the Observatorio Vulcanológico y Sismológico de Manizales, the Washington Volcanic Ash Advisory Center (VAAC) notices, and various satellite data.

Summary of activity during December 2017-December 2019. Although data is incomplete, the current eruptive period is considered to have begun with the emission of an ash plume on 18 December 2017. The Washington VAAC issued an advisory that day for an ash plume to 6 km that was moving west and dispersing, further describing it as a "thin veil of volcanic ash and gasses" that was seen in visible satellite imagery, NOAA/CIMSS, and supported by webcam imagery.

Reports of significant ash plumes visible in satellite imagery were infrequent in 2018 and 2019, with a few notable pulses in July 2018, February-March 2019, and August-September 2019 (figure 95). Sentinel-2 thermal satellite data in comparison with Suomi NPP/VIIRS sensor data, and the MODVOLC algorithm for MODIS data registered infrared thermal hotspots intermittently throughout 2018 to 2019 with more frequent anomalies during January-March 2018, August 2018, October 2018-February 2019, and November-December 2019; observations during March-June of each year were low. Identification of SO₂ emissions were frequent and consistent during all of 2018-2019 (figure 96).

Figure 95. Timeline summary of observed activity at Nevado del Ruiz from January 2018 through December 2019. VAAC reports typically indicate a significant ash plume. Satellite-based SO2 data is variable with respect to volume of emitted gas, but reflects a point source at the volcano. For Sentinel-2, MODVOLC, and VIIRS data, the dates indicated represents detected thermal anomalies. White areas indicate no activity was observed, which may also be due to meteoric clouds. Data courtesy of Washington VAAC, NASA Goddard Space Flight Center, Sentinel Hub Playground, HIGP, and NASA Worldview using the "Fire and Thermal Anomalies" layer.

Figure 96. Examples of SO2 plumes from Nevado del Ruiz detected by the Aura/OMI instrument during 12 May (top left), 7 October (top middle), and 29 November 2018 (top right) and 9 January (bottom left), 30 March (bottom middle), and 6 October 2019 (bottom right). Courtesy of NASA Goddard Space Flight Center.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows weak thermal anomalies within 5 km of the summit occurring dominantly between October 2018 through March 2019 (figure 97). Between April and October 2019, the number of thermal anomalies was low, registering eight during this time. The number of thermal signatures increased at the beginning of November 2019 and continued through the rest of 2019.

Figure 97. Weak thermal anomalies at Nevado del Ruiz for 25 September 2018 through December 2019 as recorded by the MIROVA system (log radiative power) occurred mostly during December 2018 through March 2019. Activity was low during April to October 2019 with renewed signatures in November 2019. Courtesy of MIROVA.

Seismicity that occurred during 2018-2019 was located mainly in the Arenas crater and consisted of low-frequency (LF) and very low-frequency (VLF) earthquakes and volcanic tremors, many of which were associated with minor gas-and-steam and ash emissions confirmed through webcams. The number of earthquakes reported by SGC fluctuated each week, but the energy remained relatively consistent. The highest magnitude earthquake that occurred during 2018 was on 26 October reaching 3.1 ML (local magnitude) and during 2019 the largest was 2.8 ML on 21 April.

Activity during 2018. Throughout 2018, gas-and-steam plumes, mostly composed of water vapor and sulfur dioxide frequently occurred, rising to a maximum of 2.2 km above the Arenas crater on 24 March. Weak thermal anomalies were seen intermittently in thermal satellite imagery from Sentinel-2 and NASA Worldview during 4 January through March and September to December (figure 98). Activity during March to April 2018 was relatively low and consisted dominantly of gas-and-steam emissions, low-energy seismicity, and intermittent thermal anomalies. Between 9 May and 5 August, no thermal signatures were detected.

Figure 98. Sentinel-2 thermal satellite imagery detected thermal anomalies (bright yellow-orange) within the Arenas crater at Nevado del Ruiz that were mostly visible during the beginning and last months of 2018. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

Ash plumes were seen in GOES-EAST satellite imagery, through webcams, and by SGC personnel. The first ash plume of 2018 occurred on 21 April at 0800, six days after NASA Worldview detected a thermal anomaly within the Arenas crater. The plume rose 6 km altitude and drifted NW as seen in GOES-EAST satellite imagery and reported by the Washington VAAC. Weak gas-and-steam and ash emissions were confirmed by webcams on 22 July, associated with a volcanic tremor. On 11 August 2018, another ash plume was reported in a VAAC notice rising 6.7 km altitude drifting W. During the week of 21 August, SGC reported that seismicity in the Arenas crater was associated with minor gas-and-steam and ash emissions, as confirmed by webcams.

The number of ash plumes increased during September (figure 99), one of which reached a maximum altitude of 7.3 km on 2 September. On 5 September, a continuous volcanic tremor occurred and was accompanied by an ash plume rising 7 km altitude drifting W, according to a Washington VAAC report. Ashfall was observed during the week of 11 September in Manizales (30 km NW) and Villamaría (27 km NW). A new volcanic tremor occurred on 15 September and was accompanied by various ash emissions reaching 1.4 km above the crater and drifting NW as confirmed by PNNN, inhabitants within the vicinity of the volcano, and the Washington VAAC. Seismicity continuing into the weeks of 25 September and 2 October was also accompanied by ash emissions, rising to an altitude of 1.4 km above the crater on 22 September. The number of reported gas-and-steam and ash emissions decreased after September; ash emissions were reported by SGC on 19, 22, 26, and 31 October, 6, 9, and 17 November, and 14 December.

Figure 99. Webcam images of gas-and-steam and ash plumes rising from Nevado del Ruiz during 2018. Courtesy of Servicio Geologico Colombiano.

Activity during 2019. Gas-and-steam and ash emissions continued intermittently through 2019, with an increased number of ash emissions compared to the previous year. Infrared hotspots were detected in Sentinel-2 satellite imagery primarily during January-February 2019 and December 2019, often accompanied by gas-and-steam emissions (figure 100). An ash plume was seen in GOES-EAST satellite imagery on 2 January 2019, rising to an altitude of 5.8 km and drifting NW, according to a Washington VAAC report. On 7 January, ashfall in Manizales and Villamaría was observed. A thermal hotspot was detected in multispectral imagery, according to a Washington VAAC report on 29 January. Slight ground deformation was observed by GNSS and electronic inclinometers during the weeks of 29 January and 10 September. Volcanism was relatively low during February to March and consisted of mostly gas-and-steam emissions and rare ash plumes; these ash emissions were reported on 2 and 9 February and 16 March by the Washington VAAC rising between 5.8-6.7 km altitude. Gas-and-steam emission were detected on 6 and 17 February and 17 and 21 March.

Figure 100. Sentinel-2 thermal satellite imagery detected thermal anomalies (bright yellow-orange) mostly visible within the Arenas crater at Nevado del Ruiz during the last three months of 2019 and were accompanied by gas-and-steam emissions. Sentinel-2 atmospheric penetration (bands 12, 11, 8A) images courtesy of Sentinel Hub Playground.

The number of ash emissions detected in satellite imagery increased after March, occurring on 4, 7, 16, 17-19, and 23-26 April and 2 and 4-5 May. Ash plumes were detected on 27 June, 4, 7, 8, and 29 July, 1 August, and on 19, 29, and 30 September. Los Nevados National Natural Park (PNNN) personnel reported that the ash plume on 8 July was accompanied by gas-and-steam emissions and a continuous tremor occurring at 0722 (figure 101). These emissions rose 450 m above the crater and drifted W. On 29 September, a tremor associated with an ash plume occurred at 2353. The ash plume rose to a maximum altitude of 8.5 km drifted NW, resulting in ashfall confirmed by PNNN, GOES-EAST satellite imagery, and SGC personnel in the field.

Seismicity increased during the week of 1 October compared to the previous week, which was accompanied by several gas-and-steam and ash emissions rising 1 km altitude drifting NW observed by webcams, PNNN personnel, and GOES-EAST satellite imagery. An ash plume rising 7 km altitude drifting NW on 4 October resulted in fine ashfall in Manizales. Ash plumes rose to an altitude of 7.3 km drifting N on 5, 9, and 16 October and was seen in the GOES-EAST satellite according to Washington VAAC notices. Ash emissions were observed frequently during November; 11 Washington VAAC notices, the most for any month during 2019, reported emissions ranging 5.8 to 7 km altitude drifting in different directions. Gas-and-steam plumes rose to a maximum of 2.4 km above the crater during 14 and 30 November. The number of reported emissions decreased during December with one ash emission observed on 4 December.

Figure 101. Webcam images of gas-and-steam and ash plumes rising from Nevado del Ruiz during 2019. Courtesy of Servicio Geologico Colombiano.

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: Servicio Geologico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Antarctica's Ross Island, 35 km SSW. Because of the remoteness of the volcano, activity is primarily monitored using satellites (figure 27), including MODIS infrared detectors aboard the Aqua and Terra satellites and analyzed using the MODVOLC algorithm.

Figure 27. Satellite image of Erebus (on left) acquired on 19 October 2019 by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite. The false-color combines visible and near-infrared wavelengths of light (ASTER bands 3, 2, 1). The area was just days away from constant 24-hour sunlight when this image was acquired, with the Sun angle low enough to cast a long shadow towards the west. The blue patches are areas clear of surface snow, exposing glacial ice. Nearby areas that appear smooth are the snow- and ice-topped waters of McMurdo Sound. Courtesy of NASA Earth Observatory: image by Joshua Stevens, using data from NASA/METI/AIST/Japan Space Systems and U.S./Japan ASTER Science Team; description by Kathryn Hansen.

Available since 2000, MODIS-MODVOLC data have shown a strong and nearly continuous thermal signal through 2019. A compilation of thermal alert pixels during 2017-2019 (table 5, continuing the table in BGVN 44:01) shows a wide range of detected activity in 2019, with a high of 162 in April. Infrared satellite imagery from Sentinel-2 identified one or two lava lakes during January-March and September-December 2019; a few of the images showed gas emissions, possibly from melted snow (figure 28).

Table 5. Number of monthly MODVOLC thermal alert pixels recorded at Erebus from 1 January 2017 to 31 December. Table compiled using data provided by the Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System.

Figures 28. Sentinel-2 satellite image of Erebus in color infrared (bands 8, 4, 3) on 20 October 2019 showing two lava lakes in the summit crater. Courtesy of Sentinel Hub Playground.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); 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/).

Frequent activity at Ecuador's Sangay has included pyroclastic flows, lava flows, ash plumes, and lahars since 1628. Its remoteness on the east side of the Andean crest has made ground observations difficult until recent times. The current eruption began in March 2019; this report covers ongoing activity from July through December 2019. Information is provided by Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), and a number of sources of remote data including the Washington Volcanic Ash Advisory Center (VAAC), the Italian MIROVA Volcano HotSpot Detection System, and Sentinel-2 satellite imagery.

The eruption that began in March 2019 continued during July-December 2019 with activity focused on two eruptive centers at the summit, the Cráter Central and the Ñuñurco (southeast) vent. The Cráter Central produced explosive activity which generated small ash emissions that rose up to 3.2 km above the crater and were frequently directed towards the W and SW. Associated with these emissions in early November, ashfall was reported in Chimborazo province and elsewhere, and ejecta from explosions was deposited on all the upper flanks. At the Ñuñurcu vent, effusive activity resulted in an almost continuous emission of material down the SE flank. Small rockfalls and pyroclastic flows along the fronts and sides of the flows reached the basin and upper channel of the Volcán river which flows into the Upano river. These deposits were remobilized by rainfall and formed mud and debris flows (lahars) in the Volcán river, which caused damming at the confluence with the Upano river downstream. Increased thermal activity was recorded by the MIROVA system from mid-May 2019 through the end of the year, corresponding to the ongoing lava flow and explosive activity (figure 36).

Figure 36. Increased heat flow at Sangay was recorded beginning in mid-May 2019 and continued steadily through the end of the year as seen in this graph of Log Radiative Power produced by the MIROVA project. Courtesy of MIROVA.

Activity during July-September 2019. Several ash emissions were reported by the Washington VAAC during the first part of July 2019. On 1 July a plume rose to 6.7 km altitude and extended 45 km WSW from the summit. During 3-4 July a plume rose 6.4 km and drifted WNW; it included occasional discrete emissions that extended approximately 35 km from summit. The VAAC recorded a bright hotspot in SWIR imagery on 4 July. On 11 July a 7.3-km-altitude ash plume detached from the summit and extended from immediately W of the summit S past Segu. Webcam and satellite imagery on 11 July demonstrated the continuing thermal activity of the lava flow on the SE flank and ash emissions drifting W (figure 37). On 29 July a plume rose to 7.6 km altitude and drifted 65 km WSW. Later in the day continuous emissions were drifting SW from the summit at 5.8 km altitude before dissipating. The first satellite images of 30 July showed a plume extending 110 km WSW from the summit at 7 km altitude. Activity decreased later in the day and the plume extended W about 45 km from the summit at 6.4 km altitude. Composite satellite imagery on 31 July showed almost constant ash emissions extending over 150 km W of the summit (figure 38).

Figure 37. The local webcam at Sangay (left) and Sentinel satellite imagery (right, bands 12, 11, and 8A) both confirmed the high heat output from the active lava flow on the SE flank on 11 July 2019. The flow is about 2 km long. A plume of steam and ash also drifted W from the summit (right). Courtesy of IG-EPN (left) and Sentinel Hub Playground (right).

Figure 38. An ash emission from Sangay on 29 July 2019 drifted tens of km WSW as seen in the webcam (left). Two days later on 31 July a small dark ash plume was visible above the dense cloud cover in Sentinel satellite imagery; the VAAC reported ash drifting W throughout the day. Courtesy of IG-EPN (left) and Sentinel Hub Playground (right).

During an overflight on 6 August 2019 scientists from IG-EPN observed ash emissions from the Cráter Central, and the lava flow continuing from the Vento Ñuñurco in a similar location to where it was in May 2019 (figure 39). Light-colored sediments filled much of the upper basin of the Volcán river. Thermal images of the area also showed that some of the deposits were elevated in temperature, even in the riverbed (figure 40).

Figure 39. The E and SE flanks of Sangay showed continuing activity during August 2019 (right) that was similar to activity going on during May (left). In May, steam issued from the Cráter Central and a lava flow descended the SE flank from Vento Ñuñurco (photo by M. Almeida, IG-EPN). In August, diffuse ash and steam issued from the Crater Central, and a new flow descended from the same area of the Vento Ñuñurco seen in May (photo by P. Ramón , IG-EPN). Courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 5, Quito, 13 de noviembre del 2019).

Figure 40. The upper Volcán River basin was filled with deposits of pyroclastic material associated with the most recent activity at Sangay when observed during an overflight on 6 August 2019 (left). Thermal analysis of the drainage indicated that several of the deposits were still hot, as was the active flow (right). Left photo by P. Ramón, thermal image by Silvia Vallejo; courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 5, Quito, 13 de noviembre del 2019).

Frequent ash emissions continued during August 2019. Diffuse ash was seen moving W from the summit at 5.8 km altitude on 1 August. Another short-lived plume was observed extending 15 km WSW the next day at 5.8-6.1 km altitude. Continuous ash emissions were visible in satellite imagery extending 35 km SW from the summit at 6.1 km altitude on 5 August. During the next two days, the emissions extended 45 km WSW and a prominent hot spot was visible through the meteoric clouds. The ash plume altitude rose to 6.7 km on 8 August and a larger ash emission extended more than 100 km WSW. A new emission the next day drifted 25-35 km W at 6.1 km altitude. A well-defined hotspot seen in shortwave imagery on 10 August accompanied an ash emission that extended 35 km WSW from the summit at 6.7 km altitude. On 12 August a plume drifted 65 km due W at 6.4 km altitude; emissions continued the next day in the same direction at 6.1 km altitude. An ash plume extended 100 km WNW of the summit at 5.8 km altitude on 18 August. A very bright hotspot was observed in infrared imagery the next day. The ash emissions continued to be visible in satellite imagery through 20 August.

An ash plume extending 10 km N from the summit on 25 August coincided with the appearance of a vivid hot spot, according to the Washington VAAC. The plume was initially reported at 7.6 km altitude and later in the day was at 6.7 km altitude. The leading edge of an ash emission reported on 31 August was 350 km W of the summit late that day moving at 5.8 km altitude, and over 950 km WSW before it dissipated on 1 September. Fewer ash emissions were reported during September 2019. The leading edge of a plume extended about 160 km W from the summit on 2 September at 7.6 km altitude; a second emission that day moved NE at 6.4 km altitude. On 4 September a small emission rose to 6.4 km altitude and drifted SW; on 9 September a plume was observed moving W at 5.5 km. A new emission on 19 September was seen in satellite imagery moving in many different directions (N, NE, E, and SE) at 6.7 km altitude. The lava flow on the SE flank produced a strong thermal signature that appeared unchanged from late August through late September (figure 41).

Figure 41. The thermal signature from the lava flow on the SE flank of Sangay appeared unchanged from late August (top left) to late September 2019 (bottom right) in Sentinel-2 imagery (bands 12, 11, and 8A); an ash emission drifted in multiple directions on 19 September 2019. Courtesy of Sentinel Hub Playground and IG-EPN.

Activity during October-December 2019. Pulses of ash were reported during 1, 9-11, 14, 26, and 31 October 2019 by the Washington VAAC. On 1 October the plume rose to 5.8 km altitude and drifted NE. A narrow plume on 9 October extending 55 km NW corresponded with a bright hotspot at its source. Concentrated emissions the next day rose to 7.3 km altitude and extended over 200 km WNW. Later in the day on 10 October emissions were reported at 5.8 km drifting W. A substantial thermal anomaly and a constant plume of diffuse ash appeared in satellite imagery on 14 October at 6.1 km altitude drifting 15 km W. Diffuse emissions on 26 October appeared 35 km NW of the summit at 5.8 km altitude. The intensity of the thermal anomaly from the lava flow on the SE flank remained strong during the month, and emissions of steam and ash were also visible in satellite images (figure 42). In a site visit on 19 October 2019, IG-EPN scientists measured a recent lahar deposited near the confluence of the Volcán and Upano rivers. It was full of sand-sized particles and approximately 30 cm thick at the river’s edge (figure 43).

Figure 42. The thermal anomaly from the lava flow on the SE flank of Sangay remained strong during October 2019, and both ash and steam emissions were seen in Sentinel-2 satellite images (bands 12, 11, and 8A). The lava flow is about 2 km long. Courtesy of Sentinel Hub Playground.

Figure 43. A lahar deposit at the confluence of Río Volcán and Río Upano at Sangay was about 30 cm thick on 19 October 2019. Photograph by Francisco Vasconez, courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 5, Quito, 13 de noviembre del 2019).

Ash emissions during 10-26 November 2019 were reported daily by the Washington VAAC, each lasting for less than 24 hours before dissipating. The first report of ash detected in satellite imagery on 10 November indicated that the plume extended 25 km WSW at 6.7 km altitude. On the subsequent days, the plumes drifted in many different directions at altitudes of 5.8-7.3 km, usually around 6.4 km. The plumes generally drifted 25-45 km from the summit, although some were still visible over 100 km away, depending on weather conditions. The highest plume reached 7.3 km altitude on 18 November and drifted W. The plume on 26 November rose to 6.4 km altitude and was last seen 140 km SW of the summit before it dissipated. Pyroclastic flows were witnessed on 20 November 2019 (figure 44). The last plume of the month, on 29 November, rose to 6.4 km altitude and drifted 65 km W, dissipating quickly, and was accompanied by a very bright thermal anomaly.

Figure 44. Ash plumes from Sangay rose to 5.8 km altitude on 20 November 2019 and drifted 25 km NE before dissipating, according to the Washington VAAC. Pyroclastic flows appeared on the flank that day. Courtesy of Walter Calle C.

Ashfall was reported during November in the provinces of Chimborazo (Alao, 20 km NW, Cebadas, 35 km NW, and Guaguallá), Morona Santiago (Macas, 40 km SE), and Azuay (120 km SW). Samples of ash collected from two locations indicated that the amount of material was very small (less than10 g/m2) with a high content of extremely fine ash (between 40 and 60% ash less 63 μm in diameter). The larger fraction over 63 μm was mainly composed of juvenile magma (80%) and a small fraction of free crystals (10% plagioclase and pyroxenes), oxidized fragments (5%), and gray lithics (5%) (figure 45).

Figure 45. Photos from a binocular microscope of the greater than 63 μm fraction of ash from Sangay collected in Macas and at the SAGA station during November 2019. See text for details. Courtesy if IG (Informe Especial del Volcán Sangay - 2019 - No 6, 4 de diciembre del 2019).

In a report issued in early December 2019 the IG-EPN noted that eruptive activity which increased in May 2019 was continuing (figure 46); a small amount of inflation was observed during November. Explosive activity continued at the Cráter Central with ash plumes reaching 2 km above the summit, and plumes drifting frequently towards the NE causing small amounts of ash to fall in the Chimborazo, Morona Santiago, and Azuay provinces. Effusive activity from the Ñuñurco vent produced almost continuous lava that flowed down the SE flank. Small pyroclastic flows around the margins of the lava flows reached the basin and the upper channel of the Volcán river, causing temporary dams that turned to mudflows during rain events.

Figure 46. IG-EPN published this multi-parameter chart of activity of the Sangay volcano from May to 1 December 2019. a: seismic activity (number of events per day) detected at the PUYO station (source: IG-EPN); b: SO2 emissions (tons per day) detected by the Sentinel-5P satellite sensor (source: MOUNTS); c: height of ash clouds (m above crater level) detected by the GOES-16 satellite sensor (source: Washington VAAC); d: thermal emission power (megawatt) detected by the MODIS satellite sensor (source: MODVOLC) and estimated accumulated lava volume (million m3, dotted lines represent the error range). Courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 6, 4 de diciembre del 2019).

During an overflight on 3 December 2019 a strong smell of sulfur was noted 1 km above the summit. The Ñuñurco vent continued to emit lava with a maximum apparent temperature of 100 to 210°C (figure 47). IG-EPN scientists concluded that approximately 58 ± 29 million m³ of lava had been emitted through 3 December.

Figure 47. Views of the SE flank of Sangay on 3 December 2019 with visible (left) and thermal (right) imagery. Photograph by C. Viracucha, thermal analysis by F. Naranjo; courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 6, 4 de diciembre del 2019).

Recurring lahars in the Río Volcán during the period occasionally reached the Rio Upano (figure 48). By late November, they had partially dammed the Upano river (figure 49). On 26 November 2019 when IG-EPN and Sangay National Park officials inspected the area, they recorded deposits more than 2 m thick at the confluence of the two rivers (figure 50). During an overflight the next day, additional deposits were identified along 16 km upstream. The total volume of the lahar deposits was estimated at 5 million m³ to date.

Figure 48. Inferred lahar deposits at Sangay along the Río Volcán from the foot of the volcano up to its confluence with Río Upano shown in red. Courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 6, 4 de diciembre del 2019).

Figure 49. Lahar deposits at Sangay filled Río Volcán and dammed part of the confluence where it joins río Upano when photographed during an overflight on 26 November 2019. Photographs by Pedro Espín; courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 6, 4 de diciembre del 2019).

Figure 50. Lahar deposits from Sangay exceeded 2 m in thickness at the confluence of the Upano and Volcán rivers on 26 November 2019. Photography by Pedro Espín; courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2019 - No 6, 4 de diciembre del 2019).

Another extended period of ash emissions began on 4 December 2019 and continued daily through 19 December. The Washington VAAC reported that an ash plume was initially at 6.7 km altitude drifting S on 4 December. Continuous emissions were observed at 4.6 km altitude later in the day and were visible in satellite images located 25 km S at 5.8 km altitude that evening. The drift directions were initially mostly SW in early December, but migrated to mostly SE during 10-16 December, then back to SW. Plume altitudes ranged from 5.8 to 7.3 km and satellite images revealed ash as far as 160 km away; most plumes were visible to about 25 km before dissipating or disappearing into meteoric clouds. IG-EPN reported steam and gas emissions with small amounts of ash on 13 December that drifted SE (figure 51). Small block avalanches from the active flow were also observed on the SE flank. The next day, ash and gas emissions rose to 1,170 m above the summit and drifted NE while the lava flow appeared incandescent on the SE flank.

During the night of 14-15 December ashfall was reported in San Isidro in the Province of Morona Santiago (30 km SE). Ash plumes rose 870 m above the summit on 15 December and 1,470 m high the next day. Ashfall was reported in the Guasuntos (60 km SW) and Llagos (80 km SW) areas of the Chimborazo province on the morning of 16 December. The next day plumes drifted SE and SW, and minor ashfall was reported that night (16-17 December) in Macas (40 km SE), Morona Santiago province. Satellite images captured gas and ash emissions on 25 December, and ashfall was reported in Alausí (60 km SW) in the province of Chimborazo. An explosion on 29 December produced an ash plume that rose to 6.1 km and first drifted WNW then in an arc to the SW almost 185 km to the coast. Multiple plumes at 5.8-6.7 km drifted westerly for tens of kilometers that day and the next. Prominent thermal anomalies were noted in satellite imagery on 8, 15, 17, and 30 December.

Figure 51. Numerous explosions produced ash emissions from Sangay during 4-30 December 2019, shown here on days 13, 14, 16, and 25. Courtesy of IG-EPN (Informe Diario del Estado del Volcán Sangay No. 2019-1, 13 Diciembre; No. 2019-2, 14 Diciembre; No. 2019-5, 17 Diciembre; No. 2019-13, 25 Diciembre 2019).

By late December 2019, the lahar deposits in Rio Volcán had backed up noticeably further into the Upano river from a month earlier (figure 52). Sulfur dioxide emissions were not recorded during July through August 2019, but small, pulsing plumes were captured in satellite images during September, October and November, gradually increasing in density. Several plumes were detected hundreds of kilometers from the volcano before dissipating; by December, larger, more frequent pulses of SO₂ were measured during many days when ash emissions were reported (figure 53).

Figure 52. Lahar deposits from Sangay in the Rio Volcán (right) continued to dam up the Rio Upano into late December 2019. Compare with figure 49 taken one month earlier. Photo by WJ Hernandes, courtesy of Edgar Chulde, posted online 21 December 2019.

Figure 53. Sulfur dioxide emissions from Sangay were weak but persistent during September-November 2019 (top row), often drifting in narrow plumes with distinct pulses. During December, the density of the SO2 emissions increased noticeably (bottom row). Columbia’s Nevado del Ruiz was also producing plumes of SO2 at the same time. Courtesy of NASA Goddard Space Flight Center.

Geologic Background. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within horseshoe-shaped calderas of two previous edifices, which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been sculpted by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of a historical eruption was in 1628. More or less continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: Instituto Geofísico (IG-EPN), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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/); Walter Calle C., Macas, Ecuador (Twitter: @walterc333; URL: https://twitter.com/walterc333/status/1197273200822046720); Edgar Chulde, Quito, Ecuador (Twitter: @EdgarChulde2; URL: https://twitter.com/EdgarChulde2/status/1208547471024173056).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Simon Plank, German Aerospace Center (DLR) German Remote Sensing Data Center, Geo-Risks and Civil Security, Oberpfaffenhofen, 82234 Weßling (URL: https://www.dlr.de/eoc/en/desktopdefault.aspx/tabid-5242/8788_read-28554/sortby-lastname/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Planet Labs, Inc. (URL: https://www.planet.com/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Planet Labs, Inc. (URL: https://www.planet.com/).

Seismicity remained higher than usual, with 13 swarms recorded in April, each lasting for about 5 hours. Twelve explosions occurred . . . in April, . . . producing ash clouds to 2,500 m (on 2 April).

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.

Minor ash emission . . . continued through late April. Residents of Akutan village (16 km NE of the volcano) suggested that ash emission may have occurred on 20 or 21 April. A dark streak that was presumed to be ash was visible on the E flank when weather cleared 22 April. A videotape taken by Reeve Aleutian Airways personnel on 26 April showed vigorous steaming from the prominent cinder cone in the summit crater, and fresh ash on the snowfields S of the cone. That day, a pilot saw ash rising to ~2.5 km altitude (roughly 1.2 km above the summit), but no ashfall was reported from Akutan village. Activity was next seen on 21 May, when Mark Owen (Trident Seafoods, Akutan village) observed fresh ash on the snow-covered flank during the early morning, and brief emissions of dark ash that rose an estimated 250-300 m above the volcano at about 1000 and 1400.

Geologic Background. One of the most active volcanoes of the Aleutian arc, Akutan contains 2-km-wide caldera with an active intracaldera cone. An older, largely buried caldera was formed during the late Pleistocene or early Holocene. Two volcanic centers are located on the NW flank. Lava Peak is of Pleistocene age, and a cinder cone lower on the flank produced a lava flow in 1852 that extended the shoreline of the island and forms Lava Point. The 60-365 m deep younger caldera was formed during a major explosive eruption about 1600 years ago and contains at least three lakes. The currently active large cinder cone in the NE part of the caldera has been the source of frequent explosive eruptions with occasional lava effusion that blankets the caldera floor. A lava flow in 1978 traveled through a narrow breach in the north caldera rim almost to the coast. Fumaroles occur at the base of the caldera cinder cone, and hot springs are located NE of the caldera at the head of Hot Springs Bay valley and along the shores of Hot Springs Bay.

Information Contacts: AVO.

Block lava extrusion from the summit area began soon after the start of the eruption in 1968. Since then, an extensive lava field has developed on the W side of the volcano. The northernmost lobe of the W-flank lava flow stopped growing in April. Its southernmost lobe, however, continued a slow advance to 775 m elevation, reaching the forest, and burning some 20,000 m² of fields. Strombolian explosions continued, at intervals of several minutes to hours. During 12-22 April fieldwork by OVSICORI and SI scientists and volunteers, 539 seismic events were recorded. The majority of the signals were associated with gas and ash emissions with locomotive and jet-engine sounds (figure 47). Continuous tremor of low, medium, and high frequency, associated with lava extrusion, was recorded almost 24 hours/day during this period. Seismicity decreased moderately from previous months (recorded at the ICE station "Fortuna", 4 km E of the summit), with a maximum of 16 earthquakes/day (18 April), and an average of 6/day. High levels of continuous tremor were recorded on 9-15, 21, 23, 25, and 27-28 April.

Figure 47. Seismicity at Arenal, 12-21 April 1992. The 21 April data only include 17 hours of measurements. Courtesy of OVSICORI.

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

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto and R. Barquero, ICE; W. Melson, SI.

The crater lake was visited on 11 May, following a sudden drainage of ~80% of the lake (from ~3.5 x 10⁶ m³ to 0.7 x 10⁶ m³) on 1 February. Water temperature was 31.1°C and pH was 2-3, similar to 4 March values (17:02), cooler but more acid than the 36°C and pH 5 measured in February. Fumaroles along the inner N wall of the crater emitted steam that rose 25-40 m and had temperatures of 70-92°C. Active solfataras, with temperatures of 70.6-97.4°C, had left substantial sulfur along the S and E walls. In the SE section of the crater, a deep vent 20 m in diameter produced a thick 50-m-high steam cloud that smelled of sulfur and was accompanied by an audible boiling sound. The presence of lithic ejecta around the vent suggested that it had been formed by a phreatic explosion.

Tectonic and volcanic A-type earthquakes were recorded at the volcano every month during January 1991-January 1992; volcanic A-type events ranged from 2 to 18/month.

Geologic Background. The massive Gunung Awu stratovolcano occupies the northern end of Great Sangihe Island, the largest of the Sangihe arc. Deep valleys that form passageways for lahars dissect the flanks of the volcano, which was constructed within a 4.5-km-wide caldera. Powerful explosive eruptions in 1711, 1812, 1856, 1892, and 1966 produced devastating pyroclastic flows and lahars that caused more than 8000 cumulative fatalities. Awu contained a summit crater lake that was 1 km wide and 172 m deep in 1922, but was largely ejected during the 1966 eruption.

Information Contacts: W. Modjo, VSI; UPI

Seismicity was more vigorous during the 1991-92 austral summer than in previous years and was associated with increased fumarolic activity, gravity changes, and deformation. The following supplements the report in 17:1.

Seismic monitoring, with similar instruments at the same locations as in previous years, yielded 766 events between 21 December 1991 and 23 February 1992, many in clusters lasting up to several days (figure 6 and table 1). The most vigorous activity occurred around 10 January and in late January/early February. More than half of the recorded events were followed by another with similar characteristics within an hour. Most had magnitudes of 0.8-2. Of the 15 shocks exceeding this range, four were felt, all were M >3, and all were within 100 km of the station at the Argentine base. RSAM data (figure 6) show both individual events and swarms, but geologists did not believe that the observed acceleration was large enough to suggest that an eruption was imminent.

Figure 6. Gravity data (top), Real-time Seismic Amplitude Measurements (RSAM) (middle), and the number of seismic events per day (bottom) recorded at Deception Island, 22 December 1991-24 February 1992. Gravity data are compensated for instrument deflection, which consistently indicated uplift in the direction of Cerro Caliente. Courtesy of Ramón Ortiz.

Table 1. Summary of increased seismic activity at Deception Island, December 1991-February 1992.

Figure 7. Gravity data (top) and Real-time Seismic Amplitude Measurements (bottom) recorded at Deception Island, 17-19 January 1992, showing the anomalous gravitmetric deflection associated with the 18-19 January swarm. Courtesy of Ramón Ortiz.

Gravity data showed an increase until after the 30 January seismic crisis, then a decrease until the end of monitoring on 22 February (figure 6). Small gravity irregularities corresponded with successive seismic swarms. Compensations in the gravity data were required to allow for deflections that indicated uplift toward Cerro Caliente (figure 8), the area identified as the center of renewed activity. Uplift was visible in the nearby Fumarole Bay area and in the vicinity of the Argentine station, where the helicopter landing field was noticeably tilted. Uplift totaled ~20 cm in the past year.

Figure 8. Sketch map of Deception Island, showing the area of recent uplift and new fumaroles, early 1992. Courtesy of José Viramonte.

The area of hot soil near Fumarole Bay and Cerro Caliente appeared to be larger than in previous years. Fumarolic emission from the top of Cerro Caliente had increased noticeably, as had the concentration of sulfur compounds in Fumarole Bay. Some of the seismic swarms were associated with deaths of krill (small shrimp-like marine organisms) near fumarolic areas.

Geologists noted that the activity . . . could be related to a magmatic intrusion in the main fracture system of the Fumarole Bay area. No acceleration in any of the measured parameters (uplift, earth tides, seismicity, magnetic field, and temperature) was evident, so an eruption was not thought to be imminent. However, the possibility of a small phreatic eruption from the hot area on the beach near Cerro Caliente could not be ruled out. Because the Argentine station is near the potentially active area, an evacuation camp was established in a valley almost 4 km E of Cerro Caliente (in the Crater Lake area) and separated from it by highlands. The valley rises from 15 to 50 m elevation (offering tsunami protection), glaciers are poorly developed above the valley (reducing the risk of lahars), the terrain allows easy helicopter operation, and easily traveled routes lead to the inner coast for possible evacuation by sea.

Geologic Background. Ring-shaped Deception Island, one of Antarctica's most well known volcanoes, contains a 7-km-wide caldera flooded by the sea. Deception Island is located at the SW end of the Shetland Islands, NE of Graham Land Peninsula, and was constructed along the axis of the Bransfield Rift spreading center. A narrow passageway named Neptunes Bellows provides entrance to a natural harbor that was utilized as an Antarctic whaling station. Numerous vents located along ring fractures circling the low, 14-km-wide island have been active during historical time. Maars line the shores of 190-m-deep Port Foster, the caldera bay. Among the largest of these maars is 1-km-wide Whalers Bay, at the entrance to the harbor. Eruptions from Deception Island during the past 8700 years have been dated from ash layers in lake sediments on the Antarctic Peninsula and neighboring islands.

Information Contacts: A. García and R. Ortiz, Museo Nacional de Ciencias Naturales, Spain; C. Risso, Instituto Antártico Argentino; J. Viramonte, Univ Nacional de Salta, Argentina.

A sudden gas emission occurred at about 1600 on 18 March from a fractured and altered zone in a river valley 200 m W of Sikidang Crater. After the gas emission, one person was found dead in the stream, and two others were hospitalized after trying to rescue him. Surface gas measurements the next day indicated high concentrations of CO₂ and O₂ (40 and 15 weight %, respectively), and lesser concentrations of H₂S and HCN (200 and 197 ppm, respectively).

Steam emission continued from Sileri Crater (~3 km NNW of Sikidang), rising 40-60 m in mid-April. An average of one A-type and seven B-type volcanic earthquakes were recorded daily during mid-April, an increase from earlier in the month.

Geologic Background. The Dieng plateau in the highlands of central Java is renowned both for the variety of its volcanic scenery and as a sacred area housing Java's oldest Hindu temples, dating back to the 9th century CE. The Dieng volcanic complex consists of two or more stratovolcanoes and more than 20 small craters and cones of Pleistocene-to-Holocene age over a 6 x 14 km area. Prahu stratovolcano was truncated by a large Pleistocene caldera, which was subsequently filled by a series of dissected to youthful cones, lava domes, and craters, many containing lakes. Lava flows cover much of the plateau, but have not occurred in historical time, when activity has been restricted to minor phreatic eruptions. Toxic gas emissions are a hazard at several craters and have caused fatalities. The abundant thermal features and high heat flow make Dieng a major geothermal prospect.

Information Contacts: W. Modjo, VSI; T. Casadevall, USGS.

Lava has emerged from a SE-flank fissure in the W wall of the Valle del Bove since 15 December, covering an estimated 7.3 km² with ~ 100 x 10⁶ m³ of lava. A well-developed tube system carried lava downslope, threatening the town of Zafferana Etnea and prompting attempts at lava diversion (figure 47). The lava production rate, as observed through numerous skylights along the main lava tube, has remained relatively constant, but distal flow fronts advanced at varying rates. The apparent intensity of gas emission from the eruptive fissure changed with weather conditions. During the last 10 days of April, fumarolic activity was observed in the W wall of the Valle del Bove, extending upslope from the eruptive fissure along its NNW trend. This zone was active on 14 December during the initial phase of the eruption.

Figure 47. Topographic sketch map showing Etna's 1989-92 lava flows, with preliminary locations of the 1991-92 eruptive fissures, and the barrier constructed in January in Val Calanna. Areas covered by lava since 14 January and 10 March are shown in separate patterns. Asterisks mark sites of lava diversion experiments in April and May. Courtesy of R. Romano, T. Caltabiano, P. Carveni, and M.F. Grasso.

Lava overwhelmed a series of barriers in early April, and advanced 1 km down a gorge (within the Valle di Portella Calanna) toward Zafferana during the second week in April. This flow stopped on 15 April at 750 m elevation, roughly 1.5 km from the inhabited center of Zafferana. Numerous ephemeral vents began to form below 1,000 m elevation on 19 April (on the E edge of Val Calanna, in which a barrier had been built in early January). Flows from these vents covered lava from previous days along the gorge below Portella Calanna. The longest stopped during the evening of 25 April at 755 m altitude, ~ 7.5 km from the eruptive fissure and 1.5 km from the center of Zafferana. Lava flows originated from a large ephemeral vent at the head of Val Calanna in the beginning of May, passing Portella Calanna atop previous flows on 6 May and reaching 850 m asl that evening.

Ephemeral vents also developed upslope, within the wide lava field that had formed in the S part of the Valle del Bove during previous months. The first formed around 1,900 m altitude (at Monte del Rifugio Menza) on 22 April, and a second occurred near the center of the lava field, at around 1,550 m elevation (near Poggio Canfareddi) on 25 April. Flows from these vents were not very substantial and were no longer active a few days later. On 5 May, only a modest active vent at the N edge of the lava field (around 1,600 m elevation) was observed.

Experiments with the use of explosives, cement blocks, and, more recently, lava blocks continued at skylights in the main lava tube (in the upper Valle del Bove, at around 2,100 m altitude, on 17, 21, and 29 April, and 4 May) and at ephemeral vents (near Portella Calanna on 15 April and in Val Calanna on 6 May). These were designed to cause lava overflows and thus reduce the amount of lava carried in tubes toward inhabited areas. As of early May, it was difficult to evaluate whether these experiments had favorably affected the course of the eruption. On 4 May an overflow began from a skylight in the main lava tube at around 2,100 m altitude, where blocks of cement had been dropped and explosives detonated on previous days. The modest overflow moved over the lava field that had formed in the preceding months. However, during this time, numerous ephemeral vents, varying daily in number and location, remained active above Zafferana at the head of Val Calanna (in the Salto della Giumenta).

Slow, weak degassing continued through early May from the summit craters. Weak ash ejections, caused by internal collapse, were observed only from Northeast Crater, on 22 April. Early April-early May seismic activity was much reduced from previous months.

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: R. Romano and T. Caltabiano, IIV; P. Carveni and M. Grasso, Univ di Catania.

Gas and ash were emitted daily in April, accompanied by strong sulfurous odors, with some explosions audible to 500 m away. Activity was concentrated in the W part of the crater and 1991 dome, along a fracture to the NW, and in Portillas crater. Sulfur deposits were visible around gas vents at the extreme SE and S margins of the dome, where explosive activity was minor.

Long-period seismicity in April was similar to the previous several months (figure 53), although a slight increase in amplitude, released energy, and number of events was noted. The majority of the signals arrived in groups, and tended to occur at certain hours of the day. Tremor, generally spasmodic in character, remained at low levels, but at higher amplitudes than previous months. Eleven high-frequency earthquakes (M 1.1-2.5) were recorded in April, centered principally in the W part of the summit crater. Electronic tiltmeter measurements indicated little deformation since early 1992, with only minor amounts of deflation recorded [at Crater Station].

Figure 53. Daily number of long-period earthquakes at Galeras, 27 February 1989-24 April 1992. An arrow marks the first observation of the lava dome, on 9 October 1991. Courtesy of INGEOMINAS.

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

Information Contacts: J. Romero, INGEOMINAS-Observatorio Vulcanológico del Sur.

Dense steam emissions continued through mid-Apr, rising 50-300 m above the crater rim. Earthquakes averaged 12-13/day in mid-Apr, an increase from early April, but only half the early March rate (17:02).

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: W. Modjo, VSI; UPI.

The main crater's fumarolic activity continued in April. Although temperatures remained similar at 88-91.5°C, the jet-engine sound heard in prior months was no longer audible. Between 12 February and 2 April, the crater lake's water level dropped 16 cm and the lake diameter shrank by 2 m. Cold and warm springs around the volcano showed no measurable changes in temperature or pH. The portable seismic net continued to record low-frequency earthquakes of low energy. Fewer than 10 events were recorded in April (at station "ICR", near the summit).

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: G. Soto and R. Barquero, ICE.

A pyroclastic flow, triggered by collapse of a lava flow front, killed six people on 11 May.

After an increase in seismicity to as many as 4 events/week in April-May 1991, ash explosions began in the main central crater, ejecting incandescent projectiles to 50-75 m height (16:08). Strombolian activity lasted until August, when lava emission began in the main crater. During September, explosive activity decreased to ash emissions 25-75 m high, accompanied by audible explosions and some incandescence.

Activity increased in February 1992, and incandescent ash emissions became continuous. An estimated 6 x 10⁶ m³ of lava had accumulated in the crater and lava flows began to advance down the S flank's Kali Keting valley. On 2 March, VSI and local authorities warned farmers along the upper Kali Keting to be prepared for the possibility of collapse of the lava flow front and subsequent generation of pyroclastic flows. This region was designated on the 1989 VSI volcano hazard map as being at highest risk of destruction (by pyroclastic flows). At 1330 on 11 May, a pyroclastic flow caused by the collapse of the lava flow front traveled 4 km from the main crater down the Kali Keting, burning seven farmers (six of whom later died in hospitals) and destroying >30 houses and ~2 km² of coconut, cassava, and nutmeg farms. The pyroclastic-flow deposit had a volume of ~1.2 x 10⁶ m³ (roughly 20% of the lava in the crater). The eruption was continuing as of 19 May, as indicated by the increasing number of volcanic earthquakes and seismically recorded degassing events (table 2).

Table 2. Monthly seismicity at Karangetang, February-19 May 1992. Courtesy of VSI.

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: W. Modjo, VSI; UPI.

Episode 51 . . . continued through early May with two pauses, each lasting less than a week. During the first half of April, E-51 vents on the W flank of Pu`u `O`o (figure 85) fed lava N to a perched pond on the small shield built by the recent activity, and to a large channel that carried flows southward. This channel, active since the brief pause at the end of March, had roofed over to form a tube by 6 April. Flows advancing through the tube reached the edge of the lava field on 13 April and began to burn trees in Hawaii Volcanoes National Park, > 1 km from the vent. During this period, several smaller flows were active on the shield, some fed by the perched lava pond. The E-51 vents remained active, sustaining periodic low fountains, until the eruption halted on the evening of 19 April. No large flows were observed the next day, although small aa flows continued to drain lava stored in the pond area.

The eruption resumed on 23 April, as two vents along the E-51 fissure fed the pond and a channelized flow that headed S. Its aa front advanced rapidly and began burning vegetation in the national park by the next day. The lava pond and main channel also fed large shelly pahoehoe flows that moved N and W. Small, apparently tube-fed aa flows continued to break out on the shield. By 28 April, the main channel was beginning to roof over, but lava production stopped at 1130 that day, the channel drained, and lava flows stagnated.

The level of the small lava lake in Pu`u `O`o fluctuated between 36 and 53 m below the crater rim in April, sustaining numerous overflows onto the crater floor and vigorous spattering as it remained active throughout the month. After lava production stopped at the fissure vent on 28 April, the Pu`u `O`o lava lake rose until it spilled onto the crater floor on 3 May, and was still overflowing when the eruption resumed from the E-51 fissure vent the next day. Flows from the fissure vent generally remained on top of earlier lava during the following week, while the Pu`u `O`o lava lake withdrew into the conduit, to nearly 70 m below the crater rim.

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

Information Contacts: T. Mattox, HVO.

Steam emission continued steadily in April to 100-200 m height. A light dusting of ash was noted on leaves in the crater during a 6 April visit. The maximum measured fumarole temperature was 96°C. Seismicity was at low levels in April, but a weak tremor episode was recorded on the 11th. A monthly total of 23 small earthquakes was recorded, almost unchanged from March. Similar activity continued through early May.

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

Information Contacts: JMA.

A seismic swarm occurred 21-25 April, centered a few kilometers NW of the island. Some of the shocks were felt by island residents; the largest, M 3.6, occurred on 23 April. Another swarm was recorded on 8 May, centered E of the island (maximum M 3.9). No surface anomalies were observed.

Geologic Background. A cluster of rhyolitic lava domes and associated pyroclastic deposits form the small 4 x 6 km island of Kozushima in the northern Izu Islands. Kozushima lies along the Zenisu Ridge, one of several en-echelon ridges oriented NE-SW, transverse to the trend of the northern Izu arc. The youngest and largest of the 18 lava domes, 574-m-high Tenjoyama, occupies the central portion of the island. Most of the older domes, some of which are Holocene in age, flank Tenjoyama to the north, although late-Pleistocene domes are also found at the southern end of the island. Only two possible historical eruptions, from the 9th century, are known. A lava flow may have reached the sea during an eruption in 832 CE. Tenjosan lava dome was formed during a major eruption in 838 CE that also produced pyroclastic flows and surges. Earthquake swarms took place during the 20th century.

Information Contacts: JMA.

"Moderate eruptive activity continued during April. Crater 2 emitted moderate volumes of pale grey ash and vapour, and occasionally there were stronger explosions that propelled ash clouds several kilometers above the summit. Ashfalls to 10 km from the source were common. Explosions were heard at the observation post . . . on most days between 1 and 11 April. Rumbling and roaring sounds were heard on 27-30 April. Steady, weak crater glow was seen on most nights. Crater 3 activity was mild at the beginning of the month, and only weak white emissions were seen. Lava flows that began 6 March ceased on 1 April. Crater 3 became more active on 6 April; however, the ash content of emissions remained low. Incandescent lava ejections and/or glow were reported on most nights beginning 6 April. The ejections rose as much as 500 m above the crater. Beginning on 9 April, the explosive activity was stronger and emissions contained more ash. Explosion noises were reportedly loud at the observation post.

"In early April, seismicity appeared to mainly reflect the activity at Crater 2, while during 7-14 April, most of the seismicity was associated with Crater-3 activity. All seismic monitoring ceased on 19 April with the failure of both seismic stations."

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

Information Contacts: C. McKee, RVO.

"The eruption continued strongly in April with new paroxysmal phases of activity at Southern Crater and activation of Main Crater, which emitted a lava flow for the first time since 1960. The intensity of the eruption was declining in late March and early April, and activity at this time was restricted to Southern Crater. The moderate Strombolian activity there consisted of ash emissions that were rising to ~1 km over the crater at the beginning of April, but by 8 April were only rising a few hundred meters. On 8 April, Strombolian activity began at Main Crater, which became active for the first time in the eruption. The ash content of emissions was low. Ejections of incandescent lava fragments were visible at night, rising 100-200 m above the crater.

"Seismicity began to increase on 9 April as sub-continuous, irregular tremor became progressively stronger. This coincided with stronger explosive activity at both craters as ash clouds rose ~1 km and sound effects were more prominent. This buildup culminated in a paroxysmal phase of activity at Southern Crater starting about 0300 on 11 April. For about 2 hours, there were nearly continuous strong explosions at Southern Crater, projecting incandescent lava fragments to ~1 km above the rim. Ash clouds rose considerably higher. This activity seems to have been significantly stronger than the previous paroxysmal phase on 23 March. Scoria flows were directed into both the SE and SW valleys, although the SE valley was the main pathway. The flow deposits extended ~3.5 km down the SE valley to a point ~270 m asl. The volume of the flow deposits is estimated to be ~100,000 m³.

Coarse tephra were confined to a stream channel on the S side of the valley but the overriding ash clouds left thin deposits of fine ash in relatively narrow zones (up to 100 m wide) bordering the coarse flow deposits. Vegetation damage ranged from scorching to complete destruction. Scorching was evident to the top of the SE Valley's S wall (100-200 m above the floor). The bulk of scoria-flow deposits in the SW valley are within ~500 m of the base of its near-vertical headwall. A more surprising product of this phase of activity was a lava flow in the SW valley, which is detached from its source at Southern Crater. The lava flow disintegrated as it descended the steep headwall of the SW valley. A prominent channel near the center of the headwall was scoured out by the cascade of lava. On reaching the base of the headwall, the lava flow was reconstituted. The ribbon-like body of lava that now fills the main drainage channel in the upper part of the SW Valley is ~900 m long. Its width ranges from ~20 to 40 m, and its thickness is ~10 m. From these dimensions, the volume of the lava flow is estimated to be ~300,000 m³.

"There was a decline in Southern Crater activity after the paroxysmal phase, although ash emissions were reported to have risen ~2 km over the vent during 11 and 12 April. Southern Crater activity ceased sometime overnight on 12-13 April, and the focus of activity shifted to Main Crater, where bright fluctuating glow was reported the same night. On 13 April, the first reports of a lava flow from Main Crater were received. Lava was flowing into the NE valley for the first time since 1960, following a stream channel on the valley's N side. On 16 April, the terminus of the flow was ~2 km from the source, at ~600 m above sea level. The source of the flow was a breach in the flank of an ejecta cone that infilled a large portion of the previously deep, funnel-shaped Main Crater. Daylight incandescence was visible in the lava flow for ~800 m from the source and at several points farther downslope. With the advent of lava effusion from Main Crater, seismicity rose to its highest level of the eruption. Seismicity remained high until 30 April when lava effusion ceased temporarily. Throughout this period (13-30 April) explosive activity at Main Crater was mild. Frequent ejections lofted incandescent lava fragments 100-200 m, and ash clouds ascended to 500-1,000 m above the crater. The content of ash in the emissions was low. The explosive activity at Main Crater was continuing at the end of April, while Southern Crater remained inactive."

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

Information Contacts: C. McKee, RVO.

During a 26 April visit to Santiago Crater, extremely weak emissions were observed from two or 3 small, quiet fumaroles at the base of the talus in the inner crater and up the W wall (toward Nindirí Crater). COSPEC measurements indicated an SO₂ flux of <10 metric tons/day (t/d), compared to 1500-2000 t/d during lava lake activity in 1980 (SEAN 05:12). Simultaneous use of SO₂ and HCl INTERSCAN instruments at the crater indicated HCl concentrations several times greater than SO₂. A drive on the WSW (downwind) ridge, the site of extensive acid gas deposition and acid rain during the early 1980's (SEAN 05:12, 06:12, and 07:08), showed that vegetation had recovered somewhat; the same stark deforested appearance was still evident, but low shrubs were healthier and larger.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Historical lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: S. Williams, Arizona State Univ; Martha Navarro C. and Silvia Arguello G., INETER.

Dome growth continued through early May, reaching an estimated volume of 4 x 10⁶ m³. Combined rockfall and pyroclastic-flow volumes were estimated to be <10⁶ m³. The 1992 dome covered the remnant of the 1957 lava dome that had formed the NW crater rim, causing a shift in the primary direction of glowing rockfalls from W to NW, down the upper Senowo River valley. No pyroclastic flows have been observed since mid-Apr (17:03).

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

Information Contacts: S. Bronto, MVO.

Ending 21 years of quiet, violent Strombolian explosions began at about 2320 on 9 April, shortly after 5 felt earthquakes (BGVN 17:03). The initial explosive phase produced a plume 7-7.5 km high, and deposited ash to the W and WSW (figure 5), before ceasing at about 1800 on 12 April. Between 6,000 and 9,000 people were evacuated (corrected from BGVN 17:03), and numerous houses and buildings collapsed under the weight of the accumulated ash. Reduced explosive activity (plumes to 3.5 km high) resumed at around 2200-2300 on 13 April, gradually decreasing until about 1730 on 14 April (figure 6), when all activity ceased.

Figure 5. Preliminary isopach map of 9-14 April 1992 ashfall deposits from Cerro Negro. Prepared by INETER.

Figure 6. View of Cerro Negro's eruption from 4 km NW, 14 April 1992. Courtesy of G. Soto.

Field observations, 13-14 April 1992. The volcano was visited by a team from the National Seismic Network (ICE-UCR) of Costa Rica, who operated two short-period portable seismometers on 13 and 14 April, 1 and 4 km from the crater. The following is from Gerardo Soto.

According to reports by residents near the volcano, the initial explosive activity on 9 April seemed to have been preceded by low-magnitude earthquakes. These occurred during the few minutes immediately preceding the explosion, and were only felt within 5 km of the volcano, neither causing damage nor alarming area residents. No seismic records exist for the period 9-12 April. During the 30 hours of seismic observations by the ICE-UCR team, periods of calm alternated with explosive activity. Observations can be summarized as follows: a) virtually no volcano-tectonic (A-type) seismicity was recorded; b) only a few small low-frequency events (durations <1 minute) and small tremor episodes were recorded during most of the quiet period before explosive activity resumed at 2200 on 13 April; c) a progressive increase in recorded tremor activity began at 2000 on 13 April, culminating in high-energy tremor completely saturating the record; d) continuous high-energy tremor was recorded during the roughly 19 hours of explosive activity that followed, with intermittent pulses of increased amplitude and energy (approximately 10 pulses/hour). Geologists interpreted the seismicity as indicating an open system, allowing the magma a direct and rapid ascent to the surface.

Tephra emitted during the eruption was carried predominantly W of the volcano, although very fine ash was reported at 13 km altitude over northern Nicaragua, probably carried by high-altitude winds from the Pacific Ocean. Two granulometric analyses were conducted: 1) ash collected on 13 April, 0.5 km NW of the crater, on Cerro La Mula; and 2) ash collected on 14 April, 21 km along the axis of dispersal, in León (table 1). The ejected ballistic tephra were abundant within 0.5 km of the crater, and less common to 1 km radius. Shrubs were severely damaged within this area. The ballistic clasts are gray-black porphyritic olivine basalts, with phenocrysts of plagioclase (about 30%), olivine (about 5%), and pyroxene (about 1%). Scoriae are black and strongly vesiculated, whereas the gray cognate blocks are poorly vesiculated. The fine ash deposited along the axis of dispersal was rich in millimeter-sized glassy scoria fragments, and isolated crystals of plagioclase and olivine.

Table 1. Granulometric analyses of April 1992 bulk airfall deposit samples from Cerro Negro, collected on 13 April 1992 at Cerro la Mula (0.5 km NW of the crater, and on 14 April at León (21 km SW of the crater). * indicates that the largest clast was smaller than 2.0 mm. Courtesy of ICE-UCR.

During the first three days of the eruption, government officials reported severe damage from ashfall over an area of about 186 km², of which about 116 km² are corn, cotton, sesame, beans, and other grain cultivation, and 70 km² are sugar cane. In addition, numerous cattle were affected. The most severe damage extended from the volcano to the city of León. Primary damage to home, commercial, and industrial infrastructure, estimated at $3,000,000, was mostly caused by roof collapse on the first day of the eruption. As of 15 April, there were no reports of injuries directly related to the eruption. The 50 indirectly related injuries occurred predominantly during evacuations, and from falls as people cleaned their roofs; 1-2 people died from falls. The primary medical complaints were eye and respiratory problems. Refugees were evacuated to 4 centers distant from the most affected area.

Field observations, 23-29 April. The following, from S. Williams, describes fieldwork during 23-29 April.

A rusty brown plume was emanating from the region close to Cerro Negro during the approach to Managua on 23 April. The plume reached 1-2 km, extending W to >100 km, causing the pilot to detour W, far out to sea, to avoid it. During a 1 1/2 hour visit later that day to a site about 3 km SW of the crater, there was no visible gas emission, noise, landslides, or felt seismicity, suggesting that the plume consisted entirely of re-suspended fine ash from the regional tephra blanket. COSPEC measurements from the same site the next day yielded a barely discernible SO₂ signal; calculations suggested a maximum SO2 flux of <25 metric tons/day (t/d). Again, there were no visible signs of activity from the volcano.

On 28 April, the summit was clear for 40 minutes during a helicopter overflight. The crater appeared to be very quiet, with 3 main regions of low-pressure degassing, occupying essentially the same sites known from before the 1992 eruption. The crater was estimated to have widened somewhat, to at least 300 m in diameter (but had not tripled in width as indicated in BGVN 17:03), and was about 2x deeper than formerly. The inner-crater walls were extremely steep (about 60° in places) and cut outward-dipping tephra beds of the old cone. In the lower portion of the crater, a large dike was plainly visible. Concentrations of large boulders (>2 m in diameter) were scattered at the E base of the cone, apparently after being ejected beyond the crater rim during the eruption and rolling down the cone's outer slopes.

The distribution and impact of the ash blanket were visible when flying over León. The collapsed structures tended to be public (e.g. schools) or large businesses (e.g. cotton warehouses) and apparently represented the failure to clean accumulated scoriae and ash from their roofs. León was still the site of major cleanup efforts; most households removed all of their clay roof tiles, cleaned them and replaced them. There appeared to be only minor danger of mudflow activity, reflecting the low topographic relief of the area, the fact that no major drainage begins on Cerro Negro and continues through León, and the relatively thin deposits (about 3 cm in León). Selected drainages were to be cleaned out, for fear that even small mudflows might damage bridges on the Pan American highway.

The crater was visited on 29 April by a group of Nicaraguan, American, and Russian scientists. A fault circled virtually the entire crater rim, with vertical and horizontal offset of ~25 cm. The fault was the site of minor degassing and fumarole sublimate deposition; maximum gas temperature was 189°C. The very steep slopes and nearly complete fault suggested that the crater will undergo massive slumping, probably after the first rains soak the scoriae, and will resume its more typical broad, shallow, bowl-like form. The rim was notably more uneven than the pre-1992 rim, reflecting heavy accumulation of scoriae on the W margin; the low point on the rim was about 675 m, and the high point might have reached 850 m. No agglutinate was found anywhere in the crater or on the rim.

A descent was made to the principal accessible fumarole field, adjacent to the dike on the SW side of the crater. The dike was oriented at 290° and extended intermittently across the crater, becoming more pipe-like (~6 m in diameter) at the center, where it appeared to represent the primary eruptive conduit. All of the remaining degassing occurred along the dike, centered on a small zone about 10 m long on its SE portion, at the central pipe, and on its NW portion. The maximum measured temperature was 350° and abundant sublimate deposition was evident (possibly thenardite). Degassing was quite diffuse and passive, and the overall appearance was remarkably like that seen in a previous visit in 1982. There was a strong smell of HCl, with notable SO₂ odor but no H₂S.

Geologic Background. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.

Information Contacts: G.J. Soto and R. Barquero, ICE; Sergio Paniagua and Hector Flores, Sección de Sismología, Vulcanología y Exploración Geofísica, Escuela Centroamericana de Geología, Univ de Costa Rica, Apdo. 35 UCR, San José, Costa Rica; S. Williams, Arizona State Univ; Martha Navarro C. and Silvia Arguello G., INETER, Apdo. 2110, Managua, Nicaragua; Josephine Malilay, Health Studies Branch, F-28, Centers for Disease Control, 1600 Clifton Road, N.E., Atlanta, GA 30333 USA.

A persistent, very small gas plume was visible in late April, rising from the NE margin of the 1-km fissure formed in 1952. Weak activity has been reported from this fumarole since 1980.

Geologic Background. Las Pilas volcanic complex, overlooking Cerro Negro volcano to the NW, includes a diverse cluster of cones around the central vent, Las Pilas (El Hoyo). A N-S-trending fracture system cutting across the edifice is marked by numerous well-preserved flank vents, including maars, that are part of a 30-km-long volcanic massif. The Cerro Negro chain of cinder cones is listed separately in this compilation because of its extensive historical eruptions. The lake-filled Asososca maar is located adjacent to the Cerro Asososca cone on the southern side of the fissure system, south of the axis of the Marrabios Range. Two small maars west of Lake Managua are located at the southern end of the fissure. Aside from a possible eruption in the 16th century, the only historical eruptions of Las Pilas took place in the 1950s from a fissure that cuts the eastern side of the 700-m-wide summit crater and extends down the N flank.

Information Contacts: S. Williams, Arizona State Univ.

The crater lake's water level dropped approximately 8 m between January and 7 April, revealing sediments (gypsum, amorphous silica, and sulfur) and numerous mud pots around its edges. The lake was emerald green, with a temperature of 77°C (7 April). Emissions from fumaroles in the lake increased in April, producing jet-engine sounds audible at the tourist overlook, and a 1-km-high plume. Fumaroles on the dome S of the lake had temperatures of <83.5°C. The increase in emissions was reflected in an increase in the effects of acid rain, which damaged trees in the national park, and coffee plantations, cypress trees, and pasture beyond the park boundaries. Residents on the W and SW flanks reported sulfur odors, and skin and eye irritation. A daily average of 250 low-frequency earthquakes was recorded in April (at the UNA station POA2, 2.7 km SW).

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

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSCIORI; G. Soto and R. Barquero, ICE.

"Seismic activity was at a low level in April. The month's total number of caldera earthquakes was 166 . . .. The highest daily totals were 43 and 32, recorded on 28 and 29 April, and consisted mostly of events in the W part of the caldera seismic zone, near Vulcan cone. Three of these events were felt, all registering at M_L 2.8. Other earthquakes were located in the S and E parts of the caldera seismic zone."

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

Information Contacts: C. McKee, RVO.

A moderately large steam plume, with the same general appearance as in 1982, was observed many days during 23-29 April fieldwork at nearby Cerro Negro. Geologists were unable to measure the SO₂ flux, but the plume was estimated to represent several hundred t/d.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: S.N. Williams, Arizona State Univ.

Although no detailed observations were made, no significant plume was emitted during 23-29 April fieldwork. Fumarolic activity that was vigorous in June 1989 (SEAN 14:02 and 14:06) had decreased notably by February 1990 (BGVN 16:02). Fumarole temperatures had also decreased, from around 550°C (9-10 March 1989) to <=246°C on 13 June 1990.

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

Information Contacts: S.N. Williams, Arizona State Univ.

Low-temperature (89°C) fumarolic activity continued in and between the central and SW craters. Low- and medium-frequency earthquakes were recorded sporadically, totalling 32 in April.

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: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto and R. Barquero, ICE.

Lava dome growth continued through mid-May, accompanied by frequent avalanches and pyroclastic flows produced by partial collapse of the lava dome complex (800 m E-W, 650 m N-S, and 350 m high by mid-April). The youngest dome (7), which first appeared on 6 April, grew slightly faster than material was removed by collapse along the leading edge. Its viscous lava formed "banana peel-like" structures with several radial lobes. These structures had typically been observed on the surfaces of newly extruded lava that overrode older domes (dome 3 over dome 2, and dome 5 over dome 4). By mid-May, dome 7 was about 200 m long, 110 m wide, and 90 m high, and had the highest extrusive vent of the dome complex.

The cryptodome that formed among domes 3, 4, 6, and 7 continued to swell, and small reddish oxidized pieces, probably from its interior, were exposed on the surface. Intrusion and erosion rates were balanced, keeping the peak shape and height nearly stable. Gullies developed around the cryptodome, extensively eroding its ENE side, where rockfalls frequently occurred. The rockfalls commonly traveled E and NE, and rarely N, producing reddish to pink-colored ash clouds. Many cracks appeared on the head of dome 6, pushed by the swelling cryptodome. The foot of dome 6 reappeared from the talus deposits, while dome 4 (E of the cryptodome) was buried by talus. Dome 2 was similarly buried in mid-December 1991. The E part of dome 3 was also pushed, and covered by reddish lava blocks from the cryptodome.

The daily number of seismically recorded pyroclastic flows ranged from 4 to 28, almost unchanged from previous months, totaling 322 in April . . . . Flows originating at dome 7 traveled down the SE flank of the dome complex toward Iwatoko-yama and the Akamatsu River valley. Pyroclastic deposits buried the gentle slope along the Akamatsu River valley. April's longest flow extended 3.5 km E from the dome. Although ash clouds (generally 500-1,000 m high, maximum 1,500 m) extended to > 500 m beyond the main flow deposits, they had only minor effect, neither burning nor toppling the trees that they passed. Geologists suggested that this may indicate a decrease in the auto-explosivity of the lava blocks.

On 22 April, repeated lava-block falls from the toe and sides of dome 7 generated multiple pyroclastic flows that cascaded down the steep SE slope made of pre-1990 volcanics, forming a gully 50 m deep, 100 m wide, and up to 400 m long. By mid-May, the gully had been buried by rockfall talus.

From mid-April to mid-May, blue gas was emitted continuously from dome 3, and sporadically from the heads of domes 6 and 7, and the cryptodome. Ash emission was weak and infrequent. Small earthquakes continued to occur beneath and within the dome complex, fluctuating between 20 and 200/day during April and early May, and totaling 3,053 in April (down from ~ 4,000-6,000 in prior months). Roughly 7,600 people remained evacuated as of early May.

Analyses of lava samples collected from pyroclastic-flow deposits generated by collapse of domes 6 and 7 indicate compositions similar to those of the other domes; ~ 65 weight % SiO₂, with ~ 20 volume % of plagioclase, hornblende, and biotite phenocrysts. The samples had specific gravities of 2.1-2.2, also similar to the other domes, implying that vesicularity of the dome lavas has remained nearly constant throughout the 1991-92 eruption. Bombs erupted directly from the magma conduit on 8 and 11 June 1991 had specific gravities of 0.8-2.6.

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: S. Nakada, Kyushu Univ; JMA.

Continued eruptive activity in March and April was dominated by fine ash emission from Wade Crater (figure 17). Numerous E-type (eruption) earthquake sequences were recorded, but no new deposits of ballistic ejecta were evident. A new collapse crater (named Princess) developed in the SE part of the 1978/90 Crater complex in mid-April, and subsidence occurred over a substantial area nearby.

Figure 17. Sketch map of the 1978/90 Crater complex and adjacent parts of the Main Crater floor, mid-April 1992, showing the new Princess Crater and the neighboring subsided area. Contour interval, 40 m. Courtesy of DSIR.

During 15 April fieldwork, Wade Crater emitted a gas column that included a little ash, while dense white steam emerged from nearby TV1 Crater ... . Sounds from Wade Crater appeared to have a deeper origin than previously, suggesting that the level of the magma column had dropped. Fumarolic activity NW of the 1978/90 Crater complex occurred with a strong, high-pitched roar. Fine, green-brown ash coated much of the Main Crater floor and walls. Stratigraphy of a pit dug roughly 200 m SE of Wade Crater included 57 mm of fine ash overlying a block/lapilli layer erupted in early March. The coarsest fraction of surface ash collected at the site was dominated by fresh crystals of plagioclase, orthopyroxene, and clinopyroxene, with minor amounts of vesiculated brown glass and a little altered lithic material. Fewer than 10 fresh impact craters were observed near the tephra pit; one contained a scoriaceous bomb, the others dense lava blocks. A detailed infrared survey was flown over White Island, but no data were available at press time.

The new crater was not present during 15 April fieldwork, but collapse had occurred by the time R. Fleming visited the island during the late morning of 17 April. The only significant seismicity during the 2-day interval was a 37-minute E-type event, similar to many others recorded during 1992, that began at 0002 on 17 April. Light ash emission was occurring from Wade Crater on 17 April, and ash fallout N of the island was too heavy to sail through on 18-19 April. Red ash was emerging from Wade Crater on 20 April, and an eruption column rose ~1,500 m on 21 April at 1445.

The new collapse crater was 60-70 m in diameter with nearly vertical walls, when first observed by geologists on 23 April. No coarse ejecta appeared to have been erupted during its formation. More than 4,000 m² of the adjacent Main Crater floor had subsided and sagged toward the new crater. The head of the subsided area had a vertical scarp 1-1.5 m high, and its center had dropped an estimated 8-10 m. Given its shape, geologists suggested that the subsided area had previously been underlain by a SE-trending cavity.

Gray-brown ash collected near the subsided area on 23 April appeared to be mainly derived from crater-fill material involved in the formation of Princess Crater, with only a minor magmatic component. The sample's coarse fraction was dominated by white altered lithic material, although small amounts of black scoria and brown vesiculated glass were present. Many crystals had abraded surfaces and did not appear as fresh as those in the ash collected 15 April. Some fine pyrite clasts were also found in the sample.

Seismicity through 21 March was characterized by 2-5 A-type events/day. Significant volcanic tremor that was dominantly of medium frequency (3-5 Hz) resumed on 21 March. Individual tremor episodes lasting 2-9.5 hours continued through 31 March. A shallow ML 5 earthquake centered ~12 km NW of White island occurred on 26 March at 0527, followed by ~90 aftershocks in the next nine days. High-frequency A-type volcanic earthquakes began to increase on 29 March, averaging 7/day until a swarm of >150 events occurred on 3 April, with 58 more the next day; the largest reached ML 3.8. Seismicity remained elevated 5-12 April at >10 recorded events per day, then declined to more normal levels of 3-4/day. B-type events continued to be detected about every other day. No E-type earthquakes occurred from 18 March until one was recorded 2 April, but there were at least 20 from 6 to 16 April, sometimes accompanying the beginning or end of volcanic tremor episodes. Another E-type event was recorded on 21 April.

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. Scott, DSIR Rotorua.

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).