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/).

Mt. Cameroon began erupting during the night of 28 May 2000. On 29 May, following a violent explosion, red-tinged fumaroles were observed at an elevation of 3,300 m. On May 30, an earthquake shook the provincial capital of Buea, located to the SE of the volcano. Volcanic ash and gases that were vented during the course of the eruption were blown to the W coast by NE winds.

The eruption occurred at two principle sites separated by 3 km. These sites lie on the central portion of the upper SE flank, upslope of the town of Buea and in the vicinity of vents active in 1904 and 1922. The first site, located at latitude 04°12'40" N and longitude 09°10'45" E and an elevation of 4,000 m, is composed of two craters aligned NE to SW. Juvenile material comprises less than 1% of the total volume of the pyroclastic material surrounding the vents. The larger pyroclasts were found farther away from the vent, while the finer material was deposited closest to the crater. The NE crater, with slopes that are fissured and unstable, showed relatively little activity compared with its neighbor to the SW. The eruption at the SW crater was characterized by sporadic explosions of gas and pyroclastic materials, including juvenile materials such as volcanic bombs, blocks, and scoria. There were no lava flows reported from this site.

The second vent lies at 04°11'15" N and 09°10' E at an elevation of 3,300 m. This site consists of a large open fissure oriented at N 40° E. The following features at the site run NE to SW along the fissure: two lava lakes surrounded by spatter cones, two craters in the process of forming cones with fluid lava, and several hectometer-sized (104 m²) fissure lava flows. The spatter cones are about 40 m from the associated lava lake.

The NE lava lake forms a 60 x 40 m ellipse. This lava lake was the source of the lava flows that moved towards the ocean to the S and away from many of the inhabited parts of the volcano's flanks. There were sporadic explosions at the lava lakes.

Contrary to some media reports that suggested the lava was advancing at rates up to 20-25 m/hour, a scientist from the Ministry of Territorial Administration reported that the lava moved at ~5 m/hour. The scientist also indicated that the lava flows were far from populated areas.

However, on 8 June, various news reports placed the lava flows within 5-7 km of the town of Buea. Reuters reported on 9 June that geologist Isaac Konifer Nijah, a member of a scientific team monitoring the volcano, considered the Buea area a high risk zone. Concern for the residents in this town prompted an evacuation plan for ~3,000 residents to the towns of Limbe to the SW and Tiko to the SE. However, the evacuation plan was not implemented because on 10 June the lava front halted its advance on the town.

The BBC reported that on 19 June, the Prime Minister of Cameroon, Peter Mafany Musonge, visited the village of Bokwango, which is on the outskirts of Buea. News reports stated that at this point the lava flows were 4 km from the edge of the village. However, no new activity had been reported by seismologists for several days preceding the visit.

Thanks to Pierre Vincent and the company ELF Aquitaine, an initially proprietary report on Mount Cameroon geology, eruptions, and hazards (including a geological map) were recently made available to the Smithsonian (Vincent, 1980). The same author has some earlier published work on this volcano (Vincent, 1971).

References. Vincent, Pierre M., 1980, GNL Project in Cameroon, geology and volcanology of Mount Cameroon: Report for ELF Aquitaine (in French), 11 p., appendices, and map (plate).

Vincent, Pierre M., 1971, New data about Cameroon Mountain volcano: 6th Colloquium on African Geology, Leicester, UK, April 1971, Jour. Geol. Soc. London 127, p. 414-415.

Geologic Background. Mount Cameroon, one of Africa's largest volcanoes, rises above the coast of west Cameroon. The massive steep-sided volcano of dominantly basaltic-to-trachybasaltic composition forms a volcanic horst constructed above a basement of Precambrian metamorphic rocks covered with Cretaceous to Quaternary sediments. More than 100 small cinder cones, often fissure-controlled parallel to the long axis of the 1400 km3 edifice, occur on the flanks and surrounding lowlands. A large satellitic peak, Etinde (also known as Little Cameroon), is located on the S flank near the coast. Historical activity was first observed in the 5th century BCE by the Carthaginian navigator Hannon. During historical time, moderate explosive and effusive eruptions have occurred from both summit and flank vents. A 1922 SW-flank eruption produced a lava flow that reached the Atlantic coast, and a lava flow from a 1999 south-flank eruption stopped only 200 m from the sea. Explosive activity from two vents on the upper SE flank was reported in May 2000.

Information Contacts: US State Department, 2201 C St., NW, Washington, DC 20520 USA (URL: http://www.state.gov/); BBC (URL: http://news.bbc.co.uk/); Reuters (URL: http://www.reuters.com).

The following summarizes activity at Colima during the period from August 1999 to May 2000. As previously mentioned (BGVN 24:08), outbursts occurred on 5 and 17 July 1999. However, in the months that followed, August 1999 through May 2000, little activity occurred on Colima. Microearthquakes, sporadic eruptions and lahars were the most common events during these months.

During August through December 1999 Colima maintained low levels of seismicity, with few explosions or mudflows. Due to heavy precipitation on 2 September a lahar traveled under the Cordoban bridge without causing damage. Residents of Yerbabuena, La Becerrera, and Rancho El Jabali were told to avoid activities on the S-flank stream beds of the Cordoban, La Lumbre, San Antonio, and Montegrande rivers.

Landslides and lahars on the S and SW flanks during 5-6 September were quickly dispersed into La Lumbre and Cordoban drainages due to intense rains. Other monitored parameters showed no significant changes. During the week of 10 September seismicity remained low, with no degassing events or important explosions noted.

On 6 October at about 0120, residents from the village La Yerbabuena (8 km SW of the summit) reported a very short and light ashfall. The ashfall lasted only a few minutes, and prior to the fall residents reportedly heard "jet" sounds coming from the crater. Before the described events, the telemetered seismic network alerted the civil protection authorities, who then notified nearby villages of the activity. At 1700 on 12 October there were ground reports of an eruption that sent an ash cloud ~6 km.

During the first two weeks of November Colima ejected steam-and-ash an average of once per day. The estimated height of the columns varied from 200 to 1,000 m above the summit. Neither ballistic ejecta nor pyroclastic flows were observed. On 17 December seismicity remained stable, but some fumarolic and explosive emissions took place.

Beginning in January and continuing through May, ash explosions and steam emissions became frequent. Seismicity on 18 March remained low, yet Colima continued to produce fumes and explosions that were considered to be a high risk to the surrounding population. The evacuation of populations within a radius of 6.5-8.5 km from the summit was maintained by the State Systems of Civil Protection and the Mexican Army. After some explosions on 25 May these evacuations were again enforced.

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

Information Contacts: Colima Volcano Observatory, University of Colima, Ave. 25 de Julio 965, Colima 28045 México (URL: https://portal.ucol.mx/cueiv/).

An eruption of Copahue (figure 5) began on 1 July 2000. Ash-and-gas emissions, which have continued into late July, are considered to be Copahue's most vigorous activity in the past century. Reports were received from geologists in Argentina and Chile. Except where otherwise noted, Argentine geologists Adriana Bermúdez (CONICET) and Daniel Delpino (Civil Defense of Neuquén Province) reported information for 1-9 July, and Chilean geologists José Naranjo and Gustavo Fuentealba (both of SERNAGEOMIN) reported information from 10-13 July. The scientists submitted joint reports beginning on 13 July. All time references are to Argentina local time; Chilean time is one hour earlier (GMT - 4 hours).

Figure 5. Preliminary geologic map of Copahue, showing outlines of Pliocene and Pleistocene calderas and post-caldera lava flows. Contour interval, 100 m. Modified from a previous map in BGVN 17:10. Courtesy of A. Bermúdez and D. Delpino.

Initial explosions, 1-2 July. Although visibility was poor in late June, at 0030 and at 0430 on 1 July local Argentine police and gendarmerie (National Guard) reported ash mixed with heavy snowfall, as well as a strong sulfur smell. At around 1145, lapilli and ashfall became heavier, eventually covering the snow and the products of previous eruptions around the summit. At 1200 the gendarmerie reported lapilli falling 7.5 km NE of the volcano, in the village of Copahue, Argentina. The alert status was set at yellow; the village's emergency committee restricted tourist access and helped to evacuate 200 people.

Explosions continued throughout 2 July with increasing intensity. Lapilli, ash, and sporadic bombs (15 cm in diameter) fell 8-9 km E on the town of Caviahue, Argentina, with up to 15 cm of materials from the day's explosions eventually being deposited in some areas (figure 6). Until 2345 there were explosions of varying intensities. Preliminary results of an examination of the deposits revealed that they were composed of a very fine silica, sulfur particles, accidental rock fragments from the conduit, and juvenile materials. In Caviahue, visibility was practically zero due to ash particles in the air, and heavy ashfall cut off power for several hours. By midday, eruption plumes blowing SE reached Loncopué, a small village 50 km from the volcano.

Figure 6. Ashfall from the frequent eruptions that began [1 July] at Copahue and heavy snowfall have affected the reliability of power and potable water resources in the town of Caviahue, a popular ski area 8-9 km E of the volcano. Although the town is no longer under official evacuation, many inhabitants have not returned to battle current conditions. Courtesy of A. Bermúdez and D. Delpino.

Alert status was raised to orange on 2 July when ash was dispersed as far as 100 km away from the crater and the plume covered a total area of 2,000 km². Maximum ash accumulation of 3-5 cm occurred over an area of 6 km², including the town of Caviahue and the W sector of Lake Caviahue. Due to the ashfall, the surface of Lake Caviahue changed color from its normal deep blue to gray-green, and a water sample taken had a pH of 2.l.

Tests by Argentine geologists on ash samples deposited in Caviahue revealed a grain-size distribution of 15% coarse ash (> 1 mm), 80% fine ash (0.5-1.0 mm), and 5% fine ash dust (< 0.5 mm). The coarse ash contained a small quantity of juvenile and lapilli-sized (3-6 mm) accidental fragments; the juvenile materials were dark gray vitric scoria. Non-juvenile accessory materials accounted for 7-10% of the coarse ash and consisted primarily of white-gray silica from the bottom of the crater lake. The fine ash-sized particles had similar components and characteristics.

Irregularly shaped dark gray scoriae, 3-8 cm in size, were found as far as 12 km N of the crater; scoriae completely covered the area within a 1.0-1.5 km radius around the crater. The scoriae contained spherical vesicles 3-5 mm in diameter. Cooling cracks marked the scoriae's surfaces and their shapes had been modified during flight.

Ashfall was also reported 60 km SE of the volcano in the town of Loncopué, where the stream closest to the volcano had cloudy brown-gray waters.

Continuing activity through 25 July. Activity decreased after 2345 on 2 July. The only explosion of 3 July, at 1720 in the main crater, deposited tephra on the flanks and generated a dense, dark gray ash plume that blew NW and produced a local ashfall. According to the Buenos Aires Volcanic Ash Advisory Center, the ash plume reached an altitude of 10.6 km and blew NE. On 4 July there were explosions at 1030, 1830, and 2000. In the town of Caviahue, Delpino noted a strong sulfur smell and great booming sounds that caused windows to shake. A dark gray ash plume rose 2 km above the summit. Bermúdez and Delpino reported that at 0020 on 5 July a new cycle of rhythmic explosions began; by 1325 a total of 37 explosions had occurred. The biggest explosion, at 0515, generated a pyroclastic surge down the E and N slopes.

A report was received on 5 July from Ralco-Lepoy, a town 30 km SW of the volcano, indicating that dead fish had washed up along the banks of the Lomín river. The Lomín, as well as the Agrio river, which drain the acidic, active crater, were marked by a deep, dark-colored gully but there was no evidence of lahars. However, it is possible that ashfall covered up the evidence. The dead fish found along the Lomín River on 5 July confirmed that acidic mudflows from the crater had been channeled down this river. Chilean geologists Naranjo and Fuentealba recommended that states bordering the Lomín river (to the SW) and Queuco to Trapa-Trapa (to the N) be alerted that an acidic mudflow was moving down the river. Accordingly, authorities noted that inhabitants should be evacuated outside of an enforced safety radius. It was also recommended that professionals regularly measure the pH of affected Lomín drainages, meteorological reports be kept up to date, and that town officials periodically reevaluate the yellow alert.

Naranjo and Fuentealba also noted that at 2030 on 5 July a patrol of carabineros (Chilean National Guard) approached the volcano on horseback and observed small dark ash emissions moving SE from the volcano.

Observers in Argentina during the night of 5-6 July reported an incandescent pyroclastic emission flowing down the cone and, at one point, a white light emanating from the crater for ~15 seconds. In the same time interval, gendarmerie officers from Copahue village described "an orange-red light coming up from the crater." It is thought that the light was produced when magma rose to the surface but did not spill over the crater walls. They also noted the vertical ejection of large incandescent blocks that fell back into the crater, as well as smaller incandescent fragments that fell onto the volcano's slopes, rolled downhill, and broke up into smaller pieces.

On 6 July, Delpino reported to Naranjo and Fuentealba from Caviahue that the eruption was Strombolian with explosion pulses every 1-2 hours. Winds blew ash S of Caviahue without any ashfall in the town. There was no evidence of lahars or floods. Throughout the morning of 6 July snow continued, and there was zero visibility of the volcano.

Bermúdez and Delpino reported that during 0100-1020 on 7 July, loud explosions and ash emissions occurred at 15-minute intervals. At about 2000, the wind changed, blowing W, and ash began falling over Caviahue. About 1 mm of ashfall was observed from 20 km W of the crater.

The same day, ice blocks 15-20 cm in diameter, as well as ash and lapilli, were carried down the swollen Agrio river from the volcano's permanent ice cap. At 1300, a sample of the river water taken at the bridge near Caviahue had a pH of 2, and at 2000 a sample from the same location had a pH of 1.5. The Dulce stream source lies 4.5 km E of the cone and it flows 5.5 km W of the cone into Lake Caviahue. Ashfall altered the stream's typical pH of 7 to a pH of 2.5. Preliminary investigations by Argentina's Provincial Water Division also indicated an increased iron content.

A loud explosion summit at 0300 on 8 July awakened citizens of Caviahue; a day-long ash emission moved SE through clear skies. On 9 July at 0100 a glowing light was seen over the crater, but cloud cover obscured visual observations throughout the day.

Naranjo and Fuentealba reported that on 10 July, explosions were gray to dark brown and it is thought that the ash fell over a 25 km² area to the W, in the direction of Chile. Ash reached the summit of neighboring Callaqui volcano, covering it in gray ash. Samples from this ashfall taken 4 km W of the active crater were found to contain juvenile volcanic glass fragments, 0.3-0.5 mm in diameter.

During 1200-1230 on 12 July, a Chilean overflight revealed that explosions inside the active crater (El Agrio) occurred at 1- to 3-minute intervals, ejecting fine material up to 500 m above the crater. This material was dispersed via a plume of fine ash and gases moving NNE for more than 250 km. Observers reported that 1-2 mm of fine ash was deposited in the village of Copahue. Throughout the day, activity increased and, at 2300, there was an explosion heard in Caviahue that was thought to have deposited 1-2 cm of ash 5 km NNE of Copahue. On 12 July, scientists noted that Copahue was in an eruptive phase of lower intensity (a Volcano Explosivity Index, VEI, of 1) compared to that seen on 1-2 July (an inferred VEI of 2).

At 1100 on 13 July, explosions generated white-gray to bluish gas emissions rising 200-300 m over the crater. A gas cloud with a strong sulfur odor remained trapped in the Agrio valley over a 10 km² area; it later descended, and strong winds spread it over a 20 km² area. At 2310, an explosion produced a 1-km-high plume and incandescent fragments were ejected onto the flanks of the cone reaching up to 1 km from the crater. The plume covered Caviahue, obscuring the moon, but there was no ashfall on the town.

A Chilean helicopter flight on the morning of 13 July observed explosions emitting pale gray ash columns up to 300 m above the crater rim. Winds dispersed the ash ENE to Caviahue. Carabineros sampling water at the source of the Lomín river found it slightly acidic (pH = 5-6).

At 1250 on 13 July, an eruption plume that rose 3-5 km over the crater was reported by military and civilian pilots. The column dispersed to the NE and was a reddish-brown color. Reports from Caviahue stated that on 15 July the eruption stayed at the same intensity as previous days, and fine ash was dispersed to the N. Ash samples from 13 July were found to have an andesitic composition and to include juvenile fragments, the presence of which indicates the volcano's potential to produce even larger explosions. Water samples from the Lomín river on the same date revealed high fluorine and sulfate levels.

At 1700-1730 on 16 July, and also between 0300 and 0400 on 17 July, a dusting of ash fell over Caviahue and there was a strong sulfur smell in the air. At 0905 on 18 July, a civilian pilot reported a pale gray ash column at 3.5-4 km above sea level (just over the top of the cordillera) dispersed over 10 km to the volcano's NNW. At this time, the ongoing eruptions were considered to be of VEI 1. Ash from the weak explosions was dispersed by low winds as it escaped from the crater.

At 2206 on 19 July, members of the gendarmerie reported that a series of explosions continued to generate columns of ash and water vapor 0.5-1.0 km above the crater. The plumes dispersed to the NE depositing a fine dusting of ash over the village of Copahue. A strong sulfurous odor was reported at 2100 in Caviahue. On 20 July activity remained low, and no noises or odors were detected. Winds carried the gas-and-ash plume NNE, depositing a light ashfall over the N sector of Caviahue.

On 21 July, light ashfall dusted Caviahue and, although the crater was obscured, ash columns were sighted rising above the summit and through the clouds to heights of 700-1,000 m. At 1048 (Argentina), Caviahue residents heard a series of rhythmic explosions occurring every 2-5 minutes for one hour. The plume carried ash NNE toward Trapa-Trapa. The volcano was obscured by cloud cover on 22 July but intermittent explosions continued emitting ash plumes carried NE toward Trapa-Trapa.

A seismological team from the Southern Andes Volcanological Observatory (OVDAS) installed a portable seismic station on 21 July at a spot ~2 km NNW of the active crater in the vicinity of Trapa-Trapa, Chile. After taking 15 hours of readings, the team left on 23 July after cold temperatures had prematurely reduced battery power. These readings were fortunately during a time of elevated activity, and registered seismic events generally correlated with visual observations. Despite this similarity, it was impossible to establish an exact correlation between the periodicity of the explosions (occurring every 1-3 minutes) and their microseismic signals at distance.

During the stay of the seismic team, no ashfall was reported in the Queco river region and no correlation was established between seismicity and sporadic thundering sounds reported by villagers in the area. These sounds have been attributed to chunks of the ice cap breaking off and rolling down Copahue's flanks. Due to over 3 m of snowfall, access to the area is difficult.

Explosions of low to intermediate intensity continued emitting ash-and-gas plumes on 23 July. The clouds continued to partially obscure the volcano, but at 1930 an ash column blew E toward Caviahue. On 24 July, the active crater was producing small explosions and dark gray ash emissions; a dusting of ash fell over Caviahue. When the Argentina gendarmerie and the Chilean carabineros compared respective observations no discrepancies were found.

Two pilots reported a strong sulfur odor at 1.8-2.1 km altitude, ~250 km WSW of Copahue on 25 July. At 1000 another pilot reported an ash plume extending 200 km WNW from the summit; plume height was ~2 km and width was 10-15 km. Although this explosion was not seen from Caviahue, a light ashfall fell over the town.

Due to the continued frequent ashfalls over Caviahue, town officials decided to reestablish a yellow alert. The prolonged fall of fluorine-rich ash has posed a possible problem for grazing animals in the affected fields, but heavy snowfall has made it less likely that vegetation will absorb the fluorine.

Background. Volcan Copahue is a composite cone constructed along the Chile-Argentina border. The cone lies within an 8-km-wide caldera formed 0.6 million years ago at a spot near the NW rim of the Pliocene, 20 x 15 km Del Agrio caldera. Copahue's eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains an acidic crater lake (also referred to as Del Agrio) and displays intense fumarolic activity. Infrequent explosive eruptions have been recorded since the 18th century. Eruptions in 1992 and 1995 produced several phreatic and phreatomagmatic explosions and emissions that contained higher levels of water vapor but lower ash particle content than the current eruption. The current eruption has been of longer duration than either of the previous two.

The Agrio river emerges from a crack in the edifice of the volcano 50 m below the active El Agrio crater. The river water is highly acidic and has a yellow color. Near Caviahue, the Agrio river enters the Caviahue lake basin. The lake is formed by 2 glacial finger lakes over a 9.2 km² area and is a reservoir of acidic water.

Most residents of Copahue village leave each winter, but Caviahue's population of 400 can grow to 10,000 during the ski season. Eruption-related damage has cut off power and potable water, and there remains an inability to keep ski slopes cleared of ash. In late July there were reportedly only about 419 people staying in Caviahue.

Geologic Background. Volcán Copahue is an elongated composite cone constructed along the Chile-Argentina border within the 6.5 x 8.5 km wide Trapa-Trapa caldera that formed between 0.6 and 0.4 million years ago near the NW margin of the 20 x 15 km Pliocene Caviahue (Del Agrio) caldera. The eastern summit crater, part of a 2-km-long, ENE-WSW line of nine craters, contains a briny, acidic 300-m-wide crater lake (also referred to as El Agrio or Del Agrio) and displays intense fumarolic activity. Acidic hot springs occur below the eastern outlet of the crater lake, contributing to the acidity of the Río Agrio, and another geothermal zone is located within Caviahue caldera about 7 km NE of the summit. Infrequent mild-to-moderate explosive eruptions have been recorded since the 18th century. Twentieth-century eruptions from the crater lake have ejected pyroclastic rocks and chilled liquid sulfur fragments.

Information Contacts: Adriana Bermúdez, National Council of Scientific and Technical Research (CONICET) and the National University of Comahue, Buenos Aires 1400, Neuquén Capital, Argentina; Daniel Delpino, Advisor to the Civil Defense of Neuquén Province, Argentina and the National University of Comahue, Buenos Aires 1400, Neuquén Capital, Argentina; José Naranjo, National Geology and Mining Service (SERNAGEOMIN), P.O. Box 10465, Avda. Santa Maria 0104, Providencia, Santiago, Chile; Gustavo Fuentealba, Southern Andes Volcanological Observatory (OVDAS), SERNAGEOMIN, P.O. Box 10465, Avda. Santa Maria 0104, Providencia, Santiago, Chile; Buenos Aires Volcanic Ash Advisory Center, Argentina (URL: http://www.ssd.noaa.gov/ VAAC/OTH/AG/messages.html).

Between March and June 2000, Etna's activity was characterized by several Strombolian eruptions and high gas emissions predominantly at the Southeast Crater (SEC). Sixty-four strong eruptive episodes have occurred since the new eruptive series began on 26 January 2000 (BGVN 25:03), with 19 episodes between March and June. The information for the following report is based on official weekly monitoring reports posted on the Poseidon website.

Activity during 29 March-April. Through March lava flows and ash emissions occurred frequently, and on 29 March at about 1900, lava flows were generated on the S sector of the SEC. Shortly after 0730 on 1 April intermittent ash emissions rose to ~3 km and fell on the E flank. An episode on 3 April produced strong rumblings that were felt in the area of Zafferana Etnea, with ashfall in the area of Giardini (NE sector). On 6 April, between 1010 and 1130, explosive activity produced a lava fountain and lava flows. Over the following days the only activity at the volcano was abundant emissions of steam from Bocca Nuova (BN).

On 10 and 11 April, modest Strombolian activity was observed at BN, which became more sporadic in the following days then quieted on the evening of 14 April. On 15 April, at about 1700, weak effusive activity resumed from the vent at the S foot of SEC. At 0928 explosive activity recommenced with abundant lava emission. Ash also erupted from SEC's summit and reached 2 km altitude. Intense but irregular explosive activity was also present at the BN. Activity peaked at 1235 with an eruptive column that enveloped the SEC and rose to an estimated height of 6 km; the column produced abundant fall of ash and lapilli on the E slope. The episode ended abruptly at 1250. During this time Voragine (VOR) exhibited slow steam emission.

At 0545 on 26 April, intense Strombolian activity began and was followed at 0637 by an ash emission that rose several kilometers. In addition, a series of lava flows occurred from the SEC. Beginning at 0723, explosive activity diminished and had ended by 0740. In the following days there were no further eruptive events except for occasional, and sometimes intense, gas emissions from the BN.

Activity during May 2000. During 1-7 May, there was strong gas emission. On 5 May, a strong new gas emission phase began at the SEC, representing the 52nd episode since 26 January 2000. A dense eruptive column rose several kilometers over the volcano's summit and deposited several centimeters of ash on local villages to the SE. At about 1800 the volcanic tremors and eruptive column waned, leaving weak Strombolian activity that ended around 1824. After 5 May, the SEC returned to a state of quiet. The Northeast Crater (NEC) showed intense gas emission, with varied ash content. Weak Strombolian activity persisted at the BN.

Eruptive activity during 8-14 May consisted of abundant steam emissions, mainly from BN and NEC. The BN was the most active crater, emitting copious amounts of steam from at least two vents. The NEC also had abundant steam emissions with varied ash content. Meanwhile, VOR emitted modest amounts of gas and SEC virtually nothing.

During 15-21 May there were four strong gas emissions from the SEC. During the first strong episode, on 15 May, tephra covered the E flank of the volcano. A second episode during the night of 15-16 May consisted of a violent emission of tephra from 2100-2150 that covered the SE flank. The third episode began with Strombolian activity at the SEC then changed rapidly to well-developed lava fountains between 2240 and 2300. Activity abruptly decreased and ended completely within the space of a few minutes. A fourth strong episode occurred about 2145 on 19 May with increased activity from the lava flow on the N flank of the SEC. Violent gas emissions occurred shortly after 2200 and ended within an hour. Significant eruptive activity continued from the NEC, though more discontinuous than during the preceding weeks. The abundant emissions of ash increased significantly beginning 17 May, continuing for several hours. The ash emissions from the NEC were independent of the concurrent increase of volcanic tremors and activity of the SEC, except for occasional temporal coincidence. Steam emissions from the BN were also intense, sometimes associated with weak Strombolian intracrater activity. Slow gas emissions appeared from the VOR.

Two strong episodes occurred at SEC on 23 and 27 May. Activity at the other craters consisted of above normal ash emissions from NEC, intense gas emissions at BN, and weak fumarolic activity at VOR. The 57th eruptive episode of the series began on 23 May with strong explosive activity between 0301 and 0329 accompanied by lava flows down the S flank of the volcano. An episode on 27 May was obscured by poor meteorological conditions.

Activity through June 2000. Two eruptive episodes occurred at SEC on 1 June. First, at 0814, sustained lava fountains began, with some reaching an altitude of 600-700 m before ending around 0832. The column of ash and steam rose for several thousands of meters over the summit and produced a fall of fine pyroclastic material over much of the countryside on Etna's S slope, as far as Catania. At 1930 on 1 June another episode began with a considerable increase in the flow of lava.

On 5 June a strong gas emission at SEC went on for about thirty minutes, during which an ash-and-steam cloud rose to ~3-4 km. The ashfall covered an ample sector of the SE and S region, extending to the Plain of Catania and creating difficulties in air traffic to and from Fonatanrossa and Sigonella airports. As with preceding episodes, the gas emissions were associated with lava flows, primarily on the N slope of the SEC. Just after 1230 on 8 June, an increase in this same lava flow announced another strong gas emission phase beginning with a Strombolian eruption. There was a progressive increase in the explosive activity which reached its peak between 1356 and 1426. The fallout from the eruptive cloud was distributed toward the N.

Another strong gas emission began on 14 June at about 0700 with Strombolian characteristics. Ash emissions reached a climax between 0920 and 0940. On 24 June the 64th episode of activity at SEC occurred when a strong gas emission issued from NEC and VOR. This episode began with an increase of lava flow activity from the fracture on the N flank of the SEC. Later, Strombolian activity at the SEC's summit crater made a transition at about 2130 to a more violent, continuous gas emission phase which reached a peak about 2144, before ending shortly thereafter. After the 24 June activity there were no eruptions the rest of the month, but sporadic ash emissions occurred at all summit craters, particularly at BN and VOR.

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: Sistema Poseidon, a cooperative project supported by both the Italian Government and the Sicilian Regional Government, and operated by several scientific institutions (URL: http://www.ct.ingv.it/en/chi-siamo/la-sezione.html).

This report discusses activity at Guagua Pichincha during the months of June and July 2000. A Washington Volcanic Ash Advisory Center (VAAC) advisory was issued at 1337 on 2 June after a minor ash explosion propelled a plume to 7.3 km altitude above the summit. Another small eruption occurred one week later at 0941 on 9 June. Emissions from this second eruption did not rise more than 5 km, but more earthquakes and rockfalls indicated increasing instability of the January 2000 lava dome.

At 0953 on 12 July the dome experienced a partial collapse on its W side. This is the area of the dome closest to the W opening of the horseshoe-shaped caldera. High on the slope of the volcano's W flank, just below the caldera's opening, is the origin of the Cristál river. A long-period (LP) earthquake with a reduced displacement of 14.5 cm² probably destabilized the dome and caused the partial collapse. Judging by seismic data, the ash plume may have risen ~12 km above the crater, but cloud cover inhibited visual observations. A strong wind blew most ash W, away from the city of Quito, and very fine ash blanketed the caldera. Seismicity remained low after the eruption, but a slight increase in the number of rockfalls indicated that the dome was still unstable.

Two other events occurred during July. An ash plume was also sighted at 0900 on 23 July at an estimated height of 6.1 km moving W. An aviation notice at 0900 on 24 July described ash from six emissions over the course of the previous night that reached 4.8 km altitude, a height comparable to the volcano's summit elevation.

Over 14,530 LP events were registered in the month of March and this number decreased to 6,892 in April; there was a reported average of 271 LP events daily for the year 2000. The number of monthly explosions dropped to almost zero during the period of January to April; this was the first time there have been so few explosions since the month of July 1998. Volcano-tectonic seismicity also dropped dramatically during January-July 2000, averaging approximately the same number of monthly events as seen prior to activity that began in October 1999. The number of rockfall events has remained high since dome growth began in January 2000; thus far in the year 2000 there has been an average of 72 daily rockfalls. Beginning around June 2000 these events have occurred 100-200 times per day. Two main seismic centers have been inferred at Guagua Pichincha from data; one center is less than 1 km below the crater surface and the second ~2-4 km deeper. Continued fumarolic activity has been moderate but variable.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately to the W of Ecuador's capital city, Quito. A lava dome is located at the head of a 6-km-wide breached caldera that formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent in the breached caldera consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the central lava dome. One of Ecuador's most active volcanoes, it is the site of many minor eruptions since the beginning of the Spanish era. The largest historical eruption took place in 1660, when ash fell over a 1000 km radius, accumulating to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity, causing great economic losses.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Associated Press.

NASA's Dryden Flight Research Center (DFRC) advised that information concerning two flights of their DC-8 aircraft as reported in BGVN 25:02 contained errors and requested that the information be corrected with additional details as follows:

"For approximately seven minutes starting at 0510 during a transit flight on 29 February to Kiruna, Sweden, a NASA DC-8 aircraft with a payload of SOLVE (SAGE III Ozone Loss and Validation Experiment) sensors flew through the plume ~11.3 km NNE of Iceland at 76 °N and 5 °W, just off the Greenland coastline. The plume extended up to ~13 km altitude, well into the lower stratosphere. The aircraft passed thorough the volcanic ash far N and W, and at a flight level much higher, than the predictions reported by the Volcanic Ash Advisory Center (VAAC), London. Instruments measured many in situ trace gases, SO₂, HNO₃, NO, NOy, O₃, volatile and non-volatile aerosols, and aerosol size distribution. The scientific team reported substantial increases in CN, NOy, HNO₃, CO, and particle counts, O₃ went to nearly zero, H₂O increased, and strong scattering layers up to 13 km were detected.

"A flight on 5 March detected enhanced aerosols and SO₂ at 1301, but by that time the plume was so diluted that it represented no danger to the aircraft. During the three weeks following the initial encounter the DC-8 detected remnants of the plume trapped within the polar vortex. The resulting analysis concluded that volatile aerosols increased and the sizes of non-volatile large aerosols decreased."

NASA-DFRC also advised that the statement about the plume being a "very impressive, orange, airfoil-shaped feature in the pre-dawn sky" was erroneous. Post-flight interviews with the pilot indicated that there was no moon out, therefore pitch black sky conditions existed at the time of the encounter. The pilots had no visual evidence of flying into the plume.

Geologic Background. One of Iceland's most prominent and active volcanoes, Hekla lies near the southern end of the eastern rift zone. Hekla occupies a rift-transform junction, and has produced basaltic andesites, in contrast to the tholeiitic basalts typical of Icelandic rift zone volcanoes. Vatnafjöll, a 40-km-long, 9-km-wide group of basaltic fissures and crater rows immediately SE of Hekla forms a part of the Hekla-Vatnafjöll volcanic system. A 5.5-km-long fissure, Heklugjá, cuts across the 1491-m-high Hekla volcano and is often active along its full length during major eruptions. Repeated eruptions along this rift, which is oblique to most rifting structures in the eastern volcanic zone, are responsible for Hekla's elongated ENE-WSW profile. Frequent large silicic explosive eruptions during historical time have deposited tephra throughout Iceland, providing valuable time markers used to date eruptions from other Icelandic volcanoes. Hekla tephras are generally rich in fluorine and are consequently very hazardous to grazing animals. Extensive lava flows from historical eruptions, which date back to 1104 CE, cover much of the volcano's flanks.

Information Contacts: Gary Shelton, NASA, Dryden Flight Research Center, P.O. Box 273, Edwards, CA 93523-0273 USA.

This report covers January-June 2000. In January seismographic station IRZ2 (5 km SW of the active crater) recorded seven small-magnitude earthquakes. During February and March no activity was recorded. In April, May, and June, respectively, IRZ2 recorded 10, 12, and 30 earthquakes. The latter month included low-frequency events.

During May the level of the crater lake decreased by 50 cm. During the dry period, the lake's color was yellow/green, and a significant amount of algae covered its surface. On the lake's NE and S shore lines constant bubbling continued; the temperature of the lake was 18°C. The E, N, and W crater walls continued sliding toward the lake. Fumarolic activity on the NE flank continued at a low level.

In June the crater lake's surface rose 40 cm in comparison to May. The lake color was now green and its surface was still covered by abundant algae. The NE crater wall continued sliding, partly covering some fumaroles while others completely disappeared. Also, along the NE wall three new thermal features appeared with temperatures that fluctuated between 22 and 54°C. On the NE and S shore the bubbling stopped during June.

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: Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

The period from 1 May through 17 July 2000 was characterized by frequent surface flows and earthquakes. On 9 May a thick steam and sulfur dioxide fume formed SW of Pu`u `O`o; such fumes, or vog, have often obscured the crater for the past few months. The prominent fumes came from skylights (holes in roofs of lava tubes) along the active tubes leading to a narrow dark aa flow that emerged onto the surface on 6-7 May.

On 15 May lava broke frequently onto the surface, widening the active flow field toward the E. During 16-25 May very little activity took place. On 26 May at 0457, heavy vog hung over Pulama pali and slowly drifted downslope. The ocean entry at Waha`ula remained vigorous over the past several weeks, building a bench 40-45 m seaward of the former coastline (figure 147).

Figure 147. Map of Kilauea showing lava flows (black) on Pulama pali and the coastal plain active since October 1999 through 1 July 2000, as well as flows erupted earlier from Pu`u `O`o and Kupaianaha. Courtesy of the USGS Hawaiian Volcano Observatory.

On the afternoon of 29 May two successive earthquakes occurred on Kilauea's S flank. The earthquakes had a preliminary magnitude of 4 and were felt in the town of Hilo 45 km NW of Kilauea.

Observations of the Pu`u `O`o cone on 1 June revealed no significant changes in the crater or collapse pits on the S and W flanks (figure 148). On the E crater rim, gentle "sloshing" sounds were heard, indicating lava at a shallow level. Direct observation into the vent was prevented by heavy fume. The Pu`u `O`o crater contains three pond vents and two hornitos. Most of these originated during September-November 1999 intracrater activity. Since then the crater has often been obscured by fume, but occasionally HVO observers have witnessed active lava within these vents.

Figure 148. A diagram of the Pu`u `O`o cone and surroundings at Kilauea as of March 2000 showing the area covered by lava since February 1997 during episode 55 (light gray). Inside the crater of Pu`u `O`o, the "trough" is the drained lava pond of September-October 1999. The central portion of the trough was briefly filled with active lava in February 2000. Puka Nui is the prominent collapse pit on the SW flank of Pu`u `O`o, which was floored with lava during September-October 1999. Puka Nui is a slowly expanding collapse crater that has consumed part of the tephra cone and surrounding shield on Pu`u `O`o's SW flank. Flank vents active in 1997 have built the south shield, minishield, and 55 cone. Courtesy of Steven Brantley and Christina Heliker, USGS Hawaiian Volcano Observatory.

The S shield (figure 148) has about 20 m of relief; the minishield, less than 10 m. The episode 55 cone was about 10 m high; yet has subsided into a slowly expanding collapse crater. The cracks adjacent to the pit wall show the expansion of the 55 cone's pit. These cracks are as wide as 1-2 m and some have slight vertical offsets. Major subsidence occurs in abrupt stages. Entire collapse craters 10-30 m deep and 50 m across form in a few hours or less. The cracked ground then remains stable for weeks or months. The W gap, which formed in January 1997, is the result of the subsidence along the E-rift axis. An E-rift intrusion in September 1999 led to a temporary shutdown of volcanic activity at Pu`u `O`o. When activity resumed, new small spatter cones were active briefly, shedding the lava flows shown as 1999 flows on the sketch map.

Throughout the week of 11-17 June activity remained stable. Lava continued to flow to the sea from Waha`ula entry, and from the entry to its W. Surface lava flows were visible sporadically on Pulama pali and elsewhere. Volcanic tremor near Pu`u `O`o remained weak to moderate.

On 13 June rain cleared vog from Holei Pali and enabled good views of the flow field in the morning. Lava continued to enter the ocean, not only at the Waha`ula entry but also at other entries a few hundred meters to the W (figure 147). Surface flows were apparent several hundred meters inland, and visitors reported breakouts near the western edge of the present flow field for the past several days. Pulama pali remained dark, but the fumes rolling down the pali came from active lava tubes feeding the active ocean entries and surface breakouts. Due to rain clouds and volcanic gas in the crater center, Pu`u `O`o was dark on the morning of 14 June. Seismicity was low across the island. Volcanic tremor near Pu`u `O`o remained weak to moderate. Kilauea's summit tilt and the tilt near and on Pu`u `O`o and all along the E rift zone were flat and stable.

Two moderate steam plumes rose from coastal entries on the afternoon of 15 June. Summit and rift-zone tilt remained steady, volcanic tremor at Pu`u `O`o was moderate and continued, and there was no unusual earthquake activity. Apparently on 15 June the eruption continued through tubes, with relatively little entering the sea.

On 16-17 June the lava bench at the Waha`ula entry was 30-50 m wide. On top of Pulama pali lava moved through the tube at a speed of ~10 km/hour. On 17 June, from 1330 to 1415, observations during a helicopter flight revealed more lava on the flow field a few hundred meters inland of Waha`ula. As movement of lava continued in Waha`ula, for the first time in several weeks a surface breakout was visible on Pulama pali between 1830 and 2030 on the evening of 17 June. The lava appeared from a distance to be aa and moved slowly down the middle third of the pali, near the eastern edge of the flow field W of Royal Gardens. On the evening of 17 June the Waha`ula entry , and another entry ~800 m to the W became active for several hours.

No breakouts were visible on 20 June on Waha`ula, Pulama pali, or the coastal flat. Fume continued to blanket the flow path down the pali. Above Pulama pali a new ledge was observed on 25 June, only ~1 m below the surface, at 642 m elevation. The ledge indicated that the level of lava in the tube rose temporarily and then subsided, and a breakout was observed at 686 m elevation.

During July there were frequent surface flows. On 6 July a substantial new pahoehoe flow began from a breakout point at about 200 m elevation on Pulama pali. The flow was ~500 m long and 150-200 m wide. Lava continued to spill into the sea at three sites. The most vigorous entry remained at Waha`ula, which generated two steam plumes on 6 July. The Kamokuna entry, the westernmost active bench, was less vigorous than Waha`ula but created a substantially larger steam plume. During mid-day 16 July, several entries were active: Waha`ula was the most active and Kamokuna the second most active. Several moderate-size surface flows were active in the eastern part of the flow field, between Royal Gardens and the coast. Heavy fume continued to flow down Pulama pali above the lava tube system.

Overall the seismicity and volcanic tremor for the months of May through July remained moderate and stable in the area around Kilauea's summit. Within the summit of Kilauea activity has remained slightly elevated.

Background. Kilauea is one of five coalescing volcanoes that comprise the island of Hawaii. Historically its eruptions originate primarily from the summit caldera or along one of the lengthy E and SW rift zones that extend from the caldera to the sea. The latest Kilauea eruption began in January 1983 along the E rift zone. The eruption's early phases, or episodes, occurred along a portion of the rift zone that extends from Napau Crater on the uprift end (towards the summit) to ~8 km E on the downrift end (towards the sea). Activity eventually centered on the area and crater that was later named Pu`u `O`o.

Between July 1986 and January 1992, the Kupaianaha lava lake was active ~3 km NE of Pu`u `O`o. It was during this period that the town of Kalapana and a majority of the 181 homes lost were destroyed. In December 1991, one month prior to the shutdown of Kupaianaha, eruptive activity returned to Pu`u `O`o. More than 1 km³ of lava has erupted during the 14 years of activity (January 1983-January 1997).

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: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

At about 1044 on 20 July 2000, an eruption began at Lascar volcano that lasted until 1509. The Washington VAAC reported an ash advisory at 1509 for an ash plume that extended 660 km to the E, stretching from N Chile across S Bolivia and N Argentina and into W central Paraguay. At that time, the plume was traveling at speeds of up to 130 km/hour, reached altitudes of 10.7-13.7 km, and was reported to be 103 km wide.

Residents of the village of Jama, located 60 km ENE of the volcano on the Argentina-Chile border, reported feeling an earthquake before seeing a white mushroom cloud that rose 4-5 km high and rapidly blew E, depositing 1-2 mm of ash over the village. Several explosions were felt and heard 160 km ESE in San Antonio de los Cobres, but there were no reports of any injuries or damage. Activity continued into 21 July with small explosions producing plumes 200-300 m above the summit. The volcano is in a sparsely populated area so no evacuations were necessary.

According to Matthews and others (1997) Lascar has undergone four recognized cycles between 1984 and 1993. In each of these cycles, a lava dome is extruded in the active crater accompanied by vigorous degassing through high-temperature, high-velocity fumaroles on and around the dome. The dome then subsides into the conduit while the velocity and gas output of the fumaroles decrease; the cycle ends with violent explosive activity. No new lava was immediately extruded after the dome collapsed in the explosive 1993 eruption, thus breaking the previous pattern.

Background. Lascar is the most active volcano of the northern Chilean Andes; it is characterized by its persistent fumarolic activity, steam eruptions, and occasional vulcanian eruptions. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters along a NE-SW trend.

Matthews and others (1997) discussed Lascar's evolution in four phases starting at ~50 ka. During phase I, an edifice was established on the E side, and pyroxene andesite lavas erupted. Phase II saw the development of the W edifice with a subglacial andesitic eruption, and the destruction of a substantial dome, arguably the volcano's most explosive event. In Phase III, a stratocone was constructed and a major andesitic explosive eruption generated scoria flows, known as the Tumbres deposits, dated at 9.2 ka. Phase IV activity shifted back to the E, leaving pyroclastic deposits dated at 7.1 ka. Prominent Phase IV lava flows extended NW and were later truncated by the formation of three deep collapse craters that mark the W migration of the active center. The current active vent discharges in the deepest of these craters, which is 800 m in diameter and 300 m deep. Frequent explosive eruptions have been recorded since the mid-19th century.

Reference. Matthews, S.J., Gardeweg, M.C., and Sparks, R.S.J., 1997, The 1894 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions: Bulletin of Volcanology, v. 59, p. 72-82.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: José Viramonte, Universidad Nacional de Salta and CONICET, Buenos Aires 177 -4400 Salta, Argentina; George Stephens, NOAA Operational Significant Events Imagery Support Team, World Weather Bldg., 5200 Auth Road, Rm. 510, NOAA/NESDIS, Camp Springs, MD 20748 (URL: https://www.nnvl.noaa.gov/); Associated Press.

The 27 June 2000 water discoloration ~1 km off the W shore of the island of Miyake-jima (BGVN 25:05) prompted considerable investigation. Remote Operation Vehicle (ROV) work and multi-beam side-scan sonar revealed fractures and what appeared to be three ocean-floor craters around the area of discoloration. Crustal deformation found in this region implies that cracks have opened under the W flank of the volcano. Magma intrusion was confirmed to have occurred in the W flank of the volcano around the time of the 27 June event. The absence of scoria or other eruptive products makes it likely that the event was thermal water released due to intrusion.

Magma intrusion is also thought to be the cause of a series of earthquakes that began on 26 June. Hypocenters migrated from a central position under the island in a curve to the W, NW, and N, reaching a position ~70 km NNW of the island by 21 July (figure 5).

Figure 5. A map showing Miyake-jima (lower right-hand corner) and the NW migration of hypocenters, 26 June-21 July 2000. Hypocenters were centered under the summit when activity began and then migrated to a submarine location ~ 10 km NW. This movement was thought to be related to magma intruding to the W. Labels for the higher-magnitude events indicate the month/day, magnitude, and hypocenter depth. Courtesy of the Japan Meteorological Agency.

Miyake-jima's mayor, Naoyuki Hirose, lifted the evacuation order for the SE district of Tsubota on 29 June, permitting hundreds of the almost 2,000 evacuees to return home. Approximately half of the island's population of 4,000 had been evacuated on 26 June.

At 0414 on 7 July, an eruption from the summit crater sent ash and rock into the sky; plumes dispersed ash over wide areas of the island. The eruption continued until 1110 and about 140 residents had to be evacuated from the N sector of the island to protect them from heavy ashfall. A second eruption at 1550 sent an ash column 1 km above the crater, ejected rocks, and produced loud booming noises. On 8 July there was a weak yellow-colored emission. Closer inspection of this last eruption revealed that very little material had been ejected, but a pit crater ~200 m in diameter and 100-200 m deep had formed. It is thought that the pit crater marked an empty cavity left when magma progressed from the summit area and intruded to the W.

The month-long crisis (figure 5) involved more than 17,500 earthquakes, including 5,480 strong enough to be felt by humans. The Miyake-jima earthquake swarm included a 7 July, M 5.2 event centered 25 km NW of Miyake-jima under the young volcanic islands of Nii-jima and Kozu-shima at 10 km depth, and a 1 July, M 6.4 event that killed one man on Kozu-shima by rockfall.

Geologic Background. The circular, 8-km-wide island of Miyakejima forms a low-angle stratovolcano that rises about 1100 m from the sea floor in the northern Izu Islands about 200 km SSW of Tokyo. The basaltic volcano is truncated by small summit calderas, one of which, 3.5 km wide, was formed during a major eruption about 2500 years ago. Parasitic craters and vents, including maars near the coast and radially oriented fissure vents, dot the flanks of the volcano. Frequent historical eruptions have occurred since 1085 CE at vents ranging from the summit to below sea level, causing much damage on this small populated island. After a three-century-long hiatus ending in 1469, activity has been dominated by flank fissure eruptions sometimes accompanied by minor summit eruptions. A 1.6-km-wide summit caldera was slowly formed by subsidence during an eruption in 2000; by October of that year the crater floor had dropped to only 230 m above sea level.

Information Contacts: Geological Survey of Japan, Higashi 1-1-3, Tsukuba, Ibaraki 305 Japan; Japan Meteorological Agency, Tokyo, Japan; Associated Press; Reuters.

Seismicity remained stable between November 1999 and April 2000. In May 2000 a seismic swarm began near the volcano, and in June there was heightened seismicity.

During 9-11 June the INETER seismic network registered over 500 earthquakes near Momotombo, 100 of which were located. Many of the earthquakes were between M 3.4 and 4.1 (figure 8), and occurred at depths less than 5 km. The small epicentral area was directly under a geothermal plant on the S slope of the volcano, between Momotombo's crater and the coast of Lake Managua. A similar area was the site of seismic swarms in past years, with the most recent occurrence in May 2000. Some of the earthquakes on 9 June were felt by the personnel of the geothermal plant 5 km SW of the crater and one was felt by several people in the town of Nagarote. INETER stated that an eruption could affect the geothermal plant's 96 employees, as well as residents of towns bordering the volcano. Continuous seismic tremor was also observed at the volcano, which was attributed to volcanic processes rather than movements at tectonic faults. The number of seismic events began to decrease on 11 June. From 12 to 13 June, 60 earthquakes occurred with seven epicenters located. In comparison, 150 earthquakes occurred from 9 to 10 June with 38 epicenters located. After 13 June the number of earthquakes gradually decreased to normal levels.

Figure 8. Locations of earthquake epicenters at Momotombo with magnitudes less than 3.8 (circles), and magnitudes between 3.8 and 4.1 (stars) from 9 to 16 June 2000. Courtesy of INETER.

Julio Alvarez, Jorge Cross, Arming Saballos (all INETER/Vulcanología), and Eduardo Mayorga visited the volcano on 15 June to measure the temperature of fumaroles in the crater zone. Temperature measurements conducted at fumaroles in the volcano's dome yielded values between 255 and 933 °C (figure 9). The highest temperature was found near the N edge of the crater.

Figure 9. Sketch of Momotombo's active crater showing fumarole temperatures on 15 June 2000. Areas of fumarolic activity are gray. View is towards the S; the crater is ~ 150 m wide. Courtesy of INETER.

Geologic Background. Momotombo is a young stratovolcano that rises prominently above the NW shore of Lake Managua, forming one of Nicaragua's most familiar landmarks. Momotombo began growing about 4500 years ago at the SE end of the Marrabios Range and consists of a somma from an older edifice that is surmounted by a symmetrical younger cone with a 150 x 250 m wide summit crater. Young lava flows extend down the NW flank into the 4-km-wide Monte Galán caldera. The youthful cone of Momotombito forms an island offshore in Lake Managua. Momotombo has a long record of Strombolian eruptions, punctuated by occasional stronger explosive activity. The latest eruption, in 1905, produced a lava flow that traveled from the summit to the lower NE base. A small black plume was seen above the crater after a 10 April 1996 earthquake, but later observations noted no significant changes in the crater. A major geothermal field is located on the south flank.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

A blocky lava flow fed from the Caliente vent, active since July 1999 (see BGVN 24:12), had advanced nearly 2.5 km by the end of January 2000. The thermal anomaly related to this flow as measured on the 23 January Landsat 7 Enhanced Thematic Mapper (ETM+) is ~2,370 m long and 60-120 m wide. The flow extended S down the flank of the Santiaguito dome complex before being deflected SW by a low ridge and moving over the top of the 1986-89 flow (figure 29). A ~50 m-wide axial zone of the flow was very steep with a front slope of 60-70°. This ~30-m high axial zone advanced downward and collapsed into the sheer-sided ravine that forms the upper reaches of the Río Nimá II. The marginal flow front is ~18 m thick and its slope is smaller (~32°). As 2- to 5-m-wide sections of the flow front moved, minor collapses occurred at a rate of 1 to 2 per minute. Ash clouds generated by these collapses had temperatures of 185°C, and flow temperatures as high as 531°C were measured at a freshly exposed section of the axial zone. Temperatures for the blocky crust capping the flow front were lower, typically 34-76°C.

Figure 29. Sketch map of Santiaguito showing the January 2000 location of the blocky lava flow that began in July 1999. Also marked are lava flows emplaced between 1990 and 1999, as identified from an analysis of a Thematic Mapper time-series of 13 images. Using this time series the blocky flow which breached the 1902 crater rim is believed to have occurred during 1996-97, where "a" indicates the new aggradation load supply to Río Nimá I. Courtesy of Eddie Sánchez, Otoniel Matías, Andy Harris, Luke Flynn, Bill Rose, James Vallance, Edouard Gegout.

On 23 January, the Caliente vent was full. The 23 January ETM+ image shows this zone as an intense, thermal anomaly, 120-150 m in diameter. Small ash eruptions occurred at a rate of 1-2 events per hour producing ash plumes that extended kilometers above the vent. More powerful events generated small pyroclastic flows as well as rock falls. Both the dome and upper flow area collapse frequently produced audible rock falls that could be heard from a distance of ~1.5 km. Thirty-seven (37) rockfalls were heard on 23 January; 7 of which were incandescent as hot blocks from the dome and upper flow bounced down the flank of the dome.

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

Information Contacts: Eddie Sánchez and Otoniel Matías, Instituto Nacional de Sismología, Vulcanología, Meteorología e Hydrología (INSIVUMEH), Ministerio de Communicaciones, Transporte y Obras Publicas, 7A Avenida 14-57, Zona 13, Guatemala City, Guatemala; Andy Harris and Luke Flynn, IGP/SOEST, University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA; Bill Rose, Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA; James Vallance, Department of Civil Engineering and Applied Mechanics, McGill University, Montreal, Quebec H3A 2K6, Canada; Edouard Gegout, c/o European Volcanological Society, C.P.1-1211 Geneva 17, Switzerland.

During June-July 2000 seismicity was generally at background levels with occasional weak fumarolic activity; the hazard level was Green. However at 0447 on 30 June, visual reports indicated a short-lived explosive eruption and an ash-gas plume that rose to about 8 km altitude; in response, the hazard status was raised to Yellow. Similar reports indicated that a short-lived explosive eruption at 1644 on 1 July sent and an ash-gas plume to ~6 km altitude. The mushroom-shaped plume extended to the W and at 2034, satellite imagery showed the arched plume extending 70 km NW. At 1728 on 1 July seismic data indicated a less intensive short-lived explosion, and on 2 July several weak explosions occurred and a gas-steam plume rose 300-700 m extending 3-5 km to the W and E. On 3 July seismicity under the volcano returned to background levels and the hazard status was reduced to Green.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA (URL: http://www.avo.alaska.edu/).

This report covers the period of 1 May to 3 July 2000. Tiltmeter readings from 1-3 May showed a decrease in both the x-axis (25 µrad) and y-axis (40 µrad on the SW side of the summit, indicating deformation due to magma rising towards the surface. Magma continued to rise, but there was no increase in earthquakes registered at the Soputan Post Observatory (SPO) in Maliku. Nevertheless, seismic data from both satellite-telemetered and SPO's instruments contained an increasing trend in cumulative energy that could have been the result of tectonic earthquakes. A 5 May MR 6.5 earthquake in Banggai, ~325 km SW of Soputan, is thought to have been a precursor to a 13 May eruption.

At 1250 on 13 May, an eruption began with the ejection of incandescent materials and the emission of a thick, black ash cloud that rose 1,000 m above the summit and drifted NE. There were reports of ashfall up to 2 cm thick in the towns of Malompar and Tombatu, ~9 km S of the summit.

In the weeks following this event, seismicity remained elevated, with tectonic earthquakes dominating activity. Sporadic emissions of thin, white ash-and-steam plumes rose up to 100 m, but no explosions were reported. By 22 June, scientists were reporting several small explosions and avalanches, as well as a significant increase in the number of volcanic tremors and avalanche earthquakes.

At 1200 on 1 July, continuous tremor earthquakes reached amplitudes of 20-50 mm. Later that day, at 2232, two loud booms were heard and at 2255, lava was seen flowing up to 200 m to the W of Soputan's summit, covering over 13-14 May lava flows. Lightning was also seen around the crater and the rising plume. At 0200 on 2 July, Strombolian lava fountains were seen spewing lava 10-50 m above the crater. Later in the day, a thick gray ash plume was seen as it reached ~1,000 m altitude and slowly changed color to a dark brown. The volcano continued to produce ash plumes and persistent booming that indicated explosions were taking place although they could not be seen. The number of earthquakes reached over 100 events per day, indicating that lava dome growth continued. Observations made at both SPO and the Lokon Post Observatory, ~30 km N in Tomohon, gave the government reason to have concern for inhabitants' safety and, on 3 July, the alert level was raised from 2 to 3 (on a scale of 4).

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is located SW of Riendengan-Sempu, which some workers have included with Soputan and Manimporok (3.5 km ESE) as a volcanic complex. It was constructed at the southern end of a SSW-NNE trending line of vents. During historical time the locus of eruptions has included both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

This report covers activity from 26 May to 21 July 2000. During this interval, the lava dome continued to grow; however, between 26 May and 2 June, the direction of the dome's growth changed. Although it continued to grow vertically, the majority of growth appears to have redirected from the E and NE to the S and possibly the W.

Visual observations were severely limited due to clouds throughout the early part of this period. However, during the week of 23-30 June a "rough, spiny area" appeared high on the E face of the dome at the top of the Tar River Valley. The week of 9-16 June, the dome grew to about 914 m. By 25 June, the dome had surpassed the height attained prior to the 20 March 2000 collapse. During this event, instruments for measuring dome volume were damaged. Observations from 30 June through 7 July showed that the area of dome growth had changed to a more slab-like appearance. A new area of spiny growth was first seen on 10 July. This growth appeared on the NE flank at 940 m elevation, which was thought to be the highest point on the dome. On 17 July, a large area of new growth was reported on the S and W sectors of the dome, attaining a height of 950 m.

Pyroclastic flows were reported to the ENE in the Tar River, between 9 and 16 June. The following week, pyroclastic flows were reported in the Gages valley to the W. Additional pyroclastic flows during the week of 7 July went NE into the upper Tar valley; some, if not all, of the flow material originated from the remains of the 1995-98 dome. On 21 July at 0620, there was a small pyroclastic flow with an explosive start. During an observation flight later that day, evidence of pyroclastic flows was observed to the SW in the upper region of the White River valley.

Rockfalls occurred throughout the reporting period (table 34). However, the week of 23 to 30 June was characterized by nearly constant rockfalls and small pyroclastic flows. These rockfalls were concentrated on the E side of the dome and talus accumulated much more slowly to the S above the White River. Prior to this, during the week of 9 to 16 June, the rockfalls occurred almost exclusively in the Tar River valley. During 30 June to 14 July, rockfalls occurring to the E of the dome were infrequent despite the presence of large blocks at the top of the steep E slope. The majority of the rockfall events at this point were occurring to the S and to the W of the dome.

Table 34. Seismic data for Soufriere Hills during 26 May-21 July 2000. Courtesy of MVO.

Seismic records (table 1) revealed a sharp increase in the number of long-period (LP) earthquakes after 2 June. The frequency of LP events continued to increase until its peak during 23-30 June. This same week marked the low point in the number of hybrid earthquakes. The number of volcano-tectonic earthquakes increased towards the end of the reporting period.

A steady production of ash during the week of 9-16 June maintained a dilute ash plume that moved W towards Plymouth and off the coast. Neither this ash plume nor the smaller ash clouds produced by rockfalls during the preceding weeks affected the inhabited parts of the island. During the week of 30 June to 7 July, abundant steaming was observed on the W flanks of the dome. The following week, steaming occurred on the N side between the main masses of the old dome. During this same week, ash venting was also observed from the S side of the dome.

The sharp increase in the number of LP and hybrid earthquakes after 2 June was taken to indicate increasing pressure in the dome. In addition, the dome's filling in of the crater on all sides suggests that rockfalls and pyroclastic flows will increase in the future. These events are expected to affect not only the Tar River valley, but also several other surrounding valleys, particularly Tuitt's Ghaut, White River valley, and Gages valley. These observations also lead to increased concern over the possibility of a substantial dome collapse in the near future.

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

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).

Usu's multi-vent NW-flank eruption that began on 31 March 2000 continued until at least 10 July (see previous report and map in BGVN 25:03). By 10 July the eruption had lost considerable vigor. The last noteworthy ash fall took place on 6 April; a small one occurred on 7 April. Several excellent reports were published rapidly by the Geological Survey of Hokkaido (GSH, 2000a, b). This article provides a summary of those Japanese-language reports as well as excerpts from a formal statement discussing Usu's behavior through 10 July. Satellite imagery also provided ashfall data. The active dome and associated vent group was incorrectly spelled "Konpira-yama" in previous Bulletin reports. According to formal rules of translation this name should be "Kompira-yama."

Prior to the eruption, geological mapping and bore holes had delineated portions of Usu's edifice and surrounding subsurface, enabling workers to draw a generalized cross section (figure 22). In addition to these background studies, as Usu entered an eruptive phase on 31 March a comprehensive suite of monitoring instruments were in place.

Figure 22. A schematic cross section across the flank of Usu showing boreholes, subsurface rock units (unlabeled), and areas of the two active vent groups with their plumes. The schematic illustrates the inferred zone of phreatic eruptions, estimated at 200-1000 m in depth. The groundwater surface was drawn as the distinctly heavier line connecting to the Toya lake on the right and at a depth of a few tens of meters above the "0" datum on the left). Arrows show the idealized paths of groundwater moving through the rock. After GSH (2000a, b).

Four days prior to the eruption, groundwater levels in instrumented wells (the potentiometric surface) around the volcano began to change (figure 23). A day or two later, these perturbations escalated rapidly. Data from five wells (figure 23) show that at least four underwent roughly synchronous offsets that grew to reach the 2- to 10-m range. These dramatic offsets were inferred to have been driven by the influx of magma. Also, water temperatures increased at hot springs. The level of the groundwater surface in the instrumented wells peaked near the time the eruption started. For the wells with post-eruptive data on figure 23, the groundwater surface began a comparatively gradual steady decrease soon after the eruption started. Ancillary details on well locations and behavior appear in the cited reports.

Figure 23. Perturbations to the groundwater surface level in monitored water wells around the time of the initial Usu eruptions; common vertical scale bar at upper right shows relative magnitude of displacement with strong offset beginning around 27 March 2000. Small arrow labeled "3/31" indicates the point of initial eruption (31 March 2000). After GSH (2000a, b).

Global Positioning System (GPS) data helped predict the 31 March eruption. GPS station KMK is near Hokkaido's N coast and ~7 km E of Usu's active vent groups. KMK was compared with three other stations near Usu beginning around 30 March (figure 24). The comparison revealed large vertical motions-tens of centimeters per day- including some beginning on 29 March (not shown). Figure 24 shows how the rates of vertical motion declined in early April at all three close-in stations. The reports also noted horizontal motions measuring tens of centimeters per day.

Figure 24. Relative vertical position of the land surface near Usu during 30 March-9 May 2000. The comparison is between three close-in GPS stations with respect to station KMK, ~ 7 km E of the Kompira-yama and western Nishi-yama vent groups. After GSH (2000a, b).

Clear precursory seismicity appeared at Usu (figure 25). The maxima, ~150 earthquakes in a 2-hour interval (i.e., ~75 earthquakes/hour), occurred ~1 day prior to the eruption. The eruptive pulses on 1 April took place during an interval with comparatively low seismicity.

Figure 25. Seismic overview of Usu for 28 March to 7 April 2000 portraying multifold increases in the number of earthquakes prior to the 31 March eruption. Bars are for 2-hour intervals with the maximum values representing ~75 earthquakes/hour. The arrows indicate the date of the first three eruptions of the episode. For comparison, the perturbations of hydraulic head were strongest during 27-31 March. After GSH (2000a, b).

Figure 26 provides a graphical summary of the episode's eight modest but identifiable ash falls. Most of the ash blew E, but an eruption on 1 April blew SE and one on 4 April blew N. GSH (2000a) features a time line for the two vent groups in eruption, graphically portraying 31 March-7 April plume observation. Figure 27 presents a sample of this larger time line: the portion for 1 April 2000. Figures such as this provide a particularly apt summary of complex phenomena.

Figure 26. Limits of ash fall distribution seen for Usu's outbursts (31 March-7 April 2000). The date convention is month/day. After GSH, 2000a, b.

Figure 27. A time line of activity at Usu on 1 April 2000 portraying the character of eruptive plumes from the Kompira-yama (upper line) and western Nishi-yama (lower line) vent groups. Plume symbols are shown in two sizes and colors, representing larger (>1-km-tall), smaller (< 1-km-tall), black, and white plumes. The shaded area bracketed by a solid line above (about 1145-1545) indicates an interval of dominantly visual plume observations. The arrow at 11:30 represents the time of onset for an eruption. The given compass directions (eg., E~SE) indicate the direction of ashfall from the vent groups. The original full-length version (31 March-7 April, in Japanese) includes numerous other notes and comments. After GSH (2000a).

Committee's announcement. The Usu eruption committee chaired byYoshiaki Ida made a formal announcement on 10 July. They noted that on this date Usu's phreatic eruption continued at the Kompira-yama and western Nishi-yama vent groups, but the supply of magma from depth had almost ceased. Accordingly, they anticipated a gradual decrease in volcanism.

The committee indicated that the current eruption occurred due to upward movement of magma from depths of ~10 km reaching a shallow reservoir around 4-5 km. Portions of the shallow reservoir traveled NW and then to the vents where magma escaped. The committee noted that on 10 July uplift still continued at western Nishi-yama (~5 cm/day) but that its areal extent and rate were decreasing. The committee noted that by 10 July small faults associated with the eruption ceased moving. They appeared as visible fault traces cutting across roads and other infrastructure (see photos in GSH, 2000a, b).

The committee also noted that the early phases of the eruption had ejected portions of juvenile material, whereas by 10 July the eruptions mainly discharged steam. Similarly, with time, cloud height and explosive vigor decreased. On 10 July Nishi-yama still gave off intermittent weak ash; Kompira-yama still emitted loud blasts with glowing volcanic rocks. But by this time such activities had decreased and ballistic bombs continued to fall several hundred meters from the Kompira-yama vent group. Earthquakes continued on the SW flank of Usu, but by 10 July they became increasingly scarce. The committee suggested a pyroclastic surge was unlikely in the near future .

Satellite imagery. A satellite image from 3 April shows Usu's ash blanketing parts of the largely snow-covered landscape (figure 28). The image caption states that the team planned to image Usu every 3-4 days. The images were captured on ASTER (The Advanced Spaceborne Thermal Emission and Reflection Radiometer), a Japanese-built instrument that obtains high-resolution (15-90 m²/pixel) images at 14 wavelengths from visible to thermal infrared. ASTER registers land surface temperature, emissivity, reflectance, and elevation; it flies on the Terra platform where it serves as a zoom lens for the other Terra instruments. ASTER has the ability to change viewing angles, enabling it to make stereoscopic images and detailed terrain height models. NASA terms the Terra satellite the flagship of the EOS mission. The latter is an effort to better understand planet Earth's atmosphere, land, and oceans, as well as their interactions with solar radiation and with one another.

Figure 28. A false-color image taken on 3 April by the Terra satellite's ASTER instrument showing the ash-darkened snow resulting from complex eruptions at Usu volcano's multiple vents. N is towards the top of the image. Usu and many of the visible deposits lie immediately S of Lake Toya, a circular 10-km-diameter caldera lake with a central island. The Pacific Ocean lies towards the S (the image's lower left-hand corner) and in this region enters Uchiura-Wan bay. (ASTER record identification, 257). Courtesy of NASA.

References. Geological Survey of Hokkaido (GSH), 2000a, Observations of Usu's volcanic eruption, 2000, Preliminary Report (in Japanese), 53 p. (in color on the GSH website and available as a 47 M file.

Geological Survey of Hokkaido (GSH), 2000b, Usu eruption in 2000, GSH News, 2000, 5, vol. 16, (ISSN 1345-1138), (text and captions in Japanese), 4 p. (1 additional sheet with 8 color plates)

Geologic Background. Usuzan, one of Hokkaido's most well-known volcanoes, is a small stratovolcano located astride the southern topographic rim of the 110,000-year-old Toya caldera. The center of the 10-km-wide, lake-filled caldera contains Nakajima, a group of forested Pleistocene andesitic lava domes. The summit of the basaltic-to-andesitic edifice of Usu is cut by a somma formed about 20-30,000 years ago when collapse of the volcano produced a debris avalanche that reached the sea. Dacitic domes erupted along two NW-SE-trending lines fill and flank the summit caldera. Three of these domes, O-Usu, Ko-Usu and Showashinzan, along with seven crypto-domes, were erupted during historical time. The 1663 eruption of Usu was one of the largest in Hokkaido during historical time. The war-time growth of Showashinzan from 1943-45 was painstakingly documented by the local postmaster, who created the first detailed record of growth of a lava dome.

Information Contacts: Masahiro Yahata, Geological Survey of Hokkaido, Kita 19, Nishi 12, Kita-Ku, Sapporo, 060-0819, Japan; Yoshiaki Ida, University of Tokyo, Earthquake Research Institute, Yayoi 1-1-1 Bunkyo-Ku, Tokyo 113, Japan; NASA Terra Project, NASA Goddard SFC, MC 613, Greenbelt, MD 20771 USA (URL: https://terra.nasa.gov/); Yasushi Yamaguchi, Japan Outreach Center for ASTER, Nagoya University, Earth & Planetary Sci Dept/Faculty Sci, Furou-cho Chikusa-ku, Nagoya 464-01; Usu Volcano Observatory, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sohbetsu-cho, Usu-gun, Hokkaido, 052-0103, Japan (URL: http://www.sci.hokudai.ac.jp/isv/english/).

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/).