Frequent historical eruptions from Tanzania's Ol Doinyo Lengai have been recorded since the late 19th century. Located near the southern end of the East African Rift in the Gregory Rift Valley, the unique low-temperature carbonatitic lavas have been the focus of numerous volcanological studies; the volcano has also long been a cultural icon central to the Maasai people who live in the region. Following explosive eruptions in the mid-1960s and early 1980s the volcano entered a phase of effusive activity with the effusion of small, fluid, natrocarbonatitic lava flows within its active north summit crater. From 1983 to early 2007 the summit crater was the site of numerous often-changing hornitos (or spatter cones) and lava flows that slowly filled the crater. Lava began overflowing various flanks of the crater in 1993; by 2007 most flanks had been exposed to flows from the crater.

Seismic and effusive activity increased in mid-2007, and a new phase of explosive activity resumed in September of that year. The explosive activity formed a new pyroclastic cone inside the crater; repeated ash emissions reached altitudes greater than 10 km during March 2008, causing relocation of several thousand nearby villagers. Explosive activity diminished by mid-April 2008; by September new hornitos with small lava flows were again forming on the crater floor. Periodic eruptions of lava from fissures, spatter cones, and hornitos within the crater were witnessed throughout the next decade by scientists and others occasionally visiting the summit. Beginning in 2017, satellite imagery has become a valuable data source, providing information about both the thermal activity and the lava flows in the form of infrared imagery and the color contrast of black fresh lava and whiter cooled lava that is detectable in visible imagery (BGVN 43:10). The latest expeditions in 2018 and 2019 have added drone technology to the research tools. This report covers activity from September 2018 through August 2019 with data and images provided from satellite information and from researchers and visitors to the volcano.

Summary and data from satellite imagery. Throughout September 2018 to August 2019, evidence for repeated small lava flows was recorded in thermal data, satellite imagery, and from a few visits to or overflights of the summit crater by researchers. Intermittent low-level pulses of thermal activity appeared in MIROVA data a few times during the period (figure 187). Most months, Sentinel-2 satellite imagery generated six images with varying numbers of days that had a clear view of the summit and showed black and white color contrasts from fresh and cooled lava and/or thermal anomalies (table 27, figures 188-191). Lava flows came from multiple source vents within the crater, produced linear flows, and covered large areas of the crater floor. Thermal anomalies were located in different areas of the crater; multiple anomalies from different source vents were visible many months.

Figure 187. Intermittent low-level pulses of thermal activity were recorded in the MIROVA thermal data a few times between 21 October 2018 and the end of August 2019. Courtesy of MIROVA.

Table 27. The number of days each month with Sentinel-2 images of Ol Doinyo Lengai, days with clear views of the summit showing detectable color contrasts between black and white lava, and days with detectable thermal anomalies within the summit crater. A clear summit means more than half the summit visible or features identifiable through diffuse cloud cover. Information courtesy of Sentinel Hub Playground.

Figure 188. Sentinel-2 imagery of Ol Doinyo Lengai from September 2018 showed examples of the changing color contrasts of fresh black lava which quickly cools to whitish-brown (top row) and varying intensities and numbers of thermal anomalies on the same days (bottom row). It is clear that the color and thermal patterns change several times during the month even with only a few days of available imagery. Dates of images from left to right are 11, 16, and 21 September. The summit crater is 300 m across and 100 m deep. The top row is with Natural color rendering (bands 4, 3, 2) and the bottom row is with Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Figure 189. Contrasting patterns of dark and light lava flows within the summit crater of Ol Doinyo Lengai on 1 (left) and 11 (right) October 2018 show how quickly new dark flows cool to a lighter color. The flow on 1 October appears to originate in the E part of the crater; the flow in the crater on 11 October has a source in the N part of the crater. These Sentinel-2 images use Natural color rendering (bands 4,3,2). Courtesy of Sentinel Hub Playground.

Figure 190. A large flow at Ol Doinyo Lengai on 3 February 2019 filled most of the summit crater with lobes of black lava (top left) and generated one of the strongest thermal signatures of the period (top right) in these Sentinel-2 satellite images. On 20 March 2019, a small dark area of fresh material contrasted sharply with the surrounding light-colored material (bottom left); the thermal image of the same data shows a small anomaly near the dark spot (bottom right). The left column is with Natural color rendering (bands 4, 3, 2) and the right column is with Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Figure 191. The dark lava spots at Ol Doinyo Lengai on 18 June 2019 (top left) and 28 July 2019 (top center) produced matching thermal anomalies in the Sentinal-2 imagery (bottom left and center). On days when the summit was partly obscured by clouds such as 27 August (top right), the strong thermal signal from the summit still confirmed fresh flow activity (bottom right). The top row is with Natural color rendering (bands 4, 3, 2) and the bottom row is with Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Information from site visits and overflights. Minor steam and gas emissions were visible from the summit crater during an overflight on 29 September 2018. Geologist Cin-Ty Lee captured excellent images of the W flank on 20 October 2018 (figure 192). The large circular crater at the base of the flank is the 'Oldoinyo' Maar (Graettinger, 2018a and 2018b). A view into the crater from an overflight that day (figure 193) showed clear evidence of at least five areas of dark, fresh lava. An effusive eruption was visible on the crater floor on 2 March 2019 (figure 194).

Figure 192. A large maar stands out at the base of the SW flank of Ol Doinyo Lengai on 20 October 2018. Courtesy of Cin-Ty Lee (Rice University).

Figure 193. A view into the summit crater of Ol Doinyo Lengai on 20 October 2018 shows clear evidence of recent flow activity in the form of multiple dark spots of fresh lava that has recently emerged from hornitos and fissures. The lava cools to a pale color very quickly, forming the contrasting background to the fresh flows. The summit crater is 300 m across and 100 m deep. Courtesy of Cin-Ty Lee (Rice University).

Figure 194. A view into the crater floor at Ol Doinyo Lengai on 2 March 2019 showed a vent with both fresh (dark brown) and cooled (gray-white) carbonatite lavas and hornitos on the floor of the crater. The darkest material on the crater floor is from recent flows. Courtesy of Aman Laizer, Tanzania.

Research expedition in July-August 2019. In late July and early August 2019 an expedition, sponsored by the Deep Carbon Observatory (DCO) and led by researchers Kate Laxton and Emma Liu (University College London), made gas measurements, collected lava samples for the first time in 12 years, and deployed drones to gather data and images. The Ol Doinyo Lengai sampling team included Papkinye Lemolo, Boni Kicha, Ignas Mtui, Boni Mawe, Adadeus Mtui, Emma Liu, Arno Van Zyl, Kate Laxton, and their driver, Baraka. They collected samples by lowering devices via ropes and pulleys into the crater and photographed numerous active flows emerging from vents and hornitos on the crater floor (figure 195). By analyzing the composition of the first lava samples collected since the volcano's latest explosive activity in 2007, they hope to learn about recent changes to its underground plumbing system. A comparison of the satellite image taken on 28 July with a drone image of the summit crater taken by them the next day (figure 196) confirms the effectiveness of both the satellite imagery in identifying new flow features on the crater floor, and the drone imagery in providing outstanding details of activity.

Figure 195. Researchers Kate Laxton and Emma Liu collected gas and lava samples at the summit of Ol Doinyo Lengai during their 26 July-4 August 2019 expedition. They sent gas sampling devices (small white "hamster ball" in center of left image) and lava sampling devices (right) down into the crater via ropes and pulleys. The crater is 300 m across and 100 m deep. Courtesy of Kate Laxton (University College London).

Figure 196. A clear view by drone straight down into the crater at Ol Doinyo Lengai on 29 July 2019 provides valuable information about ongoing activity at the remote volcano. N is to the top. The summit crater is 300 m across and 100 m deep. The same configuration of fresh and cooled lava can be seen in Sentinel-2 imagery taken on 28 July 2019 (inset, N to the top). Courtesy of Emma Liu (University College London) and Sentinel Hub Playground.

With the drone technology, they were able to make close-up observations of features on the north crater floor such as the large hornito on the inner W wall of the crater (figure 197), an active lava pond near the center of the crater (figure 198), and several flows resurfacing the floor of the crater while they were there (figure 199). A large crack that rings the base of the N cone had enlarged significantly since last measured in 2014 (figure 200).

Figure 197. A closeup view of the large hornito in the W wall of the Ol Doinyo Lengai summit crater on 26 July 2019 shows recent activity from the vent (dark material). See figure 197 for location of hornito against W wall. View is to the NW. Courtesy of Emma Liu (University College London).

Figure 198. Incandescence from the lava pond in the center of the crater was still visible at 0627 on 29 July 2019 at Ol Doinyo Lengai; incandescence from the large hornito in the NW quadrant (behind the lava pond) had been visible when the researchers arrived at the summit at about 0500 that morning. The crater floor is continually resurfaced by ultra-low viscosity natrocarbonatite lava flows. The lava hydrates on contact with air within hours, changing color from black to grey/white in a very short time. View towards the N. Courtesy of Kate Laxton (University College London).

Figure 199. On 30 July 2019 a lava flow from a hornito cluster resurfaced the NE quadrant of the crater floor at Ol Doinyo Lengai. The initial outbreak occurred at 0819, was vigorous, and ended by 0823. Lava continued to flow out of the hornito cluster at intervals throughout the day. Image facing NE, courtesy of Kate Laxton (University College London).

Figure 200. The circumferential crack near the base of the N cone of Ol Doinyo Lengai is seen here being inspected by Emma Liu on 30 July 2019 where it intersects the Western Summit Trail. View is to the S. Significant widening of the crack is seen when compared with a similar image of the same crack from March 2014 (figure 172, BGVN 39:07). Local observers reported that the crack continued to widen after July 2019. Courtesy of Kate Laxton (University College London).

The color of the flows on the crater floor changed from grays and browns to blues and greens after a night of rainfall on 31 July 2019 (figure 201). Much of the lava pond surface was crusted over that day, but the large hornito in the NW quadrant was still active (figure 202), and both the pond and another hornito produced flows that merged onto the crater floor (figure 203).

Figure 201. The active crater at Ol Doinyo Lengai is on the north side of and slightly below the topographic summit of the mountain (in the background). After overnight rain, lava flows on the crater floor turned various shades of greys, whites, blues, and greens on 31 July 2019. View to the SW, drone image. Courtesy of Emma Liu (University College London).

Figure 202. A closeup view to the NW of the Ol Doinyo Lengai north crater on 31 July 2019 shows the blue and green tones of the hydrated lavas after the previous night's rains. The lava pond is at high-stand with much of the surface crusted over. The adjacent hornito is still active and breached to the NE. Courtesy of Emma Liu (University College London).

Figure 203. Two fresh lava flows merge over the hydrated crater floor of the north crater at Ol Doinyo Lengai on 31 July 2019. One comes from a small hornito just out of view to the SW (lower right) and the other from the overflowing lava pond (left), merging in the SE quadrant. The colors of the two flows differ; the pond lava appears jet black, and the hornito lava is a lighter shade of brown. View to the SE, courtesy of Emma Liu (University College London).

On 1 August 2019 much of the crater floor was resurfaced by a brown lava that flowed from a hornito E of the lava pond (figure 204). Images of unusual, ephemeral features such as "spatter pots," "frozen jets," and "frothy flows" (figure 205) help to characterize the unusual magmatic activity at this unique volcano (figure 206).

Figure 204. On 1 August 2019 at Ol Doinyo Lengai brown lava flowed from a hornito directly E of the lava pond (above the pond in figure 203) and resurfaced much of the S portion of the crater floor. At the far left of the image, the white (hydrated) lava jet aimed away from the hornito was solidified in mid-flow. View to the SE, courtesy of Emma Liu (University College London).

Figure 205. Frothy pale-brown lava flowed across the SE quadrant of the crater floor (right) at Ol Doinyo Lengai on 4 August 2019 from an uncertain source between the adjacent hornito and lava pond which appears nearly crusted over. Spattering from a "spatter pot" (inset) and a small flow also headed NE from the hornito cluster E of the pond (behind pond). Courtesy of Kate Laxton (University College London).

Figure 206. A view from the summit peak of Ol Doinyo Lengai on 4 August 2019 looking at the entire N cone and the swale between it and the peak. The crack shown in figure 201 rings the base of cone; the main summit trail intersects the crack near the bottom center of the cone. The researcher's campsite on the W flank (left) shows the scale of the cone. The East African Rift wall and Lake Natron are visible in the background on the left and right, respectively. Courtesy of Kate Laxton (University College London).

References: Graettinger, A. H., 2018a, MaarVLS database version 1, (URL: https://vhub.org/resources/4365).

Graettinger, A. H., 2018b, Trends in maar crater size and shape using the global Maar Volcano Location and Shape (MaarVLS) database. Journal of Volcanology and Geothermal Research, v. 357, p. 1-13. https://doi.org/10.1016/j.jvolgeores.2018.04.002.

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

Information Contacts: Cin-Ty Lee, Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main St., Houston, TX 77005-1827, USA (URL: https://twitter.com/CinTyLee1, images at https://twitter.com/CinTyLee1/status/1054337204577812480, https://earthscience.rice.edu/directory/user/106/); Emma Liu, University College London, UCL Hazards Centre (Volcanology), Gower Street, London, WC1E 6BT, United Kingdom (URL: https://twitter.com/EmmaLiu31, https://www.ucl.ac.uk/earth-sciences/people/academic/dr-emma-liu); Kate Laxton, University College London, UCL Earth Sciences, Gower Street, London, WC1E 6BT, United Kingdom (URL: https://twitter.com/KateLaxton, https://www.ucl.ac.uk/earth-sciences/people/research-students/kate-laxton); Deep Carbon Observatory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, DC 20015-1305, USA (URL: https://deepcarbon.net/field-report-ol-doinyo-lengai-volcano-tanzania); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Aman Laizer, Volcanologist, Arusha, Tanzania (URL: https://twitter.com/amanlaizerr, image at https://twitter.com/amanlaizerr/status/1102483717384216576).

Typical activity at Ulawun consists of occasional weak explosions with ash plumes. During 2018 explosions occurred on 8 June, 21 September, and 5 October (BGVN 43:11). The volcano is monitored primarily by the Rabaul Volcano Observatory (RVO) and Darwin Volcanic Ash Advisory Centre (VAAC). This report describes activity from November 2018 through August 2019; no volcanism was noted during this period until late June 2019.

Activity during June-July 2019. RVO reported that Real-time Seismic-Amplitude Measurement (RSAM) values steadily increased during 24-25 June, and then sharply increased at around 0330 on 26 June. The RSAM values reflect an increase in seismicity dominated by volcanic tremor. An eruption began in the morning hours of 26 June with emissions of gray ash (figure 17) that over time became darker and more energetic. The plumes rose 1 km and caused minor ashfall to the NW and SW. Local residents heard roaring and rumbling during 0600-0800.

Figure 17. Photograph of a small ash plume rising from the summit crater of Ulawun taken by a helicopter pilot at 1030 local time on 26 June 2019. According to the pilot, the amount of ash observed was not unusual. Image has been color adjusted from original. Courtesy of Craig Powell.

The Darwin VAAC issued several notices about ash plumes visible in satellite data. These stated that during 1130-1155 ash plumes rose to altitudes of 6.7-8.5 km and drifted W, while ash plumes that rose to 12.8-13.4 km drifted S and SW. A new pulse of activity (figures 17 and 18) generated ash plumes that by 1512 rose to an altitude of 16.8 km and drifted S and SE. By 1730 the ash plume had risen to 19.2 km and spread over 90 km in all directions. Ash from earlier ejections continued to drift S at an altitude of 13.4 km and W at an altitude of 8.5 km. RVO stated that RSAM values peaked at about 2,500 units during 1330-1600, and then dropped to 1,600 units as the eruption subsided.

Figure 18. Photograph of Ulawun taken by a helicopter pilot at 1310 local time on 26 June 2019 showing a tall ash plume rising from the summit crater. Image has been color adjusted from original. Courtesy of Craig Powell.

Figure 19. Photograph of Ulawun taken by a helicopter pilot at 1350 local time on 26 June 2019 showing a close-up view of the ash plume rising from the summit crater along with an area of incandescent ejecta. According to the pilot, this was the most active phase. Image has been color adjusted from original. Courtesy of Craig Powell.

According to RVO, parts of the ash plume at lower altitudes drifted W, causing variable amounts of ashfall in areas to the NW and SW. A pyroclastic flow descended the N flank. Residents evacuated to areas to the NE and W; a news article (Radio New Zealand) noted that around 3,000 people had gathered at a local church. According to another news source (phys.org), an observer in a helicopter reported a column of incandescent material rising from the crater, residents noted that the sky had turned black, and a main road in the N part of the island was blocked by volcanic material. Residents also reported a lava flow near Noau village and Eana Valley. RVO reported that the eruption ceased between 1800 and 1900. Incandescence visible on the N flank was from either a lava flow or pyroclastic flow deposits.

On 27 June diffuse white plumes were reported by RVO as rising from the summit crater and incandescence was visible from pyroclastic or lava flow deposits on the N flank from the activity the day before. The seismic station 11 km NW of the volcano recorded low RSAM values of between 2 and 50. According to the Darwin VAAC a strong thermal anomaly was visible in satellite images, though not after 1200. Ash from 26 June explosions continued to disperse and became difficult to discern in satellite images by 1300, though a sulfur dioxide signal persisted. Ash at an altitude of 13.7 km drifted SW to SE and dissipated by 1620, and ash at 16.8 km drifted NW to NE and dissipated by 1857. RVO noted that at 1300 on 27 June satellite images captured an ash explosion not reported by ground-based observers, likely due to cloudy weather conditions. The Alert Level was lowered to Stage 1 (the lowest level on a four-stage scale).

RSAM values slightly increased at 0600 on 28 June and fluctuated between 80 to 150 units afterwards. During 28-29 June diffuse white plumes continued to rise from the crater (figure 20) and from the North Valley vent. On 29 June a ReliefWeb update stated that around 11,000 evacuated people remained in shelters.

Figure 20. Photograph of the steaming summit crater at Ulawun taken by a helicopter pilot at 0730 local time on 29 June 2019. Image has been color adjusted from original. Courtesy of Craig Powell.

According to RVO, diffuse white plumes rose from Ulawun's summit crater and the North Valley vent during 1-4 July and from the summit only during 5-9 July. The seismic station located 11 km NW of the volcano recorded three volcanic earthquakes and some sporadic, short-duration, volcanic tremors during 1-3 July. The seismic station 2.9 km W of the volcano was restored on 4 July and recorded small sub-continuous tremors. Some discrete high-frequency volcanic earthquakes were also recorded on most days. Sulfur dioxide emissions were 100 tonnes per day on 4 July. According to the United Nations in Papua New Guinea, 7,318 people remained displaced within seven sites because of the 26 June eruption.

Activity during August 2019. During 1-2 August RVO reported that white-to-gray vapor plumes rose from the summit crater and drifted NW. Incandescence from the summit crater was visible at night and jetting noises were audible for a short interval. RSAM values fluctuated but peaked at high levels. During the night of 2-3 August crater incandescence strengthened and roaring noises became louder around 0400. An explosion began between 0430 and 0500 on 3 August; booming noises commenced around 0445. By 0600 dense light-gray ash emissions were drifting NW, causing ashfall in areas downwind, including Ulamona Mission (10 km NW). Ash emissions continued through the day and changed from light to dark gray with time.

The eruption intensified at 1900 and a lava fountain rose more than 100 m above the crater rim. A Plinian ash plume rose 19 km and drifted W and SW, causing ashfall in areas downwind such as Navo and Kabaya, and as far as Kimbe Town (142 km SW). The Darwin VAAC reported that the ash plume expanded radially and reached the stratosphere, rising to an altitude of 19.2 km. The plume then detached and drifted S and then SE.

The Alert Level was raised to Stage 3. The areas most affected by ash and scoria fall were between Navo (W) and Saltamana Estate (NW). Two classrooms at the Navo Primary School and a church in Navo collapsed from the weight of the ash and scoria; one of the classroom roofs had already partially collapsed during the 26 June eruption. Evacuees in tents because of the 26 June explosion reported damage. Rabaul town (132 km NE) also reported ashfall. Seismicity declined rapidly within two hours of the event, though continued to fluctuate at moderate levels. According to a news source (Radio New Zealand, flights in and out of Hoskins airport in Port Moresby were cancelled on 4 August due to tephra fall. The Alert Level was lowered to Stage 1. Small amounts of white and gray vapor were emitted from the summit crater during 4-6 August. RVO reported that during 7-8 August minor emissions of white vapor rose from the summit crater.

Additional observations. Seismicity was dominated by low-level volcanic tremor and remained at low-to-moderate levels. RSAM values fluctuated between 400 and 550 units; peaks did not go above 700. Instruments aboard NASA satellites detected high levels of sulfur dioxide near or directly above the volcano on 26-29 June and 4-6 August 2019.

Thermal anomalies, based on MODIS satellite instruments analyzed using the MODVOLC algorithm, were observed at Ulawun only on 26 June 2019 (8 pixels by the Terra satellite, 4 pixels by the Aqua satellite). The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected three anomalies during the reporting period, one during the last week of June 2019 and two during the first week of August, all three within 3 km of the volcano and of low to moderate energy.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it); ReliefWeb (URL: https://reliefweb.int/); Radio New Zealand (URL: https://www.rnz.co.nz); phys.org (URL: https://phys.org); United Nations in Papua New Guinea (URL: http://pg.one.un.org/content/unct/papua_new_guinea/en/home.html).

Sarychev Peak, located on Matua Island in the central Kurile Islands of Russia, has had eruptions reported since 1765. Renewed activity began in October 2017, followed by a major eruption in June 2009 that included pyroclastic flows and ash plumes (BGVN 43:11 and 34:06). Thermal anomalies, explosions, and ash plumes took place between September and October 2018. A single ash explosion occurred in May 2019. Another ash plume was seen on 11 August, and small thermal anomalies were present in infrared imagery during June-October 2019. Information is provided by the Sakhalin Volcanic Eruption Response Team (SVERT) and the Tokyo Volcanic Ash Advisory Center (VAAC), with satellite imagery from Sentinel-2.

Satellite images from Sentinel-2 showed small white plumes from Sarychev Peak during clear weather on 4 and 14 August 2019 (figure 27); similar plumes were observed on a total of nine clear weather days between late June and October 2019. According to SVERT and the Tokyo VAAC, satellite data from HIMAWARI-8 showed an ash plume rising to an altitude of 2.7 km and drifting 50 km SE on 11 August. It was visible for a few days before dissipating. No further volcanism was detected by SVERT, and no activity was evident in a 17 August Sentinel-2 image (figure 27).

Figure 27. Small white plumes were visible at Sarychev Peak in Sentinel-2 satellite images on 4 and 14 August 2019 (left and center). No activity was seen on 17 August (right). All three Sentinel-2 images use the "Natural Color" (bands 4, 3, 2) rendering; courtesy of Sentinel Hub Playground.

Intermittent weak thermal anomalies were detected by the MIROVA system using MODIS data from late May through 7 October 2019 (figure 28). Sentinel-2 satellite imagery from 28 June, 13 and 23 July, 9 August, and 21 October showed a very small thermal anomaly, but on 28 September a pronounced thermal anomaly was visible (figure 29). No additional thermal anomalies were identified from any source after 7 October through the end of the month.

Figure 28. Thermal anomalies detected at Sarychev Peak by the MIROVA system (Log Radiative Power) using MODIS data for the year ending on 9 October 2019. Courtesy of MIROVA.

Figure 29. Sentinel-2 satellite images of Sarychev Peak on 23 June and 28 September 2019. A small thermal anomaly is visible on the eastern side of the crater on 23 June (left, indicated by arrow), while the thermal anomaly is more pronounced and visible in the middle of the crater on 28 September (right). Both Sentinel-2 satellite images use the "False Color (Urban)" (bands 12, 11, 4) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. Sarychev Peak, one of the most active volcanoes of the Kuril Islands, occupies the NW end of Matua Island in the central Kuriles. The andesitic central cone was constructed within a 3-3.5-km-wide caldera, whose rim is exposed only on the SW side. A dramatic 250-m-wide, very steep-walled crater with a jagged rim caps the volcano. The substantially higher SE rim forms the 1496 m high point of the island. Fresh-looking lava flows, prior to activity in 2009, had descended in all directions, often forming capes along the coast. Much of the lower-angle outer flanks of the volcano are overlain by pyroclastic-flow deposits. Eruptions have been recorded since the 1760s and include both quiet lava effusion and violent explosions. Large eruptions in 1946 and 2009 produced pyroclastic flows that reached the sea.

Information Contacts: Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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).

Asamayama (also known as Asama), located in the Kanto-Chubu Region of Japan, previously erupted in June 2015. Activity included increased volcanic seismicity, small eruptions which occasionally resulted in ashfall, and SO₂ gas emissions (BGVN 41:10). This report covers activity through August 2019, which describes small phreatic eruptions, volcanic seismicity, faint incandescence and commonly white gas plumes, and fluctuating SO₂ emissions. The primary source of information for this report is provided by the Japan Meteorological Agency (JMA).

Activity during October 2016-May 2019. From October 2016 through December 2017, a high-sensitivity camera captured faint incandescence at night accompanied by white gas plumes rising above the crater to an altitude ranging 100-800 m (figure 44). A thermal anomaly and faint incandescence accompanied by a white plume near the summit was observed at night on 6-7 and 21 January 2017. These thermal anomalies were recorded near the central part of the crater bottom in January, February, and November 2017, and in May 2019. After December 2017 the faint incandescence was not observed, with an exception on 18 July 2018.

Figure 44. A surveillance camera observed faint incandescence at Asamayama in February 2017. Left: Onimushi surveillance camera taken at 0145 on 5 February 2017. Right: Kurokayama surveillance camera taken at 0510 on 1 February 2017. Courtesy of JMA (Monthly Report for February 2017).

Field surveys on 6, 16, and 28 December 2016 reported an increased amount of SO₂ gas emissions from November 2016 (100-600 tons/day) to March 2017 (1,300-3,200 tons/day). In April 2017 the SO₂ emissions decreased (600-1,500 tons/day). Low-frequency shallow volcanic tremors decreased in December 2016; none were observed in January 2017. From February 2017 through June 2018 volcanic tremors occurred more intermittently. According to the monthly JMA Reports on February 2017 and December 2018 and data from the Geographical Survey Institute's Global Navigation Satellite Systems (GNSS), a slight inflation between the north and south baseline was recorded starting in fall 2016 through December 2018. This growth become stagnant at some of the baselines in October 2017.

Activity during August 2019. On 7 August 2019 a small phreatic eruption occurred at the summit crater and continued for about 20 minutes, resulting in an ash plume that rose to a maximum altitude of 1.8 km, drifting N and an associated earthquake and volcanic tremor (figure 45). According to the Tokyo Volcanic Ash Advisory (VAAC), this plume rose 4.6 km, based on satellite data from HIMAWARI-8. A surveillance camera observed a large volcanic block was ejected roughly 200 m from the crater. According to an ashfall survey conducted by the Mobile Observation Team on 8 August, slight ashfall occurred in the Tsumagoi Village (12 km N) and Naganohara Town (19 km NE), Gunma Prefecture (figure 46 and 47). About 2 g/m2 of ash deposit was measured by the Tokyo Institute of Technology. Immediately after the eruption on 7 August, seismicity, volcanism, and SO₂ emissions temporarily increased and then decreased that same day.

Figure 45. Surveillance camera images of Asamayama showing the small eruption at the summit crater on 7 August 2019, resulting in incandescence and a plume rising 1.8 km altitude. Both photos were taken on 7 August 2019.Courtesy of JMA (Monthly Report for August 2019).

Figure 46. A photomicrograph of fragmented ejecta (250-500 µm) from Asamayama deposited roughly 5 km from the crater as a result of the eruption on 7 August 2019. Courtesy of JMA (Monthly Report for August 2019).

Figure 47. Photos of ashfall in a nearby town NNE of Asamayama due to the 7 August 2019 eruption. Courtesy of JMA (Daily Report for 8 August 2019).

Another eruption at the summit crater on 25 August 2019 was smaller than the one on 7 August. JMA reported the resulting ash plume rose to an altitude of 600 m and drifted E. However, the Tokyo VAAC reported that the altitude of the plume up to 3.4 km, according to satellite data from HIMAWARI-8. A small amount of ashfall occurred in Karuizawa-machi, Nagano (4 km E), according to interview surveys and the Tokyo Institute of Technology.

Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the horseshoe-shaped remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asama-yama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has an historical record dating back at least to the 11th century CE. Maekake has had several major plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

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

Villarrica is a frequently active volcano in Chile with an active lava lake in the deep summit crater. It has been producing intermittent Strombolian activity since February 2015, soon after the latest reactivation of the lava lake; similar activity continued into 2019. This report summarizes activity during March-August 2019 and is based on reports from the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN), Projecto Observación Villarrica Internet (POVI), part of the Fundacion Volcanes de Chile research group, and satellite data.

OVDAS-SERNAGEOMIN reported that degassing continued through March with a plume reaching 150 m above the crater with visible incandescence through the nights. The lava lake activity continued to fluctuate and deformation was also recorded. POVI reported sporadic Strombolian activity throughout the month with incandescent ejecta reaching around 25 m above the crater on 17 and 24 March, and nearly 50 m above the crater on the 20th (figure 76).

Figure 76. A webcam image of Villarrica at 0441 on 20 March 2019 shows Strombolian activity and incandescent ejecta reaching nearly 50 m above the crater. People are shown for scale in the white box to the left in the blue background image that was taken on 27 March. Photos taken about 6 km away from the volcano, courtesy of POVI.

There was a slight increase in Strombolian activity reported on 7-8 April, with incandescent ballistic ejecta reaching around 50 m above the crater (figure 77). Although seismicity was low during 14-15 April, Strombolian activity produced lava fountains up to 70 m above the crater over those two days (figure 78). Activity continued into May with approximately 12 Strombolian explosions recorded on the night of 5-6 May erupting incandescent ejecta up to 50 m above the crater rim. Another lava fountaining episode was observed reaching around 70 m above the crater on 14 May (figure 79). POVI also noted that while this was one of the largest events since 2015, no significant changes in activity had been observed over the last five months. Throughout May, OVDAS-SERNAGEOMIN reported that the gas plume height did not exceed 170 m above the crater and incandescence was sporadically observed when weather allowed. SWIR (short-wave infrared) thermal data showed an increase in energy towards the end of May (figure 80).

Figure 77. Strombolian activity at Villarrica on 7-8 April 2019 producing incandescent ballistic ejecta reaching around 50 m above the crater. Courtesy of POVI.

Figure 78. Images of Villarrica on 15 April show a lava fountain that reached about 70 m above the crater. Courtesy of POVI.

Figure 79. These images of Villarrica taken at 0311 and 2220 on 14 May 2019 show lava fountaining reaching 70-73 m above the crater. Courtesy of POVI.

Figure 80. This graph shows the variation in short-wave infrared (SWIR) energy with the vertical scale indicating the number of pixels displaying high temperatures between 23 June 2018 and 29 May 2019. Courtesy of POVI.

Ballistic ejecta were observed above the crater rim on 17 and 20 June 2019 (figure 81), and activity was heard on 20 and 21 June. Activity throughout the month remained similar to previous months, with a fluctuating lava lake and minor explosions. On 15 July a thermal camera imaged a ballistic bomb landing over 300 m from the crater and disintegrating upon impact. Incandescent material was sporadically observed on 16 July. Strombolian activity increased on 22 July with the highest intensity activity in four years continuing through the 25th (figure 82).

Figure 81. Ballistic ejecta is visible above the Villarrica crater in this infrared camera (IR940 nm) image taken on 17 June 2019. Courtesy of POVI.

Figure 82. Strombolian activity at Villarrica on 22, 23, and 24 July with incandescent ballistic ejecta seen here above the summit crater. Courtesy of POVI.

On 6 August the Alert Level was raised by SERNAGEOMIN from Green to Yellow (on a scale of Green, Yellow, Orange, and Red indicating the greatest level of activity) due to activity being above the usual background level, including ejecta confirmed out to 200 m from the crater with velocities on the order of 100 km/hour (figure 83). The temperature of the lava lake was measured at a maximum of 1,000°C on 25 July. POVI reported the collapse of a segment of the eastern crater rim, possibly due to snow weight, between 9 and 12 August. The MIROVA system showed an increase in thermal energy in August (figure 84) and there was one MODVOLC thermal alert on 24 July.

Figure 83. Observations during an overflight of Villarrica on 25 July 2019 showed that ballistic ejecta up to 50 cm in diameter had impacted out to 200 m from the crater. The velocities of these ejecta were likely on the order of 100 km/hour. The maximum temperature of the lava lake measured was 1,000°C, and 500°C was measured around the crater. Courtesy of SERNAGEOMIN.

Figure 84. Thermal activity at Villarrica detected by the MIROVA system shows an increase in detected energy in August 2019. Courtesy of MIROVA.

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

Information Contacts: Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/).

The andesitic Volcán El Reventador lies east of the main volcanic axis of the Cordillera Real in Ecuador and has historical eruptions with numerous lava flows and explosive events going back to the 16th century. An eruption in November 2002 generated a 17-km-high eruption cloud, pyroclastic flows that traveled 8 km, and several lava flows. Eruptive activity has been continuous since 2008. Daily explosions with ash emissions and ejecta of incandescent blocks rolling hundreds of meters down the flanks have been typical for many years. Alameida et al. (2019) provide an excellent summary of recent activity (2016-2018) and monitoring. Activity continued during February-July 2019, the period covered in this report, with information provided by Ecuador's Instituto Geofisico (IG-EPN), the Washington Volcano Ash Advisory Center (VAAC), and infrared satellite data.

Persistent thermal activity accompanied daily ash emissions and incandescent block avalanches during February-July 2019 (figure 111). Ash plumes generally rose 600-1,200 m above the summit crater and drifted W or NW; incandescent blocks descended up to 800 m down all the flanks. On 25 February an ash plume reached 9.1 km altitude and drifted SE, causing ashfall in nearby communities. Pyroclastic flows were reported on 18 April and 19 May traveling 500 m down the flanks. Small but distinct SO₂ emissions were detectible by satellite instruments a few times during the period (figure 112).

Figure 111. The thermal energy at Reventador persisted throughout 4 November 2018 through July 2019, but was highest in April and May. Courtesy of MIROVA.

Figure 112. Small SO2 plumes were released from Reventador and detected by satellite instruments only a few times during February-July 2019. Columbia's Nevada del Ruiz produced a much larger SO2 signal during each of the days shown here as well. Top left: 26 February; top right: 27 February; bottom left: 3 April; bottom right: 4 April. Courtesy of NASA Goddard Space Flight Center.

The Washington VAAC issued multiple daily ash advisories on all but two days during February 2019. IGEPN reported daily ash emissions rising from 400 to over 1,000 m above the summit crater. Incandescent block avalanches rolled 400-800 m down the flanks on most nights (figure 113). Late on 8 February the Washington VAAC reported an ash plume moving W at 5.8 km altitude extending 10 km from the summit. Plumes rising more than 1,000 m above the summit were reported on 9, 13, 16, 18, 19, and 25 February. On 25 February the Washington VAAC reported an ash plume visible in satellite imagery drifting SE from the summit at 9.1 km altitude that dissipated quickly, and drifted SSE. It was followed by new ash clouds at 7.6 km altitude that drifted S. Ashfall was reported in San Luis in the Parish of Gonzalo Díaz de Pineda by UMEVA Orellana and the Chaco Fire Department.

Figure 113. Emission of ash from Reventador and incandescent blocks rolling down the cone occurred daily during February 2019, and were captured by the COPETE webcam located on the S rim of the caldera. On 1 February (top left) incandescent blocks rolled 600 m down the flanks. On 13 February (top right) ash plumes rose 800 m and drifted W. On 16 February (bottom left) ash rose to 1,000 m and drifted W. On 18 February (bottom right) the highest emission exceeded 1,000 m above the crater and was clearly visible in spite of meteoric clouds obscuring the volcano. Courtesy of IGEPN (Daily reports 2019-32, 44, 47, and 49).

Ash plumes exceeded 1,000 m in height above the summit almost every day during March 2019 and generally drifted W or NW. The Washington VAAC reported an ash plume visible above the cloud deck at 6.7 km altitude extending 25 km NW early on 3 March; there were no reports of ashfall nearby. Incandescent block avalanches rolled 800 m down all the flanks the previous night; they were visible moving 300-800 m down the flanks most nights throughout the month (figure 114).

Figure 114. Ash plumes and incandescent block avalanches occurred daily at Reventador during March 2019 and were captured by the COPETE webcam on the S rim of the caldera. On 3 March (top left) a possible pyroclastic flow traveled down the E flank in the early morning. Ash plumes on 17 and 18 March (top right, bottom left) rose 900-1,000 m above the summit and drifted W. On 23 March (bottom right) ash plumes rose to 1,000 m and drifted N while incandescent blocks rolled 600 m down the flanks. Courtesy of IGEPN (Daily reports 2019 62, 76, 77, and 82).

During April 2019 ash plume heights ranged from 600 to over 1,000 m above the summit each day, drifting either W or NW. Incandescent avalanche blocks rolled down all the flanks for hundreds of meters daily; the largest explosions sent blocks 800 m from the summit (figure 115). On 18 April IGEPN reported that a pyroclastic flow the previous afternoon had traveled 500 m down the NE flank.

Figure 115. Ash plumes and incandescent block avalanches occurred daily at Reventador during April 2019. On 3 April, ash emissions were reported drifting W and NW at 1,000 m above the summit (top left). On 14 April ash plumes rose over 600 m above the summit crater (top right). The 3 and 14 April images were taken from the LAVCAM webcam on the SE flank. Incandescent block avalanches descended 800 m down all the flanks on 15 April along with ash plumes rising over 1,000 m above the summit (bottom left), both visible in this image from the COPETE webcam on the S rim of the caldera. A pyroclastic flow descended 500 m down the NE flank on 17 April and was captured in the thermal REBECA webcam (bottom right) located on the N rim of the caldera. Courtesy of IGEPN (Daily reports 2019-93, 104, 105, and 108).

On most days during May 2019, incandescent block avalanches were observed traveling 700-800 m down all the flanks. Ash plume heights ranged from 600 to 1,200 m above the crater each day of the month (figure 116) they were visible. A pyroclastic flow was reported during the afternoon of 19 May that moved 500 m down the N flank.

Figure 116. Even on days with thick meteoric clouds, ash plumes can be observed at Reventador. The ash plumes reached 1,000 m above the crater on 8 May 2019 (top left). The infrared webcam REBECA on the N rim of the caldera captured a pyroclastic flow on the N flank on the afternoon of 19 May (top right). Strong explosions on 23 May sent incandescent blocks and possible pyroclastic flows at least 800 m down all the flanks (bottom left). Ash plumes reached 1,000 m above the summit on 27 May and drifted W (bottom right). Images on 8, 23, and 27 May taken from the COPETE webcam on the S rim of the caldera. Courtesy of IGEPN (Daily Reports 2019-128, 140, 143, and 147).

Activity diminished somewhat during June 2019. Ash plumes reached 1,200 m above the summit early in June but decreased to 600 m or less for the second half of the month. Meteoric clouds prevented observation for most of the third week of June; VAAC reports indicated ash emissions rose to 5.2 km altitude on 19 June and again on 26 June (about 2 km above the crater). Incandescent blocks were reported traveling down all of the flanks, generally 500-800 m, during about half of the days the mountain was visible (figure 117). Multiple VAAC reports were also issued daily during July 2019. Ash plumes were reported by IGEPN rising over 600 m above the crater every day it was visible and incandescent blocks traveled 400-800 m down the flanks (figure 118). The Darwin VAAC reported an ash emission on 9 July that rose to 4.9 km altitude as multiple puffs that drifted W, extending about 35 km from the summit.

Figure 117. Activity diminished slightly at Reventador during June 2019. Incandescent material was visible on the N flank from infrared webcam REBECA on the N rim of the caldera on 6 June (top left). On 7 June ash rose over 1,000 m above the summit and drifted N and W (top right) as seen from the COPETE webcam on the S rim of the caldera. Incandescent block avalanches rolled 600 m down all the flanks on 8 June (bottom left) and were photographed by the LAVCAM webcam located on the SE flank. An ash plume rose to 1,000 m on 25 June and was photographed from the San Rafael waterfall (bottom right). Courtesy of IGEPN (Daily Reports 2019-157, 158, 159, and 176).

Figure 118. Daily explosive activity was reported at Reventador during July 2019. On 9 and 10 July ash plumes rose over 600 m and drifted W and incandescent blocks descended 800 m down all the flanks (top row), as seen from the LAVCAM webcam on the SE flank. On 27 July many of the large incandescent blocks appeared to be several m in diameter as they descended the flanks (bottom left, LAVCAM). On 1 August, a small steam plume was visible on a clear morning from the CORTESIA webcam located N of the volcano. Courtesy of IGEPN Daily reports (2019-190, 191, 208, and 213).

References: Almeida M, Gaunt H E, and Ramón P, 2019, Ecuador's El Reventador volcano continually remakes itself, Eos, 100, https://doi.org/10.1029/2019EO117105. Published on 18 March 2019.

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 (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/); 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/).

Raikoke in the central Kuril Islands lies 400 km SW of the southern tip of Russia's Kamchatcka Peninsula. Two significant eruptive events in historical times, including fatalities, have been recorded. In 1778 an eruption killed 15 people "under the hail of bombs" who were under the command of Captain Chernyi, returning from Matua to Kamchatka. This prompted the Russian military to order the first investigation of the volcanic character of the island two years later (Gorshkov, 1970). Tanakadate (1925) reported that travelers on a steamer witnessed an ash plume rising from the island on 15 February 1924, observed that the island was already covered in ash from recent activity, and noted that a dense steam plume was visible for a week rising from the summit crater. The latest eruptive event in June 2019 produced a very large ash plume that covered the island with ash and dispersed ash and gases more than 10 km high into the atmosphere. The volcano is monitored by the Sakhalin Volcanic Eruption Response Team, (SVERT) part of the Institute of Marine Geology and Geophysics, Far Eastern Branch of the Russian Academy of Sciences (IMGG FEB RAS) and the Kamchatka Volcanic Eruption Response Team (KVERT) which is part of the Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences (IVS FEB RAS).

A brief but intense eruption beginning on 21 June 2019 sent major ash and sulfur dioxide plumes into the stratosphere (figures 1 and 2); the plumes rapidly drifted over 1,000 km from the volcano. Strong explosions with dense ash plumes lasted for less than 48 hours, minor emissions continued for a few more days; the SO₂, however, continued to circulate over far eastern Russia and the Bering Sea for more than three weeks after the initial explosion. The eruption covered the island with centimeters to meters of ash and enlarged the summit crater. By the end of July 2019 only minor intermittent steam emissions were observed in satellite imagery.

Figure 1. On the morning of 22 June 2019, astronauts on the International Space Station captured this image of a large ash plume rising from Raikoke in the Kuril Islands. The plume reached altitudes of 10-13 km and drifted E during the volcano's first known explosion in 95 years. Courtesy of NASA Earth Observatory.

Figure 2. A large and very dense SO2 plume (measuring over 900 Dobson Units (DU)) drifted E from Raikoke in the Kuril Islands on 22 June 2019, about 8 hours after the first known explosion in 95 years. Courtesy of NASA Goddard Space Flight Center.

Summary of 2019 activity. A powerful eruption at Raikoke began at 1805 on 21 June 2019 (UTC). Volcano Observatory Notices for Aviation (VONA's) issued by KVERT described the large ash plume that rapidly rose to 10-13 km altitude and extended for 370 km NE within the first two hours (figure 3). After eight hours, the plume extended 605 km ENE; it had reached 1,160 km E by 13 hours after the first explosion (figure 4). The last strong explosive event, according to KVERT, producing an ash column as high as 10-11 km, occurred at 0540 UTC on 22 June. SVERT reported a series of nine explosions during the eruption. Over 440 lightning events within the ash plume were detected in the first 24 hours by weather-monitoring equipment. The Japanese Ministry of Transportation reported that almost 40 planes were diverted because of the ash plume (figure 5).

Figure 3. A dense ash plume drifted E from Raikoke on 22 June 2019 from a series of large explosions that lasted for less than 24 hours, as seen in this Terra satellite image. The plume was detected in the atmosphere for several days after the end of the eruptive activity. Courtesy of NASA Earth Observatory.

Figure 4. The ash plume from Raikoke volcano that erupted on 21 June 2019 drifted over 1,000 km E by late in the day on 22 June, as seen in this oblique, composite view based on data from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite. Courtesy of NASA Earth Observatory.

Figure 5. Numerous airplanes were traveling on flight paths near the Raikoke ash plume (black streak at center) early on 22 June 2019. The Japanese Ministry of Transportation reported that almost 40 planes were diverted because of the plume. Courtesy of Flightradar24 and Volcano Discovery.

On 23 June (local time) the cruise ship Athena approached the island; expedition member Nikolai Pavlov provided an eyewitness account and took remarkable drone photographs of the end of the eruption. The ship approached the W flank of the island in the late afternoon and they were able to launch a drone and photograph the shore and the summit. They noted that the entire surface of the island was covered with a thick layer of light-colored ash up to several tens of centimeters thick (figure 6). Fresh debris up to several meters thick fanned out from the base of the slopes (figure 7). The water had a yellowish-greenish tint and was darker brown closer to the shore. Dark-brown steam explosions occurred when waves flowed over hot areas along the shoreline, now blanketed in pale ash with bands of steam and gas rising from it (figure 8). A dense brown ash plume drifted W from the crater, rising about 1.5 km above the summit (figure 9).

Figure 6. The entire surface of the island of Raikoke was covered with a thick layer of light-colored ash up to several tens of centimeters thick on 23 June 2019 when photographed by drone from the cruise ship Athena about 36 hours after the explosions began. View is of the W flank. Photo by Nik Pavlov; courtesy of IVS FEB RAS.

Figure 7. Fresh ash and volcanic debris up to several meters thick coated the flanks of Raikoke on 23 June 2019 after the large explosive eruption two days earlier. View is by drone of the W flank. Photo by Nik Pavlov; courtesy of IVS FEB RAS.

Figure 8. The 21 June 2019 eruption of Raikoke covered the island in volcanic debris. The formerly vegetated areas (left, before eruption) were blanketed in pale ash with bands of steam and gas rising all along the shoreline (right, on 23 June 2019) less than two days after the explosions began. The open water area between the sea stack and the island was filled with tephra. Photos by Nik Pavlov; courtesy of IVS FEB RAS.

Figure 9. At the summit of Raikoke on 23 June 2019, a dense brown ash plume drifted W from the crater, rising about 1.5 km, two days after a large explosive eruption. Drone photo by Nik Pavlov; courtesy of IVS FEB RAS.

Early on 23 June, the large ash cloud continued to drift E and then NE at an altitude of 10-13 km. At that altitude, the leading edge of the ash cloud became entrained in a large low pressure system and began rotating from SE to NW, centered in the area of the Komandorskiye Islands, 1,200 km NE of Raikoke. By then the farthest edge of ash plume was located about 2,000 km ENE of the volcano. Meanwhile, at the summit and immediately above, the ash plume was drifting NW on 23 June (figures 9 and 10). Ashfall was reported (via Twitter) from a ship in the Pacific Ocean 40 km from Raikoke on 23 June. Weak ashfall was also reported in Paramushir, over 300 km NE the same day. KVERT reported that satellite data from 25 June indicated that a steam and gas plume, possibly with some ash, extended for 60 km NW. They also noted that the high-altitude "aerosol cloud" continued to drift to the N and W, reaching a distance of 1,700 km NW (see SO₂ discussion below). By 27 June KVERT reported that the eruption had ended, but the aerosols continued to drift to the NW and E. They lowered the Aviation Alert Level to Green the following day.

Figure 10. The brown ash plume from Raikoke was drifting NW on 23 June 2019 (left), while the remnants of the ash from the earlier explosions continued to be observed over a large area to the NE on 25 June (right). The plume in the 23 June image extends about 30 km NW; the plume in the 25 June image extends a similar distance NE. Natural color rendering (bands 4, 3, 2) of Sentinel-2 imagery, courtesy of Sentinel Hub Playground.

Tokyo and Anchorage VAAC Reports. The Tokyo VAAC first observed the ash plume in satellite imagery at 10.4 km altitude at 1850 on 21 June 209, just under an hour after the explosion was first reported by KVERT. About four hours later they updated the altitude to 13.1 km based on satellite data and a pilot report. By the evening of 22 June the high-level ash plume was still drifting ESE at about 13 km altitude while a secondary plume at 4.6 km altitude drifted SE for a few more hours before dissipating. The direction of the high-altitude plume began to shift to the NNW by 0300 on 23 June. By 0900 it had dropped slightly to 12.2 km and was drifting NE. The Anchorage VAAC reported at 2030 that the ash plume was becoming obscured by meteorological clouds around a large and deep low-pressure system in the western Bering Sea. Ash and SO₂ signals in satellite imagery remained strong over the region S and W of the Pribilof Islands as well as over the far western Bering Sea adjacent to Russia. By early on 24 June the plume drifted NNW for a few hours before rotating back again to a NE drift direction. By the afternoon of 24 June, the altitude had dropped slightly to 11.6 km as it continued to drift NNE.

The ash plume was still clearly visible in satellite imagery late on 24 June. An aircraft reported SO₂ at 14.3 km altitude above the area of the ash plume. The plume then began to move in multiple directions; the northern part moved E, while the southern part moved N. The remainder was essentially stationary, circulating around a closed low-pressure zone in the western Bering Sea. The ash plume remained stationary and slowly dissipated as it circulated around the low through 25 June before beginning to push S (figure 11). By early on 26 June the main area of the ash plume was between 325 km WSW of St. Matthew Island and 500 km NNW of St. Lawrence Island, and moving slowly NW. The Anchorage VAAC could no longer detect the plume in satellite imagery shortly after midnight (UTC) on 27 June, although they noted that areas of aerosol haze and SO₂ likely persisted over the western Bering Sea and far eastern Russia.

Figure 11. This RGB image created from a variety of spectral channels from the GOES-17 (GOES-West) satellite shows the ash and gas plume from Raikoke on 25 June 2019. The brighter yellows highlight features that are high in SO2 concentration. Highlighted along the bottom of the image is the pilot report over the far southern Bering Sea; the aircraft was flying at an altitude of 11 km (36,000 feet), and the pilot remarked that there were multiple layers seen below that altitude which had a greyish appearance (likely volcanic ash). Courtesy of NOAA and Scott Bachmeier.

Sulfur dioxide emissions. A very large SO₂ plume was released during the eruption. Preliminary total SO₂ mass estimates by Simon Carn taken from both UV and IR sensors suggested around 1.4-1.5 Tg (1 Teragram = 10⁹ Kg) that included SO₂ columns within the ash plume with values as high as 1,000 Dobson Units (DU) (figure 12). As the plume drifted on 23 and 24 June, similar to the ash plume as described by the Tokyo VAAC, it moved in a circular flow pattern as a result of being entrained in a low-pressure system in the western Bering Sea (figure 13). By 25 June the NW edge of the SO₂ had reached far eastern Russia, 1,700 km from the volcano (as described by KVERT), while the eastern edges reached across Alaska and the Gulf of Alaska to the S. Two days later streams of SO₂ from Raikoke were present over far northern Siberia and northern Canada (figure 14). For the following three weeks high levels of SO₂ persisted over far eastern Russia and the Bering Sea, demonstrating the close relationship between the prevailing weather patterns and the aerosol concentrations from the volcano (figure 15).

Figure 12. A contour map showing the mass and density of SO2 released into the atmosphere from Raikoke on 22 June 2019. Courtesy of Simon Carn.

Figure 13. Streams of SO2 from Raikoke drifted around a complex flow pattern in the Bering Sea on 23 and 24 June 2019. Data from TROPOMI instrument on the Sentinel-5P satellite, courtesy of NASA Goddard Space Flight Center and Simon Carn.

Figure 14. SO2 plumes from Raikoke dispersed over a large area of the northern hemisphere in late June 2019. By 25 June (top) the SO2 plumes had dispersed to far eastern Russia, 1,700 km from the volcano, while the eastern edges reached across Alaska and the Gulf of Alaska to the S. By 27 June (bottom) streams of SO2 were present over far northern Siberia and northern Canada, and also continued to circulate in a denser mass over far eastern Russia. Data from TROPOMI instrument on the Sentinel-5P satellite, courtesy of NASA Goddard Space Flight Center and Simon Carn.

Figure 15. For the first two weeks of July 2019, high levels of SO2 from the 21 June 2019 eruption of Raikoke persisted over far eastern Russia and the Bering Sea entrained in a slow moving low-pressure system, demonstrating the close relationship between the prevailing weather patterns and the aerosol concentrations from the volcano. Data from TROPOMI instrument on the Sentinel-5P satellite, courtesy of NASA Goddard Space Flight Center.

Changes to the island. Since no known activity had occurred at Raikoke for 95 years, the island was well vegetated on most of its slopes and the inner walls of the summit crater before the explosion (figure 16). The first clear satellite image after the explosion, on 30 June 2019, revealed a modest steam plume rising from the summit crater, pale-colored ash surrounding the entire island, and new deposits of debris fans extending out from the NE, SW, and S flanks. Part of a newly enlarged crater was visible at the N edge of the old crater. Two weeks later only a small steam plume was present at the summit, making the outline of the enlarged crater more visible; the extensive shoreline deposits of fresh volcanic material remained. A clear view into the summit crater on 23 July revealed the size and shape of the newly enlarged summit crater (figure 17).

Figure 16. Changes at Raikoke before and after the 21 June 2019 eruption were clear in Sentinel-2 satellite imagery. The island was heavily vegetated on most of its slopes and the inner walls of the summit crater before the explosion (top left, 3 June 2019). The first clear satellite image after the explosion, on 30 June 2019 revealed a steam plume rising from the summit crater, pale-colored ash surrounding the entire island, and new deposits of debris fans extending out from the NE, SW, and S flanks (top right). Part of a newly enlarged crater was visible at the N edge of the old crater. Two weeks later only a small steam plume was present at the summit, making the outline of the enlarged crater more visible; the extensive shoreline deposits of fresh volcanic material remained (bottom right, 13 July 2019). A clear view into the summit crater on 23 July revealed the new size and shape of the summit crater (bottom left). Natural Color rendering (bands 4, 3, 2), courtesy of Sentinel Hub Playground.

Figure 17. Sentinel-2 satellite imagery of the summit crater of Raikoke before (left) and after (right) the explosions that began on 21 June 2019. The old crater rim is outlined in red in both images. The new crater rim is outlined in yellow in the 23 July image. Natural Color rendering (bands 4, 3, 2), courtesy of Sentinel Hub Playground.

References: Gorshkov G S, 1970, Volcanism and the Upper Mantle; Investigations in the Kurile Island Arc, New York: Plenum Publishing Corp, 385 p.

Tanakadate H, 1925, The volcanic activity in Japan during 1914-1924, Bull Volc. v. 1, no. 3.

Geologic Background. A low truncated volcano forms the small barren Raikoke Island, which lies 16 km across the Golovnin Strait from Matua Island in the central Kuriles. The oval-shaped basaltic island is only 2 x 2.5 km wide and rises above a submarine terrace. An eruption in 1778, during which the upper third of the island was said to have been destroyed, prompted the first volcanological investigation in the Kuril Islands two years later. Incorrect reports of eruptions in 1777 and 1780 were due to misprints and errors in descriptions of the 1778 event (Gorshkov, 1970). Another powerful eruption in 1924 greatly deepened the crater and changed the outline of the island. Prior to a 2019 eruption, the steep-walled crater, highest on the SE side, was 700 m wide and 200 m deep. Lava flows mantle the eastern side of the island.

Information Contacts: Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Sakhalin Volcanic Eruption Response Team (SVERT), Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Science, Nauki st., 1B, Yuzhno-Sakhalinsk, Russia, 693022 (URL: http://www.imgg.ru/en/, http://www.imgg.ru/ru/svert/reports); Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NOAA, Cooperative Institute for Meteorological Satellite Studies (CIMSS), Space Science and Engineering Center (SSEC), University of Wisconsin-Madison, 1225 W. Dayton St. Madison, WI 53706, (URL: http://cimss.ssec.wisc.edu/); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn); Scott Bachmeier, Cooperative Institute for Meteorological Satellite Studies (CIMSS), Space Science and Engineering Center (SSEC), University of Wisconsin-Madison, 1225 W. Dayton St. Madison, WI 53706; Flightradar24 (URL: https://www.flightradar24.com/51,-2/6); Volcano Discovery (URL: http://www.volcanodiscovery.com/).

Indonesia's Sinabung volcano in north Sumatra has been highly active since its first confirmed Holocene eruption during August and September 2010. It remained quiet after the initial eruption until September 2013, when a new eruptive phase began that continued uninterrupted through June 2018. Ash plumes often rose several kilometers, avalanche blocks fell kilometers down the flanks, and deadly pyroclastic flows traveled more than 4 km repeatedly during the eruption. After a pause in eruptive activity from July 2018 through April 2019, explosions took place again during May and June 2019. This report covers activity from July 2018 through July 2019 with information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), referred to by some agencies as CVGHM or the Indonesian Center of Volcanology and Geological Hazard Mitigation, the Darwin Volcanic Ash Advisory Centre (VAAC), and the Badan Nacional Penanggulangan Bencana (National Disaster Management Authority, BNPB). Additional information comes from satellite instruments and local news reports.

After the last ash emission observed on 5 July 2018, activity diminished significantly. Occasional thermal anomalies were observed in satellite images in August 2018, and February-March 2019. Seismic evidence of lahars was recorded almost every month from July 2018 through July 2019. Renewed explosions with ash plumes began in early May; two large events, on 24 May and 9 June, produced ash plumes observed in satellite data at altitudes greater than 15 km (table 9).

Table 9. Summary of activity at Sinabung during July 2018-July 2019. Steam plume heights from PVMBG daily reports. VONA reports issued by Sinabung Volcano Observatory, part of PVMBG. Satellite imagery from Sentinel-2. Lahar seismicity from PVMBG daily and weekly reports. Ash plume heights from VAAC reports. Pyroclastic flows from VONA reports.

No eruptive activity was reported after 5 July 2018 for several months, however Sentinel-2 thermal imagery on 30 August indicated a hot spot at the summit suggestive of eruptive activity. The next distinct thermal signal appeared on 6 February 2019, with a few more in late February and early March (figure 66, see table 9).

Figure 66. Sentinel-2 satellite imagery on 30 August 2018, 6 February, and 8 March 2019 showed distinct thermal anomalies suggestive of eruptive activity at Sinabung, although no activity was reported by PVMBG. Images rendered with Atmospheric Penetration, bands 12, 11, and 8A. Courtesy of Sentinel Hub Playground.

PVMBG reported the first ash emission in 11 months early on 7 May 2019. They noted that an ash plume rose 2 km above the summit and drifted ESE. The Sinabung Volcano Observatory (SVO) issued a VONA (Volcano Observatory Notice for Aviation) that described an eruptive event lasting for a little over 40 minutes. Ashfall was reported in several villages. The Jakarta Post reported that Karo Disaster Mitigation Agency (BPDB) head Martin Sitepu said four districts were affected by the eruption, namely Simpang Empat (7 km SE), Namanteran (5 km NE), Kabanjahe (14 km SE), and Berastadi (12 km E). The Darwin VAAC reported the ash plume at 4.6 km altitude and noted that it dissipated about six hours later (figure 67). The TROPOMI SO₂ instrument detected an SO₂ plume shortly after the event (figure 68).

Figure 67. Images from the explosion at Sinabung on 7 May 2019. Left and bottom right photos by Kopi Cimbang and Kalak Karo Kerina, courtesy of David de Zabedrosky. Top right photo courtesy of Sutopo Purwo Nugroho, BNPB.

Figure 68. The TROPOMI instrument on the Sentinel-5P satellite captured an SO2 emission from Sinabung shortly after the eruption on 7 May 2019. Courtesy of NASA Goddard Space Flight Center.

On 11 May 2019 SVO issued a VONA reporting a seismic eruption event with a 9 mm amplitude that lasted for about 30 minutes; clouds and fog prevented visual confirmation. Another VONA issued the following day reported an ash emission that lasted for 28 minutes but again was not observed due to fog. The Darwin VAAC did not observe the ash plumes reported on 11 or 12 May; they did report incandescent material observed in the webcam on 11 May. Sutopo Purwo Nugroho of BNPB reported that the 12 May eruption was accompanied by incandescent lava and ash, and the explosion was heard in Rendang (figure 69). The Alert Level had been at Level IV since 2 June 2015. Based on decreased seismicity, a decrease in visual activity (figure 70), stability of deformation data, and a decrease in SO₂ flux during the previous 11 months, PVMBG lowered the Alert Level from IV to III on 20 May 2019.

Figure 69. Incandescent lava and ash were captured by a webcam at Sinabung on 12 May 2019. Courtesy of Sutopo Purwo Nugroho, BNPB.

Figure 70. The summit of Sinabung emitted only steam and gas on 18 May 2019, shortly before PVMBG lowered the Alert Level from IV to III. Courtesy of PVMBG (Decreased G. Sinabung activity level from Level IV (Beware) to Level III (Standby), May 20, 2019).

A large explosion was reported by the Darwin VAAC on 24 May 2019 (UTC) that produced a high-altitude ash plume visible in satellite imagery at 15.2 km altitude moving W; the plume was not visible from the ground due to fog. The Sinabung Volcano Observatory reported that the brief explosion lasted for only 7 minutes (figure 71), but the plume detached and drifted NW for about 12 hours before dissipating. The substantial SO₂ plume associated with the event was recorded by satellite instruments a few hours later (figure 72, left). Another six-minute explosion late on 26 May (UTC) produced an ash plume that was reported by a ground observer at 4.9 km altitude drifting S (figure 72, right). About an hour after the event, the Darwin VAAC observed the plume drifting S at 6.1 km altitude; it had dissipated four hours later. Sumbul Sembiring, a resident of Kabanjahe, told news outlet Tempo.com that ash had fallen at the settlements. Two more explosions were reported on 27 May; the first lasted for a little over 12 minutes, the second (about 90 minutes later, 28 May local time) lasted for about 2.5 minutes. No ash plumes were visible from the ground or satellite imagery for either event.

Figure 71. A brief but powerful explosion at Sinabung in the early hours of 25 May 2019 (local time) produced a seven-minute-long seismic signal and a 15.2-km-altitude ash plume. Courtesy of MAGMA Indonesia and Volcano Discovery.

Figure 72. Two closely spaced eruptive events occurred at Sinabung on 24 and 26 May UTC (25 and 27 May local time). The 24 May event produced a significant SO2 plume recorded by the TROPOMI instrument a few hours afterwards (left), and a 15.2-km-altitude ash plume only recorded in satellite imagery. The event on 26 May produced a visible ash plume that was reported at 6.1 km altitude and was faintly visible from the ground (right). SO2 courtesy of NASA Goddard Space Flight Center, photograph courtesy of PVMBG and Øystein Lund Andersen.

An explosion on 9 June 2019 produced an ash plume, estimated from the ground as rising to 9.5 km altitude, that drifted S and E; pyroclastic flows traveled 3.5 km SE and 3 km S down the flanks (figure 73). The explosion was heard at the Sinabung Observatory. The Darwin VAAC reported that the eruption was visible in Himawari-8 satellite imagery, and reported by pilots, at 16.8 km altitude drifting W; about an hour later the VAAC noted that the detached plume continued drifting SW but lower plumes were still present at 9.1 km altitude drifting W and below 4.3 km drifting SE. They also noted that pyroclastic flows moving SSE were sending ash to 4.3 km altitude. Three hours later they reported that both upper level plumes had detached and were moving SW and W. After six hours, the lower altitude plumes at 4.3 and 9.1 km altitudes had dissipated; the higher plume continued moving SW at 12.2 km altitude until it dissipated within the next eight hours. Instruments on the Sentinel-5P satellite captured an SO₂ plume from the explosion drifting W across the southern Indian Ocean (figure 74).

Figure 73. A large explosion at Sinabung on 9 June 2019 produced an ash plume that rose to 16.8 km altitude and also generated pyroclastic flows (foreground) that traveled down the S and SE flanks. Left image courtesy of Sutopo Purwo Nugroho, Head of the BNPB Information and Public Relations Data Center. Right image photo source PVMBG/Mbah Rono/ Berastagi Nachelle Homestay, courtesy of Jaime Sincioco.

Figure 74. An SO2 plume from the 9 June 2019 explosion at Sinabung drifted more than a thousand kilometers W across the southern Indian Ocean. Courtesy of Sentinel Hub and Annamaria Luongo.

The SVO reported continuous ash and gas emissions at 3.0 km altitude moving ESE early on 10 June; it was obscured in satellite imagery by meteoric clouds. There were no additional VONA's or VAAC reports issued for the remainder of June or July 2019. An image on social media from 20 June 2019 shows incandescent blocks near the summit (figure 75). PVMBG reported that emissions on 25 June were white to brownish and rose 200 m above the summit and drifted E and SE.

Figure 75. Incandescent blocks at the summit of Sinabung were visible in this 20 June 2019 image taken from a rooftop terrace in Berastagi, 13 km E. Photo by Nachelle Homestay, courtesy of Jaime Sincioco.

PVMBG detected seismic signals from lahars several times during the second week of July 2019. News outlets reported lahars damaging villages in the Karo district on 11 and 13 July (figure 76). Detik.com reported that lahars cut off the main access road to Perbaji Village (4 km SW), Kutambaru Village (14 km S), and the Tiganderket connecting road to Kutabuluh (17 km WNW). In addition, Puskesmas Kutambaru was submerged in mud. Images from iNews Malam showed large boulders and rafts of trees in thick layers of mud covering homes and roads. No casualties were reported.

Figure 76. Lahars on 11 and 13 July 2019 caused damage in numerous villages around Sinabung, filling homes and roadways with mud, tree trunks, and debris. No casualties were reported. Courtesy of iNews Malam.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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/); The Jakarta Post (URL: https://www.thejakartapost.com/news/2019/05/07/mount-sinabung-erupts-again.html); Detikcom (URL: https://news.detik.com/berita/d-4619253/hujan-deras-sejumlah-desa-di-sekitar-gunung-sinabung-banjir-lahar-dingin); iNews Malam (URL: https://tv.inews.id/, https://www.youtube.com/watch?v=uAI4CpSb41k); Tempo.com (URL:https://en.tempo.co/read/1209667/mount-sinabung-erupts-on-monday-morning); David de Zabedrosky, Calera de Tango, Chile (Twitter: @deZabedrosky, URL: https://twitter.com/deZabedrosky/status/1125814504867160065/photo/1, https://twitter.com/deZabedrosky/status/1125814504867160065/photo/2); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Øystein Lund Andersen? (Twitter: @OysteinLAnderse, URL: https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com image at https://twitter.com/OysteinLAnderse/status/1132849458142572544); Jaime Sincioco, Phillipines (Twitter: @jaimessincioca, URL: https://twitter.com/jaimessincioco); Annamaria Luongo, University of Padua, Venice, Italy (Twitter: @annamaria_84, URL:https://twitter.com/annamaria_84).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Krakatau Islands KPHK, Conservation Area Region III Lampung, BKSDA Bengkulu-Ministry of LHK, (URL: https://www.instagram.com/krakatau_ca_cal); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); BBC News, (URL: https://www.bbc.com, article at https://www.bbc.com/news/science-environment-46743362); Planet Labs Inc. (URL: http://www.planet.com/); Sutopo Purwo Nugroho, BNPB (Twitter: @Sutopo_PN, URL: https://twitter.com/Sutopo_PN, image at https://twitter.com/Sutopo_PN/status/1101007655290589185/photo/1); Øystein Lund Andersen? (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com, image at https://twitter.com/OysteinLAnderse/status/1107479025126039552/photo/1); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/), images at https://www.volcanodiscovery.com/krakatau/news/80657/Krakatau-volcano-Indonesia-activity-update-and-field-report-increasing-unrest.html; Devy Kamil Syahbana, Volcanologist, Bandung, Indonesia, (URL: https://twitter.com/_elangtimur, video at https://twitter.com/_elangtimur/status/1143372011177033728); The Daily Mail (URL: https://www.dailymail.co.uk, article at https://www.dailymail.co.uk/sciencetech/article-6910895/FORTY-volcanoes-world-potential-Anak-Krakatoa-eruptions.html) published 11 April 2019.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Continuous gas and tephra emission was accompanied by sporadic Strombolian eruptions. Blocks and bombs fell as much as 1,200 m from the active crater (C) to 880 m elevation. The activity occasionally produced small nuées ardentes, such as one observed on 9 November at 0230 that traveled some 400 m down the S flank from Crater C. Lava flows moved down the NE, N, and NW flanks. Some of the flows covered and burned vegetation on parts of the NE and N flanks. Fumarolic activity continued from the summit crater (D).

Winds carried ash columns toward the NW, W, and SW flanks, the area most affected by gases. During November, the mean pH of rain in this area was 3.8. Vegetation on the E flank continued to be affected by gases, acid rain, and falling blocks. The combination of vegetation damage, steep slopes, poorly consolidated material, and heavy precipitation has caused strong erosion, producing small cold avalanches in the canyons Calle de Arenas, Guillermina, and Río Agua Caliente.

During observations by geologists 9-10 January, loud explosions (Type 1 in 14:06) were heard, on average, about every 1.5 hours. Ash columns associated with five of the explosions were seen through breaks in the cloud cover. The loudest explosion during the period, on 10 January at 1720, was associated with the largest observed ash column, which rose ~1 km and was videotaped from the N side of Lake Arenal, ~20 km from the volcano. On 9 January at 2240, another Type 1 eruption ejected incandescent blocks and bombs onto the upper 2/3 of the N flank. A lava flow from the summit area descended the N flank, spalling blocks from its surface and levees.

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

Information Contacts: E. Fernández, E. Duarte, V. Barboza, J. Barquero, and J. Bravo, OVSICORI; S. Halsor, Wilkes Univ, USA; C. Chesner, Eastern Illinois Univ, USA.

"Mild but steady eruptive activity continued as a result of slow extrusion of blocky lava in the summit crater. The main visible activity consisted of frequent rockfalls of (night-glowing) rock avalanches from the summit onto all flanks of the volcano. New pulses of lava were seen spilling onto the SE flank and into the [1966-75] lava channel at 4-5-day intervals. Abundant fumaroles are present within the crater which is weakly glowing at night. Two low explosions were reported on the 10th and 27th. Seismicity continued to be dominated by rockfall events (several tens/day), but occasional B-type events were also recorded. Seismic monitoring . . . ceased on the 24th, due to the loss of telemetry as the result of the current civil disturbance . . . ."

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

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

"A swarm of earthquakes took place in the Esa'Ala area in mid-December 1989. Earthquakes were felt with intensities MM III-V by villagers at Esa'Ala, Dobu Island, and on SE Fergusson Island at Oiau, Deidei, and Numa Numa. The local volcano observatory at Esa'Ala noted felt events on the 10th (26 events), 11th (12), 12th (6), and the 13th (3).

"Reports received at RVO on 11 and 13 December prompted a 5-day on-site inspection and local seismic recording by three RVO officers, 13-18 December. When local recording resumed at Esa'Ala Volcano Observatory (on the 13th), 30 sharp volcano-tectonic earthquakes (A-type with S-P

Table 1. Number of A- and B-type events recorded at the Esa 'Ala Observatory, 13-17 December 1989.

"The deployment of two portable seismographs at Numa Numa and Waiope Island, together with the seismograph at the Esa'Ala Volcano Observatory, allowed location of some of the larger earthquakes. Of the five earthquakes located, three occurred in the Dobu passage, another under Dobu Island, and the 5th between Dobu Island and Neumara Island (figure 1). These locations lie [near the outline] of an inferred submerged caldera.

Figure 1. Map of the Dawson Strait Group area, showing volcanic vents, the volcano observatory (with permanent seismograph), sites of two portable seismographs deployed 13-18 December 1989, and the five earthquakes located during that period. Courtesy of RVO.

"The strongest earthquake caused minor landslides from 4-30-m cliffs of pyroclastic deposits along the N and E shores of Dobu Island, and one landslide inland, from a steep gulley on the E flank of Dobu. Ground cracks, 1-4 m inland and 0.5-2 cm wide, reported at Kenaie village on the NE side of Dobu Island were related to the collapse of marine cliffs parallel to their edges.

"Off the N coast of Dobu Island on the afternoon of the 11th, a team of professional divers filmed and photographed very turbulent springs (28°C) and degassing vents through sand holes up to 1 m in diameter, at a depth of 6 m of water. They also reported subcontinuous booming and roaring underwater sounds (at an estimated 30-40 db level).

"The December 1989 earthquake swarm was the first episode of unrest [in this area] since April 1969. At that time (1969), over a period of a few days, 5-7 shocks were felt daily, accompanied by noises. This prompted spontaneous evacuation of villagers on Dobu Island and around Oiau Volcano. Recording at the time was from one station only (ESA). On the basis of the S-P interval (1-1.4 seconds) and reports of earthquake intensities, the swarm was presumed to have originated from the Dobu Passage."

Geologic Background. The Dawson Straits, located between eastern Fergusson and western Normanby Islands in the D'Entrecasteaux island group, contains a volcanic field with several centers that define a possible partly submerged caldera. There have been no historical eruptions, but morphology suggests an extremely young age for some lava flows, and the area displays vigorous thermal activity. The most prominent volcanic centers are Mounts Lamonai and Oiau, located about 10 km apart on the SW tip of Fergusson Island. The summit of Lamonai is capped by a steep-walled crater, and rhyolitic lava flows are exposed on the NE side of the cone. The dominantly volcaniclastic Oiau cone has also produced obsidian lava flows. Dobu Island to the south is formed of coalescing volcanic centers and likewise has produced youthful rhyolitic obsidian flows.

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

Summit activity. (S. Calvari, M. Coltelli, O. Consoli, M. Pompilio and V. Scribano.) Eruptive activity that resumed in December at Southeast Crater continued in January, with explosive episodes of Strombolian activity and lava fountaining 4-5, 12, and 14-15 January. The first episode of Strombolian activity deposited 1 to a few cm of ash on the NW flank. Lava poured from the S crater rim, reaching the 1971 eruptive craters (at least 200 m from the vent). A smaller amount of tephra ejected by similar activity on 12 January was carried a few hundred meters WNW by the wind. Lava spilled over the N crater rim, producing a flow 1.5 km long and 20-30 m wide that traveled NE along the September-October fracture system to ~ 2,700 m altitude. When activity ceased, Southeast Crater was completely obstructed by a solidified crust of lava.

The next day, Strombolian activity gradually increased, reaching maximum intensity on the morning of the 15th. Observations from a helicopter on 16 January revealed a new lava flow 50-100 m wide (apparently erupted 15 January) that had flowed over the E crater rim and traveled about 2.5 km SE down the Valle del Bove, stopping at ~2,000 m altitude (S of Sierra Gianicola Piccola).

Sporadic Strombolian explosions with variable intensities resumed 19 January and continued throughout the month. Bad weather prevented field surveys at the other active summit craters, but observations by helicopter showed degassing at the two central vents (Bocca Nuova and La Voragine) and Northeast Crater.

Seismic activity. (E. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montalto, D. Patane, A. Pellegrino, and S. Spampinato.) January seismicity increased from previous months. Tremor amplitude fluctuated, with increases on 5, 11-12 and 14-15 January, associated with strong explosive activity and lava emission at Southeast Crater. The number of low-frequency events of M<=1 increased from the single shock recorded in December, often becoming more numerous before and after variations in tremor amplitude. A large increase in the number of low-frequency shocks was recorded 19-20 January, but was not accompanied by variation in the tremor amplitude or an increase in energy release. A sequence of 18 events on the NNW side of the volcano at 10-15 km depth had a large energy release. At least nine events reached or exceeded M 2, with 2 main shocks (at 1200 and 1336 on the 28th) reaching M 2.7.

Ground deformation. (A. Bonaccorso, O. Campisi, G. Falzone, B. Puglisi, and R. Velardita.) January tilt recorded at the SPC and CDV borehole tilt stations on the volcano's S flank showed no significant variation from the previous month. EDM surveys across the fracture on the S side (along SP92) and on the N part of the Etna Sud trilateration network showed variations within instrumental error limits.

Summit crater SO₂ flux. (T. Caltabiano and R. Romano.) Samples collected from the summit craters on 8, 12, 16-18, 24, and 31 January showed increased SO₂ emission preceding eruptive activity. Emissions rose from ~4,000 t/d on 29 December to ~ 26,000 t/d measured 16 January following Southeast Crater explosive and effusive activity on the 14-15th. Emissions returned to moderate values on 17 January (~5,000 t/d) and remained near that level during measurments on 18, 24, and 31 January.

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: R. Santacroce, IIV.

[This preliminary report is supplemented by detailed information in 15:2]. An eruption began on 18 January at 1124 from the SE area of the central (Dolomieu) crater and from its upper SE flank. The eruption was preceded by three months of significant seismicity. Vigorous 50-100-m fountaining from roughly NW-SE-trending fractures was observed until about 1500. Activity had completely stopped by 0630 on 19 January.

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

Information Contacts: J. Toutain, P. Taochi, J-L. Cheminée, IPGP; P. Bachelery, Univ de la Réunion.

Geologists observed a vigorous and persistent gas plume that extended several tens of kilometers W from Fuego's summit crater during work at neighboring Acatenango on 6 January between 0900 and 1400. Frequent rock avalanches occurred in the upper parts of Barranca Honda, a steep E flank canyon.

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

Information Contacts: S. Halsor, Wilkes Univ; C. Chesner, Eastern Illinois Univ.

The number of high- and low-frequency earthquakes increased in January (see figure 18). Pulses of spasmodic tremor accompanied ash emission on the 7th. A swarm of high-frequency events occurred W of the crater at depths of 3-5 km on the 10th (figure 14). The electronic tiltmeter showed an increase of 3 µrad on 26 January, coinciding with an M 2 earthquake beneath the tiltmeter station. Gas emission was not measured in January.

Figure 14. Epicenters of 95 seismic events near Galeras, January 1990. Courtesy of INGEOMINAS.

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

Information Contacts: INGEOMINAS, Pasto; INGEOMINAS, Manizales.

A strong explosive eruption on 10 February produced a large cloud and heavy tephra falls. Although evacuation began before the onset of the eruption, more than 30 people were reported killed in the densely populated area near the volcano.

Local seismicity increased to ~10x the normal rate between the middle and end of November, then returned to background levels. A second episode of increased seismicity occurred during the second and third weeks of January, accompanied by a rise in crater lake temperature from 32 to 38°C. Enhanced earthquake activity was again detected 7 February, and lake temperature peaked at 39-41°C. Measurements on 8 February indicated that lake pH had decreased to 4.2 from its usual 4.9, and hydrophones in the lake recorded noise at 3x normal levels. However, seismicity and hydrophone noise declined that day and remained relatively quiet until the onset of the eruption. During the night of 9-10 February, transmission from the lake instruments (via the ARGOS satellite) ceased. VSI issued an evacuation warning on 10 February at 1000. Press sources estimated the number of evacuees at 60,000.

The eruption began 10 February at 1141, with the strongest explosive phase at 1250, and activity continued until 1700. Pyroclastic flows traveled [5-6] km down the steep-walled valley on the E flank (from the breach in the summit crater), filling it to a depth of ~10 m. The crater lake was emptied during the eruption. However, most of the damage and casualties were attributed to heavy tephra falls that reached 20-30 cm thickness. In Nglegok (~55 km SW of the volcano) residents reported falls of fist-sized tephra, with some material that was head-sized. About 15 cm of ash accumulated at Blitar, ~25 km SW of the summit. Ashfalls were also reported from Malang, ~35 km E of Kelut. The weight of the tephra caused houses to collapse, trapping their occupants. As of 20 February, the official death toll had risen to 32, with >500 homes and 50 schools destroyed and many others damaged.

Tracking of the plume by satellite was made difficult by heavy weather cloud activity in the area. Imagery from Japan's GMS satellite at 1300 showed a bright cloud 53 km across centered over the volcano. At 1347, a NOAA polar orbiter image revealed that the plume, still roughly circular and centered over the volcano, had grown to ~160 km in diameter. Preliminary temperature analysis suggested that the top of the dense cloud was at ~12 km altitude, although diffuse material could have extended higher. By 1600, GMS data indicated that the plume had drifted slightly WSW and was ~310 km long. Very diffuse-appearing material could be seen extending to the NW coast of Australia, ~1,400 km from Kelut, on a NOAA polar orbiter image the next day at 1347. Dispersed remnants of the plume could be traced on GMS images until 1900.

Small explosions with associated glow and/or lightning were continuing during the night of 11-12 February, accompanied by strong seismic activity. A second strong explosive episode occurred between 1512 and 1827 on 12 February. Incandescent tephra rose 7 km above the crater. Rains triggered a major lahar on 15 February that moved downslope at 40-60 km/hour, inundating parts of subdistricts ~16 km NNW (Kepung) and 17 and 28 km NW (Ploso Klaten and Guran) of the summit, forcing residents to flee to high ground. Further heavy rains during the night of 16-17 February forced 50,000 Kepung subdistrict residents to flee. Minor eruptive activity for ~2 hours that night sent thick ash clouds to ~600 m above the vent. Additional lahars the next day moved downslope at ~45 km/hour, destroying hundreds of hectares of agricultural land.

Further Reference. Sudradjat, Adjat, 1991, A preliminary account of the 1990 eruption of the Kelut volcano: Geol. Jahrbuch, v. A127, p. 447-462.

Geologic Background. The relatively inconspicuous Kelut stratovolcano contains a summit crater lake that has been the source of some of Indonesia's most deadly eruptions. A cluster of summit lava domes cut by numerous craters has given the summit a very irregular profile. Satellitic cones and lava domes are also located low on the E, W, and SSW flanks. Eruptive activity has in general migrated in a clockwise direction around the summit vent complex. More than 30 eruptions have been recorded from Gunung Kelut since 1000 CE. The ejection of water from the crater lake during the typically short but violent eruptions has created pyroclastic flows and lahars that have caused widespread fatalities and destruction. After more than 5000 people were killed during an eruption in 1919, an ambitious engineering project sought to drain the crater lake. This initial effort lowered the lake by more than 50 m, but the 1951 eruption deepened the crater by 70 m, leaving 50 million cubic meters of water after repair of the damaged drainage tunnels. After more than 200 deaths in the 1966 eruption, a new deeper tunnel was constructed, and the lake's volume before the 1990 eruption was only about 1 million cubic meters.

Information Contacts: VSI; Y. Sawada, JMA; O. Karst, SAB; U.S. Embassy, Jakarta; T. Casadevall, USGS; Jakarta Post; UPI.

The eruption . . . continued in January. Tube-fed lava flowed into the sea at one active entry and surface breakouts occurred upslope. The pond level remained ~32 m below Kupaianaha's N rim throughout the month. Glowing vents in the talus pile on the crater floor of Pu`u `O`o . . . were reported on the 7th, the only day during the month when weather conditions allowed observations.

On 12 January, a 50-100-m inflated area formed over the lava tube at the 575 m (1,900 ft) elevation. The bulge ruptured the next morning and produced aa lava that flowed SE across Kupaianaha's lava field (figure 66). By the 14th, the flow had formed a tube and had a pahoehoe front. It entered the forest on 17 January, moving between the 1977 aa flow and the December 1986 Kupaianaha flow (which destroyed 17 homes in and near Kalapana Gardens). The new flow, ~100-150 m wide, was advancing slowly near the 230 m (750 ft) elevation by the end of the month.

Figure 66. Lava flows produced by Kilauea's Kupaianaha vent, July 1986-March 1990. The dotted line indicates the active lava tube system, dashed lines are flows active before the eruption's 19 March hiatus, and arrows show flow fronts active in late March. Courtesy of C. Heliker.

Minor surface breakouts at 480-360 m (1,600-1,200 ft) elevation fed small hummocky pahoehoe flows along the W margin of the flow field. A long-lived surface breakout from the main tube at the 180 m (600 ft) elevation (active since late October) sent flows W into Royal Gardens, destroying a house (just below Orchid St.). Surface flows on the coastal plain were limited to the area immediately inland of the Poupou entry . . . , where moderate littoral explosions hurled spatter 10-15 m into the air and built a 2-3-m littoral cone.

Eruption tremor continued at low levels . . . near Pu`u `O`o and Kupaianaha. Tremor of relatively steady amplitude was punctuated by intermittent high-frequency bursts of seismic energy related to crustal adjustments near both the Kalalua (KLC, ~1 km from Kupaianaha) and Steam Cracks (STC, a few kilometers from Pu'u 0'o) seismometers. The most significant of these were two strong 15-minute avalanches registered on the STC instrument at 0240 on 18 January and at 1305 on 21 January. Shallow microearthquake activity in the East rift zone and at the summit was average throughout the month. The daily number of intermediate-depth (5-15 km) long-period events was erratic, with > 100/day occurring 12-14 and 16-18 January. A gradual increase from levels of tens/day to approaching 100/day began 27 January.

Addendum: The first hiatus . . . since April 1988 occurred 7-9 February [but see 15:2]. Kupaianaha's magma supply decreased on the 6th and the flow that had formed 13 January stagnated. By the next day, tremor had generally declined, all flows had stopped, and only small pahoehoe lobes were active around margins of crusted lava in Kupaianaha. Activity resumed late on the 9th, with strong night glow from Pu`u `O`o and a slight increase in tremor. By the next day, lava had returned to Pu`u `O`o and Kupaianaha, and had reoccupied the tube system, with several surface breakouts forming vigorous pahoehoe flows.

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: C. Heliker and P. Okubo, HVO.

Explosive activity from summit and flank vents emitted ash and lava October-December 1989 and January 1990. Up to 5 ejections/minute from three vents in the summit crater sent ash to 2 km above the summit in December, with a visible plume stretching 20 km from the volcano. Lava fountaining, 30-50 m high from SE flank vents at 4100-4200 m altitude, fed two lava flows that moved SE and E to 2,500 m altitude. Violent explosive activity from the summit crater resumed 29 January, ejecting a 6-km ash plume that extended 60 km from the volcano [see also 15:3]. An incandescent cloud 600-1,500 m high pulsated at the base of the plume, which generated lightning at its top. Ashfall on the Bering Islands (~375 km SE of the volcano) was reported 1 February. Activity also occurred from a SE-flank crater.

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

Information Contacts: B. Ivanov and E. Zhdanova, IV.

"Activity remained at a moderate level throughout January. Weak red glow was observed over Crater 2 on every clear night of the month. Vapour was released in weak to moderate amounts. The seismicity consisted of numerous daily B-type events of small amplitude and an occasional Vulcanian explosion shock. Crater 3 was inactive except for weak fumarolic emissions."

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

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

Landsat TM data recorded on 28 November and 14 December show the continued presence of a short-wavelength infrared thermal anomaly in the center of the active crater, despite the deflation of the lava dome that had occurred by 19 October. A roughly circular area four 30 x 30 m pixels in diameter was radiant in TM bands 5 (1.55-1.75 mm in wavelength) and 7 (2.08-2.35 mm) on both images. Several pixels were saturated in both bands, but there was no thermal radiance for the next shorter wavelength (band 4, 0.76-0.09 mm). Temperatures and radiant flux have not yet been calculated, but the general appearance of the data is similar to that of November 1987. There had been no Landsat TM observations of Lascar since then, but it seemed likely that the radiant anomaly has persisted for 5 years, since the first TM image of the volcano was recorded in December 1984.

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: D. Rothery, Open Univ.

Ground and aerial observations on 20 January showed water vapor and other gases emerging from the main northern summit crater, while fumaroles on the summit of the southern cone emitted only water vapor. Water vapor fumaroles were also present on the southern summit's upper SE flank, where an older crater had collapsed and was breached to the SE during the February 1957 eruption. Fumaroles on the southern summit have been increasing in number and intensity since 1984.

Geologic Background. Llaima, one of Chile's largest and most active volcanoes, contains two main historically active craters, one at the summit and the other, Pichillaima, to the SE. The massive, dominantly basaltic-to-andesitic, stratovolcano has a volume of 400 km3. A Holocene edifice built primarily of accumulated lava flows was constructed over an 8-km-wide caldera that formed about 13,200 years ago, following the eruption of the 24 km3 Curacautín Ignimbrite. More than 40 scoria cones dot the volcano's flanks. Following the end of an explosive stage about 7200 years ago, construction of the present edifice began, characterized by Strombolian, Hawaiian, and infrequent subplinian eruptions. Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

Information Contacts: H. Moreno, Univ de Chile; J. Naranjo, SERNAGEOMIN, Santiago.

"Swarm-like activity of magnitude greater than or equal to 2 earthquakes along the S margin of the resurgent dome and S moat continued to increase through January and early February (figure 11). During the same period, Mammoth Mountain, on the SW rim of the caldera, remained relatively quiet except for a brief burst of activity beneath the mountain's N flank on 19 January that included a locally felt earthquake of about M 3." A preliminary report on the Mammoth Mountain swarm will appear in the April issue of the Bulletin of the Seismological Society of America (see Reference, below).

Figure 11. Preliminary values for cumulative number of earthquakes top and their magnitudes bottom, 2 January-4 February 1990. Only events with magnitudes greater than or equal to 0.5 are shown. Courtesy of D. Hill.

"Activity within the caldera continues to occur in much the same area as the recurring swarms of the early 1980's, although the current activity shows a tendency to be concentrated somewhat farther N beneath the resurgent dome and along its W and E margins. Focal depths range from roughly 8 to <3 km as during the earlier activity. This increased swarm activity within the caldera no doubt reflects the 5-fold increase in extensional strain rate across the resurgent dome, detected by repeated measurements of the 2-color geodimeter network that spans the central section of the caldera. The strain rate increase began in September and has continued through January at a rate of ~5 microstrain/year."

Reference. Hill, D.P., Ellsworth, W.L., Johnston, M.J.S., Langbein, J.O., Oppenheimer, D.H., Pitt, A.M., Reasenberg, P.A., Sorey, M.L., and McNutt, S.R., 1990, The 1989 Earthquake Swarm Beneath Mammoth Mountain, California: An Initial Look at the 4 May through 30 Sep Activity: Bulletin of the Seismological Society of America, v. 80, p. 325-339.

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: D. Hill, USGS Menlo Park.

During observations on 10 January from 1200 to 1530, lava emission continued from the vent at the NE foot of Navidad Cone. The black aa lava had a surface velocity of 70 m/hour as it moved through a pre-existing channel, which widened progressively from 7 to 12 m over a distance of 50 m from the source. The vent was triangle-shaped, 7 m wide and 4-5 m high, elongating within to a glowing tunnel that had not been exposed during 20 November fieldwork. Activity in the main crater was limited to fumarolic emission that varied gradually from white-yellowish to blue. A vapor cloud was distinctly visible 300-500 m above the cone.

Geologists climbed to the cone's summit on 20 January between 1145 and 1630. Blue fumarolic emissions were continuous from the central crater, nested within the eastern part of the formerly horseshoe-shaped main crater. Annular fractures surrounding the central crater had been emitting vapor on 20 November, but vapor emission had ceased by 20 January after a period of relatively dry weather. Collapse had occurred along some of the fractures, widening the funnel-shaped crater to 60-70 m. A hole 3-4 m in diameter, formerly the main steam jet vent, was clearly visible in the N inner wall, without gas emission.

Between 10 and 20 January, the rate of lava production had clearly decreased, and the 14-m-high wall of the drainage channel had partially collapsed. This process triggered discontinuous slumping of flank material directly above the vent, producing dense dust clouds. The slumped debris was transported on the lava surface, aligned in arched transverse ridges representing different slumping episodes. By the end of field observations at 1630 on 20 January, these materials had been carried as much as 500 m from the vent. An overflight the same day at about 1730 showed that advance had ceased along the entire flow front, including the lobe in the Lolco River valley, which remained 10.7 km from the vent as on 20 November.

Major and trace element analyses of lava and bombs sampled in July and November show no significant variation, being very similar to samples from early in the eruption (Moreno and Gardeweg, 1989).

Geologic Background. Lonquimay is a small, flat-topped, symmetrical stratovolcano of late-Pleistocene to dominantly Holocene age immediately SE of Tolguaca volcano. A glacier fills its summit crater and flows down the S flank. It is dominantly andesitic, but basalt and dacite are also found. The prominent NE-SW Cordón Fissural Oriental fissure zone cuts across the entire volcano. A series of NE-flank vents and scoria cones were built along an E-W fissure, some of which have been the source of voluminous lava flows, including those during 1887-90 and 1988-90, that extended out to 10 km.

Information Contacts: J. Naranjo, SERNAGEOMIN, Santiago; H. Moreno, Univ de Chile.

"Activity remained at a low level in January, as it has for the last 6 months. Emissions from both Southern and Main Craters consisted mainly of weak white vapour. Rumbling noises were occasionally heard on the 2nd. There was no significant tilt change."

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

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

Gas bursts from MacKenney Crater's inner crater vent were observed during fieldwork between 1500 and 1730 on 4 January. The gas bursts, of varying intensity and duration, occurred about every 15 minutes. Some were accompanied by Strombolian spattering; the most energetic lasted several seconds and ejected spatter to 40 m height. A mound of agglutinated spatter was centered around the 1-m-wide vent. The inner crater was about 60 m in diameter and 30 m deep, with its S wall against the inner wall of MacKenney Crater. Rockfalls were frequent on the steep inner crater walls. Near the inner crater rim, the floor of MacKenney Crater was muddy and strewn with small blocks of spatter. Residents of the area said that this part of the crater floor had recently been occupied by a small lake. Active fumaroles were abundant along the upper E wall of MacKenney Crater. No active lava flows were evident.

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: S. Halsor, Wilkes Univ; C. Chesner, Eastern Illinois Univ.

The level of the crater lake descended 1.5 m between November 1989 and January 1990. Its mean temperature was about 80°C and mean pH was 0.5. Degassing was continuous, and winds carried gases toward the W and SW flanks. Ejecta from phreatic eruptions reached 50 m height before falling back into the lake. Seismicity remained at low frequency (<3 Hz), with 9,434 recorded events in November (314/day), 10,164 in December (328/day), and 5,115 through 21 January (243/day) (figure 25). Geologists attributed the shallow, low-frequency seismicity to the volcano's continued degassing. Fumarolic activity continued on the remnants of the 1953-55 dome.

Figure 25. Number of recorded seismic events/day at Poás, 1 November 1989-21 January 1990. Courtesy of the Observatorio Vulcanológico y Sismológico de Costa Rica.

Geologists visited the volcano on 12 January. The roughly circular crater lake was about 75 m in diameter and appeared to be only a few meters deep. Its water was grayish-green, had a high concentration of mud, and contained small pools and ribbons of yellow or green liquid sulfur. Water temperature was 77°C. Vigorous boiling occurred at three sites associated with occasional geyser activity; one geyser event ejected lake water to 45 m height. Dense gas plumes and thin concentric bands of liquid sulfur were also associated with the boiling zones.

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

Information Contacts: E. Fernández, E. Duarte, V. Barboza, J. Barquero, and J. Bravo, OVSICORI; S. Halsor, Wilkes Univ; C. Chesner, Eastern Illinois Univ.

"January marked the end of a period of minor, short-term unrest that started in October 1989 and was comparable to August-November 1988 activity. Seismicity in January (401 recorded events) had declined markedly since December (table 2). The background level ranged from 1 to 15 events/day, compared to 5-40/day in December. Three small earthquake swarms occurred January 1-2 (69 events), 13 (39), and 23 (79) from the N (Greet Harbour) and E (Blanche Bay) sides of the caldera (table 2 and figure 11). The increased rate of ground deformation recorded in December apparently stabilized in January, although a complete survey is required to assess the amount of caldera-wide elevation and tilt change (table 2)."

Table 2a. Summary of monthly seismicity during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.

Table 2b. Summary of local earthquake swarms during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.

Table 2c. Summary of ground deformation at at Matupit Island during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.

Table 2d. Summary of tilt changes during the October 1989-January 1990 period of unrest at Rabaul. Courtesy of RVO.

Figure 11. Epicenters of seismic events at Rabaul, October 1989-January 1990. Courtesy of RVO.

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

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

AVO's detailed summary of the eruption through late January appears in the 13 February issue of EOS, accompanied by a second paper on satellite observation of the December plumes (see References, below).

Airfall tephra. Initial tephra studies suggest that airfall deposits have been relatively modest. At 8-11 km from the crater, most sectors showed 4-6 or more tephra layers in the snowpack, from <1 to 20 cm thick. One of the 15 December explosions generated pumiceous tephra, including fist-sized bombs that melted their way into the snow. Other explosive episodes appeared to have been dominated by accretionary lapilli and fine lithic ash.

8 January plume data. A monitoring aircraft operated by the Univ of Washington reached the 8 January plume about 2 1/2 hours after the onset of activity. Dimensions of the plume were measured with upward-looking lidar over the Kenai Peninsula (60.96°N, 151.17°W to 60.81°N, 150.02°W) between 1246 and 1257. The base of the plume was at about 3.85 km altitude and its top at 9.0-9.7 km, with the main mass of material at approximately 6.4 km. Mass median diameter of particles was about 7 µm. Inter-eruptive emissions monitored from the same aircraft before 8 January were SO₂-rich, perhaps totaling several thousand metric tons/day, but attempts to collect COSPEC data failed because of low sun angles.

Lava extrusion and minor venting, mid January-mid February. A new lava dome was first seen during an overflight on 18 January, although seismic evidence suggested the onset of extrusion by 13 January, before the 16 January explosion. Vigorous vapor emission limited observations of the dome in late January, and growth was difficult to discern. Rockfalls off the dome were extensive, but no new lobes or fissures were evident. It appeared to be slightly less than 100 m high and roughly 1/4-1/3 the size of the late December dome, which had an estimated volume of 25 x 10⁶ m³. Additional breakage of the glacier that remained above the vent was indicated by new crevasses and ice avalanches into the crater. Ice blocks descended in stairstep fashion toward the dome from a vertical icewall. Most of the vapor emission originated from the area between the ice and the dome, obscuring the near-dome region and the interaction between ice and lava. A stream of meltwater flowed from the crater.

A minor seismic event began at 2315 on 7 February and lasted for about 20 minutes, suggesting vigorous steam venting. It was detected only by the summit station, indicating that little if any ash was ejected. The dome appeared nearly unchanged during an overflight the next day. Shallow earthquake activity recorded near the summit increased 12 February, suggesting a higher rate of dome growth. A tremor-like episode on the seismic station nearest the summit began at 0305 on 13 February and lasted nearly 30 minutes. The seismicity was probably associated with vigorous gas venting, perhaps including a little ash. Shallow summit-area seismicity declined somewhat that day, but remained elevated, as dome growth apparently continued.

Explosive episode, 15 February. A moderately large explosive episode that began at 0403 on 15 February produced a steam and ash column that rose to more than 10 km altitude and was carried rapidly SE by 160 km/hour winds. A spectacular display of lightning and thunder that lasted for about an hour awakened many people on the Kenai Peninsula. Satellite data 7 minutes after the onset of explosive activity showed a possible low-level pyroclastic flow or surge extending 7-10 km from the crater on Redoubt's NW quadrant. The temperature of the warmest 1 x 1 km pixel in this area was +25°C, with ambient air reported below -10°. By 0500, satellite images showed a plume extending about 120 km SE with an average width of more than 70 km. Plume temperatures of -49°C yielded an estimated altitude of 10.5-11.5 km for the top of the dense portion of the cloud. Airline pilots reported ash to 12 km altitude in the Homer area (roughly 120 km SE of Redoubt) and more than 1 cm of ash on Homer's airport runway. Anchorage's international airport remained open, but the FAA warned aircraft to avoid the area near the volcano. Most personnel were evacuated from the Drift River oil facility, but the 4 remaining crew members reported an increased flow of silty water in both the Drift River and Rust Slough, a small-capacity stream W of the oil facility into which much of the Drift River's flow was diverted by earlier mudflow-induced channel changes. The seismic station nearest the summit was destroyed, presumably by lightning, but seismicity decreased to low levels on flank stations 18 minutes after the onset of the eruption.

An overflight the next day revealed that at least 75% of the lava dome had been destroyed. Pyroclastic flows had swept across the glacier's Piedmont lobe, and had entered small, previously unaffected drainages. A pyroclastic surge had moved about 2500 m up the N wall of the steep canyon that leads from Redoubt's breached crater. Flood deposits, slightly less extensive than those of 2 January, were primarily dark brown sediment, locally containing abundant 1-2 m boulders. Water had flowed around an L-shaped levee on the upriver side of the oil facility, and a small amount of water had penetrated the containment dike protecting the southernmost of the facility's 7 storage tanks, but no oil was spilled. Aerial observations on 18 February showed that a small new dome, with a volume of less than 1 x 10⁶ m³, had begun to grow in the vent area. [See also 15:2].

Explosive episode, 21 February. Another vigorous explosion occurred on 21 February, beginning at [0032] and lasting for about [12] minutes. Winds carried the plume rapidly NE, and by 0300 satellite images showed its front 700 km from the volcano, where it was about 80 km wide. Pilots reported that the base of the cloud was at about 7.5-9 km altitude E of Anchorage, and NOAA estimated that its top was at about 9-11.5 km. Light ashfalls were reported at Kenai (80 km E of Redoubt), Nikishka (80 km NE), Hope (175 km ENE), and Girdwood (200 km ENE). [See also 15:2].

References. Alaska Volcano Observatory Staff, 1990, The 1989-1990 Eruption of Redoubt volcano: EOS, v. 71, no. 7, p. 265, 272-275.

Kienle, J., Dean, K.G., Garbeil, H., and Rose, W.I., 1990, Satellite surveillance of volcanic ash plumes, application to aircraft safety: EOS, v. 71, no. 7, p. 266.

Geologic Background. Redoubt is a glacier-covered stratovolcano with a breached summit crater in Lake Clark National Park about 170 km SW of Anchorage. Next to Mount Spurr, Redoubt has been the most active Holocene volcano in the upper Cook Inlet. The volcano was constructed beginning about 890,000 years ago over Mesozoic granitic rocks of the Alaska-Aleutian Range batholith. Collapse of the summit 13,000-10,500 years ago produced a major debris avalanche that reached Cook Inlet. Holocene activity has included the emplacement of a large debris avalanche and clay-rich lahars that dammed Lake Crescent on the south side and reached Cook Inlet about 3,500 years ago. Eruptions during the past few centuries have affected only the Drift River drainage on the north. Historical eruptions have originated from a vent at the north end of the 1.8-km-wide breached summit crater. The 1989-90 eruption had severe economic impact on the Cook Inlet region and affected air traffic far beyond the volcano.

Information Contacts: AVO Staff; SAB; W. Scott, CVO; R. Ferek, Univ of Washington; UPI; AP.

The first phreatic eruptions since July 1989 began in January, with episodes on the 7th (1215-1230), 12th (1100 and 1355), and 14th (1000, 1200, and 1232). The eruption at 1232 on the 14th sent water to 3-5 m and a nearby rockfall suggested that it may have been preceded by a small earthquake.

Intermittent 2-Hz tremor was recorded 30 December 1989-12 January 1990, and 5 hours of 1-Hz tremor, believed to reflect a deeper source, began on 10 January at 2300. Tremor amplitude increased ~3 hours before the 7 January eruption, but no changes accompanied the eruption, and geologists did not believe that the tremor increase was significant. A new episode of lake heating began around 5 January as Crater Lake's temperature rose from ~15°C on the 5th to 27° on the 11th.

When B. Scott flew over Ruapehu on 7 January at about 0915, a black slick, 100 m in diameter, covered the center of Crater Lake. The remainder of the lake was uniformly gray except for an area (50-100 m wide) of blue-green water extending from the NW through the NE quadrant. Later that day between 1215 and 1230, the first phreatic eruption ejected steam and water to 50-60 m, with a steam cloud 200 m in diameter. The activity turned the lake gray and formed a sulfur strand on the shore.

During 11 January fieldwork, sulfur strands up to 5 m from the lake's edge (~0.75 m above the current lake level) gave further evidence of the 7 January eruption. Lake temperatures of 25.3 and 26.8°C were measured at two locations. The lake was battleship gray and minor upwelling at the N vents formed a yellow slick. Upwelling over the main vent that produced a dark area strengthened by midafternoon, and at 1611 a 10-20-second audible burst of upwelling occurred.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The 110 km3 dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake, is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P. Otway, DSIR Wairakei.

Seismicity was at low levels during the first ten days of January, then the number and energy release of high-frequency events increased significantly. Swarms of high-frequency events, mostly centered in a zone ~6 km N of the crater at ~6 km depth, caused four energy release peaks (figure 36). A small number of low-frequency events were recorded during the month. Short pulses of low-energy tremor were not associated with ash emission. Deformation measurements showed no significant changes in January. Five SO₂ measurements during January yielded an average of 980 t/d, only about half that of the previous month.

Figure 36. Daily seismic energy release at Ruiz, January 1990. Low-frequency events are shown by a dashed line, high-frequency events by a solid line. Courtesy of INGEOMINAS.

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

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

Geologists noted several low-density ash ejections from Caliente Vent during 1 1/2 hours of observations beginning at 0945 on 3 January. The largest, at 1010, produced a gas/ash plume 1,000 m high. Subsequent episodes were smaller, ejecting plumes to a few hundred meters above the vent. A small lava flow that was emerging from Caliente Vent moved slowly down the dome's upper SE flank. The flow was a few tens of meters long, with associated rock avalanche deposits extending several hundred meters from its front. Recent avalanche deposits were also visible S and SW of the vent.

Caliente Vent, on the E side of Santiaguito dome, has been the site of continuous lava extrusion since 1975. A strong explosion from the Caliente Vent area on 19 July 1989 may have been the source of small lower stratospheric aerosol layers detected at several sites in August and September (see Atmospheric Effects, 14:8-9).

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: S. Halsor, Wilkes Univ; C. Chesner, Eastern Illinois Univ.

"Activity was at a low level throughout January, as it has been since July 1989. The summit crater emitted white vapour in weak to moderate amounts. The seismicity was also at a very low level, with only a few volcanic (B-type) events of very small amplitude/day. On the 4th, however, an earthquake (ML 4.2) originating 25 km away provoked the onset of a 10-hour swarm of small B-type events (~150) that ended abruptly after a string of a dozen larger events."

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

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/).