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/).

Regular plume emissions seen in satellite imagery and by aviators during March-May 2006 (BGVN 31:05) apparently ended in June, with the last reported activity being a pilot report of an ash cloud on 26 June that reached 3 km altitude. A report issued by the U.S. Geological Survey (USGS) on 7 December noted that the Alert Level was being lowered to Green and that seismic activity at Anatahan was very low during late November and early December, although diffuse steam-and-gas plumes were occasionally visible on recent satellite images or by aviators.

According to the USGS, seismometers recorded tremor starting on 24 February (UTC) that continued at high levels through 17 March. During that time, recorded tremor occasionally increased to much higher values. In addition, OMI satellite spectrometer data showed occasionally high amounts of sulfur dioxide over Anatahan. Tremor levels increased significantly starting at 1625 on 9 March (UTC) and continued for over 40 hours. As of 13 March the tremor bursts were infrequent, and some were high amplitude. In addition, a distinct gas plume was visible in Moderate Resolution Imaging Spectroradiometer (MODIS) imagery, suggesting increased emissions. On that day the Alert Level was raised to Advisory.

The MODIS flying onboard the Aqua satellite captured a view of the plume on 18 March 2007 as emissions continued. In the image, the volcanic plume headed SE, then changed direction slightly and trended towards for the islands of Saipan and Tinian. The same day MODIS acquired this image, the U.S. Air Force Weather Agency reported an odor of sulfur, which would also suggest the presence of vog (volcanic smog) on Guam, ~200 km SW of Saipan. USGS and Emergency Management Office air quality instruments on Saipan recorded a maximum 5-minute average of 959 ppb sulfur dioxide and 99 ppb hydrogen sulfide on 18 March.

As of 24 March, the USGS was reporting that tremor levels after 17 March had remained low at pre-24 February levels. The plume visible in MODIS imagery had also remained weak but distinct since 18 March. On 24 March the Alert Level was lowered to Normal, with an aviation color code of Green. No confirmed ash eruptions had occurred after 3 September 2005.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: Juan Takai Camacho and Ramon Chong, Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO-CNMI), PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/); Frank Trusdell, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3591, USA (URL: https://volcanoes.usgs.gov/nmi/activity/); U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA; NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

The 10-day-long eruption of Etna's Southeast Crater (SEC) in mid-July 2006 (BGVN 31:08 and 31:10) was considered by scientists at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) to represent a distinct phase of 2006 activity for Etna. They identified a very different phase when eruptive activity shifted to SEC's summit vent between 31 August and early 15 September 2006. The latter activity led to lava overflows and repeated collapse on SEC's E side. The seven eruptive activity episodes previously described (BGVN 31:10) have since been renumbered slightly, with Episode 1 taking place between 31 August and 16 September.

The following report was compiled from recent reports by Boris Behncke and Sonia Calvari, based on daily observations by numerous staff members of the INGV Catania (INGV-CT). This issue overlaps with our previous Bulletin reports and then goes on through the end of 2006.

Overview of the 31 August to 14 December eruption. Figure 117 indicates key vents and lava flows during the period 4 September-7 December 2006. It excludes lavas emitted during the short but intense final episode (Episode 20, 11-14 December 2006), but they did not significantly extend beyond flow margins shown here. The longest lava flows of the reporting interval reached ~ 4.7 km SE from their source vent (figure 117).

Figure 117. Map Etna showing lava flows and their corresponding periods of activity: (1) lavas from the summit and flanks of the SEC, 4 September-3 December 2006; (2) lavas from the 2,800-m vent, 13 October-7 December 2006; (3) lavas from the 3,050-m vent, 27 October-27 November 2006; and (4) lavas from 3,180-m vent, 8-27 November 2006. The capital letters indicate the most persistent eruption sources: (A) SEC summit; (B) 2,800?m vent; (C) 3,050-m vent; (D) 3,180-m vent; (E) 3,100-m vent (active between 30 November and 3 December 2006); and (F) the foundation crater of the 23 October 2006 activity (which developed a pit that was also active between 24 November and 7 December 2006). Courtesy of INGV-CT; Behncke, Branca, Neri, and Norini (2006).

Table 9 summarizes the 20 episodes of recent eruptive activity, as currently identified by the INGV staff. Note, however, that episode numbers have changed since discussed in BGVN 31:10. One earlier episode has been added (31 August-15 September). Former Episodes 1-7 as listed in BGVN 31:10 based on earlier INGV reports, have been renumbered to Episodes 2-8. Subsequent episodes (9 through 20) are the main subject of this report.

Table 9. List of eruptive episodes (1-20) at Etna as reported by INGV-CT for the interval 31 August-December 2006. "Former number" refers to the episode numbers stated in BGVN 31:10 but here revised. Geberal morning and afternoon times are indicated by am and pm, respectively. Courtesy of INGV-CT.

Episode 9. Although there were no real paroxysms of Strombolian activity or lava fountaining at the SEC during 26 October-4 November, clear pulses of activity occurred at the effusive vents at 2,800 and 3,050 m elevation, accompanied by ash emission or weak Strombolian explosions at the SEC. These events defined Episode 8, on 27 October, and Episode 29, which took place during 29-30 October. The clear pattern of distinct paroxysms from the SEC finally returned on 5 November and lasted through late that month, before the activity became again more continuous early in December.

Episode 10. Following one week of intermittent ash emissions and weak Strombolian activity on late 4 November, a new strong eruptive episode started at the SEC summit vent at 2004 on 5 November and continued with some fluctuations and intermittent ash emissions for the next 9.5 hours. Light ashfalls occurred over populated areas to the SE. At about 2147 on 5 November, the effusion rate increased at a vent at 3,050 m elevation at the S base of the central summit cone (C on figure 117) which had been continuously active since 27 October. A new lobe of lava traveled S of the summit cone complex across a flat area known as the Cratere del Piano.

An apparent increase in the effusion rate was also noted at the effusive fissure at 2,800 m elevation on the ESE flank (B on figure 117), with active lava lobes extending downslope. Lava effusion from the 3,050-m vent ended during the morning of 6 November, and for the following 48 hours, lava emission continued only at the 2,800-m vent.

Episode 11. Ash emissions from the summit of the SEC occurred on 8 November 2006, followed by vigorous Strombolian activity that continued until about 2200. Around 1600, lava started to flow from a new vent located in the saddle between the SEC cone and the adjacent main summit cone, at an elevation of ~ 3,180 m (D on figure 117). The lava reached the SW base of the SEC cone in a few minutes, where it bifurcated into several short lobes, the largest and westernmost lobe stopping at the E margin of the lava flow field from the 3,050-m vent. Lava from the 3,180-m vent had ceased flowing by about 1845, whereas spattering and lava effusion continued at the 3,050-m vent for some time. Spattering ended at that vent around 1930, but lava continued to flow for another 24 hours.

Episode 12. At 2100 on 10 November 2006, tremor amplitude rapidly increased. Bad weather hampered visual observations until 11 November, when it became evident that this episode was quite similar to its predecessor, with lava emission occurring from both the 3,050-m and 3,180-m vents. Strombolian activity from the SEC summit ceased at 1100 on 11 November. Lava emission from the 3,050-m vent continued until the following night, and the associated lava flow field grew mainly on its W side, with flow fronts descending to ~ 2,800 m. For the next five days, lava emission continued unabated from the 2,800-m-vent, whereas the SEC and all other vents remained inactive.

Episode 13. Following a sharp increase in tremor amplitude at 0500 on 16 November, vigorous ash emissions started at the SEC summit at 0507 and were gradually replaced by intense Strombolian bursts, marking the onset of this eruptive episode.

Very early during the episode, lava issued from the 3,180-m vent, forming a lobe ~ 100 m long before activity at this vent ceased.

Lava effusion from the summit started at 0615 on 16 November and triggered a series of rockfalls down the SE flank of the SEC cone, before the lava descended on the same flank. At 0626, brownish ash was emitted from a spot next to the effusive vent, and major rockfalls and avalanches started shortly thereafter. These originated at the S rim of what remained of the 2004/2005 collapse pit on the E flank of the SEC (see BGVN 30:01 and 30:12). Plumes rising from the descending avalanches contained both brownish ash and white steam. Avalanching was most intense between 0631 and 0640, after which the new lava flow rapidly descended the lower SE flank of the cone and began to extend beyond its base toward the area of the 2,800-m vent. At the same time, strong emissions of black ash marked the opening of another explosive vent next to the summit, and a third explosive vent became active in the same area. For the next several hours, the vents continued to eject ash and occasionally bombs, and to produce vigorous Strombolian activity.

At 0700 on 16 November emissions of white vapor occurred from the SE flank of the SEC cone; a few minutes later large rock avalanches started to descend that flank. Simultaneously a fissure began to open near the summit to downslope on the SSE flank, triggering local rockfalls and dust avalanches. This fissure initially propagated ~ 100 m downslope, then it temporarily stopped; but at 0720, it propagated another 150 m downslope. During the following 15 minutes, another fissure perpendicular to the earlier one cut SE across the flank, generating more rockfalls and dust avalanches. The resulting fissure system had the form of an inverted Y delimiting a block that was actively pushed outward by magma intruding into the cone's flank.

Lava began to issue from the lower end of the W branch of the fissure system at about 0810 on 16 November. At approximately the same time, the 3,050-m vent started to emit lava. By this time, the upper portion of the fissure cutting the SSE flank of the SEC cone had significantly enlarged and became a deep trench. Dense volumes of steam were emitted from this trench at 0831 and were followed a few minutes later by another series of rockfalls and avalanches. Direct observation from ~ 700 m showed that the most energetic of these avalanches resulted from the collapse of low fountains of gas and tephra at the lower end of the large trench. The avalanches and rockfalls lasted about 15 minutes, then a voluminous surge of lava issued from the lower end of the opening trench.

Over the next few hours this sequence of events (vapor emission?rockfalls and avalanches?lava emission) was repeated several times as the trench widened and propagated further downslope. During the few moments when steam and dust clouds cleared and the interior of the trench became visible, a cascade of very fluid lava was seen in the center of the trench. Apparently, the lava issued from a source high in the head wall of the trench, and at times spurted from the vent like a firehose.

At 1100 on 16 November, white steam plumes, rockfalls, and dust avalanches appeared high on the SE flank of the SEC cone, in the area where the summit lava flow was emitted. These phenomena marked a major collapse of the E wall of the trench, which eventually cut into the descending summit lava flow, diverting it into the trench. The original flow, which had descended immediately S of the 2,800-m vent down to ~ 2,600 m elevation, rapidly stopped, although lava continued to drain from the main flow channel and accumulated in a thickening lobe at the cone's base.

At about 1425 on 16 November, several vertical jets of black tephra shot upward from an area at ~ 150 m distance from the cone's base. These emissions were very distinct in color from the brownish dust clouds, which at the same time descended from the trench. The activity at the new site appeared to migrate rapidly both toward the SEC as dark plumes began to rise closer to the cone, while a ground-hugging plume of white vapor shot in the opposite direction. A few ten's of seconds later, very dense clouds of dark brown material began to appear at the base of the surging white cloud and formed a distinct flow that rapidly overtook the front of the white cloud while speeding toward SE. At the slope break along the W rim of the Valle del Bove (~ 2,800 m elevation), both clouds disappeared from view in weather clouds, but at the site where the activity had originated, a huge plume of white vapor soared skyward. White vapor continued to rise from the area and from the path of the white and dark brown clouds for more than 15 minutes.

Another explosive emission of white steam and dark brown plumes occurred at about 1455. Like the 1425 event, it generated ground-hugging clouds of steam and dark brown material, the latter again traveling faster. During the following hours, activity at the SEC gradually decreased, with several spectacular cascades of lava descending through the trench on the cone's SSE side. Steam explosions and rock avalanches occurred at the lower termination of the trench at 1525. Strombolian activity ceased at 1500 on 16 November, but lava emission continued until about midnight. This lava does not seem to have extended far from the base of the SEC cone, since investigation during the following day failed to reveal any fresh lava on top of the debris deposits emplaced during the major explosive events at 1425 and 1455. A minor lava flow was also fed from a new short fissure ~ 80 m E of the 3,050-m vent. During the evening a small lobe of lava was emitted from the accumulation at the SEC cone's base.

Fieldwork and aerial surveys during the two days following 16 November revealed that the 1425 and 1455 explosions and related volcaniclastic density currents (figure 118) had left two main types of deposit. One was of lobate shape and extended a few hundred meters from the source of the explosions to the SE, covering a footpath established by mountain guides to allow tourists to approach the persistently active 2,800-m vent.

Figure 118. One of the peculiar density currents at Etna that occurred during Episode 13, 16 November 2006. The photo was taken from the N side of the large 2002-2003 cone complex, ~ 1.3 km S of the SEC. Seen in the photo are strong emissions of dark gray ash from two vents at the summit (a third caused intense Strombolian activity, but not in the moment shown in the photo). A huge gash carved out of the near right side of the cone emitted a lot of white vapor, with lava flowing from its lower end, and a ground-hugging brownish ash cloud spilling downslope on top of the flowing lava. Photo courtesy of INGV-CT.

On the ground the deposit consisted of very fine grained reddish-brown ash made up almost exclusively of lithic fragments. To the N the deposit gradually thickened and larger clasts were found on its surface, some of which represented fresh magmatic material. Close to the 2,800-m vent, the deposit abruptly graded into a sort of debris flow rich in lithics but with up to 25% of fresh magmatic clasts. These latter showed a peculiar flattened-out morphology. Where this deposit overlay the tourist path near the 2,800-m vent it was 1.52 m thick. In one place the flow had surrounded a plastic-coated sign warning tourists to stay on the path. The plastic lacked evidence of strong heating, indicating that the flow was relatively cool at this point along its path.

Volcanic tremor amplitude began to increase during the late afternoon of 18 November and, during a helicopter flight at 1800, the 2,800-m vent showed vigorous spattering. Active lava from the vent traveled ~ 3 km to Monte Centenari. Bright incandescence was also noted within the 3,180-m vent during this overflight.

Episode 14. At 0400 on 19 November, Strombolian activity at the SEC occurred from 2 vents at the summit while lava flowed through the 16 November trench and divided into numerous braiding lobes on top of the debris deposited 3 days earlier. The longest lobe traveled along the prominent channel in the main debris flow, passing immediately to the S of the 2,800-m vent and extending to an elevation of ~ 2,600 m. This episode was much less violent than its predecessor and lacked the explosions, surges, and flows characteristic of that event. Strombolian activity continued until the late evening, while lava effusion ended early on 20 November. As during previous episodes, lava had also briefly issued from the 3,050-m and 3,180-m vents. In addition, a flow of a few meters in length started from another fissure that opened at ~ 3,200 m, on the saddle between Bocca Nuova and SEC. This upper flow merged with the flow coming out from the 3,180-m vent.

Episode 15. This eruptive episode at the SEC started at 1200 on 21 November 2006, but direct observations were thwarted by inclement weather through nightfall. At about 1500, a black ash plume was seen rising above the cloud cover to ~ 1.5 km above the summit. Light ashfalls occurred along the Ionian coast near Giarre and further N, while at Rifugio Citelli (~ 6 km NE of the SEC), ash deposition was nearly continuous.

After 1900, the cloud cover gradually opened, allowing direct views of the strong Strombolian explosions generating jets sometimes over 300 m high. Lava once more flowed through the 16 November trench on the cone's SSE flank toward the 2,800-m vent. Likewise, the 3,050-m and 3,180-m-vents reactivated, although the latter apparently ceased erupting early during the episode. Lava flowed from the trench until shortly after midnight on 22 November. Bad weather precluded observations until the evening, when all activity was again limited to the 2,800-m vent.

Episode 16. At 0219 on 24 November, there began ash emissions mixed with Strombolian explosions. These were recorded by the INGV-CT thermal camera in Nicolosi (~ 15 km S of the SEC) with a significant anomaly occurring at the SEC summit. Strombolian activity at 0320 was accompanied by voluminous ash emission, which formed a plume that rose ~ 2 km above the summit before being blown to SE.

Two particularly powerful explosions occurred at 0452 and 0455. The latter was followed by lava extruding from a vent presumably located within the 16-November trench. At around 0535, lava began to issue from the 3,050-m vent, forming a small flow on the W side of the lava flow field emplaced since 26 October. A second minor flow issued from another vent located ~ 80 m SE of the 3,050-m vent. Vigorous ash emission from the summit of the SEC caused light ashfalls over populated areas between Zafferana and Acireale (figure 119).

Figure 119. Dark ash plume rising from Etna's SEC during eruptive Episode 16 on the morning of 24 November, photographed from a helicopter provided by the Italian Department of Civil Protection (Dipartimento di Protezione Civile) during that day's particularly explosive episode. A small steam plume at left rises from the area of the 2,800-m vent. More diffuse gas emitted from active lava flows engulfs the photo's extreme left. Etna's other summit craters (Northeast Crater foremost, with Voragine and Bocca Nuova behind) are in the lower right corner of the image, showing normal degassing activity. View is approximately to the S. Courtesy of INGV-CT.

A fracture opened at about 0817 at the SSE base of the SEC cone, producing a violent explosion and a rock avalanche that descended at a speed of several ten's of km/h toward the Valle del Bove, following the path of similar avalanches that had occurred on 16 November. Lava effusion continued from vents at the cone's base, where mild spattering was observed. Upslope from the effusive vent at 2,800 m elevation, a second fracture formed and commenced spattering and lava emission.

During the early afternoon a change in the wind direction drew the plume from its earlier SE-ward course toward Catania and adjacent areas, forcing the closure of the Fontanarossa International airport of Catania. The activity began to diminish, and by 1530 all explosive phenomena ceased. For several more hours lava continued to issue from two vents at the SEC cone's base.

Late in the afternoon of 24 November, weak sporadic Strombolian explosions occurred from a pit located on the E flank of the SEC cone, which had formed during the 23 October eruptive episode (hereafter, '23 October pit' identified as F on figure 117). On 25 November this vent produced pulsating ash emissions that continued intermittently for the next two days.

Episode 17. At around 0410 on 27 November, eruptive activity occurred at the SEC and the thermal monitoring camera at Nicolosi began to record a significant thermal anomaly at the crater and a W-drifting ash plume. Visual observations were hampered by inclement weather. Around 0730, the thermal camera at Nicolosi disclosed lava emission on the W side of the SEC cone, possibly from the vent at 3,180 m elevation in the saddle between the SEC and the Bocca Nuova. About 45 min later, lava emission became evident at the cone's SE base. No further visual observations were available after 0845, but the tremor amplitude remained high until the afternoon, when a sharp drop indicated the end of this eruptive episode.

Bad weather persisted until early on 29 November when observers saw ash emissions from the '23 October pit.' These emissions became more intense after 0545, and the tremor amplitude began to increase rapidly during the late morning. Intermittent, weak Strombolian activity from the '23 October pit' was visible after nightfall; this became notably stronger shortly after 0100 on 30 November and reached its highest intensity around 0130, after which there was a notable decrease. Ash emissions occurred from the same pit at dawn and again from 1240 onward, producing low ash plumes.

Episode 18. At around 1600 on 30 November 2006, lava fountains began to rise from the 2,800-m vent. Two hours later the '23 October pit' emitted a dense ash plume, and Strombolian explosions reached up to 150 m above the vent. At 2045, a fissure opened at ~ 3,100 m elevation, venting spatter several ten's of meters high and releasing a short lava flow towards the 2,800-m vent. After about 10 min the effusion rate at this new fissure diminished, but lava continued to escape at a decreasing rate for ~ 1 hour. The '23 October pit' remained vigorously active for the next 5 hours, producing incandescent jets and a dense tephra plume.

The new fissure at 3,100 m elevation revived around 0115 on 1 December, with vigorous spattering and a new surge of similarly directed lava. At the same time, the '23 October pit' emissions strongly increased. Like on the evening before, the new fissure at 3,100 m elevation remained active only for a short time; lava emission ceased by 0200 on 1 December.

The 2800-m vent produced the largest lava flows during the entire period of activity, in this episode extending lava flows to ~ 1,500 m elevation on the Valle del Bove floor, to a distance of ~ 4.7 km from their source.

Between 1-3 December, the '23 October pit' remained active with nearly continuous emissions of ash interspersed with Strombolian activity. This was accompanied by the 3,100-m fissure emitting low fountaining and lava; lava flows from that fissure were generally short and did not extend far beyond the 2,800-m vent. The last observed activity at the 3,100-m vent occurred during the morning of 3 December. Ash emissions from the '23 October pit' continued for another few days but became progressively weaker; likewise the lava emission at the 2,800-m vent diminished gradually.

Episode 19. Weak Strombolian activity and ash emission occurred at the SEC on the afternoon of 6 December, evidenced by increased tremor, but the amplitude dropped rapidly to very low levels implying that the SEC ceased erupting late on 6 December. Minor lava emissions continued from the 2,800-m vent. On the morning of 8 December, no eruptive activity was visible at any of the numerous vents of the previous weeks. Following several days of very low tremor amplitude, it began to increase again late on 10 December.

Episode 20. Eruptive activity resumed around 0330 on 11 December 2006 from the '23 October pit' on the SEC, with Strombolian explosions documented by INGV-CT's monitoring cameras. Simultaneously, lava emission started from the area of the 2,800-m vent, forming a flow that slowly descended toward the Valle del Bove. Bad weather hampered observations during the following days, but occasional clear views revealed ash emissions from the '23 October pit.' In addition, there were voluminous lava emissions from the 2,800-m vents, feeding a broad lava flow adjacent the N margin of the lava flowfield produced from the same vent between mid-October and early December. The 2,800-m vents generated vigorous Strombolian explosions from two vents that built up a pair of large hornitos, and lava emissions came from a third vent located on the lower E flank of the larger, more easterly of the hornitos. No activity occurred from any other of the numerous vents that had been active during the previous weeks at the summit and in the vicinity of the SEC. Late in the afternoon of 14 December, a sharp drop in tremor amplitude indicated that the end of this final eruptive episode was imminent, and field observations made on the following morning revealed the absence of eruptive activity.

INGV considered Etna's 2006 summit eruptions during 14 July-14 December and made a rough estimate of erupted lava volumes. The total volume produced during those 5 months amounted to ~ 15-20 x 10⁶ m³.

There was a single, relatively small ash emission from Bocca Nuova on 19 March 2007, discharged without an associated seismic signal. This was followed ten days later by a brief episode of violent lava fountaining and tephra emission from the SEC. Details on that and subsequent activity will be reported in a future Bulletin.

References. Behncke, B., and Neri, M., 2006, Mappa delle colate laviche aggiornata al 20 Novembre 2006 (1 page PDF file on the INGV website) and Carta delle colate laviche emesse dall'Etna dal 4 Settembre al 7 Dicembre 2006 (Map of lava flow emissions at Etna from 4 September to 7 December 2006).

Behncke, B., Branca, S., Neri, M., and Norini, G., 2006, Rapporto eruzione Etna: mappatura dei campi lavici aggiornata al 7 Dicembre 2006 (Report of Etna eruption: map of lava flows up to 7 December 2006): INGV report WKRVGALT20061215.pdf.

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: Sonia Calvari and Boris Behncke, Istituto Nazionale di Geofisica e Vulcanologia-Catania (INGV-CT), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

Scientists from Simon Fraser and McGill universities conducted preliminary geophysical and geochemical field studies at Ijen (figure 4) between 13 and 26 August 2006. During this period, volcanic activity was low and restricted to persistent degassing of the solfatara in the SE part of the crater.

Figure 4. Photograph of the acid crater lake and solfatara (bottom left) in the active crater at Ijen, August 2006. View is from the E crater rim. Courtesy of G. Mauri.

Measurements of temperature and pH were made every morning during 14-19 August at four locations: the Banyupuhit River, ~ 5 km from the Banyupuhit River source, the acid lake in the summit crater, and the E shore of the crater lake. Temperatures of the Banyupuhit River were 16-20°C, always above atmospheric temperature by ~ 1-3°C; the pH was ~ 0.4. Lake temperatures varied between 31 and 43°C and the pH was -0.02. The color of the crater lake was generally homogeneous, although large black to brown linear patches, probably sulfur deposits from the solfatara, were observed on the turquoise-green surface. These ephemeral patches were of variable size (e.g. several ten's of meters long and a few meters wide) and moved across the lake during the course of the day, but were not always evident throughout the day. The area near the E shore appeared lighter than the rest of the lake, probably due to a spring at the bottom of the inner E slope.

Pipes driven into the fumaroles are used to extract gases for sulfur mining (figure 5). Temperatures measured 50 cm down into four of those pipes ranged from 224 to 248°C. These measurements almost certainly represent minimum estimates of the true temperatures due to heat loss along the length of the extraction pipes. After the gases had exited less than 50 cm from the pipes, temperatures had dropped below 120°C, the melting point of native sulfur.

Figure 5. Close-up view of the solfatara at Ijen with fumarole temperature of more than 220°C. Note pipes for extracting sulfur gases. Courtesy of G. Mauri.

A survey of sulfur dioxide (SO₂) fluxes made by a portable spectrometer (FLYSPEC) on 21 and 23 August along the SE rim of the crater consisted of seven and twelve walking traverses through the plume, respectively. The gas plume produced directly from the active solfatara near the lake surface rose buoyantly before flowing over the crater rim. During the first survey (conducted over a 2-hour period), the concentration-pathlength of the gas in the plume fluctuated between 1,000 and 2,500 ppm-m. The wind speed (measured by handheld anemometer at plume height) during this time averaged 6.1 m/s and the resultant SO₂ flux was therefore calculated to average 412 metric tons per day (t/d) with a standard deviation of 154 t/d. On 23 August, gas concentrations were somewhat lower, ranging between 500 and 2,000 ppm-m. The average wind speed during the survey period (2 hours) was 3.9 m/s and the resultant SO₂ flux averaged 254 t/d, with a standard deviation of 117 t/d. Based on this very limited survey, the flux of SO₂ was estimated to be 330 t/d.

Gravity surveys (Bouguer and dynamic) were conducted in the active crater and seven gravity stations were selected for future dynamic gravity monitoring. A digital elevation map was prepared (using digital photogrammetric mapping methods) to provide the spatial framework required for interpretation of the geophysical surveys.

The scientists also applied the self-potential (SP) method, also know as spontaneous potential, that measures electrical potentials developed in the Earth by electrochemical action between minerals and solutions with which they are in contact. SP mapping of the active summit crater showed two main hydrologic structures (figure 6). The first is a hydrogeologic zone on the E and NE rim characterized by a negative SP anomaly with a minimum at -100 mV (millivolts), an inverse SP/elevation gradient of -1.6 mV/m, and length of 1,500 m. This almost certainly represents inflow of meteoric water and groundwater.

Figure 6. Self-potential survey results shown on a topographic map of the active crater of Ijen, August 2006. All the SP data were referenced at the Banyupuhit River and at a spring on the inner E slope of the crater. Contour line intervals are 100 m. Courtesy of G. Williams-Jones.

The second structure is the main hydrothermal system located S, W, and N of the crater as well as in the southern inner slope of the crater, places where the surface expressions are solfataras. The SP maxima range between 48 and 60 mV and are located on the slope of the river below a dam on the outer W slope (+52 mV), on the N rim (+48 mV) and in the S part of the solfatara (+ 59 mV). Processing of the SP data along the crater profile by continuous wavelet transform (Mauri and others, 2006) shows that the hydrothermal fluid cells are near the surface (less than 200 m below the topographic surface) suggesting that the hydrothermal system is under high pressure with significant heat flux, as shown by the solfatara.

Reference. Mauri, G., Saracco, G., and Labazuy, P., 2006, Volcanic activity of the Piton de la Fournaise volcano characterized by temporal analysis of hydrothermal fluid movement, 1992 to 2005: AGU, Eos Trans, v. 87, no. 52, Fall Meet. Suppl., Abstract V51A-1653.

Geologic Background. The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the large 20-km-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the caldera rim is buried by post-caldera volcanoes, including Gunung Merapi, which forms the high point of the complex. Immediately west of the Gunung Merapi stratovolcano is the historically active Kawah Ijen crater, which contains a nearly 1-km-wide, turquoise-colored, acid lake. Picturesque Kawah Ijen is the world's largest highly acidic lake and is the site of a labor-intensive sulfur mining operation in which sulfur-laden baskets are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of cones forms an E-W zone across the southern side of the caldera. Coffee plantations cover much of the caldera floor, and tourists are drawn to its waterfalls, hot springs, and volcanic scenery.

Information Contacts: Guillaume Mauri and Glyn Williams-Jones, Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada (URL: http://www.sfu.ca/earth-sciences.html); Willy (A.E.) Williams-Jones, Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada (URL: http://www.mcgill.ca/eps/); Deddy Mulyadi, Center of Volcanology and Geological Hazard Mitigation (CVGHM), Diponegoro 57, Bandung, Jawa Barat 40122, Indonesia (URL: http://vsi.esdm.go.id/).

After a year of quiet following ash ejections from Canlaon in May 2005 (BGVN 30:06), the Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported that a new period of activity began on 3 June 2006. In total, twenty-three ash ejections occurred between 3 June and 25 July 2006. These outbursts were all water-driven in nature, characterized by emission of ash and steam that rose up to 2 km above the active crater. The prevailing winds dispersed ash in all directions. The seismic network, however, did not detect significant seismic activity before or after the ash emissions, supporting the idea that the explosions were very near-surface hydrothermal events.

Four explosive episodes that occurred over the days 3, 10, and 12 June ejected mainly steam with some ash, and affected only the summit crater and upper SW slopes. The event at 1430 on 3 June sent dirty white to grayish steam 800 m above the summit. The activity was observed until 1445 when thick clouds covered the summit. Another emission started at 2316 on 10 June and lasted until 0030 the next morning. The plume was estimated to attain heights of 700-1,000 m before drifting SW. After the ash emission, moderate to wispy steam plumes escaped, to maximum heights of 600 m above the summit. Another steam-and-ash episode during 0515-0535 on 12 June caused a plume to rise about 600 m before drifting SW. After the ash emission, generally weak to moderate steaming to a height of ~ 400 m returned. Plumes rose 600-1,000 m and drifted SW; ashfall was confined to the upper slopes. This new period of low-level unrest prompted PHIVOLCS to raise the hazard status to Alert Level 1 on 12 June, suspending all visits to within 4 km of the summit.

Three small steam-and-ash emissions without recorded seismicity occurred again between the afternoon of 13 June and the morning of the 14th. The grayish steam clouds rose ~ 900 m above the active crater and drifted NE and NW. Only traces of ash were observed over the N upper slope. An explosion from 0845 to 0924 on 14 June produced an ash and steam cloud, which rose up to 1.5 km above the summit and drifted N, affecting mainly the upper slopes. Voluminous grayish steam plumes were then seen rising up to 1.5 km above the summit crater after 1640 through the next morning. The seismic network detected only two low-frequency volcanic earthquakes. Kanlaon City proper experienced light ashfall starting at 1630 on 15 June after voluminous dirty white steam was observed rising 1.5-2 km above the summit crater a few hours earlier (from 1346 to 1520). As of 1800, ashfall was still wafting through the city.

The character of this episode changed on the afternoon of 19 June when two episodes of steam-and-ash emission sent clouds 600 m above the crater that drifted SW. Weak to moderate steaming was observed after the second explosion and during the morning observation on the 20th. The initial explosion was recorded by the Cabagnaan station's seismograph as low-frequency tremor with a duration of 13 minutes. One minute of tremor was recorded at the time of the second explosion. No precursor seismicity was detected. Traces of ashfall and sulfurous odors were reported at Barangay Cabagnaan proper in La Castellana. During the 24 hours before 0730 on 20 June, the seismic network detected two cases of low-frequency tremor and three small low-frequency volcanic earthquakes.

An additional six short steam-and-ash emissions took place during 21-25 June. The explosions produced grayish columns that rose 800-1,500 m above the crater and drifted NW, SW, and SSW. Volcanic seismicity was not associated with these events except for a single harmonic tremor before the emission on 25 June. Light ashfall was reported at Upper Cabagnaan in La Castellana. Weak to moderate steaming was observed after the explosions.

Steam-and-ash emissions were not reported again until the afternoon of 2 July. The grayish steam clouds then rose to heights of up to 1,000 m above the active crater and generally drifted NW. Another episode on the morning of 3 July produced a column to a height of 500 m above the crater. The seismograph at Cabagnaan recorded ten volcanic earthquakes while the seismograph at Sto. Bama near Guintubdan in La Carlota City recorded eight local seismic events during the 24 hour observation period that included these emissions.

An explosion-type earthquake with a 10 min, 25 sec duration was recorded at 0426 on 23 July, but cloud cover prevented observations. Traces of ash fell up to about 9 km ENE from the crater, affecting Barangays Pula, Malaiba, and Lumapao. When clouds cleared during 0630-0800 on 25 July, ash-laden steam clouds were seen rising up to 300 m above the crater drifting ENE and SE. Light ashfall was experienced at Gabok, Malaiba, and Lumapao of Kanlaon City, about 9 km from the crater. This emission was not reflected on the seismic record as only two small volcanic earthquakes were detected during the preceding 24 hours. Dirty white steam was observed on the morning of the 26th rising to a maximum of 100 m above the crater.

Explosions ceased after 25 July, and other activity, such as weak steaming and minor seismicity, showed a general trend towards quiescence. After three months with no further explosive emissions, on 2 November 2006 PHIVOLCS lowered the hazard status from Alert Level 1 to Alert Level 0, meaning the volcano has returned to normal conditions.

Geologic Background. Kanlaon volcano (also spelled Canlaon), the most active of the central Philippines, forms the highest point on the island of Negros. The massive andesitic stratovolcano is dotted with fissure-controlled pyroclastic cones and craters, many of which are filled by lakes. The largest debris avalanche known in the Philippines traveled 33 km SW from Kanlaon. The summit contains a 2-km-wide, elongated northern caldera with a crater lake and a smaller, but higher, historically active vent, Lugud crater, to the south. Historical eruptions, recorded since 1866, have typically consisted of phreatic explosions of small-to-moderate size that produce minor ashfalls near the volcano.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, Univ. of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/).

Moderate activity occurred at Langila between January and March 2006 (BGVN 31:05), with eruptive activity accompanied by a continuous ashfall, rumbling, and weak emissions of lava fragments. Since March 2006, activity has continued at Crater 2.

According to the Darwin Volcanic Ash Advisory Center (VAAC), eruptions at Crater 2 occurred in August 2006 and from October 2006 through March 2007, with explosions of incandescent lava fragments, roaring noises at regular intervals, and continuous emissions of gray-to-brown ash plumes. Plumes generally reached 2.3-3.3 km altitude, although on 31 October a small ash plume rose to an altitude of 4.6 km. Ash plumes were occasionally visible on satellite imagery. During October and through the first part of January 2007, plumes generally drifted N, NW, W, WNW, and NE; between the end of January and March, plumes drifted SE and SW.

Thermal anomalies detected by MODIS instruments on the Terra and Aqua satellites were absent after 2 January 2006 until 21 July 2006. The same system (the HIGP Thermal Alerts System) identified anomalies again on 24 and 31 October, 12 and 21 November, 16 and 27 December 2006, 6 January, 8 March, and 18 March 2007.

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: Rabaul Volcano Observatory (RVO), PO Box 386, Rabaul, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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 rarely visited Lastarria has not erupted in historical time, but has displayed strong fumarolic activity (figure 1) for at least 67 years. This is the first Bulletin report ever issued on this volcano; it presents new images of the steaming edifice. On 2 February 2007, a group of scientists from the Servicio Nacional de Geología y Minería (SERNAGEOMIN) and the Corporación Nacional Forestal (CONAF) observed the fumarolic activity from a distance. The scientists were on a field trip to count flamingos and other Andean birds at Ramsar sites. The Ramsar Convention on Wetlands (http://www.ramsar.org/), named after a city in Iran, is an intergovernmental treaty that provides the framework for national action and international cooperation for the conservation and wise use of wetlands and their resources. The group noted steam plumes blowing NE at mid-day from ~ 47 km SW. Fumarolic gases were again seen, from ~ 35 km WSW, slowly moving down the W slope of the cone (figure 2). Steam plumes were seen intermittently throughout the afternoon.

Figure 1. Lastarria imaged by satellite on an unknown date. Fumaroles can be seen on the SW and SE crater rims. Crater width (E-W) is ~600 m. Courtesy of Google Earth and DigitalGlobe.

Figure 2. Photograph showing Lastarria from ~35 km WSW, 2 February 2007. Fumarolic gases can be seen rising above the cone and moving down the W flank. Courtesy of Héctor Cepeda.

Jose Antonio Naranjo, who has worked at Lastarria since 1983, is very familiar with its spectacular fumarolic activity. He confirmed that the observations of February 2007 reflect Lastarria's normal intense fumarolic emissions. Such activity has continued since at least 1940, when observed by Danko Slozilo. Naranjo noted that in 2007 he saw the same fumarole locations as those he observed in 1983 and in October 2002 (figure 3). The temperatures of these fumaroles were unchanged between 1983 and 2002.

Figure 3. Photograph of the Lastarria cone showing the lava dome overlapping the N crater rim and fumaroles along the rim, October 2002. View is from the N. Courtesy of Jose Antonio Naranjo.

References. Naranjo, J.A., 1985, Sulphur flows at Lastarria volcano in the North Chilean Andes: Nature, v. 313, no. 6005, p. 778-780.

Naranjo, J.A., 1986, Geology and evolution of the Lastarria volcanic complex, north Chilean Andes: Unpublished M Phil. Thesis, The Open University, England, 157 p.

Naranjo, J.A., and Francis, P., 1987, High velocity debris avalanche at Lastarria volcano in the north Chilean Andes: Bull. Volcanol., v. 49, p. 509-514.

Naranjo, J.A., 1988, Coladas de azufre de los volcanes Lastarria y Bayo en el norte de Chile: reologia, genesis e importancia en geologia planetaria: Revista Geologica de Chile, v. 15, no. 1, p. 3-12.

Naranjo, J.A., 1992, Chemistry and petrological evolution of Lastarria volcanic complex in the north Chilean Andes: Geol. Magazine, v. 129, p. 723-740.

Geologic Background. The NNW-trending edifice of 5706-m-high Lastarria volcano along the Chile-Argentina border contains five nested summit craters. The youngest feature is a lava dome that overlaps the northern crater rim. The large andesitic-dacitic Negriales lava field on the western flanks was erupted from a single SW-flank vent. A large debris-avalanche deposit is found on the SE flank. Recent pyroclastic-flow deposits form an extensive apron below the northern flanks of the volcano. Although no historical eruptions have been recorded, the youthful morphology of deposits suggests activity during historical time. Persistent fumarolic activity occurs at the summit and NW flank, and sulfur flows have been produced by melting of extensive sulfur deposits in the summit region.

Information Contacts: Héctor Cepeda and Margaret Mercado, Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile; Jorge Carabantes, Cristian Rivera, Eric Díaz, and Juan Soto, Corporación Nacional Forestal (CONAF), Chile; Jose Antonio Naranjo, Volcano Hazards Programme, Servicio Nacional de Geologia y Mineria, Chile.

The previous Bulletin report (BGVN 31:03) discussed an unusually vigorous eruption during late March and early April 2006. This report revisits the March 2006 eruption and continues to the beginning of 2007, thanks in large part to the reports of many observers posted by Frederick Belton on his website.

March-April 2006 eruption. The March 2006 eruption was initially characterized in the Arusha Times as being more massive than the one in 1966. However, Celia Nyamweru noted that subsequent information indicated that the 2006-2007 event was smaller than the 1966-1967 event. During the March-April 2006 event, the volcano was reported to have emitted "red-hot rivers of molten rock and scalding fumes." Ibrahim Ole Sakay, a resident of Ngaresero (-1.3 km from the volcano) reported that the eruption began on the night of 24 March 2006, continuing the following day, and marked by "rumbling and spitting lava for more than a week."

Several news sources, including CNN, reported that on 30 March 2006 the eruption led to the evacuation of up to 3,000 people from several villages, some quite distant from the volcano. As of 5 April, there was a great deal of contradictory information about this eruption. Belton noted that news media and people distant from the volcano reported explosions, but that people living and working nearby reported a "smoke column" followed by a very large lava flow down the W flank, but no explosions or ash. All evidence now indicates that there was no explosive activity and that this was only a very large eruption of lava.

Visitor observations. Belton posted reports from a number of persons who observed the volcano before and shortly after the March 2006 eruption. One observer, Christoph Weber, drew a new map of the crater in February 2006 (figure 91). Belton visited the volcano in August 2006 and provided (figure 92) an update to Weber's February map as well as a photo of the recent changes (figure 93). The following text and table 11 were taken from observations by visitors, as reported by Belton on his website.

Figure 91. Sketch map showing features in Ol Doinyo Lengai's active crater as of February 2006 (i.e., before the March-April 2006 eruption). Courtesy of Christoph Weber.

Figure 92. Sketch map showing features in Ol Doinyo Lengai's active crater as of August 2006 (i.e., after the March-April 2006 eruption). Courtesy of Frederick Belton, based on update of the map by Christoph Weber.

Figure 93. Photo of Ol Doinyo Lengai's active crater as seen 7 August 2006, looking N from the S rim. To elucidate recent changes in the crater, see maps in figures 91 and 92 [and earlier maps and photos from BGVN 31:03 (March 2006), 30:10 (October 2005), and 30:04 (April 2005)]. The tall cone is T49B. Slightly to its front and to the right, note the large collapse zone that grew in the spot where cones T56B, T58B, and T58C once stood. The dark lava on the right (E side of crater) was believed to have erupted around 20 June 2006 from T37B. The dark lava to the lower left probably dates from early April 2006. Although it appears dark and fresh here, it had already been highly weathered and easily crumbled into powder if touched. Courtesy of Frederick Belton.

Table 11. Summary of visitors to Ol Doinyo Lengai and their brief observations (from a climb, crater overflight photos, or from the flank) from January 2006 to February 2007 (see figures 91 and 92 for crater features). Detailed observations prior to March 2006 were reported in BGVN 31:03; most of the later observations were detailed in the text. Courtesy of Frederick Belton.

When Rick and Heidi Rosen flew over on 13 March 2006, there appeared to be no activity and many lava flows had turned white. Several flows still contained dark areas, their surface color indicating that they were then only a few days old. Narrow flows extended in all directions from the central cone mound, and a small flow originating on the upper part of T49B extended across the NW crater rim overflow and a short distance down that flank. Lava also appeared to have reached the E crater rim overflow. Most of the flows appeared to have been subject to the same amount of weathering, except for the flow down the NW flank, which looked more recent.

After a 1 April 2006 climb, Matt Jones reported that there was a fairly large lava flow down the W flank. Residents in nearby Ngaresero village and the Ngorongoro District Commissioner said that activity started on 27 March 2006. At the summit in the dark, Jones noted no glowing from lava emissions. The new eruption left a big hole to the left of the climbing path to the crater that emitted a plume of steam. On the following day, abundant steam came from the hornitos and from fissures all around the rim. Two central hornito's had been blown open relatively recently.

According to people interviewed by Amos Bupunga, who visited later, lava had flowed out on 29 (30?) March 2006 and extended to ~ 2 km from a Maasai family village (boma) at Ol Doinyo Lengai's foot. Bupunga heard that residents did not vacate their village. In the crater, lava of unstated ages covered almost all of the NW to SE regions of the crater to a depth of 2 m. At its outlet over the crater's W rim, one or more lava flows was 2.5 m deep and 3 m wide.

On 4 April 2006, Michael Dalton-Smith flew over and observed a very large lava flow that traveled over 1 km down the mountain and into a gorge. He reported that a bush pilot observed a 30 March eruption consisting of a fountain and lava flow, without an ash cloud. Local pilots also noted that on 4 April the eruption stopped. No steam was seen, nor any evidence that the large lava flow was still hot or moving.

On 5 April, Dalton-Smith drove to the foot of the volcano and saw a huge lava fountain coming from one of the summit hornitos. The fountain stopped before he could photograph it, but from the previous overall structure of the hornitos, it appeared that a new one had been building. All hornitos emitted black plumes, and there appeared to be a lake at the summit about the size of the large hornito.

Amos Bupunga visited the crater on 7 or 8 April 2006, and, in addition to the above-mentioned information he gathered relevant to 29 or 30 March, he saw that the fresh lava coming to the surface remained inside the new lava lake.

Table 12 summarizes annual measurements from 2000 to 2006 of widths of lava flows leaving the crater at various rim overflows. The number and size of the overflows have generally grown, although the width of the NW overflow has remained 135 m since 2002.

Table 12. Annual crater rim overflow measurements taken during 2000 to 2006. Stated values are the width of the crater outflow area at the crater rim. Courtesy of Frederick Belton.

Aerial photos made on 1 April by Dean Polley showed that there had been a huge collapse of the upper parts of hornitos T56B and T58B, which merged together and probably contained a lava lake (figure 94); as noted earlier, photos by Rick Rosen showed that the collapse had not occurred by 13 March 2006.

Figure 94. Aerial photo of Ol Doinyo Lengai, taken 1 April 2006, viewing the central crater looking toward the S. The very recent collapse of hornitos T56B and T58B, which appear to have merged together, is evidenced by the depressions sharp edges. Courtesy of Dean Polley.

Polley's 1 April photos show that at the SE base of T58C (just behind the collapse pit) there appeared to be a new vent with prominent lava channels leading away to the SE. Lava from this vent seemingly filled up the low lying areas in the S crater, spilled across the W overflow and down the flank. A similar eruption probably occurred again on 3 April. It was likely that a large amount of the lava was flowing through buried tubes, typical during an eruption of long duration.

From 6-11 May 2006, Jean Perrin and four others from Reunion Island visited ol Doinyo Lengai and reported an absence of active lava flows but small gaseous emissions at some hornitos and plausible rare explosions (which may have also been the sound of rocks collapsing). Due to the very large collapse mentioned above, hornitos T56B, T58B, T58C, and T57B no longer existed. No lava lake activity was seen or heard in the collapsed area. The crater floor was covered with a thick ash layer and looked considerably different than before.

On 12-13 May 2006, Tobias Fischer reported seeing no activity, but the crater was filled with old lava much higher than what was seen the previous year. A very large collapsed cone with sharp rugged edges was noticed in the T58B area. Sulfur dioxide (SO₂) flux was measured using a differential optical absorption spectrometer (mini-DOAS), but the fluxes measured were low, the same as in 2005. Sampled lava were later analyzed and their carbonatite compositions were identical to 2005 lavas. Some possible carbonatite tephra was also sampled. Coming from deep inside the volcano there were discrete rumblings lasting for several seconds and up to 10 seconds; these repeated up to 15 times per hour.

Matthieu Kervyn reported that during his visit to the volcano, 21-28 May 2006, he noted no eruptive activity at all except for fumaroles from cracks in the rim and from most of the hornitos (especially in the afternoons). The collapse pit in the middle was enlarging through rim collapse. Visual inspection showed that the collapse pit might soon cause instability of the very high T49B cone. Maasai guides were also expecting T49B to collapse soon. There were some tremors felt several times per hour within the N crater, as if rocks were collapsing beneath the crater.

During 13-15 July 2006, Steve Beresford, Michelle Carey, and Mark and Rene Tait visited the active crater. Activity at that time was limited to abundant fumarolic degassing from the crater rim and central hornitos. They noted a recent (several days old) major lava flow in the SE part of the N crater, its path emanating from the S end of the lava lake at the crater dominating the central N crater. The pre-March 2006 morphology of the N crater had been the scene of a prominent central hornito cluster (figure 91). During 13-15 July the group found much of that cluster destroyed, with the dominant feature on 13 July being a wide (120 x 120 m) crater hosting a recently active lava lake. The hosting crater's S margin was very unstable and periodic collapse of the crater walls was common over the two days of observation. The crater's N margin was marked by a steep collapse scarp in the T49B hornito. Talus breccia from this scarp partially infilled the N part of the lava lake. Numerous scarp collapses (associated with abundant seismic activity) highlighted the ephemeral nature of the current crater/lava lake outline. Marks around the lava lake recorded former high-stands of lava during recent months. SE- and S-draining tubes were present, both testifying to the lateral draining of lava.

The above group saw the S tubes that emanated from the central lava lake appeared to connect to the T37B hornito. The majority of the lava flow of the March-April eruption appeared to have come from this hornito. The reduction in lava lake level and southerly flow direction suggested that the lava lake dramatically drained to the S and may have provided the lava that escaped in the T37B eruption. Pyroclastics surrounding T37B suggested that early mild Strombolian/Hawaiian style activity preceded or accompanied effusion, as was typical of recent N crater volcanism. The lava flow itself was dominantly slabby to spiny pahoehoe with many aa and frothy pahoehoe breakouts along the E margin. This flow appeared similar to an inflated slabby pahoehoe flow field. Very small toothpaste pahoehoe flows emanated from the slabby pahoehoe flow front.

August 2006 map and its interpretation. During 4-8 August 2006, Fred Belton and Peter and Jennifer Elliston camped on the volcano. The visitors found degassing cones and fumaroles; no lava erupted. Occasional rockfalls occurred in the collapse zone.

To explain the August map and field relationships (figure 21), Belton and the visitors provided the following synopsis of the most recent activity and collateral observations. Some of the following revisits observations already discussed, but other points are new to this report and convey the significance of this stage where substantial lava flows descend out of the summit crater.

Prior to their arrival, lava had flowed from T37B and CP2 and spread over the SE part of the crater floor. Thermal anomaly satellite sensing data from MODIS, analyzed by Matthieu Kervyn, indicated that the eruption probably occurred on 20 June (UTC). An Aster image from June 29 shows new dark lava in the SE part of the crater. During the eruption, lava lakes existed in CP1 and CP2 and lava flowed from CP2 and T37B and covered most of the crater floor lying between T45, T37B, T37, and the crater rim. Lava also flowed across the E overflow and down the flank. The flow was composed of at least two distinct, differently weathered lavas that may have occurred within days or hours of one another. The first eruption phase produced a fine-textured aa no more than 40 cm thick and was the more extensive of the two flows, covering a large area of the crater floor and crossing the E rim overflow. The second phase produced a less extensive but much thicker flow, nearly 2 m deep in places, that stopped before reaching the crater rim or the E overflow. It consisted of broken, ropy pahoehoe slabs. Lava from this eruption and possibly from prior activity completely covered cone T24, which was no longer visible. The collapse of the E half of T46 has revealed an interior cave containing long thin stalactites.

Since March 2006, ~ 8,000 m² of the central crater floor had collapsed. Photographs by several observers indicated that the collapse began just prior to or during the eruption of late March through early April 2006 and continued as an ongoing process. The current collapse zone consisted of two collapse pits, designated CP1 and CP2 in figure 92, plus a fractured area between the two pits and S of CP1 where large sections of terrain had broken away from the crater floor proper and subsided by 1-3 m. The displaced sections had tilted at various angles and were separated from one another and the crater floor by 1- to 2-m-wide fissures. The fissures contain numerous large boulders composed of lavas that were altered by weathering and then lithified.

Cones T58C, T56B, and T58B had collapsed into CP1 and were completely gone. Further enlargement of CP1 claimed the SW half of T57B, the SE base of T49B, and the E half of T46. The SW half of T37B had collapsed into CP2. Tall cone T49B, visible from the Rift Valley floor, appeared likely to collapse in the near future. Failure of its SE base resulted in a talus slope that spilled out onto the floor of CP1. CP1 and CP2 were each ~ 10 m deep with respect to the lowest point on their rims. CP2's floor and E side were talus-covered, but CP1 had a bi-level floor of slabby pahoehoe lava, the surface of a frozen lava lake. A wide lava channel exited CP2 to the SE, near the base of T37B, indicating that it contained a lava lake, which had overflowed onto the crater floor during the March-April eruption. From the lowest point of CP2, a tunnel sloped upward to CP1, connecting the pits. The floor of the tunnel was covered by talus from its unstable walls and roof.

A prominent open lava channel, with a smaller channel diverging from it, led SSE from CP1 past T37 and then wound W and NW to the W overflow, recording the route of the lava that flowed from T58C to Ol Doinyo Lengai's base during the exceptionally strong discharges of roughly 25 March-5 April 2006. Near CP1 the channel's path had thermally eroded to a depth of ~ 3 m, and remained nearly closed at the top. An overhanging ledge contained stalactites. The channel became indistinct in the S part of the crater, but regained prominence near the W overflow, where in places it attained a width of ~ 5 m and depth of ~ 2.5 m. A large chasm just below the W overflow carved by thermal erosion extended ~ 20 m down the flank, with a depth of 5 m and a width of ~ 12 m. Its sides appeared unstable and prone to collapse. Immediately downslope of the chasm, the lava entered an existing gully and could not be easily seen again until the slope moderated near the base of the volcano, at which place the lava chilled only a few meters from the climbing track. From there its path continued into an aa field at its terminus, ~3 km from the summit.

The terminus of the flow lies within 1 km of a Masai boma on the flank, the only habitation evacuated as a result of the eruption. The lava channel near the climbing track was ~ 3 m high and at one point formed a tumulus ~ 5 m in height (tumulus, an elliptical, domed structure formed on the surface of a pahoehoe flow on flat or gentle slopes, created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward). A video of this segment of the lava flow (made during the eruption viewed from the escarpment to the W) showed a rapid, turbulent flow with blobs of lava becoming airborne. The lava near the base of Ol Doinyo Lengai had a dark gray-black coloration and appeared less weathered than might be expected based on its age of 4 months.

Lava flows from the same eruption also covered much of the S part of the crater floor to a depth of at least 2 m. Based on the indistinctness of the main lava channel in the S part of the crater, it appeared likely that the low areas of the S part of the crater were filled by lava prior to spilling over the W crater rim overflow and down the flank. Hornitos T27 and T30, formed in 1993, were completely covered by this flow.

Satellite IR data for 2006 (MODIS and MODLEN). Remote thermal monitoring by satellite using an algorithm called MODLEN was analyzed by Matthieu Kervyn. The analysis suggested an increase in volcanism around 11-13 March 2006. MODLEN is the name of a semi-automated algorithm using MODIS night-time imagery to record thermal activity and detect abnormally high-intensity eruptive events. It is built upon MODVOLC, an algorithm developed by the University of Hawaii, which provides a fully-automated global-coverage hot-spot-detection system. MODLEN was specifically tailored to Ol Doinyo Lengai's low-temperature and small scale eruptive activity (Kervyn and others, 2006a and 2006b).

Table 13 shows the MODIS/MODVOLC thermal anomalies for the year 2006. MODIS thermal alerts on 25, 27, and 29 March 2006 indicated a small but intense area of activity, possibly in the form of a large lava lake. A thermal alert at about 2255 on 29 March was consistent with eye-witness reports and air photos by Polley (mentioned above). A thermal alert for a large area of the flank on 3 April probably indicated a second lava flow to the base of the volcano.

Table 13. MODIS thermal anomalies detected at Ol Doinyo Lengai during 2006. Courtesy of Hawai'i Institute of Geophysics and Planetology.

Kervyn reported that the MODIS algorithm indicated a strong thermal anomaly in the crater on 20 June 2006 (table 13). He interpreted this anomaly as likely thermal signatures from new lava in the SE part of the crater and the lava lakes that later observers reported. No thermal alerts were detected through the remainder of 2006.

Early 2007 observations. Tom Pfeiffer reported that during a visit from 31 January-2 February 2007, no lava erupted from the summit vents. According to local Masai guides, the form of the central area of the crater with the large collapse pit near the tall hornito T49b appeared unchanged since the summer of 2006. From an open vent in the NE corner at the bottom of the pit at the base of the hornito, continuous sounds of loud sloshing suggested mobile lava in some caverns just beneath that area, an assumption confirmed by the glow of lava visible at night from a second, smaller vent located about 30 m S of the large vent in the base of the collapse pit. One guide confirmed he had seen spattering of lava from this vent some two weeks earlier. In addition to the loud sound of moving lava underground, a constant, deep rumbling could be heard from the ground, resembling the sounds of very distant thundering. It was strongest in the NW area of the crater between the collapse pit and the fissure vents of the March 2006 lava flow.

References. Kervyn, M., Harris, A.J.L., Mbede, E., Jacobs, P., and Ernst, G.G.J., 2006a, MODIS thermal remote sensing monitoring of low-intensity anomalies at volcanoes: Oldoinyo Lengai (Tanzania) and the MODLEN algorithm: Geophysical Research Abstracts, v. 8, p. 03887.

Kervyn, M., Harris, A.J.L., Mbede, E., Jacobs, P., and Ernst, G.G.J., 2006b, MODLEN: A semi-automated algorithm for monitoring small-scale thermal activity at Oldoinyo Lengai Volcano, Tanzania: International Association for Mathematical Geology XIth International Congress, Université de Liège, Belgium, 3-8 September 2006, paper SO9-15.

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: Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA (URL: http://oldoinyolengai.pbworks.com/); Christoph Weber, Volcano Expeditions International, Muehlweg 11, 74199 Untergruppenbach, Germany (URL: http://www.v-e-i.de/); Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617, USA (URL: http://blogs.stlawu.edu/lengai/); Matthieu Kervyn, University of Ghent, Geology Department, Ghent, Belgium (URL: http://homepages.vub.ac.be/~makervyn/); Arusha Times, Arusha, Tanzania (URL: http://www.arushatimes.co.tz/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/).

Volcanic activity from Lopevi has continued intermittently since November 1998 (BGVN 24:02). Though there are no permanent residents on the island, which is known as Vanei Vollohulu in the local language, the nearby islands of Epi (~ 17 km SW) and Paama (~ 10 km WNW) are heavily populated. Ambrym, another active volcanic island 18 km NNW, is also at risk of ashfall from Lopevi. Ash plumes during active periods are often reported by aviators, and thermal anomalies are frequently detected by the MODIS instrument on the Terra and Aqua satellites. Ash plumes and lava flows have most recently been reported in January, May, and July 2006.

Activity during 2006. Vertical plumes were observed by aviators reaching altitudes of 2.1-2.4 km on the morning of 24 January, and ~ 2.7 km the next morning. Further advisories issued by the Wellington VAAC reported that "smoke" plumes with a "steady rate of growth" rose to ~ 2.1 km on the morning of 26 January and drifted S. Lava flowing down the S flank was also reported on the 26th.

Based on information from a pilot report, the Wellington VAAC reported that on 7 May 2006 a small ash plume was visible below an altitude of ~ 3 km and an active lava flow was observed. On 10 May, a slow moving plume reached 3 km altitude. The next day a plume rose to 4.6 km and trended SE. During 12-13 May, the plume heights lessened to 3 km as the eruption vigor reportedly decreased. News media also reported heavy ashfall on Ambrym and Paama from an eruption on 15 May. An official spokesperson for Vanuatu's National Disaster Management Office reported no new ashfall during 17-22 May.

A situation report from the UN Office for the Coordination of Humanitarian Affairs (OCHA) noted that the May eruptive episode caused heavy ashfall on Paama and SE Ambrym, affecting water supplies and crops. The total population of Paama is 1,572, comprised of 23 villages and 511 households. On the island of Paama, the two main cash crops of vanilla and pepper were damaged badly. On both islands, staple foods such as wild yams, kumala, taros, bananas, and coconut trees were either damaged or destroyed. Residents experienced health problems caused by the consumption of contaminated food and water as well as the inhalation of ash. Head pain, skin infections, diarrhea, vomiting and respiratory difficulties were reported.

The Wellington VAAC received pilot reports of an eruption plume on 5 July that reached an unknown altitude. Another pilot report indicated that the eruption may have started on 27 June. The eruption continued over the next few days, with dark ash plumes reaching altitudes of 3.7 km and drifting E and SE. No plumes were reported after the morning of 10 July.

MODIS thermal anomalies during 2005-2006. Thermal anomalies were detected by MODIS during 26-31 March 2005, though no corresponding explosive activity was reported. No hot spots were identified at Lopevi again until 27 October 2005, after which anomalies were present on most days through 26 January 2006; ash plumes were not reported until the end of this period, 24-26 January.

Later in 2006, thermal anomalies were detected by MODIS on most days during 25-28 April, 2-16 May, 25-28 May, 26 June-9 July, and 18 July-1 August 2006. The largest number of alert pixels (24) during this time occurred at 2225 on 2 May. These data indicated two significant episodes of activity that included both explosive activity and probably lava emission during 25 April-28 May and 26 June-1 August. Two periods of plumes observations discussed previously, during 7-15 May and 27 June-10 July, fall within these longer episodes defined by the thermal data. No MODIS thermal anomalies were detected between 2 August 2006 and mid-March 2007.

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

Information Contacts: Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://vaac.metservice.com/); MODVOLC Alerts Team, Hawai'i Institute of Geophysics and Planetology (HIGP), SOEST, University of Hawaii and Manoa, 168 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Department of Geology, Mines, and Water Resources, PMB 01, Port-Vila, Vanuatu (URL: http://www.suds-en-ligne.ird.fr/fr/volcan/vanu_eng/lopevi1.htm); Port Vila Presse, PO Box 637, Port Vila, Efate, Vanuatu (URL: http://www.news.vu/en/); ReliefWeb, Office for the Coordination of Humanitarian Affairs, United Nations, New York, NY 10017, USA (URL: https://reliefweb.int/).

Merapi, one of the most dangerous volcanoes in the world owing to its perched lava dome and location in populous central Java, underwent vigorous dome growth during early to mid-2006, and its increasingly unstable summit dome released numerous pyroclastic flows and incandescent avalanches. Thousands of residents evacuated and the volcano became prominent in international news. The longest pyroclastic flows of mid-2006 took place on 8 and 14 June, with respective run-out distances from the summit area of ~ 5 and 7 km. Merapi's summit lies 32 km N of the large city of Yogyakarta.

This report contains summary notes on activity during 7 March to 1 July 2006. These notes were assembled and reported by scientists from the Merapi Volcano Observatory and the Center of Volcanology and Geological Hazard Mitigation (CVGHM), formerly the Volcanological Survey of Indonesia, and augments material presented previously (BGVN 31:05 and 31:06).

The USGS provided a satellite image with labels showing key drainages and features near the summit (figure 27). The dome's instability leads to pyroclastic flows and various kinds of rockfalls and other mass wasting episodes down the labeled drainages. During the 7 March to 1 July reporting interval, pyroclastic flows followed the headwaters of the Gendol , Krasak, Boyong, and Sat rivers, which trend to the SE, SW, SSW, and W, respectively.

Figure 27. An annotated Ikonos satellite image of Merapi taken 10 May 2006. Image resolution is 2 m; N is to the top, and the scale is such that the entire distance N-S on the image is approximately 1 km. The labeled arrows indicate key rivers into which upslope avalanche shoots drain. Multiple drainage names are separated by a slash, and many western headwaters descend into the Woro river. The "K." stands for Kali, Indonesian for stream. Lava domes and viscous flows ("L") are labeled with the year of extrusion. The Gegerbuaya ridge was formed by 1911 lavas. Garuda, Woro, and Gendol identify headwaters. Letters reference locations used by scientists to facilitate communication. The Kaliurang Observatory lies ~ 4 km to the SE of the summit. The labeled image was a collaborative effort provided here courtesy of John Pallister, USGS. Image copyright 2006, GeoEye.

Tectonic earthquake on 27 May 2006. The tectonics of Java are dominated by the subduction of the Australia plate to the NNE beneath the Sunda plate with a relative velocity of ~ 6 cm/year. The Australia plate dips NNE from the Java trench, attaining depths of 100-200 km beneath the island of Java, and depths of 600 km to the N of the island. The earthquake of 27 May 2006 occurred at shallow depth in the overriding Sunda plate, well above the dipping Australia plate.

The pace of volcanism and the intensity of the regional crisis increased after 27 May 2006. At 0553 that day, a destructive M_w 6.3 earthquake occurred leaving damage across central Java's southern coastal and inland areas (figure 28). The earthquake occurred at 10 km focal depth. The epicenter (at 7.962°S, 110.458°E) was 20 km SSE of Yogyakarta (population, 511,000; 6 million in the larger metro area). Some initial estimates put the earthquake at MR 5.9; this was later revised and even the newer (above-stated) seismic parameters are preliminary.

Figure 28. Epicenter of the 27 May 2006 earthquake in Central Java, including impact on regions around Merapi. The histograms show numbers of people killed (on left bar) and injured (right bar). As mentioned in text, some of the seismic parameters stated were later revised. Modified from a UN OCHA ReliefWeb Map Centre (1 June 2006) map in a 2006 United Nations report (see References).

A US Geological Survey (USGS) summary stated that the earthquake caused 5,749 deaths, 38,568 injuries, and led to as many as 600,000 people displaced in the Bantul-Yogyakarta area. The shaking left more than 127,000 houses destroyed and an additional 451,000 houses damaged in the area, with the total loss estimated at ~3.1 billion US dollars. Modified Mercalli intensities were as follows: at Bantul and Klaten, IX; at Sleman and Yogyakarta, VIII; at Surakarta, V; at Salatiga and Blitar, IV; and at Surabaya, II. The earthquake was felt in much of Java and at Denpasar, Bali. The website of the US Geological Survey's Earthquake Hazards Program features a large number of photos (captioned in English) depicting various aspects of the earthquake.

Events during 7 March-1 July 2006. Tables 17 and 18 summarize some of the details during the reporting interval. Merapi's activity had increased to include volcanic earthquakes and deformation of the summit area a year earlier (in July 2005). Although the number of daily lava avalanches and pyroclastic flows had increased almost a week earlier, a tectonic earthquake, MR 6.3 (Richter scale magnitude), at 0555 (local time, WIB) on 27 May was followed by another significant increase in those events for another week (tables 17 and 18). Pyroclastic flows and lava avalanches between 10 May and 30 June were rare in the W-flank Sat drainage (31 May, 2 June, and 10 June), and did not descend into the Boyong drainage (SSW) after 4 June (table 18). The Krasak river drainage (SW) had material entering it on an almost daily basis after 27 May, except for a brief time during 14-19 June, with maximum run-out distances of 4 km. The Gendol drainage (SE) also experienced daily pyroclastic flows and lava avalanches starting on 28 May. Most of these flows to the SE did not extend more than 5 km, but on 14 June a pyroclastic flow descended 7 km.

Table 17. A compilation of seismic events at Merapi during 7 March to 1 July 2006. In creating this table Bulletin editors merged the category "landslides" with the category "lava avalanches". Similarly, the category "hot cloud reports" was interpreted to be equivalent to "pyroclastic flow" and those were also merged. Those mergers were driven by sudden shifts in terminology found in CVGHM reports. No data was available for 26-27 April, 29 April-5 May, 8 May, 12-21 May, 24-26 May, 9 June, or 16-18 June. * Earthquake, MR 6.3 (Richter scale magnitude) recorded at 0555 (local time, WIB). ** Incomplete data only 0000-0600 (local time). All data courtesy of CVGHM.

Table 18. Record of run out distances (km) of pyroclastic flows and lava avalanches (the latter, in parentheses) toward river drainages on Merapi from 10 May to 30 June 2006. No data was reported for 16-18 June, and weather obscured views on21-22 June. Courtesy of CVGHM.

Because of the vigor of activity, the Alert Level rose in several steps as follows: 19 March (Green to Yellow), 12 April (Yellow to Orange), and 13 May (Orange to Red). The step to Red (which is the highest alert level, and sometimes also referred to as Level 4) followed clear deformation at the dome during elevated seismicity. On 28 April, a new lava dome emerged. By 20 May, pyroclastic flows several kilometers long were regularly seen passing down several key drainages (table 18). Figure 29 shows a 15 May pyroclastic flow (seen two days after the alert status rose to red).

Figure 29. A photo taken on 15 May 2006 (0555 local time) of a pyroclastic flow traveling down the W flank of Merapi (the Krasak headwaters). Photo taken from the Kaliurang Observatory; courtesy of CVGHM.

Volcano enthusiasts and photographers Martin Rietze and Tom Pfeiffer viewed Merapi on the morning of 27 May, during the destructive earthquake, from a high-elevation parking area ~ 4 km S of the summit. Prior to the earthquake, Rietze took several spectacular photos of incandescent avalanches pouring down avalanche shoots (figure 30 A-B). During the earthquake, he described horizontal swinging motion and dull rumbling sounds lasting perhaps 20 seconds. Dust rose from the volcano. Plants rubbing together also produced a rustling noise. Cries and engine noises in the background came from distant residents responding to the earthquake. At ~1-minute intervals, Merapi emitted about six pyroclastic flows and a substantial ash cloud grew overhead, reaching several kilometers in altitude above them. The photo in figure 30 C depicts the scene on Merapi around that time (which Rietze lists as 0555 on 27 May). His companion, Tom Pfeiffer, also took photos just after the large earthquake (e.g., figure 30 D).

Figure 30. (A and B) Pre-dawn shots of incandescent material traveling down S-flank avalanche shoot(s) at Merapi on 27 May 2006 (prior to the M ~ 6 earthquake). (C) A photo of Merapi's response at 0555 on 27 May during or just after the M ~ 6 earthquake, with several pyroclastic flows clearly visible. (D) A second photo of the scene on Merapi during or just after the earthquake. This photo captured the chaotic scene at the summit and upper slopes, including a complex array of billowing ash clouds seemingly from multiple sources, and suspended dust hanging over many parts of the volcano (particularly distinguishable along the photo's lower central and right-hand areas). Copyrighted photos; those labeled A-C, used with permission of Martin Rietze; the one labeled D, with permission of Tom Pfeiffer.

During early June the activity level of Merapi remained at red and on 4 June, the increase in volume of the new lava dome had caused the southern part of the crater wall called Gegerbuaya (1910 lavas) to collapse. Prior to its collapse, Gegerbuaya had functioned as a barrier to prevent pyroclastic flows moving southward from entering the Gendol River, which they did later in June.

On 8 June, multiple pyroclastic flows reached 4 km from the Krasak and Boyong Rivers and up to 4.5 km down the Gendol River. On 9 June, ash drifted W and NW and accumulated as ashfall ~ 1.5 mm thick. Pyroclastic flows traveled as far as 4 km toward the Gendol River. Figures 31 and 32 show pyroclastic flows on 7 and 10 June.

Figure 31. A pyroclastic flow at Merapi at 08:54:37 on 7 June 2006 shown traveling down Merapi's upslope region in a generally SE direction. Photo credit to BPPTK (The Research and Technology Development Agency for Volcanology, Yogyakarta). Provided courtesy of CVGHM.

Figure 32. A Merapi pyroclastic flow in its early stages as seen at 08:50:53 on 10 June 2006. Photo credit to BPPTK; provided courtesy of CVGHM.

In the period after the hazard level was raised to red, the lava dome grew and by 22 May its volume was ~ 2.3 million cubic meters. The M 6.3 earthquake in S-Central Java on 27 May triggered additional activity at Merapi. The dome's growth rate increased from the previous rate of around 100,000 cubic meters/day, leading to a lava dome volume on 8 June 2006 of ~4.3 million cubic meters. That lava dome stood 116 m above the nominal summit elevation of Merapi's peak (Garuda peak).

Dome collapse created the longest pyroclastic flow of the reporting interval, which took place on 14 June 2006. That pyroclastic flow attained a run-out distance of 7.0 km (table 18, figures 33 and 34, and previously reported in BGVN 31:05).

Figure 33. Deserted houses and dislodged lumber amid ash and volcanic rocks from Merapi (left-background) as seen in the village of Kaliadem (E of Kinahrejo near Bebeng, on the SE flank ~ 5 km from the summit) shortly after the 14 June 2006 pyroclastic flows passed through the settlement. Courtesy of Agence France Presse (photo by Tarko Sudiarno).

Figure 34. Night photo of Merapi (unknown date) showing incandescence on the slopes and, in the foreground, the large pyroclastic flow deposited on 14 June 2006. This photo is taken from nearly the same spot as the photos of 27 May (figure 30, above). Copyrighted photo used with permission of Tom Pfeiffer.

At least in part owing to loss of topographic relief at the Gegerbuaya ridge along the S crater wall (figure 27), the 14 June pyroclastic flow took a different path. It crossed the former barrier and descended the Gendol drainage. As previously noted (BGVN 31:05), the 14 June pyroclastic flow took two lives when the underground bunker where the victims sought refuge was buried by the pyroclastic flow.

The bunker overridden on 14 June resides in Kaliadem village (~ 5 km SE of the summit). News stories showed pictures of the rescue attempt with initial digging commencing using picks and shovels, with the excavation by soldiers wearing dust masks and standing on boards or wooden platforms, presumably to reduce the heat flow from the fresh deposit. The article also noted that the soldiers wore heat-retardant clothes. A report from the Taipei Times of 16 June 2006 and credited to the Associated Press said that "The fierce heat melted the troops' shovels and the tires on a mechanical digger brought in to plow through more than 2 m of volcanic debris covering the bunker, built for protection from volcanic eruption . . .." Later news reports noted that authorities unearthed the bunker, which lay beneath more than 2 m of steaming pyroclastic flow deposit. The two bodies had suffered burns and the facility's door was ajar. A BBC report showed deeper portions of the hole being excavated by a large backhoe. They also noted that upon deeper excavation a probe into the deposit with a hand-held digital thermometer apparently indicated temperatures reached ~ 400°C. Several grim photographs circulated in the press showing the excavated entrance of the bunker and a team in the process of removing the victim's bodies. No report has been found discussing the exact reason for the bunker's failure, although several comments in the press suggested it was not designed to withstand burial by a pyroclastic flow.

Prior to that, on 13 June, the alert status dropped to orange, but it rose back to red again the next day after the pyroclastic flow and increases in multi-phased earthquakes. Activity remained stable but high through June 29 but began to decrease after 30 June. During July the intensity and frequency of pyroclastic flows and rock falls decreased. On 10 July, authorities reduced the alert status to orange on all but the S slopes. By the end of July 2006, pyroclastic flows had ceased.

Merapi's long-term dome growth continued at low to modest levels during the rest of 2006 and early 2007. The Darwin Volcanic Ash Advisory Center noted a plume to 6.1 km altitude drifting NE on 19 March 2007. These later incidents will be discussed in more detail in a forthcoming issue of the Bulletin.

MODVOLC Thermal Alerts. The Hawai'i Institute of Geophysics and Planetology MODIS Thermal Alert System web site lacked any thermal alerts for over a year preceding May 2006. Thermal alerts over Merapi began 14 May 2006 and extended through early September 2006 on nearly a daily basis. The alerts continued intermittently into 2007.

Reference. United Nations, 2006, Indonesia Earthquake 2006 Response Plan: United Nations, OCHA Situation Report No. 5, Issued 31 May 2006, GUDE EQ-2006-000064-IDN, 42 p.

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

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); United Nations-Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017, USA; National Earthquake Information Center, US Geological Survey, PO Box 25046, Denver Federal Center MS967, Denver, CO 80225, USA (URL: http://earthquake.usgs.gov/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/advisories/); John Pallister, Volcano Disaster Assistance Program, USGS Cascades Volcano Observatory, 1300 SE Cardinal Court, Suite 100, Vancouver, WA 98683-9589, USA (URL: http://volcanoes.usgs.gov/); Tom Pfeiffer and Martin Rietze, Volcano Discovery (URL: http://www.decadevolcano.net/), http://www.tboeckel.de/); Tarko Sudiarno, Agence France Presse (AFP) (URL: http://www.afp.com/english/home/); Taipei Times (URL: http://www.taipeitimes.com/); Associated Press (URL: http://www.ap.org/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/).

As previously reported, the Rabaul Volcano Observatory noted a large, sustained Vulcanian eruption at Rabaul on 7 October 2006. Since that initial event at the Tavurvur cone, activity has varied in intensity (BGVN 31:10). During 13 December 2006 through the end of March 2007, generally mild eruptive activity continued, often with loud roaring noises and in some cases with ash plumes rising 1.5 to 3.7 km above Tavurvur's summit.

During December 2006, there was only low level seismicity, including high-frequency earthquakes and mild eruptive activity. During 24-29 December, ash clouds rose 1-3.7 km above the summit before being blown variably to the NE and SW. On 25, 27, and 28 December, fine ash fell downwind, including in Rabaul Town, and occasional roaring noises were heard. Seismic activity continued at low levels. No high-frequency earthquakes were recorded. Low seismicity continued during most of January.

During 4-10 January 2007 plumes occasionally bearing ash rose 0.9-3.3 km above the cone and drifted E and NE. Vapor emissions accompanied by pale gray ash clouds occurred on 13, 16, and 24 January. The emissions rose 0.4- 2.5 km above Tavurvur's summit and blew E, NE, and N. During 24-25 January there were nine low-frequency earthquakes recorded. Ground deformation measurements showed no significant movement apart from a slight deflation of about 1 cm during the last few days of January. From 29 January onwards, seismicity increased to a moderate level. Three high-frequency earthquakes were recorded, one on 27 January, and two on 30 January, all originating NE of the caldera. Low-frequency earthquakes began 24 January. A total of 16 events were recorded during 24-28 January, and a further 50-60 small events 29-31 January.

Two small explosions occurred at 0448 and 0548 on 27 January and a large explosion occurred at 0130 on 31 January. The latter explosion showered the cone's flanks. The accompanying ash clouds rose a couple of hundred meters straight above the summit. Fine ashfall occurred at Rabaul Town and surrounding areas.

Mild eruptive activity continued during early February with associated seismicity at very low levels. The small low-frequency earthquakes had declined in number by about half. Ground deformation data indicated a noticeable deflation of the caldera. Mild eruptive activity continued intermittently during the latter half of February, associated with low seismicity. Ash fell on surrounding villages on 20 February. On 16, 19, and 21 February, low-frequency earthquakes and white vapor emissions containing very low ash content rose as high as 3 km above Tavurvur's summit. The emissions were not accompanied by high-frequency signals or significant ground deformation.

Moderate explosions occurred on 21, 26, and 27 February. A larger explosion, at 1150 on 28 February, showered the cone's flanks with lava fragments. Thick ash clouds rose 2 km above the summit and blew NE.

Between 3 and 4 March, multiple explosions occurred; the biggest on 3, 4, and 8 March. The explosion's shockwaves rattled houses in Rabaul Town and surrounding villages. Thick ash and lava fragments showered the flanks of the cone. Other emissions consisted of white gray ash clouds that drifted E and SE. On 4 and 6 March ash plumes rose as high as 2.7 km above the summit. A weak glow was visible only during forceful emissions. During 6 to 21 March, ash plumes intermittently rose as high as 3.7 km. From 16 to 25 March, multiple explosions again produced shockwaves felt in Rabaul Town, and ash fell in surrounding villages. Incandescent material was seen rolling down the cone's flanks. During the period 27-30 March only low level vapor emissions rising to 400 m above the cone were visible. Seismic activity continued to remain at a very low level, with just three or four short (< 30 second) low-frequency events. There were no high-frequency events.

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: Steve Saunders and Herman Patia, Rabaul Volcanological Observatory (RVO), Department of Mining, Private Mail Bag, Port Moresby Post Office, National Capitol District, Papua, New Guinea; Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Darwin, Australia.

A moderate volcanic earthquake struck Ruapehu at 2230 on 4 October 2006. The M 2.8 event falsely triggered the lahar warning system. A visit to the crater lake on 7 October revealed evidence that a small hydrothermal eruption had occurred. Wave action reached up to 4-5 m above the lake surface around the basin, but was insufficient to overflow the tephra dam where it might have formed a lahar on the outer slopes. Since the last measurement (date not specified) the lake's temperature rose ~8°C, and the water level increased ~ 1 m. Both of these effects were expected. Seismic activity remained at typical background levels on 7 October 2006.

At about 1300 on 18 March 2007, Crater Lake partly emptied and its runoff traveled rapidly downstream as a powerful lahar. A subsequent issue will discuss that dramatic event and its impact.

Since the last report in February 2004 (BGVN 29:02), from May 2003 to October 2006, there were eight alerts issued by the Institute of Geological & Nuclear Sciences (IGNS, table 12), indicating appreciable changes in both the level of the lake and its temperature; these alerts can be compared with the temperature data (table 13).

Table 12. Institute of Geological & Nuclear Sciences (IGNS) alerts posted for Ruapehu volcano, May 2003 to October 2006. Compiled from IGNS reports.

Table 13. Lake temperature data recorded at Ruapehu during 2003-2006. Some months have multiple sets of readings. Data were rounded to two significant figures. Compiled from IGNS reports.

Volcanic tremor was recorded during July 2005 and continued at varying levels. Although tremor is not unusual at Ruapehu, this was the strongest recorded since November 2004. Prominent steam plumes rose above Ruapehu on the morning of 13 September 2005. The crater lake temperature had recently risen from 23°C in August 2005 (table 13) to 39°C in early September 2005. By 12 October 2005 it had fallen to 30°C, indicating the end of the heating cycle. Thereafter, another cycle of lake heating took place in middle to late October 2005. During the period when the lake was at its hottest, steam plumes appeared on several days, but no eruptive activity was observed. Seismic activity continued at about normal levels except for a slight increase in the occurrence of volcanic earthquakes over the previous two weeks.

Lahar hazard. The last report on Ruapehu (BGVN 29:02) reviewed the government of New Zealand's efforts to lessen potential damage and loss of life from the possible collapse of the ash dam surrounding the lake that sits directly within the crater. An illustrative model of the most likely potential lahar was presented in the previous Bulletin (BGVN 29:02). Figure 27 provides more details on the regional geography.

Figure 27. Composite maps of the Ruapehu area modified from part of a lahar hazards poster titled "How will the Lahar Affect Me?" The schematic map (at left) shows that the Tongariro river trends N, crosses State Highway 1 two times, and eventually enters Lake Taupo. The shaded relief map (right) of Ruapehu and adjacent flanks along its E-sector. Note the multiple chutes created to divert flood waters and lahars toward the S on the Whangaehu river. These chutes are intended to protect the Tongariro river's headwaters. Courtesy of the NZ Department of Conservation.

According to IGNS and related government websites, the most likely lahar's path starts from a 7-m-thick tephra dam sitting above bedrock along the low point in Ruapehu's crater rim. This path descends along the Whangaehu valley, a drainage that initially travels radially down the cone to the E. Where the Whangaehu reaches beyond ~ 10 km from the rim (figure 27), the channel curves sharply S and then SW, ultimately crossing Ruapehu's S side. In contrast, just upstream of the above-mentioned bend, the intersecting Tongariro river flows N. At that connection between the two drainages (a divide), engineers added a 300-m-long embankment (a levee or bund), to keep substantial material from entering the Tongariro drainage. Engineers also added one or more chutes to direct some of the Whangaehu river S and away from the critical junction. Protecting the Tongariro river from sudden influx of water and debris protects infrastructure along and downstream of that river. For example, the Tongariro river enters Lake Taupo, a 30 x 40 km caldera lake. Lake Taupo drains to the N along the Waikato river and dams along that river generate hydroelectric power.

According to the Institute of Geological & Nuclear Sciences (IGNS), about 60 lahars have swept down the mountain's southern side in the past 150 years. Lahars are not limited to the Whangaehu valley as eruptive and mass wasting processes can result in sudden influx of water and debris in other drainages as well. Lahar episodes since 1945 appear on figure 28.

Figure 28. Lahar episodes occurring at Ruapehu since 1945, as grouped into four categories. The categories are those associated with an extended eruption, a sudden (blue-sky) eruption, rain mobilization, and dam break or failure. From Harry J. R. Keys (date unknown), Department of Conservation (see Reference, below).

Figure 29 contains plots of the crater lake's surface elevation during the past several years. The plot is part of a poster available on the Department of Conservation website. The poster also notes the approximate volume of the crater lake, 107 m³. The tephra dam allows lake water to seep through it, considerably complicating estimates of the late-stage-filling rates, and any predicted date of overflow or related failure. Derek Cheng wrote an 8 January 2007 New Zealand Herald news piece stating that the lake then stood ~2.7 m below the dam's top. According to Chang's news story, the tephra dam allowed lake water to seep through it at a rate of ~10 L per second.

Figure 29. A plot of the surface elevation with time (1996 to mid-2006) of Ruapehu's crater lake. Absolute lake elevations in meters above sea-level apply to the curve labeled "Lake level" and correspond to the y-axis scale at the right. Indices of lake fullness (percent above or below the elevation 2,440 m) apply to the curve describing "Lake volume as percent of fullness." This curve corresponds to the y-axis at left (i.e., 0 % full = 2,440 m a.s.l.; 100% full = 2,529.3 m a.s.l.). The dotted horizontal line indicates the elevation of the base of the tephra dam that lies over the rim's low point. This plot came directly from an informative poster on the lahar available online at the Department of Conservation website (Keys, (date unknown), in reference list below).

Crater Lake observations. Ruapehu's Crater Lake had warmed following periods of volcanic tremor, with heating cycles getting to temperatures ranging from about 15 to 40°C (eg., 39°C during February 2004 and ~36°C during late October 2006; table 13). The IGNS website notes that Ruapehu's heating cycles typically occur every 9-12 months and normally last 1-3 months.

An innovative approach to covering the current lahar hazard status can be found at the Department of Conservation website. As of early February 2007 the reports were "updated every 1-2 weeks depending on weather conditions and [field] site visits."

Reference. Keys, H.J.R., (date unknown), Lahars from Mount Ruapehu—mitigation and management; NZ Dept. of Conservation website (a poster conveyed as a PDF file; creation/publication date unknown) (URL: http://www.doc.govt.nz/templates/summary.aspx?id=42442).

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: Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/, https://www.geonet.org.nz/); New Zealand Department of Conservation, Private Bag, Turangi, New Zealand (URL: http://www.doc.govt.nz/).

A previous report (BGVN 31:02) described small earthquakes on 1-2 March 2006, accompanied by "gray-blue emissions." Subsequent ongoing eruptions continued at Ulawun through 18 January 2007, generating almost daily aviation reports describing plumes blowing W to NW and of generally modest height (table 3). The tallest plume of the reporting interval rose to 4.6 km altitude.

Table 3. A summary of key events at Ulawun observed during the reporting interval 22 March 2006-18 January 2007. Reported plumes did not attain an altitude of over 4 km except on 12 November, when they reached an altitude of 4.6 km. Information based primarily on satellite data and pilot reports from the Darwin VAAC and in a few cases, the US Air Force Weather Agency (AFWA).

No MODIS thermal alerts were identified between March 2006 and January 2007 on the Hawai'i Institute of Geophysics and Planetology MODIS Thermal Alert System web site. The lack of thermal anomalies may indicate explosive eruptions, and not lava emissions. However, such activity has occurred at the summit in the past. One such episode, in November 1985, generated Strombolian activity and pyroclastic flows (figure 11).

Figure 11. Photograph of Ulawun taken from a helicopter on 25 November 1985. The view from the NE shows emission of large clots of molten lava into the air above the vent and pyroclastic flows (right). The other large stratovolcano in the background is 2,248-m-tall Bamus. Photographs were taken and provided by James Mori, Disaster Prevention Research Institute, Kyoto University.

Four Volcanic Ash Advisory Centers (VAAC): Tokyo, Washington, Darwin, and Wellington, have an interest in this volcano, because plumes may enter their areas of responsibility (figure 12). The VAACs came into existence to keep aviators informed of volcanic hazards. A key player in their development was the International Civil Aviation Organization (ICAO), a United Nations Related Agency that is the recognized international authority regarding a large number of aviation isses. Nine VAAC were created, in Anchorage (Alaska), Buenos Aires (Argentina), Darwin (Australia), London (England), Montreal (Canada), Tokyo (Japan), Toulouse (France), Washington (United States), and Wellington (New Zealand). These centers are tasked with monitoring volcanic ash plumes and providing Volcanic Ash Advisories (VAA) whenever those plumes enter their assigned airspace. The VAACs are often integrated with aviation weather centers; many have developed back-up sites. For example, the Washington VAAC is backed-up by the US Air Force Weather Agency; the Tokyo by Japan Meteorological Association Headquarters, and Darwin by the National Meteorological & Oceanographic Centre.

Figure 12. Map of Indonesia and Papua New Guinea showing selected volcanoes, including Ulawun on New Britain (right center), with areas of responsibility for local VAACs. Courtesy of Darwin VAAC.

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), P. O. Box 386, Rabaul, 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/); US Air Force Weather Agency (AFWA), Satellite Applications Branch, Offutt AFB, NE 68113-4039, USA; Hawai'i Institute of Geophysics and Planetology (HIGP) 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/); James Mori, Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan (URL: http://eqh.dpri.kyoto-u.ac.jp/~mori/).

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/).