The lava lake in the summit crater of Erebus has been active since at least 1972. Located in Antarctica overlooking the McMurdo Station on Ross Island, it is the southernmost active volcano on the planet. Because of the remote location, activity is primarily monitored by satellites. This report covers activity during 2023.

The number of thermal alerts recorded by the Hawai'i Institute of Geophysics and Planetology’s MODVOLC Thermal Alerts System increased considerably in 2023 compared to the years 2020-2022 (table 9). In contrast to previous years, the MODIS instruments aboard the Aqua and Terra satellites captured data from Erebus every month during 2023. Consistent with previous years, the lowest number of anomalous pixels were recorded in January, November, and December.

Table 9. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2023. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the HIGP – MODVOLC Thermal Alerts System.

Sentinel-2 infrared images showed one or two prominent heat sources within the summit crater, accompanied by adjacent smaller sources, similar to recent years (see BGVN 46:01, 47:02, and 48:01). A unique image was obtained on 25 November 2023 by the OLI-2 (Operational Land Imager-2) on Landsat 9, showing the upper part of the volcano surrounded by clouds (figure 32).

Figure 32. Satellite view of Erebus with the summit and upper flanks visible above the surrounding weather clouds on 25 November 2023. Landsat 9 OLI-2 (Operational Land Imager-2) image with visible and infrared bands. Thermal anomalies are present in the summit crater. The edifice is visible from about 2,000 m elevation to the summit around 3,800 m. The summit crater is ~500 m in diameter, surrounded by a zone of darker snow-free deposits; the larger circular summit area is ~4.5 km diameter. NASA Earth Observatory image by Lauren Dauphin, using Landsat data from the U.S. Geological Survey.

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

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: https://earthobservatory.nasa.gov/images/152134/erebus-breaks-through).

Rincón de la Vieja is a volcanic complex in Costa Rica with a hot convecting acid lake that exhibits frequent weak phreatic explosions, gas-and-steam emissions, and occasional elevated sulfur dioxide levels (BGVN 45:10, 46:03, 46:11). The current eruption period began June 2021. This report covers activity during July-December 2023 and is based on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Numerous weak phreatic explosions continued during July-December 2023, along with gas-and-steam emissions and plumes that rose as high as 3 km above the crater rim. Many weekly OVSICORI-UNA bulletins included the previous week's number of explosions and emissions (table 9). For many explosions, the time of explosion was given (table 10). Frequent seismic activity (long-period earthquakes, volcano-tectonic earthquakes, and tremor) accompanied the phreatic activity.

Table 9. Number of reported weekly phreatic explosions and gas-and-steam emissions at Rincón de la Vieja, July-December 2023. Counts are reported for the week before the Weekly Bulletin date; not all reports included these data. Courtesy of OVSICORI-UNA.

Table 10. Summary of activity at Rincón de la Vieja during July-December 2023. Weak phreatic explosions and gas emissions are noted where the time of explosion was indicated in the weekly or daily bulletins. Height of plumes or emissions are distance above the crater rim. Courtesy of OVSICORI-UNA.

According to OVSICORI-UNA, during July-October the average weekly sulfur dioxide (SO₂) flux ranged from 68 to 240 tonnes/day. However, in mid-November the flux increased to as high as 334 tonnes/day, the highest value measured in recent years. The high SO₂ flux in mid-November was also detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 43).

Figure 43. Sulfur dioxide (SO2) maps from Rincón de la Vieja recorded by the TROPOMI instrument aboard the Sentinel-5P satellite on 16 November (left) and 20 November (right) 2023. Mass estimates are consistent with measurements by OVSICORI-UNA near ground level. Some of the plume on 20 November may be from other volcanoes (triangle symbols) in Costa Rica and Nicaragua. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/).

Bezymianny, located on Russia’s Kamchatka Peninsula, has had eruptions since 1955 characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. Activity during November 2022-April 2023 included gas-and-steam emissions, lava dome collapses generating avalanches, and persistent thermal activity. Similar eruptive activity continued from May through October 2023, described here based on information from weekly and daily reports of the Kamchatka Volcano Eruptions Response Team (KVERT), notices from Tokyo VAAC (Volcanic Ash Advisory Center), and from satellite data.

Overall activity decreased after the strong period of activity in late March through April 2023, which included ash explosions during 29 March and 7-8 April 2023 that sent plumes as high as 10-12 km altitude, along with dome growth and lava flows (BGVN 48:05). This reduced activity can be seen in the MIROVA thermal detection system graph (figure 56), which was consistent with data from the MODVOLC thermal detection system and with Sentinel-2 satellite images that showed persistent hotspots in the summit crater when conditions allowed observations. A renewed period of strong activity began in mid-October 2023.

Figure 56. The MIROVA (Log Radiative Power) thermal data for Bezymianny during 20 November 2022 through October 2023 shows heightened activity in the first half of April and second half of October 2023, with lower levels of thermal anomalies in between those times. Courtesy of MIROVA.

Activity increased significantly on 17 October 2023 when large collapses began during 0700-0830 on the E flanks of the lava dome and continued to after 0930 the next day (figure 57). Ash plumes rose to an altitude of 4.5-5 km, extending 220 km NNE by 18 October. A large explosion at 1630 on 18 October produced an ash plume that rose to an altitude of 11 km (8 km above the summit) and drifted NNE and then NW, extending 900 km NW within two days at an altitude of 8 km. Minor ashfall was noted in Kozyrevsk (45 km WNW). At 0820 on 20 October an ash plume was identified in satellite images drifting 100 km ENE at altitudes of 4-4.5 km.

Figure 57. Sentinel-2 satellite images of Bezymianny from 1159 on 17 October 2023 (2359 on 16 October UTC) showing a snow-free S and SE flank along with thermal anomalies in the crater and down the SE flank. Left image is in false color (bands 8, 4, 3); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Lava flows and hot avalanches from the dome down the SE flank continued over the next few days, including 23 October when clear conditions allowed good observations (figures 58 and 59). A large thermal anomaly was observed over the volcano through 24 October, and in the summit crater on 30 October (figure 60). Strong fumarolic activity continued, with numerous avalanches and occasional incandescence. By the last week of October, volcanic activity had decreased to a level consistent with that earlier in the reporting period.

Figure 58. Daytime photo of Bezymianny under clear conditions on 23 October 2023 showing a lava flow and avalanches descending the SE flank, incandescence from the summit crater, and a small ash plume. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 59. Night photo of Bezymianny under cloudy conditions on 23 October 2023 showing an incandescent lava flow and avalanches descending the SE flank. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 60. Sentinel-2 satellite images of Bezymianny from 1159 on 30 October 2023 (2359 on 29 October UTC) showing a plume drifting SE and thermal anomalies in the summit crater and down multiple flanks. Left image is in true color (bands 4, 3, 2); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Aviation warnings were frequently updated during 17-20 October. KVERT issued a Volcano Observatory Notice for Aviation (VONA) on 17 October at 1419 and 1727 (0219 and 0527 UTC) raising the Aviation Color Code (ACC) from Yellow to Orange (second highest level). The next day, KVERT issued a VONA at 1705 (0505 UTC) raising the ACC to Red (highest level) but lowered it back to Orange at 2117 (0917 UTC). After another decrease to Yellow and back to Orange, the ACC was reduced to Yellow on 20 October at 1204 (0004 UTC). In addition, the Tokyo VAAC issued a series of Volcanic Ash Advisories beginning on 16 October and continuing through 30 October.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).chr

Kīlauea is the southeastern-most volcano in Hawaii and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has recently been characterized by lava effusions, spatter, and sulfur dioxide emissions in the active Halema’uma’u lava lake (BGVN 47:08). Lava effusions, some spatter, and sulfur dioxide emissions have continued during this reporting period of July through December 2022 using daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Summary of activity during July-December 2022. Low-level effusions have continued at the western vent of the Halema’uma’u crater during July through early December 2022. Occasional weak ooze-outs (also called lava break outs) would occur along the margins of the crater floor. The overall level of the active lava lake throughout the reporting period gradually increased due to infilling, however it stagnated in mid-September (table 13). During September through November, activity began to decline, though lava effusions persisted at the western vent. By 9 December, the active part of the lava lake had completely crusted over, and incandescence was no longer visible.

Table 13. Summary of measurements taken during overflights at Kīlauea that show a gradual increase in the active lava lake level and the volume of lava effused since 29 September 2021. Lower activity was reported during September-October. Data collected during July-December 2022. Courtesy of HVO.

Activity during July 2022. Lava effusions were reported from the western vent in the Halema’uma’u crater, along with occasional weak ooze-outs along the margins of the crater floor. The height of the lava lake was variable due to deflation-inflation tilt events; for example, the lake level dropped approximately 3-4 m during a summit deflation-inflation event reported on 1 July. Webcam images taken during the night of 6-12 July showed intermittent low-level spattering at the western vent that rose less than 10 m above the vent (figure 519). Measurements made during an overflight on 7 July indicated that the crater floor was infilled about 130 m and that 95 million cubic meters of lava had been effused since 29 September 2021. A single, relatively small lava ooze-out was active to the S of the lava lake. Around midnight on 8 July there were two brief periods of lava overflow onto the lake margins. On 9 July lava ooze-outs were reported near the SE and NE edges of the crater floor and during 10-11 July they occurred near the E, NE, and NW edges. On 16 July crater incandescence was reported, though the ooze-outs and spattering were not visible. On 18 July overnight webcam images showed incandescence in the western vent complex and two ooze-outs were reported around 0000 and 0200 on 19 July. By 0900 there were active ooze-outs along the SW edge of the crater floor. Measurements made from an overflight on 19 July indicated that the crater floor was infilled about 133 m and 98 million cubic meters of lava had erupted since 29 September 2021 (figure 520). On 20 July around 1600 active ooze-outs were visible along the N edge of the crater, which continued through the next day. Extensive ooze-outs occurred along the W margin during 24 July until 1900; on 26 July minor ooze-outs were noted along the N margin. Minor spattering was visible on 29 July along the E margin of the lake. The sulfur dioxide emission rates ranged 650-2,800 tons per day (t/d), the higher of which was measured on 8 July (figure 519).

Figure 519. Minor spattering rising less than 10 m was visible at the E end of the lava lake within Halema‘uma‘u, at the summit of Kīlauea on 8 July 2022. Sulfur dioxide is visible rising from the lake surface (bluish-colored fume). A sulfur dioxide emission rate of approximately 2,800 t/d was measured on 8 July. Courtesy of K. Mulliken, USGS.

Figure 520. A helicopter overflight on 19 July 2022 allowed for aerial visible and thermal imagery to be taken of the Halema’uma’u crater at Kīlauea’s summit crater. The active part of the lava lake is confined to the western part of the crater. The scale of the thermal map ranges from blue to red, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

Activity during August 2022. The eruption continued in the Halema’uma’u crater at the western vent. According to HVO the lava in the active lake remained at the level of the bounding levees. Occasional minor ooze-outs were observed along the margins of the crater floor. Strong nighttime crater incandescence was visible after midnight on 6 August over the western vent cone. During 6-7 August scattered small lava lobes were active along the crater floor and incandescence persisted above the western vent through 9 August. During 7-9 August HVO reported a single lava effusion source was active along the NW margin of the crater floor. Measurements from an overflight on 4 August indicated that the crater floor was infilled about 136 m total and that 102 million cubic meters of lava had been erupted since the start of the eruption. Lava breakouts were reported along the N, NE, E, S, and W margins of the crater during 10-16 August. Another overflight survey conducted on 16 August indicated that the crater floor infilled about 137 m and 104 million cubic meters of lava had been erupted since September 2021. Measured sulfur dioxide emissions rates ranged 1,150-2,450 t/d, the higher of which occurred on 8 August.

Activity during September 2022. During September, lava effusion continued from the western vent into the active lava lake and onto the crater floor. Intermittent minor ooze-outs were reported through the month. A small ooze-out was visible on the W crater floor margin at 0220 on 2 September, which showed decreasing surface activity throughout the day, but remained active through 3 September. On 3 September around 1900 a lava outbreak occurred along the NW margin of the crater floor but had stopped by the evening of 4 September. Field crews monitoring the summit lava lake on 9 September observed spattering on the NE margin of the lake that rose no higher than 10 m, before falling back onto the lava lake crust (figure 521). Overflight measurements on 12 September indicated that the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had been erupted since September 2021. Extensive breakouts in the W and N part of the crater floor were reported at 1600 on 20 September and continued into 26 September. The active part of the lava lake dropped by 10 m while other parts of the crater floor dropped by several meters. Summit tiltmeters recorded a summit seismic swarm of more than 80 earthquakes during 1500-1800 on 21 September, which occurred about 1.5 km below Halema’uma’u; a majority of these were less than Mw 2. By 22 September the active part of the lava lake was infilled about 2 m. On 23 September the western vent areas exhibited several small spatter cones with incandescent openings, along with weak, sporadic spattering (figure 522). The sulfur dioxide emission rate ranged from 930 t/d to 2,000 t/d, the higher of which was measured on 6 September.

Figure 521. Photo of spattering occurring at Kīlauea's Halema’uma’u crater during the morning of 9 September 2022 on the NE margin of the active lava lake. The spatter material rose 10 m into the air before being deposited back on the lava lake crust. Courtesy of C. Parcheta, USGS.

Figure 522.The active western vent area at Kīlauea's Halema’uma’u crater consisted of several small spatter cones with incandescent openings and weak, sporadic spattering. Courtesy of M. Patrick, USGS.

Activity during October 2022. Activity during October declined slightly compared to previous months, though lava effusions persisted from the western vent into the active lava lake and onto the crater floor during October (figure 523). Slight variations in the lava lake were noted throughout the month. HVO reported that around 0600 on 3 October the level of the lava lake has lowered slightly. Overflight measurements taken on 5 October indicated that the crater floor was infilled a total of about 143 m and that 111 million cubic meters of lava had been effused since September 2021. During 6-7 October the lake gradually rose 0.5 m. Sulfur dioxide measurements made on 22 October had an emission rate of 700 t/d. Another overflight taken on 28 October showed that there was little to no change in the elevation of the crater floor: the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had erupted since the start of the eruption.

Figure 523. Photo of the Halema’uma’u crater at Kīlauea looking east from the crater rim showing the active lava lake, with active lava ponds to the SE (top) and west (bottom middle) taken on 5 October 2022. The western vent complex is visible through the gas at the bottom center of the photo. Courtesy of N. Deligne, USGS.

Activity during November 2022. Activity remained low during November, though HVO reported that lava from the western vent continued to effuse into the active lava lake and onto the crater floor throughout the month. The rate of sulfur dioxide emissions during November ranged from 300-600 t/d, the higher amount of which occurred on 9 November.

Activity during December 2022. Similar low activity was reported during December, with lava effusing from the western vent into the active lava lake and onto the crater floor. During 4-5 December the active part of the lava lake was slightly variable in elevation and fluctuated within 1 m. On 9 December HVO reported that lava was no longer erupting from the western vent in the Halema’uma’u crater and that sulfur dioxide emissions had returned to near pre-eruption background levels; during 10-11 December, the lava lake had completely crusted over, and no incandescence was visible (figure 524). Time lapse camera images covering the 4-10 December showed that the crater floor showed weak deflation and no inflation. Some passive events of crustal overturning were reported during 14-15 December, which brought fresh incandescent lava to the lake surface. The sulfur dioxide emission rate was approximately 200 t/d on 14 December. A smaller overturn event on 17 December and another that occurred around 0000 and into the morning of 20 December were also detected. A small seismic swarm was later detected on 30 December.

Figure 524. Photo of Halema’uma’u crater at Kīlauea showing a mostly solidified lake surface during the early morning of 10 December 2022. Courtesy of J. Bard, USGS.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Nyamulagira (also known as Nyamuragira) is a shield volcano in the Democratic Republic of Congo with the summit truncated by a small 2 x 2.3 km caldera with walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from numerous flank fissures and cinder cones. The current eruption period began in April 2018 and has more recently been characterized by summit crater lava flows and thermal activity (BGVN 48:05). This report describes lava flows and variable thermal activity during May through October 2023, based on information from the Observatoire Volcanologique de Goma (OVG) and various satellite data.

Lava lake activity continued during May. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system recorded moderate-to-strong thermal activity throughout the reporting period; activity was more intense during May and October and relatively weaker from June through September (figure 95). The MODVOLC thermal algorithm, detected a total of 209 thermal alerts. There were 143 hotspots detected during May, eight during June, nine during September, and 49 during October. This activity was also reflected in infrared satellite images, where a lava flow was visible in the NW part of the crater on 7 May and strong activity was seen in the center of the crater on 4 October (figure 96). Another infrared satellite image taken on 12 May showed still active lava flows along the NW margin of the crater. According to OVG lava effusions were active during 7-29 May and moved to the N and NW parts of the crater beginning on 9 May. Strong summit crater incandescence was visible from Goma (27 km S) during the nights of 17, 19, and 20 May (figure 97). On 17 May there was an increase in eruptive activity, which peaked at 0100 on 20 May. Notable sulfur dioxide plumes drifted NW and W during 19-20 May (figure 98). Drone footage acquired in partnership with the USGS (United States Geological Survey) on 20 May captured images of narrow lava flows that traveled about 100 m down the W flank (figure 99). Data from the Rumangabo seismic station indicated a decreasing trend in activity during 17-21 May. Although weather clouds prevented clear views of the summit, a strong thermal signature on the NW flank was visible in an infrared satellite image on 22 May, based on an infrared satellite image. On 28 May the lava flows on the upper W flank began to cool and solidify. By 29 May seismicity returned to levels similar to those recorded before the 17 May increase. Lava effusion continued but was confined to the summit crater; periodic crater incandescence was observed.

Figure 95. Moderate-to-strong thermal anomalies were detected at Nyamulagira during May through October 2023, as shown on this MIROVA graph (Log Radiative Power). During late May, the intensity of the anomalies gradually decreased and remained at relatively lower levels during mid-June through mid-September. During mid-September, the power of the anomalies gradually increased again. The stronger activity is reflective of active lava effusions. Courtesy of MIROVA.

Figure 96. Infrared (bands B12, B11, B4) satellite images showing a constant thermal anomaly of variable intensities in the summit crater of Nyamulagira on 7 May 2023 (top left), 21 June 2023 (top right), 21 July 2023 (bottom left), and 4 October 2023 (bottom right). Although much of the crater was obscured by weather clouds on 7 May, a possible lava flow was visible in the NW part of the crater. Courtesy of Copernicus Browser.

Figure 97. Photo of intense nighttime crater incandescence at Nyamulagira as seen from Goma (27 km S) on the evening of 19 May 2023. Courtesy of Charles Balagizi, OVG.

Figure 98. Two strong sulfur dioxide plumes were detected at Nyamulagira and drifted W on 19 (left) and 20 (right) May 2023. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 99. A map (top) showing the active vents (yellow pins) and direction of active lava flows (W) at Nyamulagira at Virunga National Park on 20 May 2023. Drone footage (bottom) also shows the fresh lava flows traveling downslope to the W on 20 May 2023. Courtesy of USGS via OVG.

Low-level activity was noted during June through October. On 1 June OVG reported that seismicity remained at lower levels and that crater incandescence had been absent for three days, though infrared satellite imagery showed continued lava effusion in the summit crater. The lava flows on the flanks covered an estimated 0.6 km². Satellite imagery continued to show thermal activity confined to the lava lake through October (figure 96), although no lava flows or significant sulfur dioxide emissions were reported.

Geologic Background. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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/); NASA 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Charles Balagizi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

The remote volcano of Bagana is located in central Bougainville Island, Papua New Guinea. Recorded eruptions date back to 1842 and activity has consisted of effusive activity that has built a small lava dome in the summit crater and occasional explosions that produced pyroclastic flows. The most recent eruption has been ongoing since February 2000 and has produced occasional explosions, ash plumes, and lava flows. More recently, activity has been characterized by ongoing effusive activity and ash emissions (BGVN 48:04). This report updates activity from April through September 2023 that has consisted of explosions, ash plumes, ashfall, and lava flows, using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

An explosive eruption was reported on 7 July that generated a large gas-and-ash plume to high altitudes and caused significant ashfall in local communities; the eruption plume had reached upper tropospheric (16-18 km altitude) altitudes by 2200, according to satellite images. Sulfur dioxide plumes were detected in satellite images on 8 July and indicated that the plume was likely a mixture of gas, ice, and ash. A report issued by the Autonomous Bougainville Government (ABG) (Torokina District, Education Section) on 10 July noted that significant ash began falling during 2000-2100 on 7 July and covered most areas in the Vuakovi, Gotana (9 km SW), Koromaketo, Laruma (25 km W) and Atsilima (27 km NW) villages. Pyroclastic flows also occurred, according to ground-based reports; small deposits confined to one drainage were inspected by RVO during an overflight on 17 July and were confirmed to be from the 7 July event. Ashfall continued until 10 July and covered vegetation, which destroyed bushes and gardens and contaminated rivers and streams.

RVO reported another eruption on 14 July. The Darwin VAAC stated that an explosive event started around 0830 on 15 July and produced an ash plume that rose to 16.5 km altitude by 1000 and drifted N, according to satellite images. The plume continued to drift N and remained visible through 1900, and by 2150 it had dissipated.

Ashfall likely from both the 7 and 15 July events impacted about 8,111 people in Torokina (20 km SW), including Tsito/Vuakovi, Gotana, Koromaketo, Kenaia, Longkogari, Kenbaki, Piva (13 km SW), and Atsinima, and in the Tsitovi district, according to ABG. Significant ashfall was also reported in Ruruvu (22 km N) in the Wakunai District of Central Bougainville, though the thickness of these deposits could not be confirmed. An evacuation was called for the villages in Wakunai, where heavy ashfall had contaminated water sources; the communities of Ruruvu, Togarau, Kakarapaia, Karauturi, Atao, and Kuritaturi were asked to evacuate to a disaster center at the Wakunai District Station, and communities in Torokina were asked to evacuate to the Piva District station. According to a news article, more than 7,000 people needed temporary accommodations, with about 1,000 people in evacuation shelters. Ashfall had deposited over a broad area, contaminating water supplies, affecting crops, and collapsing some roofs and houses in rural areas. Schools were temporarily shut down. Intermittent ash emissions continued through the end of July and drifted NNW, NW, and SW. Fine ashfall was reported on the coast of Torokina, and ash plumes also drifted toward Laruma and Atsilima.

A small explosive eruption occurred at 2130 on 28 July that ejected material from the crater vents, according to reports from Torokina, in addition to a lava flow that contained two lobes. A second explosion was detected at 2157. Incandescence from the lava flow was visible from Piva as it descended the W flank around 2000 on 29 July (figure 47). The Darwin VAAC reported that a strong thermal anomaly was visible in satellite images during 30-31 July and that ash emissions rose to 2.4 km altitude and drifted WSW on 30 July. A ground report from RVO described localized emissions at 0900 on 31 July.

Figure 47. Infrared (bands B12, B11, B4) satellite images showed weak thermal anomalies at the summit crater of Bagana on 12 April 2023 (top left), 27 May 2023 (top right), 31 July 2023 (bottom left), and 19 September 2023 (bottom right). A strong thermal anomaly was detected through weather clouds on 31 July and extended W from the summit crater. Courtesy of Copernicus Browser.

The Darwin VAAC reported that ash plumes were identified in satellite imagery at 0800 and 1220 on 12 August and rose to 2.1 km and 3 km altitude and drifted NW and W, respectively. A news report stated that aid was sent to more than 6,300 people that were adversely affected by the eruption. Photos taken during 17-19 August showed ash emissions rising no higher than 1 km above the summit and drifting SE. A small explosion generated an ash plume during the morning of 19 August. Deposits from small pyroclastic flows were also captured in the photos. Satellite images captured lava flows and pyroclastic flow deposits. Two temporary seismic stations were installed near Bagana on 17 August at distances of 7 km WSW (Vakovi station) and 11 km SW (Kepox station). The Kepox station immediately started to record continuous, low-frequency background seismicity.

Satellite data. Little to no thermal activity was detected during April through mid-July 2023; only one anomaly was recorded during early April and one during early June, according to MIROVA (Middle InfraRed Observation of Volcanic Activity) data (figure 48). Thermal activity increased in both power and frequency during mid-July through September, although there were still some short gaps in detected activity. MODVOLC also detected increased thermal activity during August; thermal hotspots were detected a total of five times on 19, 20, and 27 August. Weak thermal anomalies were also captured in infrared satellite images on clear weather days throughout the reporting period on 7, 12, and 17 April, 27 May, 1, 6, 16, and 31 July, and 19 September (figure 48); a strong thermal anomaly was visible on 31 July. Distinct sulfur dioxide plumes that drifted generally NW were intermittently captured by the TROPOMI instrument on the Sentinel-5P satellite and sometimes exceeded two Dobson Units (DUs) (figure 49).

Figure 48. Low thermal activity was detected at Bagana during April through mid-July 2023, as shown on this MIROVA graph. In mid-July, activity began to increase in both frequency and power, which continued through September. There were still some pauses in activity during late July, early August, and late September, but a cluster of thermal activity was detected during late August. Courtesy of MIROVA.

Figure 49. Distinct sulfur dioxide plumes rising from Bagana on 15 July 2023 (top left), 16 July 2023 (top right), 17 July 2023 (bottom left), and 17 August 2023 (bottom right). These plumes all generally drifted NW; a particularly notable plume exceeded 2 Dobson Units (DUs) on 15 July. Data is from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.0

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

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/); 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/); NASA 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Autonomous Bougainville Government, P.O Box 322, Buka, AROB, PNG (URL: https://abg.gov.pg/); Andrew Tupper (Twitter: @andrewcraigtupp); 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); Radio NZ (URL: https://www.rnz.co.nz/news/pacific/494464/more-than-7-000-people-in-bougainville-need-temporary-accommodation-after-eruption); USAID, 1300 Pennsylvania Ave, NW, Washington DC 20004, USA (URL: https://www.usaid.gov/pacific-islands/press-releases/aug-08-2023-united-states-provides-immediate-emergency-assistance-support-communities-affected-mount-bagana-volcanic-eruptions).

Mayon is located in the Philippines and has steep upper slopes capped by a small summit crater. Historical eruptions date back to 1616 CE that have been characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during June through October 2022 and consisted of lava dome growth and gas-and-steam emissions (BGVN 47:12). A new eruption was reported during late April 2023 and has included lava flows, pyroclastic density currents, ash emissions, and seismicity. This report covers activity during April through September 2023 based on daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

During April through September 2023, PHIVOLCS reported near-daily rockfall events, frequent volcanic earthquakes, and sulfur dioxide measurements. Gas-and-steam emissions rose 100-900 m above the crater and drifted in different directions. Nighttime crater incandescence was often visible during clear weather and was accompanied by incandescent avalanches of material. Activity notably increased during June when lava flows were reported on the S, SE, and E flanks (figure 52). The MIROVA graph (Middle InfraRed Observation of Volcanic Activity) showed strong thermal activity coincident with these lava flows, which remained active through September (figure 53). According to the MODVOLC thermal algorithm, a total of 110 thermal alerts were detected during the reporting period: 17 during June, 40 during July, 27 during August, and 26 during September. During early June, pyroclastic density currents (PDCs) started to occur more frequently.

Figure 52. Infrared (bands B12, B11, B4) satellite images show strong lava flows descending the S, SE, and E flanks of Mayon on 13 June 2023 (top left), 23 June 2023 (top right), 8 July 2023 (bottom left), and 7 August 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 53. Strong thermal activity was detected at Mayon during early June through September, according to this MIROVA graph (Log Radiative Power) due to the presence of active lava flows on the SE, S, and E flanks. Courtesy of MIROVA.

Low activity was reported during much of April and May; gas-and-steam emissions rose 100-900 m above the crater and generally drifted in different directions. A total of 52 rockfall events and 18 volcanic earthquakes were detected during April and 147 rockfall events and 13 volcanic events during May. Sulfur dioxide flux measurements ranged between 400-576 tons per day (t/d) during April, the latter of which was measured on 29 April and between 162-343 t/d during May, the latter of which was measured on 13 May.

Activity during June increased, characterized by lava flows, pyroclastic density currents (PDCs), crater incandescence and incandescent rockfall events, gas-and-steam emissions, and continued seismicity. Weather clouds often prevented clear views of the summit, but during clear days, moderate gas-and-steam emissions rose 100-2,500 m above the crater and drifted in multiple directions. A total of 6,237 rockfall events and 288 volcanic earthquakes were detected. The rockfall events often deposited material on the S and SE flanks within 700-1,500 m of the summit crater and ash from the events drifted SW, S, SE, NE, and E. Sulfur dioxide emissions ranged between 149-1,205 t/d, the latter of which was measured on 10 June. Short-term observations from EDM and electronic tiltmeter monitoring indicated that the upper slopes were inflating since February 2023. Longer-term ground deformation parameters based on EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano remained inflated, especially on the NW and SE flanks. At 1000 on 5 June the Volcano Alert Level (VAL) was raised to 2 (on a 0-5 scale). PHIVOLCS noted that although low-level volcanic earthquakes, ground deformation, and volcanic gas emissions indicated unrest, the steep increase in rockfall frequency may indicate increased dome activity.

A total of 151 dome-collapse PDCs occurred during 8-9 and 11-30 June, traveled 500-2,000 m, and deposited material on the S flank within 2 km of the summit crater. During 8-9 June the VAL was raised to 3. At approximately 1947 on 11 June lava flow activity was reported; two lobes traveled within 500 m from the crater and deposited material on the S (Mi-isi), SE (Bonga), and E (Basud) flanks. Weak seismicity accompanied the lava flow and slight inflation on the upper flanks. This lava flow remained active through 30 June, moving down the S and SE flank as far as 2.5 km and 1.8 km, respectively and depositing material up to 3.3 km from the crater. During 15-16 June traces of ashfall from the PDCs were reported in Sitio Buga, Nabonton, City of Ligao and Purok, and San Francisco, Municipality of Guinobatan. During 28-29 June there were two PDCs generated by the collapse of the lava flow front, which generated a light-brown ash plume 1 km high. Satellite monitors detected significant concentrations of sulfur dioxide beginning on 29 June. On 30 June PDCs primarily affected the Basud Gully on the E flank, the largest of which occurred at 1301 and lasted eight minutes, based on the seismic record. Four PDCs generated between 1800 and 2000 that lasted approximately four minutes each traveled 3-4 km on the E flank and generated an ash plume that rose 1 km above the crater and drifted N and NW. Ashfall was recorded in Tabaco City.

Similar strong activity continued during July; slow lava effusion remained active on the S and SE flanks and traveled as far as 2.8 km and 2.8 km, respectively and material was deposited as far as 4 km from the crater. There was a total of 6,983 rockfall events and 189 PDCs that affected the S, SE, and E flanks. The volcano network detected a total of 2,124 volcanic earthquakes. Continuous gas-and-steam emissions rose 200-2,000 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 792-4,113 t/d, the latter of which was measured on 28 July. During 2-4 July three PDCs were generated from the collapse of the lava flow and resulting light brown plumes rose 200-300 m above the crater. Continuous tremor pulses were reported beginning at 1547 on 3 July through 7 July at 1200, at 2300 on 8 July and going through 0300 on 10 July, and at 2300 on 16 July, as recorded by the seismic network. During 6-9 July there were 10 lava flow-collapse-related PDCs that generated light brown plumes 300-500 m above the crater. During 10-11 July light ashfall was reported in some areas of Mabinit, Legazpi City, Budiao and Salvacion, Daraga, and Camalig, Albay. By 18 July the lava flow advanced 600 m on the E flank as well.

During 1733 on 18 July and 0434 on 19 July PHIVOLCS reported 30 “ashing” events, which are degassing events accompanied by audible thunder-like sounds and entrained ash at the crater, which produced short, dark plumes that drifted SW. These events each lasted 20-40 seconds, and plume heights ranged from 150-300 m above the crater, as recorded by seismic, infrasound, visual, and thermal monitors. Three more ashing events occurred during 19-20 July. Short-term observations from electronic tilt and GPS monitoring indicate deflation on the E lower flanks in early July and inflation on the NW middle flanks during the third week of July. Longer-term ground deformation parameters from EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano was still generally inflated relative to baseline levels. A short-lived lava pulse lasted 28 seconds at 1956 on 21 July, which was accompanied by seismic and infrasound signals. By 22 July, the only lava flow that remained active was on the SE flank, and continued to extend 3.4 km, while those on the S and E flanks weakened markedly. One ashing event was detected during 30-31 July, whereas there were 57 detected during 31 July-1 August; according to PHIVOLCS beginning at approximately 1800 on 31 July eruptive activity was dominated by phases of intermittent ashing, as well as increased in the apparent rates of lava effusion from the summit crater. The ashing phases consisted of discrete events recorded as low-frequency volcanic earthquakes (LFVQ) typically 30 seconds in duration, based on seismic and infrasound signals. Gray ash plume rose 100 m above the crater and generally drifted NE. Shortly after these ashing events began, new lava began to effuse rapidly from the crater, feeding the established flowed on the SE, E, and E flanks and generating frequent rockfall events.

Intensified unrest persisted during August. There was a total of 4,141 rockfall events, 2,881 volcanic earthquakes, which included volcanic tremor events, 32 ashing events, and 101 PDCs detected throughout the month. On clear weather days, gas-and-steam emissions rose 300-1,500 m above the crater and drifted in different directions (figure 54). Sulfur dioxide emissions averaged 735-4,756 t/d, the higher value of which was measured on 16 August. During 1-2 August the rate of lava effusion decreased, but continued to feed the flows on the SE, S, and E flanks, maintaining their advances to 3.4 km, 2.8 km, and 1.1 km from the crater, respectively (figure 55). Rockfall and PDCs generated by collapses at the lava flow margins and from the summit dome deposited material within 4 km of the crater. During 3-4 August there were 10 tremor events detected that lasted 1-4 minutes. Short-lived lava pulse lasted 35 seconds and was accompanied by seismic and infrasound signals at 0442 on 6 August. Seven collapses were recorded at the front of the lava flow during 12-14 August.

Figure 54. Photo of Mayon showing a white gas-and-steam plume rising 800-1,500 m above the crater at 0645 on 25 August. Courtesy of William Rogers.

Figure 55. Photo of Mayon facing N showing incandescent lava flows and summit crater incandescence taken at 1830 on 25 August 2023. Courtesy of William Rogers.

During September, similar activity of slow lava effusion, PDCs, gas-and-steam emissions, and seismicity continued. There was a total of 4,452 rockfall events, 329 volcanic earthquakes, which included volcanic tremor events, two ashing events, and 85 PDCs recorded throughout the month. On clear weather days, gas-and-steam emissions rose 100-1,500 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 609-2,252 t/d, the higher average of which was measured on 6 September. Slow lava effusion continued advancing on the SE, S, and E flanks, maintaining lengths of 3.4 km, 2.8 km, and 1.1 km, respectively. Rockfall and PDC events generated by collapses along the lava flow margins and at the summit dome deposited material within 4 km of the crater.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); William Rogers, Legazpi City, Albay Province, Philippines.

Nishinoshima, located about 1,000 km S of Tokyo, is a small island in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent submarine peaks to the S, W, and NE. Eruptions date back to 1973 and the current eruption period began in October 2022. Recent activity has consisted of small ash plumes and fumarolic activity (BGVN 48:07). This report covers activity during May through August 2023, using information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports and satellite data.

Activity during May through June was relatively low. The Japan Coast Guard (JCG) did overflights on 14 and 22 June and reported white gas-and-steam emissions rising 600 m and 1,200 m from the central crater of the pyroclastic cone, respectively (figure 125). In addition, multiple white gas-and-steam emissions rose from the inner rim of the W side of the crater and from the SE flank of the pyroclastic cone. Discolored brown-to-green water was observed around almost the entire perimeter of the island; on 22 June light green discolored water was observed off the S coast of the island.

Figure 125. A white gas-and-steam plume rising 600 m above the crater of Nishinoshima at 1404 on 14 June 2023 (left) and 1,200 m above the crater at 1249 on 22 June 2023 (right). Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, June, 2023).

Observations from the Himawari meteorological satellite confirmed an eruption on 9 and 10 July. An eruption plume rose 1.6 km above the crater and drifted N around 1300 on 9 July. Satellite images acquired at 1420 and 2020 on 9 July and at 0220 on 10 July showed continuing emissions that rose 1.3-1.6 km above the crater and drifted NE and N. The Tokyo VAAC reported that an ash plume seen by a pilot and identified in a satellite image at 0630 on 21 July rose to 3 km altitude and drifted S.

Aerial observations conducted by JCG on 8 August showed a white-and-gray plume rising from the central crater of the pyroclastic cone, and multiple white gas-and-steam emissions were rising from the inner edge of the western crater and along the NW-SE flanks of the island (figure 126). Brown-to-green discolored water was also noted around the perimeter of the island.

Figure 126. Aerial photo of Nishinoshima showing a white-and-gray plume rising from the central crater taken at 1350 on 8 August 2023.

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity), showing an increase in both frequency and power beginning in July (figure 127). This increase in activity coincides with eruptive activity on 9 and 10 July, characterized by eruption plumes. According to the MODVOLC thermal alert algorithm, one thermal hotspot was recorded on 20 July. Weak thermal anomalies were also detected in infrared satellite imagery, accompanied by strong gas-and-steam plumes (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Nishinoshima during May through August 2023, showing an increase in both frequency and power in July, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 128. Infrared (bands B12, B11, B4) satellite images showing a small thermal anomaly at the crater of Nishinoshima on 30 June 2023 (top left), 3 July 2023 (top right), 7 August 2023 (bottom left), and 27 August 2023 (bottom right). Strong gas-and-steam plumes accompanied this activity, extending NW, NE, and SW. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of Strombolian eruptions and ash plumes (BGVN 48:07). This report describes lower levels of activity consisting of ash and white gas-and-steam plumes during May through August 2023, based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, and satellite data.

Activity was relatively low during May and June. Daily white gas-and-steam emissions rose 25-200 m above the crater and drifted in different directions. Five ash plumes were detected at 0519 on 10 May, 1241 on 11 May, 0920 on 12 May, 2320 on 12 May, and at 0710 on 13 May, and rose 1-2.5 km above the crater and drifted SW. A webcam image taken on 12 May showed ejection of incandescent material above the vent. A total of nine ash plumes were detected during 6-11 June: at 1434 and 00220 on 6 and 7 June the ash plumes rose 500 m above the crater and drifted NW, at 1537 on 8 June the ash plume rose 1 km above the crater and drifted SW, at 0746 and at 0846 on 9 June the ash plumes rose 800 m and 3 km above the crater and drifted SW, respectively, at 0423, 1431, and 1750 on 10 June the ash plumes rose 2 km, 1.5 km, and 3.5 km above the crater and drifted NW, respectively, and at 0030 on 11 June an ash plume rose 2 km above the crater and drifted NW. Webcam images taken on 10 and 11 June at 0455 and 0102, respectively, showed incandescent material ejected above the vent. On 19 June an ash plume at 0822 rose 1.5 km above the crater and drifted SE.

Similar low activity of white gas-and-steam emissions and few ash plumes were reported during July and August. Daily white gas-and-steam emissions rose 25-300 m above the crater and drifted in multiple directions. Three ash plumes were reported at 0843, 0851, and 0852 on 20 July that rose 500-2,000 m above the crater and drifted NW.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during May through August 2023 (figure 140). Although activity was often obscured by weather clouds, a thermal anomaly was visible in an infrared satellite image of the crater on 12 May, accompanied by an eruption plume that drifted SW (figure 141).

Figure 140. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during May through August 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 141. A single thermal anomaly (bright yellow-orange) was visible at Krakatau in this infrared (bands B12, B11, B4) satellite image taken on 12 May 2023. An eruption plume accompanied the thermal anomaly and drifted SW. Courtesy of Copernicus Browser.

Geologic Background. The renowned Krakatau (frequently mis-named as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones 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 caused more than 36,000 fatalities, most as a result of 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 Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Villarrica, in central Chile, consists of a 2-km-wide caldera that formed about 3,500 years ago and is located at the base of the presently active cone at the NW margin of a 6-km-wide caldera. Historical eruptions eruptions date back to 1558 and have been characterized by mild-to-moderate explosive activity with occasional lava effusions. The current eruption period began in December 2014 and has recently consisted of nighttime crater incandescence, ash emissions, and seismicity (BGVN 48:04). This report covers activity during April through September 2023 and describes occasional Strombolian activity, gas-and-ash emissions, and nighttime crater incandescence. Information for this report primarily comes 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) and satellite data.

Seismicity during April consisted of long period (LP) events and tremor (TRE); a total of 9,413 LP-type events and 759 TR-type events were detected throughout the month. Nighttime crater incandescence persisted and was visible in the degassing column. Sulfur dioxide data was obtained using Differential Absorption Optical Spectroscopy Equipment (DOAS) that showed an average value of 1,450 ± 198 tons per day (t/d) during 1-15 April and 1,129 ± 201 t/d during 16-30 April, with a maximum daily value of 2,784 t/d on 9 April. Gas-and-steam emissions of variable intensities rose above the active crater as high as 1.3 km above the crater on 13 April. Strombolian explosions were not observed and there was a slight decrease in the lava lake level.

There were 14,123 LP-type events and 727 TR-type events detected during May. According to sulfur dioxide measurements taken with DOAS equipment, the active crater emitted an average value of 1,826 ± 482 t/d during 1-15 May and 912 ± 41 t/d during 16-30 May, with a daily maximum value of 5,155 t/d on 13 May. Surveillance cameras showed continuous white gas-and-steam emissions that rose as high as 430 m above the crater on 27 May. Nighttime incandescence illuminated the gas column less than 300 m above the crater rim was and no pyroclastic emissions were reported. A landslide was identified on 13 May on the E flank of the volcano 50 m from the crater rim and extending 300 m away; SERNAGEOMIN noted that this event may have occurred on 12 May. During the morning of 27 and 28 May minor Strombolian explosions characterized by incandescent ejecta were recorded at the crater rim; the last reported Strombolian explosions had occurred at the end of March.

Seismic activity during June consisted of five volcano-tectonic (VT)-type events, 21,606 LP-type events, and 2,085 TR-type events. The average value of sulfur dioxide flux obtained by DOAS equipment was 1,420 ± 217 t/d during 1-15 June and 2,562 ± 804 t/d, with a maximum daily value of 4,810 t/d on 17 June. White gas-and-steam emissions rose less than 480 m above the crater; frequent nighttime crater incandescence was reflected in the degassing plume. On 12 June an emission rose 100 m above the crater and drifted NNW. On 15 June one or several emissions resulted in ashfall to the NE as far as 5.5 km from the crater, based on a Skysat satellite image. Several Strombolian explosions occurred within the crater; activity on 15 June was higher energy and ejected blocks 200-300 m on the NE slope. Surveillance cameras showed white gas-and-steam emissions rising 480 m above the crater on 16 June. On 19 and 24 June low-intensity Strombolian activity was observed, ejecting material as far as 200 m from the center of the crater to the E.

During July, seismicity included 29,319 LP-type events, 3,736 TR-type events, and two VT-type events. DOAS equipment recorded two days of sulfur dioxide emissions of 4,220 t/d and 1,009 t/d on 1 and 13 July, respectively. Constant nighttime incandescence was also recorded and was particularly noticeable when accompanied by eruptive columns on 12 and 16 July. Minor explosive events were detected in the crater. According to Skysat satellite images taken on 12, 13, and 16 July, ashfall deposits were identified 155 m S of the crater. According to POVI, incandescence was visible from two vents on the crater floor around 0336 on 12 July. Gas-and-ash emissions rose as high as 1.2 km above the crater on 13 July and drifted E and NW. A series of gas-and-steam pulses containing some ash deposited material on the upper E flank around 1551 on 13 July. During 16-31 July, average sulfur dioxide emissions of 1,679 ± 406 t/d were recorded, with a maximum daily value of 2,343 t/d on 28 July. Fine ash emissions were also reported on 16, 17, and 23 July.

Seismicity persisted during August, characterized by 27,011 LP-type events, 3,323 TR-type events, and three VT-type events. The average value of sulfur dioxide measurements taken during 1-15 August was 1,642 ± 270 t/d and 2,207 ± 4,549 t/d during 16-31 August, with a maximum daily value of 3,294 t/d on 27 August. Nighttime crater incandescence remained visible in degassing columns. White gas-and-steam emissions rose 480 m above the crater on 6 August. According to a Skysat satellite image from 6 August, ash accumulation was observed proximal to the crater and was mainly distributed toward the E slope. White gas-and-steam emissions rose 320 m above the crater on 26 August. Nighttime incandescence and Strombolian activity that generated ash emissions were reported on 27 August.

Seismicity during September was characterized by five VT-type events, 12,057 LP-type events, and 2,058 TR-type events. Nighttime incandescence persisted. On 2 September an ash emission rose 180 m above the crater and drifted SE at 1643 (figure 125) and a white gas-and-steam plume rose 320 m above the crater. According to the Buenos Aires VAAC, periods of continuous gas-and-ash emissions were visible in webcam images from 1830 on 2 September to 0110 on 3 September. Strombolian activity was observed on 2 September and during the early morning of 3 September, the latter event of which generated an ash emission that rose 60 m above the crater and drifted 100 m from the center of the crater to the NE and SW. Ashfall was reported to the SE and S as far as 750 m from the crater. The lava lake was active during 3-4 September and lava fountaining was visible for the first time since 26 March 2023, according to POVI. Fountains captured in webcam images at 2133 on 3 September and at 0054 on 4 September rose as high as 60 m above the crater rim and ejected material onto the upper W flank. Sulfur dioxide flux of 1,730 t/d and 1,281 t/d was measured on 3 and 4 September, respectively, according to data obtained by DOAS equipment.

Figure 125. Webcam image of a gray ash emission rising above Villarrica on 2 September 2023 at 1643 (local time) that rose 180 m above the crater and drifted SE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 02 de septiembre de 2023, 17:05 Hora local).

Strong Strombolian activity and larger gas-and-ash plumes were reported during 18-20 September. On 18 September activity was also associated with energetic LP-type events and notable sulfur dioxide fluxes (as high as 4,277 t/d). On 19 September Strombolian activity and incandescence were observed. On 20 September at 0914 ash emissions rose 50 m above the crater and drifted SSE, accompanied by Strombolian activity that ejected material less than 100 m SSE, causing fall deposits on that respective flank. SERNAGEOMIN reported that a Planet Scope satellite image taken on 20 September showed the lava lake in the crater, measuring 32 m x 35 m and an area of 0.001 km². Several ash emissions were recorded at 0841, 0910, 1251, 1306, 1312, 1315, and 1324 on 23 September and rose less than 150 m above the crater. The sulfur dioxide flux value was 698 t/d on 23 September and 1,097 t/d on 24 September. On 24 September the Volcanic Alert Level (VAL) was raised to Orange (the third level on a four-color scale). SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

During 24-25 September there was an increase in seismic energy (observed at TR-events) and acoustic signals, characterized by 1 VT-type event, 213 LP-type events, and 124 TR-type events. Mainly white gas-and-steam emissions, in addition to occasional fine ash emissions were recorded. During the early morning of 25 September Strombolian explosions were reported and ejected material 250 m in all directions, though dominantly toward the NW. On 25 September the average value of sulfur dioxide flux was 760 t/d. Seismicity during 25-30 September consisted of five VT-type events, 1,937 LP-type events, and 456 TR-type events.

During 25-29 September moderate Strombolian activity was observed and ejected material as far as the crater rim. In addition, ash pulses lasting roughly 50 minutes were observed around 0700 and dispersed ENE. During 26-27 September a TR episode lasted 6.5 hours and was accompanied by discrete acoustic signals. Satellite images from 26 September showed a spatter cone on the crater floor with one vent that measured 10 x 14 m and a smaller vent about 35 m NE of the cone. SERNAGEOMIN reported an abundant number of bomb-sized blocks up to 150 m from the crater, as well as impact marks on the snow, which indicated explosive activity. A low-altitude ash emission was observed drifting NW around 1140 on 28 September, based on webcam images. Between 0620 and 0850 on 29 September an ash emission rose 60 m above the crater and drifted NW. During an overflight taken around 1000 on 29 September scientists observed molten material in the vent, a large accumulation of pyroclasts inside the crater, and energetic degassing, some of which contained a small amount of ash. Block-sized pyroclasts were deposited on the internal walls and near the crater, and a distal ash deposit was also visible. The average sulfur dioxide flux measured on 28 September was 344 t/d. Satellite images taken on 29 September ashfall was deposited roughly 3 km WNW from the crater and nighttime crater incandescence remained visible. The average sulfur dioxide flux value from 29 September was 199 t/d. On 30 September at 0740 a pulsating ash emission rose 1.1 km above the crater and drifted NNW (figure 126). Deposits on the S flank extended as far as 4.5 km from the crater rim, based on satellite images from 30 September.

Figure 126. Webcam image of a gray ash plume rising 1.1 km above the crater of Villarrica at 0740 (local time) on 30 September 2023. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 30 de septiembre de 2023, 09:30 Hora local).

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) showed intermittent thermal activity during April through September, with slightly stronger activity detected during late September (figure 127). Small clusters of thermal activity were detected during mid-June, early July, early August, and late September. According to the MODVOLC thermal alert system, a total of four thermal hotspots were detected on 7 July and 3 and 23 September. This activity was also intermittently captured in infrared satellite imagery on clear weather days (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Villarrica during April through September 2023, according to this MIROVA graph (Log Radiative Power). Activity was relatively low during April through mid-June. Small clusters of activity occurred during mid-June, early July, early August, and late September. Courtesy of MIROVA.

Figure 128. Consistent bright thermal anomalies (bright yellow-orange) were visible at the summit crater of Villarrica in infrared (bands B12, B11, B4) satellite images, as shown on 17 June 2023 (top left), 17 July 2023 (top right), 6 August 2023 (bottom left), and 20 September 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on 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. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Av. Beauchef 1671, Santiago, Chile (URL: https://web.senapred.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Merapi, located just north of the major city of Yogyakarta in central Java, Indonesia, has had activity within the last 20 years characterized by pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome. The current eruption period began in late December 2020 and has more recently consisted of ash plumes, intermittent incandescent avalanches of material, and pyroclastic flows (BGVN 48:04). This report covers activity during April through September 2023, based on information from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data.

Activity during April through September 2023 primarily consisted of incandescent avalanches of material that mainly affected the SW and W flanks and traveled as far as 2.3 km from the summit (table 25) and white gas-and-steam emissions that rose 10-1,000 m above the crater.

Table 25. Monthly summary of avalanches and avalanche distances recorded at Merapi during April through September 2023. The number of reported avalanches does not include instances where possible avalanches were heard but could not be visually confirmed as a result of inclement weather. Data courtesy of BPPTKG (April-September 2023 daily reports).

BPPTKG reported that during April and May white gas-and-steam emissions rose 10-750 m above the crater, incandescent avalanches descended 500-2,000 m on the SW and W flanks (figure 135). Cloudy weather often prevented clear views of the summit, and sometimes avalanches could not be confirmed. According to a webcam image, a pyroclastic flow was visible on 17 April at 0531. During the week of 28 April and 4 May a pyroclastic flow was reported on the SW flank, traveling up to 2.5 km. According to a drone overflight taken on 17 May the SW lava dome volume was an estimated 2,372,800 cubic meters and the dome in the main crater was an estimated 2,337,300 cubic meters.

Figure 135. Photo showing an incandescent avalanche affecting the flank of Merapi on 8 April 2023. Courtesy of Øystein Lund Andersen.

During June and July similar activity persisted with white gas-and-steam emissions rising 10-350 m above the crater and frequent incandescent avalanches that traveled 300-2,000 m down the SW, W, and S flanks (figure 136). Based on an analysis of aerial photos taken on 24 June the volume of the SW lava dome was approximately 2.5 million cubic meters. A pyroclastic flow was observed on 5 July that traveled 2.7 km on the SW flank. According to the Darwin VAAC multiple minor ash plumes were identified in satellite images on 19 July that rose to 3.7 km altitude and drifted S and SW. During 22, 25, and 26 July a total of 17 avalanches descended as far as 1.8 km on the S flank.

Figure 136. Photo showing an incandescent avalanche descending the flank of Merapi on 23 July 2023. Courtesy of Øystein Lund Andersen.

Frequent white gas-and-steam emissions continued during August and September, rising 10-450 m above the crater. Incandescent avalanches mainly affected the SW and W flanks and traveled 400-2,300 m from the vent (figure 137). An aerial survey conducted on 10 August was analyzed and reported that estimates of the SW dome volume was 2,764,300 cubic meters and the dome in the main crater was 2,369,800 cubic meters.

Figure 137. Photo showing a strong incandescent avalanche descending the flank of Merapi on 23 September 2023. Courtesy of Øystein Lund Andersen.

Frequent and moderate-power thermal activity continued throughout the reporting period, according to a MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 138). There was an increase in the number of detected anomalies during mid-May. The MODVOLC thermal algorithm recorded a total of 47 thermal hotspots: six during April, nine during May, eight during June, 15 during July, four during August, and five during September. Some of this activity was captured in infrared satellite imagery on clear weather days, sometimes accompanied by incandescent material on the SW flank (figure 139).

Figure 138. Frequent and moderate-power thermal anomalies were detected at Merapi during April through September 2023, as shown on this MIROVA plot (Log Radiative Power). There was an increase in the number of anomalies recorded during mid-May. Courtesy of MIROVA.

Figure 139. Infrared (bands B12, B11, B4) satellite images showed a consistent thermal anomaly (bright yellow-orange) at the summit crater of Merapi on 8 April 2023 (top left), 18 May 2023 (top right), 17 June 2023 (middle left), 17 July 2023 (middle right), 11 August 2023 (bottom left), and 20 September 2023 (bottom right). Incandescent material was occasionally visible descending the SW flank, as shown in each of these images. Courtesy of Copernicus Browser.

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 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent 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.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Øystein Lund Andersen (URL: https://www.oysteinlundandersen.com/, https://twitter.com/oysteinvolcano).

Ebeko, located on the N end of Paramushir Island in Russia’s Kuril Islands just S of the Kamchatka Peninsula, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Observed eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruptive period began in June 2022, consisting of frequent explosions, ash plumes, and thermal activity (BGVN 47:10, 48:06). This report covers similar activity during June-November 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Moderate explosive activity continued during June-November 2023 (figures 50 and 51). According to visual data from Severo-Kurilsk, explosions sent ash 2-3.5 km above the summit (3-4.5 km altitude) during most days during June through mid-September. Activity after mid-September was slightly weaker, with ash usually reaching less than 2 km above the summit. According to KVERT the volcano in October and November was, with a few exceptions, either quiet or obscured by clouds that prevented satellite observations. KVERT issued Volcano Observatory Notices for Aviation (VONA) on 8 and 12 June, 13 and 22 July, 3 and 21 August, and 31 October warning of potential aviation hazards from ash plumes drifting 3-15 km from the volcano. Based on satellite data, KVERT reported a persistent thermal anomaly whenever weather clouds permitted viewing.

Figure 50. Ash explosion from the active summit crater of Ebeko on 18 July 2023; view is approximately towards the W. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Figure 51. Ash explosion from the active summit crater of Ebeko on 23 July 2023 with lightning visible in the lower part of the plume. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/).

During August-October 2000 there were no reports of unusual volcanic activity occurring at Esa'ala (also called the Dawson Strait group). RVO had a 1960s-vintage seismic recorder at Esa'ala until 1994. Since then, maintenance and funding problems have meant it has neither functioned nor been replaced. Discussion with Professor Abe following a seismic survey in the area in the second part of 1999 revealed that he had seen continued seismicity at the Esa'ala base station.

The last notable seismic swarm at Esa'ala before the RVO instrument broke down was in November-December 1992. Another prior swarm of earthquakes took place in mid-December 1989 (BGVN 15:01). RVO maintains a part-time observer at Esa'ala who keeps track of felt earthquakes. He typically reports that no felt earthquakes have occurred.

General References. Davies, H.L., 1973, Fergusson Island, Papua New Guinea-1:250,000 Geological Series: Bur. Miner. Resour. Aust. explan. Notes, SC/56-5.

Smith, I.E.M., 1976, Peralkaline rhyolites from the D'Entrecasteaux Islands, Papua New Guinea, in Johnson, R.W., ed., Volcanism in Australasia: Elsevier, Amsterdam, p. 275-285.

Smith, I.E.M., 1981, Young volcanoes in eastern Papua in Johnson, R.W., ed., Cooke-Ravian Volume of Volcanological Papers: Geological Survey of Papua New Guinea Memoir 10, p. 257-265.

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

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

This summary of Sistema Poseidon reports covers the period from July to November 2000. The summit craters discharged several minor lava flows, some Strombolian eruptions, and frequent degassing. The Bocca Nuova (BN) vent was particularly active.

During July and well into August the summit craters displayed comparatively low activity. During July at BN three different vents were degassing. During July at NEC emissions came from one primary vent. Emissions were robust on 18 August, and commonly bore light-brown ash.

During late August, Southeast Crater (SEC) renewed emission of a weak lava flow from a fracture on the N side. The lava stream, which flowed into the Valle de Bove, persisted throughout 27 August, and increased progressively on the night of 27-28 August.

At 0135 on 28 August fairly sustained degassing occurred at SEC with initially violent Strombolian emissions. Beginning at about 0600, the explosive Strombolian activity changed rapidly to violent lava fountains, which generated an eruptive cloud rising thousands of meters above the summit. Ash and lapilli fell on the Etna's E slopes. This phase lasted about one hour, and was analogous to what had been observed during episodes in the first half of 2000.

The lava flow, despite appearing larger during the more violent degassing phase, moved little on its farthest-advanced fronts, which along the W face of the Valle del Bove reached to about 2,200-2,300 m elevation. Rather, the flow tended to widen in the zone between 2,800 and 2,700 m. The lava emission rate at the vent appeared to be drastically reduced at the end of this degassing phase.

A new degassing episode was confirmed on 29 August. This was characterized by its brevity and by the way in which it manifested itself, producing explosive Strombolian blasts (rhythmic expulsion of pyroclastics) rather than true lava fountains.

As frequently observed for the last episode, this one also started with a glimmer of light on the N flank of the SEC announcing the beginning of a new lava emission. Eruptive activity increased between 22 and 28 August, while the volcanic tremor first showed a modest increase at 0339 on 29 August, when sporadic explosions from the SEC summit crater began. Only after 0530 did the explosive activity reach a continuous intensity. It concluded at about 0610. Peak activity did not reach the same levels as the preceding phase, but ejected pyroclastics ~200 m above the crater rim. The finer portions were carried several hundred meters and dispersed E, without reaching residential areas.

Observations at the conclusion of the late-August explosive phase showed the new lava flow still spreading over the N flank of the SEC, but new lava had ceased venting. This new flow overrode the one from 28 August, and descended to ~2,100 m on the W face of the Valle del Bove.

The other craters in the volcano's summit area chiefly slowly emitted gas vapors, with the exception of one of BN's vents, which frequently ejected brown ash. The emission of ash from this vent intensified during the week. As September began, BN continued to produce abundant steam and ash emissions, which at times seemed aided by elevated atmospheric humidity and by infiltration of recent precipitation. This effect continued later into September.

In mid-September, BN produced generally mild degassing. During 19, 22, and 23 September nearly continuous ash emission took place. Primarily dark gray and sometimes brownish colored plumes were visible for many kilometers. For the preceding weeks these plumes had vented at two distinct crater cavities on the inside of the BN. The larger cavity lies in BN's center and discharged gaseous blue-white emissions. The smaller cavity lay near BN's internal SW wall, and it expelled ash. During this same time, as in past weeks during the month, the Voragine and Northeast Crater continued to emit abundant steam. The SEC weakly degassed from fumaroles.

October activity continued as in past months with ash emissions at the BN. These were particularly visible on 3-6 October. At night it was possible to observe light coming from the crater cavity on the inside of the BN, suggesting weak Strombolian activity. Mid-October behavior included explosive Strombolian eruptions from both crater cavities; incandescent bombs occasionally fell outside of the crater. Milder episodes occurred on 17 and 21 October. Between 24 and 29 October two stronger episodes took place.

At the Voragine and the NEC, the early days of October showed rather sustained steam emissions, in part accentuated by the first snowfalls and by the elevated humidity on the summit. The SEC displayed mostly fumarolic activity. Later, the Voragine gave off copious steam, but at the SEC and NEC weak degassing occurred.

The last days of October and the early days of November were distinguished by a decline of the explosive Strombolian activity from the two emission points within BN. Strombolian activity sent tephra ~100-150 m high, which still frequently fell outside of the BN crater.

During November, BN continued to produce modest explosive Strombolian activity that sometime spewed incandescent material of moderate size outside the crater walls. Observers continued to note two distinct cavities in BN.

In the early hours of 29 November observers noted the presence of a small lava flow at the base of the SEC. Upon close viewing, observers found that the flow gushed from the base of a fracture on the N sector of the cone at the SEC and continued downslope for ~200 m. Although lava continued to flow in the succeeding days, atmospheric conditions obscured later views of this area. No relevant activity aside from a constant steam emission occurred either at the Voragine or at the NEC during this time.

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of 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 caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

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

Piton de la Fournaise erupted several times during 2000; 14 February to 4 March (BGVN 25:01), 23 June to 30 July (BGVN 25:07), and in October. The last eruption in 2000 began on 12 October after two periods of inflation, high pre-eruptive radon emissions, and three weeks of increased seismicity beneath the volcano.

During the two months prior to the eruption, two tiltmeter stations, "Dolomieu Sud," located at the volcano's summit and "Château Fort" on its southern base, showed tilt variations of up to 50 µrad, which indicated a clear inflation of the S flank. In addition, extensometer data at Château Fort showed that fissure openings had significantly increased since the preceding eruption in June 2000. The fissure expansions confirmed that inflation was occurring.

Three weeks prior to the eruption high seismicity occurred under the volcano, with 10 to 20 earthquakes per day. A small seismic crises that consisted of 57 earthquakes occurred on 6 October (figure 56). Thereafter, the number of seismic events returned to the high levels that had been recorded during the previous 3 weeks until the number of earthquakes significantly increased on 12 October, marking the beginning of the eruption. All of the 278 seismic events that occurred between the end of September and 12 October were of very low energy, usually with magnitudes less than 0.7. Only seven earthquakes were recorded with higher magnitudes, ranging between 0.9 and 1.7.

Figure 56. The number of daily seismic events recorded at seismic stations at Piton de la Fournaise during 10 September through 21 October 2000. Courtesy of OVPDLF.

In addition to increased seismicity, high radon activity was measured at the volcano. Three different probes in soil and old eruption vents at the "Bory" station on the W rim of the summit crater showed a high mean level of radon activity. The "Bory 3" radon probe showed about 40 counts per day, which was 2.7 times higher than during January-May 2000. OVPDLF scientists determined that the high counting rates indicated a general increase in volcanic gas emissions from the volcano, reflecting the presence of degassing magma.

At 0401 on 12 October a seismic crisis began that consisted of 201 low-energy events (figure 56). All but five events had magnitudes less than or equal to 1.1, with the largest being 1.6. The seismic crisis lasted 64 minutes and at 0505 a strong eruption tremor, which was localized on the E flank of the volcano, appeared at the summit stations. Visual observations helped to constrain the eruption site between "Signal de l'Enclos" and "Le Langlois" craters, and above "Piton Pârvédi" crater, which formed during the previous eruption in June 2000 (figure 57). Field observations conducted with a hand-held GPS receiver allowed scientists to precisely locate the two fissures where lava was emitted during the eruption. The smaller fissure (fissure 1) was several tens of meters long, located at 2,260 m in altitude, and emitted a small, 50-100 m long aa lava flow. The other fissure (fissure 2) was 680 m long and ran continuously between 2,220 m and 2,000 m in altitude.

Figure 57. Sketch map showing the location of craters of the 12 October activity and fissures where lava flows were emitted. PDN (Piton de Neiges) is a coordinate system used on Reunion Island by IGN and other scientists. In general, IGN maps include both PDN and international ellipsoid coordinates. Courtesy of OVPDLF.

Almost all of the lava-flow activity occurred at fissure 2. At 1100 on 12 October, lava fountaining still occurred within the lower 350 m of fissure 2, and lava output was relatively high. A large network of numerous aa lava flows of up to 200 m width traveled down the SE flank of the volcano towards "Piton Pârvédi" and continued in a single, large lava flow for 5.5 km on the southern border of the June lava flow until reaching 400 m in altitude. At 2100 on 13 October, about 40 hours after the eruption began, the rate of lava emission was still high with an estimated rate of 40-60 m³/s. A continuous incandescent lava flow, at least 2 km long, was visible.

The following day volcanic activity was focused on the lower end of fissure 2, and a crater began to build up. It was named "Piton Morgabim." Initially the crater was U-shaped with an opening towards the ESE. Throughout the entire period of activity a permanent lava lake was present within the crater, and lava flows were observed on the downhill (SE) side of the crater. During the first week of November the crater closed so that the lava lake was no longer visible, and the upper crater walls were high and sub-vertical. Several tunnels began to form and a tumulus that was several tens of meters high piled up in front of "Piton Morgabim" (figure 58). Since the end of October pahoehoe lava flows appeared in the upper part of the initial aa lava flows and surrounded "Piton Pârvédi" crater to the N and S.

Figure 58. Photograph of the eruption, taken from the SW at 0943 on 9 November from a helicopter. The photograph shows the initial crater ("Piton Morgabim") and the new vent (circled to the left) and an active incandescent lava flow channel. The pahoehoe lava flows above "Piton Pârvédi" that began in late October can be distinguished (gray area), as well as the tumulus in front of "Piton Morgabim." Courtesy of OVPDLF.

Since 29 October, tremor began to increase until it reached the same high value as during the first minutes of the eruption. Tremor remained at high levels for the following 5 days. Beginning on 5 November strong degassing and liberation of H₂S occurred just above "Piton Morgabim." On 8 November the upper crater walls collapsed and the [lava] lake, which was ~40 m in diameter, was visible again. On 9 November an intense explosion occurred ~50 m NW of "Piton Morgabim" crater, and rocks and lava were ejected up to 200 m in altitude. A second vent formed in this area and both it and "Piton Morgabim" were simultaneously active for several tens of hours (figure 58 and 59). From 12 November, explosions and black ash were observed at the upper vent, which were most likely phreatomagmatic features. Lava bombs were ejected up to 250 m away from the vent. Both vents fused together, and the initial crater raised up, finally forming one single large crater named "Piton Morgabim" (figure 59). Figures 16 and 17 show different stages of the vents growing together. During the period of increased tremor, new several-km-long pahoehoe flows formed. Again they surrounded Piton Pârvédi to the N and S and covered large parts of the June 2000 lava flow. In particular, one pahoehoe lava flow extended beyond the front of the June eruption in the "Grand Brûlé" by ~500 m length down to 370 m elevation.

Figure 59. Photograph of "Piton Morgabim" and a second crater coalescing at Piton de la Fournaise. The photograph was taken on 11 November from the E flank of the volcano. The saddle-shaped separation between the two craters disappeared during the next days. Bright spots to the left and right of the craters were emanations from the lava flow and fissure 2, respectively. Courtesy of P. Morin.

The high level of tremor suddenly disappeared at 2310 on 13 November, marking the end of the eruption. By this time the remaining crater, "Piton Morgabim," was ~100 x 75 m across and 30-40 m deep (figure 60). On 15 November, the lava flow SE of the crater was still hot; a temperature of ~800°C was measured 40 cm below the surface.

Figure 60. Photograph of the surface of the affected area of Piton de la Fournaise after the eruption. The black line shows the outline of the lava flow. Courtesy of OVPDLF.

Basalt samples were collected throughout the eruption. The initial basalt was apheric, near the end of October olivine crystals appeared, and near the end of the eruption the basalt had numerous centimeter-sized olivine crystals.

Digital photos were analyzed in order to map the lava flow and to obtain an estimate of it's erupted volume. The total erupted volume was estimated to be on the order of 5 x 10⁶ m³, which is a typical value for eruptions at Piton de la Fournaise.

Correction. In BGVN 25:07 the area of the entire lava flow from the 23 June-30 July 2000 eruption of Piton de la Fournaise was reported as being 3 x 10² m², when it was actually 3 x 10⁶ m².

Geologic Background. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: Thomas Staudacher, Jean Louis Cheminée, Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, Institut National des Sciences de l'Univers, 14 RN3 - Km 27, 97418 La Plaine des Cafres, Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/observatoire-volcanologique-piton-de-fournaise).

A minor explosion occurred at 1915 on 11 January 2001. The explosion ejected ash that coated Bajawa (~8 km from the summit) with an ash layer less than 0.5 mm thick. Increased activity after 11 January prompted the VSI to set the volcano's hazard status to 3 (on a scale of 1-4). Three explosions occurred at about 0700 on 13 January, sending ash 300-1,000 m above the crater rim. Workers at the volcano's observatory post, located ~7.5 km from the summit, subsequently heard thundering sounds. Ash, which appeared dense and light in color, blew E to Toa and S to Boya, Bolodio, and Bajawa. By 15 January, a seismograph recorded continuous tremor with an amplitude of 2 mm in addition to 59 explosion earthquakes with amplitudes of 2-14 mm.

Ash emission was ongoing as of 16 January, and ranged from 100 to 1,000 m above the summit. VSI workers observed two new large craters trending SE-NW. The top of the SE crater measured 50 m in diameter, narrowed to 25 m at its base, and was 10 m deep. It emitted an audible sound and ejected an ash plume from its N wall with variable pressure. Winds tended to blow ash toward the S. The NW crater was 20 m in diameter and 1.1 m deep. The temperature of a fumarole measured 95°C, and nearby ground temperature measured 89°C.

During 16-22 January, explosions produced both ash and lapilli. Light gray ash fell around the main crater within a 10-20 m radius. Lapilli, which had a maximum size of 50 cm, fell up to 500 m from the main crater.

Geologic Background. Inielika is a broad, low volcano in central Flores Island that was constructed within the Lobobutu caldera. The complex summit contains ten craters, some of which are lake filled, in a 5 km2 area north of the city of Bajawa. The largest of these, Wolo Runu and Wolo Lega North, are 750 m wide. A phreatic explosion in 1905 formed a new crater, and was the volcano's only eruption during the 20th century. Another eruption took place about a century later, in 2001. A chain of Pleistocene cinder cones, the Bajawa cinder cone complex, extends southward to Inierie.

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

Heightened activity continued at Karangetang in late December 2000-late January 2001, following a year of frequent activity in 2000 (BGVN 25:11). The main crater and Crater II sent a white, variably-thick ash plume up to 600 m above the summit during 19-25 December. Plume illumination up to 150 m above the craters was visible at night. Lava flows occurred on 21-22 December and reached as far as 1,250 m laterally along the SW flank. The seismic record also showed increased activity with multi-phase earthquakes predominating.

Activity, however, tailed off during 26 December-1 January before increasing again with renewed vigor from 2 to 8 January. At 1258 on 2 January an explosion produced a white-gray ash plume that rose ~500 m above the summit. At 1845 on the same day, workers observed a glowing lava avalanche issuing from the main crater and moving 50 m from the summit down toward the Naitu River. A larger explosion on 7 January sent gray ash 1,500 m above Karangetang. A coeval Strombolian eruption cloud rose 200 m. Ashfall occurred W of the volcano, coating Pahe, Lehi, Mini, and Kinali villages. Lava flowed down to the Tanitu River as far a 1 km from the summit. Tectonic earthquakes dominated seismicity during the week, and a significant number of tremor earthquakes also occurred.

A minor explosion occurred on 10 January; ash rose and subsequently fell back into the crater. Tectonic earthquakes again overshadowed all other types during 9-15 January. At 0845 on 17 January an explosion generated a small ash plume and a lava avalanche. Ash fell on Salili and Beong villages; lava flowed down both the E and W flanks of the volcano. Seismicity remained elevated with earthquake distributions similar to the previous week. The VSI maintained a hazard status of 2 (on a scale of 1-4) for Karangetang throughout the report period.

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

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

Increasing crater lake temperature, water level, and inflation have been observed since 19 January 2001. Water temperature in the crater lake rose to 47.5-49.1°C. On 21 January water level rose 5 cm. Leveling measurement showed 5.5-6 mm of inflation. During 16-22 January, seismographs recorded 20 tectonic earthquake events. These observations prompted the VSI to increase Kelut's hazard status from 1 to 2 (on a scale of 1-4).

Geologic Background. The relatively inconspicuous Kelud stratovolcano contains a summit crater lake that has been the source of some of Indonesia's most deadly eruptions. A cluster of summit lava domes cut by numerous craters has given the summit a very irregular profile. Satellitic cones and lava domes are also located low on the E, W, and SSW flanks. Eruptive activity has in general migrated in a clockwise direction around the summit vent complex. More than 30 eruptions have been recorded since 1000 CE. The ejection of water from the crater lake during the typically short but violent eruptions has created pyroclastic flows and lahars that have caused widespread fatalities and destruction. After more than 5,000 people were killed during an eruption in 1919, an engineering project to drain the crater lake lowered the surface by more than 50 m. The 1951 eruption deepened the crater by 70 m, leaving 50 million cubic meters of water after the damaged drainage tunnels were repaired. Following more than 200 deaths in the 1966 eruption, a new deeper tunnel was constructed, and the lake's volume before the 1990 eruption was only about 1 million cubic meters.

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

Activity has continued intermittently since early August 1999 (BGVN 24:11). Celia Nyamweru compiled a report based on observations and interpretations of photographs taken of the crater on various dates between 2 September 1999 and 29 July 2000, and observations made during a 23-26 July 2000 visit to the summit crater. Observations and photos during this period are from the mountain guide Burra Ami Gadiye unless otherwise noted. The report for July 2000 also includes the observations of Fred Belton. A report of fieldwork performed in early October 2000 was prepared by Christoph Weber.

Some cones have been renumbered according to the system agreed on by Nyamweru, Belton, and Weber in October 2000. Under the revised system a new eruption center is assigned a new T-number (e.g. T49). New cones at the flank of an existing cone, and clearly fed by this, will be identified with letters appended to the T-number (e.g T49B, T49C, etc.). This leads to the following renumbering: T52 (formerly T52C), T52B (formerly T52W), T52C (formerly T52E), T37 (formerly T37S), T37B (formerly T37N1), T37C (formerly T37N2), and T37D (formerly T37E and also formerly T5/9). This report reflects the new cone names.

Activity during September-December 1999. Reports and photographs from Gadiye on 2 and 10 September 1999 showed no eruptive activity, almost no dark lava visible, and steaming from cone T47. Colors of the crater floor ranged from white to light gray and light brown, with a slightly darker (chestnut brown) patch of lava around the lower slopes of T45. Small dark patches on the upper slopes of T40 might have been fresh spatter. There were no new cones or significant changes since July 1999. On 10 October Gadiye reported similar activity, plus steam from T37 and T47. A small flat-topped cone W of T37 and T45 may be new, though it is pale gray.

Gadiye's notes from 9 November refer to a cone (most likely T40) throwing out lava. The eruption appeared to be small, with lava bubbling at the top of a hornito on the lower slope of the cone. Some small flows appeared to have come from the central part of the slopes of one or two of the large cones, so possibly there had been some minor activity since 10 October 1999. Joerg Keller reported about a 23 December visit to the crater by Michael Kraml, Ralf Gertisser, Marika Vespa, and Andrea Bull. Patches of relatively fresh looking lava were seen around cones T48 and T49. Between cones T48 and T49 there was a 30-cm-thick layer of natrocarbonatite tear-drop lapilli, relatively fresh in appearance and about a week old. No new cone was present on the W side of the crater floor.

Activity during January-20 July 2000. Gadiye's notes from 2 January refer to "a completely new cone that appeared in December 1999" shown in several photos. In one, he describes it as "throwing out lava violently" although this is not entirely clear from the photograph. This cone, towards the W side of the crater, has been named T51. Most of the crater floor was white to light gray and light brown, with pahoehoe lobes and other well-defined structures. Located close to the W wall, T51 had regular, rather steep slopes and was surrounded with fresh lava. These flows are pahoehoe but appear rather thick in comparison to their length, possibly 10-20 cm thick rather than the 2-5 cm layers observed elsewhere. Emission of steam occurred from several cones in the central cluster.

On 6 January Gadiye noted that very dark fresh-looking lava seemed to originate from a small vent between the E (T37-T45) and W (T47-T49) cone clusters. This may be the beginning of the activity that produced the T52 group described in July 2000. Overall the colors of the crater floor ranged from white to light gray and light brown. The lower slopes of T45 look significantly darker than the pale gray lava surrounding them.

Gadiye's photographs from 24 January showed continued activity from the area between the E and W cone clusters. The source of four long narrow tongues of lava seemed to be a low flat-topped cone between T48 and T37B. Other photographs show a large patch of dark lava between the older cone clusters and possibly a small flow from it towards the NE. Colors of the crater floor ranged from white to light gray, except for the dark patches around T52 and the new flows. The lava at the E overflow was white and showed little or no change since 1999.

Aerial photographs taken on 8 February by Benoit Wangermez showed no visible eruptive activity. No significant changes seemed to have occurred since late January 2000. Gadiye noted no eruptive activity on 12 March. T40 showed little change, there was a small pale brown hornito between T40 and the NW overflow, and T51 was rather pale brown with no sign of dark lava flows around it. Photographs by David Bressler in late April/early May revealed a cone with fresh lava on its slopes, probably T49B. No eruptive activity could be seen on an aerial photograph taken by Nigel Pavitt on 2 May (figure 65). Gadiye also did not report any activity on 7 May. No fresh lava flows were visible in his photographs; the flows originating from the T52 vent(s) were mid-brown in color.

Figure 65. An aerial photograph of Ol Doinyo Lengai taken on 2 May 2000 from the W showing the upper part of the cone with Ketumbeine mountain in the background. The summit crater is white, with no details of individual cones visible. The outer walls on the NW and W side are clearly visible, along with the NW overflow, the breakout lava flow of June 1993 on the W wall, and the numerous erosional gullies that expose white ash. The similarity of color between the weathered lava and the weathered ash in the gullies makes it very difficult to determine how far down the slopes the lava flows have extended. Photograph by Nigel Pavitt; courtesy of Celia Nyamweru.

No activity was obvious in 20 July aerial photographs from Luigi Cantamessa (Geo-Decouverte SA), though some very fresh lava may have been present around T51. A near-vertical view of the crater floor and outer W and NW slopes showed the NW overflow clearly, with very pale brown seeming to extend several hundred meters down the outer slope. The breakout lava flow of June 1993 was also visible on the W slope, and seemed shorter than the NW overflow. A photo taken from over the summit looking N revealed a variety of colors on the crater floor, evidence of many lava flows. One medium-sized brown flow extended S from T46, ending in a broad front of rounded lobes. A near-vertical view with the NW wall in the foreground showed several flows of pale brown and pale gray lava that had moved across the crater rim. The darkest patches on the crater floor were NNW of T49B and W of T51. The patch near T51 was small with very narrow lava tongues radiating outwards. These appeared to be very recent, as such small very narrow flows would not remain dark for very long once the eruption ended.

Activity during 23-30 July 2000. Observations and photographs were made during summit visits by C. Nyamweru (23-26 July) and F. Belton (23-30 July).

Observations made by Nyamweru of the crater floor on 24 July (figure 66) showed that the N part was mostly pale gray, pale brown, or white in color, with no sign of recent lava flows. The youngest lava flows were in the S and E parts of the crater floor. Flow 1, originating from a small vent on the slopes of T46, was probably several weeks old; it still retained its form, pahoehoe surface texture, and a slightly darker brown color than its surroundings. It had recently been partly covered by Flow 2, which was probably less than 24 hours old when Nyamweru's group arrived at the summit on 23 July. On the morning of 23 July it could just be touched with a bare hand; most of the flow was very dark brown with a small amount of whitening around the edges of the slabs. It retained significant warmth and cracking sounds could frequently be heard from within this flow. Flow 2 was ~1.5 m thick with a rough surface composed of broken, tilted pahoehoe slabs, covering much of the S crater floor. It appeared to have originated from the small T37D vent. The lava flowed S and E, surrounding two remnant slabs of the old crater wall and pouring down in a 'lava fall' between T24 and the crater wall to the lower level of the S crater floor. It flowed into T24 and partly buried it, and also flowed around T26, T27, and T30. 'Lava strandlines' were visible around the crater wall E of T24 where the lava lake had been held at a higher level. Flow 3 moved from the N slope of T37B to the N and E to within a few meters of the E overflow. It was still very hot about 1330 on 23 July. Several large blocks of older lava on the upper part of Flow 3 had probably been part of the top of T37B.

Figure 66. A sketch of crater features at Ol Doinyo Lengai made from the summit on 24 July 2000. Notable features include three recent lava flows, large cracks in the crater floor and walls, and a lapilli field. Courtesy of C. Nyamweru.

The N and W crater floor were crossed by radiating cracks, some of which continued from the floor up through the crater wall. Some of them emitted steam and sulfur fumes, and in places the ground along the cracks was bright yellow with sulfur crystals. Such cracks have long been features of the crater floor, but compared with earlier years there were more of them and they were wider. Nyamweru estimated one crack on the N wall as being ~1 m wide; Belton measured a crack between T40 and T49 and obtained a width of 60 cm and a depth of 4.1 m. Four or five big cracks continued from the floor up onto the N crater wall; this was not something that had been obvious in earlier years. A deep crack extended from T51 across the SW crater floor and up onto the crater wall.

At the NW overflow a photograph taken from the road N of the cone showed what may be a very narrow tongue of white lava (?), not present in July 1999. It appeared to have flowed down a gully in which whitish patches of ash are visible lower down. There is no evidence of major lava flows spilling out of the crater during the last few months, possibly even during the last year. The most recent flows to have crossed the rim may be towards the S end of the overflow, and are small, discolored pahoehoe flows that may have emerged from T51 since January 2000. A crack over 20 cm wide emitting sulfur fumes and steam ran from T49B NW towards the crater rim at the overflow.

In a photograph of the E overflow taken from the road E of the cone no changes were evident since July 1999. However in July 1999 Nyamweru measured the width of this overflow as 22 m, whereas in July 2000 it was 38 m. In July 2000 there was no sign of any fresh lava approaching the overflow apart from Flow 3, which reached within 20 m of the N side of the overflow on 23 July. The low point on the SW rim had changed little since July 1999; small pahoehoe flows from T51 had reached the SW crater wall, but not close to this low point.

T51 was the new cone on the W side of the crater, probably formed in late December 1999. T47/T36/T39 showed little or no change. T46/T44 showed little or no change, but was the source of Flow 1. T48 had collapsed. T49 has been joined by a well-defined cone to its W, called T49B. Three new cones (T52 group) formed E of T48; the western one was a jagged brown cone with no signs of recent activity; the two eastern ones were younger, with smoother shapes and dark gray to black in color, possibly the source of some small lava flows within the last few days. T37B's big open vent had collapsed. The small T37D cone, visible in 1999 photographs, seemed to have been very active in the few days preceding the visit, in particular as the source of Flow 2.

The lapilli field covered an area some tens of meters across, S of T45 and E of T37D. Here the lapilli, well-formed spheres and ovals less than 2 mm in diameter, were black and still warm on 23 July, forming a layer ~8 cm thick. In this area the lapilli overlay some recent lava but in turn were overlain by small pahoehoe flows. Nearby the surface layer of lapilli had already turned white, but below 1-cm depth they were still warm and black. Elsewhere, smaller quantities of lapilli very similar in appearance occurred on the crater floor and on the E crater wall. Lapilli extended ~130 m across the crater floor from the N end of the E overflow, lying in small depressions. In this area the lapilli were pale gray on the afternoon of 23 July. Small amounts of similar gray lapilli were seen on the surface to about a quarter to a third of the way up the SE crater rim.

Belton provided detailed descriptions of the activity at the cones during this period. Activity was nearly continuous at T49B, but varied considerably in nature and intensity. The cone degassed frequently, sometimes emitting loud jets of steam and lava fragments, other times producing a steady output of invisible gases. The degassing alternated with lava splashes that coated the sides of the cone. Eruptions usually occurred 4-5 times per minute. On the night of 23 July cone T49B produced several short aa flows. Rockfall from the top of T49B was also common, with some lava boulders 30 cm in diameter rolling up to 7 m from the base. Throughout the week the summit vent(s) of T49B frequently changed size and location.

The T51 cone built up a low shield in the WNW part of the crater. Lava overflowed from the summit vent of T51 several times during the early morning of 23 July. In a much larger eruption at 1130, lava of very low viscosity cascaded down the N flank and formed pahoehoe flows at its base. A similar but smaller eruption occurred at 1900. From 24 through 27 July cone T51 contained lava at depth (5 m). At 0600 on 28 July a lava pond was 2 m below the rim of the 1-m-diameter summit vent. The pond degassed with increasing vigor and gradually rose closer to the top of the vent. At 1645 lava overflowed the N side, forming channel-fed pahoehoe flows (figure 67). Similar activity continued through the night and into 29 July. Numerous small cones formed above the lava tubes and erupted highly vesicular lava, really nothing more than brown foam. Around 1300 on 29 July surges about once per minute caused the pond to overflow. The eruption continued through 0800 on 30 July (figure 68). One flow traveled 75 m NW to within 16 m of the NW crater rim breach. During the 39 hours of activity, T51 grew in height by at least 1.5 m and its summit vent was reduced in size by ~75%.

Figure 67. Cone T51 in the Ol Doinyo Lengai summit crater overflowed at 1645 on 28 July, sending lava flows down the N and NW slopes. The shape of the small lava shield that T51 has constructed is apparent here. The summit of Ol Doinyo Lengai is just behind the hornito. Courtesy of F. Belton.

Figure 68. Photograph showing the summit area of cone T51 in the crater of Ol Doinyo Lengai on the morning of 29 July. After erupting through the night of 28 July, the lava dropped to a lower level inside T51 by 0800 on the 29th. A climb to the top revealed beautiful lava stalactites around the interior rim of the ~ 1-m-diameter summit vent. Courtesy of F. Belton.

At around 1300 on 23 July a short (about one minute) unusually violent eruption from T37B sprayed ejecta ~25 m above the cone. It is probable that this activity also created Flow 3, a fast moving 15-cm-thick flow of ropy lava that moved to the E. Minor activity also occurred in this part of the crater on 25 July between about 1500 and 1600 when a 10-m-long pahoehoe flow emerged from a small ground-level vent just E of T37D.

Activity during October 2000. An expedition organized by Chris Weber from 3 to 11 October 2000 consisted of a film team and four scientists, led by Joerg Keller. Observations were made by J. Keller, A. Zaytsev, D. Wiedenmann, J. Klaudius, D. Szczepanski, M. Szeglat, and C. Weber. The best-known track is on the WNW flank. Two other different routes were taken during descents following the visit (figure 69). The track down the NE flank (named Dorobo-Route) and a second track starting halfway between the W crater wall and the summit and descending the WSW flank to pass the Kirurum crater (named Reck-Route) were followed by different expedition members. An overnight camp was made at the Kirurum crater to give time for fieldwork. GPS data and barometric measurements gave new information about the elevations of various points on Ol Doinyo Lengai. The summit peak is approximately 2,955 m (2,950-2,960 m) elevation, cone C on the N crater rim is approximately 2,835 m, and the crater floor was approximately 2,925 m at the NW and E overflows.

Figure 69. Sketch map of the Ol Doinyo Lengai area showing nearby geographic features and climbing routes, October 2000. Courtesy of C. Weber.

Flow 1 (figure 70) still had a brown appearance, but continued to weather and was lighter in color than on 30 July. The younger Flow 2 was partly black to gray in the joints and cracks of the aa flow field. Some smaller flows around T49 and T49B were slightly black, though probably only a few hours old. Hydration of fresh lava flows (especially under high humidity) can cause a black surface to turn white within 24 hours. NW of T49B another cone appeared after 30 July and was named T49C. T51 was surrounded by flat pahoehoe flows and had grown since 30 July. There was a new cone in the collapsed T48 with some small light gray lava flows close to the cone. During this year many new cracks (at maximum up to 1 m wide and 5 m deep) had opened all across the crater floor. Most of the V-shaped cracks pointed to the T52 and T49 clusters, roughly the center of the major cone concentration. Some of the cracks broke through the crater rim. Other cracks were filled or covered by young lava flows.

Figure 70. Sketch maps of the Ol Doinyo Lengai crater, October 2000, showing cracks (left) and recent lava flows (right). Courtesy of C. Weber.

Between 1200 and 1350 on 3 October spattering occurred from a small vent in the saddle between T49 and T49B. Two small lava flows were observed at the N and S flank of T49 during that time. At 1350 the W side of T49B collapsed, creating a ~6-m-wide and 5-m-high opening from which a sudden flash flood of lava was released. Parts of the collapsed wall of T49B were washed towards the W as big blocks. Within a few seconds the flow had reached halfway between the cone and the NW overflow. After 5 minutes the flow (Flow 4A) had reached its final extent ~40 m short of the NW overflow (figure 70). The lava flow was up to 5 cm thick and later aa flows were several decimeters thick. Until 8 October lava spattering and small lava flows had nearly closed the gap in the W wall of T49B. On the morning of 9 October at 1035 the W flank again collapsed in the same manner as on 3 October, leaving a 7-m-wide and 7-m-high gap. A flash flood of lava moved NW (Flow 4B) within seconds and stopped just 10 m from the NW overflow, covering Flow 4A. During the afternoon of 9 October T48 had strong degassing and for 10 minutes ejected tear-drop lapilli; no further activity was seen. Right after sunset of 9 October a crack opened at the SSW base of T49C with a noisy gas jet followed by a 10-minute spray of lava droplets and spherical lapilli up to ~10 m high. Small lava flows (Flow 4C) were emitted and moved NW. No more flows were observed through 11 October, but the lava lake inside T49B was splashing and degassing.

Between 3 and 9 October 2000 temperature measurements were made by three different instruments and gave consistent values. A digital thermometer (TM 9¹⁴C with a K-type stab feeler) was used in the 0-1,200°C mode, taking readings by inserting the feeler 10 cm into still-moving and liquid lavas (10 times on various days) and as deep as possible into the fumaroles (five times on various days). Calibration was by the Delta-T method: values are ± 6°C in the 0-750°C range. All values were recorded by four repeat measurements at one spot. The pahoehoe lava flow (15 m below outflow from the T49B lava pond in an closed lava tube) was at 507°C. An aa flow front in slow motion (shortly after escaping an enclosed lava tube near T49B, 25 cm thick) was 496°C. The fumarole 25 m NNW of T49C in a crater crack towards the rim was at 75°C. The fumarole at the NW overflow inside the old crater rim was at 69°C. The fumarole on the NW flank of T48 was at 95°C.

On the evening of 3 October, one of the scientists (Jurgis Klaudius) accidentally stepped in a fresh but already solid-looking lava flow (~25 cm thick) at the W slope of T49B. This can easily happen in the dark when it is difficult to discern between solid and fresh black flows. In this case it caused a serious second-degree burn around his left ankle up to his lower leg. The lava, at a temperature of about 500°C, burned away all of the light plastic parts of his sport shoe, leaving the leather parts and the sole. On 6 October evacuation was necessary because of the risk of infection. He managed to slide down the steep slopes on his hands and right foot for most of the steep upper track, but was finally carried the rest of the way down in a seat on the shoulders of four porters. He was brought to a hospital in Arusha and then flown to Germany 24 hours later. Klaudius is recovering very well following skin grafts and will not suffer lasting damage.

Lengai is as dangerous as any other active volcano. Activity includes explosive eruptions, suddenly appearing lava fountains, several cone collapses, lava flash floods, and flows of enormous quantity. A lava temperature of 500°C is hot enough to burn someone seriously and because of the very low viscosity, this natrocarbonatite lava is extremely fluid and can flow very fast. Visits are not recommend without a guide.

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: Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton NY 13617 USA (URL: http://blogs.stlawu.edu/lengai/); Joerg Keller, IMPG, Albert-Ludwig-University Freiburg, Albertstrasse 23b, 79104 Freiburg, Germany; Christoph Weber, Volcano Expeditions International, Friesenstrasse 20, 42107 Wuppertal, Germany (URL: http://www.Vulkanexpeditionen.de); Frederick Belton, 3555 Philsdale Ave., Memphis, TN 38111 USA (URL: http://oldoinyolengai.pbworks.com/); Marc Szeglat (contact for video-film clips), Duelmenerstrasse 11, 46117 Oberhausen, Germany (URL: http://www.vulkane.net).

Eruptive activity increased markedly at Merapi during the period of 26 December 2000-22 January 2001. Instrumental monitoring first recorded a significant increase in seismicity, expressed in both shallow and deep volcanic earthquakes, during 26 December-1 January. Visual observations were hindered during this time because of hazy conditions, and VSI maintained a hazard status of 2 (on a scale of 1-4) for Merapi.

Activity continued to increase during 2-8 January. Atmospheric conditions were clearer, allowing observation of a 1,500-m-high plume above the summit. Lava avalanches flowed ~1 km from the summit down to the Sat River. Seismicity remained high, again with a significant number of shallow and deep volcanic earthquakes, and was dominated by multi-phase and avalanche earthquakes.

During 9-15 January, activity again increased with respect to the previous week. Accordingly, VSI elevated Merapi's hazard status to 3. Observers noted a light-colored, variable-density, low-pressure ash plume that rose 500 m above the summit. Glowing lava avalanches flowed into the headwaters of the Lamat, Sat, and Senowo Rivers, up to 2 km from the summit. On 14 January, 29 pyroclastic flows traveled down the volcano's flanks into the three above-mentioned rivers and reached up to 4 km from their source. During the week, lava avalanches and pyroclastic flows occurred with an average interval of 0.5-1 hours.

Visual observations from several post observatories during 16-22 January revealed ash eruptions, glowing lava flows and avalanches, and pyroclastic flows. Merapi ejected a dense, light-colored ash plume under medium to high pressure. Ash rose 850-1,300 m above the summit, with an estimated emission volume of 95 metric tons/day. Ashfall occurred on the surrounding areas of Babadan, Kaliurang, and Ngepos. Glowing lava avalanches, with more than 150 occurring per day, reached as far as 3.5 km from the summit into the Bebeng, Sat, and Senowo Rivers. Observers suggested more than one source vent for these flows. More than 20 pyroclastic flows occurred daily during the week, sending ash and gas a maximum of 3 km down the Bebeng River, 4.5 km down the Sat River, and an unreported distance down the Senowo River.

The Darwin VAAC issued an ash advisory on 19 January to advise pilots of ash emanating from Merapi. The advisory reported an ash plume up to an altitude of ~3,400 m. Prevailing winds were projected to carry ash to the E or SE; cloud cover prevented any further descriptions.

A new lava dome, termed "2001," grew on top of the 1998 dome that had collapsed around 16 January. Growth appeared continuous with the glowing dome visible at night. Researchers speculated that the failure of the 1998 dome and the instability of the new dome accounted for the high frequency and volume of pyroclastic flows.

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 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent 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.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 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/).

This report discusses previously unreported information about the activity during March and April 2000. This interval of low activity occurred prior to one with heightened seismicity during May and June 2000 (BGVN 25:06).

The seismic swarm that began in May 2000 reached its peak during 9-11 June when the INETER seismic network registered over 500 earthquakes (BGVN 25:06). Many of the earthquake magnitudes were between 3.4 and 4.1, and the small epicentral area was directly under a geothermal plant on the S slope of the volcano. INETER reported that prior to the seismic activity, in March 2000, seismicity was low, with only two seismic events during the month. They did not visit the volcano during March.

On 9 April, Pierre Delmelle of the Université de Montréal along with local guides visited the volcano's crater. According to Delmelle, the crater was horseshoe-shaped and recent landslides had occurred down the crater's walls. He also noted that the majority of the fumarolic activity took place in the bottom of the crater. Gas was released from the fumaroles with very weak pressure, and temperatures ranged from 100 to 460°C. INETER personnel made a previous trip to the crater interior in September 1998 and found a lack of fresh landslides down the crater walls; fumarolic gas temperatures were 79 to 235°C.

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

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Pierre Delmelle, Département de Géologie, Université de Montréal, Montréal, Québec H3C 3J7, Canada.

Around the end of the year 2000 and in January 2001 Popocatépetl extruded dome lavas at record-setting rates and amassed the largest active dome ever recorded on the volcano. The seismic energy released in one 25-hour interval in mid-December was greater than the accumulated energy for any entire year for which measurements are available. The highest plume of the interval rose to ~8 km above the summit crater.

During late 2000 into January 2001, but particularly in December, tremor reached the biggest amplitudes yet recorded during this multi-year crisis; it was felt by people 12-14 km distant, and one tremor episode prevailed for ~10 hours. Another episode saturated instruments to the point of damage and drove tiltmeters in dramatic oscillations.

Although impressive plumes had been seen before in this crisis, for the first time hot ash and gases began escaping the summit crater regularly, accompanied by pyroclastic flows and mudflows. The longest pyroclastic flow reached a runout distance of ~8 km. Preliminary photo analysis made during episodes of harmonic tremor in mid-December led to lava extrusion-rate estimates that were more than an order of magnitude higher than those typically seen at stratovolcanoes. During mid-December, sulfur dioxide (SO₂) fluxes reached ten to twenty times larger than the volcano's typical ~5,000 tons/day.

Although later Bulletin reports will provide more details, what follows here are critical highlights for assessing the behavior through 29 January. The report was provided by Servando de la Cruz-Reyna, Carlos Valdés-Gonzalez, Roberto Quaas-Weppen, and affiliated CENAPRED scientists noted below.

Relative quiet followed by unrest. The previous episode of dome growth took place in February 2000, resulting in the smallest of all domes grown since 1996 (see BGVN 25:01). After a period of relative quiescence, unrest followed at Popocatépetl in early September 2000 (BGVN 25:10). This marked the beginning of a new episode.

September unrest was marked by two seismic observations. First, harmonic tremor appeared in the peak of the exhalation signals. Second, tectono-volcanic earthquakes below the crater were followed by long-duration explosive eruptions that generated higher-altitude plumes. GOES satellite imagery depicted strong thermal anomalies in the crater. Still, only comparatively minor dome growth was detected in mid-September, and this same pattern continued during early November. A variable, somewhat reduced level of activity continued into early December.

Escalation in December 2000. As discussed below, RSAM values climbed precipitously during a 7-day interval in mid-December. Prior to that, on 2 December an ash emission of moderate-to-large size lasted about 90 minutes. On 6 December, nine low-magnitude earthquakes (M ~1.7-2.4) occurred followed by a similar swarm on 8-9 December. These earthquakes, in turn, were followed by a period of low-frequency harmonic tremor that lasted about 5 minutes. Although brief, this tremor had the largest amplitude recorded since this eruptive period began in 1994.

Particularly during December, tiltmeters, for the first time since their installation, registered all of the large tremor signals (figure 30). Tilt oscillation amplitudes were typically in the range of 100 µrad, reaching peak-to-peak values near 200 µrad. Seismicity during 11-18 December was extremely high (figures 31 and 32).

Figure 30. Tilt at Popocatépetl recorded by various stations for 11-20 December 2000. The large tilt displacements occurred in conjunction with high-amplitude tremor. The x and y directions are neither radial nor strictly tangential in orientation. The two axes lie at right angles on a horizontal plane such that a line 45 degrees away and bisecting both these axes trends through the center of the volcano. Courtesy of CENAPRED.

Eruptive activity increased on 12 December 2000 with frequent ash-bearing emissions (up to 200 per day), some of them reaching about 5-6 km above the volcano's summit. During the following night observers saw incandescence and small amounts of hot debris. Similar activity and longer-duration eruptions during 13-15 December produced light ashfalls on towns around the volcano.

Early on 15 December more episodes of high-amplitude, low-frequency harmonic tremor were detected, lasting a few minutes. At 1404, the low-frequency harmonic tremor grew to a continuous signal, with amplitudes peaking on all the monitoring stations, including the most distant one. These signals were strong enough to be felt by residents 12-14 km away, and to be detected at stations of the Mexican Seismological Network as far as 150 km from the volcano. This tremor episode remained at constant intensity for about 10 hours, and may have stemmed from very high rates of lava extrusion.

Starting early on 16 December activity underwent a dramatic drop that was reversed 16 hours later by a return of low-frequency harmonic tremor of increasing amplitude. This tremor again saturated all monitoring stations; it lasted about 9.5 hours. The amplitudes of the signals were so high that pen drivers and several styli of the paper-drum recorders were damaged. A still-larger tremor episode took place on 18-19 December.

Figures 31 and 32 illustrate the seismic traces and cumulative RSAM data. RSAM peaked during an interval of slightly over 7 days in mid-December, when low-frequency tremor prevailed for ~25 hours and the seismic energy released exceeded that of the rest of the year 2000. Actually, the peak surpassed that accumulated during any previous entire year for which records exist (including 1997, see figure 32).

Figure 31. Paper-drum records from Popocatépetl photographed while laid out on a flat surface. The records depict the record-setting seismic signals at Canario station (PPPN) on 30 June 1997 (labeled "a") and on 18-19 December 2000 (labeled "b"). Some of the records in the latter set (b, central to upper left) were re-scaled when the maximum pen displacement was shifted from 8 cm to 4 cm in order to stop damaging pens and motors during ongoing saturating oscillations. It is clear that the amplitude and duration of the 18-19 December 2000 events greatly exceeded those from 30 June 1997. Prior to mid-December, the 30 June 1997 events represented the largest amplitude tremor seen since 1994. Courtesy of CENAPRED.

Figure 32. Real-time seismic amplitude measurement (RSAM) plots for two Popocatépetl stations for the years 1997-8 and 2000, illustrating the extremely high seismic energy release seen in a time interval just over 7 days long (11-18 December 2000). This interval includes the acute increase seen on 18 December 2000. This ~7-day interval's energy release was eightfold larger than the total annual release in 1997. Courtesy of CENAPRED.

The episodes of quiescence and high-amplitude, low-frequency harmonic tremors occurred in such a pattern that they could be described as a load-and-discharge model, as suggested by the time-predictable model of Shimazaki and Nagata (1980). Using this paradigm, workers forecast the onset of the 18 December eruption and tremor episode.

Aerial photos taken on 16 December showed significant dome growth inside the crater (figure 33) and allowed correlation of the episodes of high amplitude, low-frequency harmonic tremor with periods of lava extrusion at very high rates. Analysis of the photos indicated that the dome grew at an average rate of ~180-200 m³/s during the episodes of intense harmonic tremor. This rate, which was not sustained, was about two orders of magnitude higher than any other previously observed.

Figure 33. A photograph taken looking into the crater at Popocatépetl, as viewed from the N on 16 December 2000. The substantial glacier on the N side lies covered by ash. Courtesy of CENAPRED.

At Popocatépetl, correlation spectrometer (COSPEC) measurements of SO₂ flux have had yearly averages on the order of 5,000 metric tons/day (t/d). In contrast, during 13-19 December the estimates were in excess of 50,000 t/d. On 19 December the reported value was near 100,000 t/d.

Civil authorities were made aware of the high magnitude of the monitoring signals, the very high rate of lava production, and the growth of the largest dome yet observed. This motivated them to constitute, on 15-16 December, an emergency board. They declared a further increase in the alert level and defined a security radius of 13 km. This radius was suggested to include at least some of the most vulnerable towns, like Santiago Xalitzintla (centered ~15 km NE of the crater) and San Pedro Benito Juarez (with a few residences 10 km SE of the crater, but the main town at 12 km from it). Santiago Xalitzintla sits downstream of the E side of the largest glacier along one of the main N-flank drainages. San Pedro Benito Juarez lies on a fracture zone on the SE flank, an area where many of the largest tectono-volcanic earthquakes were located. Additionally, increased deformation was also detected using the geodetic network located on that fault. San Pedro Benito Juarez is an isolated town closest to a notch in the SE crater rim. This notch is believed to have formed by collapse on 24 February 1664 during an eruption similar to the current one.

Preventive evacuation of Santiago Xalitzintla, San Pedro Benito Juarez, and other towns began on late 15 December and early 16 December. The decisions regarding which other towns should be evacuated were made by authorities at the state and municipal level. This caused some towns, well outside the security radius of 13 km, to also be evacuated by decision of their mayors. About 41,000 people left the area. Around half left the region by their own will and means. The other half used resources provided by local civil protection authorities. Of these, ~14,000 accepted transportation to shelters where they remained for about 10 days. Others moved to stay with relatives or friends.

The total volume of fresh lava accumulated within the crater of Popocatépetl was estimated to be between 15 and 19 million cubic meters on 18 December, exceeding the combined volume of all the previous domes (figures 33 and 35). The estimated vertical growth rate of the dome was such that another 20 or 30 hours of tremor associated with the above-mentioned lava production rate could potentially have enabled the dome to begin escaping the confines of the crater. The rate slowed, however, and the dome's upper surface remained well within the crater (figures 33 and 35).

As anticipated by the applying the above-mentioned model, after a three-day period of relative quiescence, on the afternoon of 18 December, a new eruption began. The relatively low-explosivity, yet long-lasting eruption of 18-19 December (figure 34) ejected large amounts of hot debris on the flanks of the volcano in three episodes of incandescent fountaining. Ejected hot debris is believed to have ultimately flowed a maximum distance of 5-6 km from the crater. Some images of these eruptions were distributed by some news media, which had installed cameras around the volcano and broadcast images in real time. After 19 December activity decreased noticeably. The next expected period of unrest, suggested by the time-predictable model to ensue near 23 December (figure 35), did not occur, likely indicating that the rate of magma supply had changed. What was believed to be the first dome-destruction explosion of this episode occurred on 24 December, ejecting incandescent debris to a distance of 3.5 km from the volcano, and producing an ash plume estimated to reach 5 km above the crater. When the nature and size of this event was understood, authorities reduced the security radius to 12 km. No towns lie within that radius, and accordingly many people returned to their homes.

Figure 34. An ash-bearing eruption column rises from Popocatépetl on 19 December 2000, viewed from the N. This kind of activity was common during the energetic mid-December time interval and stimulated international attention (e.g. the media and websites of Reuters, Stromboli Online, and others). Courtesy of CENAPRED.

Figure 35. Aerial photograph taken looking into the crater at Popocatépetl, as seen from the NE on 23 December. Courtesy of CENAPRED.

Later explosive events failed to excavate substantial portions of the new dome. The current estimate as of 16 January 2001 was that ~10-20% of the new dome volume has been blown out by explosions recorded after 18 December 2000. In many of the previous dome growth-and-destruction episodes since 1996, most of the dome mass has been removed by small to moderate (VEI <= 2) explosions; a similar scenario may play out in the near future.

After several weeks of relative calm, significant activity resumed at Popocatépetl on 22 January. At 1458 a M 2.8 volcano-tectonic earthquake occurred on the E flank. This event was possibly a precursor to a large ash emission that started at 1615, and initially produced an ash plume several kilometers in height. Eight minutes later observers saw a more explosive phase throwing incandescent fragments around the crater. After several more minutes, pyroclastic flows were generated and moved 4-6 km down several ravines on the N flank. Ash emission from the crater was continuous and punctuated by intermittent explosions. By 1640, the ash plume towered more than 8 km above the summit crater. At 1800 fluctuating harmonic tremor, similar to that of December, was registered. At times the signals again reached saturation amplitudes; the tremor could have been associated with magma intrusion into the base of the crater, an idea also suggested to explain previous tremor events. Harmonic tremor lasted for ~30 minutes. Ashfall was documented in Santiago Xalitzintla, Atlixico, and parts of Puebla and Tetela del Volcán. At 2200 it was possible to see ejected incandescent fragments that fell up to 1 km from the crater. On 29 January (figure 36), pyroclastic flows caused some glacial melting. The pyroclastic flows initially reached up to 8 km from their source, halting in the drainage upstream of Santiago Xalitzintla. They triggered some glacial melting and in early February their deposits were remobilized and came to rest about 15 km from the crater, about 2 km upstream of Santiago Xalitzintla. As of 29 January Popocatépetl remained at a Stage 3 Yellow alert.

Figure 36. Popocatépetl on 29 January photographed looking S. The image captured the forceful ejection of an ash-laden cloud. Courtesy of CENAPRED.

Reference. Shimazaki and Nagata,1980, Time-predictable recurrence model for large earthquakes: Geophys Res. Lett. 7, p. 279-282.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Carlos Valdés-Gonzalez, Roberto Quaas-Weppen, E. Guevara, A. Martinez, G. Castelán, S. Alcocer, C. Gutiérrez, G. Espitia, F. Galicia, M. Galicia, A. Gomez, G. Jiménez, C. Morquecho, J. Ortiz, E. Ramos, H. Romero, Centro Nacional de Prevencion de Desastres (CENAPRED), Delfin Madrigal 665, Col. Pedregal de Santo Domingo, Coyoacán, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); Servando de la Cruz-Reyna, Instituto de Geofisica, UNAM, Coyoacán 04510, México D.F., México.