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/).

Based on information from the Japanese Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC) reported that on 3 December 2003 at 2025 ash was emitted from Sakura-jima, rose to ~2.5 km a.s.l., and extended to the S. An eruption on 12 January 2004 at ~1430 produced an ash cloud that rose higher than 2 km altitude. On 19 and 20 February, explosions produced ash clouds that rose to unknown heights. No ash was visible on satellite imagery. Based on JMA information, the Tokyo VAAC reported that explosions on 26 March at 1715 and 27 March at 0607 produced plumes that extended S and rose to ~2.5 km and ~2 km altitude, respectively.

An eruption on 17 April produced a gas-and-ash plume that rose to ~3 km altitude and extended W. Another eruption on 25 April produced an ash plume that rose to ~2.4 km altitude. and extended N. The Tokyo VAAC reported, based on information from the JMA, that an eruption occurred on 28 April at 1820. It produced a plume that rose to ~2.4 km altitude and drifted SE. No ash was visible on satellite imagery.

According to the Har-Tass news agency, JMA reported a powerful ash-bearing discharge on 15 May at 1107. Specialists stated that the activity was the most intensive in four years. There were no reports of damage or injuries. The explosion registered as 'large' on the JMA's scale for both the sound and the strength of the tremor it caused, according to a quoted official at the local agency office in Kagoshima.

The Tokyo VAAC said the ash plume rose to more than 1.8 km altitude. An explosion occurred on 17 May at 1946, sending an ash plume to a height of 2.1 km altitude. On 18 May a pilot reported ash at a height of ~1.2 km altitude and ~23 km S of the Amori region. During 19-24 May, several explosions produced ash clouds. The highest reported ash cloud reached ~2.4 km altitude on 24 May. An explosion on 20 June at 1523 produced an ash cloud that rose to an unknown height.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: Naokuni Uchida, Japan Meteorological Agency (JMA), Fukuoka, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC) (URL: https://ds.data.jma.go.jp/svd/vaac/data/).

Ambrym (last reported in BGVN 29:03) exhibited high levels of activity in March and April 2004. During March, an active lava lake was present in Mbwelesu crater, one of the active summit craters. As of 27 March, there were reports that the people of Craig Cove in West Ambrym were suffering from the effects of the ongoing volcanic eruption on the island. Gas and acidic rainfall from the active vents on the volcano were threatening to destroy the local food gardens. The island was still recovering from the effects of Cyclone Ivy, which caused widespread damage two weeks earlier; the added affects of the eruption prompted Vanuatu's leaders to request emergency relief assistance from national and local authorities.

As of 3 April, reports confirmed by the Darwin VAAC and J. Seach described continuing lava lake activity at Ambrym. On 27 April, a large ash plume was recorded drifting 150 km NW of the volcano, passing the northern tip of Malekula Island and almost reaching Malo Island. Eruptions were still continuing up to 2 May.

NASA's Earth Observatory posted two images of Ambrym and its plume as they appeared on 27 April 2004 (figure 11). The pair of images came from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite. A large plume of volcanic ash blew westward from the volcano, which appears at the center right edge of figure 11 (top). The plume was mixing with clouds, and was more apparent as a bright, reddish orange color in the false-color image (below). Figure 11 (bottom) shows a wider area at the same spatial resolution.

Figure 11. Ambrym volcano in two MODIS images (top and bottom). See text for discussion. Image courtesy Jeff Schmaltz, MODIS Rapid Response Team, NASA-GSFC.

Geologic Background. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides Arc. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: John Seach, PO Box 4025, Port Vila, Vanuatu (URL: http://www.volcanolive.com/); Darwin VAAC (URL: http://www.bom.gov.au/info/vaac/); Jeff Schmaltz, MODIS Rapid Response Team, NASA-GSFC; Holli Riebeek, NASA Earth Observatory (URL: https://earthobservatory.nasa.gov/).

The first recorded historical eruption at Anatahan Island began on 10 May 2003 (BGVN 28:04-28:06 and 28:09). More volcanism accompanied increased seismicity beginning 30 March 2004 (BGVN 29:04). Lava was noted in the crater on 15 April. During an overflight on 24 April scientists reported fresh lava within the inner crater. Seismic activity increased abruptly at 1052 on 24 April, escalating to levels higher than recorded since summer 2003, and a moderate eruption initially produced a light ash cloud that rose to altitudes below 2 km. The cloud persisted for only a day or so.

The seismicity level increased further on 24 and 25 April. On 26 April, a flat-shaped dome was observed within the inner crater. On the evening of April 28, the seismicity level peaked, then decreased slowly to about 40% of its peak value by 29 May. That seismicity resulted from strombolian bursts every minute or so that ejected material some hundreds of meters out of the crater, and steam and ash to several hundred meters. After a two-day-long decrease, the seismicity surged on 30-31 May to double the value of the previous few days, resulting from more frequent small explosions (occurring every few tens of seconds) as well as increased tremor.

On 7 and 8 June a 100-km-long, light-colored plume of steam and ash blew W. This was reported by the U.S. Air Force Weather Agency based on Defense Meteorological Satellite Program (DMSP) satellite images (figure 12).

Figure 12. Visual (0.3 nm) image of plume from Anatahan volcano taken from Defense Meteorological Satellite Program satellite on 7 June 2004 at 2139 hours GMT (8 June 2004 at 0739 hours local time). Note that the plume length at this time, measured by the U.S. Air Force Weather Agency, was ~104 km (~56 nautical miles). Courtesy of Charles R. Holliday, U.S. Air Force Weather Agency.

Juan Camacho of the Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) visited the island on 10 June and reported an active spatter cone, from which continuous strombolian explosions threw material as high as 100 m every 10 seconds to one minute. By 15 June, the amplitude and number of discrete events appeared to have decreased slightly.

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

Information Contacts: Juan Takai Camacho and Ramon Chong, Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO), Saipan, MP 96950 USA (URL: http://www.cnmihsem.gov.mp/); Frank Trusdell, U.S. Geological Survey, Hawaiian Volcano Observatory (HVO), PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/nmi/activity/); Hawaii Volcano Observatory (HVO), U.S. Geological Survey (USGS), Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/nmi/activity/)update.html); Charles R. Holliday, Air Force Weather Agency, Offutt Air Force Base, Nebraska 68113 USA.

A dome-extruding eruption occurred in the previouly lake-bearing summit crater of Mount (Gunung) Awu, a stratovolcano in Northern Indonesia off the N end of the island of Sulawesi (Celebes). Details of the eruption are still emerging, but an early dome had clearly extruded by 2 June 2004 (figure 1).

Figure 1. A close up of Awu's new dome as it appeared amid loose tephra and through a steam-laden atmosphere on 2 June 2004, soon after emplacement began. Courtesy Penduduk.

Prior to the eruption the crater contained a green lake. Before 1992, water volume was 35,000 x 10³ m ³, but it decreased continuously, and in 2003, only 50 x 10³ m ³ remained. Research carried out in 1993-1995 attributed the water loss to active faulting beneath the crater. Water inside the crater was of great concern because of its potential to produce lahars that could threaten settlements around Awu. Prior to the eruption, thick vegetation covered the crater's inner and outer rims.

Awu's previous eruption took place 12 August 1966. It took 39 lives, injured more than 1,000, and forced ~11,000 evacuations.

Signs of Awu threatening to erupt became clear mid-May 2004. They included a 15 May (felt, I MMI) tectonic earthquake, followed by two volcanic earthquakes. On 16 May, there were 12 volcanic earthquakes recorded, events interpeted as a signs of fluid moving up, and supported by the appearence of tremor with peak-to-peak amplitude of 8 mm. A gas plume rose 75 m above the crater's rim. On 17 May there were 4 volcanic earthquakes; peak-to-peak tremor amplitudes had dropped to 5 mm. This pattern continued through 18 May, with the number of volcanic earthquakes typically standing at ~6 and tremor amplitudes at 5 mm. The S minus P (S - P) times dropped from 2.0-1.75 sec to 0.5 sec, suggesting a shallower earthquake source, a possible indication of stress moving towards the surface.

In addition to the above observations, VSI scientists regarded the shortest historical repose time at Awu volcano as 25 years, an interval that had passed since the last eruption, and this became an additional reason for raising the alert level on 18 May.

Figure 1, a photo from the VSI website shows a close-up of a dome on 2 June 2004 with intense steam escaping, indicating that at least portions of a dome had emerged by that time. In figure 1, the dome and surrounding tephra predominanly appear as gray, darker-colored spines and angular blocks and fragments, but occasional clasts of large white fragments, presumably pumice, lie sprinkled across the surface.

Seismicity increased on 4-5 June during 2330-0130 when more than 30 shallow volcanic (Type A) earthquakes occurred. In contrast, typical May seismicity only included one earthquake per day. On 5 June during 1000-1300, instruments recorded 85 earthquakes. On 6 June during 0200-0430, they recorded 50; and during 0900-1010, they recorded 2-3 earthquakes per minute. Tremor followed, with maximum peak-to-peak amplitude of 24 mm. The hazards status quickly increased to its highest level ('IV,' WITA).

At 1230 on 6 June, explosion earthquakes of small size occurred, followed by a rain of thin ash, which fell to the N. Visible white ash reached 500-750 m above the summit. An explosion sent ash 1 km above the crater rim, and the ash fell around the summit. Tremor prevailed until 2000, with maximum amplitude of 5 mm. At this point, 20,000 residents had already been evacuated.

Seismicity increased on 7 June; during the period 0000 to 0800 hours seismometers recorded 165 deep volcanic earthquakes, 18 shallow volcanic earthquakes, and continuous volcanic tremor-amplitude maxima exceeded 46 mm.

At 1117 on 7 June, an eruption began at 1800 hours, with ash plumes rising 1 km above the summit. After the eruption on 7 June, seismic signals similar to tremor occured (at 1807), with continuous, peak-to-peak amplitudes of about 12-45 mm (maximum).

During 7-8 June from 2000 to 0600, visual observers noted that 500- to 700-m-high ash clouds still hung over the summit. For the interval 0600-0600 8-9 June, VSI reported, "All day long there were many explosions." In additon, five major explosions were noted, at 1510, 1630, and 1730 on 8 June, and at 0606 and 0910 on 9 June. Presumably due to each of those larger outbursts, dark gray ash plumes rose up 1-2 km above the summit.

Ash thickness at Tahuna was about 0.5-1.5 mm. Beginning on 8 June 2004 at 0800, Tahuna airport was closed. VSI noted that the ash rain could have reached Tabukan Utara and part of Kendahe, caused by the wind to the SW.

At 0529 on 10 June, Awu began a sustained eruption, described as the climax, lasting 34 minutes (figure 2). That event sent a column of gray to black ash to 3 km above the summit. The outburst was accompanied by low rumbling sounds and tephra.

Figure 2. Ash plume at Mount Awu at 0529 on 10 June 2004. Courtesy of Wittiri, VSI, Directorate of Volcanology and Geological Hazard Mitigation.

By 11 June, explosions and seismicity decreased drastically, with tremor amplitudes of only 2-3 mm. Until 13 June VSI recorded no deep volcanic earthquakes. At 0600 on 13 June authorities reduced the hazard status and some W- and ESE-flank residents returned home.

Figure 3 documents fresh deposits, the presumably new dome, and denuded vegetation. Ash generally fell to the ESE. During the first eruption, ash fell on Tahuna city and its vicinity with a thickness of 0.5-1 mm. Surrounding villages received ash deposits as follows: Lenganeng, 2 mm; Naha, 2 mm; Bahang, 1.5 mm; Kalakuhe, 1.5 mm; and Mala, 1.5 mm.

Figure 3. Recent tephra deposited below the Mount Awu lava dome, 12 June 2004. For scale, note the backpack-clad person standing on fresh tephra and amid stripped vegetation in the right-central foregound. Courtesy of A. Solihin, VSI, Directorate of Volcanology and Geological Hazard Mitigation (DVMBG).

Inspection of the crater at an undisclosed time revealed a lava dome 300 x 250 m in plan view and 40 m in height. It is uncertain whether these values represent an early dome (figure 1) or larger, later dome (figure 3).

On 14 June, observers saw a thin white plume rising 50-100 m above the crater. Beginning 17 June, the hazard status dropped to level II (Waspada). Following 18 June, seismicity declined, and instruments no longer recorded tremor. The latest Awu report, which discussed the interval 28 June-4 July, noted level II hazard status, plumes 50-200 m tall, and the observation of incandescent material, suggesting continued dome growth.

UN Reports. According to an 8 June report from the UN Office for the Coordination of Humanitarian Affairs, the evacuation process triggered by Awu's eruption started on the evening of 6 June and continued through at least 8 June. The total number of people expected to be evacuated was ~27,000 (12,065 from Tahuna, 5,690 from Kendahe, and 9,248 from Tabukan Utara). As of 8 June, 17,326 people had been evacuated. These displaced people were accommodated in government buildings, schools, and houses of prayer. The Directorate of Vulcanology strongly advised the temporary halting of flights from Manado (at the N end of Sulawesi Island) to Sangihe Island.

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

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Directorate of Volcanology and Geological Hazard Mitigation, Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017 USA.

The Rabaul Volcano Observatory (RVO) received a report on 28 April from a pilot of the Hevi Lift helicopter company stating that new lava had come from Bagana volcano the day before. RVO has had no monitoring equipment at Bagana since 1989. Although they hope to again install monitoring instruments in the future, they could not confirm the visual observations instrumentally.

Bagana has been in long-term eruption since 1972, although reports ceased in 1995 because of political and economic unrest. MODIS satellite observations began in 2000, and almost monthly thermal alerts have been recorded since September 2000.

According to a news article, on 2 May local volcanologists and a team of provincial disaster delegates conducted an aerial inspection of the area around Bagana. At that time, the team concluded that the lava flows were not an immediate threat to the safety of villagers near the volcano. According to news reports a spokesperson for Papua New Guinea's national Disaster Center said the aerial inspection team noted a continual effusion of lava flowing in a southwesterly direction, but there was a great deal of vegetation in the area which acted as a buffer.

A later news article also noted that in the long term the lava flows could expose local hamlets to danger. The hamlets were constructed in the 1990s by people displaced by civil unrest.

RVO staff sent a series of photos and brief notes regarding their visit. Ima Itikarai commented that during his trip clouds affected the quality of the photos. Figure 3 shows a hamlet, which sits 3 km from the active block-lava flow front and 6.5 km from the summit, well within reach of pyroclastic flows similar to those in 1952, 1960, and 1966.

Figure 3. An overview of the scene on the SW region surrounding Bagana illustrating a potentially threatened hamlet and the erupting volcano and block-lava flow in the background. Courtesy of Ima Itikarai, RVO.

At about the same time but in clearer weather, another photographer, Peter Mildner, took the photo in figure 4. It shows Bagana's summit and the active block-lava flow at a point where the levees had become 'bank full.' Figure 5 shows the lava flow pouring over the levees at various points. The lava flow's toe was also being overridden.

Figure 4. Bagana summit and upper flanks as seen in April 2004 showing the active block-lava flow on the SW side (steaming, at left center). A second block-lava flow path may have begun to descend the leveed banks on the right (note abundant steam on upper right-hand slopes). Copyrighted photo by Peter Mildner provided courtesy of Ima Itikarai, RVO.

Figure 5. Closer view of the block-lava flow down Bagana's SW flank taken in April or May 2004, on a day with considerable low clouds. The initially confined lava flow followed the leveed path and then began to escape at several places. The fresh block lava's darker color stands in mild contrast to sparsely vegetated, older levee banks, which have a speckled appearance. Courtesy of Ima Itikarai, RVO.

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: Ima Itikarai and Herman Patia, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea; Papua New Guinea Post-Courier; The National; The Australian.

Eruptions associated with extrusion of viscous lavas continued at Bezymianny into June 2004. Since observers last reported on Bezymianny (BGVN 29:03) they noted substantial ash plumes occurring in June 2004 (table 2). The summary below chiefly comes from weekly reports made by Kamchatka Volcanic Eruptions Response Team (KVERT) and disseminated through the Alaska Volcano Observatory (AVO).

Table 2. A synopsis of some recent eruptions distinguished at Bezymianny (the first three were previously discussed, BGVN 28:10 and 29:03). Taken from KVERT reports.

This report concerns the most recent eruption of Bezymianny which occurred on 19 June 2004. Increased activity on the volcano began during 11 to 14 June, when seismicity rose above background level and 2-3 shallow earthquakes occurred daily.

By 16 June, KVERT elevated Bezymianny's hazard status, raising the Concern Color Code from Yellow to Orange (table 3), signifying that an eruption could occur at any time. On 19 June, the Code was raised to Red, the highest level.

Table 3. The significance of various hazard status categories on the KVERT Concern Color Code Key. This key is regularly posted with their reports.

Explosive activity began at 0840 on 19 June, and according to seismic data, it produced an ash plume that rose ~8-10 km altitude. Satellite imagery revealed that by 1319, the plume had extended ~200 km. The more concentrated portion of the plume was in the zone of ~ 167-189 km from the volcano. At 1439, a large local ash cloud moved to the NNE towards Bering Island. Later in the day, the seismicity level decreased, and KVERT reduced the Concern Color Code to Orange. During 18 to 19 June, an ash cloud extended over 1,000 km E and SE of the volcano, and "possible ash deposits" were inferred 190 km SE of the lava dome. The last time an ash cloud was noted near Korovin Island was on 20 June.

Around this time KVERT noted viscous lava flows at the lava dome. They documented weak, 1- to 4-pixel thermal anomalies over the dome. In the wake of the eruption KVERT reported gas-steam plumes extending ~3.5 km S, NE, and ESE. Following that, they reported no other activity as recently as 25 June.

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: Olga A. Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

The Bulletin staff received numerous photographs of currently non-eruptive El Chichón from amateur photographer King Freeland; a few are included below. Some of Freeland's photos are wide-angle montages compiled from multiple photos using image-processing software. The photograph in figure 5 was taken in April 2004 and shows the warm, bright green, acidic crater lake. Figure 6 illustrates the central crater as it looked in May 2004 from the SE side of the volcano looking towards the WSW.

Figure 5. Photograph taken in April 2004 from the SW crater rim of El Chichón showing the crater lake. Courtesy of King Freeland.

Figure 6. The S side of El Chichón's inner crater, May 2004, looking toward WSW. Courtesy of King Freeland.

Figure 7 depicts a sequence of what resemble stair steps developed on the pyroclastic deposits. Yuri Taran estimated the approximate height of these "steps" as 0.5-1 m, but the photographer Freeland suggested a height of up to 3 m. Taran and Freeland both offered that the features may result from the work of water, and Taran also suggested wind as a possibility.

Figure 7. Broken 'stair-stepped' surface developed on pyroclastic deposits at El Chichón. The inset at upper right is a blow-up of a representative area in the photo's center. The photo was taken in 2004. Courtesy of King Freeland.

Taran lamented the lack of people studying these features, even though they appeared quite spectacular, stating "We need a team of geographers to study this type of erosion, soil formation..." Many of Freeland's other photos in our archives also depict fumarolic and hydrothermal features.

Yuri Taran from the Institute of Geophysics, Universidad Nacional Autonoma de Mexico (UNAM), has been studying El Chichón volcano and its hydrothermal activity for almost 10 years (see Capaccioni and others, 2004). Taran noted that three groups of hot springs exist on the slopes of the volcano: Agua Caliente, S of the volcano with a maximum temperature of 74°C; Agua Salada, at the base of the dome, SSW from the crater, with a maximum temperature of 55°C; and Agua Tibia, NW of the crater with an estimated maximum temperature of ~ 51°C. The crater lake has variable shape and size, depending on the flow rate of a boiling spring in the crater that feeds the lake. When this spring sometimes disappears, the lake becomes smaller until an equilibrium develops between precipitation, evaporation, and seepage through the lake bottom. This 'equilibrium' lake size is quite small. Taran noted that during the period of his study the lake was smallest in November 1998 (at the end of the rainy season), and it was very shallow, ~1.3 m deep.

References. Capaccioni, B., Taran, Y., Tassi, F., Vaselli, O., Mangani, G., and Macias, J.L., 2004, Source conditions and degradation processes of light hydrocarbons in volcanic gases: an example from El Chichón volcano (Chiapas State, Mexico), Chemical Geology, v. 206, nop. 1-2, p.81-96.

Geologic Background. El Chichón is a small trachyandesitic tuff cone and lava dome complex in an isolated part of the Chiapas region in SE México. Prior to 1982, this relatively unknown volcano was heavily forested and of no greater height than adjacent non-volcanic peaks. The largest dome, the former summit of the volcano, was constructed within a 1.6 x 2 km summit crater created about 220,000 years ago. Two other large craters are located on the SW and SE flanks; a lava dome fills the SW crater, and an older dome is located on the NW flank. More than ten large explosive eruptions have occurred since the mid-Holocene. The powerful 1982 explosive eruptions of high-sulfur, anhydrite-bearing magma destroyed the summit lava dome and were accompanied by pyroclastic flows and surges that devastated an area extending about 8 km around the volcano. The eruptions created a new 1-km-wide, 300-m-deep crater that now contains an acidic crater lake.

Information Contacts: King Freeland, Distrito Reynosa 157, Fracc. Pages Llergo, Villahermose, Tabasco, CP86125, México; Yuri Taran, Instituto de Geofisica, Universidad Nacional Autonoma de México (UNAM), Ciudad Universitaria, Coyaocan 04510, México D.F., México (URL: http://www.geofisica.unam.mx/).

A March 2004 observatory report noted that one year of explosive activity had passed, an interval that began in February 2003 after the termination of lava emission (BGVN 28:06). During March-May 2003 there was an increase in the number of small explosions. During the year, seismometers recorded ~ 1,500 small explosions (figure 68). After that, the frequency of explosions became stable, with 3-5 daily explosions (figures 69 and 70). Four relatively significant explosions occurred during 2003 on 17 July, on 2 and 28 August (BGVN 28:08), and on 15 November, although there was no change in the daily number of events.

Figure 68. Daily variations in the number of small explosions recorded by the seismic network Red Sismica de Colima (RESCO) of Colima University from January 2003 to February 2004. The termination of the effusive stage is shown by the open arrow; four significant explosions are shown by filled arrows. Courtesy of Colima Volcano Observatory.

Figure 69. A typical daily seismogram with the records of small explosions (27 February 2004). RESCO seismic station Soma, at a distance of about 1.7 km from the crater. Courtesy of Colima Volcano Observatory.

Figure 70. A typical view of a small explosion at Colima. Photo was taken on 1 February 2004. Courtesy of Colima Volcano Observatory.

The sequence of explosions destroyed the former lava dome. Although the depth of the crater floor increased slightly as a result, the crater's dimensions changed little (figure 71).

Figure 71. A view of Colima's crater floor from the S, taken on 27 February 2004. Courtesy of Colima Volcano Observatory.

A later observatory report also noted that a significant explosion took place at 1228 on 12 June 2004. During preceding days, the volcano continued to show low-intensity activity, with an average of under three ash explosions per day. The heights of the columns did not exceed 2,000 m above the crater; they blew mainly to the W.

The exclusionary zone for both States adjoining Colima volcano remained 6.5 km from the summit. Also, the alert radius covered distances of up to 11.5 km from the summit, in order to include residents of Causentla, Cofradia de Tonila, Atenguillo, El Saucillo, El Fresnal, and El Embudo. Warnings to avoid lingering were also applied to the valleys of La Lumbre, El Cordobán, San Antonio and Monte Grande, El Muerto, La Tuna, Santa Ana, El Cafecito, La Arena, and Beltrán-Duranzno.

The Washington Volcanic Ash Advisory Center (VAAC) for aviation safety issued many reports ("Volcanic Ash Advisories") for Colima during 2003 and 2004, including over 30 during 2004. The bulk of the 2004 reports came out in February, March, April, and as recently as 14 May; no reports were issued since then to the late June date of this Bulletin. A sampling of the 2004 VAAC reports and associated graphics indicated several plumes to over 6 km altitude had been seen via satellite.

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

Information Contacts: Observatorio Vulcanológico de la Universidad de Colima, Colima, Col., 28045, México; Washington Volcanic Ash Advisory Center (VAAC), NOAA-NESDES, Satellite Analysis Branch, 5200 Auth Road, Camp Springs, Maryland 20746 USA.

After the eruption of December 2003 (BGVN 29:03), Piton de la Fournaise underwent a month of high seismic activity in April 2004. The activity consisted of 10-30 earthquakes per day with two minor seismic crises, and was accompanied by continuous inflation of the summit. On 2 May a new seismic crisis started at 1903. At 1936 eruption tremor appeared. The high intensity of tremor near the Bory crater (2,632 m) indicated that eruption had most likely started within or very close to the crater.

No activity was visible in the crater on 3 May. An overflight planned by the Observatoire Volcanologique du Piton de la Fournaise (OVPF) with the help of local police militia was unable to take place due to bad weather and rain on the volcano. The initial assessments of the observatory indicated the opening of eruptive cracks in the higher of the two craters. A long crack on the SW side extended from 2,500 m to at least 2,300 m elevation. During an observational visit by OVPF volcanologists on 4 May, a fissure was observed to have opened between 2,800 m and 2,200 m elevation. The fissure was inactive at the time of observation but much lava ejecta covered the surrounding area. A second fissure, opened during the night between Sunday and Monday, was active. As of 4 May, activity continued from three eruptive vents located between Chateau-Fort crater and Piton Bert. Tremor remained stable. During the night of 11-12 May, the single remaining active fissure projected lava ejecta onto the slopes of the cone in the SW area of the crater. The eruption continued on 15 May but moved from the summit of the volcano toward its lower slopes. Flows accumulated within the crater, and a large flow with an estimated length of 300 m was seen coming from a ~ 2.5 km-long tunnel, originating at the floor of the Enclos Fouqué caldera and issuing at the surface near the Nez du Tremblet and in the Grandes Pentes area. Further downslope, burning vegetation was observed, indicating the presence of lava flows far from the point of emission. The larger flow reached an elevation of 1,150 m, putting it 4 km from National Route (NR) 2. At 1200, the lava flow was 2.5 km from NR 2. Scientists at the observatory expected the flow's advance to slow due to the shallowing of the slope starting at 900 m elevation, and because the eruptive tremor, though it had increased slightly the day before, remained at a moderate level.

On 16 May, the lava flow stopped 1.8 km from NR 2 at 460 m elevation. A second fissure produced a second lava flow parallel to the first. Tremor increased in the crater, indicating a renewal of activity, and lava ejecta were erupted from the two cones. The OVPF reported on 17 May that the eruption was still continuing. Lava fountains from the main eruptive cone rose several tens of meters above the vent. That evening, lava flows were visible on the upper part of the Grandes Pentes. Pélé's hair had fallen in the town of St. Rose. Seismicity remained on a moderate level. At about 1500 on 18 May, the OVPF's network recorded a progressive increase in the tremor over a twenty-minute period; then at 1552, the tremor decreased dramatically. By 1615, any trace of tremor had disappeared from the recordings. On 21 May at 1500, a lava front was observed flowing at 1150 m elevation, within ~4 km of National Route 2. Volcanic tremor increased slightly, but remained at a moderate level.

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, Observatoire Volcanologique du Piton de la Fournaise Institut de Physique du Globe de Paris, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/observatoire-volcanologique-piton-de-fournaise).

The following is a summary of Hill (2004) and Sorey, Hill, and McConnell (2000), reports that collectively concluded that with the close of 2003, Long Valley Caldera had sustained nearly five years of relative quiescence. This marked the longest such interval since the onset of unrest in 1978. A summary of 2001-2002 activity was published in March 2003 (BGVN 28:03).

The slow inflation of the resurgent dome at a rate of ~ 1 cm/year that persisted through most of 2002 leveled off in early 2003 with essentially no change through the end of the year. At the end of 2003, the center of the resurgent dome stood only about 0.5 cm higher than in early 1999. It remained roughly 80 cm higher than in the late 1970s.

Seismic activity within the caldera remained low through 2003 as it has for the previous four years, averaging fewer than five earthquakes per day large enough to be located by the realtime computer system (M 0.5 and above). As in the past, most of these earthquakes were confined to the S moat and the S margin of the resurgent dome. The largest intra-caldera earthquake during the year was a M 2.4 event on 19 September 2003 at 0751, associated with a cluster of smaller events in the S moat beneath the E margin of Mammoth Lakes. An earthquake sequence of comparable intensity was centered beneath the SE margin of the resurgent dome on 8 November. This sequence included three M > 2 earthquakes, the largest of which was a M 2.2 earthquake at 2102.

Most of the earthquake activity in the Sierra Nevada block S of the caldera continued to be concentrated in the N-NE lineation of epicenters that represents the aftershock zone of the three M > 5 earthquakes of June and July 1998 and May 1999 (figure 29). A notable exception was the M 4.0 earthquake of 8 March (0735) that was located 1 km S of Laurel Mountain (~5 km S of the caldera boundary and 11 km ESE of Mammoth Lakes). This earthquake was felt in the Mammoth Lakes area and was accompanied by over 50 smaller earthquakes, the largest of which was a M 3.2 event. The Grinnell Lake area near the S end of the seismicity lineation in the Sierra Nevada was one of the more persistently active areas through the year. It produced M 3.2 earthquakes on 15 June and 18 August as well as a host of smaller earthquakes.

Figure 29. Earthquake epicenters in the Long Valley region for 2003 (from Hill, 2003).

Occasional M 3 earthquakes elsewhere in the region included: a M 3.2 earthquake on 23 January 3 km E of Red Slate Mountain (midway along the seismicity lineation in figure 29), a M 3.0 earthquake on 18 March located beneath the Volcanic Tableland 10 km E of Crowley Lake, a M 3.1 earthquake on 31 August located 2 km E of Lake Dorothy in the Sierra Nevada, a M 3.0 earthquake on 26 October located 20 km W of Bishop, and a M 3.5 earthquake on November 10 in Round Valley. Altogether, ten earthquakes of M 3 or greater occurred in the area during 2003, the largest being the M 4.0 event on 8 March near Laurel Mountain. The mid-crustal (10- to 25-km-deep) long period (LP) volcanic earthquakes, which began during the 1989 Mammoth Mountain earthquake swarm, continued beneath the SW margin of Mammoth Mountain but at a much-reduced rate with respect to the activity levels during the first half of 1997. LP activity for 2003 was limited to the first and last quarters of the year with no LP earthquakes detected from April through September.

The carbon dioxide (CO₂) emissions from the tree-kill areas around the flanks of Mammoth Mountain remained similar over the last several years. In particular, data from the CO₂ sensors at Horseshoe Lake were relatively flat and uneventful for 2003 except for the normal winter excursions due to snow accumulation. A soil CO₂ efflux survey of Horseshoe Lake in August gave an emission rate of 135 tons/day, which is slightly higher than the rate for 2002. However, the emission rate trend from 1995 through 2003 based on linear regression was relatively flat at ~100 tons/day, suggesting continued CO₂ emissions. The Horseshoe Lake tree-kill area produces roughly one third of the total CO₂ flux from the flanks of Mammoth Mountain.

Intra-caldera sites contained dead vegetation, elevated soil temperatures, and CO₂ concentrations consistent with ongoing geothermal activity. The areas that produced the greatest CO₂ emissions were in the vicinity of the geothermal plant and have been known for some time. Initially the formation of these areas likely occurred as a result of superficial changes linked to increases in geothermal fluid production in the late 1980s and early 1990s. Some recently identified sites displayed elevated soil temperatures on the resurgent dome above Fumarole Canyon; these may reflect a delayed response to the 1997 earthquake swarm activity in the area. Total CO₂ emissions at these sites are marginally above background levels.

Hydrologic monitoring data show that declining fluid pressures in key monitoring wells over the past several years continued through 2003. Fluid pressures in four of five key monitoring wells during 2003 were at the lowest values since 1995 and for three of these wells the pressures were the lowest since the late 1980s. The data also show a sharp decline in thermal-water discharge from springs in Hot Creek Gorge, an event that began in August 2003 and persisted to the end of 2003. The decline in discharge was ~18% of the long-term mean discharge.

The decline in thermal-water discharge from Hot Creek Gorge springs was consistent with the low fluid pressures recorded in wells CW3 and CH10B, both of which tapped the S-moat hydrothermal system. The reason for this decline was unclear. Geothermal production from the Casa Diablo power plant has not changed significantly over the past year and the caldera has shown no significant unrest.

New instrumentation and an interdisciplinary workshop. During the week of 2 August 2003, a team of scientists and drilling experts from the oil industry successfully installed a 30-m-long geophysical instrument string at a depth ~2.4 km in the Long Valley Exploratory Well (LVEW). The instrument string includes two three-component seismometers (4 Hz natural frequency, one at 2592 m and the other at 2264 m depths), a dilatometer (at 2254 m depth), a 48-m-long vertical-axis optical-fiber strainmeter (centered at 2150 m depth), and pass-through tubes designed to track pore pressure in the open hole beneath the instrument package. As signals from the remaining components of the LVEW deep borehole observatory come on line over the next few months, they will greatly enhance the power of the LVO network as both a monitoring and research tool.

Instrumentation of LVEW as a deep-borehole observatory represents the final stage of a major drilling project that began in the mid-1980s with multi-agency support (Sorey and others, 2000).

A four-day workshop was held 8-12 October 2003. The title was "Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Process in Long Valley Caldera-Mono Craters."

References.Hill, D.P., 2003, Long Valley Observatory quarterly report October-December 2003 and annual summary for 2003: Long Valley Observatory, U.S. Geological Survey, Menlo Park, CA (URL: http://lvo.wr.usgs.gov/Quarterly/qrt_rpt_4-03.htm).

Sorey, M.L., Hill, D.P., and McConnell, V.S., 2000, Scientific drilling in Long Valley Caldera, California—an update, in California Geology, California Geological Survey, v. 53, pp. 4-11, URL: http://www.consrv.ca.gov/cgs/information/publications/california_geology_magazine.htm.

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

Information Contacts: David Hill, Long Valley Observatory, Volcano Hazards Program, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/); Deborah Bergfeld, Jim Howle, Chris Farrar, and William Evans, U. S. Geological Survey, Menlo Park, and Carnelian Bay, CA.

When last reported (BGVN 29:04), Nyamuragira was in the midst of an eruption that had begun on 8 May 2004 and continued through 12 May. The Toulouse Volcanic Ash Advisory Center (VAAC) reported that satellite imagery showed a weak ash eruption on 25 May and that from 26 May to 1 June there were weak but steady emissions from Nyamuragira and neighboring Nyiragongo (~13 km SE of Nyamuragira). The Goma volcano observatory confirmed that ash fell within a radius of 60 km of both volcanoes. VAAC reports on 1 June said that satellite imagery indicated the eruptions at Nyamuragira had ceased.

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: Baluku Bajope and Kasereka Mahinda, Observatoire Volcanologique de Goma, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Toulouse Volcanic Ash Advisory Center (VAAC), Météo-France, 42 Avenue Gaspard Coriolis, F-31057 Toulouse cedex, France (URL: http://www.meteo.fr/vaac/); TOMS Volcanic Emissions Group (URL: https://so2.gsfc.nasa.gov/).

With the exception of strong ash explosions and related seismic activity on 9-10 May (described below), unrest at Shiveluch during 9 April-27 May 2004 was similar to that described in our last report (BGVN 29:03).

In effect, observers noted above-background seismicity, lava dome growth, and associated pyroclastic flows. Steam plumes rising as high as 3.5 km altitude, and ash plumes rising 4-7 km altitude, were seen frequently. Earthquakes occurred at depths of 0-5 km and had local magnitudes (Ml) of 1.25 - 2.25 while spasmodic tremor varied between 0.1-0.9 µm/sec.

During the period, U.S. and Russian satellites repeatedly detected 1- to 9-pixel thermal anomalies. Accordng to ground-based observers, the volcano was obscured by clouds throughout much of the report period.

Less than ten strong earthquakes were recorded each week in April. However, activity increased during the week ending 6 May when 35 strong earthquakes were recorded. According to seismic data, from 0210 to 0730 on 10 May, a series of strong ash explosions occurred at the lava dome. Continuous tremor at 14.8 µm/sec occurred during that time, decreasing to 0.3 µm/sec by 0940. Seismic activity increased again during 2150-2325, and tremor was 5-6 µm/sec. According to video and visual observation, explosions sent ash to altitudes of 8-11 km. American and Russian satellite data recorded a 9-pixel thermal anomaly over the lava dome at 2336 on 9 May and a 6-pixel anomaly at 0642 on 10 May. Around this time, authorities temporarily raised the level of concern from orange to red.

From 0725 through 1502 on 10 May an ash plume extended over 450 km to the SE and ash deposits were observed on 11 May over a wide sector to the SE at distances over 100 km. At 0914, pyroclastic- and mud-flow deposits were observed on the SE slopes of the volcano extending to distances of ~7-8 km.

At Ust-Kamchatsk (coastal settlements ~100 km ENE of Bezymianny), the thickness of orange-brown ash deposits on 10-11 May was ~1-2 mm. On 10 May, the airport at Ust-Kamchatsk was closed and the road and the dam in the area of the Bekesh River were destroyed by mud flows.

On 10 May seismic activity continued with 27 and 21 strong earthquakes recorded, respectively, during the subsequent two weeks. The number of thermal anomalies reported from satellite observations also increased to as many as 36 during the week ending 13 May.

By 27 May, activity had returned to levels typical of April (and earlier). On 21 May, the lava dome and pyroclastic-flow deposits were observed from a helicopter and from the ground. A part of the dome had been destroyed. Deposits were gas-rich, high-temperature juvenile pyroclastic flows in the central sector of the S slope of the volcano. The temperature of the main flow was ~ 300°C at a depth of 15 cm. According to satellite data, 1-20 pixel thermal anomalies were observed over the lava dome during the week.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Olga A. Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Although seismicity and volcanism were generally low during this reporting interval, mid-January to early June 2004, several episodes of elevated activity occurred. Weekly summaries for the early part of 2004 are presented in tables 54 and 55. The tables include a summary of seismicity, SO₂ emissions, and forward-looking infrared (FLIR) measurements of the HCl/SO₂ ratio.

Table 54. Summary of seismicity recorded at Soufrière Hills, 16 January to 4 June 2004. Courtesy of Montserrat Volcano Observatory.

On 18 January a low-amplitude swarm of long-period (LP) earthquakes comprised of 1000 separate events began and continued for ~36 hours. A similar swarm occurred on 30 January, lasting for ~30 hours. On 21 February a period of low-level tremor, including many small LP earthquakes, began at ~0600 and continued for ~36 hours.

A period of low-level tremor began on 2 March and continued until 1444 on 3 March when seismic activity increased significantly and an explosion and collapse event occurred. According to the Washington Volcanic Ash Advisory Center (VAAC), the ash clouds associated with the explosion reached an altitude of ~7 km. During 1445-1500 pyroclastic flows were observed in the Tar River, reaching the sea at the Tar River fan on at least two occasions. Seismicity returned to near background levels by 1525, but vigorous ash venting continued until ~0700 on 4 March. Visual observations reported that the explosion removed the small dome that had grown in the collapse scar in late July 2003, as well as a portion of the NW remnant of the 1995-1998 dome.

[A small amount of ash venting from the volcano's summit occurred on 2 May around 1815.] Episodes of tremor . . . continued until 7 May. During this period, tremor amplitude varied from low to moderate, and tremor duration varied from several days (continuous background) to a few seconds. Tremor peak frequencies were in the 1-10 Hz range. Subsequently, the activity level was low (table 54). The SO₂ flux level dropped to 146 metric tons/day on 13 May (table 55), the lowest value recorded since before the collapse event of 12-15 July 2003. For the remainder of the report period, activity remained at a low level. The seismic network recorded several hybrid earthquakes but also a number of 'mixed' events, characterized by emergent onsets and relatively short durations (~30 seconds) with broad frequency spectra (1-10 Hz), peaking at ~10 Hz.

Table 55. Summary of SO₂ emissions and the HCl/SO₂ ratio recorded at SoufriPre Hills, 16 January to 4 June 2004. Courtesy of Montserrat Volcano Observatory.

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

Information Contacts: Gill Norton, Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).

Reports of the Volcanological Survey of Indonesia (VSI) stated that Mount Bromo produced a phreatic eruption at 1526 on 8 June 2004. The eruption killed two people and injured several others. The Bromo cone is the youngest and most active volcano within the larger Tengger caldera complex. Bromo also resides within an inner caldera (Sandsea caldera).

The eruption, which vented at the crater, had a duration of ~20 minutes. Ash rose up to 3 km above the crater rim (figure 6) and was blown to the WNW and detected at the Mount (Gunung) Kelud observatory (~ 75 km away). Lapilli and ash spread out over a radius of ~ 300 m from the crater's center.

Figure 6. Bromo's 8 June 2004 eruption as seen 2.2 km away, looking from the N. The photographer, K. Nishi, was in the seismic station and saw evidence of the eruption on the seismic system. He ran to the adjacent crater rim and took a series of shots. This one was taken 9 minutes after the eruption began. The prominent cone on the right is Batok. Courtesy of K. Nishi.

Bromo was closed to the public until further notice. Its hazard status was set to the elevated state of 'Alert Level III' (on a scale with a maximum of IV). Search and rescue teams were advised to stay away from the volcano until declaration of safe approach.

John Seach reported that many buildings in the nearby towns of Malang and Probolinggo were covered by a light coating of ash 2 hours after the eruption. The neighboring towns of Lumajang and Pasuruan were also affected by the eruption.

From 0600 on 9 June to 0600 on 10 June, visual observations disclosed a thin white and slightly red cloud about 25-50 m above the crater, moving W. Seismic records were dominated by tremor with peak-to-peak amplitudes ranging from ~ 1-4 mm. Seismometers also registered 123 emission earthquakes and 15 type-A volcanic earthquakes.

During 1800 on 10 June through 0600 on 11 June, the activity of Bromo was dominated by 'smoke emissions' of low-to-medium intensity reaching heights of ~ 25-100 m. Shallow volcanic earthquakes increased, and continuous tremor occurred with a peak-to-peak amplitude of 6.0 mm. Four volcanic earthquakes were detected within about 8-15 minutes, followed by tremor for 18 minutes, after which came 8 volcanic earthquakes. Despite all of the tremor and earthquakes, however, no explosion followed. When the weather was clear, VSI scientists could see white, thick 'smoke' emissions and smelled sulfur.

At 0819 on 14 June 2004, there was an ash explosion, accompanied by a plume that rose to 100 m. Pre-explosion spectrometer measurements suggested SO₂ fluxes of 200 tons/day. During 13-14 June the seismic record contained emission and tectonic earthquakes, as well as a half hour of continuous tremor with a peak-to-peak amplitude of 6.0 mm. Deformation measurement using electronic distance meters (EDM) and global positioning systems (GPS) implied deflations of about 2-6 mm and 2-15 mm, respectively.

By 0630 on 15 June 2004, activity at Bromo had generally decreased, and the Alert Level was reduced to Level II. During that day emissions of white thin smoke rose ~ 25-150 m above the summit and the seismograph recorded 24 emission earthquakes and 1 tectonic earthquake. Deformation measured by EDM and GPS implied respective deflations of 1.0-5.0 mm and 0.2-6.2 mm.

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

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Directorate of Volcanology and Geological Hazard Mitigation, Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Heri Retnowate, Reuters; Derwin Pereira, The Straits Times; John Seach, P.O. Box 842, Southport BC 4215, Queensland, Australia (URL: http://www.volcanolive.com); 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/).