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/).

Our last report focused on the VEI 2 eruption of August 2010 as well as results from regular monitoring through May 2011 by the Instituto Colombiano de Geología y Minería (INGEOMINAS) based in Pasto, the provincial capital located ~10 km E of Galeras. Here we discuss the continuing efforts to monitor Galeras from June 2011 through April 2012. We highlight regular measurements from telemetered tiltmeter data, SO₂ flux values, and earthquake cataloging and analysis. Additional monitoring activities, including radon gas assessments and thermal measurements, were conducted by INGEOMINAS and reported in weekly and monthly reports online. We briefly mention ash explosions that began on 13 May 2012.

April 2011-April 2012 Seismicity. During this reporting period, INGEOMINAS characterized five types of earthquake events: volcano-tectonic (VT), long-period (LP), tremor (TRE), hybrid (HYB), and tornillo (TOR). This data is available in online reports on the INGEOMINAS website for various years.

Earthquakes during this time were rarely deeper than 20 km and clustered ~2 km below the summit, and at times, ranging 5-8 km (table 13). Seismicity was dominated by hybrid events, signals characterizing fracturing and fluid movement. Tremor frequently occurred from May-July 2011 and December 2011-January 2012. From January-March 2012, the duration of tremor was longer than 800 minutes/month (table 13). LP events occurred most frequently in April 2011 and February 2012; VT events primarily appeared in March and April 2012. Tornillo events had been rare in 2011 but were the cause for alarm in November 2011 when INGEOMINAS detected 18 events. The seismic pattern of "tornillo-type" earthquakes has been associated with pre-eruptive conditions - in particular, explosive activity in 1992 and 2010 was preceded by episodes of tornillos (BGVN 34:12). The Alert Level was raised in November (to Orange, on a four-color scale) but lowered again in December (to Yellow) when these signals disappeared from the records; only two events were recorded in December 2011, then again in February 2012.

Table 13. Seismicity at Galeras from April 2011 through April 2012. Earthquake counts for five types of events: volcano-tectonic (VT), long-period (LP), tremor (TRE), hybrid (HYB), and tornillos (TOR). The Alert Level was raised to Orange in November (highlighted in red). Tornillos occurred rarely during this reporting period; "-" indicates events were not reported. Courtesy of INGEOMINAS.

June 2011. INGEOMINAS reported that seismic energy was relatively low this month compared to May 2011. Inflation and deformation events were recorded by two tilt stations (Cráter and Calabazo); other stations, however, were stable (see figure 116 for monitoring station locations). The most proximal tilt station, Cráter, recorded the largest changes in deformation, and especially the radial component (often an order of magnitude larger than the tangential component). During this reporting period, INGEOMINAS frequently included data from the two component tiltmeters and calculated the vectors for Cráter (see INGEOMINAS online reports).

Figure 116. Map of station locations for the INGEOMINAS Pasto monitoring network (from the April 2012 online monthly report). Instrumentation includes: seismometers (SP = short period, BB = broadband), tiltmeters, acoustic flow, ScanDOAS, electromagnetic potential, and Global Positioning System (GPS) stations, as indicated in the legend. This map does not include all monitoring sites, for example fixed stations for Radon and EDM are also part of the network with results posted online. Courtesy of INGEOMINAS.

Large amounts of steam and gas rose from Galeras' crater in June; a plume was frequently observed with a height up to 400 m above the crater. The plume was primarily water vapor, and measurements of SO₂ flux showed high variability. INGEOMINAS reported values from ScanDOAS and MobileDOAS ranging from 41-1,455 tons/day; a total of 22 measurements with wind direction and velocity were taken between 1-30 June. The maximum measurement of SO₂ flux was made by ScanDOAS from the Santa Bárbara station located 7.0 km NNW from the summit. The minimum value was measured along a traverse with MobileDOAS between the towns La Buitrera and Sandoná (see figure 116 for locations, La Buitera is beyond the map).

July 2011. Seismic energy was 75% higher in July compared with calculations in June. A low-energy seismic swarm of LP events was recorded during 18-19 July. Seismic swarms have occurred periodically at Galeras, the last episode was recorded in early April 2011; this was also the last time tornillo earthquakes were detected (table 13). Deformation continued with fluctuations, however, fieldwork was necessary to reinstall the Cráter tiltmeter (located 0.8 km E of the main crater and at 4,060 m above sea level) when it was disrupted by electric storms on 11 July 2011; the tiltmeter was back online on 20 July.

During clear conditions, a steam plume was visible from Galeras which reached a maximum of 1.5 km above the crater. The maximum SO₂ flux for July was 1,080 tons/day which was obtained on 11 July at the Santa Bárbara station with ScanDOAS. A total of 15 measurements with wind direction and velocity were taken between 6 and 23 July. The minimum measurement of SO₂ flux was made on 19 July by ScanDOAS, also from the Santa Bárbara station (stations Alto Jiménez and Alto Tinajillas were also recording values).

August 2011. An hour-long seismic swarm was recorded starting at 1800 on 24 August. INGEOMINAS classified these earthquakes as primarily long-period, suggesting that hydrothermal processes were active beneath Galeras. Three of the tiltmeters (Cráter, Huairatola, and Calabozo) indicated deformation and two stations (Peladitos and Cobanegra) showed no change.

Emissions continued to be visible from the crater; a white plume was frequently observed that rose 800 m above the crater rim. SO₂ levels were significantly low in August; INGEOMINAS calculated the maximum SO₂ flux as 185 tons/day from the Santa Bárbara station on 3 August. A total of 26 measurements were recorded from 1 to 31 August. The lowest value, 25 tons/day, was recorded during a traverse along the northeastern route (between the towns of Genoy and Nariño) on 9 August with MobileDOAS.

September 2011. Seismicity continued at low levels and few earthquakes were large enough to locate (table 13). On 6 September a swarm of hybrid earthquakes was recorded; this was a small episode that occurred between 0600 and 0800. Tilt stations Cráter and Huairatola recorded fluctuations while Calabozo, Peladitos, and Cobanegra showed no significant changes.

The summit was visible for much of September; the plume rose typically less than 500 m above the crater. According to INGEOMINAS, SO₂ levels were low in September. A total of 16 measurements were recorded by ScanDOAS from one fixed station (Santa Bárbara station), flux ranged from 51-225 tons/day.

October 2011. INGEOMINAS reported that an earthquake swarm occurred during 25-30 October. Events were characterized as hybrids, suggesting fluid movement and hydrothermal processes; hypocenters were very shallow, less than 2 km beneath the crater. Tilt stations Cráter, Huairatola, and Calabozo recorded fluctuations while Peladitos and Urcunina showed no significant changes.

In October, conditions were favorable for observing the summit area of Galeras. A column of white vapor was visible during most of the month; the plume rose to a height of 800 m above the rim. SO₂ flux was relatively low; 19 values were recorded between 1-31 October. The maximum value was 340 tons/day as recorded on 1 October by the Alto Jiménez station (located 10.8 km NW of the summit). The lowest value, 32 tons/day, was recorded at the Santa Bárbara station on 10 October.

November 2011.INGEOMINAS continued registering swarms of shallow VT earthquakes. These events were primarily located at depths less than 1 km from the crater with magnitudes

Figure 117. Seismogram, energy peaks, and spectrogram of the frequency of a tornillo event recorded on 14 November 2011 at 2328 from Galeras. Courtesy of INGEOMINAS.

Several overflights of the crater were conducted in November by INGEOMINAS along with the Colombian Air Force (figure 118). During these flights, staff observed conditions within the crater and noted a strong sulfur odor. Thermal anomalies were detected with a forward-looking infrared (FLIR) camera; on 2 and 26 November, investigators recorded maximum temperatures around 200°C.

Figure 118. An aerial view of Galeras looking S toward a police station and towers on the crater rim. Photo taken during reconnaissance by INGEOMINAS on 2 November 2011.

INGEOMINAS reported significant changes in tilt from the Cráter station (figure 119). Between 7 September and 30 November, there were variations between 3,720 and 920 µrad with increasing and decreasing trends for tangential and radial components, respectively. Trends were also recorded from stations: Peladitos, Huairatola, and Cobanegra. Stations Calabozo and Urcunina showed small fluctuations and were considered stable.

Figure 119. Galeras tiltmeter data (Radial and Tangential components are 'C.Rad' and 'C.Tang', respectively) from stations Cráter, Peladitos, Huairatola, and Cobanegra from April through November 2011. Courtesy of INGEOMINAS.

INGEOMINAS reported that SO₂ flux in November ranged from 5 to 178 tons/day. The highest values were recorded by stations implementing ScanDOAS; the Alto Jiménez station recorded the maximum on 5 November. The lowest value was from a MobileDOAS traverse along the Sandoná route on 30 November.

December 2011. The Alert Level was lowered from Orange to Yellow on 6 December due to reduced seismicity; tornillo events were no longer recorded. The tilt station Cráter continued to register changes. INGEOMINAS determined that the NE sector of the volcano exhibited deflation from 7 September to 24 November (figure 119) and beginning on 24 November a change occurred and inflation began. The records suggested that the Huairatola station was detecting deflation of the NE sector from 6 August to 31 December. Data from Cobanegra, from 28 February to 31 December, was also consistent with showing changes in the NE. The Peladitos, Urcunina, and Cóndor stations showed small variations and were considered stable.

In collaboration with the Colombian Air Force, INGEOMINAS conducted an overflight of the crater on 6 December. Several thermal images were taken with a FLIR camera (figure 120). The highest temperature recorded was 200°C.

Figure 120. On 6 December 2012 INGEOMINAS retrieved thermal images of Galeras. In the FLIR image on the right, three maximum temperatures were captured: 116.8°C, 98.7°C, and 74.4°C. Courtesy of INGEOMINAS.

Increased degassing was noted from two sites on the N edge of the crater, Paisita and Chavas (for a crater location map, see figure 87 in BGVN 23:01). SO₂ flux was measured by three fixed ScanDOAS stations; a total of 12 measurements were recorded during 1-22 December. Emissions were low and ranging from 21 to 310 tons/day. INGEOMINAS recorded the maximum value from Alto Tinajillas (located 13.3 km W of the crater, figure 116) on 14 December; the minimum was from Santa Bárbara on 9 December.

January 2012. On 31 January, INGEOMINAS reported that a seismic swarm dominated by short-period VT events was recorded. Deformation detected by the Cráter station suggested three unique episodes where radial tilt was increasing, stabilized, and decreased. The tangential component exhibited an inversion of this trend: decreasing, stabilization, and increasing. INGEOMINAS calculated 657 µrad of inflation within the central crater, followed by stabilization and later, deflation measured as 264 µrad.

Steam continued to rise from Paisita and Chavas craters. A white plume was typically visible low over the crater however, on 5, 11, and 21 January, the plume height varied between 500 and 800 m. INGEOMINAS reported that SO₂ flux continued at low levels, ranging from 32-259 tons/day. A total of 12 values were obtained from fixed ScanDOAS stations. The maximum value was recorded at the Santa Bárbara station on 27 January.

February 2012. Seismic swarms occurred in February consisting primarily of small, shallow events. At 2148 on 27 February the short-period seismic station Anganoy (located 0.8 km E of the crater, figure 116) recorded an event INGEOMINAS characterized as a 'pseudo-Tornillo'; this event had a dominant frequency of 4.1 Hz and a duration of 36 seconds.

Pseudo-tornillos appeared to be rare events and had occurred previously in November 2011. These have much shorter codas (tails) compared to those of the tornillo signals. The latter last up to several minutes, have small amplitudes compared to duration, and generally decay progressively so their seismic traces appear screw-like in appearance (tornillos is Spanish for screw). These features and various other subtypes and their diagnostic signal characteristics and names are discussed in Narváez and others (1997).

Deformation measured by the Cráter station recorded 774 µrad of deflation in the central crater. The Cobanegra station registered decreasing trends; the stations Peladitos, Urcunina, and Cóndor were considered stable.

A white plume from the crater was visible by webcameras and reached heights less than 800 m above the rim. SO₂ flux in February remained low and ranged from 8 to 498 tons/day. A total of 27 values were recorded from fixed ScanDOAS and MobileDOAS measurements. The maximum flux was recorded on 22 February at the Alto Jiménez station.

March 2012. In March, seismic energy decreased by 89.1% compared to February, and few earthquakes were located. However, tremor continued (table 13). The Cráter tiltmeter recorded variability in early March, and from 22 to 31 March, 1,440 µrad of inflation was recorded within the central crater. The Cobanegra station recorded decreasing trends with both components while the stations Peladitos, Urcunina, Cóndor, Calabozo, and Arlés were considered stable.

A white plume was visible during most of the month except for four days. Plume height was maintained below 1.9 km. On 2 March, the National Park reported strong sulfur odors and also received alerts from the municipal committee of Sandoná that gas was noticeable.

Based on fixed and mobile detectors, INGEOMINAS reported that SO₂ flux increased dramatically in March. A maximum of 3,390 tons/day was recorded by the Alto Jiménez station on 15 March. The lowest value recorded was 305 tons/day during a traverse along the Consaca-Sandoná route on 30 March. A total of 33 measurements were collected from 1 to 31 March.

April 2012. INGEOMINAS reported that seismic swarms occurred during 5-8 and 11-16 April consisting primarily of small, shallow VT events. The Cráter and Huairatola tilt stations registered variability suggesting inflation in the W sector of Galeras, an area known for high seismicity. The Cobanegra station recorded decreasing trends from both components between 85 and 430 µrad. The other stations were considered stable.

A white plume was frequently visible above the crater in April. Webcameras and observers recorded a maximum height of 2,000 m. On 16 April, the local committee for the prevention of disasters (CLOPAD) of the provincial capital, Pasto, received reports from inhabitants near the N flank of Galeras; gas emissions were visible and people could hear noises from the crater.

SO₂ flux continued at elevated levels in April. INGEOMINAS recorded 33 measurements during April. A maximum of 1,477 tons/day was recorded at the Alto Jiménez station on 2 April. The highest levels of SO₂ emissions were recorded within the first week of April, averaging 1,012 tons/day. The lowest value was recorded on 13 April, 10 tons/day, along the La Florida-Sandoná route with MobileDOAS.

Editor's Note: INGEOMINAS and the Washington Volcanic Ash Advisory Center (VAAC) reported that ash emissions were detected in early May 2012 (figure 121) and continued into early June.

Figure 121. An ash explosion from Galeras was captured by the "Barranco" webcamera on 27 May 2012. This high-resolution camera was located on the NW rim of the crater. Timing of photo sequence: A. 09:37:53; B. 09:39:08; C. 09:40:15; D. 09:41:40. Courtesy of INGEOMINAS.

Reference. Narváez, L.M., Torres, R.A., Gómez, D.M., Cortez, G.P., Cepeda, H.V., and Stix, J., 1997. 'Tornillo'-type seismic signals at Galeras volcano, Colombia, 1992-1993, Journal of Volcanology and Geothermal Research, 77: 159-171.

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

Information Contacts: Instituto Colombiano de Geologia y Mineria (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Pasto, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

This report mentions a possible ash plume from Galunggung volcano in July 2008 and various other anomalies, including discolored crater lake water during parts of 2011 and 2012. Our last report on Galunggung was in 1984 (SEAN 09:02), following a deadly eruption that began in mid-1982 and ended in early 1983.

The following background information on the volcano was provided in 13 February and 28 May 2012 reports from the Indonesian Center of Volcanology and Geological Hazard Mitigation (CVGHM). According to the latest report from CVGHM, the present-day lake in the conical crater of Galunggung volcano has a diameter of 1 km and a typical depth of 11 m. In the middle of the lake sits a small, 30 m high, 250 x 165 m scoria cone which was produced during the final stage of the 1982-83 eruption. Galunggung's hazards include phreatic and phreatomagmatic eruptions capable of draining the lake and producing mud flows.

As further background, some of the historical eruptions were explosive, centered at the volcano's crater lake. These eruptions occurred four times, in 1822, 1894, 1918, and 1982-1983. The eruption of 1982-1983 occurred over a period of 21 months, from 5 April 1982-8 January 1984 (SEAN 07:04, 07:06, 07:07, 07:08, 07:09, 07:10, 07:11, and 07:12). In late June 1982, a British Airways jumbo jet encountered an ash cloud that stalled all four of its engines and abraded its windshield and wing surfaces. The aircraft lost 7.5 km of altitude before the engines could be restarted, but it landed safely in Jakarta (SEAN 07:06).

Incorrect report of 2002 eruption; questionable one in 2008. Based on erroneous information from a pilot report, the Darwin Volcanic Ash Advisory Centre (VAAC) stated that an eruption occurred at Galunggung at 1748 hr on 23 August 2002. It produced a W-drifting low-level plume. No ash was visible on satellite imagery. Subsequently, Dali Ahmad of CVGHM had advised Dan Shackelford (amateur volcanist, now deceased) that the report of an eruption on 23 August 2002 was incorrect. It turned out that the likely cause of the incident was a bushfire near the volcano that led observers to believe that an eruption was occurring.

Based on a pilot report and inconclusive observations of satellite imagery, the Darwin VAAC reported that on 17 July 2008 a possible ash plume from Galunggung rose to an altitude of 5.5 km and drifted SW. However, CVGHM did not report eruptive activity and advised that the volcanic activity status was "normal" at that time.

2011-2012 observations. CVGHM reported that from September 2011 to 8 February 2012 the crater lake water at Galunggung was discolored. In addition, a sudden increase in water temperature was measured, from 27° C on 5 February to 40° C on 8 February. Based on seismic data and crater lake observations, CVGHM raised the Alert Level from 1 to 2 (on a scale of 1-4) on 12 February and recommended that people stay at least 500 m away from the lake shore.

CVGHM reported that after the Alert Level was raised, seismic activity at Galunggung decreased drastically through 27 May 2012. Moreover, on 27 April, plants around the crater area looked green and lush, small fish were swimming in the water, and insects around the crater were active. Based on seismic data, crater lake water temperature and pH data, and visual observations, CVGHM lowered the Alert Level from 2 to 1 on 28 May 2012.

MODVOLC satellite thermal alerts were absent at Galunggung during 2011-2012 (and at least since 2000). CVCHM noted in its 28 May 2012 report that throughout the first half of 2012 Galunggung volcano was often covered in mist.

Geologic Background. The forested slopes of Galunggung in western Java SE of Bandung are cut by a 2-km-wide collapse scarp open towards the ESE. The "Ten Thousand Hills of Tasikmalaya" dotting the plain below the volcano are debris-avalanche hummocks from the collapse about 4,200 years ago. An eruption in 1822 produced pyroclastic flows and lahars that killed over 4,000 people. A series of major explosive eruptions starting in April 1982 destroyed a number of villages, killed as many as 30 people, and forced over 60,000 to evacuate. Pyroclastic flows and heavy widespread ash caused significant damage. A large passenger jet that encountered the ash plume on 24 June lost power to all four engines but managed to land safely in Jakarta. The 1982 activity destroyed a 1918 dome and formed the Warirang crater, almost as wide as the valley, about 2 km down from the summit.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM) (URL: http://www.vsi.esdm.go.id); Darwin Volcanic Ash Advisory Centre (VAAC) (URL: http://www.noaa.gov/VAAC/OTH/AU/messages.html).

Previous Bulletin reports on Gamkonora highlighted an eruption in 1981, minor explosions in April 1987 (SEAN 06:07), and a phreatic eruption in early July 2007 (BGVN 32:10). Reports by the Center of Volcanology and Geological Hazard Mitigation (CVGHM) noted tiny diffuse white plumes in 2009 and again in 2011 when the observatory recorded an average of 2 volcanic earthquakes per day. During mid-2011 through mid-2012, in addition to intervals with several shallow volcanic earthquakes per day, instruments also recorded increasing tremor and hundreds of signals of inferred emissions described as hot-air blasts. The hazard status rose accordingly and remained elevated as this report goes to press on 29 June 2012 at Alert Level 3 (on a scale of 1-4).

As this report goes to press a potentially inaccurate news report indicated an eruption starting 13 June 2012 (see subsection below). That behavior remained unconfirmed by CVGHM or the Darwin Volcanic Ash Advisory Centre (VAAC) as discussed further in a subsection below.

Figures 2-4 provide broad regional context on Gamkonora near the northern margin of Indonesia. A previous map (figure 1 in BGVN 32:10) shows Gamkonora and other Holocene volcanoes on a map of Halmahera and adjacent islands.

Figure 2. Indonesian volcanoes with eruptions since 1900 A.D. as compiled from Simkin and Siebert (1994) by Lyn Topinka (USGS-Cascades Volcano Observatory). Halmahera island and Gamkonora volcano appear in the upper (N) part of the map (see figures 3 and 4). Courtesy of the USGS.

Figure 3. Gamkonora and the situation there associated with unrest in July 2007 (BGVN 32:10). Note the globe showing Indonesia at upper right. On the main map, most of the unshaded and unlabeled islands situated NW of Gamkonora belong to the Philippines. Courtesy of Relief Web.

Figure 4. A UNOSAT product made 12 July 2007 addressing Gamkonora's crisis around that time. The scale and details highlight the local setting; the box at upper left mentions 2004 population estimates and notes that there were 35,000 residents within 20 km of the volcano. Courtesy of UNOSAT.

CVGHM reports were scarce during 1982-2011. One report noted that seismic activity increased somewhat on 24 March 2008. The increase included an episode of continuous tremor.

On 23 March 2009, CVGHM lowered the Alert Level from 2 to 1 based on visual observations and decreased seismicity since January. Diffuse white plumes rose 50-150 m above the crater. Residents and visitors were reminded not to approach or climb into the crater.

CVGHM reported that during January-April 2011, diffuse white plumes rose 25-100 m above Gamkonora's crater rim. Seismicity increased during 29 April-3 May 2011.

On 1 May, white plumes rose 150 m above the crater rim. The next day, white plumes were observed rising 300 m above the crater rim and observers saw incandescence from the crater. Residents near the volcano's base noted a sulfur smell. On 3 May 2011 the Alert level was raised to 2.

Various types of earthquakes were noted during January to April 2011. They included shallow volcanic earthquakes (2 per day average), deep volcanic earthquakes (once per day average), local tectonic earthquakes (1-7 per day average), and far tectonic earthquakes (4 per day average).

A 13 June 2012 CVGHM report noted that during May and June 2012 the emissions were sparsely to medium white in color and rising 75 to 200 m above the crater rim. Absent were sulfurous smells, open flames, eruptive noises, and other similar anomalous symptoms.

The same CVGHM report noted that seismic signals since 3 May 2010 included emission signals (hot-air blasts, averaging 10-12 daily), harmonic tremor (averaging 10-15 events daily), shallow volcanic earthquakes (averaging 2 daily, but for the one specific case given, during the interval 31 May to 11 June 2012, only 1 occurred), and distant tectonic earthquakes (averaging 4 daily). Table 1 presents a breakdown of the interpreted seismic signals during 1 May to 12 June 2012.

Table 1. Seismic data released on 13 June 2012 for Gamkonora. The entries represent total events during specified intervals during May and early June 2012 ("--" signifies absence of data). Courtesy of CVGHM.

The authors of the 13 June report made no further comment about the air-blast signals that had become common at the volcano (table 1). They did note that since the beginning of May 2012, tremor had increased. They interpreted this and the overall seismicity as due to magma intruding upward and approaching shallow depths within the volcano. The authors noted that intrusions could lead to increased pressure within the volcano, although they viewed this pressure as yet relatively small.

As previously noted, starting on 3 May 2011, the volcano's hazard status rose to Alert Level 2. On 3 May 2012 it rose to Level 3, where it remained at least as late as 29 June 2012. The Level 3 status excluded residents, visitors, and tourists from approaching closer than 3 km from the summit. The report also prompted local governments to coordinate with the volcano's monitoring post, which is located in the village of Gamsungi (or with CVGHM's main office in Bandung).

News claims of eruption on 13 June 2012. The English language version of Antara News released a report (edited by Ella Syafputri) at 1913 on 13 June stating that Gamakonora had erupted that afternoon. The eruption, if it did occur, escaped clear mention in available CVGHM reports. The news report said that the eruption sent a plume of undisclosed type or color 3 km "into the sky" (a term that could imply a plume to 3 km altitude or could mean a plume 3 km over the ~1.6 km summit, in effect to ~4.6 km altitude). The news report said the event had the effect of "forcing hundreds of residents living on the volcano's slope to evacuate to safer areas."

Despite the headline "Mount Gamkonora erupts" and directly under that, the sentence "The volcanic ash spread to as far as Tobelo, the capital of North Halmahera district", the two quotes referred to events at two separate volcanoes. In the 5th paragraph of the article the topic shifted to Dukono, another volcano in the region, which turned out to have been the source of the ash (not Gamkonora).

The news report spawned no fewer than 10-20 English-language reports on as many websites. Some of these derivative reports continued to mistakenly attribute Dukono ashfall to Gamkonora, and in some cases they added further errors.

Reference. Simkin, T. and Siebert, L., 1994, Volcanoes of the World: a Regional Directory, Gazetteer, and Chronology of Volcanism During the Last 10,000 Years. (2nd ed.) Geoscience Press, Tucson, 368 pp.

Geologic Background. The shifting of eruption centers on Gamkonora, the highest peak of Halmahera, has produced an elongated series of summit craters along a N-S trending rift. Youthful-looking lava flows originate near the cones of Gunung Alon and Popolojo, south of Gamkonora. Since its first recorded eruption in the 16th century, typical activity has been small-to-moderate explosive eruptions. Its largest recorded eruption, in 1673, was accompanied by tsunamis that inundated villages.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Lyn Topinka, United States Geological Survey, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA, 98683; UNOSAT (URL: https://unitar.org/unosat/); 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/); Antara News (URL: http://www.antaranews.com/en/news/).

Iliamna was last discussed in September 1997 (BGVN 22:09). This report is largely based on seismic data extracted from Alaska Volcano Observatory (AVO) yearly reports for 1997 to 2011, with the exception of an increase of seismicity during early 2012 that was reported by various sources. From the start of 2012, both rockfalls and seismicity progressively increased; this prompted AVO to increase the Alert Level to Advisory in March 2012. A map showing the location of Iliamna in relation to nearby volcanoes and communities is depicted in figure 1. Figures 2 and 3 are topographic maps showing Iliamna's known debris avalanches and rockfall deposits.

Figure 1. Map of Iliamna and nearby volcanoes and communities. Iliamna is in SW Alaska near the mouth of the Cook Inlet, and W of the Kenai Peninsula. Courtesy of AVO.

Figure 2. Iliamna topographic mapping of known debris-avalanche and rockfall deposits. As indicated in the explanation (bottom), red triangles indicate debris avalanches associated with Iliamna, pale orange triangles indicate debris avalanches associated with Iliamna that have been reworked by glaciers, green triangles indicate debris avalanches not associated with Iliamna, green and red dashed lines indicate the maximum likely extent of debris avalanches with relatively long and short runouts, respectively, potential pathways of debris avalanches are indicated by red arrows, and orange shaded areas indicate the generalized extent of rockfall debris on glacier surfaces. Courtesy of Waythomas (1999).

Figure 3. View of the SE flank of Iliamna Volcano showing debris-avalanche deposits from 1997 (solid red line), the fumarole zone near the summit (yellow dashed line), and the older avalanche scar at the head of Red Glacier (red dashed line). Photo undated; courtesy of Waythomas (1999).

Most of the upper edifice exposes highly altered, unstable rock and shows scars from mass wasting. The E scar has been the source of frequent non-volcanic gravitational collapses producing mixed avalanches of ice, snow, rock, and mud that typically extend several kilometers downslope. Some are large enough to be visible from the Kenai Peninsula (Neal and others, 1995; McGimsey and Wallace, 1999).

Reports on Iliamna's seismicity since early 1997 are sparse. According to AVO, a pilot reported a fresh deposit of mud and rock on the upper NE flank on 6 July 1999. However, spring and summer avalanches are common on the glacier-dominated summit.

On 25 July 2003, an avalanche of snow, ice, and rock occurred. The event lasted four minutes and was recorded by seismometers located 75 km away on Augustine volcano. The avalanche presumably originated from the same vicinity as in previous years, a steep portion of the SE flank adjacent to an extensive permanent fumarolic zone above a debris-avalanche deposit (figure 3; Neal and others, 1995; McGimsey and Wallace, 1999; McGimsey and others, 2004).

On 15 May 2005, AVO seismologists noted a swarm of unusual seismic activity at Iliamna. The events were emergent and prolonged (the longest lasted 5-8 minutes) and were strongest at seismic station ILS, located on the S flank of South Twin (figure 4). The activity began at about 1250 UTC and tapered off at 1718 UTC. Analysis revealed that the signals most likely were caused by a surficial process, such as a snow avalanche (a common occurrence on Iliamna), but this particular event lacked the usual precursory seismicity preceding other Iliamna snow and ice avalanches ( Caplan-Auerbach and others, 2004; J. Caplan-Auerbach, written commun., 2005; Caplan-Auerbach and Huggel, 2007).

Figure 4. Iliamna volcano topographic map showing the location of the 15 May 2005 rockslide as a thick black line on the S flank of South Twin and the seismic station ILS as a red dot. Lake Clark National Park boundary shown as a thin black line. Base map provided by C. Waythomas, AVO/USGS; courtesy of McGimsey (2008).

During an overflight on 16 May 2005, Lee Fink of Lake Clark National Park observed a large, fresh rock slide (not a snow or ice avalanche) SE of Iliamna that began at ~1,980 m elevation on the SE flank of South Twin, and ran down to ~365 m elevation (figure 5a). Along the lengthy ridge extending S of Iliamna (including both South Twin, North Twin, and a large unnamed massif) are steep, exposed sections of bedrock. The 15 May rockfall occurred below the ridge (figure 5b).

Figure 5. (a) Rock avalanche on SE flank of S Twin (topographic high at upper center) beginning at ~2 km elevation and running down to ~0.37 km elevation. Photo by Page Spencer, Lake Clark National Park, 16 May 2005. (b) Iliamna from the E captured on 12 July 2006. The arrows mark the location of the 15 May 2005 rock avalanche. Photo by Christinia Neal, AVO/USGS. Courtesy of McGimsey (2008).

During Iliamna's mid-May 2005 rock slide, earthquakes at Augustine volcano, ~100 km SSW of Iliamna in the Cook Inlet, increased from 2 per day in April to 70 per day by the end of the year (McGimsey, 2008). However, no evidence exists that this increase disturbed Iliamna. Other factors such as temperature changes, ice and snow mass (and other conditions) would have contributed to the weakening of the summit material at Iliamna.

According to AVO, earthquake numbers increased significantly between 2008-2009, but returned to near-normal levels in 2010 (table 1).

Table 1. Numbers and types of earthquakes at Iliamna between 2008 and 2010. Key: VT, volcanic tremor; LF, low frequency; Mc, magnitude of completion (lowest magnitude detectable); and '--', not reported. Courtesy of AVO.

Early 2012 elevated seismicity. AVO reported that during December 2011-February 2012, earthquake activity steadily increased. During the first week of March 2012, numerous earthquakes occurred that varied in number and magnitude. According to a press account (Alaskan Dispatch), on 8 March, a moderate M 4.1 earthquake struck the region. On 9 March, AVO increased the Alert Level to Advisory and the Aviation Color Code to Yellow. AVO reported that the increased activity was a significant change, but also noted that a similarly energetic episode of seismic unrest from September 1996 to February 1997 did not lead to an eruption.

Between 9 March through at least 3 April 2012, seismicity remained above background levels. Satellite images acquired during 9-16 March showed a plume drifting 56 km downwind that was likely water vapor. An AVO report noted that long-lived fumaroles at the summit of Iliamna frequently produced visible plumes, but the current plume appeared to be more robust than usual. Scientists aboard an overflight on 17 March observed vigorous and plentiful fumaroles at the summit, consistent with elevated gas emissions. Gas measurements indicated that the volcano was emitting elevated levels of SO₂ and CO₂, consistent with a magmatic source. During the overflight, scientists did not observe obvious signs of recent rockfalls, such as large areas of newly exposed bedrock or unusual disturbance of the glacial ice. Some deformation of the ice at the headwall of the Red Glacier on the E side of the summit was observed, but it is not clear that this was related to the current volcanic unrest; glacier avalanching is common on this very steep area and was last seen in 2008. During 25-27 March, activity declined somewhat to just above background levels. When not obscured by clouds, satellite and web camera views showed nothing unusual.

References. Caplan-Auerbach, J., Prejean, S.G., and Power, J.A., 2004, Seismic recordings of ice and debris avalanches of Iliamna Volcano (Alaska): Acta Vulcanologica, v. 16, n. 1-2, p. 9-20.

Caplan-Auerbach, J., and Huggel, C., 2007, Precursory seismicity associated with frequent, large ice avalanches on Iliamna volcano, Alaska, USA: Journal of Glaciology, v. 53, n. 180, p. 128-140.

Detterman, R.L., and Hartsock, J.K., 1966, Geology of the Iniskin-Tuxedni region, Alaska: U.S. Geological Survey Professional Paper 512, 78 p.

Dixon, J.P., and Stihler, S.D., 2009, Catalog of earthquake hypocenters at Alaskan volcanoes: January 1 through December 31, 2008: U.S. Geological Survey Data Series 467, 88 p. Available at http://pubs.usgs.gov/ds/467/

Dixon, J.P., Stihler, S.D., Power, J.A., and Searcy, C.K., 2010, Catalog of earthquake hypocenters at Alaskan volcanoes: January 1 through December 31, 2009: U.S. Geological Survey Data Series 531, 84 p. Available online at http://pubs.usgs.gov/ds/531/

McGimsey, R.G., and Wallace, K.L., 1999, 1997 volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 99-0448, 42 p.

McGimsey, R.G., Neal, C.A., and Girina, O., 2004, 1999 Volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 2004-1033, 49 p.

McGimsey, R.G., Neal, C.A., Dixon, J.P., and Ushakov, S., 2008, 2005 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2007-5269, 94 p.

Neal, C.A., Doukas, M.P., and McGimsey, R.G., 1995, 1994 volcanic activity in Alaska-Summary of events and response of Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 95-271, 18 p. [Iliamna, p. 4-5].

Waythomas, C.F. and Miller, T.P., 1999, Preliminary Volcano-Hazard Assessment for Iliamna Volcano, U.S. Geological Survey Open-File Report OF 99-373.

Geologic Background. Iliamna is a prominentglacier-covered stratovolcano in Lake Clark National Park on the western side of Cook Inlet, about 225 km SW of Anchorage. Its flat-topped summit is flanked on the south, along a 5-km-long ridge, by the prominent North and South Twin Peaks, satellitic lava dome complexes. The Johnson Glacier dome complex lies on the NE flank. Steep headwalls on the S and E flanks expose an inaccessible cross-section of the volcano. Major glaciers radiate from the summit, and valleys below the summit contain debris-avalanche and lahar deposits. Only a few major Holocene explosive eruptions have occurred from the deeply dissected volcano, which lacks a distinct crater. Most of the reports of historical eruptions may represent plumes from vigorous fumaroles E and SE of the summit, which are often mistaken for eruption columns (Miller et al., 1998). Eruptions producing pyroclastic flows have been dated at as recent as about 300 and 140 years ago, and elevated seismicity accompanying dike emplacement beneath the volcano was recorded in 1996.

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

In our last report on Masaya volcano, we reviewed field investigations and gas measurements from 2008-2011 including the attempt to launch a small Zeppelin as an experiment to measure gas emissions in March 2011 (BGVN 36:11). Here we present results from monitoring efforts focused on the elevated activity that has continued from Masaya's Santiago crater, one of the nested summit craters in Nindirí cone (figure 30). New gas measurements and field observations have become available from the Instituto Nicaragüense de Estudios Territoriales (INETER) from November 2011 through March 2012. Reports were also available for Masaya's Comalito cinder cone, a site of continuous gas emissions and elevated temperatures. In February 2012, INETER met with collaborators from both Simon Fraser University (Canada) and The Open University (UK). We highlight some of the results from these collaborators including mapping and modeling of Masaya's hydrothermal complex, results from long-term SO₂ flux monitoring, and a conceptual model that links magma chamber dynamics with intermittent explosive activity.

Figure 30. In this false-color image, Masaya caldera is well-defined. Landsat bands 4,3,2 emphasize vegetation (red) and soil (brown to yellow) and the panchromatic analysis improved the distinction between dark rock (lava) and water (Masaya lake, at the E edge of the caldera) (NASA Landsat Program, 2007). Annotation is based on sketch maps by Mooser and others (1958) and Girard and van Wyk de Vries (2005); image processed by GVP.

The false-color image of Masaya (figure 30) and the surrounding area is a standard composite image (bands 4,3,2) captured by Landsat on 25 March 2001, during Nicaragua's dry season (November through April). Here, vegetation appears in shades of red (darker in areas with denser vegetation), urban areas are cyan blue, and soils vary from dark to light browns. Located just 500 m E of Santiago crater, Masaya crater is distinguished by older deposits, last active around 150 AD, and contains a ring of vegetation (which appears as a pale pink circle). Masaya's recent lava flows have been contained within the larger caldera except for those dating from 1670 when lava ponded along the northern caldera rim and spilled over to cover more than 1 km² outside the caldera.

In November 2011, INETER recorded little activity from Masaya. No field visits were made and no earthquakes were large enough to locate hypocenters. Seismicity that month was low, at 50 RSAM units.

On 12 December 2011, INETER conducted site visits to Masaya's active crater (Santiago) and Comalito cinder cone. With an infrared thermometer, temperatures were measured from vents within Santiago crater; the highest temperatures measured were 42 and 45°C. The field investigators learned from National Park personnel that, recently during a 2-hour period, booming noises were heard from Santiago crater. INETER suggested that the noise resulted from strong gas release from deep within the crater - no visible material was ejected during the episodes. Areas of gas release could be visually identified within the crater; these were also locations where debris had been shed from the S and SW walls. Rockfalls from these locations were likely affecting gas emissions.

Additional visits to Comalito cone (figure 30), a satellite cone located less than 2 km NE of Santiago crater, allowed in situ measurements of fumarole temperatures. Four sites were measured; the highest temperature was 79°C, the lowest was 75°C (fumaroles 4 and 1 respectively). These temperatures were considered typical compared to others during 2011 (as compiled by INETER; figure 31). The lowest temperatures of the year 2011 were recorded in May and July with some values as low as 60-65°C.

Figure 31. Temperature measurements made by INETER during 2011 at Masaya's Comalito cone. Four fumaroles were measured consistently throughout the year. Courtesy of INETER.

To quantify SO₂ gas emissions, INETER used a mobile Mini DOAS throughout the year transported on two different routes. The road between Ticuantepe and San Juan de la Concepción was the closest route available when the plume trended SW. An additional route, at a greater distance (figure 25 from BGVN 36:11) was available between Las Quatro Esquinas and El Crucero. On 13 December, cloud cover limited the number of successful traverses; however, an average SO₂ flux of 648 metric tons per day (t/d) was calculated from three of the six traverses. This was a significant increase compared to values obtained in October 2011 when 13 successful traverses that month yielded an average of 153 t/d. These values (and others in this report) have not been corrected for meteorological conditions and error calculations were not available during this reporting period.

On 23 January 2012 INETER conducted traverses below Masaya's plume with a Mini DOAS. Measurements along both routes, proximal (Ticuantepe and San Juan de la Concepción) and distal (Las Quatro Esquinas and El Crucero) were attempted. From 10 calculations, SO₂ flux from the proximal route yielded 801 t/d. From the distal route, the average flux rate was 543 t/d.

INETER conducted fieldwork during 30-31 January 2012, visiting Santiago crater and Comalito cone. Temperatures from fumarole sites on Comalito had maximum temperatures of 70°C (fumarole 4) and 78°C (fumarole 2) on 30 January. The maximum temperature measured from Santiago crater had increased to 95°C.

On 1 February 2012, INETER visited Comalito cone and reported fumarole temperatures. The highest temperature was 97°C (fumarole 1); on 23 February the highest temperature was 86°C (fumarole 2). Fieldwork also included visits to Santiago crater; temperatures within the crater were relatively low, 75 and 70°C (from 1 February and 23 February, respectively). SO₂ flux from Mini DOAS from the closest route (Ticuantepe and San Juan de la Concepción) yielded an average of 943 t/d based on 12 traverses, continuing the trend of increased SO₂ emissions since December 2011.

In March 2012, National Park personnel reported that acoustic noise from the crater was less frequent compared to the previous month. Also, visible gas emissions appeared concentrated at the SW and innermost portions of Santiago crater. On 12 March 2012, INETER visited Masaya and measured temperatures from Santiago crater. A wide range of values was recorded: 100°C to 43°C. Relatively stable temperatures were measured from Comalito cone: 73°C to 76°C. The highest temperatures were measured at fumaroles 3 and 4.

On 20 March INETER conducted Mini DOAS traverses beneath Masaya's SW-trending gas plume. On the proximal route, 12 traverses were successful and determined an average SO₂ flux of 1002 t/d suggesting the increasing trend that began in early December 2011 was continuing. Without error calculations and assessing meteorological conditions, however, this trend could not be directly interpreted.

Geohydrology. Long-term interest in diffuse CO₂ gas emissions spurred recent investigations into Masaya's hydrothermal system. Mauri and others (2012) found active hydrothermal anomalies under many of the cinder cones and investigated these conditions with field measurements of soil CO₂ concentration, self-potential (SP), soil temperatures, and flow-path modeling (figure 32). Self potential methods make observations "of the static natural voltage existing between sets of points on the ground (Sheriff, 1982)". From Comalito cone, Nindirí cone, and the lower slopes of Masaya, CO₂ gas concentrations ranged from 26 to 43 ppm (mean values). During a 5-year investigation, the authors collected SP geophysical data over extensive transects within the caldera. The datasets yielded significant correlations between high CO₂ soil concentrations and SP anomalies. Water depths were determined by processing the SP data with mathematical techniques (wavelets from the Poisson kernel family). They concluded that interconnected structures (ring faults, fissures, and dikes) serve as flow paths for gas, fluids, and heat. These structures also have the potential to block groundwater flow, a conclusion suggested by their models of groundwater contributions to Masaya Lake (Laguna de Masaya) (figure 32).

Figure 32. Groundwater flow model for Masaya volcano taken from Mauri and others (2012). (a) A map indicating key geographic and geologic features including groundwater flow. (b,c) Two vertical profiles with a legend at the bottom. The groundwater was mapped using two geo-electrical prospecting techniques. The self-potential (SP) technique yielded data processed with multi-scale wavelet tomography (MWT). The second technique was the transient electromagnetic method (TEM) (see key and text).

In Figure 32a, we see the spatial localization of uprising fluids associated with hydrothermal activity (green diamonds) and gravitational water flow (blue squares) within Masaya caldera for which depths have been determined. The names of volcanic cones are in blue; crater names and ground structures are in dark red; dark green dashed lines are the fissure vent structures; solid red lines represent the inferred structures (faults, fissures) based on previous work by Crenshaw and others (1982) and Harris (2009). The red dashed lines are the hypothetical structures (faults, fissures) (Crenshaw and others, 1982). The black dashed line is the inferred limit of the caldera.

The three segments traced in Figure 32a correspond to cross-sections along A-D-B (figure 32b) and C-D-B (figure 32c). Cross-section A-D-B represents the water flow direction across the caldera while the cross-section along profile C-D-B represents the water flow direction through the active Santiago crater and across the caldera. The dashed red lines represent underground structures in cases where the dip orientation is unknown and are based on the work of Williams (1983) and Crenshaw and others (1982). Blue lines with a single dot above the center represent water flow having a flow direction different than the cross-section view. Solid arrows represent the flow direction inferred from the self potential/elevation gradient. Elevations of the shallow flow direction (blue and solid green arrows) were estimated from multi-scale wavelet tomography (MWT). MWT is a signal processing method based on waves that allow for location of dipole and monopole sources which correspond to the electrical anomalies generated by water flow through bedrock. The dashed grey line and dashed blue arrows are deep hypothetical flows from the transient electromagnetic method (TEM) results (MacNeil and others, 2007). TEM results were considered in this study because they offered a different level of sensitivity to SP method and, at the time of the study, direct well data was not available to correlate results, making it difficult to determine which model (MWT or TEM) best represented the true water depth.

Long-term SO₂ fluxes and windspeed-induced errors. Nadeau and Williams-Jones (2009) consolidated data spanning three decades (figure 33) and assessed current methods for constraining uncertainties in SO₂ data collected on traverses with UV correlation spectrometers (COSPEC/DOAS/FLYSPEC). The authors agreed with previous investigators that the following factors contribute to uncertainties: variable local windspeed, emission rate, dry deposition of sulfur from the plume, and conversion of SO₂ to sulfate aerosols within the plume. Of these factors, the authors stressed that for low-lying volcanoes such as Masaya, the local wind patterns cause the largest errors. "One must be wary of using one blanket plume speed value for all data collected at different locations, as it can result in misleading variations within the SO₂ flux dataset (Nadeau and Williams-Jones, 2009)." At Masaya, this led to as much a 50% apparent decrease in measured SO₂ flux between the proximal and distal routes.

Figure 33. Mean SO2 fluxes grouped by month from numerous field campaigns at Masaya. Error bars represent 1 standard deviation of 1 month of measurements. Note the break in the x-axis. Data from Nadeau and Williams-Jones (2009), which expanded on previous work by numerous investigators listed in that publication.

Modeling Masaya's magma system. Glyn Williams-Jones from Simon Fraser University visited Masaya with student researchers on 21 February 2012. At the National Park facilities, this group presented recent research and results from the 8-year-long collaborative effort between Simon Fraser University, The Open University, and INETER. Williams-Jones reviewed the primary monitoring techniques applied to Masaya and preliminary results regarding the environmental impact of the persistent degassing. In particular, gravity measurements, GPS, and DOAS/FLYSPEC have been used to characterize activity. SO₂ flux and air quality measurements have been part of an additional effort to characterize environmental impacts related to resident's health. The varying trend in the SO₂ flux observed since 1976 has been interpreted as being related to varying rates of magma convection in the volcanic plumbing system, as opposed to models invoking intermittent magma supply (Williams-Jones and others, 2003; Stix, 2007).

The model invoking convection within the system links Masaya's periodic explosive activity with intense, long-term degassing (Williams-Jones and others, 2003; Stix, 2007). The accumulation of a gas-rich magma within a shallow reservoir could develop a buoyant, pressurized foam. This setting would be susceptible to disruptions (by convection cells or structural adjustments, for example) and could be destabilized, leading to explosive activity.

References. Crenshaw, W.B., Williams, S.N., and Stoiber, R.E., 1982, Fault location by radon and mercury detection at an active volcano in Nicaragua, Nature, 300: 345?346.

Harris, A.J.L., 2009, The pit-craters and pit-crater-filling lavas of Masaya volcano, Bulletin of Volcanology, 71(5): 541?558.

MacNeil, R.E., Sanford, W.E., Connor, C.B., Sandberg, S.K., and Diez, M., 2007, Investigation of the groundwater system at Masaya Caldera, Nicaragua, using transient electromagnetics and numerical simulation, Journal of Volcanology and Geothermal Research, 166(3?4): 216?232.

Mauri, G., Williams-Jones, G., Saracco, G., and Zurek, J.M., 2012, A geochemical and geophysical investigation of the hydrothermal complex of Masaya volcano, Nicaragua, Journal of Volcanology and Geothermal Research, 227?228: 15?31.

Nadeau, P.A. and Williams-Jones, G., 2009, Apparent downwind depletion of volcanic SO₂ flux-lessons from Masaya Volcano, Nicaragua, Bulletin of Volcanology, 71: 389?400.

NASA Landsat Program, 2007, Landsat ETM+ scene 7dx20010325, Orthorectified, USGS, Sioux Falls, Mar. 25, 2001.

Sheriff, R.E., 1982, Encyclopedic Dictionary of Exploration Geophysics, Eighth Edition, Society of Exploration Geophysicists, Tulsa, OK, 266 pp.

Stix, J., 2007, Stability and instability of quiescently active volcanoes: the case of Masaya, Nicaragua. Geology, 35(6):535?538.

Williams, S.N., 1983, Geology and eruptive mechanisms of Masaya Caldera complex, Nicaragua [PhD Thesis]: Hanover, New Hampshire, Dartmouth College, 169 p.

Williams-Jones, G., Rymer, H., and Rothery, D.A., 2003, Gravity changes and passive SO₂ degassing at the Masaya caldera complex, Nicaragua, Journal of Volcanology and Geothermal Research, 123: 137?160.

Geologic Background. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of observed eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Glyn Williams-Jones, Department of Earth Sciences, Simon Fraser University, Burnaby, Canada (URL: http://www.sfu.ca/earth-sciences.html); Hazel Rymer, Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK (URL: http://www8.open.ac.uk/science/environment-earth-ecosystems/).

Semeru is one of the most active volcanoes worldwide and is of special concern because the drainage area is heavily populated. The volcano has a steep canyon that extends from the summit to the SE, which has funneled pyroclastic flows and lahars into populated areas. The decades-long seismicity from Semeru has typically included mildly explosive Strombolian style eruptions, earthquakes and tremor, ash plumes, and occasional pyroclastic flows (BGVN 32:03, 34:05, and 35:08). See the location of Semeru with respect to the regional setting in figure 17.

Figure 17. Index map of Semeru (red triangle) with respect to other Holocene regional volcanoes (black triangles). Courtesy of CVGHM and VDAP.

According to reporting by the Center of Volcanology and Geological Hazard Mitigation (CVGHM) and the USGS Volcano Disaster Assistance Program (VDAP), six large explosions between 1981 and 2002 resulted in many fatalities. They noted that since 1995, pyroclastic flows have been restricted to S drainages such as Kali Kembar; however, a small proportion of recent flows have entered the headwaters of Kali Koboan on the SE, which leads to heavily populated areas, including Sumberrejo and Candipuro (figure 18). This report discusses activity between February 2010 (the end of the previous report) and 2 May 2012.

Figure 18. 2010 map of Semeru and adjacent area, showing drainage channels from the summit and nearby population centers. Note the location of the 2012 lava flows just S and SE of the volcano. The area around the SE quadrant is heavily populated with a Volcano Population Index (VPI10) of 7,000. In previous eruptions, lahars reached as far as 30 km from the summit. Should similar lahars occur in the future, as many as 150,000 more inhabitants along major drainages could be affected. Based in part on a summary of activity by CVGHM and VDAP. Modified from Siswowidjoyo and others (1997) and Thouret and others (2007); VPI10 was calulated using LandScan 2010.

On 4 November 2010, CVGHM reported that from August to October 2010 seismic activity at Semeru had increased, and "smoke" and occasional gas plumes rose 400-500 m above the crater. During September incandescent avalanches traveled 400 m SE into the Besuk Kembar drainage on three occasions. Incandescence from the crater was observed in October. Incandescent avalanches traveled 600 m into Besuk Kembar on 2 November. Two days later, they reached 4 km into the Besuk Kembar and Besuk Bang (S) drainages (figure 18). CVGHM noted that the lava dome in the Jonggring Saloko crater was growing. The Alert Level remained at 2 (on a scale of 1-4).

According to the Darwin Volcanic Ash Advisory Centre (VAAC), during 18-19 November 2010, ash plumes rose to an altitude of 4.6 km and drifted 75-110 km N and NW. Sulfur dioxide gas was detected 75 km SW.

According to Volcano Discovery, the group observed 2-3 small-to-medium ash explosions per day during a photo expedition in May 2011, but noted that activity had increased during the past weeks.

In an account posted online by Volcano Discovery on 15 September 2011, the group visited the volcano and noted that an active lava dome was growing inside the crater and that 3-4 eruptions occurred daily. They inferred that the size and frequency of the eruptions had apparently increased in the past days (figure 19).

Figure 19. Photo of Semeru's crater on 1 September 2011, with a lava dome. Courtesy of Volcano Discovery.

CVGHM reported that on 29 December 2011, both earthquakes and tremor increased, and dense white-and-gray plumes rose as high as 600 m above the active crater. During January 2012, crater incandescence was observed, and avalanches carried incandescent material 200-400 m away from the crater. According to a 4 January 2012 article in the Jakara Globe, a government official indicated that authorities had closed the trail to the peak of Semeru because of heavy rain and an increased danger of landslides.

On 2 February 2012 a large explosion was reported and incandescent material fell up to 2.5 km from the crater. Tables 20 and 21 indicate the types and numbers of earthquakes and other seismic events reported by CVGHM for February to April 2012. Based on the increased seismic activity and visual observations, CVGHM raised the Alert Level from 2 to 3 on 2 February 2012.

Table 20. Types and numbers of earthquakes and plumes observed at Semeru during February-April 2012. Courtesy of CVGHM.

Table 21. Observed Semeru plumes during February-April 2012. Data from CVGHM. The only other plume noted by the Darwin VAAC between February 2010 and May 2012 was on 18-19 November 2010; this plume was noted in the text. Courtesy of CVGHM.

CVGHM reported that during 1-29 February 2012 multiple pyroclastic flows from Semeru traveled 500 and 2,500 m into the Besuk Kembar and Besuk Kobokan rivers (on the S flank), respectively. Government officials set up an exclusion zone on the SE flank where pyroclastic flows had occurred.

During 1 February-30 April 2012, dense gray-to-white plumes rose 100-500 m above Jongring Seloko crater and drifted W and N. Incandescence was visible up to 50 m above the crater during 1 February-31 March. Seismicity decreased toward the end of April, although the lava dome grew that month.

According to a news account (People's Daily Online) on 1 March 2012, seismic activity had increased from 28 to 38 tremors per day. According to the news account, Dr. Surono, head of CVGHM, stated that the volcano was erupting daily, emitting ash plumes, and tremor occurred every 15-30 minutes. He also noted that the volcanic dome was increasing in size.

According to Volcano Discovery, an expedition leader visiting Semeru observed frequent explosions every few minutes on 27 March 2012, with many powerful enough to eject glowing bombs that produced small glowing avalanches down the S flank.

According to CVGHM and VDAP, a new lava dome started to extrude in late 2011 directly over a dome formed in 2010. The new dome probably will not completely fill the summit crater because it is being drained by two new lava flows, both flowing SE. The longer of the two lava flows extended about 1.9 km from the summit vent. Pyroclastic flows are being generated by collapse of the steep termini of the lava flows, and their deposits extend to 3.2 km from the summit, i.e. 0.7 km from the front shown in figure 18. In addition, the collapsing lava flow fronts are resulting in high levels of avalanche and rockfall activity. According to CVGHM and VDAP, the closest villages in the highest-risk areas on the S and SE flanks are about 10 km from the summit.

On 2 May 2012 CVGHM lowered the Alert Level to 2, but reminded the public not to approach the crater within a 4-km radius.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Volcano Disaster Assistance Program (VDAP), US Geological Survey (USGS), 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683; 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); Jakarta Globe (URL: http://www.thejakartaglobe.com); People’s Daily Online (URL: english.peopledaily.com; Volcano Discovery (URL: http://mobile.volcanodiscovery.com).

Our previous report of Soputan volcano chronicled activity during July-September 2011 (BGVN 36:11). Table 9 gives a brief history of activity and highlights activity through early May 2012. The data sources are the Darwin Volcanic Ash Advisory Centre (VAAC) for satellite monitoring of ash plumes and the Indonesian Center of Volcanology and Geological Hazard Mitigation (CVGHM) for seismic monitoring and assignment of alert levels. According to a 28 May 2012 report by CVGHM, Soputan's activities are characterized by the growth of lava domes that have been accreting steadily since 1991. The accretion of these lava domes has been frequently accompanied by ash/cinder eruptions.

Table 9. Summary of volcanic ash observations and other activity at Soputan volcano from late June 2011 through mid-2012. 'VA' refers to volcanic ash. Courtesy of Darwin VAAC and CVGHM.

On 28 May 2012, CVGHM raised the Alert Level of Soputan from 2 to 3 (on a scale of 1-4) following increasing sesimic activity. According to CVGHM, increasing activity had been observed from 21-27 May, when the volcano spewed out white smoke to heights of between 50 to 150 m above the summit. Seismicity increased significantly on 25 May.

CVGHM called on local residents to stay beyond a 6 km radius from the volcano's summit. It also warned residents of the threat of a lahar, urging people living near Ranowangko, Pentu, Lawian and Popang rivers to remain alert and aware.

MODVOLC Thermal Alerts. MODVOLC satellite thermal alerts were measured at Soputan on 2-3 July, 9 July, and 14-15 August 2011, all on the volcano's W flank. These were the first such measurements since the volcano's last eruption, during late October to early November 2008 (BGVN 33:09). Since 8 August 2011 to early March 2012, no alerts have been measured.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is the only active cone in the Sempu-Soputan volcanic complex, which includes the Soputan caldera, Rindengan, and Manimporok (3.5 km ESE). Kawah Masem maar was formed in the W part of the caldera and contains a crater lake; sulfur has been extracted from fumarolic areas in the maar since 1938. Recent eruptions have originated at both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Volcanological Survey of Indonesia (VSI), Jalan Diponegoro 57 Bandung, Jawa Barat 40122, Indonesia (URL: http://vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac); MODVOLC, Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Jakarta Post (URL: http://www.thejakartapost.com).