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/).

Small to moderate explosive eruptions have been common at Colima since 1999, some blasting material as high as 11 km altitude and at times sending pyroclastic flows to 5 km runout distances. Between these explosive eruptions, andesitic lava from the main intracrater vent sometimes formed small, short-lived lava domes. The feeder lavas, cryptodomes, and occasional domes were blasted out during subsequent eruptions. A table of significant eruptive events at Colima during July 1999 to June 2005 (Luhr and others, in press) produced this tally for the number of days where plumes went over 2 km above the summit (~6 km altitude): in the latter half of 1999, three days; 2000, one day; 2001, four days; 2002, four days; 2003, 15 days; 2004, ~ 24 days; and in the first half of 2005, 31 days. Eruptions discussed in aviation reports from the Washington Volcanic Ash Advisory Center (VAAC) became a significant source of data starting in 2003, and formed the basis of many entries in the subsequent years.

Extrusions during September-November 2004 formed a new lava dome in the active crater, and two lava flows descended from that crater along the N and WNW flanks (BGVN 30:01). After lava effusion ceased, intermittent explosions and exhalations followed. In the same pattern mentioned above, the dome was later destroyed by Vulcanian-style explosions that produced eruption plumes and in some cases, pyroclastic flows (BGVN 30:03).

The number of seismic events decreased during December 2004-February 2005 (figure 77), and with some important exceptions, remained under 10 events per day until as late as the end of June 2005. During this reporting interval, April-June 2005, intermittent explosions continued (figure 77). Explosions that generated pyroclastic flows were known to have continued through at least 5 July.

Figure 77. The number of daily earthquakes ascribed to rockfalls and pyroclastic flows (heavy line) and to explosions and exhalations (dashed line) at Colima during September 2004-June 2005. Double arrows show the beginning (B) and the end (E) of the lava extrusion in late 2004. A label indicates the period when occasional large explosions took place (an interval that began on 10 March and continued through June 2005). Courtesy of Colima Volcano Observatory.

Comparatively large explosions began to occur starting 10 March 2005 (BGVN 30:03). The largest, accompanied by pyroclastic flows, were particularly vigorous from 24 May to 5 June. As in March 2004 the explosions consisted of Vulcanian-style gas-and-ash explosions. Some of the April-June explosions issued material that reached as high as ~ 10 km altitude, and pyroclastic flow runout distances reached up to ~ 5.1 km, an increase over those in March 2004 (when maximum runout distances only reached ~ 2.8 km).

When photographed on 25 May 2005 the dome and unconsolidated material filled much of the crater, although the intracrater area was anything but flat (figure 78). By comparison, a photo of the crater taken on 16 June 2005, following many large Vulcanian explosions, shows its upper portion to be essentially empty (figure 79).

Figure 78. At Colima on 25 May 2005 the crater contained considerable dome and unconsolidated material, filling it to near the rim. Several weeks later, after further explosions had driven considerable material out, the upper crater was left with substantial open space (see next photo). Courtesy of Colima Volcano Observatory.

Figure 79. Photo of Colima's crater after the comparatively large explosions that began in March 2005. This photo was taken on 16 June looking from the S. Eruptions had removed much of the crater fill and a small dome from the upper crater. Small impact craters pocked the crater floor. An erosion channel had developed across crater's S rim, presumably due to the passage of pyroclastic flows associated with the recent explosions. The notch in the rim has been prominent since 2004 and has emptied and perhaps grown considerably since the photo taken 25 May 2005. Despite the changes seen in this photo, the explosions had left the crater walls intact and without evidence of fractures. Courtesy of Colima Volcano Observatory.

The March-June explosive sequence removed the 2004 lava dome, and left a crater ~ 260 m across and ~ 30 m deep (figure 79). No significant deformation of the volcanic edifice was recorded before or during the large explosions (table 17). After the explosion of 5 June, residents were evacuated from Juan Barragán, a small village ~ 10 km SE of the summit. Smaller explosions at Colima typically take place at the rate of several per day.

Table 17. Main characteristics of the largest explosions seen at Colima during May-June 2005. Column heights and ash cloud velocities came from remote-sensing data and reports furnished by the Washington VAAC. The highest velocity, 15 m/s, corresponds to 54 km/hour. Courtesy of Colima Volcano Observatory.

Reference. Luhr, J., Navarro-Ochoa, C., and Savov, I., (in press), Petrology and mineralogy of lava and ash erupted from Volcán Colima, México, during 1999-2005: Special Volume on the Colima Volcano, from the University of Guadalajara (edited by Francisco Nuñez-Cornú).

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

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

A few gas-and-steam plumes from Ebeko were reported during February-April 2004 (BGVN 29:04). The most recent previous eruption was in January 1991. On 30 January 2005 the Kamchatka Volcanic Eruptions Response Team (KVERT) raised the Concern Color Code at Ebeko from Green to Yellow after reports of a strong smell of sulfur on 27 and 28 January in the town of Severo-Kurilsk, ~ 7 km from Ebeko. Observations by Leonid and Tatiana Kotenko in Severo-Kurilsk during May-July 2004 included occasional gas-and-steam plume rising as high as 250 m above the volcano during clear weather and fumarolic plumes moving close to the ground. There was no visible activity in August, but a few plumes were seen again from September to November.

During 28 January, a white gas-and-steam plume was seen from Severo-Kurilsk rising 400 m above the volcano. Summit observations the next day revealed a yellow-gray, 5-m-diameter, column rising 300 m from a vent on the NE side of the active crater. Three ash layers 2-3 mm thick were noted 10 m from the vent, and ash extended ~ 500 m E into the crater. At this time a new 7 x 12 m turquoise lake had developed in the SW part of the active crater. The lake disappeared on 30 January, and there was intensive fumarolic activity where it had been. Shallow earthquakes were recorded at the Severo-Kurilsk seismic station.

On 1 February gas-and-steam plumes rose to 450 m above Ebeko's crater and drifted NE. On 7 February a small emission of steam, gas, and possibly ash rose ~ 1 km above the crater and drifted ~ 12 km SE. On 8 and 9 February plumes rose to 600 m and thin ash deposits were noted in the town of Severo-Kurilsk.

The following information came to KVERT from observers in Severo-Kurilsk (Leonid and Tatiana Kotenko). On 15-16 February a dark-gray column rose up to 500 m above the crater. A dark-gray plume extended 6 km E and a light-gray plume 7 km SE. On 16 February ashfall together with snowfall was noted over the strait to the E of Paramushir Island. On 17 February a white column up to 250 m above the crater was observed. On 12 February and 16-17 February a strong smell of a H₂S was noted at Severo-Kurilsk. On 18-19 February white gas-and-steam columns 5 m in diameter rose from the two vents up to 450 m above the crater and a new lake (10 x 10 m) on the floor of the active crater was observed. On 25 February white gas-and-steam plumes rose to 450 m and 1,000 m above the crater. Gas-and-steam plumes were also observed on 1-2, 4-5, and 9 March. No ash was seen. A strong smell of H₂S was noted at Severo-Kurilsk on 25 February and 2 March.

About 20 seismic events of less than Ml 2.0 were observed during 1-9 March at the Severo-Kurilsk seismic station. No seismic activity was observed from 12 to 14 March. On 15 March two seismic events were noted. There was no seismicity during 18-25 March, so KVERT reduced the hazard status from Yellow to Green, the lowest level.

The Russian Emergency Situations Ministry's Sakhalin department reported renewed activity on 27 June in the form of emission clouds rising to a maximum height of 200 m above the crater and drifting SW. KVERT did not report any activity, and the Concern Color Code for Ebeko remained at Green.

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

Throughout May 2005, PHIVOLCS noted that ash-and-steam emissions from Canlaon produced plumes to 500-1,000 m above the volcano. The hazard status remained at Alert Level 1. The SO₂ flux remained above the 'normal' level of 500 metric tons/day (t/d) with values of 2,700 t/d on 1 May, 2,080 on 22 May, and 1,400 on 26 May. According to news reports, flights to and from nearby Kalibo airport were suspended on 3 May due to reduced visibility.

Although voluminous white steam continued to be discharged from the active vent early in June 2005, after 25 May ash ejections stopped and ash contents in the steam plume were significantly reduced. On [30 June] PHIVOLCS lowered the hazard status of Canlaon from Alert Level 1 to Alert Level Zero, listing a variety of reasons. For one, they noted the downtrend in the SO₂ gas emission rate from a high of about 4,900 t/d, to the prevailing level of 1,500 t/d. For another, they noted the absence of significant seismic activity before, during, and after the ash emissions. And finally, they cited a lack of significant observations indicating near-surface hydrothermal activity. Since Canlaon has a history of sudden outbursts, the public was reminded to refrain from entering the 4-km-radius Permanent Danger Zone (PDZ) and to coordinate with PHIVOLCS and Disaster Management Councils in any attempt to climb the volcano.

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

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, Univ. of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Chris Newhall, USGS, Box 351310, University of Washington, Seattle, WA 98195-1310, USA; Philippine Star (URL: http://www.philstar.com/).

During 1 January to mid-April 2004 (BGVN 29:04), ash-and-gas explosions and gas plumes were observed and seismicity remained generally above background levels. From May to the beginning of September 2004, seismic activity remained above background levels, varying over this time from 100-800 small shallow earthquakes per day. Ash-and-gas explosions and gas plumes to a maximum height of 7.5 km were frequent. On 1 September 2004 an increase in activity led the Kamchatka Volcanic Eruptions Response Team (KVERT) to raise the Concern Color Code from Yellow to Orange. From September to December 2004, seismicity remained above background levels, and ash-and-gas explosions and ash plumes were frequent. On 12 November the hazard status was lowered to Yellow.

Increasing seismicity, rock avalanches and possible ash plumes to 2.5 km altitude led KVERT to raise the Concern Color Code to Orange again on 7 December 2004. On 28 December, an observed eruption at Karymsky produced a plume composed primarily of gas and steam, but with some ash, that rose to ~ 1 km above the crater. Thermal anomalies were also visible on satellite imagery on 27 and 28 December. On 30 December the Tokyo VAAC reported that a plume was present up to ~ 8 km altitude extending SW.

There were no seismic data from 12 December 2004 till late January 2005. Through January and February thermal anomalies were frequently visible on satellite imagery. Seismicity remained above background levels from February 2005 through July 2005.

Through March and April 2005, ash-and-gas explosions and gas plumes were frequent. Ash deposits extended 10-15 km S and SW of the volcano. On 20 April, volcanic bombs rose to 50 m above the crater, and ash fell to the NE on 21 April. On 26 and 27 April, Strombolian activity was seen in two of the volcano's craters; volcanic bombs rose to ~ 300 m above the craters. Ash fell to the SE on 22-23 April and pyroclastic-flow deposits were seen on the NNW flank of the volcano. During May 2005, ash-and-gas explosions and plumes were again frequent, and a thermal anomaly continued to be visible on satellite imagery.

Due to a decrease in seismic and volcanic activity during 3-10 June, KVERT decreased the alert level from Orange to Yellow. Seismic activity increased starting on 22 June. Ash explosions up to 3,000 m altitude traveling SW were observed by pilots. According to seismic data, about 10 ash-and-gas plumes and avalanches occurred at the volcano. On 23 June KVERT increased the alert level to Orange. Satellite imagery of Karymsky showed a narrow ash-and-gas plume at a height of ~ 3.5 km altitude on 30 June. Based on interpretations of seismic data, ash-and-gas plumes may have reached 3 km above the crater.

The Tokyo VAAC posted four messages on Karymsky during the 90 days prior to 8 August 2005; in each, ash was not identifiable from satellite. The earliest, 18 May was similar to the last one, on 23 June. Both noted a reported plume to FL100 ('flight level 100' signifies 10,000 feet; 3.05 km altitude). Reports on 22 and 24 May both noted ash to FL 120 (3.65 km altitude).

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: KVERT (URL: http://www.kscnet.ru/ivs/kvert/); Tokyo Volcanic Ash Advisory Center (VAAC), Japan Meteorological Agency, Tokyo Aviation Weather Service Center, Haneda Airport 3-3-1, Ota-ku, Tokyo 144-0041, Japan (URL: https://ds.data.jma.go.jp/svd/vaac/data/).

Activity at Kīlauea through October 2004 was previously reviewed in reports that included maps showing the extent of key lava flows through most of August 2004 (BGVN 29:09). During November 2004 through January 2005, lava flows were abundant and made complex patterns. Their overall advance can be seen by comparing maps of the extent of the lava flows as of late August 2004 (figure 169) and 2 February 2005 (figure 170).

Figure 169. Kīlauea lava flows erupted during activity from 1983-August 2004 of Pu`u `O`o and Kupaianaha. Note the location of Kupaianaha, the active vent area during 1986-1992, ~ 4 km ENE of Pu`u `O`o. Courtesy of the U.S. Geological Survey's Hawaiian Volcano Observatory.

Figure 170. Kīlauea lava flows erupted during activity from 1983-2 February 2005 of Pu`u `O`o and Kupaianaha. Courtesy of the U.S. Geological Survey's Hawaiian Volcano Observatory.

On 4 November 2004 lava from the Prince Kuhio Kalaniana `ole (PKK) flow entered the sea, forming a new delta seaward of the E end of the old Lae'apuki delta. The PKK flow has been continuously active since 26 July 2004, and lava continued to enter the sea through 26 November 2004. This was the first time lava entered the sea since the Banana lava flow ceased in early August 2004. The Banana flow developed from breakouts when lava escaped from the confines of the Mother's Day lava tube, emerging near the former Banana Tree kipuka. This flow stagnated early in September 2004, and the Mother's Day tube ceased carrying lava late in 2004.

During the first week in December 2004, the lava flow at Lae'apuki abated. Activity resumed during the second week along all areas of the PKK flow from high on the Pulama pali fault scarp. By 13 December lava again entered the sea at the East Lae'apuki delta. The flow moderated during the second half of December with only several areas of visible surface lava apparent on the Pulama pali fault scarp and on the coast.

New vents opened at the southern base of Pu`u `O`o on 19 January 2004 and fed the Martin Luther King (MLK) flows (figure 11). The PKK flow originated from two vents ~ 250 m S of the base of Pu `u `O`o. By 2 February 2005 the PKK flow had entered the sea at West Highcastle, Lae'apuki, and Ka`ili`ili (figure 11).

During January 2005, surface lava was visible along the three main arms of the PKK flow as they advanced downslope towards the coast (figure11). The middle arm of the PKK flow was comparatively small, and it failed to reach the ocean during this reporting interval; it remained high on Pulama pali. In contrast, lava from the E and W arms of the PKK flow began to enter the ocean on 31 January. The large E arm of the PKK lava flow fed the larger Ka`ili`ili entry. The W branch of the PKK lava flow once supplied lava to Lae'apuki (an E branch of the W arm), but later also began feeding the West Highcastle ocean entry (the W branch of the W arm, figure 11).

Seismicity. After seven months of relative quiescence renewed seismicity and numerous small long-period (LP) events again became visible in November 2004 on the North Pit seismogram. Elevated activity began on 16 November, peaking at over 2,000 events a day by late November (figure 171). Nearly all of these earthquakes were too small to catalog. To obtain this plot, a daily event count was extrapolated from a representative part of the North Pit (NPT) seismogram. Scientists combined the counts for two shallow (0-5 km deep) earthquake types, those designated by HVO as short-period summit or short-period caldera (SPC) and those designated as shallow, long-period (long-period caldera A, LPC-A) earthquakes. The similar frequency content of these two kinds of earthquakes make them difficult to distinguish on the drum record. In addition, small-magnitude deeper earthquakes, designated as long-period earthquakes originating at depths over 5 km, may have also registered within the summit caldera to appear on the plot, although they would be expected to contain a lower dominant frequency of oscillation than the LPC-A earthquakes. Tremor episodes were rare or absent.

Figure 171. A time series of Kīlauea's daily earthquakes (SPC, LPC-A, and possibly LPC-C types) registered at the summit during October 2004 through January 2005. Courtesy of U.S. Geological Survey's Hawaiian Volcano Observatory.

A minor peak in seismicity occurred in later January, during the two days before and after the 25 January inflation-deflation event. Most of the events on 25 January appeared to be of the SPC variety.

Tilt and deformation. The tiltmeter record at Kīlauea summit (UWE) and Pu`u `O`o (POC) showed numerous correlated tilt changes, with a short time delay between UWE and POC stations and larger magnitude delays at POC (figure 172). One of the largest of these deformations took place on 25-26 November and resulted in about 3 microradians of tilt at UWE, and 5 microradians at POC. This was similar in character to the tilt events of recent months, starting with fairly rapid deflation, followed by a similar rate and magnitude of inflation. Though they differ in character from the deflation-inflation-deflation (DID) cycles of the past few years, they seem to be originating from the same shallow storage area near Halemaumau, the crater at Kīlauea's summit.

Figure 172. Electronic tiltmeter records from the N flank of Pu`u `O`o cone (POC) and NW rim of Kīlauea caldera (UWE) for (A) October and November 2004 and (B) December 2004 through January 2005. Only the radial component is plotted, i.e., the direction that maximizes signal from the most common sources of tilt at both locations. Courtesy of U.S. Geological Survey's Hawaiian Volcano Observatory.

Kīlauea continued to inflate over this reporting period. The extension rate across the summit increased dramatically in early January 2005, from an average rate of about 8 cm/yr to over 40 cm/yr. There was a short inflation-deflation event on 25 January, followed by about 2-3 days of extremely rapid movement of the S flank; continuous GPS stations on the S coast were displaced by up to 2 cm. The pattern and rate of motion are very similar to the slow earthquake of November 2000. The slip event occurred during a swarm of earthquakes (see seismic section above), but the cumulative magnitude of these earthquakes was not nearly as great as the estimated equivalent moment magnitude of the slip.

Other large episodes of correlated multistation tilt occurred on 14 December 2004 and 25 January 2005. In December, both UWE and POC recorded deflationary tilts of about 4 and 2.5 microradians, respectively, over about 12 hours. In mid-January, the summit started showing a high rate of inflationary tilt, coinciding with the increase in cross-summit extension, measured by continuously recording GPS. In the early morning of 25 January, summit tiltmeters and POC recorded a rapid inflation (about 5.5 microradians in an hour at UWE, 2 at POC) followed by an equal amount of deflation over the next day. The event was similar to the fairly frequent deflation-inflation-deflation (DID) events at Kīlauea. Similarities included the apparent source regions of the inflation, the seismic signature, the delay time between the summit and the rift zone, and the timing of increased activity.

SO₂ emission rate measurements. Summit SO₂ emission rates for October/November ranged from 80 to 130 metric tons per day (t/d) with an average of 105 t/d (standard deviation, s.d.=20 t/d for 36 measurements made over 6 days). Although this represents a slight decrease over emission rates measured during the previous reporting period, it does not represent a significant change. Correlation spectrometer (COSPEC) SO₂ measurements along the Chain of Craters Road yielded SO₂ flux rates of 1,080-1,660 t/d with a mean value of 1,270 t/d (s.d. of 260 t/d for 27 measurements made over 4 days). The drop in emissions, which began in May 2004, had continued through November 2004. A lack of trade winds hindered SO₂ flux measurements during November and December. Six traverses on 6 December yielded an emission rate of 105 t/d (s.d.=10 t/d) consistent with the more frequent measurements made during September-October 2004. The return of the tradewinds in early February allowed measurements to resume and showed that summit emissions had decreased markedly, likely due to the heavy rainfall on 4 February.

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, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

The following report comes from Matt Patrick of the HIGP Thermal Alerts Team. Two night-time ASTER images (Band 10, 8.3 microns, at 90 m pixel size) of McDonald Island show activity centered on the NW shore of the island. The December 2002 image was examined some months ago, but it was not determined whether the long-wave infrared (IR) anomaly was genuine, since it was relatively low intensity and there was no anomaly in the shortwave IR. The most recent ASTER image (12 July 2005) shows a somewhat larger long-wave IR anomaly, but more importantly, there are five pixels in the shortwave IR (Band 9, 2.4 microns; not shown) which are saturated, indicating this is a significantly hot target. Based upon McDonald's typical activity, the anomaly probably reflects low-level effusive activity.

The first and only MODVOLC alert pixel showed up in November 2004 (BGVN 29:12). These ASTER images show that recent activity is centered around the NW flank of the island, very close to shore. Comparing the July 2005 image with the December 2002 image, there might be an indication of the shoreline growing westward, but it is hard to tell for sure with this resolution (90 meters). The location of this activity is generally consistent with recent BGVN reports: in 1999 steaming was observed on the N-NE part of the island (BGVN 24:01), and a recent Landsat ETM image indicated that island construction over the last two decades has expanded the northern portion of the volcano (BGVN 26:02 and 27:12).

Andrew Tupper noted that he found the hot spot identification plausible. The question of edifice collapse and possible tsunami generation associated with McDonald Islands has recently been a subject of interest but little technical information is available on topics such as edifice morphology and slope stability.

Geologic Background. Historical eruptions have greatly modified the morphology of the McDonald Islands, located on the Kerguelen Plateau about 75 km W of Heard Island. The largest island, McDonald, is composed of a layered phonolitic tuff plateau cut by phonolitic dikes and lava domes. A possible nearby active submarine center was inferred from phonolitic pumice that washed up on Heard Island in 1992. Volcanic plumes were observed in December 1996 and January 1997 from McDonald Island. During March 1997 the crew of a vessel that sailed near the island noted vigorous steaming from a vent on the N side of the island along with possible pyroclastic deposits and lava flows. A satellite image taken in November 2001 showed the island to have more than doubled in area since previous reported observations in November 2000. The high point of the island group had shifted to the McDonald's N end, which had merged with Flat Island.

Information Contacts: Matt Patrick, HIGP Thermal Alerts Team, Hawai'i Institute of Geophysics and Planetology (HIGP) / School of Ocean and Earth Science and Technology (SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Commonwealth Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

Following explosions from Shiveluch during 25 February to 4 March 2005 ash fell in Ust'-Hairyuzovo, about 250 km W (BGVN 30:02). From March 2005 until July 2005, Shiveluch remained at Concern Color Code Orange. Throughout March 2005 the lava dome at Shiveluch continued to grow and on several days ash-and-gas plumes and gas-and-steam plumes rose to a maximum of ~ 2.8 km above the dome. Satellite imagery showed a thermal anomaly at the dome during the first week of March and a large thermal anomaly over the recent pyroclastic-flow deposit during 11-12 March. Between 5-28 March a new lava extrusion added ~ 50 m height to the SW part of the dome.

During April 2005, intensive growth of the new extrusion at the W part of the dome continued, and the E and W parts of the lava dome became nearly level. Gas-and-steam plumes rose to a maximum of ~ 1.2 km above the dome during April 2005. Satellite imagery showed a large thermal anomaly at the dome during mid-April and a small anomaly associated with a pyroclastic flow on 19 April. On 25 April, a hot avalanche on the dome's W side produced an ash plume that rose ~ 2 km above the 2.5-km-high lava dome. Growth of the dome continued during May 2005 with a new extrusion to the W. Ash-and-gas plumes, some rising 2 km above the dome, were frequent. Satellite imagery showed a persistent thermal anomaly at the lava dome throughout May.

The dome continued to grow during June 2005. During 3-10 June, two shallow M 1.6-1.7 earthquakes occurred 0-5 km beneath the active dome. Gas-and-steam plumes rose as high as 400 m above the dome during June. A persistent thermal anomaly was visible throughout June. Fumarolic activity was reported during the week of 18-24 June. During the last week of June, satellite imagery showed a persistent thermal anomaly, and fumarolic activity produced steam to 4-5 km altitude. On 30 June, ash-and-gas plumes rose 3-5 km altitude. and drifted NW. Hot avalanches of volcanic material were also recorded. On 6 July ash-and-gas plumes rose to ~ 7 km altitude and drifted NW. On 7 July an 11-minute-long seismic event occurred, and ash-and-gas plumes may have reached a height of 10 km altitude. Around 8 July, KVERT raised the Concern Color Code from Orange to Red, the highest level. On 8 July 2005, video footage showed weak gas-and-steam plumes rising to ~ 5 km altitude. On 9 July 2005, the Concern Color Code was reduced to Orange.

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

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

Soufrière Hills was last reported on in BGVN 30:03, covering November 2004 to March 2005, during which time the volcano remained quiet, with seismic signals, gas emissions and rockfalls all decreasing. This report, from Montserrat Volcano Observatory (MVO), covers the period from late March 2005 to July 2005. The volcano continued to be relatively quiet through April and early May, with activity increasing somewhat through June and several explosive events in late June and in July. Table 60 summarizes the seismicity and SO₂ emissions during the period of this report.

Table 60. Geophysical and geochemical data recorded at Soufrière Hills, 25 March 2005 to 15 July 2005. * Only measurement during report period. **12-hour system failure may have caused events to be missed. Courtesy of MVO.

Seismic activity at Soufrière Hills remained at low levels throughout March and most of April 2005. Beginning on 15 April, vigorous steam-and-ash venting occurred on the NW side of Soufrière Hills crater and continued throughout the period of this report. Average daily SO₂ emissions were generally lower than the long-term eruption average of 500 tons/day, but increased in July to above the average.

On 13 June at 0600 an ash plume reached a height of ~ 2.4 km altitude and drifted NE, depositing light ash in Lookout, Geralds, and St. Peters.

Starting around 10 June, seismic and volcanic activity were at elevated levels. The ash venting that began on 13 June declined in intensity during the following week. The ash venting was caused by the rapid release of steam and other volcanic gases, possibly triggered by intense rainfall on the night of 12 June. Ash analyses from this episode did not indicate fresh magma.

On 27 June a steam and ash cloud at ~ 3 km altitude was reported to be drifting W. By 28 June satellite imagery showed a plume of ash and steam at ~ 1.8 km altitude extending NW. Periodic episodes of intense ash venting continued, culminating in an explosive event on 28 June at 1306. During the event, ballistics were ejected onto the Farrell's plain (to the NW), and a column collapse produced pyroclastic flows. The pyroclastic flows reached the sea at the Tar River delta (to the NE), and a smaller volume of material flowed into the top of Tyre's Ghaut (to the N). Ash showed no evidence of fresh magma.

Preliminary analysis of recent ground deformation data from the GPS network at the volcano showed that deflation during April to mid June 2005 had later reversed, and the volcano appeared to be inflating. Periodic ash venting continued and an explosion occurred on 3 July at 0130, which was similar to the explosion on 28 June.

An explosive event at 0301 on 18 July caused widespread ash fallout between Fogarty Hill on the island's NW and Brodericks Yard on the island's SW and almost certainly led to pyroclastic flows to the sea in Tar River. This explosion was similar to, but slightly bigger than, the explosion on 3 July, and ash venting and pyroclastic flows combined to cause dramatic ash clouds which reached to at least 6 km. Winds blew the ash plume in a NW direction causing significant ash fall in Old Towne, Iles Bay, Salem, Olveston, Woodlands and St Peters. The maximum depth of ash measured by scientists in inhabited areas was 1.5 to 2.0 mm; the deepest ash was recorded at Weekes. Activity subsequently returned to background levels.

The MVO collected ash samples from the affected areas to determine whether it was new material from depth or older material from the dome. Ash collected after the 28 June and 3 July 2005 events showed no evidence of new magmatic material.

On 28 July 2005, the Moderate Resolution Imaging Spectroradiometer (MODIS) flying onboard the Aqua satellite acquired an image of a plume of volcanic ash drifting westward in a slightly curving shape as it departs Soufriere Hills (in the middle of the image, figure 61).

Figure 61. A MODIS image of an ash plume from Soufrière Hills acquired on 28 July 2005. N is towards the top. The plume was visible for over 100 km, but conspicuous portions of the plume continued beyond the W (left) side of this image between the arrows. A Washington VAAC report from that day suggested a plume to ~ 5 km altitude and 70-300 km long, blown W. Several islands neighboring Montserrat (M) are labeled: A, Antigua; B, Barbuda; G, Guadeloupe; N, Nevis; and SK, St. Kitts. For scale, the distance between the centers of the islands of Montserrat and Antigua is ~ 55 km. Some islands are ringed in bright blue-green, the possible result of coral reefs in shallow water, sediment, phytoplankton, or some combination of these conditions. Image and some elements of the caption courtesy of Jeff Schmaltz, MODIS Rapid Response Team, NASA.

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

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/).

Throughout the period covered by this report, March 2005 to July 2005, growth of the new lava dome inside the crater of St. Helens continued, accompanied by low rates of both seismicity and gas and ash emissions. The hazard status remained at 'Volcano Advisory' (Alert Level 2); aviation color code Orange. Results from a digital elevation model produced from imagery taken on 21 February showed the highest part of the new lava dome was 12 m higher than on 1 February; during that 3 week period the volume of dome and surrounding uplift had increased by 3 million cubic meters. The average rate of growth continued at ~2 m³/s. Figure 52 shows four views of changes to the lava dome during the period of this report. Figure 53 shows the seismicity and the time of the larger recognized explosions.

Figure 52. Four views into St. Helens's crater from different perspectives and dates, focusing on the new dome. A: 15 March 2005, view from NE. The whaleback is close to its maximum length of 500 m. Note that the glacier's heavily crevassed, half-moon shaped, E (left) arm lies squeezed between the growing dome and crater wall. Vent is steaming at lower-right whaleback. B: 3 May 2005, view from N. The whaleback has been breaking apart for several weeks. Note the large slab of smooth gouge-covered surface moving E (left). C: 21 June 2005, view from NW. Note the development of broad talus on W (right) flank of dome. An isolated body of smooth gouge-covered surface to the right of the main spine is emerging from talus. D: 26 July 2005, view from E crater rim. The smooth, gouge-covered spine continues to crumble as a result of M > 3 earthquakes and rockfalls. A large slab of March whaleback is visible at left. Most of the dome surface is now talus and disintegrating older whalebacks. By the end of July, the spine had been reduced to a highly fractured stump. All photos courtesy of USGS CVO.

Figure 53. Magnitude of located earthquakes at Mount St. Helens through 27 July 2005 (Pacific Northwest Seismograph Network). Vertical lines represent the time of moderate explosions. Note periods of earthquakes M > 3 that accompanied dome break-ups in December, April, and July. Courtesy of CVO and the Pacific Northwest Seismograph Network.

During 2-7 March, dome growth accompanied low rates of both seismicity and gas and ash emissions. Parts of the growing lava dome continued to crumble, forming rockfalls and generating small ash clouds that drifted out of the crater. The bulging ice on the deformed E arm of the glacier in the crater continued to move rapidly N at about 1.2 m per day (figure 54).

Figure 54. A view of the growing dome at St. Helens from the Sugar Bowl camera just before the 8 March 2005 explosion. The Sugar Bowl digital camera takes a picture every hour from its housing on the NE flanks. The image data are transmitted to a more accessible spot immediately after the pictures are taken. Courtesy of CVO.

A small explosive event began at approximately 1725 on 8 March. The eruption lasted about 30 minutes with intensity gradually declining throughout; a fine dusting of ash from this event later fell ~ 100 km to NW (in Yakima, and Toppenish, Washington). By 0200 on 9 March, the leading edge of the faint, diffuse plume had reached ~ 300 km to the E (over western Montana). After the explosion scientists found the lava dome intact. They recognized ballistics (up to ~ 1 m in diameter) as far as the N flank of the old lava dome and a lack of them along or beyond the crater rim. The explosion vented from the NNW side of the new lava dome, very near the source of the 1 October 2004 and 16 January 2005 explosions (figure 55).

Figure 55. The 8 March 2005 explosion at St. Helens viewed from the Sugar Bowl camera. This shot was taken at about 1727 hours and 42 seconds on 8 March. Courtesy of CVO.

The explosion on 8 March was one of the largest steam-and-ash emissions to occur since renewed activity began in October 2004. The Cascades Volcano Observatory (CVO) lost radio signals from three monitoring stations in the crater soon after the event started. The event followed a few hours of slightly increased seismicity not then interpreted as precursory. There were no other indications of an imminent change in activity.

After the 8 March explosion, St. Helens only emitted steam, and seismicity dropped to a level similar to that during the several hours prior to the explosion. Gas emissions were very low, essentially unchanged from those measured in late February. The hazard status for the ongoing eruption, 'Volcano Advisory (Alert Level 2),' mentioned the possibility of events like the 8 March explosion occurring without warning. That assessment remained unchanged and the hazard status stayed the same.

Analysis of aerial photographs indicated that as of 10 March the topographic changes in the crater resulting from growth of the new dome and consequent glacier deformation had a combined volume of about 45 million m³. The current eruption contributed new materials amounting to about two-thirds the volume of the old lava dome.

From March 2005 through July 2005, growth of the new lava dome continued. Rates remained low for both seismicity and gas and ash emissions. CVO noted that during such eruptions, episodic changes in the level of activity can occur over days to months. During about 26-27 March, a group of M 2 to M 3 earthquakes occurred beneath the volcano, a level of activity considered normal during dome-emplacing volcanism.

A series of large (M >=3) earthquakes occurred during 3-4 April, in addition to the typical array of smaller events. Observations on 6 April revealed that the smooth whaleback-shaped portion of the growing lava dome was broken by numerous fractures, and the edges had crumbled greatly. Several deep gashes on the E, N, and W sides frequently produced rockfalls and accompanying ash clouds. On 10 April the new dome continued to fracture and spread laterally. As a consequence, the dome's summit dropped by a few tens of meters over 2-3 weeks, leaving isolated high-standing remnants. This broken pattern was apparent in a photograph on 3 May (figure 5B).

Earthquakes steadily decreased in magnitude and number through mid-April. A GPS receiver 200 m N of the new dome crept steadily NNW at ~ 10 cm per day. The combination of the GPS measurements adjacent to the lava dome and the qualitative estimate of lateral spreading suggested that extrusion of new lava continued during April.

On the morning of 28 April there were reports of minor amounts of ashfall in the eastern part of the Portland metropolitan area, ~ 80 km SSW of St. Helens. There was no evidence of a new explosion. CVO scientists determined that large convective storms over the Cascades on 27 April entrained ash generated by the frequent hot rockfalls from the growing lava dome and kept it in suspension to fall out as far away as Portland.

During early May poor weather obscured the volcano. Seismic and ground deformation activity remained unchanged. Through much of the night of 4-5 May, however, VolcanoCam images detected intermittent glow from the new dome. The camera is mounted at the Johnston Ridge Observatory at an elevation of 1,400 m and ~ 6.5 km NNW of the volcano, a spot W of the S part of Spirit Lake. During 11-12 May images from the mouth of the crater showed the new spine of lava at the N end of the dome continuing to grow. Data from seismic and GPS instruments in the crater and on the outer flanks continued to lack significant changes over the past few weeks. Through the end of May, lava extrusion continued at the N end of the new lava dome, while the high spines continued to crumble. Other parts of the lava dome moved at the relatively low velocity of about 30 cm/day or remained stagnant. Table 7 compares the older dome with the new one as of 3 May 2005.

Table 7. A comparison of the old (1980-86) and new (2004-) domes at St. Helens. The new dome (unofficially called "the whaleback") started in October 2004, and the reported data reflects conditions seen until 1 February 2005. Courtesy of CVO.

Around 4 June the rate of motion of a GPS unit on the NE part of the new dome slowed slightly, continuing to creep eastward and northward at a rate of several centimeters per day, but no longer rising vertically. The lava spine, however, continued to grow. Through the end of June 2005, seismic and deformation data continued trends similar to the previous few weeks, with small earthquakes approximately every 5 minutes, little to no movement of the old lava dome, minor movement of the N end of the new lava dome, and continued growth of the lava spine. Observations made on 15 June revealed that the lava spine continued to grow and that temperatures in cracks near the base of the spine were near 700°C. Thermal data from 15 June suggested that much of the W part of the dome was moving upward, as well as southward. During the last week of June, the smooth lava spine continued to grow at a rate of about 1.8-3.7 m per day. Rockfalls from the top of the spine kept its height from increasing by that same rate. Analysis of a digital elevation model made from imagery acquired on 15 June showed that the total volume addition to the crater since September 2004 had reached almost 60 million cubic meters.

On 2 July at 0630 a rockfall from the growing lava dome removed a large piece of the dome's top, producing an ash plume that rose above the crater rim and generating a substantial seismic signal. Persistent smaller rockfalls from the growing lava dome built talus aprons on the W and NE flanks of the dome.

On 12 July, CVO reported that rates of seismicity and ground deformation at Mount St. Helens had declined during the previous two weeks to some of the lowest levels since the eruption began in September 2004. A similar lull occurred in December 2004.

Beginning 15 July and continuing through the end of the month, the growing spine and other high areas of the dome to the south produced numerous large rockfalls, most of which were associated with earthquakes of about M 3 (figure 56). Diffuse ash plumes that rose hundreds of meters above the rim were produced by the larger rockfalls. By the end of July most of the smooth gouge-covered surface of the spine had disintegrated, and the spine was reduced to a highly fractured, but still-extruding, stump surrounded by rapidly growing aprons of rockfall debris.

Figure 56. Rockfall and accompanying ash cloud on 26 July 2005 as viewed from station Brutus on the crater's E rim. Rockfall originated from the steep, fractured top of an inclined spine. Note boulders (light-colored specks against shadow) shooting ahead of ash cloud. Another spine is extruding from ground just behind the lower end of the ash cloud. Courtesy USGS and CVO.

Geologic Background. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismograph Network (PNSN), Seismology Lab, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/).

The eruption of Tungurahua that began at the end of December 2003 (BGVN 28:11) continued through January 2004 (BGVN 29:01). Figure 25 shows an ash plume emitted on January 2004 in a Moderate Resolution Imaging Spectroradiometer (MODIS) image.

Figure 25. A NASA MODIS image showing an ash plume from Tungurahua acquired 14 January 2004. N is up; the plume's height and length were undisclosed. Arrow points to Tungurahua and is along the approximate trend of the densest portion of the plume. The plume blew NE across the Andes and remained visible well over the thickly vegetated lowlands farther E. (Visible Earth v1 ID 26233.) Courtesy of NASA. Inset map showing major active Ecuadorian volcanoes courtesy of the USGS.

On 5 February 2004 there was a slight increase in seismic activity at Tungurahua; steam emissions rose to low levels, and small lahars traveled down the volcano's W flank via the Achupashal and Chontapamba gorges. On 9 February emissions of steam, gas, and moderate amounts of ash occurred, deposited to the W in the sectors of Pillate and San Juan. During mid February, several avalanches of incandescent volcanic blocks traveled ~ 1 km down the volcano's flank. During late February through mid April 2004, degassing continued at Tungurahua with occasional explosions of steam, gas, and ash, producing plumes to ~ 500 m above the volcano.

On 2, 11, and 15 March lahars traveled through the Pampas sector. During the night of 28-29 March incandescent material was observed avalanching on the upper slopes. From 30 March to 3 April, volcanic activity was at relatively low levels, but emissions of steam and ash occurred, and incandescence was visible in the crater. On 4 April at 1902 an explosion produced a plume containing a moderate amount of ash that rose to 800 m above the crater, and on the evenings of 10 and 11 April, incandescence was visible in the crater.

Sulfur-dioxide flux measurements taken on 11 April were the highest measured for several weeks; 1,600-1,700 metric tons per day. Heavy rain during the afternoon and night of 13 April triggered a lahar that cut the La Pampa section of the Baños-Pelileo road.

Volcanic activity at Tungurahua at the end of April 2004 was at moderate levels. On 21 April, a column of steam, gas, and ash rose to a height of ~ 1 km above the volcano and drifted NW. Ash fell in Bilbao, Cusúa, San Juan, Cotaló, Pillate, and Juive sectors. A plume reached ~ 0.5 km on 22 April and deposited ash in the towns of Ambato (to the NW) and Baños (to the N). During the evening of 24 April, incandescence was visible in the crater, and incandescent blocks rolled a few meters down the volcano's NW flank.

Volcano-tectonic earthquakes on 27 and 28 April preceded a slight increase in the number of sudden explosions at Tungurahua on 30 April. According to news reports, ash fell in the towns of Cotaló, and San Juan (W of the volcano) on 1 and 2 May. The level of seismicity at Tungurahua decreased on 4 May. On 12 May, an explosion produced an ash cloud to ~ 3 km above the volcano that drifted SW, and on 13 May seismicity increased moderately, related to the increased numbers of emissions. Incandescence was visible at the lava dome during some nights.

From mid May through June, small-to-moderate emissions of gas, steam, and ash continued at Tungurahua. The highest rising plume reached ~ 2.5 km above the volcano on 23 May. On the morning of 19 May a mudflow occurred in the Pampas sector, but it did not affect the highway. Strombolian activity was visible in the crater on the evening of 23 May. During 2-8 June, activity remained moderate with several weak to moderate explosions recorded per day. Sporadically observed gas-and-ash and gas-and-steam plumes rose up to 1 km above the summit. A strong explosion on 5 June produced a gas-and-ash plume that rose 2 km above the summit. All plumes drifted W. Seismicity remained at moderate levels. On 3 June, possible lahars were noted on the N and NW flanks.

Several explosions occurred on 10 June, with the largest rising ~ 3 km above Tungurahua's summit and drifting W. A small amount of ash fell in the Pillate area, and a lahar destroyed a bridge in the Bibao zone. During mid to late June, there was a slight increase in volcanic activity at Tungurahua in comparison to the previous weeks. There were several emissions of steam, gas, and moderate amounts of ash, and 5-10 explosions occurred daily. Seismicity was characterized by long-period earthquakes.

From July through December 2004 the level of volcanic and seismic activity diminished at Tungurahua, with sporadic moderate explosions of ash and gas. The highest rising plume reached ~ 1.5 km above the volcano. Seismicity was at relatively low levels. Incandescence in the crater was observed at night on several occasions. Some explosions on 20 September generated plumes with ash, causing ashfall in Bilbao and Pondoa, and on the evening of 21 September, Strombolian activity was seen, with volcanic blocks thrown as high as 200 m above the volcano. On 27 October an explosion produced an ash column to a height of ~ 3.5 km above the volcano. During the evening, ash fell in the towns of Baños, Runtón, and El Salado. Explosions on 31 October also deposited small amounts of ash in Bilbao and Motilone, and on 15 November, incandescence was observed in the crater of the volcano and explosions generated steam columns with moderate ash content that rose ~ 2 km above the crater and drifted S. During 22-27 December, activity at Tungurahua consisted of small-to-moderate explosions, several long- period earthquakes, and episodes of tremor. Emissions of steam, gas, and small amounts of ash rose a maximum of 1.5 km on 22 December.

Increased seismicity and volcanic tremor registered at Tungurahua during early January 2005. There were eleven signals suggesting volcanic emissions and one small explosion. Seismicity then returned to a low level. On 11 January, steam plumes rose ~ 300 m above the volcano and extended WNW, and incandescence was observed emanating from the crater during 12-13 January. On 14 January, a white column of steam-and-gas was observed that reached a height of 500 m above the crater and extended to the NW. A steam- and-gas plume reached a height of 200-300 m above the crater on 16 January, and extended SE.

The Washington Volcanic Ash Advisory Center (VAAC) reported 18 January that an ash plume reached ~ 5.5 km altitude and extended to the E of Tungurahua's summit for ~ 15 km. During 19-24 January 2005, there were several emissions from Tungurahua of steam, gas, and ash. The plumes that were produced rose to a maximum height of ~ 1 km above the volcano and drifted in multiple directions, small amounts of ash falling in the sectors of Agoyán, San Francisco, Runtón, Pondoa, and Baños. Seismicity was at relatively low levels. Ash emission from Tungurahua on the evening of 25 January 2005 deposited a small amount of ash in the sector of Puela; ash was deposited on the volcano's N and W flanks on 26 January. The character of the eruption changed on 30 January to low-energy emissions of predominately steam. This type of activity continued through 31 January.

Volcanic and seismic activity was at low levels at Tungurahua during the period of February-mid July 2005. Low- energy plumes were emitted, and long-period earthquakes were recorded. Ashfall was reported in towns near the volcano, including Puela (SW of the volcano), San Juan de Pillate, Cusúaua, and Quern. On 23 February the daily sulfur-dioxide flux was 1,200 tons/day. On 27 and 28 February, rains generated lahars in the W zone of the volcano into the gorges of Cusúa and Bilbao. A moderate explosion occurred 18 April at 2057 that sent incandescent volcanic blocks rolling down the volcano's flanks. Ash fell S of the city of Ambato. On 20 and 21 April rain generated lahars that traveled down the volcano's W flank near the settlement of Bilbao (8 km W). An emission on 19 May around 1200 produced an ash-and- steam plume to ~ 500 m altitude that drifted N. On 7 June fine ash fell in the Puela sector, ~ 8 km SW of the volcano. On 24 June a narrow plume was identified in multispectal satellite imagery about an hour after an ash eruption was observed by the Instituto Geofisica. The ash plume was at an altitude of ~ 5.5 km and extended 35-45 km W from the summit. On 4 July 2005, low-energy plumes were emitted that rose to a maximum of ~ 5.8 km altitude.

Table 9 gives examples of some seismic statistics for several months during the reporting period from the Instituto Geofísico-Escuela Politécnica Nacional (IG).

Table 9. Summary of available seismicity (number of events) at Tungurahua during January 2004-March 2005 as published in IG monthly reports of March 2004, October 2004, and April 2005. Courtesy of the Instituto Geofísico-Escuela Politécnica Nacional (IG).

Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC, 8800 Greenbelt Road, Greenbelt, MD 20771, USA (URL: http://earthobservatory.nasa.gov/).