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/).

Fumarolic and solfataric activity (restricted to the crater) emitted a thin white plume periodically seen from the observatory. In late July, 69 tectonic, three volcanic A-type, and six volcanic B-type events were recorded.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE rim of the Batur caldera, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

Information Contacts: VSI.

The number of volcanic earthquakes at Arenal recorded by the Red Sismológica Nacional increased on 20 July, to a daily mean of 19 for the rest of the month. Seismicity was most frequent on the 26th, when 30 events were recorded. A moderate increase in Strombolian activity was associated with the enhanced seismicity. A small lava flow descended the NW flank during July.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: R. Barquero and G. Alvarado, ICE; G. Soto, Univ de Costa Rica.

On 16 July, Aso erupted vigorously for the first time since May-June 1985, with three ash explosions from Crater 1 (at 1354, 1603, and 1625). The eruption resumed on 24 July, ejecting ash to 2,000 m above the crater rim.

Activity at Crater 1 has been gradually increasing since the end of 1988 (figure 12). Visits to the volcano October 1988-5 August 1989 revealed red glow at vents and cracks on the crater floor. After a small ash ejection on 5 April, tephra emission continued at a relatively high rate in May and June. A new vent (891) on the crater floor, first noticed during a field survey 11 June, emitted an ash-laden plume almost every day in July. Ash often fell near the vent, and daily accumulations of 7 g/m² on 6 July, 11 g/m² on 7 July, and 6 g/m² on 8 July were measured at AWS.

Figure 12. Monthly number of isolated volcanic tremor episodes, average tremor amplitude, number of volcanic earthquakes, and maximum plume heights at Aso, January 1965-July 1989. Arrows represent eruptions. Courtesy of JMA.

On 14 July, the largest daily number of isolated volcanic tremor episodes (744) in 1989 was recorded by a seismometer 0.8 km W of Crater 1 (figure 13). The same day, at 1535, the amplitude of continuous tremor increased (figure 14), and white vapor and ash from Vent 891 was ejected to 1,200 m. A 1-km area around the crater was again closed to tourists by the Aso Disaster Prevention Authority. Two days later, at 1155, tremor amplitude began to increase, decreased sharply at 1303, but increased again at 1344, 10 minutes before the onset of ash emission. Ash reached about 2,500 m height at 1355. After a sharp decrease in tremor amplitude, two ash explosions followed at 1603 and 1625, ejecting a plume to ~1,000 m.

Figure 13. Daily number of isolated tremor episodes at Aso, January 1988-July 1989. Arrows represent eruptions. Courtesy of JMA. Courtesy of JMA.

Figure 14. Daily maximum tremor amplitude (top) and average amplitude of continuous tremor (bottom) at Aso, January 1988-July 1989. Arrows represent eruptions. Courtesy of JMA.

On 18-19 July, small amounts of ash were repeatedly ejected from Vent 891 after tremor amplitude sharply decreased. During a field survey at 1920 on 22 July, a maximum brightness temperature of 506°C was measured (by an infrared radiation thermometer) at a vent on the crater floor. The amplitude of continuous tremor began to increase 21 July, and significantly increased on the night of 23 July. Ash emission resumed at 1620 on 24 July and continued for about 1 hour, attaining a maximum height of 2,000 m above the crater rim. A small ash ejection was observed during a field survey on 29 July. Brightness temperatures near the vent on the crater floor were 504°C on the 29th and 498°C on the 30th. The average amplitude of continuous tremor remained high.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

No recent large explosive eruptions have been reported and the aerosol content of the stratosphere appears to be nearing background levels. Data from a single July observation at Mauna Loa, Hawaii showed little change from the previous month. A data set collected at Hampton, VA during marginal weather conditions on 24 July showed a profile typical of recent months, with a peak backscattering ratio of 1.2 at about 20 km altitude. Preliminary plans have been made for lidar instruments at the NASA Langley Research Center and several other sites to monitor the atmosphere almost continuously for much of October.

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico's El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin. Descriptions of the initial dispersal of major eruption clouds remain with the individual eruption reports, but observations of long-term stratospheric aerosol loading will be found here.

Information Contacts: Thomas DeFoor, Mauna Loa Observatory, P.O. Box 275, Hilo, HI 96720 USA; David Woods, NASA Langley Research Center, Hampton, VA 23665 USA.

Quoted material is from RVO, with additional information on SO₂ flux supplied by S. Williams. "Moderate eruptive activity continued . . . throughout July. The summit crater released moderate to strong volumes of thick white and sometimes grey emissions. Intermittent low rumbling sounds with occasional mild explosions were heard at the observation post ~10 km S of Bagana. Occasional night glows from the summit were observed, and rockfalls continued on the active lava flow(s). The effusive activity was not clearly seen during the aerial inspection, although voluminous white emissions on the S flank . . . probably originated from a steaming lava flow."

During an SO₂ monitoring flight on the 27th from 0830 to 0910, a strong convoluted white cloud emanated from Bagana's summit, rising only slightly before being blown ~25-30 km downwind. The plume contained no ash but varied in size and opacity. Four traverses yielded SO₂ flux measurements of 4,870, 4,800, 1,930, and 2,390 t/d. The large measured variations corresponded well with visual estimates of variation in the plume size. Based on the duration of the observations and the relative times of the traverses, the estimated weighted average of the flux data was 3,230 t/d. The September 1983 data were similar (yielding a mean value of 3,100 t/d) but showed less variation (2,300, 3,000, 4,200, 2,800, 3,000 t/d) suggesting a more steady state of degassing at that time.

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: B. Talai and C. McKee, RVO; S. Williams, Louisiana State Univ.

"A brief aerial inspection of Balbi was made on the 27th. No changes were noted. Voluminous white emissions continued from the 1-km-long fumarole field in the NE part of the summit amphitheatre, and from sources in one of the summit craters (Crater B)."

Geologic Background. The complex andesitic Balbi stratovolcano on Bougainville Island includes a large number of coalesced cones and lava domes. Five well-preserved craters occupy a NW-SE-trending ridge north of the summit cone, which also contains a crater. Three large valleys with steep headwalls dissect the flanks. The age of the most recent eruption is not known precisely. An oral tradition of a major eruption during the 19th century is thought to be in error, but could refer to minor eruptive activity. Fumaroles are located within 600-m-wide Crater B and on its W flank.

Information Contacts: B. Talai and C. McKee, RVO.

By July 1989, the volcano's activity approached background level. White weak fumes reached 25-35 m above the summit crater and 5-10 m above the N and S craters during the last week in July. Three volcanic A-type, 49 volcanic B-type, one degassing, and 165 tectonic earthquakes were recorded.

Geologic Background. The 3-km-wide island of Banda Api is the northern-most volcano in the Banda arc and has a long period of recorded observation because of its key location in the Portuguese and Dutch spice trade. The basaltic-to-rhyodacitic volcano is located in the SW corner of a mostly submerged 7 km caldera. At least two episodes of caldera formation are thought to have occurred, with the arcuate islands of Lonthor and Neira considered to be pre-caldera remnants. A conical peak rises to about 600 m at the center of the island. Eruptions have been recorded since 1586 CE, mostly consisting of Strombolian eruptions from the summit crater, but larger explosive eruptions have occurred and occasional lava flows have reached the coast.

Information Contacts: VSI.

Geologists observed Strombolian activity at three summit area craters (Bocca Nuova, La Voragine, and Southeast Crater; figure 28) and an incandescent gas vent in Northeast Crater during field work 26 May-1 July. Fresh-looking tephra was abundant W and SW of the summit craters, to 800 m from Bocca Nuova.

Figure 28. Sketch map of Etna's summit region, showing features active during field work 26 May-1 July 1989. Courtesy of C. Oppenheimer and J. Murray.

Bocca Nuova. Vigorous Strombolian activity from two vents on the crater floor, both building cones, was observed 29-30 May, 5, 18, 19, and 30 June, and 1 July. Small explosions and gas bursts averaged ~70/minute. Larger explosions ejected incandescent cowdung bombs, some of which fell 100 m beyond the crater rim. Crater depth was estimated at > 120 m. On 5 June, another glowing vent was visible on or near the flank of the largest active cone. Toward the S end of the crater floor, there was an irregularly shaped area of glowing lava, possibly a ponded flow or passive lava lake. Lava in one of the intracrater vents had a maximum brightness temperature of 1,019°C when measured with a 0.8-1.1 mm infrared thermometer. On 30 June, heat from one of Bocca Nuova's active vents was felt from the observation point on the NW rim.

La Voragine. Strombolian activity occurred from crater floor vents. Loud detonations were heard from the crater rim on 29 May; 17 were counted in 2 minutes. Ejecta were heard falling on the crater floor. During observations the evening of 5 June, four vents were visible, two of which were active. The largest of the 72 explosions counted in a 2-minute period sent bombs to 30 m above the rim. On 1 July, juvenile bombs were ejected by a glowing hornito or small cone on an apparent solidified lava lake. The 0.8-1.1 mm thermometer recorded a peak temperature of 821°C.

Southeast Crater. Strombolian eruptions continued from Southeast Crater's new cone 29 May-30 June, frequently ejecting incandescent tephra above the crater rim. On 4 June, bombs rose to 100 m above the cone. During a 10-minute period around noon, geologists counted 28 eruptions that projected bombs higher than the rim.

Northeast Crater. On 8 June, a vent 2-3 m in diameter was degassing near the middle of the crater floor. A peak brightness temperature of 462°C was recorded by an 8-14 mm infrared thermometer pointed at the vent's inner wall from ~ 50 m distance. Geologists noted that this instrument probably significantly underestimated the gas temperature (see 1 July data). There was no sign of fresh tephra. On 12 June at about 0850, four white "smoke rings" rose slowly from Northeast Crater. On 1 July, glow from the vent was visible during observations in low light shortly after 0600. A 0.8-1.1 mm infrared thermometer yielded a maximum temperature of 644°C, compared to 491° with the 8-14 mm instrument. Eight gas puffs and two more vigorous exhalations were counted in 1 minute.

Geodetic measurements. Very little vertical deformation has occurred since September 1988 (14:01). Slight subsidence was measured E of the summit, with a maximum displacement of 3 cm near the 1987 fissure (relative to a reference station 2.5 km SSW of the summit). A 3.5-cm swelling was measured on the Northeast Rift. A trilateration network on the upper E flank was reoccupied for the first time since June 1988, revealing movements of as much as 4 cm to the NE at stations in the Valle del Leone. The station on Monte Simone (4 km E of the summit) showed an anomalous displacement of 2 cm to the S since September 1987. Stations at Punta Lucia (1.5 km NNW of the summit) and Belvedere (2 km SSE of the summit) were assumed to be stable. Etna's summit elevation was determined at 3,318.4 m, using the base station near Piccolo Rifugio (2,516 m altitude) as a datum. Seismicity was detected optically through the levelling instrument, as in 1988 (14:01). During the early June levelling traverse, 36 shocks were observed, all within 2.2 km of the summit. The largest amplitudes noted were 70 µrad (1.5 km NE of the summit), 111 µrad (near Southeast Crater), and 123 µrad (400 m from Bocca Nuova).

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: J. Murray and C. Oppenheimer, Open Univ; A. Jones, Univ of Lancaster; P. Cruddace, Univ of Newcastle; P. Aragno and S. Haefeli, SVG.

Thermal activity has continued since the January 1986 eruption (11:1-3). A pale green belt of discolored sea water and white bubbles was observed during a JMSA overflight on 9 May 1989. Another monitoring flight, on 14 June, revealed a 50 x 400 m zone near the vent that was discolored cobalt blue.

Geologic Background. Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the island of Minami-Ioto. Water discoloration is frequently observed, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto ("New Sulfur Island") in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

Information Contacts: JMA; D. Shackelford, Fullerton, CA.

In June and July, fumarolic activity on the SW flank of the central cone increased significantly. A new fumarolic vent (named Las Portillas) opened on the inner W wall of the central crater. The vent was ~2 m in diameter and vigorously emitted gas with temperatures >470°C on 22 July. Both Las Portillas and the 10-m-wide La Chava vent emitted superheated gases, producing a loud noise. During the last two weeks in July, numerous small fumarolic fractures on the S rim of the crater, and small radial and concentric fractures on the W crater rim, were observed.

Seismic activity during June and July was nearly stable (figure 3), except for a slight decline (20%) in the daily average of B-type events between mid-May and the last week of July. The seismic energy release associated with B-type events was also stable. The number of A-type events was low until 23 July when a swarm of 30 earthquakes occurred between 0811 and 1500, including a M 3 event at 1233. The swarm was centered about 2 km W of the crater, at 1-7.5 km depth (figure 4). The number of low-frequency events had increased slightly before the swarm. Dry tilt showed no significant changes, although electronic tilt varied slightly on the 21st (figure 5). SO₂ emissions increased slightly with an average of 600 t/d in July (see figure 12).

Figure 3. Daily seismic energy release (top) and number of events (bottom) at Galeras, 22 April-31 July. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 4. Map showing epicenters (top) and an E-W cross-section (bottom) of seismic events at Galeras in July. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 5. Deformation measurements at Galeras, July 1989, showing radial (E-W) tilt at Peladitos station, at about 3,800 m elevation 2 km E of the crater at Galeras (top), and tangential (E-W) tilt at Telecom station, at about 3,900 m elevation 2 km S of the crater (bottom). Increasing values indicate uplift to the W. Courtesy of the Observatorio Vulcanológico de Colombia.

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

Information Contacts: INGEOMINAS, Pasto.

Weak white fumes rose 150-300 m above the crater in late July. Collapse and sliding of the crater walls caused extension of the crater toward the NW and E. Fumarole temperatures at the volcano's summit were 82-90°C. On 28 and 29 July, volcanic tremor episodes with amplitudes of 0.5 mm were recorded for 11 and 16 minutes. Volcanic (16), distant tectonic (420), and local tectonic (10) earthquakes were recorded.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the extensive documentation of activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano; the S-flank Ngade maar formed after about 14,500–13,000 cal. BP (Faral et al., 2022). Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: VSI.

The light green crater lake showed no degassing or temperature increase in late July. High-pressure steam was emitted from the solfatara fields near the lake. Nine volcanic A-type, four volcanic B-type, 25 distant tectonic, and four local tectonic earthquakes were recorded.

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

Information Contacts: VSI

In July, fumarole temperatures in the crater were 60-70°C. On 15 July, an MM II earthquake was felt. The number and types of earthquakes recorded were: 131 distant tectonic, three local tectonic, two volcanic A-type, and 144 volcanic B-type. The volcano's level of activity is slightly higher than normal.

Geologic Background. Iliboleng stratovolcano was constructed at the SE end of Adonara Island across a narrow strait from Lomblen Island. The volcano is capped by multiple, partially overlapping summit craters. Lava flows modify its profile, and a cone low on the SE flank, Balile, has also produced lava flows. Historical eruptions, first recorded in 1885, have consisted of moderate explosive activity, with lava flows accompanying only the 1888 eruption.

Information Contacts: VSI.

During fieldwork in June, Irazú remained quiet. Activity was limited to low-temperature solfataras on the NW flank. There was a small emerald green lake in the main crater, active in the 1963-65 explosive eruption.

Geologic Background. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the main crater, which contains a small lake. The first well-documented eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: G. Soto, Escuela Centroamericana de Geologia & Red Sismologica Nacional, Univ de Costa Rica.

The following supplements preliminary information in 14:6. An earthquake swarm began on 30 June beneath the sea floor E of the Izu Peninsula, an area where swarms have occurred roughly once or twice a year since 1976 (figure 2). The shocks were centered on the NE side of the post-1976 epicentral zone and were shallower than those of previous swarms (figure 3). Seismicity increased sharply on 4 July (figure 4). The strongest event, M 5.5 on 9 July at 1109, reached intensity 5 (JMA scale) in the Ito area and 4 near Ajiro, and had a right-lateral focal mechanism. The number of earthquakes began to decline the next day. However, strong ground tremor, initially with continuous wave trains of varying amplitude, then almost completely saturating instruments, was recorded 11 July between 2038 and 2148 (figure 5). Strong rumbling sounds accompanied the 11 July tremor but no eruption was evident on the sea surface. Another episode of vigorous ground tremor, with sporadic amplitude increases, began about midnight 12/13 July. The explosions observed from the JMSA's RV Takuyo between 1840 and 1845 (14:06) began with updoming of the sea surface, followed by cockscomb ejections. Frequent, audible shock waves jolted the vessel, 0.5 km from the eruption site. Strong, continuous tremor between 1833 and 1917 nearly saturated local seismometers (figure 6). Small tremor episodes continued until 20 July, with the longest total tremor duration on the 14th. Although bubbling was seen at the the eruption site and other locations nearby, no further eruptions were detected at the surface.

Figure 2. Seismic activity NE of the Izu Peninsula, 1976-89. Inset box shows superimposed epicentral zones of each of the 12 swarms, with the July 1989 swarm (no. 12) highlighted. Courtesy of JMA.

Figure 3. Epicenters and approximate magnitudes of earthquakes in the vicinity of the Izu Peninsula, 30 June-27 July 1989. The approximate site of the submarine eruption is shown by a white triangle within the zone of dense seismicity. Courtesy of JMA.

Figure 4. Number of earthquakes near the Izu Peninsula recorded during each 3-hour period, 1-27 July 1989 (SEAN bars) and minutes of tremor recorded daily 10-20 July (circles connected by lines). Courtesy of JMA.

Figure 5. Seismograph record, 11 July between 2020 and 2200, showing tremor onset about 2038 and eventual saturation of instruments before tremor ended about 2148. Courtesy of JMA.

Figure 6. Seismograph record, 13 July between 1815 and 1945, including the period of eruptions visible at the sea surface. Courtesy of JMA.

JMSA bathymetric surveys of the eventual eruption site recorded a flat sea floor before 9 July, but by 13 July, just before the witnessed explosions, the same area had developed a feature ~20 m high and 250 m across at its base. Most volcanologists believed that this feature had grown from submarine eruptions that were not witnessed but probably accompanied the strong tremor of 11 and/or 12 July. A 15 July resurvey revealed an edifice only 10 m high but 450 m in basal diameter, with a crater 200 m across breached on its south side (figure 7). Bubbles rose ~90 m from the crater to the sea surface.

Figure 7. Bathymetry of Teishi-Kaikyu measured by an unmanned JMSA survey vessel, 15 July. Contour interval is 1 m. A sketch of the cone is inset at lower right. Courtesy of JMSA.

Analysis by the National Research Center for Disaster Prevention revealed that the eruption was preceded by large, apparently precursory, ground tilting ~2.5 km to the SSE. Uplift centered in the Ito area was detected by Geographical Survey Institute levelling surveys, but was generally considered to be a possible precursor to stronger tectonic seismicity. The eruption site lies almost directly between Ito City and Hatsushima Island (roughly 10 km apart), and EDM data from the Earthquake Research Institute, Univ of Tokyo, show a clear increase in the distance between them. However, geologists suspect that the distance change may have been caused by faulting associated with the M 5.5 earthquake on 9 July rather than intrusion of magma.

Further References. Okada, Y., and Yamamoto, E., 1991, Dyke intrusion model for the 1989 seismo-volcanic activity off Ito, central Japan: JGR.

Shimada, S., Fujinawa, Y., Sekiguchi, S., Ohmi, S., Eguchi, T., and Okada, Y., 1990, Detection of a volcanic fracture opening in Japan using global positioning system measurements: Nature, v. 343, p. 631-633.

Geologic Background. The Izu-Tobu volcano group (Higashi-Izu volcano group) is scattered over a broad, plateau-like area of more than 400 km2 on the E side of the Izu Peninsula. Construction of several stratovolcanoes continued throughout much of the Pleistocene and overlapped with growth of smaller monogenetic volcanoes beginning about 300,000 years ago. About 70 subaerial monogenetic volcanoes formed during the last 140,000 years, and chemically similar submarine cones are located offshore. These volcanoes are located on a basement of late-Tertiary volcanic rocks and related sediments and on the flanks of three Quaternary stratovolcanoes: Amagi, Tenshi, and Usami. Some eruptive vents are controlled by fissure systems trending NW-SE or NE-SW. Thirteen eruptive episodes have been documented during the past 32,000 years. Kawagodaira maar produced pyroclastic flows during the largest Holocene eruption about 3,000 years ago. The latest eruption occurred in 1989, when a small submarine crater was formed NE of Ito City.

Information Contacts: JMA.

White to sometimes gray emissions under low-medium pressure rose to 250 m above the crater, often accompanied by glowing lava that was clearly seen at night. Earthquakes of MM I-II were felt on 14 July at 2240, 22 July at 1323 and 26 July at 1915. The type and number of earthquakes recorded were: 45 distant tectonic, 20 local tectonic, and one degassing. The volcano's level of activity appeared to be decreasing and was lower than normal in late July.

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

Information Contacts: VSI.

"Aerial and ground inspections and ground deformation measurements were conducted on 24 and 25 July. Aerial inspection involved photography of the summit calderas while the ground inspection covered the 1979 crater and temperature measurements of fumaroles on Bagiai Cone.

"The 1979 crater presently contains a small, shallow, muddy pool and the base of the crater wall is obscured by talus from rockslides. Erosion of the talus has formed a series of channels radial to the centre of the crater. Although no landing was made on the crater floor, observation from the crater rim, and the presence of rapidly advancing vegetation on most parts of the crater wall and on the crater floor, indicated that there were no active fumaroles in the 1979 crater. Temperatures of 74-85°C were measured at the summit of Bagiai, now almost completely revegetated following denudation during the 1974 and 1979 eruptions. The part of the inner caldera wall that was deforested during the 1979 eruption is now completely covered by regrowth, with some trees as much as 6 m tall. Vegetation is also being re-established on the caldera floor.

"The ground deformation measurements involved reoccupation of four dry tilt stations at the summit and three stations near the coast, relevelling on a 1.3-km line on the inner caldera floor, and measurements on the inner caldera EDM network. In general, the tilt and levelling results indicated deflation of the summit area."

Geologic Background. Karkar is a 19 x 25 km forest-covered island that is truncated by two nested summit calderas. The 5.5 km outer caldera was formed during one or more eruptions, the last of which occurred 9,000 years ago. The steep-walled 300-m-deep, 3.2 km diameter, inner caldera was formed sometime between 1,500 and 800 years ago. Cones are present on the N and S flanks of this basaltic-to-andesitic volcano; a linear array of small cones extends from the northern rim of the outer caldera nearly to the coast. Recorded eruptions date back to 1643 from Bagiai, a pyroclastic cone constructed within the inner caldera, the floor of which is covered by young, mostly unvegetated andesitic lava flows.

Information Contacts: B. Talai and C. McKee, RVO.

. . . In July 1989, Kupaianaha's lava pond remained active, and filled by lava to within 25 m of its rim, which partially collapsed on the 9th and 13th. The earlier collapse (on the E rim) formed a talus pile near the bottom of the wall that was covered by lava, forming a 40 x 15 m ledge and altering the shape of the pond.

Lava continued to travel through the W tube system toward the coast in July (figure 63). Surface lava breakouts in the Wahaula area fed lobes that flowed from the lower Royal Gardens subdivision to the ocean (figure 61). On 5 July, lava advanced 150 m along Chain of Craters road, reaching the lower end of Prince Street (the westernmost street of the Royal Gardens subdivision) by the 23rd. A surface breakout from the W tube on the 21st at 320 m elevation (top of the pali fault scarp) reached 120 m elevation by the end of the month. The Kupapau flow that had entered the ocean on 22 June and then stagnated, reactivated 5-21 July. Flows that had merged and destroyed the visitor center on 20 June moved N and W of the Heiau (a religious site, several hundred years old), entering the ocean on 5 July. This flow, named Kailiili, remained active into early August, as did the Poupou flow, which had entered the ocean E of the Wahaula residential area on 23 June. Both flows built seacoast benches that were ~700 m long and actively growing (mostly westward) by the end of the month. The bench that began to form E of Kupapau Point in May 1988 remained active. On the 27th, a surface flow advanced over the sea cliff, covering the E half of the old bench. There were no collapses, and by the end of the month, the bench measured 200 x 50 m.

Figure 63. Flows produced by the current eruption at Kīlauea. The July 1986-July 1989 flows from the Kupaianaha vent are striped; the rest of the flows (outlined but not shaded) were erupted January 1983-July 1986, mostly from the Pu`u `O`o vent. Courtesy of Christina Heliker.

By the end of July, the number of aftershocks from the 25 June earthquake had decreased to near normal on Kīlauea's S flank. Most earthquakes were M 1.0-3.7, 5-15 km deep, and concentrated in a 45-km-long, NE-trending zone. The number of shallow earthquakes (<5 km) was above average in the East rift zone and slightly below average at Kīlauea's summit. Intermediate depth (5-15 km) long-period events continued. Low-level tremor near Pu`u `O`o and Kupaianaha had steady amplitudes, with episodic swarms of small high-frequency shocks near Kupaianaha and occasional rockfalls at Pu`u `O`o.

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: C. Heliker and R. Koyanagi, HVO.

"A steady decrease in volcanic activity was observed at Crater 2 during July, while Crater 3 remained inactive. Emissions from Crater 2 were mostly white but occasionally grey (on 7, 16, 18, 20, 21, 26, and 27 July), and weak to moderate in volume. Occasional low rumbling sounds were heard from the 1st to the 20th and on the 28th. One loud explosion on the 16th accompanied the ejection of thick brownish-grey ash, forming a column that rose 1200 m above the crater. Night glow around the crater mouth was visible only on the 1st and 5th."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: B. Talai and C. McKee, RVO.

Alex van Leerdam flew over the volcano 28 June and took a short video film of the N crater. His view was from the N and did not show the extreme N or E crater floor or inner walls. All lava on the visible portion of the crater floor was at least a few weeks old and generally pale, with a large, slightly darker area to the W that covered the former T2 cone and its surroundings. The patch of lava (F8) that formed in November 1988, S of the former saddle (13:12), was white and seemed to have enlarged slightly since December 1988 (although it had appeared unchanged in 12 January photographs). The flow that had breached the saddle in November 1988 had widened since December, and only small parts of the saddle's upper slope remained visible. Cones T11, T8, T4/T7, and T5/T9 on the crater floor were pale in color. The rim cone (C1) appeared almost unchanged, and apparently no new major cones had formed since December 1988.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, Kenyatta Univ; A. van Leerdam, Nairobi, Kenya.

The seismic swarm under the SW flank of Mammoth Mountain continued in July, and by the 31st, 960 events had been recorded by the California Division of Mines and Geology NEWT system. Most of July's 342 recorded events were M <1.0, with the largest on 20 June at 1758 (M 3.3) and 1 August at 1917 (M 3.4). Forty earthquakes were recorded 27 July (figure 8), the second most active day since the swarm began; 44 were recorded 11 June (14:6). Hypocenters remained beneath the SW flank of Mammoth Mountain at depths mostly <10 km, but two low-frequency events on 27 July were located by the USGS at 15-18 km depth. The USGS seismic station at Mammoth pass (MMP), in the epicentral area (figure 9), recorded several hundred events/day, too small to be detected by other stations.

Figure 9. Selected epicenters for swarm events 21 May-22 September 1989 beneath Mammoth Mountain, relocated using S waves. Three- and four-letter codes mark positions of seismic stations. Courtesy of S. McNutt.

Seismic activity remained relatively constant in July but the number of spasmodic tremor episodes increased. Bursts of spasmodic tremor with 7-13 subevents over 1-3-minute periods were detected 2 and 26 June, and 6, 13, 19, and 27 July. Seven events on 11 June, and one on 30 July had low frequencies of 1-3 Hz (probably due to attenuation effects) and poorly constrained depths of ~2 km (because no S-waves were detected). Mixed-frequency events, suggesting resonance of a fluid-filled cavity, had clear high-frequency P- and S-waves superimposed on 1-2-Hz waves, with locations similar to the high-frequency events. Stephen McNutt noted that the seismicity suggests the presence of a fluid phase (meteoric water or a volatile phase), possibly associated with magma.

During mid-late June and July, the USGS Devil's Postpile dilatometer, near the W foot of Mammoth Mountain, showed several short-term (1-10-day) fluctuations, with amplitudes of ~0.6 microstrain, superimposed on long-term extension. The regional 2-color laser network has recently shown some E-W compression in contrast to the extension of the last several years. USGS L-shaped leveling arrays on Mammoth Mountain and in the caldera's S moat showed small anomalies (possibly related to the earthquake swarm) consistent with intrusion under Mammoth Mountain. Two weak fumaroles had slightly increased temperatures and were sampled for He³/He4 analysis.

The swarm represents the highest level of seismicity near Mammoth Mountain since 1979 and has had an unusually long duration (three months); 80% of swarms analyzed by McNutt in volcanic areas worldwide have lasted less than two months. Many small events were still being recorded as of 7 August.

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

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento; D. Hill, USGS Menlo Park.

Weak Strombolian activity continued in July, and the rate of ashfall increased in the Lonquimay valley. As the eruption persisted and concerns remained about health effects of the volcanic gases, particularly fluorine, residents continued to leave the area. Around 8 July, a small gravitational collapse occurred at the tephra cone in the NE part of Navidad Crater. Peter Baker has analyzed 4 February lava and 28 December ash for fluorine, yielding values of 1,638 and 3,401 ppm, respectively. Oscar González-Ferrán notes that no other high-fluorine eruption has been reported from a volcano in the Andes.

A team of geologists (Marino Martini, Luciano Gianinni, Philip Kyle, Werner Giggenbach, Jon Bjornvinsson, Oscar González-Ferrán, Hugo Moreno, José Naranjo, and Rodolfo Figueroa) carried out fieldwork at Lonquimay 7-17 July, using several techniques to measure gases. Transportation was provided by the Chilean Air Force. The following is from J. Naranjo and W. Giggenbach.

Eruptive activity, 9 July 1989. Navidad Cone continued to produce small ash emissions and lava. Since early April, its eruptive activity has clearly decreased to a VEI of 1 or less. On 9 July, ash clouds were rising to 500-700 m above the crater, and after unusually strong explosions to 1,000 m. A heavy snowfall on 30 June was covered by an ash deposit that indicated a main plume direction to the SSE, between the towns of Malalcahuello and Lonquimay. Average ash accumulation rates at El Mirador, 1.5 km SSE of Navidad, were about 2.2 mm/day in early July, considerably lower than the 6 mm/day estimated for the first 3 months of the eruption. The rate of lava effusion, estimated 400 m from the source, was about 50,000 m³/hour, similar to the 60,000 m³/hour reported for the eruption's first 100 days (Moreno and Gardeweg, 1989). The lava was traveling NE down the Lolco valley, while advance of the N lobe (toward Laguna Verde) appeared to have stopped.

"Navidad's black scoria cone has grown to 300 m high, with a basal diameter of ~800 m. On 9 July at about 1100, the crater, breached to the N, contained a small lava lake halfway up the southern crater wall. Strombolian eruptions, emanating from there every few minutes, splashed lava over the inner crater walls. Bombs occasionally reached the outer slopes. A light gray steam and ash column was emitted from a vent next to the lava lake. At about 1200, another vent opened abruptly some 80 m below the active lava pool, extruding an initially dome-shaped batch of new lava accompanied by spectacular lava fountaining for about 5 minutes. The new batch of lava quickly spread downhill to join the existing lava tongue. This process appeared to have drained the lava conduit, as Strombolian eruptions from the now-empty lava lake had ceased, producing instead an almost constant stream of ash-charged gas. Strombolian explosions resumed at around 1330, the conduit having obviously refilled. Direct observation ended at 1430.

Gas chemistry. "Following reports of extensive livestock losses (cattle, sheep, and poultry) and of detrimental health effects or even deaths of inhabitants of areas affected by the volcanic plume, an effort was made to measure the possibly harmful components of gases emitted by the volcano. However, the activity made it impossible to directly approach the degassing vents and recent snowfall prevented access to the advancing, actively degassing lava fronts. The only available samples were ashes deposited since the last snowfall on 30 June. Nine samples were collected on 9 July, from the immediate vicinity of the cone (1, 2), the Mirador area (3-6), the Refugio (7, 8), and about 10 km from the vent (9). The samples are fresh, well-sorted, vesiculated scoriaceous fragments, with grain sizes decreasing from 0.3-3 mm for sample 1 to 0.1-0.3 mm for sample 9. Samples were leached with distilled water and analyzed for Li, K, Mg, Ca, Al, Si, NO₃, SO₄, F, Cl, Br, and trace constituents. Figure 13 compares relative SO₄, Cl, and F contents in the leachates with those from other volcanic ashes.

Figure 13. Relative SO4, Cl, and F contents (by weight) leached from volcanic ash near Navidad Cone (open circles; see text for numbered sample locations), compared to data from Mt. St. Helens (Nehring and Johnston, 1981); Central America (Santiaguito, Pacaya, San Miguel, Cerro Negro, and Arenal; Taylor and Stoiber, 1973); Hekla (Oskarsson, 1980); and Yasur (Giggenbach in 13:12). Courtesy of W. Giggenbach.

"Although absolute amounts of salts leached from the ash samples may vary by several orders of magnitude, relative contents are remarkably similar and fall into distinct groups. The quite narrow scatter for a given area suggests that the leachate compositions reflect actual variations in compositions of gases released from a volcanic system rather than secondary processes. The Navidad samples fall within the group represented by Hekla and Yasur, having F/Cl atomic ratios up to 100 times those of the low fluoride group (Mt. St. Helens, Central America). The detrimental effect of high F contents in volcanic ash on grazing animals is well documented for Hekla (Thorarinsson and Sigvaldason, 1972). There, leachable F reached 2,000 mg/kg ash, compared to up to 100 mg/kg at Lonquimay.

"The samples were deposited during a period of dry weather and after the eruption had subsided considerably. During the early, more active period, and under more humid conditions, leachable F contents of the Navidad ash may have been considerably higher. At the state of activity and atmospheric conditions encountered at the beginning of July, however, it is unlikely that the Navidad gases cause any harm to people or animals. It appears highly advisable, though, to monitor F contents in drinking water and animal feed, especially during the thaw of F-charged snow in spring."

SO₂ data. Philip Kyle made a series of COSPEC V measurements on 13 July between 1339 and 1513. SO₂ flux ranged from 83 to 302 t/d, with an average of 169 ± 48 t/d (161 data points). An increase in the emission rate was evident beginning at 1353, continuing through the final measurements at 1513. A stronger increase between 1412 and 1427 was superimposed on the general trend. Video tapes of the activity during this period show no visible changes that could be correlated with the increased SO₂ emission.

The following is a report from Marino Martini. "The chemical composition of water collected in the Lonquimay area does not signal important contamination from the effects of ash or acid rain (table 7). The fluorine content, in particular, does not seem to constitute a reason for alarm, and water on the slope that supplies the city does not have negative chemical characteristics. Concentrations of acid species can also be observed in snow samples (table 7), but without important effects on potable water.

Table 7. Partial chemical compositions (in ppm) of water and snow near Lonquimay. Courtesy of M. Martini.

"The Strombolian activity did not permit a direct approach to crater fumaroles, so only indirect observations were possible. Nevertheless, given previous experience, the information that could be obtained has substantial validity. The natural gas condensates, collected near the lava flow that emerges from the active crater, are impoverished in the most volatile constituents (CO₂, SO₂), but it is possible to gather sufficient general information. A comparison with natural condensates from Vulcano (Italy) indicates that in spite of the different type of volcano and the absence of a lava flow, we can ascertain relations of the same order between Cl, F, and S (the higher concentrations of B and Br at Vulcano are related to the influence of the marine atmosphere). If we consider the F/SO₂ ratio of the gases emitted by fumaroles at Vulcano with reference to the value measured for the natural condensate, we can then estimate an approximate F/SO₂ of 0.012 for the Lonquimay gases (table 8)."

Table 8. Natural condensates of fumarolic gases (in ppm) at Lonquimay and Vulcano. Courtesy of M. Martini.

References. Moreno, H., and Gardeweg, M., 1989, La erupción reciente en el Complejo Volcánico Lonquimay (Diciembre 1988-), Andes del Sur: Revista Geológica de Chile, v. 16, p. 93-117.

Nehring, N.L., and Johnston, D.A., 1981, Use of ash leachates to monitor gas emissions, in Lipman, P.W. and Mullineaux, D.R. (eds.), The 1980 eruptions of Mt. St. Helens, Washington: USGS Professional Paper 1250, p. 251-256.

Oskarsson, N., 1980, The interaction between volcanic gases and tephra: fluorine adhering to tephra of the 1970 Hekla eruption: JVGR, v. 8, p. 251-266.

Taylor, P.S., and Stoiber, R.E., 1973, Soluble material on ash from active Central American volcanoes: GSA Bulletin, v. 84, p. 1031-1042.

Thorarinsson, S., and Sigvaldason, G.E., 1972, The Hekla eruption of 1970: BV, v. 36, p. 269-288.

Geologic Background. Lonquimay is a small, flat-topped, symmetrical stratovolcano of late-Pleistocene to dominantly Holocene age immediately SE of Tolguaca volcano. A glacier fills its summit crater and flows down the S flank. It is dominantly andesitic, but basalt and dacite are also found. The prominent NE-SW Cordón Fissural Oriental fissure zone cuts across the entire volcano. A series of NE-flank vents and scoria cones were built along an E-W fissure, some of which have been the source of voluminous lava flows, including those during 1887-90 and 1988-90, that extended out to 10 km.

Information Contacts: O. González-Ferrán, Univ de Chile; J. Naranjo, SERNAGEOMIN, Santiago; W. Giggenbach, Chemistry Division, DSIR, New Zealand; M. Martini, Univ of Firenze, Italy; P. Kyle, New Mexico Institute of Mining and Technology, Socorro; P. Baker, Univ of Nottingham, UK.

"A low level of activity continued in July. Southern Crater gently released weak to moderate amounts of thick white [emissions] with occasional light grey and blue emissions. Weak deep rumbling sounds, occurring at intervals of 5-40 minutes, were commonly heard throughout the month. Main Crater was less active, releasing only small amounts of white vapours. Seismicity continued at low inter-eruptive levels, with 400-1,080 small discrete B-type events/day.

"An aerial inspection and ground deformation measurements were conducted between the 20th and 23rd. The interiors of the summit craters were totally obscured by the emissions. The only notable change was the presence of arcuate fissures in the 1987 lava flow at the summit's E platform. Ground deformation work involved EDM, dry tilt measurements, and re-installation of two dry tilt stations that were damaged in 1987. Results of these observations will appear in the next report."

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: I. Itikarai and C. McKee, RVO.

July activity was similar to that of June. The level of the crater lake had lowered about 1 m. Intense sulfur-rich fumarolic activity occurred from four principal vents within the lake, three on the N side and one in the SE. Sulfur cones had grown at the N-central and SE vents, small sulfur flows had emerged from the NE vent, and lakes of molten brownish orange sulfur were present at other sites. Pyroclastic sulfur and sublimates carpeted the bottom of the crater lake. Bubblings of mud occurred throughout the lake from fumaroles below its surface. Activity on the 1953-55 [dome] remained unchanged, characterized by low-temperature fumaroles.

The number of microseismic events recorded at the nearby Red Sismológica Nacional station VPS-2 increased in July, totaling 3,442 through the 26th, a mean of 132/day (figure 20). Seismicity was dominated by B-type shocks, with some well-defined tremor episodes and four very low-magnitude A-type events. The most active day was 21 July, with 221 volcanic microearthquakes.

Figure 20. Number of seismic events/day at Poás, recorded by Red Sismológica Nacional station VPS-2, July 1989. Courtesy of G. Soto.

Geologic Background. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: Gerardo J. Soto, Mario Fernández, and Héctor Flores, UCR.

"Activity continued at a low level. A total of 119 caldera earthquakes was recorded. The average daily number of events was four, with the highest count (15) on the 16th. Only four events were large enough to be located, and they occurred within the Greet Harbour area. No significant changes were observed on the deformation networks."

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: B. Talai and C. McKee, RVO.

During the last week of July, 45 visible explosions ejected plumes to 75-150 m above the summit before winds carried them S. Between explosions, weak white fumes reached 50 m above the crater. Recorded earthquakes were: distant tectonic (50), local tectonic (2), volcanic A-type (1), volcanic B-type (2), explosions (1,574), and volcanic tremor (1).

Geologic Background. Raung, one of Java's most active volcanoes, is a massive stratovolcano in easternmost Java that was constructed SW of the rim of Ijen caldera. The unvegetated summit is truncated by a dramatic steep-walled, 2-km-wide caldera that has been the site of frequent historical eruptions. A prehistoric collapse of Gunung Gadung on the W flank produced a large debris avalanche that traveled 79 km, reaching nearly to the Indian Ocean. Raung contains several centers constructed along a NE-SW line, with Gunung Suket and Gunung Gadung stratovolcanoes being located to the NE and W, respectively.

Information Contacts: VSI.

After a sharp increase in seismicity on 24 June and a small ash emission 2 days later, seismicity gradually decreased in late June. High- and low-frequency events stabilized at 200-300/day by the end of the month (figure 28), and tremor was almost absent. A depression, 150-200 m in diameter and 80 m deep, apparently formed on 26 June ~50-100 m SW of the principal (Arenas) crater. It probably developed because of high-pressure gas emission and destabilization of the walls of Arenas crater.

Figure 28. Daily number of high- and low-frequency events at Ruiz, July 1989. Courtesy of the the Observatorio Vulcanológico de Colombia.

Deformation measurements showed no significant changes in July. SO₂ flux averaged 1,200 t/d, a slight increase from May (1,046 t/d) and June.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

The following supplements the preliminary report in BGVN 14:06.

"On 19 July, 32 Central American volcanologists were completing a field hazard mapping project, part of a training course sponsored by the Centro de Coordinación para la Prevención de Desastres Naturales en America Central (funded by the government of Sweden). The course prepared volcanic hazard reports and maps for Cerro Quemado, Guatemala, but on 19 July participants were touring the area near El Palmar (12 km SSW of Santiaguito) to view deposits and damage caused by river aggradation associated with continual activity at Santiaguito since 1973.

"At 0915, during excellent viewing conditions, the group observed a spectacular vertical explosive eruption and pyroclastic flow from Santiaguito (figure 11). The vertical explosion and pyroclastic flow occurred simultaneously, apparently associated with a minor (?) collapse of part of the dome near Caliente Vent. The ash cloud rose about 4 km above the vent, and was clearly observed from Llano del Piñal, 6 km NNE. Ashfall occurred in the areas W and SW of the volcano. The maximum measured thickness was 1 cm at Finca Monte Bello (6 km WSW), but ash fell at least as far away as the Mexican border (65 km distant). The pyroclastic flow followed the same path as recent lava flows from Caliente Vent, descending into the valley of the Río Nimá II and forming a block-and-ash flow and ash cloud surge that mantled some of the 1987-89 lava flow. The main part of the pyroclastic flow traveled 5 km, about 1 km farther downstream than the April 1973 pyroclastic flow (figure 20; Rose, 1973; and Rose et al., 1976/7), and thus probably represents the largest since the 1929-34 activity (Sapper and Termer, 1930; Termer, 1934; and Reck and von Tuerckheim, 1935). Some of the ash cloud surge from the pyroclastic flow probably traveled a shorter distance eastward, based on distant observations of burned vegetation. The composition of hot blocks in the new block and ash flow deposit, collected on the afternoon of 19 July is dacite (64% SiO₂), identical to other recent samples and nearly all of the dome rocks extruded since 1922.

Figure 11. Photograph of Santiaguito's eruption column on 19 July 1989 at about 0920, looking N from the valley of the Río Nimá II just W of El Palmar, Guatemala. The tephra cloud and pyroclastic flow are shown about 5 minutes after onset of the eruption. Santa María volcano, in the right background, has a summit elevation of about 3,700 m. Courtesy of Mike Conway.

"Visibility was lost within 2 hours after the eruption. A much smaller vertical explosion occurred about 20 minutes after the first, followed by two smaller phreatomagmatic eruptions before 1000. A small vertical eruption was also observed at about 1615." [see also Atmospheric Effects, BGVN 14:8-11].

References. Reck, H., and von Tuerckheim, O.G., 1935, Die Zustand der Vulkane Fuego, Atitlán, und Santa María in Guatemala Ende 1934: Zeitschrift für Vulkanologie, v. 16, p. 259-263.

Rose, W.I., 1973, Nuée ardente from Santiaguito volcano, April 1973: Bull Volc, v. 37, p. 365-371.

Rose, W.I., Pearson, T., and Bonis, S., 1976/77, Nuée ardente eruption from the foot of a dacite lava flow, Santiaguito volcano, Guatemala: Bull Volc, v. 40, p. 23-38.

Rose, W.I., 1987, Volcanic activity at Santiaguito volcano, 1976-1984 in Fink, J., ed., The emplacement of silicic domes and lava flows: GSA Special Paper 212, p. 17-27.

Sapper, K., and Termer, F., 1930, The eruption of Santa María volcano in Guatemala of November 24, 1929: Zeitschrift für Vulkanologie, v. 13, p. 73-101.

Termer, F., 1934, Die Tätigkeit des Vulkans Santa María in Guatemala in den Jahren 1931-1933: Zeitschrift für Vulkanologie, v. 14, p. 43-50.

Geologic Background. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank, and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four vents, with activity progressing E towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Otoniel Matías and Jorge Girón, INSIVUMEH; W.I. Rose, F. Michael Conway, and J.W. Vallance, Michigan Tech.

The small Vulcanian explosions and avalanches . . . continued in late July. During the second week of the month, thick ashfalls occurred at Sawur (13 km SW of the summit), Tawangsongo, and Argosuko observatories. Ash and incandescent tephra rose 50-100 m above the crater through late July. Avalanches of lava debris reached 750 m from the crater, while associated [pyroclastic flows] traveled 1,000-4,000 m. No changes on the lava dome were observed. The type and number of earthquakes recorded 1-20 July were: explosion (793), collapse (1), volcanic A-type (9), volcanic B-type (8), and [pyroclastic flows] (11). Although activity has increased, it is still considered within the normal range.

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

Information Contacts: VSI.

Normal activity continued in late July, with a weak gas plume reaching 5-10 m above the crater. The temperature of the crater solfatara was 185-190°C, while two surrounding solfataric areas measured 108-110°C and 97-115°C. Twenty-nine tectonic earthquakes (but no volcanic shocks) were recorded.

Geologic Background. Sorikmarapi is a forested stratovolcano with a 600-m-wide summit crater containing a lake and substantial sulfur deposits. A smaller upper SE flank crater (Danau Merah) also contains a crater lake; these two craters and a series of smaller explosion pits occur along a NW-SE line. Several solfatara fields are located on the E flank. Phreatic eruptions have occurred from summit and flank vents during the 19th and 20th centuries.

Information Contacts: VSI.

Explosions continued at Suwanose-jima. On 22 and 23 June 1989, several tens of eruptions were reported, accompanied by audible detonations, felt air shocks, and ashfalls on the inhabited S side of the island.

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: JMA; D. Shackelford, Fullerton, CA.

Normal activity continued . . . in July, with weak white fumes rising 30-60 m above the summit. A total of 449 gas emission earthquakes were recorded.

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

Information Contacts: VSI.

The quoted material is a report from RVO, with additional information on SO₂ flux supplied by S. Williams. "Throughout July, the volcano quietly released moderate, sometimes strong, white with occasional grey or brown emissions (on the 15th, 16th, 22nd, and 24th). Volcano seismicity remained at a low level with 10-30 small discrete B-type events/day."

During airborne COSPEC measurements on the 27th from 1150 to 1240, a diffuse white plume extended 10-15 km from the crater. Four traverses yielded SO₂ emission rates of 149, 66, 120, and 134 t/d. The average flux was 120 t/d, an increase from 1983 values (97, 70, 49, 66 t/d) that yielded an average of 70 t/d. When Williams flew past the volcano on 30 July, the plume remained thin, white, and wispy, but visible for 10-20 km downwind.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: B. Talai and C. McKee, RVO; S. Williams, Louisiana State Univ.

An explosion on 3 July, marked by an E-type earthquake at 1134, produced an eruption column 2,000-2,500 m high. E-type earthquakes were also recorded 25 June at 1002 and 17 July at 1015. Other seismicity generally remained similar to previous months, with A- and B-type events most days (~5-10), and minor high-frequency volcanic tremor 5-11 and 21-32 July.

When geologists visited the volcano on 31 July, there was little evidence of significant eruptive activity since previous fieldwork on 23 June. A small amount of fine gray ash may have originated from Donald Duck since the 23 June visit, but recent rainfall erosion made this difficult to assess. No new ejecta from R.F. crater had accumulated on the 1978 Crater rim, and no ash was found on the seismic solar panels. Tephra pits showed that Donald Duck had ejected nearly all the tephra deposited 26 April-23 June in the area ~150 m W and SW. Tephra from R.F. Crater was only significant ~200 m S of Donald Duck.

The new intermittently active vent 30 m NNE of Donald Duck (first noticed 10 May; 14:06) emitted white gas, as did R.F. Crater. Activity of R.F. and Hitchhiker vents seemed related; when emissions from R.F. Crater were strong, gases from Hitchhiker seemed to be drawn back into the vent. Dark blocks and numerous small fumaroles covered R.F. crater floor. Fumarole temperatures decreased in the Donald Mound/Noisy Nellie area. Deformation studies suggested that the pattern of continuous subsidence that began in February has ended.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: I. Nairn, NZGS Rotorua; J. Cole, Univ of Canterbury, Christchurch.