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/).

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).

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/).

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/).

The last flank eruption at Etna occurred in 1993, but eruptive activity has been nearly continuous in the summit area since late July 1995. Unusually vigorous eruptions in early June 2001 (BGVN 26:08) were followed by still more energetic behavior. An earthquake swarm took place in mid-July, and large SSE-flank eruptions vented lavas in late July and early August on a scale not seen since 1983 (BGVN 26:09). Though this activity has been reported in previous Bulletin reports, what follows is a more detailed description of the summer 2001 activity.

Precursory phases. During the first half of 2001 most activity occurred at the Southeast Crater (SEC), starting with very slow lava extrusion from a vent on the NNE flank of its cone in late January. During the next three months the lava emission rate progressively increased and, in late April, weak ejections of lava fragments began to build small spatter cones (also known as hornitos) in the vent area. Strombolian activity resumed at the SEC's summit vent on 7 May, followed by an episode of vigorous Strombolian activity and lava fountaining two days later.

The activity then returned to lower levels, but for the next two months the SEC was the site of very picturesque continuous and persistent activity, with mild, discontinuous Strombolian explosions from the summit vent and vigorous lava emission (at times accompanied by lava spattering) from vent(s) on the NNE flank. At this spot, a large, steep-sided cone consisting of overlapping lava flows and spatter began to grow, and it was informally named "Levantino." From Levantino (figure 93, upper part), lava flowed NE (toward the Valle del Leone), E, and SE (toward the Valle del Bove), forming a composite lava field with numerous overlapping flow lobes. The longest flows extended up to 2 km from the vent(s). From late May to early June the continuous and relatively harmless activity attracted many visitors.

Figure 93. Inset map (upper left) and sketch map showing Etna's July-August 2001 lava flows, fissures, and related features. The 1983 lava flow-field on the S flank (lighter shading) is shown for comparison. The legend is also linked to numbers near the vents and fissures, as a way to indicate the sequence of activity. The pattern of activity lacked a simple spatial progression. For the time citations, note that the eruption occurred during daylight saving time (UTC + 2 hours). The sketch map is preliminary; it is a composite based on both fieldwork by Behnke and others, and already available maps appearing on the web, including those on the site of INGV-Sezione di Catania. Courtesy of Boris Behnke and Marco Neri, INGV.

The prelude to the main eruption began in early June, with the first strong phase on 7 June, which was mostly confined to lava fountaining at the Levantino, whereas only mild Strombolian activity occurred at the SEC summit vent. During the following six weeks the vigor of the eruptive episodes gradually increased and at times culminated in true lava fountaining from the summit vent. Fourteen such episodes occurred between 7 June and 13 July (7, 9, 11, 13, 15, 17, 19, 22, 24, 27-28 and 30 June; 4, 7 and 13 July), ending with the most violent outburst of the series. Immediately after its cessation, vigorous seismicity began, including shocks that were felt as far as the cities of Acireale and Catania (figure 93 index map).

Between the early mornings of 13 and 17 July, about 2,600 tremors, mostly unfelt, were recorded instrumentally. During this same time interval, severe ground cracking and faulting occurred in various places on the W rim of the Valle del Bove and near the "Cisternazza" pit crater on the Piano del Lago ("PDL," figure 3), a flat area at ~2,500 m elevation to the N of the Montagnola. Press sources cited local volcanologists saying that magma was rising through a dike and stagnating about 1 km below the surface. Steaming was observed at some of the new fractures, but as of the late evening of 16 July no magma had reached the surface. Seismicity continued at a slightly decreased rate.

Shortly after midnight on 17 July, yet another paroxysmal episode occurred at the SEC. This was similar to its 13 July predecessor, with vigorous lava fountaining from the summit vent and the Levantino. It was the last event that could be considered part of the prelude to the main eruption. Figure 93 shows a sketch map of the lava flows that ultimately resulted from the July-August 2001 eruption. The inset at upper left shows the 2001 lavas and locations of various towns on Etna's S, SE, and E flanks.

Eruption of 17 July-9 August 2001. As defined here, the main eruption lasted nearly 24 days, from early 17 July to late 9 August 2001. It began a few hours after the last paroxysmal eruptive episode at the SEC.

An eruptive fissure opened at about 0700 on 17 July at the SSE base of the SEC at ~ 2,950 m elevation (point 1 on figure 93). Vigorous lava spattering occurred at a number of vents, while lava flows advanced toward SE, in the direction of the Valle del Bove rim. Several hours later, at 2200, another fissure opened at an elevation of ~ 2,700 m (point 2 on figure 93), a bit to the W of a panoramic point on the W rim of the Valle del Bove known as "Belvedere," and lava began to extend S from there, across the Piano del Lago, and in the direction of the cable car and ski lifts (shown by the vertical dotted line just beneath PDL on figure 93).

At 0120 on 18 July, a third eruptive fissure became active on the S flank of the Montagnola at ~ 2,100 m elevation, slightly uphill of the Monti Calcarazzi (point 3 on figure 93). Activity at this fissure was initially weak, and a sluggish lava flow began to move from the lower end of the fissure toward the Monte Silvestri superiore, a large complex cinder cone formed in 1892. During the day, the activity gradually increased; lava flowed around the W side of the Monte Silvestri, crossed the road (which connected the Rifugio Sapienza-cable car area with the town of Zafferana) and passed close to buildings. By late evening on 18 July the flow front was at 1,800 m elevation. On 19 July, the lava flow fed by the vents at 2,100 m elevation continued to pass between numerous older cinder cones along the E margin of the 1983 lava flow-field. At the same time eruptive activity continued unabated at all eruptive fissures.

On the evening of 19 July at about 1900, a new vent opened on the Piano del Lago at ~2,570 m elevation, ~ 500 m N of the Montagnola (point 4 on figure 93; "Montagnola 2", "Monte del Lago" or "Cono del Laghetto", unfortunately, all three names are in common usage). The activity was characterized by violent phreatomagmatic explosions that produced a black, ash-laden column and ejected large blocks of older volcanic rocks. This phreatomagmatic activity might have been the result of magma-water interactions with shallow ground water. (The name, Piano del Lago, the "plain of the lake," derives from the fact that frequently during the spring snow-melt a small meltwater lake had formed in that area.) To observers familiar with footage of the 1963 Surtsey (Iceland) eruption, the steam-and-ash columns rising from the new vent appeared strikingly similar. After nightfall spectacular lightning flashed through the plume and ash fell over inhabited areas including the NE, E, SE, and S flanks. Close observation of the vent at 2,570 m elevation revealed a simple, though constantly eroding, pit. Rumors spread that the pit would eventually "make the Montagnola cave in;" however, it remained intact.

The front of the lava flow emitted at 2,100 m elevation (point 3 on figure 93) was at 1,350 m late on the evening of 19 July. It still advanced vigorously, though at reduced speed (~45 m/hour). Soon after, authorities declared a state of emergency.

By the evening of 20 July the front of the lava flow had reached 1,220 m elevation, but it had slowed in a gently sloping area. Nonetheless, the threat to Nicolosi had become international news. The main risk now came from forest and bush fires. Helicopters began to drop large quantities of water onto the burning forest (not onto the lava flow, as was widely reported). Fortunately, no fires broke out. By late 21 July the lava flow's front had reached 1,050 m elevation.

During the night of 19-20 July, the SEC reactivated, and lava began to issue from the NNE side of the Levantino, forming a small flow that advanced in the direction of the Valle del Bove.

At about 1100 on 21 July, yet another eruptive fissure formed (point 5 on figure 93). It emerged on the NE flank, far away from the earlier fissures (below the Pizzi Deneri on the floor of the Valle del Leone). Figure 93 portrays the impressive arcuate fracture system that developed. The lava first issued from the fissure's uppermost portion at ~ 2,700 m elevation, then from its eastern extension ~50 m further downslope. A fracture system formed between this northeasternmost fissure and the E side of the summit crater area, and fissures up to 1 m wide opened on the slope between the Bocca Nuova and the SEC, and began emitting dense vapor plumes.

The large pit crater on the Piano del Lago at 2,570 m elevation continued to produce dense, ash-laden plumes. In late July ash fell over the area between Zafferana and Acireale (inset, figure 93).

On 21 July, the eruption continued with unabated vigor at all eruptive fissures. Late that day, the lava from the vents at 2,700 m had not further advanced on the steep slope above the tourist complex of the Rifugio Sapienza area (shown as Rif. Sapienza, figure 93).

The next day, 22 July, a shift in the wind direction drove the dense ash plume produced by the phreatomagmatic vent at 2,570 m elevation directly over the Catania area, paralyzing the city's Fontanarossa International Airport, and dropping large amounts of fine, black, sand-sized tephra over streets and buildings. Eruptive activity continued without significant variations at all fissures, although the main lava flow directed toward Nicolosi had slowed significantly and advanced only a few meters per hour, still 4 km from the outskirts of the town. That evening, minor, though fast-moving lava tongues fed by the vents at 2,700 m elevation spilled down the steep slope above the tourist complex, destroying part of the ski lifts and threatening to overwhelm some of the poles of the cable car. However, none of these flows made it further than about halfway down that slope. Vigorous Strombolian activity continued at the fissure at 2,100 m elevation, and a cone about 20-30 m high had formed around the largest of its vents, while minor lava spattering from four vents in the lower part of the fissure was building a low-spatter rampart. The lava flow from this fissure moved as a single, broad unit (point 6 on figure 93). Throughout 23 July the activity showed little variation.

On 24 July it became clear that something was changing at the Piano del Lago crater, which until then had displayed only phreatomagmatic activity. Loud detonations came from the Piano del Lago vent on the morning of 25 July, as the activity there had now become purely magmatic.

A new cone was growing on the former Piano del Lago on 26 July produced near-continuous explosions accompanied by loud detonations that could be heard ten's of kilometers away. The cone received bombs ejected from at least three vents within it. Lava began flowing from vents on its S and N sides, spilling around the N and NW sides of the Montagnola and down the steep slope on the W side of that cone toward the Rifugio Sapienza. The new lava flow passed E of the lower cable car station, covered another portion of the road and advanced a few hundred meters further. Vigorous eruptive activity continued on 26 July at the eruptive fissure at 2,950 m elevation, while the fissure at 2,700 m elevation continued to emit lava but pyroclastic activity there had ended. Weak activity continued at the Valle del Leone fissure, with lava flowing toward Monte Simone (in the N part of the Valle del Bove), and the fissure at 2,100 m elevation showed no significant changes in its activity with respect to the previous days.

During 27-30 July the eruption produced some outstanding displays (figures 94 and 95) and caused the greatest impact on human structures. While the fissure in the Valle del Leone showed ever-lower levels of activity and the fissures at 2,900, 2,700, and 2,100 m elevation continued to erupt in a manner almost identical to the previous days, the Montagnola 2 cone and several nearby vents not only provided a show, but vents on its N and S flanks delivered vigorous lava flows that spilled down the steep slope to the W of the Montagnola in surges, presenting a continuous threat to the Rifugio Sapienza area.

Figure 94. At Etna, a night view taken from high on the S flanks near Torre del Filosofo documenting the erupting vents at 2,700 m elevation (left) and a lava fountain at the Piano del Lago cone (right), probably taken on 30 July 2001. The camera was aimed SE. Lights of towns between Acireale and Giarre are visible in the background. Photo was published in the 1 August 2001 issue of the "La Sicilia" newspaper.

Figure 95. Spectacular aerial view of the largest cone formed in Etna's eruption during July-August 2001, built on what used to be the "Piano del Lago" at ~ 2,500 m elevation on the S flank. View is to the SW. Most of the cone grew in a few days in late July; this photo shows the waning phase around 2 August when only ash was emitted. After the end of the activity the summit of the cone stood nearly 100 m higher than its base, but then partially collapsed leaving the cone ~ 80 m higher. To the left of the cone is the 1763 Montagnola crater. Between the two lies a fuming vent; it emitted one of the latest lava flows in the area (black ribbon extending to the left margin of the photo). Other lava flows that were emitted from vents at ~ 2,700 m elevation (out of the photo to the right) can be seen in the right foreground. Yet another lava lobe extends toward the lower left corner of the photo; it emerged at the E base of the Piano del Lago cone. Photograph by volcanologists of the INGV (National Institute of Geophysics and Volcanology); published on 6 August 2001 in the "La Sicilia" newspaper.

A lava flow from the southern base of the Piana del Lago cone on 27 July remained active for about 2 days (point 7 of figure 93). This cone produced three distinct lava flow units from three different vents on its SW, S, and NW sides; those from the SW vent were the most damaging. On the evening of 30 July, the eruption claimed the upper cable car station, which burst into flames as a tongue of lava invaded its interior. Behind it, the rapidly growing cone of the Piano del Lago sent its lava fountains hundreds of meters into the sky, producing ground-shaking detonations.

During late July, the rate of lava emission in the direction of the Rifugio Sapienza area diminished as a new vent opened on the S side of the Piano del Lago cone, from which a lava flow spilled over the crest of the Valle del Bove and flowed to the bottom of the Valle, covering a portion of lava erupted in 1991-1993. The Valle del Leone fissure ceased emitting lava. The Montagnola 2 had become a large cone. It again shifted from magmatic to phreatomagmatic activity. Although the level of activity at this cone was decreasing, ash columns developed, and the wind carried the ash again in the direction of Catania, again closing the Fontanarossa airport. Vigorous eruptive activity continued during the first days of August at the fissure at 2,100 m elevation, but the lava effusion rate had dropped, and only the central portion of the original flow remained active, feeding a flow that advanced on top of the larger earlier flow from the same fissure. Strong phreatomagmatic explosions occurred from time to time at the upper vents on this fissure. Lava also continued to pour from the fissure at 2,700 m elevation, and numerous active flow lobes extended SE toward Monte Nero.

Beginning on 3 August the activity at the fissure at 2,100 m elevation showed a marked decrease. During the last week of the eruption (4-10 August) lava continued to flow from that fissure at a rapidly diminishing rate, explosive activity ended, and the fissures at 2,950 and 2,700 m elevation ceased erupting. Ash emission from the Piano del Lago cone ended around 6 August. Lava was last seen flowing from the fissure at 2,100 m elevation on late 9 or early 10 August. The eruption ended after almost 24 days, much earlier than its vigorous onset had suggested.

Products of the eruption and morphological changes. The July-August 2001 eruption emitted at least eight distinct lava flows which mostly affected the S and SSW flanks of Etna. The most damaging flows came from vents near the Piano del Lago cone at 2,570 m elevation. This cone produced most of the pyroclastic material, of which the most fine-grained portion (ash) caused widespread distress.

Petrographically, the lavas fall into two main groups, one of which is essentially similar to the historical products of Etna (porphyric hawaiites with phenocrysts of plagioclase, clinopyroxene and olivine), while the other shows characteristics not seen in any Etnean lavas during the past millennia (alkalic basalts).

Several major pyroclastic edifices grew during the eruption. The largest, the Piano del Lago cone at ~2,570 m elevation, is a nearly symmetrical cone, ~80 m high, which grew mostly between 25 and 31 July. It is crowned by a large crater ~150 m across and 50-60 m deep with near-vertical walls. During a visit on 30 August a few fumaroles located on the SEC walls emitted vapor and gas without forming a visible plume. A spectacular dike was exposed at the base of the E crater wall, and a less conspicuous dike was visible at the base of the WNW crater wall. Several faults or groups of faults were visible in the E and NE crater walls; a narrow graben developed on top of the E crater wall dike. The second-largest pyroclastic edifice lies in the upper part of the fissure at 2,100 m elevation.

Extensive (non-eruptive) fracture systems formed between the various eruptive fissures. Some of them began to form before the eruption, others developed during its first week. Many of these fractures are arranged in an en echelon pattern, which is especially notable in the Valle del Leone, between the northeastern (2,600 m) vents and the SEC. Other spectacular fractures opened between the Piano del Lago cone and the eruptive fissure at 2,100 m elevation. However, the most impressive fractures developed on the E side of the former Central Crater (now occupied by the Voragine and the Bocca Nuova), where they attained widths of more than 1 m. These fractures were vigorously steaming when visited in late-August and mid-September 2001. Fracturing also affected the southern face of the SEC cone.

About 5.5 km² of Etna's upper and middle slopes were covered with new lava (compared to 7.6 km² in the 1991-1993 flank eruption). The total volume of lavas and pyroclastics emitted during the eruption is ~ 30-35 x 10⁶ m³ (25 x 10⁶ m³ of lava and 5-10 x 10⁶ m³ of pyroclastics, all calculated as dense rock equivalent), which makes this eruption a relatively modest-sized one for Etna (the 1991-1993 eruption had produced ~ 235 x 10⁶ m³ of lava). Yet the fairly high proportion of pyroclastic material distinguishes this eruption from most other recent Etnean flank eruptions.

The most productive vents were those at 2,100 m elevation, which produced the longest (~ 7 km) and most voluminous (slightly less than 14 x 10⁶ m³) single lava flow. The average combined effusion rate at all eruptive fissures throughout the eruption was ~11 m³/s, with peak rates of 14-16 m³/s. It is assumed that much of the magma that accumulated in one or more reservoirs below the base of the volcano in the years preceding the eruption was not erupted and remains available for future eruptions.

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: Boris Behncke, Dipartimento di Scienze Geologiche (Sezione di Geologia e Geofisica), Palazzo delle Scienze, Corso Italia 55, 95129 Catania, Italy.

At least two commercial aircraft flew through airborne ash near the Guatemala City airport on 21 May 1999. An eruption from Fuego that day was the first at that volcano since 1987 (BGVN 24:04). Although the aviator's reports attributed the ash from this encounter to Fuego, their aircraft intersected multiple ash plumes in widely different locations, and thus they may also have crossed plumes from Pacaya. The most likely plume near the southern approach to the airport (La Aurora, ~23 km N of Pacaya; ~40 km NE of Fuego) is from the almost constantly active Pacaya. In contrast, Fuego lies 32 km W of Pacaya and was the likely source of a plume intersected later during the flight, at higher altitude, and for much longer duration.

During the encounters the ~100 or more people on board the two aircraft, and many more on the ground, were at risk. Both encounters seriously damaged the aircraft but ended in safe landings without reported injuries. Volcanic ash can cause jet-engine failure, which creates a hazard not only to passengers, but to people on the ground as well. The risk in this situation was amplified by the airport's proximity to urban Guatemala City (population, >1.1 million).

This example leads to two conclusions discussed further below. First, ash avoidance methodology needs further refinement. Second, there exists an apparent bias towards under-reporting of aircraft-ash encounters, which could short-change their cost to air carriers, their perceived risk, and their funding allocation.

The original report of an aircraft-ash encounter was brought to our attention by Captain Edward Miller of the Air Line Pilots Association and provided on the condition that the air carrier remain anonymous. Bulletin editors also wish to acknowledge conversations with several additional anonymous contacts (commercial pilots). This case occurred in Guatemala; however, analogous situations exist at many airports adjacent to volcanoes. Modest eruptions from nearby volcanoes may be uncertain or difficult to see; they may be hard to detect or characterize; yet they still may yield mobile ash plumes carried by complex local winds to confront air traffic (Salinas, 2001; Hefter, 1998; Casadevall, 1994).

Although in most Bulletin reports we favor the use of local time to emphasize the reference frame of people on the scene, most of the source material (satellite and aviation) for this report refer to UTC (Coordinated Universal Time). UTC is the time on the Greenwich meridian (longitude zero), formerly GMT, a term which has fallen out of use. Accordingly, there are cases in this report that lack conversion to local time. Local time in Guatemala is 6 hours behind UTC.

Activity at Fuego and Pacaya. Fuego erupted on 21 May 1999 sending ash to the S, SE, and SW and ultimately dropping up to 40 cm of ash on local settlements (BGVN 24:04). The eruption occurred at 1800 local time (in terms of UTC, at 0000 the next day). Three hours later the eruption decreased and the Aeronautica Civil recommended that planes go no closer to Fuego than 40 km. One hour after that (at 2200 local, 0400 UTC), the atmospheric ash had settled, and Aeronautica Civil recommended flying no closer to Fuego than 15 km.

Bulletin reports for Pacaya in mid-1999 suggested relative calm, in harmony with the observation that it had chiefly been fuming. However, Pacaya is well-known for Strombolian outbursts. It lies directly in-line and only 23 km S of the N10°E-oriented airstrip. As is common for commercial aircraft there, the approaches described below passed very close to Pacaya. Pacaya typically has lower and smaller eruptions than Fuego, but because it lies so close to the southern approach to the airport, Pacaya's ash plumes easily enter the path of landing aircraft. The situation was particularly complex during this eruption because Fuego's ash was reported on cities that lie below the flight path as well as on the Pacaya's flanks.

Pilot's report of aircraft-ash encounter. What follows was taken from one flight crew's description of events and from later reports and dialog on the topic. In order to preserve the confidentiality of the airlines, the precise times of events on 21 May 1999 have been omitted. Pilots reported clear visibility and light wind, with both the capital and the airport in sight. They maneuvered the aircraft for a final approach from the S. Pilots were advised by air traffic control about erupted particulate ("volcanic sand") about 24 km (15 miles) SE of the airport, well away from their projected path to Runway 01. In addition, based on winds they detected as they neared the airport, the pilots concluded that the erupted particulate to the SE was downwind from their projected flight path to the airport. (In retrospect, this conclusion appears tenuous considering the possibility of either a fresh injection of ash from Fuego, or lingering ash in the trailing portion of the plume that lay to the SE.)

At ~32 km (20 miles) distance from the runway, the pilots maneuvered the plane through ~3,000 m (9,700 feet) altitude on a final approach to the airport with the plane's flaps partially extended (at ~15°, which slowed their aircraft to ~260 km/hour (160 mph), its auxiliary power unit (a small jet engine) on, and its landing gear down. Around this point in their descent, pilots saw bright yellow sparks through the windshield lasting a few seconds, a display unlike static electricity, but rather like that from a grinding wheel. This occurred again at 2,500 m (8,200 feet) altitude, but was more intense yet intermittent.

At this time, air-traffic control announced to the pilots that the aircraft landing in front of them had encountered volcanic particulate; they instructed the pilots to abort their landing and climb to 3,350 m (11,000 feet) altitude. Discussions of options and ash avoidance ensued between pilots and air-traffic control; ash had by this time accumulated on the runway, further complicating landing, even for approaches in the opposite direction. The pilots retracted the landing gear, accelerated to ~410 km/hour (~250 mph), began to climb, and after some discussion with air-traffic control, held on a course NE of the airport. They were subsequently cleared to climb to 6,700 m (22,000 feet) altitude and advised to proceed to an alternate airfield.

During the climb, at ~5,800 m (19,000 feet) altitude en-route to the alternate airport, the aircraft encountered volcanic particulate for 10 minutes. Within that interval the plane spent 2 minutes during ascent engulfed in denser and heavier particulate. During that period, window arcing was constant and the beam of their landing lights revealed a conspicuous cloud of reflecting particles. During ash ingestion the engines's speed lacked noticeable fluctuations. The aircraft exited the plume at about 6,100 m (20,000 feet) altitude and landed without encountering additional ash.

Upon landing, the pilots noticed reduced visibility through abraded windshields. Post-flight examination of the airplane revealed heavy damage, requiring the replacement of the engines (US $2 million each) and auxiliary-power-unit engine. (Note, however, that no deterioration in engine power or performance was noticed during the flight.) Other replaced parts included windshields, a heat exchanger, and coalescer bags. Minor damage was seen on the horizontal stabilizer and wing leading-edges.

Volcanic Ash Advisory Statements. The U.S. National Oceanic and Atmospheric Administration (NOAA), Satellite Services Division website contains two archived statements issued by the Volcanic Ash Advisory Center (VAAC) at Washington, D.C. for Fuego on 21-22 May 1999. The statements, issued to the aviation cummunity to warn of volcanic hazards, are intended for an audience accustomed to special terminology (figures 2 and 3). In the interest of advancing understanding of how volcanological and atmospheric data get transmitted to aviators, we offer brief explanations for many of the terms used (figure 2).

Figure 2. The first archived Volcanic Ash Advisory Statement (VAAS) for the 21 May 1999 eruption at Fuego, Guatemala. The boxes contain added notes to explain some of the basic conventions and specific details seen here. This Statement currently appears on the NOAA Satellite Services Division website (see "Information Contacts," below). Local names are frequently anglicized, dropping all accents and other non-English characters (eg. México would be written MEXICO). Later Advisories adopted the abbreviation "Z" (pronounced 'Zulu' by aviators) for UTC, as in 1300 UTC written as 1300Z.

Figure 3. A later Volcanic Ash Advisory Statement (VAAS) for the 21 May 1999 eruption at Fuego, Guatemala. The statement, which was issued the next day, discloses that the eruption had then stopped and the hazard status was lowered. The statement appears on the NOAA Satellite Services Division website (see "Information Contacts," below).

The Washington VAAC received first notification of the Fuego eruption from a routine surface weather observation from Guatemala City at 2000 local time (0200 UTC) on 22 May. They issued the first Volcanic Ash Advisory Statement a half-hour later (figure 2). Six hours later they issued the second Advisory Statement (figure 3). The Advisories were composed by staff of the Satellite Analysis Branch, one of the two NOAA components forming the Washington VAAC (Streett, 1999; Washington VAAC).

The section "Details of ash cloud" first says that the "surface observation from Guatemala City indicate that the Fuego volcano is in eruption" and that no additional information is available and then briefly describes in words observations that came from satellite imagery. The first sentence, "No eruption . . ." is self-explanatory, but highlights a limitation of the method in use that needs to be emphasized to aviators: an eruption may have occurred but its status is not revealed on the imagery. The sentence, "No eruption could be detected due to thunderstorm cloudiness covering the area around the volcano" is self-explanatory. Less clear is the term convective debris. It does not refer to ash; rather, it refers to remnants of thunderstorms. The gist of these latter two sentences is simply that the thunderstorms that covered the area made it challenging or impossible to see the ash on satellite imagery. Central American thunderstorm clouds typically can reach altitudes of more than 12,000 m (40,000 feet), and can mask or obscure airborne ash residing below that level.

Members of the Washington VAAC commented that this eruption demonstrates key problems that can arise when cloudy conditions prevent satellite detection of ash, foiling a primary mode of analysis. In such cases, they rely on ground observers (including observatories and weather observers), pilot reports, and reports from airlines. Thus, their ability to issue useful information in cloudy conditions depends on the quality of communications with local observers, the Meteorological Watch Office, volcanologists, geophysical observatories, and the aviation community.

The Advisory Statement listed México City weather balloon data acquired 1,050 km NW of Guatemala City. In retrospect, the Washington VAAC noted that they generally avoid using such distant sounding data. If a closer sounding cannot be found, they prefer to use upper-level wind forecasts taken from a numerical weather model. In any case, the scarcity of local sounding data presents a challenge to the realistic analysis of airborne ash.

The outlook section says to "see SIGMETS." SIGMETS are the true warnings to aircraft for SIGnificant METeorological events. They are issued by regional Meteorological Watch Offices (MWOs), in this case the MWO (for Guatemala) is in Tegucigalpa, Honduras. SIGMETS were lacking for this eruption; although the Washington VAAC tried to contact the MWO without response. Another complexity confronted at the VAAC is a lack of a single scale for communicating a volcano's hazard status.

Reporting of aviation ash encounters. In personal communications with Bulletin editors, airline personnel stated that many more encounters have occurred than have yet been tallied in publically accessible literature. In accord with those assertions, the 21 May 1999 encounters are absent from reports compiled by the International Civil Aviation Organization (ICAO, 2001). In that document (Appendix I, table 3A, p. I-12) Fuego fails to appear as a source vent for any aircraft-ash encounter. Pacaya is listed for two encounters, in January 1987 and in May 1998 (BGVN 23:05).

Even though the number of encounters was probably under-represented and thus reflects a minimum, ICAO (2001) notes that the international costs to aviation since 1982 summed to well in excess of $250 million. They noted, "In addition to its potential to cause a major aircraft accident, the economic cost of volcanic ash to international civil aviation is staggering. This involves numerous complete engine changes, engine overhauls, airframe refurbishing... aircraft downtime... [and] volcanic ash clearance from airports and the damage caused to equipment and buildings on the ground."

The incidents here suggest that there has been a strong bias toward under-reporting aircraft-ash encounters. If this tentative conclusion is correct, it implies consistent understatements of the hazard's magnitude. This, in turn, may have thwarted meaningful analysis of how and whether to proceed with designing more robust hazard-reduction systems. Accordingly, resources that could have been devoted to the problem have not yet been committed (see Gimmestad and others, 2001 for a discussion of a prototype on-board ash-detection instrument).

Communication challenges. While much of the aviation community needs to learn about volcanism rapidly, dependably, and with the aid of the Internet, some observers charged with reporting volcanic-ash hazards in Central and South America lack access to basic communication devices like reliable telephones and fax machines. To reduce the risks, the aviation, meteorological, remote sensing, and volcanological communities need to improve their ability to pass critical information to each other rapidly and precisely. The operational systems related to volcanic ash and aviation must transcend numerous boundaries (eg., languages, infrastructure, funding, governments, agencies, air carriers, pilots, aircraft manufactures, etc.). The systems need to portray complex, dynamic processes such as the rapid rise of an explosive plume, or large-scale ash-cloud movement.

Although the infrastructure for ash avoidance is greater than ever, members of the Washington VAAC have told Bulletin editors that they still depend heavily on people on the scene of the eruption to notify them promptly when eruptions occur. They said that thus far in parts of Central and South America a problem has been the expense of communication (eg., by phone, fax, and Internet). They also said that for the same regions the U.S. meteorological database regularly lacks pilot reports. Though serious, these problems have at least been identified and their solutions would appear to lack great technical or economic barriers.

The pilots involved in the May 1999 encounter recommended that far more emphasis be placed on forecasting and avoiding ash plumes. Other pilots cited the need for fast and accurate communications between those who observe eruptive activity and air traffic control personnel.

Issues like these continue to be an important subject at gatherings on the topic of ash hazards in aviation (Casadevall, 1994; Streett, D., 1999; Washington VAAC, 1999). The Airline Dispatcher's Federation (ADF) will participate in a 7-9 May 2002 conference and workshop: "Operational Implications of Airborne Volcanic Ash: Detection, Avoidance, and Mitigation." The gathering will provide the pilot and dispatcher with insights into volcanic ash, including its characteristics, affects on aircraft, detection/tracking, effective warning systems, and mitigation. A "hands-on" exercise will make this the first gathering of its kind to provide lab-style instruction on interpretation of satellite and wind data, and on models of ash trajectory and dispersion. Representatives from the Boeing company, and a host of US government agencies and non-governmental organizations will attend. The workshop will include lectures, demonstrations, laboratory exercises, and a simulated-eruption exercise involving volcanologists, forecasters, controllers, dispatchers, and pilots.

A second international symposium on ash and aviation safety is being planned by the U.S. Geological Survey, organized by Marianne Guffanti. It will be held in Washington, D.C. in September 2003.

References. Casadevall, T.J. (ed.), 1994, Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety: U.S. Geological Survey Bulletin 2047, 450 p.

Gimmestad, G.G., Papanicolopoulos, C.D., Richards, M.A., Sherman, D.L., and West, L.L., 2001, Feasibility study of radiometry for airborne detection of aviation hazards, NASA/CR-2001-210855; Georgia Tech Research Institute, Atlanta, Georgia, 51 p. (URL: http://techreports.larc.nasa.gov/ltrs/PDF/2001/cr/).

Hefter, J.L., 1998, Verifying a volcanic ash forecasting model, Airline Pilot, v. 67, no. 5, pp. 20-23, 54.

International Civil Aviation Organization (ICAO), 2001, Manual on volcanic ash, radioactive material, and toxic chemical clouds, doc 9691-AN/954 (first edition): 999 University St., Montreal, Quebec, Canada H3C 5H7 (purchasing information: sales_unit@icao.int).

Rose, W.I., Bluth, J.S.G., Schneider, D.L., Ernst, G.G.J., Riley, C.M., Henderson, L.J., and McGimsey, R.G., 2001, Observations of volcanic clouds in the first few days of atmospheric residence: The 1992 eruptions of Crater Peak, Mount Spurr volcano, Alaska, Jour. of Geology, v. 109, p. 677-694.

Salinas, Leonard J., 2001, Volcanic ash clouds pose a real threat to aircraft safety: United Airlines, Chicago, Illinois (URL: http://www.dispatcher.org/library/VolcanicAsh.htm).

Simkin, T., and Siebert, L., 1994, Volcanoes of the World, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Streett, D., 1999, Satellite-based Volcanic Ash Advisories and an Ash Trajectory Model from the Washington VAAC: Eighth Conference on Aviation, Range, and Aerospace Meteorology, 10-15 January 1999, Dallas, Texas; American Meteorological Society, p. 290-294.

Washington VAAC, 1999, Operations of the Washington Volcanic Ash Advisory Center, in Aeronautical meteorological offices and their functions, and meteorological observation networks: Third Caribbean/South American Regional Air Navigation Meeting (Car/sam/ran/3); Buenos Aires, Argentina, 5 - 15 October, 1999; International Civil Aviation Organization (http://www.ssd.noaa.gov/ VAAC/PAPERS/carsam.html).

Geologic Background. Volcán Fuego, one of Central America's most active volcanoes, is also one of three large stratovolcanoes overlooking Guatemala's former capital, Antigua. The scarp of an older edifice, Meseta, lies between Fuego and Acatenango to the north. Construction of Meseta dates back to about 230,000 years and continued until the late Pleistocene or early Holocene. Collapse of Meseta may have produced the massive Escuintla debris-avalanche deposit, which extends about 50 km onto the Pacific coastal plain. Growth of the modern Fuego volcano followed, continuing the southward migration of volcanism that began at the mostly andesitic Acatenango. Eruptions at Fuego have become more mafic with time, and most historical activity has produced basaltic rocks. Frequent vigorous eruptions have been recorded since the onset of the Spanish era in 1524, and have produced major ashfalls, along with occasional pyroclastic flows and lava flows.

Information Contacts: Captain Edward Miller, Air Line Pilots Association (ALPA), Air Safety & Engineering Department, 535 Herndon Parkway, PO Box 1169, Herndon, VA 22070-1169, USA; Grace Swanson and Davida Streett, Washington VAAC, Satellite Analysis Branch (NOAA/NESDIS), 4700 Silver Hill Road, Stop 9910, Washington, DC 20233-9910, USA (URL: http://www.ssd.noaa.gov/); Bill Rose, Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA.

Episodes of increased seismicity occurred during December 2000 through September 2001 (BGVN 26:08). Since then seismic activity at Karymsky has remained mostly above background levels. The Concern Color Code remained at Yellow ("volcano is restless") through at least late March 2002. Apparent steam plumes were seen in satellite imagery during January-March 2002.

On 4 January 2002, the Kamchatkan Volcanic Eruption Response Team (KVERT) reported above-background seismicity during the previous week, with 40 to 80 weak local earthquakes occurring per day. Several shallow seismic events suggested gas-ash explosions. Beginning at 1200 on 10 January the number of local earthquakes increased noticeably. During 11-14 January about 200 weak local shallow seismic events occurred per day.

During late January through at least March 2002, the seismic station typically recorded approximately 10 local shallow earthquakes per hour. Around 24 January, the earthquakes became slightly stronger. The character of the seismicity during mid-March suggested weak ash-gas explosions and avalanches.

The Tokyo VAAC reported that on 1 February at 1810 an eruption produced an ash cloud that reached ~7.5 km above the summit and drifted to the E; however, the cloud was not visible on satellite imagery. On 13 February at 0945 a pilot reported an ash cloud to ~3.5 km above the volcano extending to the W. Again, this cloud was not visible on satellite imagery; it may have been a single burst that dissipated rapidly.

No ash was detected in satellite images during the report period; only steam and possible airborne volcanic aerosols were visible during late February. Thermal anomalies and plumes were visible on AVHRR satellite imagery throughout the report period (table 1).

Table 1. Thermal anomalies and plumes visible on AVHRR satellite imagery at Karymsky during January through March 2002. Courtesy KVERT.

General Reference. Khrenov, A.P., and others, 1982, Eruptive activity of Karymsky Volcano over the period of 10 Years (1970-1980): Volcanology and Seismology, no. 4, p. 29-48.

Tokarev, P.I., 1990, Eruptions and seismicity at Karymskii volcano in 1965-1986: Volcanology and Seismology, v. 11, p. 117-134 (in English).

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

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tokyo Volcanic Ash Advisory Center (VAAC),Tokyo, Japan (URL: https://ds.data.jma.go.jp/svd/vaac/data/).

This report discusses activity at Kīlauea during mid-January through mid-April 2002. The flow of lava into the ocean in September (BGVN 26:12) at the Kamoamoa entry and, to a lesser extent, at the E Kupapa'u entry, terminated by the end of January. During February through mid-March lava flowed on the surface at elevations above or on the Pulama Pali slope and along the Kamoamoa lava tube system. Volcanic tremor occurred at moderate levels, and long-period (LP) earthquakes were registered below the caldera. In mid-March incandescence was visible from Pu`u `O`o crater. Incoming lava flooded the floor of Pu`u `O`o crater and excess lava flowed through lava tubes down the Pulama Pali slope to the coastal plain. Heightened activity continued and in early April a series of deflation and inflation events occurred. LP earthquakes increased and on 6 April observations of the crater lake in Pu`u `O`o crater revealed that the lava had risen to 17 m below the E rim.

Geophysical activity. Tiltmeters across the volcano showed no significant deformation until mid-March, although an M 4.1 earthquake had occurred at 0118 on 18 January. The earthquake was located 4 km SSE of the Pu`u `O`o crater at a depth of 9.1 km. During the following several days volcanic tremor remained moderate to strong at Pu`u `O`o. The swarm of long-period (LP) earthquakes at the summit continued through March. Just after 0300 on 27 March a small earthquake beneath the caldera triggered more than 30 minutes of increased tremor and small earthquakes.

Sharp deflation at Pu`u `O`o on 28 March accompanied a change in eruptive activity at the cone. On 31 March volcanic tremor was low-to-moderate at Pu`u `O`o and the continuing weak tremor below the caldera was broken occasionally by small LP earthquakes. Associated tilt across the volcano was flat or only changed slightly.

During 4-6 April, a series of deflation and inflation events occurred. In terms of tilt, at ~2100 on 4 April, Kīlauea's summit deflated (~1.7 µrad); 30 minutes later Pu`u `O`o followed (~9 µrad). This was reversed at 1600 on 5 April when rapid inflation began at the summit and ~12 minutes later at Pu`u `O`o. At 1700 inflation ended at the summit, and it abruptly deflated, as did Pu`u `O`o at 1800. Subsequently, tilt at Pu`u `O`o oscillated three times between rapid deflation and slower inflation. The tilt temporarily decreased, but at 0508 on 6 April another interval of 4.5 oscillations occurred followed by resumed tilt and slow, bumpy inflation. During this turbulent period LP earthquakes increased at the summit while tremor remained steady at Pu`u `O`o. By 7 April tilt was relatively steady, volcanic tremor at Pu`u `O`o was moderate, and tremor at the summit was low to moderate.

Lava flows. During mid-January surface lava traveled along the upper portion of the flow field above the Pulama pali slope and onto the coastal flat. Surface lava also emerged along the Kamoamoa lava tube system and traveled down the Pulama pali slope. These surface flows continued to be visible through February, although the flow reaching the coastal plain had stopped at the end of January. At times during early March several rootless shields (a pile of lava flows built over a lava tube rather than over a conduit feeding magma) were active. During 12-19 March a bright glow was widely visible over Kīlauea from Pu`u `O`o crater and from a rootless shield near 665 m elevation where the most intense surface activity occurred.

By 18 March lava had flooded the floor of Pu`u `O`o crater resulting in surface lava flows and lava flowing through tubes. Lava spread out on the lower fan and adjacent coastal flat, although the fronts of the flows remained ~2.3 km from the ocean. At the base of the lava fan and on the adjacent coastal flat the rootless shields remained active and small surface lava flows persisted through mid-April.

Eruptive activity changed on 28 March, as noted above, and overflight observations revealed new lava located W of the main crater. Observers also saw lava fountaining and forming a circulating pond. By 31 March a lava flow was visible on the floor of Pu`u `O`o crater and several vents were incandescent. During a brief visit to Pu`u `O`o on 6 April observers noticed that the crater lake had risen ~8 m since 29 March (the lake surface was 17 m below the E rim), several cones were active, and lava was flowing into the lava lake from two vents. By the following day activity had decreased and by 8 April incandescence was no longer visible at Pu`u `O`o.

General References. Decker, R.W., Wright, T.L., and Stauffer, P.H., 1987, (eds.), Volcanism in Hawaii: USGS Professional Paper 1350, 1667 p. (64 papers).

Ryan, M.P., 1988, The mechanics and three-dimensional internal structure of active magmatic systems: Kīlauea volcano, Hawaii: JGR, v. 93, p. 4213-4248.

Dzurisin, D., Koyanagi, R.Y., and English, T.T., 1984, Magma supply and storage at Kīlauea volcano, Hawaii, 1956-1983: JVGR, v. 21, no. 3/4, p. 177-207.

Heliker, C., Griggs, J.D., Takahashi, T.J., and Wright, T.L., 1986/7, Volcano monitoring at the U.S. Geological Survey's Hawaiian Volcano Observatory: Earthquakes and Volcanoes, v. 18, no. 1, p. 3-71.

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

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

Activity at Manam remained relatively low following a 4 June 2000 eruption (BGVN 25:07). During August and September 2001, the Rabaul Volcanological Observatory (RVO) reported occasional emissions of weak-to-moderate volumes of white vapor. Weak emissions were also observed from South Crater.

On 13 January 2002 mild eruptive activity began from South Crater. Weak gray-brown ash clouds were emitted at 5-10 minute intervals. Fine, light ashfall was reported on the NE side of the island. Ashfall was reported again on 16, 20, 21, 22, 26, 29, 30, and 31 January, produced by activity ranging from gentle emissions to forceful projections of weak-to-moderate gray-brown and occasionally dark-gray ash clouds, during periods lasting a few hours. Most of the activity was accompanied by weak, deep, low roaring and rumbling noises. Explosion noises were heard on 20 January, and incandescence was observed on 17 and 20 January. The incandescence on 20 January fluctuated but occasionally intensified and projected glowing lava fragments.

Similar mild eruptive activity continued through much of February. Fine ashfall occurred on the SE part of the island. Weak-to-bright incandescence was observed on 13 February, after a loud explosion at 0225. The explosion was also followed by weak roaring and rumbling noises at one-minute intervals lasting for ~1 hour. During 20-24 February activity decreased and continuous weak volumes of white vapor were emitted. Main Crater released only weak-to-moderate volumes of white vapor.

People living in and near the four valleys (especially those in the NE and SE) were urged to be cautious and remain away from the valleys as much as possible.

More recently, a period of activity began in December 1956 that lasted through January 1966. Lava flows and a nuee ardente from the South Crater occurred in June and December 1974, and intermittent moderate explosive activity has continued, with peaks of activity in 1982 and 1984. The water-tube tiltmeter is located at Tabele Observatory, 4 km from the summit on the SW flank.

General References. de Saint Ours, P., 1982, Potential volcanic hazards at Manam Island: Geological Survey of Papua New Guinea Report 82/22, 19 p.

Scott, B.J., and McKee, C.O., 1986, Deformation, eruptive activity, and earth tidal influences at Manam volcano, Papua New Guinea, 1957-1982: Royal Society of New Zealand Bulletin 24, p. 155-171.

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

The following report covers activity at Miyake-jima after February 2001 and through April 2002, but a brief review of previous activity is included. Volcanism renewed at Miyake-jima on 27 June 2000 with a series of underwater eruptions (BGVN 25:05-25:07). Activity through October 2000 was characterized by intrusive events, collapse of the summit crater, explosive events, and degassing. SO₂ emissions were high and low levels of ash were intermittently emitted (BGVN 25:09). During October 2000 through mid-February 2001 high volumes of volcanic gas emission continued (BGVN 26:02).

After a large eruption on 18 August 2000, large amounts of volcanic gas, especially SO₂, started to discharge. SO₂ flux monitoring by COSPEC V has been conducted since 26 August 2000. Monitoring is performed almost daily by the Japanese Meteorological Agency (JMA) and Geological Survey of Japan (GSJ). Figure 14 shows the daily average of SO₂ flux during August 2000 through April 2002. The mean flux during September-December 2000 was ~40,000 metric tons per day (t/d). During 2000, the average SO₂ flux was 40,000 t/d, and it decreased to 21,000 t/d during 2001. As of March 2002 the average SO₂ flux was 10,000-20,000 t/d. During the 26 September 2001 explosion the amount of SO₂ degassing was ~15,000 t/d.

Figure 14. Daily averages of SO2 flux at Miyake-jima during August 2000 through April 2002. Courtesy JMA, GSJ, and KSVO.

In November 2001 the GSJ reported that the strong eruptions of July-August 2000, initially thought to be solely phreatic, had been phreatomagmatic. For instance, tephra from the 18 August 2000 eruption was ~30-40% juvenile in nature. Following the 29 August 2000 eruption, activity was much weaker. A summit pit crater or caldera, formed as a result of "drain back" as magma was intruded elsewhere, stabilized in August 2000 with a diameter of 1.4 km.

On 16 March 2001 the JMA reported that the strongest tremor since 29 August 2000 had occurred. Three days later, on 19 March, an eruption produced a black ash cloud that rose 800 m above the volcano.

In mid-May 2001, based on information from the JMA, the Volcanic Research Center reported that no ash clouds had been observed since 19 March. They also reported that steam plumes with abundant SO₂ were continuously emitted from the summit caldera to 0.5-2 km above the caldera rim. Continuous SO₂ emission released as much as 33,000 to 46,000 metric tons of SO₂ per day (figure 14). Low-level seismic activity continued; on 5 May 2001 a total of 446 small low-frequency earthquakes were registered, and on 7 May an M 2.8 earthquake occurred. Global positioning system (GPS) measurements showed steady, continuous deflation of the volcano, though the rate was lower than before September 2000. Very small collapses of the caldera rims were occasionally seen during air inspections.

About 20 small eruptions were recorded by the JMA during 2001 through the end of March 2002. Major eruptions were reported on 27 May, 26-27 September, and 16 October 2001. The eruptions produced ash plumes up to 1,500 m high.

On 25 January 2002 at 1015 Air Force Weather Agency staff detected a faint E-drifting volcanic plume emanating from Miyake-jima on MODIS imagery (figure 15).

Figure 15. MODIS imagery on 25 January 2002 at 1015 shows a plume drifting E from Miyake-jima. Courtesy AFWA and NASA.

The GSJ reported that as of 1 April 2002, eruptions were continuing and significant quantities of SO₂ (10,000-20,000 t/d) discharged from the summit pit crater (figure 14). According to a news report, a minor eruption occurred at Miyake-jima on 2 April shortly after 1000. While evacuated residents were visiting the island, ash rose to ~300 m above the volcano and fell around Miyake-jima. The island is currently uninhabited because activity that began on 26-27 June 2000 prompted officials to order an evacuation on 1 September 2000.

Geologic Background. The circular, 8-km-wide island of Miyakejima forms a low-angle stratovolcano that rises about 1,100 m from the sea floor in the northern Izu Islands about 200 km SSW of Tokyo. The basaltic volcano is truncated by small summit calderas, one of which, 3.5 km wide, was formed during a major eruption about 2,500 years ago. Numerous craters and vents, including maars near the coast and radially oriented fissure vents, are present on the flanks. Frequent eruptions have been recorded since 1085 CE at vents ranging from the summit to below sea level, causing much damage on this small populated island. After a three-century-long hiatus ending in 1469 CE, activity has been dominated by flank fissure eruptions sometimes accompanied by minor summit eruptions. A 1.6-km-wide summit crater was slowly formed by subsidence during an eruption in 2000.

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); Setsuya Nakada and Hidefumi Watanabe, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Akihiko Tomiya, Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 7, Tsukuba 305-8567, Japan (URL: http://staff.aist.go.jp/a.tomiya/miyakeE.html); Charles Holliday, Air Force Weather Agency (AFWA),106 Peacekeeper Dr., Ste 2NE; Offutt AFB, NE 68113-4039 USA; National Aeronautics and Space Administration (NASA), Washington, DC 20456-0001, USA (URL: https://www.nasa.gov/); Kusatsu-Shirane Volcano Observatory (KSVO), Tokyo Institute of Technology, Kusatsu, Agatsuma-gun, Gunma 377-17, Japan; Dow Jones News.

During 17-18 January 2002 Nyiragongo, located ~18 km N of Lake Kivu in the Democratic Republic of Congo (DRC) close to the border of Rwanda, erupted an estimated 20 x 10⁶ m³ volume of lava (BGVN 26:12). Lava flows originated from the central crater and from several locations along a huge system of fractures that rapidly developed along the entire S flank of the volcano down to the city of Goma (~400,000 people) on the N shore of Lake Kivu. Goma was then invaded by extensive lava flows, which destroyed at least 13% of the surface of the city before reaching the lake. The advancing lava flows caused the spontaneous evacuation of at least 300,000 people, mostly toward the neighboring town of Gisenyi in Rwanda, and left more than 40,000 people homeless. About 100 people died as a consequence (direct or indirect) of the eruption. This disaster required an urgent public health response to evaluate the hazards to life and health. The assessment was especially important because of the humanitarian crisis in the eastern DRC that led many of the people who had fled from the lava flows to return to the city within one or two days, despite the danger of further eruptive activity or the possible risks arising from the proximity of the cooling lava flows (Baxter, 2002).

The following report, with the exception of the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery, was compiled from field visits carried out by the volcanologists representing the United Nations Office for the Coordination of Human Affairs (UN-OCHA): Dario Tedesco, Paolo Papale, Orlando Vaselli, and Jacques Durieux. The team flew over the volcano and Goma in a helicopter on 22 and 24 January, and made field observations during 23 January-4 February. One team member continued studies on Lake Kivu until 13 February. The report provides an initial reconstruction of the main events of the eruption based on reports from local witnesses, observations and reports by researchers at the Goma Volcano Observatory (GVO), a closer inspection of seismograms recorded by GVO seismic stations, and field work by the UN-OCHA team. The UN-OCHA team worked closely with a French-British team of scientists (Patrick Allard, Peter Baxter, Michel Halbwachs, and Jean-Christophe Komorowski), whose report (Allard and others, 2002) will be summarized in a future Bulletin.

Activity during December 2000-January 2002. Since December 2000 the two GVO seismic stations had recorded long-period (LP) earthquake swarms. Commonly, each LP event was shortly followed by volcanic tremor.

On 6 February 2001 Nyamuragira erupted (BGVN 26:01). Following the 2-week eruption, seismicity did not decrease. On the contrary, volcanic tremor and LP earthquakes reappeared at both stations (Bulengo, BSS, S of Nyiragongo and Katale, KSS, NE of Nyiragongo, ~40 km from Bulengo), without showing any particular pattern, from March 2001 through January 2002. During the 10 months that preceded the January 2002 eruption, it was impossible to distinguish which volcano was the source of the seismicity.

In addition to the increased seismic activity, on 5 October strong vibrations were felt and on 7 October a strong tectonic earthquake (M 3.5-4.0), unusual for this region, was felt by the population. This earthquake was followed a few hours later by high-amplitude volcanic tremor.

Field trips by GVO personnel to the summit of Nyiragongo during 29 October-1 November and 8-10 December 2001 revealed several important changes. Immediately after the October tectonic event a white plume was visible inside the 1977 eruptive fracture on top of and inside the Shaheru crater (2 km S of the summit crater) at 2,700 m elevation. A similar plume was emerging from new cracks in the inner wall of the central crater, while a dark plume was visible from the small 1995 spatter cone inside the volcano. The new fumaroles on the side of the small central crater had a temperature of 70°C; the ground temperature, which was normally at 5-9°C, was recorded as 28°C; cracks near Shaheru crater were at ~50°C.

An increase in seismicity similar to that observed during early October occurred beginning on 4 January 2002, when an M > 4 earthquake occurred. On 7 January seismic activity triggered some visible changes, including reactivation of fumarolic activity. Following the 4 January earthquake seismicity remained high until 16 January. The onset of the eruption was preceded by about 8 hours of calm consisting of very low seismicity (without tremor or LP earthquakes).

Eruption during 17-18 January 2002. Early on 17 January a huge system of fractures began to develop on the S flank and eventually extended from high elevations down to Goma and Lake Kivu. The main trend of fractures was N15°W, which is similar to the general trend of the rift valley in the Nyiragongo region. The system of fractures appears to have propagated down from the volcano, starting at 2,800-3,000 m elevation (300-400 m N of the old Shaheru crater), and covering a distance of ~10 km towards Goma in less than 8 hours. The uppermost portion of the fracture started erupting at 0835 on 17 January. A single fracture ~2 m wide was visible above and inside Shaheru. Reactivated during the January event, it corresponded to the eruptive fracture of January 1977. Below the Shaheru crater the fracture was replaced by a more complex system, with two parallel main fractures ~300 m apart, also propagating N15°W. At about 1000 the system of fractures had reached ~1,900 m elevation; they then took ~30 minutes to cross the last system of ancient cones N of Goma, at ~1,700 m. By about 1400 the fractures were in the proximity of the Munigi village on the outskirts of Goma.

As the fractures advanced, several started erupting. Beginning around 1000 and lasting for at least two hours, lava flows escaped at ~1,950-2,000 m elevation S of the ancient Kanyambuzi-Mudjoga tuff cones. Fractures on the gentle slopes N of Goma lacked venting lava along a portion a few kilometers in length, but, at several locations the tips of intruded dykes ascended within only a few ten's of centimeters to a few meters below the ground surface.

At 1610 on 17 January the southernmost system of vents opened S of Munigi village and on the outskirts of Goma at 1,570-1,580 m elevation. Lava flows from these vents affected the Goma airport and destroyed part of the city before reaching the lake. About 10-20 minutes later a new N15°W-oriented fracture began to erupt at 1,950-2,000 m elevation, located ~1.5 km W of the main fracture system. Lava flows from this fracture partially destroyed the western part of Goma without reaching the lake.

Around the same time a fracture a few hundred meters long opened on the NW flank of the cone, at ~2,700-3,000 m elevation as estimated by helicopter, producing a lava flow that did not affect inhabited areas. Unlike the southern fracture system, this fracture had an estimated N30°E to N60°E trend. The eruptive activity at several places along the fracture system on the S flank reportedly lasted through part of the night or even up to daylight on 18 January.

MODIS images on 18, 21, and 27 January showed a hot spot in the vicinity of the volcano (figure 13). The 18 January image showed a single, large hot spot (3-4 pixels wide and ~11 pixels long on the S flank of Nyiragongo. The elongate hot spot had a NNE-SSW orientation. Its proximal section was located about mid-way up the S flank of the volcano (in the vicinity of the Shaheru-Djoga cinder cones and 1977 S-flank fissure) and the distal section was directly over Goma. The hot spot decreased over the following days, and by 27 January was barely visible.

Figure 13. Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images on 18, 21, and 27 January showing a hot spot near Nyiragongo during the eruption. The 18 January image shows a single, large hot spot (3-4 pixels wide and ~ 11 pixels long) on the S flank. The hot spot decreased over the following days, and by 27 January was barely visible. Courtesy HIGP/SOEST.

Activity following the eruption. During the night of 21 January some earthquake swarms created panic in the population. During about 0600-1800 on 22 January no earthquakes were felt. However, at about 1800 a sequence began during which ~20 earthquakes were felt in less than 3 hours. The following days were characterized by periods of intense seismicity, alternating with intervals (~10-15 hours) of lower seismic activity.

Seismic events showed quite a range of magnitude, frequency content, and S-P times, suggesting several different origins. The earthquakes that occurred in the days following the eruption were ~M > 5. During the 5 days following the eruption ~100 tectonic earthquakes (M > 3.5) occurred with epicenters in the Goma-Nyiragongo region. Several houses collapsed or were seriously damaged due to seismic loading in Gisenyi, where nine people died as a direct result of roof collapse. Seismic signals showed a typical sequence of tectonic events, followed within minutes to hours by LP events and volcanic tremor lasting continuously for more than 10 hours. Such a sequence, which is identical to the typical sequence recorded during the year preceding the eruption, suggested rock fracturing followed by the intrusion and ascent of magma.

An estimate of the location of seismic events made by comparing the arrival time differences of P- and S-waves at the three seismic stations of Bulengo, Katale, and Goma (the last one provided by the French-British team) showed that on 27 and 28 January the shocks were concentrated in the area between the Nyiragongo crater and Goma, probably 4-6 km N of Lake Kivu. During the first days of February the epicenters of earthquakes were clustered in two areas. The first was along the system of fractures, stretching a few kilometers N and S of Goma (in part, beneath Lake Kivu). The second cluster lay S of the town of Sake, 15-20 km W of Goma, close to the W border of the rift.

Observations during a helicopter flight over the volcano on 21 January revealed that the crater floor (old surface of the 1994-95 lava lake) was cut by a N-S elongated depression (like a small graben) underlain by steaming cracks. The next day witnesses from villages 6 km SW and ~10 km N of the summit crater reported that at about 2100 an earthquake was felt and immediately followed by a red glow or "flames" above the crater. Later that evening fine ash fell along the road from Goma to Sake. On the morning of 23 January ash fell in Goma and at the airport. A helicopter survey revealed a layer of ash on the volcano's upper S and E flanks and devastated vegetation on the crater's N flank.

On 24 January the inside of the crater appeared as a huge cavity with an estimated depth of 800 m and a grayish-yellow colored floor. The old terraces that represented the remnants of the lava lake at different stages of its long history were nearly totally absent. The interpretation is that either the emptying of the lava lake, the fracturing of the system, or more likely both, destabilized the inner part of the crater. Seismic records showed higher-amplitude tremor, presumably related to the crater-floor collapses, starting on 22 January at 2051 and lasting for ~4 hours. During the same flight on 24 January an explosion or a new collapse at the crater produced a small cloud of steam and ash that reached a few hundred meters above the crater rim.

Gas blasts sporadically occurred, mainly after the eruption, ~100-200 m away from the main path of the lava flow in Goma causing displacement of pavement in small buildings. In a small garage in Goma a thick (~10 cm) concrete pavement was cracked and uplifted up to 50-70 cm by the gas blasts.

Observations and research at Lake Kivu. Interesting phenomena were noted on the shore of Lake Kivu in the area of Himbi. Women who used to wash clothes close to the lake observed that the water level had decreased a few centimeters 3-5 days before the eruption. During the night and early morning of 17-18 January waiters at a restaurant observed crabs and crayfish jumping out of the lake. During 20-21 January dead fish and bubbling spots were observed in the small bay close to the restaurant. Waiters stated that the temperature of the lake in those zones had increased and a brown-black stain appeared from the bottom of the lake in an area of several square meters. At the same time an odor of spoiled eggs (suggesting presence of H₂S) was present. A similar phenomenon occurred early in the morning (at about 0230) on 30 January after an M 4.5 earthquake. The owners of the restaurant suddenly awoke due to difficulty breathing and the strong smell of rotten eggs. In both cases, the "normal" conditions were reestablished in a "short" (not well-defined) time. Soil-gas measurements indicated that CO₂ concentrations reached values up to 20%.

A geochemical survey of the main fluid manifestations in and around the S part of Nyiragongo was carried out during the days after the 17 January eruption. Sampling included water from Lake Kivu, cold and thermal springs, dry vents, and soil gases. The main conclusions of this survey were: 1) contrary to rumors following the entry of lava flows into the lake, lake water only a few meters away from the lava front was not polluted; 2) the geochemical characteristics of water from the lake and from springs on land are close to those found previously by Tietze and others (1980) and Tuttle and others (1990); and 3) the analyzed gases appear to have a clear magmatic origin as revealed by their isotopic signature. An exception to the final conclusion is that gases from the Rambo sampling point, which sits next to a brewery in Gisenyi, appear to represent mixtures between biogenic (organic) and magmatic sources.

Immediately following the eruption, fishermen reported visible changes in the water level at Lake Kivu. This was confirmed by women who visited the lake shore daily for washing or collecting water. A series of measurements was started in close cooperation with the UN field officer Dominic Garcin, who has a vast knowledge of the lake. The first measurements were obtained on 28 January along the shoreline from Gisenyi to Sake. The results confirmed a significant subsidence of the ground and rise of the water level. The subsidence was highest (37 cm) in Goma, then progressively vanished towards the E (~10 cm in Gisenyi but 0 cm ~2 km farther E) and W (0 cm ~15 km W of Goma). After these first measurements, several others were systematically taken along the N shoreline of Lake Kivu as well as around the lake up to ten's of kilometers S of Goma, and at Idjwi Island, ~30 km S of Goma. The measurements showed that ground subsidence is an ongoing phenomenon in the entire rift area near Lake Kivu. One month after the eruption, maximum measured ground subsidence was 50-60 cm at the Goma harbor and at Idjwi Island (figure 14). Scientists note that the February-March westward shifting of the ground subsidence corresponds to a similar westward shifting of epicenters.

Figure 14. Plots showing cumulative, asymmetrical land subsidence in the Goma area across the far S flanks of Nyiragongo. The subsidence was measured with respect to pre-subsidence surface of Lake Kivu, and surveyed along an E-W transect. The four survey dates (all in early 2002) each established the greatest subsidence at Goma, and lesser subsidence towards the East African rift's margins. Courtesy UN-OCHA team.

Conclusions. The characteristics of the erupted products and associated lava flows, together with the available information on the state of the volcano prior the eruption, suggest different dynamics of magma emission in the upper and lower portions of the fracture system. Two different interpretations have been offered. The "magmatic" hypothesis is that the energy for intense fracturing of the volcanic apparatus came from deep magma pushing from below inside the volcanic system. The "tectonic" hypothesis is that the energy for fracturing came from a non-volcanic source and was due to a rifting event related to regional tectonics. Further discussion of both hypotheses is offered in the report by the UN-OCHA team (Tedesco and others, 2002).

Goma is considered at a very high risk should reactivation of the volcano and rift system occur. The security of its inhabitants cannot be guaranteed even with the most sophisticated instrumental network. Thousands of lives can be saved by increased surveillance, continued scientific investigation, contingency plans, and information campaigns.

An outstanding effort by the UN and several donors allowed for the development of short and long-term aid to the Goma Volcano Observatory (GVO). This effort included support for foreign volcanologists, purchase of new equipment, establishing a new monitoring network, and help with logistics (transportation, communications, salaries, etc.). GVO is now publishing weekly reports on volcano activities.

References. Allard, P., Baxter, P., Halbwachs, M., and Komorowski, J-C, 2002, Final report of the French-British scientific team: submitted to the Ministry for Foreign Affairs, Paris, France, Foreign Office, London, United Kingdom and respective Embassies in Democratic Republic of Congo and Republic of Rwanda, 24 p.

Baxter, P.J., 2002, Eruption at Nyiragongo volcano, Democratic Republic of Congo, 17-18 January 2002: a report on a field visit to assess the initial hazards of the eruption, 15 p.

Tedesco, D., Papale, P., Vaselli, O., and Durieux, J., 2002, The January 17th, 2002 eruption of Nyiragongo, Democratic Republic of Congo: UN-OCHA report, 17 p.

Tietze, K., Geyh, M., Muller, H., Schroder, L. Stahl, W., and Wehner, H., 1980, Geol. Rund., v. 69, p. 452-472.

Tuttle, M.L., Lockwood, J.P., and Evans, W.C., 1990, Natural hazards associated with Lake Kivu in Rwanda and Zaire, central Africa: U.S. Geological Survey, Open File Report 90-691, 47 p.

General Reference. Tazieff, H., 1979, Nyiragongo, the forbidden volcano, translated by J. F. Bernard: Barron's, New York, 287 p. Originally published as Nyiragongo: Flammarion, Paris, 1975.

Geologic Background. The Nyiragongo stratovolcano contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Dario Tedesco, Dipartimento Scienze Ambientali, Seconda Universita' di Napoli, Via Vivaldi 43, 81100 Caserta, Italy; Paolo Papale, Istituto Nazionale di Geofisica e Vulcanologia, via S. Maria, 53, I-56126 Pisa, Italy; Orlando Vaselli, Dipartimento Scienze della Terra, via G. La Pira, 4, 50121 Firenze, Italy; Jacques Durieux, Groupe d'Etude des Volcans Actifs, 6, Rue des Razes 69320 Feyzin, France; Dieudonné Wafula, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DRC; Andy Harris, Eric Pilger and Luke Flynn, Hawaii Institute of Geophysics and Planetology at the School of Ocean and Earth Science Technology (HIGP/SOEST), University of Hawaii, 2525 Correa Road, Honolulu, HI 96822 USA; Peter Baxter, Department of Community Medicine, Institute of Public Health, University of Cambridge, University Forvie Site, Robinson Way, Cambridge, CB2 2SR, United Kingdom.

At least two commercial aircraft flew through airborne ash near the Guatemala City airport on 21 May 1999. An eruption from Fuego that day was the first at that volcano since 1987 (BGVN 24:04). Although the aviator's reports attributed the ash from this encounter to Fuego, their aircraft intersected multiple ash plumes in widely different locations, and thus they may also have crossed plumes from Pacaya. The most likely plume near the southern approach to the airport (La Aurora, ~23 km N of Pacaya; ~40 km NE of Fuego) is from the almost constantly active Pacaya. In contrast, Fuego lies 32 km W of Pacaya and was the likely source of a plume intersected later during the flight, at higher altitude, and for much longer duration.

During the encounters the ~100 or more people on board the two aircraft, and many more on the ground, were at risk. Both encounters seriously damaged the aircraft but ended in safe landings without reported injuries. Volcanic ash can cause jet-engine failure, which creates a hazard not only to passengers, but to people on the ground as well. The risk in this situation was amplified by the airport's proximity to urban Guatemala City (population, >1.1 million).

This example leads to two conclusions discussed further below. First, ash avoidance methodology needs further refinement. Second, there exists an apparent bias towards under-reporting of aircraft-ash encounters, which could short-change their cost to air carriers, their perceived risk, and their funding allocation.

The original report of an aircraft-ash encounter was brought to our attention by Captain Edward Miller of the Air Line Pilots Association and provided on the condition that the air carrier remain anonymous. Bulletin editors also wish to acknowledge conversations with several additional anonymous contacts (commercial pilots). This case occurred in Guatemala; however, analogous situations exist at many airports adjacent to volcanoes. Modest eruptions from nearby volcanoes may be uncertain or difficult to see; they may be hard to detect or characterize; yet they still may yield mobile ash plumes carried by complex local winds to confront air traffic (Salinas, 2001; Hefter, 1998; Casadevall, 1994).

Although in most Bulletin reports we favor the use of local time to emphasize the reference frame of people on the scene, most of the source material (satellite and aviation) for this report refer to UTC (Coordinated Universal Time). UTC is the time on the Greenwich meridian (longitude zero), formerly GMT, a term which has fallen out of use. Accordingly, there are cases in this report that lack conversion to local time. Local time in Guatemala is 6 hours behind UTC.

Activity at Fuego and Pacaya. Fuego erupted on 21 May 1999 sending ash to the S, SE, and SW and ultimately dropping up to 40 cm of ash on local settlements (BGVN 24:04). The eruption occurred at 1800 local time (in terms of UTC, at 0000 the next day). Three hours later the eruption decreased and the Aeronautica Civil recommended that planes go no closer to Fuego than 40 km. One hour after that (at 2200 local, 0400 UTC), the atmospheric ash had settled, and Aeronautica Civil recommended flying no closer to Fuego than 15 km.

Bulletin reports for Pacaya in mid-1999 suggested relative calm, in harmony with the observation that it had chiefly been fuming. However, Pacaya is well-known for Strombolian outbursts. It lies directly in-line and only 23 km S of the N10°E-oriented airstrip. As is common for commercial aircraft there, the approaches described below passed very close to Pacaya. Pacaya typically has lower and smaller eruptions than Fuego, but because it lies so close to the southern approach to the airport, Pacaya's ash plumes easily enter the path of landing aircraft. The situation was particularly complex during this eruption because Fuego's ash was reported on cities that lie below the flight path as well as on the Pacaya's flanks.

Pilot's report of aircraft-ash encounter. What follows was taken from one flight crew's description of events and from later reports and dialog on the topic. In order to preserve the confidentiality of the airlines, the precise times of events on 21 May 1999 have been omitted. Pilots reported clear visibility and light wind, with both the capital and the airport in sight. They maneuvered the aircraft for a final approach from the S. Pilots were advised by air traffic control about erupted particulate ("volcanic sand") about 24 km (15 miles) SE of the airport, well away from their projected path to Runway 01. In addition, based on winds they detected as they neared the airport, the pilots concluded that the erupted particulate to the SE was downwind from their projected flight path to the airport. (In retrospect, this conclusion appears tenuous considering the possibility of either a fresh injection of ash from Fuego, or lingering ash in the trailing portion of the plume that lay to the SE.)

At ~32 km (20 miles) distance from the runway, the pilots maneuvered the plane through ~3,000 m (9,700 feet) altitude on a final approach to the airport with the plane's flaps partially extended (at ~15°, which slowed their aircraft to ~260 km/hour (160 mph), its auxiliary power unit (a small jet engine) on, and its landing gear down. Around this point in their descent, pilots saw bright yellow sparks through the windshield lasting a few seconds, a display unlike static electricity, but rather like that from a grinding wheel. This occurred again at 2,500 m (8,200 feet) altitude, but was more intense yet intermittent.

At this time, air-traffic control announced to the pilots that the aircraft landing in front of them had encountered volcanic particulate; they instructed the pilots to abort their landing and climb to 3,350 m (11,000 feet) altitude. Discussions of options and ash avoidance ensued between pilots and air-traffic control; ash had by this time accumulated on the runway, further complicating landing, even for approaches in the opposite direction. The pilots retracted the landing gear, accelerated to ~410 km/hour (~250 mph), began to climb, and after some discussion with air-traffic control, held on a course NE of the airport. They were subsequently cleared to climb to 6,700 m (22,000 feet) altitude and advised to proceed to an alternate airfield.

During the climb, at ~5,800 m (19,000 feet) altitude en-route to the alternate airport, the aircraft encountered volcanic particulate for 10 minutes. Within that interval the plane spent 2 minutes during ascent engulfed in denser and heavier particulate. During that period, window arcing was constant and the beam of their landing lights revealed a conspicuous cloud of reflecting particles. During ash ingestion the engines's speed lacked noticeable fluctuations. The aircraft exited the plume at about 6,100 m (20,000 feet) altitude and landed without encountering additional ash.

Upon landing, the pilots noticed reduced visibility through abraded windshields. Post-flight examination of the airplane revealed heavy damage, requiring the replacement of the engines (US $2 million each) and auxiliary-power-unit engine. (Note, however, that no deterioration in engine power or performance was noticed during the flight.) Other replaced parts included windshields, a heat exchanger, and coalescer bags. Minor damage was seen on the horizontal stabilizer and wing leading-edges.

Volcanic Ash Advisory Statements. The U.S. National Oceanic and Atmospheric Administration (NOAA), Satellite Services Division website contains two archived statements issued by the Volcanic Ash Advisory Center (VAAC) at Washington, D.C. for Fuego on 21-22 May 1999. The statements, issued to the aviation cummunity to warn of volcanic hazards, are intended for an audience accustomed to special terminology (figures 31 and 32). In the interest of advancing understanding of how volcanological and atmospheric data get transmitted to aviators, we offer brief explanations for many of the terms used (figure 31).

Figure 31. The first archived Volcanic Ash Advisory Statement (VAAS) for the 21 May 1999 eruption at Fuego, Guatemala. The boxes contain added notes to explain some of the basic conventions and specific details seen here. This Statement currently appears on the NOAA Satellite Services Division website (see "Information Contacts," below). Local names are frequently anglicized, dropping all accents and other non-English characters (eg. México would be written MEXICO). Later Advisories adopted the abbreviation "Z" (pronounced 'Zulu' by aviators) for UTC, as in 1300 UTC written as 1300Z.

Figure 32. A later Volcanic Ash Advisory Statement (VAAS) for the 21 May 1999 eruption at Fuego, Guatemala. The statement, which was issued the next day, discloses that the eruption had then stopped and the hazard status was lowered. The statement appears on the NOAA Satellite Services Division website (see "Information Contacts," below).

The Washington VAAC received first notification of the Fuego eruption from a routine surface weather observation from Guatemala City at 2000 local time (0200 UTC) on 22 May. They issued the first Volcanic Ash Advisory Statement a half-hour later (figure 31). Six hours later they issued the second Advisory Statement (figure 32). The Advisories were composed by staff of the Satellite Analysis Branch, one of the two NOAA components forming the Washington VAAC (Streett, 1999; Washington VAAC).

The section "Details of ash cloud" first says that the "surface observation from Guatemala City indicate that the Fuego volcano is in eruption" and that no additional information is available and then briefly describes in words observations that came from satellite imagery. The first sentence, "No eruption..." is self-explanatory, but highlights a limitation of the method in use that needs to be emphasized to aviators: an eruption may have occurred but its status is not revealed on the imagery. The sentence, "No eruption could be detected due to thunderstorm cloudiness covering the area around the volcano" is self-explanatory. Less clear is the term convective debris. It does not refer to ash; rather, it refers to remnants of thunderstorms. The gist of these latter two sentences is simply that the thunderstorms that covered the area made it challenging or impossible to see the ash on satellite imagery. Central American thunderstorm clouds typically can reach altitudes of more than 12,000 m (40,000 feet), and can mask or obscure airborne ash residing below that level.

Members of the Washington VAAC commented that this eruption demonstrates key problems that can arise when cloudy conditions prevent satellite detection of ash, foiling a primary mode of analysis. In such cases, they rely on ground observers (including observatories and weather observers), pilot reports, and reports from airlines. Thus, their ability to issue useful information in cloudy conditions depends on the quality of communications with local observers, the Meteorological Watch Office, volcanologists, geophysical observatories, and the aviation community.

The Advisory Statement listed México City weather balloon data acquired 1,050 km NW of Guatemala City. In retrospect, the Washington VAAC noted that they generally avoid using such distant sounding data. If a closer sounding cannot be found, they prefer to use upper-level wind forecasts taken from a numerical weather model. In any case, the scarcity of local sounding data presents a challenge to the realistic analysis of airborne ash.

The outlook section says to "see SIGMETS." SIGMETS are the true warnings to aircraft for SIGnificant METeorological events. They are issued by regional Meteorological Watch Offices (MWOs), in this case the MWO (for Guatemala) is in Tegucigalpa, Honduras. SIGMETS were lacking for this eruption; although the Washington VAAC tried to contact the MWO without response. Another complexity confronted at the VAAC is a lack of a single scale for communicating a volcano's hazard status.

Reporting of aviation ash encounters. In personal communications with Bulletin editors, airline personnel stated that many more encounters have occurred than have yet been tallied in publically accessible literature. In accord with those assertions, the 21 May 1999 encounters are absent from reports compiled by the International Civil Aviation Organization (ICAO, 2001). In that document (Appendix I, table 3A, p. I-12) Fuego fails to appear as a source vent for any aircraft-ash encounter. Pacaya is listed for two encounters, in January 1987 and in May 1998 (BGVN 23:05).

Even though the number of encounters was probably under-represented and thus reflects a minimum, ICAO (2001) notes that the international costs to aviation since 1982 summed to well in excess of $250 million. They noted, "In addition to its potential to cause a major aircraft accident, the economic cost of volcanic ash to international civil aviation is staggering. This involves numerous complete engine changes, engine overhauls, airframe refurbishing... aircraft downtime... [and] volcanic ash clearance from airports and the damage caused to equipment and buildings on the ground."

The incidents here suggest that there has been a strong bias toward under-reporting aircraft-ash encounters. If this tentative conclusion is correct, it implies consistent understatements of the hazard's magnitude. This, in turn, may have thwarted meaningful analysis of how and whether to proceed with designing more robust hazard-reduction systems. Accordingly, resources that could have been devoted to the problem have not yet been committed (see Gimmestad and others, 2001 for a discussion of a prototype on-board ash-detection instrument).

Communication challenges. While much of the aviation community needs to learn about volcanism rapidly, dependably, and with the aid of the Internet, some observers charged with reporting volcanic-ash hazards in Central and South America lack access to basic communication devices like reliable telephones and fax machines. To reduce the risks, the aviation, meteorological, remote sensing, and volcanological communities need to improve their ability to pass critical information to each other rapidly and precisely. The operational systems related to volcanic ash and aviation must transcend numerous boundaries (eg., languages, infrastructure, funding, governments, agencies, air carriers, pilots, aircraft manufactures, etc.). The systems need to portray complex, dynamic processes such as the rapid rise of an explosive plume, or large-scale ash-cloud movement.

Although the infrastructure for ash avoidance is greater than ever, members of the Washington VAAC have told Bulletin editors that they still depend heavily on people on the scene of the eruption to notify them promptly when eruptions occur. They said that thus far in parts of Central and South America a problem has been the expense of communication (eg., by phone, fax, and Internet). They also said that for the same regions the U.S. meteorological database regularly lacks pilot reports. Though serious, these problems have at least been identified and their solutions would appear to lack great technical or economic barriers.

The pilots involved in the May 1999 encounter recommended that far more emphasis be placed on forecasting and avoiding ash plumes. Other pilots cited the need for fast and accurate communications between those who observe eruptive activity and air traffic control personnel.

Issues like these continue to be an important subject at gatherings on the topic of ash hazards in aviation (Casadevall, 1994; Streett, D., 1999; Washington VAAC, 1999). The Airline Dispatcher's Federation (ADF) will participate in a 7-9 May 2002 conference and workshop: "Operational Implications of Airborne Volcanic Ash: Detection, Avoidance, and Mitigation." The gathering will provide the pilot and dispatcher with insights into volcanic ash, including its characteristics, affects on aircraft, detection/tracking, effective warning systems, and mitigation. A "hands-on" exercise will make this the first gathering of its kind to provide lab-style instruction on interpretation of satellite and wind data, and on models of ash trajectory and dispersion. Representatives from the Boeing company, and a host of US government agencies and non-governmental organizations will attend. The workshop will include lectures, demonstrations, laboratory exercises, and a simulated-eruption exercise involving volcanologists, forecasters, controllers, dispatchers, and pilots.

A second international symposium on ash and aviation safety is being planned by the U.S. Geological Survey, organized by Marianne Guffanti. It will be held in Washington, D.C. in September 2003.

References. Casadevall, T.J. (ed.), 1994, Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety: U.S. Geological Survey Bulletin 2047, 450 p.

Gimmestad, G.G., Papanicolopoulos, C.D., Richards, M.A., Sherman, D.L., and West, L.L., 2001, Feasibility study of radiometry for airborne detection of aviation hazards, NASA/CR-2001-210855; Georgia Tech Research Institute, Atlanta, Georgia, 51 p. (URL: http://techreports.larc.nasa.gov/ltrs/PDF/2001/cr/).

Hefter, J.L., 1998, Verifying a volcanic ash forecasting model, Airline Pilot, v. 67, no. 5, pp. 20-23, 54.

International Civil Aviation Organization (ICAO), 2001, Manual on volcanic ash, radioactive material, and toxic chemical clouds, doc 9691-AN/954 (first edition): 999 University St., Montreal, Quebec, Canada H3C 5H7 (purchasing information: sales_unit@icao.int).

Rose, W.I., Bluth, J.S.G., Schneider, D.L., Ernst, G.G.J., Riley, C.M., Henderson, L.J., and McGimsey, R.G., 2001, Observations of volcanic clouds in the first few days of atmospheric residence: The 1992 eruptions of Crater Peak, Mount Spurr volcano, Alaska, Jour. of Geology, v. 109, p. 677-694.

Salinas, Leonard J., 2001, Volcanic ash clouds pose a real threat to aircraft safety: United Airlines, Chicago, Illinois (URL: http://www.dispatcher.org/library/VolcanicAsh.htm).

Simkin, T., and Siebert, L., 1994, Volcanoes of the World, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Streett, D., 1999, Satellite-based Volcanic Ash Advisories and an Ash Trajectory Model from the Washington VAAC: Eighth Conference on Aviation, Range, and Aerospace Meteorology, 10-15 January 1999, Dallas, Texas; American Meteorological Society, p. 290-294.

Washington VAAC, 1999, Operations of the Washington Volcanic Ash Advisory Center, in Aeronautical meteorological offices and their functions, and meteorological observation networks: Third Caribbean/South American Regional Air Navigation Meeting (Car/sam/ran/3); Buenos Aires, Argentina, 5 - 15 October, 1999; International Civil Aviation Organization (http://www.ssd.noaa.gov/ VAAC/PAPERS/carsam.html).

Geologic Background. Eruptions from Pacaya are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the older Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1,500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate scarp inside which the modern Pacaya volcano (Mackenney cone) grew. The NW-flank Cerro Chino crater was last active in the 19th century. During the past several decades, activity has consisted of frequent Strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and covered the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit.

Information Contacts: Captain Edward Miller, Air Line Pilots Association (ALPA), Air Safety & Engineering Department, 535 Herndon Parkway, PO Box 1169, Herndon, VA 22070-1169, USA; Grace Swanson and Davida Streett, Washington VAAC, Satellite Analysis Branch (NOAA/NESDIS), 4700 Silver Hill Road, Stop 9910, Washington, DC 20233-9910, USA (URL: http://www.ssd.noaa.gov/); Bill Rose, Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA.

After explosions at Tavurvur during June and August 2001, activity decreased through October. During February-March 2002, the Rabaul Volcanological Observatory (RVO) reported that volcanic and seismic activity remained low, with some low-frequency earthquakes recorded. The active vent emitted weak-to-moderate amounts of white vapor. Ground deformation measurements showed no significant changes.

During mid-February, moderate-to-strong gas emissions drifted to the SE and E and damaged vegetation on the adjacent cone South Daughter, suggesting the presence of volcanic gases like sulphur dioxide within the emissions. The last ash-producing activity from Tavurvur occurred in early September 2001. RVO reported that the chance of mild ash activity occurring in the near future is very remote.

A few tectonic earthquakes were felt during mid-February. They were located 35-80 km NW and SW from the Rabaul-Kokopo area. An M 5.6 tectonic earthquake was felt at 0650 on 17 March. The earthquake was located offshore in the Pomio area. On 21 March, some high-frequency earthquakes occurred NE of Rabaul. Since 1995, these high-frequency earthquakes have been associated with eruptive activity at Tavurvur. During mid-March, some earthquakes were felt, unrelated to volcanic activity, that had magnitudes of 5.3-5.6.

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

During late January to early April 2002, seismic activity at Shiveluch remained above background levels and the lava dome in the active crater continued to grow. Explosions, avalanches, pyroclastic flows, and plumes from combinations of steam, gas, and ash, all occurred without warning and rose as high as 7-10 km altitude. Many shallow earthquakes (ML less than or equal to 2.5) occurred within the volcano's edifice along with other shallow seismic events. The Alert Level was raised from Yellow to Orange during a period of increased activity, 12 February-mid-March. Then the Level was returned to Yellow.

Typical activities from the end of January through the first half of February 2002 included weak seismic events and short-lived explosions, which sent ash-gas plumes to heights of 0.8-1.0 km above the ~2.5 km-high dome (reaching 3.3-3.5 km altitude). Occasional explosions sent plumes to higher altitudes; on 25 January a plume rose to ~4.5 km altitude, while an explosive eruption on 1 February sent ash-gas plumes to heights over 5.0 km.

For the latter eruption, Advanced Very High Resolution Radiometer (AVHRR) satellite images show thermal anomalies up to 10 pixels. Many of these "hot" pixels were at the detector saturation level of 48°C, and appeared against typical background temperatures as low as -25°C. A satellite image on 2 February revealed a 40-pixel thermal anomaly; however, only 5-7 of the pixels had temperatures above 40°C, indicating that only those pixels were influenced by hot material on the ground. The other pixels with elevated temperatures were associated with a cloud caused by hot avalanches.

On the evening of 12 February, the character of the seismicity changed suggesting the occurrence of more intense gas-ash explosions. During the next 30 days, explosions occurred frequently, producing ash-and-gas plumes that rose over 1 km above the dome and occasionally over 3 km above it. On 22 February, a series of shallow seismic events registered for ~1 hour, possibly related to ash-gas explosions or a hot rock avalanche. A similar series was recorded on the night of 27 February, and the next morning observers from Klyuchi town (46 km S) reported a 2-km-long pyroclastic flow to the SE of the dome.

AVHRR images of the resulting plumes showed that some extended to distances of 100 km in various directions, depending upon local wind conditions. A satellite image on 21 February showed a circular ash cloud, 20 km in diameter, at an altitude of ~7.1 km. By 15 March seismic activity declined but remained above background levels. Short-lived explosions continued to send plumes as high as 7.5 km altitude during the last week of March.

Thermal anomalies and pixel size on AVHRR imagery. Dave Schneider (USGS, AVO) provided an explanation of relevant aspects of pixel size and thermal anomalies. The size of a "raw" AVHRR pixel varies across and along the scan of the sensor. The instrument scans from side to side as the satellite moves in orbit. Along the nadir (the line directly beneath the satellite sensor), the pixel size is ~1.1 km on a side. At the far extreme of the scan (55° from nadir) the pixel size increases to 2.4 x 6.5 km. The raw image looks very distorted due to this change in pixel size, so the raw data are resampled to a resolution of 1 x 1 km when processing the data for display. This resampling can generate artifacts, including duplicate "hot" pixels. Analysts usually recognize and account for this problem, and then accurately report the appropriate number of hot pixels.

Another complication is the cause and significance of hot pixels. An AVHRR pixel will begin to appear anomalously warm, compared to its neighbors, when very hot material (hundred's of degrees) occupy a very small percentage of the total pixel area (much less that 1%). So, when a report mentions four hot pixels, and the pixel is 1 km square, one might be tempted to interpret this as 4 km² of hot material. However, this is not correct. Typically, only a very small portion of a pixel area is hot, but sufficiently hot to reach the pixel's saturation value of ~50°C. The numbers of hot pixels are not all that relevant in an absolute sense. In a relative sense, however, the number of hot pixels can be important, for example, during episodes where we have seen anomalies grow from 1-4 hot pixels and then reach 10-20 hot pixels after an eruption.

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

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller and Dave Schneider, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

An eruption occurred at Ulawun during 25-30 April 2001 (BGVN 26:06). Limited evacuations occurred on 3 May due to the occurrence of relatively high seismic activity and the possibility of further volcanic activity. On 14 June the Rabaul Volcanological Observatory (RVO) recommended that the Alert Level be reduced from 2 to 1. At this stage of alert, people could move back to their homes with the approval of the local disaster committee.

Volcanic tremor began at 2200 on 24 September. It peaked at about 1000 on 27 September and generally declined afterward. By 30 September seismic activity was at moderate levels. During 27-30 September a very slow deflationary trend was detected. Volcanic tremor continued to occur at moderate-to-high levels until it dropped dramatically on 11 October. After 11 October seismicity only consisted of discrete low-frequency earthquakes. On 8 and 9 October loud roaring noises emanated from the volcano.

During October through March 2002 activity was generally low. The main crater produced weak-to-moderate volumes of white and white-gray vapor. The N valley vent occasionally released weak volumes of thin white vapor. RSAM data revealed conspicuous seismicity on 26 October; incandescence was observed that night. A rapid trend of deflation beneath the summit area during the previous two weeks stopped on 24 December.

During mid-February tremor increased to moderate levels for the first time since December 2001. On 21 February weak roaring noises were heard and weak incandescence was visible for a short time. After 22 February tremor returned to background levels. Seismic activity returned to background levels after volcanic tremor ceased on 18 March. Discrete low-frequency earthquakes continued to occur in small numbers. The electronic tiltmeter continued to show long-term deflation of the summit area. Based on recent observations, activity at Ulawun is expected to remain low.

General References. Johnson, R.W., Davies, R.A., and White, A.J.R., 1972, Ulawun volcano, New Britain: Australia Bureau of Mineral Resources, Geology and Geophysics, Bulletin 142, 42 p.

McKee, C.O., 1983, Volcanic hazards at Ulawun volcano: Geological Survey of Papua New Guinea Report 83/13, 21 p.

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