The lava lake in the summit crater of Erebus has been active since at least 1972. Located in Antarctica overlooking the McMurdo Station on Ross Island, it is the southernmost active volcano on the planet. Because of the remote location, activity is primarily monitored by satellites. This report covers activity during 2023.

The number of thermal alerts recorded by the Hawai'i Institute of Geophysics and Planetology’s MODVOLC Thermal Alerts System increased considerably in 2023 compared to the years 2020-2022 (table 9). In contrast to previous years, the MODIS instruments aboard the Aqua and Terra satellites captured data from Erebus every month during 2023. Consistent with previous years, the lowest number of anomalous pixels were recorded in January, November, and December.

Table 9. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2023. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the HIGP – MODVOLC Thermal Alerts System.

Sentinel-2 infrared images showed one or two prominent heat sources within the summit crater, accompanied by adjacent smaller sources, similar to recent years (see BGVN 46:01, 47:02, and 48:01). A unique image was obtained on 25 November 2023 by the OLI-2 (Operational Land Imager-2) on Landsat 9, showing the upper part of the volcano surrounded by clouds (figure 32).

Figure 32. Satellite view of Erebus with the summit and upper flanks visible above the surrounding weather clouds on 25 November 2023. Landsat 9 OLI-2 (Operational Land Imager-2) image with visible and infrared bands. Thermal anomalies are present in the summit crater. The edifice is visible from about 2,000 m elevation to the summit around 3,800 m. The summit crater is ~500 m in diameter, surrounded by a zone of darker snow-free deposits; the larger circular summit area is ~4.5 km diameter. NASA Earth Observatory image by Lauren Dauphin, using Landsat data from the U.S. Geological Survey.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: https://earthobservatory.nasa.gov/images/152134/erebus-breaks-through).

Rincón de la Vieja is a volcanic complex in Costa Rica with a hot convecting acid lake that exhibits frequent weak phreatic explosions, gas-and-steam emissions, and occasional elevated sulfur dioxide levels (BGVN 45:10, 46:03, 46:11). The current eruption period began June 2021. This report covers activity during July-December 2023 and is based on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Numerous weak phreatic explosions continued during July-December 2023, along with gas-and-steam emissions and plumes that rose as high as 3 km above the crater rim. Many weekly OVSICORI-UNA bulletins included the previous week's number of explosions and emissions (table 9). For many explosions, the time of explosion was given (table 10). Frequent seismic activity (long-period earthquakes, volcano-tectonic earthquakes, and tremor) accompanied the phreatic activity.

Table 9. Number of reported weekly phreatic explosions and gas-and-steam emissions at Rincón de la Vieja, July-December 2023. Counts are reported for the week before the Weekly Bulletin date; not all reports included these data. Courtesy of OVSICORI-UNA.

Table 10. Summary of activity at Rincón de la Vieja during July-December 2023. Weak phreatic explosions and gas emissions are noted where the time of explosion was indicated in the weekly or daily bulletins. Height of plumes or emissions are distance above the crater rim. Courtesy of OVSICORI-UNA.

According to OVSICORI-UNA, during July-October the average weekly sulfur dioxide (SO₂) flux ranged from 68 to 240 tonnes/day. However, in mid-November the flux increased to as high as 334 tonnes/day, the highest value measured in recent years. The high SO₂ flux in mid-November was also detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 43).

Figure 43. Sulfur dioxide (SO2) maps from Rincón de la Vieja recorded by the TROPOMI instrument aboard the Sentinel-5P satellite on 16 November (left) and 20 November (right) 2023. Mass estimates are consistent with measurements by OVSICORI-UNA near ground level. Some of the plume on 20 November may be from other volcanoes (triangle symbols) in Costa Rica and Nicaragua. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/).

Bezymianny, located on Russia’s Kamchatka Peninsula, has had eruptions since 1955 characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. Activity during November 2022-April 2023 included gas-and-steam emissions, lava dome collapses generating avalanches, and persistent thermal activity. Similar eruptive activity continued from May through October 2023, described here based on information from weekly and daily reports of the Kamchatka Volcano Eruptions Response Team (KVERT), notices from Tokyo VAAC (Volcanic Ash Advisory Center), and from satellite data.

Overall activity decreased after the strong period of activity in late March through April 2023, which included ash explosions during 29 March and 7-8 April 2023 that sent plumes as high as 10-12 km altitude, along with dome growth and lava flows (BGVN 48:05). This reduced activity can be seen in the MIROVA thermal detection system graph (figure 56), which was consistent with data from the MODVOLC thermal detection system and with Sentinel-2 satellite images that showed persistent hotspots in the summit crater when conditions allowed observations. A renewed period of strong activity began in mid-October 2023.

Figure 56. The MIROVA (Log Radiative Power) thermal data for Bezymianny during 20 November 2022 through October 2023 shows heightened activity in the first half of April and second half of October 2023, with lower levels of thermal anomalies in between those times. Courtesy of MIROVA.

Activity increased significantly on 17 October 2023 when large collapses began during 0700-0830 on the E flanks of the lava dome and continued to after 0930 the next day (figure 57). Ash plumes rose to an altitude of 4.5-5 km, extending 220 km NNE by 18 October. A large explosion at 1630 on 18 October produced an ash plume that rose to an altitude of 11 km (8 km above the summit) and drifted NNE and then NW, extending 900 km NW within two days at an altitude of 8 km. Minor ashfall was noted in Kozyrevsk (45 km WNW). At 0820 on 20 October an ash plume was identified in satellite images drifting 100 km ENE at altitudes of 4-4.5 km.

Figure 57. Sentinel-2 satellite images of Bezymianny from 1159 on 17 October 2023 (2359 on 16 October UTC) showing a snow-free S and SE flank along with thermal anomalies in the crater and down the SE flank. Left image is in false color (bands 8, 4, 3); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Lava flows and hot avalanches from the dome down the SE flank continued over the next few days, including 23 October when clear conditions allowed good observations (figures 58 and 59). A large thermal anomaly was observed over the volcano through 24 October, and in the summit crater on 30 October (figure 60). Strong fumarolic activity continued, with numerous avalanches and occasional incandescence. By the last week of October, volcanic activity had decreased to a level consistent with that earlier in the reporting period.

Figure 58. Daytime photo of Bezymianny under clear conditions on 23 October 2023 showing a lava flow and avalanches descending the SE flank, incandescence from the summit crater, and a small ash plume. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 59. Night photo of Bezymianny under cloudy conditions on 23 October 2023 showing an incandescent lava flow and avalanches descending the SE flank. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 60. Sentinel-2 satellite images of Bezymianny from 1159 on 30 October 2023 (2359 on 29 October UTC) showing a plume drifting SE and thermal anomalies in the summit crater and down multiple flanks. Left image is in true color (bands 4, 3, 2); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Aviation warnings were frequently updated during 17-20 October. KVERT issued a Volcano Observatory Notice for Aviation (VONA) on 17 October at 1419 and 1727 (0219 and 0527 UTC) raising the Aviation Color Code (ACC) from Yellow to Orange (second highest level). The next day, KVERT issued a VONA at 1705 (0505 UTC) raising the ACC to Red (highest level) but lowered it back to Orange at 2117 (0917 UTC). After another decrease to Yellow and back to Orange, the ACC was reduced to Yellow on 20 October at 1204 (0004 UTC). In addition, the Tokyo VAAC issued a series of Volcanic Ash Advisories beginning on 16 October and continuing through 30 October.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).chr

Kīlauea is the southeastern-most volcano in Hawaii and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has recently been characterized by lava effusions, spatter, and sulfur dioxide emissions in the active Halema’uma’u lava lake (BGVN 47:08). Lava effusions, some spatter, and sulfur dioxide emissions have continued during this reporting period of July through December 2022 using daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Summary of activity during July-December 2022. Low-level effusions have continued at the western vent of the Halema’uma’u crater during July through early December 2022. Occasional weak ooze-outs (also called lava break outs) would occur along the margins of the crater floor. The overall level of the active lava lake throughout the reporting period gradually increased due to infilling, however it stagnated in mid-September (table 13). During September through November, activity began to decline, though lava effusions persisted at the western vent. By 9 December, the active part of the lava lake had completely crusted over, and incandescence was no longer visible.

Table 13. Summary of measurements taken during overflights at Kīlauea that show a gradual increase in the active lava lake level and the volume of lava effused since 29 September 2021. Lower activity was reported during September-October. Data collected during July-December 2022. Courtesy of HVO.

Activity during July 2022. Lava effusions were reported from the western vent in the Halema’uma’u crater, along with occasional weak ooze-outs along the margins of the crater floor. The height of the lava lake was variable due to deflation-inflation tilt events; for example, the lake level dropped approximately 3-4 m during a summit deflation-inflation event reported on 1 July. Webcam images taken during the night of 6-12 July showed intermittent low-level spattering at the western vent that rose less than 10 m above the vent (figure 519). Measurements made during an overflight on 7 July indicated that the crater floor was infilled about 130 m and that 95 million cubic meters of lava had been effused since 29 September 2021. A single, relatively small lava ooze-out was active to the S of the lava lake. Around midnight on 8 July there were two brief periods of lava overflow onto the lake margins. On 9 July lava ooze-outs were reported near the SE and NE edges of the crater floor and during 10-11 July they occurred near the E, NE, and NW edges. On 16 July crater incandescence was reported, though the ooze-outs and spattering were not visible. On 18 July overnight webcam images showed incandescence in the western vent complex and two ooze-outs were reported around 0000 and 0200 on 19 July. By 0900 there were active ooze-outs along the SW edge of the crater floor. Measurements made from an overflight on 19 July indicated that the crater floor was infilled about 133 m and 98 million cubic meters of lava had erupted since 29 September 2021 (figure 520). On 20 July around 1600 active ooze-outs were visible along the N edge of the crater, which continued through the next day. Extensive ooze-outs occurred along the W margin during 24 July until 1900; on 26 July minor ooze-outs were noted along the N margin. Minor spattering was visible on 29 July along the E margin of the lake. The sulfur dioxide emission rates ranged 650-2,800 tons per day (t/d), the higher of which was measured on 8 July (figure 519).

Figure 519. Minor spattering rising less than 10 m was visible at the E end of the lava lake within Halema‘uma‘u, at the summit of Kīlauea on 8 July 2022. Sulfur dioxide is visible rising from the lake surface (bluish-colored fume). A sulfur dioxide emission rate of approximately 2,800 t/d was measured on 8 July. Courtesy of K. Mulliken, USGS.

Figure 520. A helicopter overflight on 19 July 2022 allowed for aerial visible and thermal imagery to be taken of the Halema’uma’u crater at Kīlauea’s summit crater. The active part of the lava lake is confined to the western part of the crater. The scale of the thermal map ranges from blue to red, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

Activity during August 2022. The eruption continued in the Halema’uma’u crater at the western vent. According to HVO the lava in the active lake remained at the level of the bounding levees. Occasional minor ooze-outs were observed along the margins of the crater floor. Strong nighttime crater incandescence was visible after midnight on 6 August over the western vent cone. During 6-7 August scattered small lava lobes were active along the crater floor and incandescence persisted above the western vent through 9 August. During 7-9 August HVO reported a single lava effusion source was active along the NW margin of the crater floor. Measurements from an overflight on 4 August indicated that the crater floor was infilled about 136 m total and that 102 million cubic meters of lava had been erupted since the start of the eruption. Lava breakouts were reported along the N, NE, E, S, and W margins of the crater during 10-16 August. Another overflight survey conducted on 16 August indicated that the crater floor infilled about 137 m and 104 million cubic meters of lava had been erupted since September 2021. Measured sulfur dioxide emissions rates ranged 1,150-2,450 t/d, the higher of which occurred on 8 August.

Activity during September 2022. During September, lava effusion continued from the western vent into the active lava lake and onto the crater floor. Intermittent minor ooze-outs were reported through the month. A small ooze-out was visible on the W crater floor margin at 0220 on 2 September, which showed decreasing surface activity throughout the day, but remained active through 3 September. On 3 September around 1900 a lava outbreak occurred along the NW margin of the crater floor but had stopped by the evening of 4 September. Field crews monitoring the summit lava lake on 9 September observed spattering on the NE margin of the lake that rose no higher than 10 m, before falling back onto the lava lake crust (figure 521). Overflight measurements on 12 September indicated that the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had been erupted since September 2021. Extensive breakouts in the W and N part of the crater floor were reported at 1600 on 20 September and continued into 26 September. The active part of the lava lake dropped by 10 m while other parts of the crater floor dropped by several meters. Summit tiltmeters recorded a summit seismic swarm of more than 80 earthquakes during 1500-1800 on 21 September, which occurred about 1.5 km below Halema’uma’u; a majority of these were less than Mw 2. By 22 September the active part of the lava lake was infilled about 2 m. On 23 September the western vent areas exhibited several small spatter cones with incandescent openings, along with weak, sporadic spattering (figure 522). The sulfur dioxide emission rate ranged from 930 t/d to 2,000 t/d, the higher of which was measured on 6 September.

Figure 521. Photo of spattering occurring at Kīlauea's Halema’uma’u crater during the morning of 9 September 2022 on the NE margin of the active lava lake. The spatter material rose 10 m into the air before being deposited back on the lava lake crust. Courtesy of C. Parcheta, USGS.

Figure 522.The active western vent area at Kīlauea's Halema’uma’u crater consisted of several small spatter cones with incandescent openings and weak, sporadic spattering. Courtesy of M. Patrick, USGS.

Activity during October 2022. Activity during October declined slightly compared to previous months, though lava effusions persisted from the western vent into the active lava lake and onto the crater floor during October (figure 523). Slight variations in the lava lake were noted throughout the month. HVO reported that around 0600 on 3 October the level of the lava lake has lowered slightly. Overflight measurements taken on 5 October indicated that the crater floor was infilled a total of about 143 m and that 111 million cubic meters of lava had been effused since September 2021. During 6-7 October the lake gradually rose 0.5 m. Sulfur dioxide measurements made on 22 October had an emission rate of 700 t/d. Another overflight taken on 28 October showed that there was little to no change in the elevation of the crater floor: the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had erupted since the start of the eruption.

Figure 523. Photo of the Halema’uma’u crater at Kīlauea looking east from the crater rim showing the active lava lake, with active lava ponds to the SE (top) and west (bottom middle) taken on 5 October 2022. The western vent complex is visible through the gas at the bottom center of the photo. Courtesy of N. Deligne, USGS.

Activity during November 2022. Activity remained low during November, though HVO reported that lava from the western vent continued to effuse into the active lava lake and onto the crater floor throughout the month. The rate of sulfur dioxide emissions during November ranged from 300-600 t/d, the higher amount of which occurred on 9 November.

Activity during December 2022. Similar low activity was reported during December, with lava effusing from the western vent into the active lava lake and onto the crater floor. During 4-5 December the active part of the lava lake was slightly variable in elevation and fluctuated within 1 m. On 9 December HVO reported that lava was no longer erupting from the western vent in the Halema’uma’u crater and that sulfur dioxide emissions had returned to near pre-eruption background levels; during 10-11 December, the lava lake had completely crusted over, and no incandescence was visible (figure 524). Time lapse camera images covering the 4-10 December showed that the crater floor showed weak deflation and no inflation. Some passive events of crustal overturning were reported during 14-15 December, which brought fresh incandescent lava to the lake surface. The sulfur dioxide emission rate was approximately 200 t/d on 14 December. A smaller overturn event on 17 December and another that occurred around 0000 and into the morning of 20 December were also detected. A small seismic swarm was later detected on 30 December.

Figure 524. Photo of Halema’uma’u crater at Kīlauea showing a mostly solidified lake surface during the early morning of 10 December 2022. Courtesy of J. Bard, USGS.

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

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Nyamulagira (also known as Nyamuragira) is a shield volcano in the Democratic Republic of Congo with the summit truncated by a small 2 x 2.3 km caldera with walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from numerous flank fissures and cinder cones. The current eruption period began in April 2018 and has more recently been characterized by summit crater lava flows and thermal activity (BGVN 48:05). This report describes lava flows and variable thermal activity during May through October 2023, based on information from the Observatoire Volcanologique de Goma (OVG) and various satellite data.

Lava lake activity continued during May. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system recorded moderate-to-strong thermal activity throughout the reporting period; activity was more intense during May and October and relatively weaker from June through September (figure 95). The MODVOLC thermal algorithm, detected a total of 209 thermal alerts. There were 143 hotspots detected during May, eight during June, nine during September, and 49 during October. This activity was also reflected in infrared satellite images, where a lava flow was visible in the NW part of the crater on 7 May and strong activity was seen in the center of the crater on 4 October (figure 96). Another infrared satellite image taken on 12 May showed still active lava flows along the NW margin of the crater. According to OVG lava effusions were active during 7-29 May and moved to the N and NW parts of the crater beginning on 9 May. Strong summit crater incandescence was visible from Goma (27 km S) during the nights of 17, 19, and 20 May (figure 97). On 17 May there was an increase in eruptive activity, which peaked at 0100 on 20 May. Notable sulfur dioxide plumes drifted NW and W during 19-20 May (figure 98). Drone footage acquired in partnership with the USGS (United States Geological Survey) on 20 May captured images of narrow lava flows that traveled about 100 m down the W flank (figure 99). Data from the Rumangabo seismic station indicated a decreasing trend in activity during 17-21 May. Although weather clouds prevented clear views of the summit, a strong thermal signature on the NW flank was visible in an infrared satellite image on 22 May, based on an infrared satellite image. On 28 May the lava flows on the upper W flank began to cool and solidify. By 29 May seismicity returned to levels similar to those recorded before the 17 May increase. Lava effusion continued but was confined to the summit crater; periodic crater incandescence was observed.

Figure 95. Moderate-to-strong thermal anomalies were detected at Nyamulagira during May through October 2023, as shown on this MIROVA graph (Log Radiative Power). During late May, the intensity of the anomalies gradually decreased and remained at relatively lower levels during mid-June through mid-September. During mid-September, the power of the anomalies gradually increased again. The stronger activity is reflective of active lava effusions. Courtesy of MIROVA.

Figure 96. Infrared (bands B12, B11, B4) satellite images showing a constant thermal anomaly of variable intensities in the summit crater of Nyamulagira on 7 May 2023 (top left), 21 June 2023 (top right), 21 July 2023 (bottom left), and 4 October 2023 (bottom right). Although much of the crater was obscured by weather clouds on 7 May, a possible lava flow was visible in the NW part of the crater. Courtesy of Copernicus Browser.

Figure 97. Photo of intense nighttime crater incandescence at Nyamulagira as seen from Goma (27 km S) on the evening of 19 May 2023. Courtesy of Charles Balagizi, OVG.

Figure 98. Two strong sulfur dioxide plumes were detected at Nyamulagira and drifted W on 19 (left) and 20 (right) May 2023. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 99. A map (top) showing the active vents (yellow pins) and direction of active lava flows (W) at Nyamulagira at Virunga National Park on 20 May 2023. Drone footage (bottom) also shows the fresh lava flows traveling downslope to the W on 20 May 2023. Courtesy of USGS via OVG.

Low-level activity was noted during June through October. On 1 June OVG reported that seismicity remained at lower levels and that crater incandescence had been absent for three days, though infrared satellite imagery showed continued lava effusion in the summit crater. The lava flows on the flanks covered an estimated 0.6 km². Satellite imagery continued to show thermal activity confined to the lava lake through October (figure 96), although no lava flows or significant sulfur dioxide emissions were reported.

Geologic Background. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Charles Balagizi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

The remote volcano of Bagana is located in central Bougainville Island, Papua New Guinea. Recorded eruptions date back to 1842 and activity has consisted of effusive activity that has built a small lava dome in the summit crater and occasional explosions that produced pyroclastic flows. The most recent eruption has been ongoing since February 2000 and has produced occasional explosions, ash plumes, and lava flows. More recently, activity has been characterized by ongoing effusive activity and ash emissions (BGVN 48:04). This report updates activity from April through September 2023 that has consisted of explosions, ash plumes, ashfall, and lava flows, using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

An explosive eruption was reported on 7 July that generated a large gas-and-ash plume to high altitudes and caused significant ashfall in local communities; the eruption plume had reached upper tropospheric (16-18 km altitude) altitudes by 2200, according to satellite images. Sulfur dioxide plumes were detected in satellite images on 8 July and indicated that the plume was likely a mixture of gas, ice, and ash. A report issued by the Autonomous Bougainville Government (ABG) (Torokina District, Education Section) on 10 July noted that significant ash began falling during 2000-2100 on 7 July and covered most areas in the Vuakovi, Gotana (9 km SW), Koromaketo, Laruma (25 km W) and Atsilima (27 km NW) villages. Pyroclastic flows also occurred, according to ground-based reports; small deposits confined to one drainage were inspected by RVO during an overflight on 17 July and were confirmed to be from the 7 July event. Ashfall continued until 10 July and covered vegetation, which destroyed bushes and gardens and contaminated rivers and streams.

RVO reported another eruption on 14 July. The Darwin VAAC stated that an explosive event started around 0830 on 15 July and produced an ash plume that rose to 16.5 km altitude by 1000 and drifted N, according to satellite images. The plume continued to drift N and remained visible through 1900, and by 2150 it had dissipated.

Ashfall likely from both the 7 and 15 July events impacted about 8,111 people in Torokina (20 km SW), including Tsito/Vuakovi, Gotana, Koromaketo, Kenaia, Longkogari, Kenbaki, Piva (13 km SW), and Atsinima, and in the Tsitovi district, according to ABG. Significant ashfall was also reported in Ruruvu (22 km N) in the Wakunai District of Central Bougainville, though the thickness of these deposits could not be confirmed. An evacuation was called for the villages in Wakunai, where heavy ashfall had contaminated water sources; the communities of Ruruvu, Togarau, Kakarapaia, Karauturi, Atao, and Kuritaturi were asked to evacuate to a disaster center at the Wakunai District Station, and communities in Torokina were asked to evacuate to the Piva District station. According to a news article, more than 7,000 people needed temporary accommodations, with about 1,000 people in evacuation shelters. Ashfall had deposited over a broad area, contaminating water supplies, affecting crops, and collapsing some roofs and houses in rural areas. Schools were temporarily shut down. Intermittent ash emissions continued through the end of July and drifted NNW, NW, and SW. Fine ashfall was reported on the coast of Torokina, and ash plumes also drifted toward Laruma and Atsilima.

A small explosive eruption occurred at 2130 on 28 July that ejected material from the crater vents, according to reports from Torokina, in addition to a lava flow that contained two lobes. A second explosion was detected at 2157. Incandescence from the lava flow was visible from Piva as it descended the W flank around 2000 on 29 July (figure 47). The Darwin VAAC reported that a strong thermal anomaly was visible in satellite images during 30-31 July and that ash emissions rose to 2.4 km altitude and drifted WSW on 30 July. A ground report from RVO described localized emissions at 0900 on 31 July.

Figure 47. Infrared (bands B12, B11, B4) satellite images showed weak thermal anomalies at the summit crater of Bagana on 12 April 2023 (top left), 27 May 2023 (top right), 31 July 2023 (bottom left), and 19 September 2023 (bottom right). A strong thermal anomaly was detected through weather clouds on 31 July and extended W from the summit crater. Courtesy of Copernicus Browser.

The Darwin VAAC reported that ash plumes were identified in satellite imagery at 0800 and 1220 on 12 August and rose to 2.1 km and 3 km altitude and drifted NW and W, respectively. A news report stated that aid was sent to more than 6,300 people that were adversely affected by the eruption. Photos taken during 17-19 August showed ash emissions rising no higher than 1 km above the summit and drifting SE. A small explosion generated an ash plume during the morning of 19 August. Deposits from small pyroclastic flows were also captured in the photos. Satellite images captured lava flows and pyroclastic flow deposits. Two temporary seismic stations were installed near Bagana on 17 August at distances of 7 km WSW (Vakovi station) and 11 km SW (Kepox station). The Kepox station immediately started to record continuous, low-frequency background seismicity.

Satellite data. Little to no thermal activity was detected during April through mid-July 2023; only one anomaly was recorded during early April and one during early June, according to MIROVA (Middle InfraRed Observation of Volcanic Activity) data (figure 48). Thermal activity increased in both power and frequency during mid-July through September, although there were still some short gaps in detected activity. MODVOLC also detected increased thermal activity during August; thermal hotspots were detected a total of five times on 19, 20, and 27 August. Weak thermal anomalies were also captured in infrared satellite images on clear weather days throughout the reporting period on 7, 12, and 17 April, 27 May, 1, 6, 16, and 31 July, and 19 September (figure 48); a strong thermal anomaly was visible on 31 July. Distinct sulfur dioxide plumes that drifted generally NW were intermittently captured by the TROPOMI instrument on the Sentinel-5P satellite and sometimes exceeded two Dobson Units (DUs) (figure 49).

Figure 48. Low thermal activity was detected at Bagana during April through mid-July 2023, as shown on this MIROVA graph. In mid-July, activity began to increase in both frequency and power, which continued through September. There were still some pauses in activity during late July, early August, and late September, but a cluster of thermal activity was detected during late August. Courtesy of MIROVA.

Figure 49. Distinct sulfur dioxide plumes rising from Bagana on 15 July 2023 (top left), 16 July 2023 (top right), 17 July 2023 (bottom left), and 17 August 2023 (bottom right). These plumes all generally drifted NW; a particularly notable plume exceeded 2 Dobson Units (DUs) on 15 July. Data is from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.0

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Autonomous Bougainville Government, P.O Box 322, Buka, AROB, PNG (URL: https://abg.gov.pg/); Andrew Tupper (Twitter: @andrewcraigtupp); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn); Radio NZ (URL: https://www.rnz.co.nz/news/pacific/494464/more-than-7-000-people-in-bougainville-need-temporary-accommodation-after-eruption); USAID, 1300 Pennsylvania Ave, NW, Washington DC 20004, USA (URL: https://www.usaid.gov/pacific-islands/press-releases/aug-08-2023-united-states-provides-immediate-emergency-assistance-support-communities-affected-mount-bagana-volcanic-eruptions).

Mayon is located in the Philippines and has steep upper slopes capped by a small summit crater. Historical eruptions date back to 1616 CE that have been characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during June through October 2022 and consisted of lava dome growth and gas-and-steam emissions (BGVN 47:12). A new eruption was reported during late April 2023 and has included lava flows, pyroclastic density currents, ash emissions, and seismicity. This report covers activity during April through September 2023 based on daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

During April through September 2023, PHIVOLCS reported near-daily rockfall events, frequent volcanic earthquakes, and sulfur dioxide measurements. Gas-and-steam emissions rose 100-900 m above the crater and drifted in different directions. Nighttime crater incandescence was often visible during clear weather and was accompanied by incandescent avalanches of material. Activity notably increased during June when lava flows were reported on the S, SE, and E flanks (figure 52). The MIROVA graph (Middle InfraRed Observation of Volcanic Activity) showed strong thermal activity coincident with these lava flows, which remained active through September (figure 53). According to the MODVOLC thermal algorithm, a total of 110 thermal alerts were detected during the reporting period: 17 during June, 40 during July, 27 during August, and 26 during September. During early June, pyroclastic density currents (PDCs) started to occur more frequently.

Figure 52. Infrared (bands B12, B11, B4) satellite images show strong lava flows descending the S, SE, and E flanks of Mayon on 13 June 2023 (top left), 23 June 2023 (top right), 8 July 2023 (bottom left), and 7 August 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 53. Strong thermal activity was detected at Mayon during early June through September, according to this MIROVA graph (Log Radiative Power) due to the presence of active lava flows on the SE, S, and E flanks. Courtesy of MIROVA.

Low activity was reported during much of April and May; gas-and-steam emissions rose 100-900 m above the crater and generally drifted in different directions. A total of 52 rockfall events and 18 volcanic earthquakes were detected during April and 147 rockfall events and 13 volcanic events during May. Sulfur dioxide flux measurements ranged between 400-576 tons per day (t/d) during April, the latter of which was measured on 29 April and between 162-343 t/d during May, the latter of which was measured on 13 May.

Activity during June increased, characterized by lava flows, pyroclastic density currents (PDCs), crater incandescence and incandescent rockfall events, gas-and-steam emissions, and continued seismicity. Weather clouds often prevented clear views of the summit, but during clear days, moderate gas-and-steam emissions rose 100-2,500 m above the crater and drifted in multiple directions. A total of 6,237 rockfall events and 288 volcanic earthquakes were detected. The rockfall events often deposited material on the S and SE flanks within 700-1,500 m of the summit crater and ash from the events drifted SW, S, SE, NE, and E. Sulfur dioxide emissions ranged between 149-1,205 t/d, the latter of which was measured on 10 June. Short-term observations from EDM and electronic tiltmeter monitoring indicated that the upper slopes were inflating since February 2023. Longer-term ground deformation parameters based on EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano remained inflated, especially on the NW and SE flanks. At 1000 on 5 June the Volcano Alert Level (VAL) was raised to 2 (on a 0-5 scale). PHIVOLCS noted that although low-level volcanic earthquakes, ground deformation, and volcanic gas emissions indicated unrest, the steep increase in rockfall frequency may indicate increased dome activity.

A total of 151 dome-collapse PDCs occurred during 8-9 and 11-30 June, traveled 500-2,000 m, and deposited material on the S flank within 2 km of the summit crater. During 8-9 June the VAL was raised to 3. At approximately 1947 on 11 June lava flow activity was reported; two lobes traveled within 500 m from the crater and deposited material on the S (Mi-isi), SE (Bonga), and E (Basud) flanks. Weak seismicity accompanied the lava flow and slight inflation on the upper flanks. This lava flow remained active through 30 June, moving down the S and SE flank as far as 2.5 km and 1.8 km, respectively and depositing material up to 3.3 km from the crater. During 15-16 June traces of ashfall from the PDCs were reported in Sitio Buga, Nabonton, City of Ligao and Purok, and San Francisco, Municipality of Guinobatan. During 28-29 June there were two PDCs generated by the collapse of the lava flow front, which generated a light-brown ash plume 1 km high. Satellite monitors detected significant concentrations of sulfur dioxide beginning on 29 June. On 30 June PDCs primarily affected the Basud Gully on the E flank, the largest of which occurred at 1301 and lasted eight minutes, based on the seismic record. Four PDCs generated between 1800 and 2000 that lasted approximately four minutes each traveled 3-4 km on the E flank and generated an ash plume that rose 1 km above the crater and drifted N and NW. Ashfall was recorded in Tabaco City.

Similar strong activity continued during July; slow lava effusion remained active on the S and SE flanks and traveled as far as 2.8 km and 2.8 km, respectively and material was deposited as far as 4 km from the crater. There was a total of 6,983 rockfall events and 189 PDCs that affected the S, SE, and E flanks. The volcano network detected a total of 2,124 volcanic earthquakes. Continuous gas-and-steam emissions rose 200-2,000 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 792-4,113 t/d, the latter of which was measured on 28 July. During 2-4 July three PDCs were generated from the collapse of the lava flow and resulting light brown plumes rose 200-300 m above the crater. Continuous tremor pulses were reported beginning at 1547 on 3 July through 7 July at 1200, at 2300 on 8 July and going through 0300 on 10 July, and at 2300 on 16 July, as recorded by the seismic network. During 6-9 July there were 10 lava flow-collapse-related PDCs that generated light brown plumes 300-500 m above the crater. During 10-11 July light ashfall was reported in some areas of Mabinit, Legazpi City, Budiao and Salvacion, Daraga, and Camalig, Albay. By 18 July the lava flow advanced 600 m on the E flank as well.

During 1733 on 18 July and 0434 on 19 July PHIVOLCS reported 30 “ashing” events, which are degassing events accompanied by audible thunder-like sounds and entrained ash at the crater, which produced short, dark plumes that drifted SW. These events each lasted 20-40 seconds, and plume heights ranged from 150-300 m above the crater, as recorded by seismic, infrasound, visual, and thermal monitors. Three more ashing events occurred during 19-20 July. Short-term observations from electronic tilt and GPS monitoring indicate deflation on the E lower flanks in early July and inflation on the NW middle flanks during the third week of July. Longer-term ground deformation parameters from EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano was still generally inflated relative to baseline levels. A short-lived lava pulse lasted 28 seconds at 1956 on 21 July, which was accompanied by seismic and infrasound signals. By 22 July, the only lava flow that remained active was on the SE flank, and continued to extend 3.4 km, while those on the S and E flanks weakened markedly. One ashing event was detected during 30-31 July, whereas there were 57 detected during 31 July-1 August; according to PHIVOLCS beginning at approximately 1800 on 31 July eruptive activity was dominated by phases of intermittent ashing, as well as increased in the apparent rates of lava effusion from the summit crater. The ashing phases consisted of discrete events recorded as low-frequency volcanic earthquakes (LFVQ) typically 30 seconds in duration, based on seismic and infrasound signals. Gray ash plume rose 100 m above the crater and generally drifted NE. Shortly after these ashing events began, new lava began to effuse rapidly from the crater, feeding the established flowed on the SE, E, and E flanks and generating frequent rockfall events.

Intensified unrest persisted during August. There was a total of 4,141 rockfall events, 2,881 volcanic earthquakes, which included volcanic tremor events, 32 ashing events, and 101 PDCs detected throughout the month. On clear weather days, gas-and-steam emissions rose 300-1,500 m above the crater and drifted in different directions (figure 54). Sulfur dioxide emissions averaged 735-4,756 t/d, the higher value of which was measured on 16 August. During 1-2 August the rate of lava effusion decreased, but continued to feed the flows on the SE, S, and E flanks, maintaining their advances to 3.4 km, 2.8 km, and 1.1 km from the crater, respectively (figure 55). Rockfall and PDCs generated by collapses at the lava flow margins and from the summit dome deposited material within 4 km of the crater. During 3-4 August there were 10 tremor events detected that lasted 1-4 minutes. Short-lived lava pulse lasted 35 seconds and was accompanied by seismic and infrasound signals at 0442 on 6 August. Seven collapses were recorded at the front of the lava flow during 12-14 August.

Figure 54. Photo of Mayon showing a white gas-and-steam plume rising 800-1,500 m above the crater at 0645 on 25 August. Courtesy of William Rogers.

Figure 55. Photo of Mayon facing N showing incandescent lava flows and summit crater incandescence taken at 1830 on 25 August 2023. Courtesy of William Rogers.

During September, similar activity of slow lava effusion, PDCs, gas-and-steam emissions, and seismicity continued. There was a total of 4,452 rockfall events, 329 volcanic earthquakes, which included volcanic tremor events, two ashing events, and 85 PDCs recorded throughout the month. On clear weather days, gas-and-steam emissions rose 100-1,500 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 609-2,252 t/d, the higher average of which was measured on 6 September. Slow lava effusion continued advancing on the SE, S, and E flanks, maintaining lengths of 3.4 km, 2.8 km, and 1.1 km, respectively. Rockfall and PDC events generated by collapses along the lava flow margins and at the summit dome deposited material within 4 km of the crater.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); William Rogers, Legazpi City, Albay Province, Philippines.

Nishinoshima, located about 1,000 km S of Tokyo, is a small island in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent submarine peaks to the S, W, and NE. Eruptions date back to 1973 and the current eruption period began in October 2022. Recent activity has consisted of small ash plumes and fumarolic activity (BGVN 48:07). This report covers activity during May through August 2023, using information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports and satellite data.

Activity during May through June was relatively low. The Japan Coast Guard (JCG) did overflights on 14 and 22 June and reported white gas-and-steam emissions rising 600 m and 1,200 m from the central crater of the pyroclastic cone, respectively (figure 125). In addition, multiple white gas-and-steam emissions rose from the inner rim of the W side of the crater and from the SE flank of the pyroclastic cone. Discolored brown-to-green water was observed around almost the entire perimeter of the island; on 22 June light green discolored water was observed off the S coast of the island.

Figure 125. A white gas-and-steam plume rising 600 m above the crater of Nishinoshima at 1404 on 14 June 2023 (left) and 1,200 m above the crater at 1249 on 22 June 2023 (right). Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, June, 2023).

Observations from the Himawari meteorological satellite confirmed an eruption on 9 and 10 July. An eruption plume rose 1.6 km above the crater and drifted N around 1300 on 9 July. Satellite images acquired at 1420 and 2020 on 9 July and at 0220 on 10 July showed continuing emissions that rose 1.3-1.6 km above the crater and drifted NE and N. The Tokyo VAAC reported that an ash plume seen by a pilot and identified in a satellite image at 0630 on 21 July rose to 3 km altitude and drifted S.

Aerial observations conducted by JCG on 8 August showed a white-and-gray plume rising from the central crater of the pyroclastic cone, and multiple white gas-and-steam emissions were rising from the inner edge of the western crater and along the NW-SE flanks of the island (figure 126). Brown-to-green discolored water was also noted around the perimeter of the island.

Figure 126. Aerial photo of Nishinoshima showing a white-and-gray plume rising from the central crater taken at 1350 on 8 August 2023.

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity), showing an increase in both frequency and power beginning in July (figure 127). This increase in activity coincides with eruptive activity on 9 and 10 July, characterized by eruption plumes. According to the MODVOLC thermal alert algorithm, one thermal hotspot was recorded on 20 July. Weak thermal anomalies were also detected in infrared satellite imagery, accompanied by strong gas-and-steam plumes (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Nishinoshima during May through August 2023, showing an increase in both frequency and power in July, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 128. Infrared (bands B12, B11, B4) satellite images showing a small thermal anomaly at the crater of Nishinoshima on 30 June 2023 (top left), 3 July 2023 (top right), 7 August 2023 (bottom left), and 27 August 2023 (bottom right). Strong gas-and-steam plumes accompanied this activity, extending NW, NE, and SW. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of Strombolian eruptions and ash plumes (BGVN 48:07). This report describes lower levels of activity consisting of ash and white gas-and-steam plumes during May through August 2023, based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, and satellite data.

Activity was relatively low during May and June. Daily white gas-and-steam emissions rose 25-200 m above the crater and drifted in different directions. Five ash plumes were detected at 0519 on 10 May, 1241 on 11 May, 0920 on 12 May, 2320 on 12 May, and at 0710 on 13 May, and rose 1-2.5 km above the crater and drifted SW. A webcam image taken on 12 May showed ejection of incandescent material above the vent. A total of nine ash plumes were detected during 6-11 June: at 1434 and 00220 on 6 and 7 June the ash plumes rose 500 m above the crater and drifted NW, at 1537 on 8 June the ash plume rose 1 km above the crater and drifted SW, at 0746 and at 0846 on 9 June the ash plumes rose 800 m and 3 km above the crater and drifted SW, respectively, at 0423, 1431, and 1750 on 10 June the ash plumes rose 2 km, 1.5 km, and 3.5 km above the crater and drifted NW, respectively, and at 0030 on 11 June an ash plume rose 2 km above the crater and drifted NW. Webcam images taken on 10 and 11 June at 0455 and 0102, respectively, showed incandescent material ejected above the vent. On 19 June an ash plume at 0822 rose 1.5 km above the crater and drifted SE.

Similar low activity of white gas-and-steam emissions and few ash plumes were reported during July and August. Daily white gas-and-steam emissions rose 25-300 m above the crater and drifted in multiple directions. Three ash plumes were reported at 0843, 0851, and 0852 on 20 July that rose 500-2,000 m above the crater and drifted NW.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during May through August 2023 (figure 140). Although activity was often obscured by weather clouds, a thermal anomaly was visible in an infrared satellite image of the crater on 12 May, accompanied by an eruption plume that drifted SW (figure 141).

Figure 140. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during May through August 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 141. A single thermal anomaly (bright yellow-orange) was visible at Krakatau in this infrared (bands B12, B11, B4) satellite image taken on 12 May 2023. An eruption plume accompanied the thermal anomaly and drifted SW. Courtesy of Copernicus Browser.

Geologic Background. The renowned Krakatau (frequently mis-named as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Villarrica, in central Chile, consists of a 2-km-wide caldera that formed about 3,500 years ago and is located at the base of the presently active cone at the NW margin of a 6-km-wide caldera. Historical eruptions eruptions date back to 1558 and have been characterized by mild-to-moderate explosive activity with occasional lava effusions. The current eruption period began in December 2014 and has recently consisted of nighttime crater incandescence, ash emissions, and seismicity (BGVN 48:04). This report covers activity during April through September 2023 and describes occasional Strombolian activity, gas-and-ash emissions, and nighttime crater incandescence. Information for this report primarily comes from the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN) and satellite data.

Seismicity during April consisted of long period (LP) events and tremor (TRE); a total of 9,413 LP-type events and 759 TR-type events were detected throughout the month. Nighttime crater incandescence persisted and was visible in the degassing column. Sulfur dioxide data was obtained using Differential Absorption Optical Spectroscopy Equipment (DOAS) that showed an average value of 1,450 ± 198 tons per day (t/d) during 1-15 April and 1,129 ± 201 t/d during 16-30 April, with a maximum daily value of 2,784 t/d on 9 April. Gas-and-steam emissions of variable intensities rose above the active crater as high as 1.3 km above the crater on 13 April. Strombolian explosions were not observed and there was a slight decrease in the lava lake level.

There were 14,123 LP-type events and 727 TR-type events detected during May. According to sulfur dioxide measurements taken with DOAS equipment, the active crater emitted an average value of 1,826 ± 482 t/d during 1-15 May and 912 ± 41 t/d during 16-30 May, with a daily maximum value of 5,155 t/d on 13 May. Surveillance cameras showed continuous white gas-and-steam emissions that rose as high as 430 m above the crater on 27 May. Nighttime incandescence illuminated the gas column less than 300 m above the crater rim was and no pyroclastic emissions were reported. A landslide was identified on 13 May on the E flank of the volcano 50 m from the crater rim and extending 300 m away; SERNAGEOMIN noted that this event may have occurred on 12 May. During the morning of 27 and 28 May minor Strombolian explosions characterized by incandescent ejecta were recorded at the crater rim; the last reported Strombolian explosions had occurred at the end of March.

Seismic activity during June consisted of five volcano-tectonic (VT)-type events, 21,606 LP-type events, and 2,085 TR-type events. The average value of sulfur dioxide flux obtained by DOAS equipment was 1,420 ± 217 t/d during 1-15 June and 2,562 ± 804 t/d, with a maximum daily value of 4,810 t/d on 17 June. White gas-and-steam emissions rose less than 480 m above the crater; frequent nighttime crater incandescence was reflected in the degassing plume. On 12 June an emission rose 100 m above the crater and drifted NNW. On 15 June one or several emissions resulted in ashfall to the NE as far as 5.5 km from the crater, based on a Skysat satellite image. Several Strombolian explosions occurred within the crater; activity on 15 June was higher energy and ejected blocks 200-300 m on the NE slope. Surveillance cameras showed white gas-and-steam emissions rising 480 m above the crater on 16 June. On 19 and 24 June low-intensity Strombolian activity was observed, ejecting material as far as 200 m from the center of the crater to the E.

During July, seismicity included 29,319 LP-type events, 3,736 TR-type events, and two VT-type events. DOAS equipment recorded two days of sulfur dioxide emissions of 4,220 t/d and 1,009 t/d on 1 and 13 July, respectively. Constant nighttime incandescence was also recorded and was particularly noticeable when accompanied by eruptive columns on 12 and 16 July. Minor explosive events were detected in the crater. According to Skysat satellite images taken on 12, 13, and 16 July, ashfall deposits were identified 155 m S of the crater. According to POVI, incandescence was visible from two vents on the crater floor around 0336 on 12 July. Gas-and-ash emissions rose as high as 1.2 km above the crater on 13 July and drifted E and NW. A series of gas-and-steam pulses containing some ash deposited material on the upper E flank around 1551 on 13 July. During 16-31 July, average sulfur dioxide emissions of 1,679 ± 406 t/d were recorded, with a maximum daily value of 2,343 t/d on 28 July. Fine ash emissions were also reported on 16, 17, and 23 July.

Seismicity persisted during August, characterized by 27,011 LP-type events, 3,323 TR-type events, and three VT-type events. The average value of sulfur dioxide measurements taken during 1-15 August was 1,642 ± 270 t/d and 2,207 ± 4,549 t/d during 16-31 August, with a maximum daily value of 3,294 t/d on 27 August. Nighttime crater incandescence remained visible in degassing columns. White gas-and-steam emissions rose 480 m above the crater on 6 August. According to a Skysat satellite image from 6 August, ash accumulation was observed proximal to the crater and was mainly distributed toward the E slope. White gas-and-steam emissions rose 320 m above the crater on 26 August. Nighttime incandescence and Strombolian activity that generated ash emissions were reported on 27 August.

Seismicity during September was characterized by five VT-type events, 12,057 LP-type events, and 2,058 TR-type events. Nighttime incandescence persisted. On 2 September an ash emission rose 180 m above the crater and drifted SE at 1643 (figure 125) and a white gas-and-steam plume rose 320 m above the crater. According to the Buenos Aires VAAC, periods of continuous gas-and-ash emissions were visible in webcam images from 1830 on 2 September to 0110 on 3 September. Strombolian activity was observed on 2 September and during the early morning of 3 September, the latter event of which generated an ash emission that rose 60 m above the crater and drifted 100 m from the center of the crater to the NE and SW. Ashfall was reported to the SE and S as far as 750 m from the crater. The lava lake was active during 3-4 September and lava fountaining was visible for the first time since 26 March 2023, according to POVI. Fountains captured in webcam images at 2133 on 3 September and at 0054 on 4 September rose as high as 60 m above the crater rim and ejected material onto the upper W flank. Sulfur dioxide flux of 1,730 t/d and 1,281 t/d was measured on 3 and 4 September, respectively, according to data obtained by DOAS equipment.

Figure 125. Webcam image of a gray ash emission rising above Villarrica on 2 September 2023 at 1643 (local time) that rose 180 m above the crater and drifted SE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 02 de septiembre de 2023, 17:05 Hora local).

Strong Strombolian activity and larger gas-and-ash plumes were reported during 18-20 September. On 18 September activity was also associated with energetic LP-type events and notable sulfur dioxide fluxes (as high as 4,277 t/d). On 19 September Strombolian activity and incandescence were observed. On 20 September at 0914 ash emissions rose 50 m above the crater and drifted SSE, accompanied by Strombolian activity that ejected material less than 100 m SSE, causing fall deposits on that respective flank. SERNAGEOMIN reported that a Planet Scope satellite image taken on 20 September showed the lava lake in the crater, measuring 32 m x 35 m and an area of 0.001 km². Several ash emissions were recorded at 0841, 0910, 1251, 1306, 1312, 1315, and 1324 on 23 September and rose less than 150 m above the crater. The sulfur dioxide flux value was 698 t/d on 23 September and 1,097 t/d on 24 September. On 24 September the Volcanic Alert Level (VAL) was raised to Orange (the third level on a four-color scale). SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

During 24-25 September there was an increase in seismic energy (observed at TR-events) and acoustic signals, characterized by 1 VT-type event, 213 LP-type events, and 124 TR-type events. Mainly white gas-and-steam emissions, in addition to occasional fine ash emissions were recorded. During the early morning of 25 September Strombolian explosions were reported and ejected material 250 m in all directions, though dominantly toward the NW. On 25 September the average value of sulfur dioxide flux was 760 t/d. Seismicity during 25-30 September consisted of five VT-type events, 1,937 LP-type events, and 456 TR-type events.

During 25-29 September moderate Strombolian activity was observed and ejected material as far as the crater rim. In addition, ash pulses lasting roughly 50 minutes were observed around 0700 and dispersed ENE. During 26-27 September a TR episode lasted 6.5 hours and was accompanied by discrete acoustic signals. Satellite images from 26 September showed a spatter cone on the crater floor with one vent that measured 10 x 14 m and a smaller vent about 35 m NE of the cone. SERNAGEOMIN reported an abundant number of bomb-sized blocks up to 150 m from the crater, as well as impact marks on the snow, which indicated explosive activity. A low-altitude ash emission was observed drifting NW around 1140 on 28 September, based on webcam images. Between 0620 and 0850 on 29 September an ash emission rose 60 m above the crater and drifted NW. During an overflight taken around 1000 on 29 September scientists observed molten material in the vent, a large accumulation of pyroclasts inside the crater, and energetic degassing, some of which contained a small amount of ash. Block-sized pyroclasts were deposited on the internal walls and near the crater, and a distal ash deposit was also visible. The average sulfur dioxide flux measured on 28 September was 344 t/d. Satellite images taken on 29 September ashfall was deposited roughly 3 km WNW from the crater and nighttime crater incandescence remained visible. The average sulfur dioxide flux value from 29 September was 199 t/d. On 30 September at 0740 a pulsating ash emission rose 1.1 km above the crater and drifted NNW (figure 126). Deposits on the S flank extended as far as 4.5 km from the crater rim, based on satellite images from 30 September.

Figure 126. Webcam image of a gray ash plume rising 1.1 km above the crater of Villarrica at 0740 (local time) on 30 September 2023. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 30 de septiembre de 2023, 09:30 Hora local).

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) showed intermittent thermal activity during April through September, with slightly stronger activity detected during late September (figure 127). Small clusters of thermal activity were detected during mid-June, early July, early August, and late September. According to the MODVOLC thermal alert system, a total of four thermal hotspots were detected on 7 July and 3 and 23 September. This activity was also intermittently captured in infrared satellite imagery on clear weather days (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Villarrica during April through September 2023, according to this MIROVA graph (Log Radiative Power). Activity was relatively low during April through mid-June. Small clusters of activity occurred during mid-June, early July, early August, and late September. Courtesy of MIROVA.

Figure 128. Consistent bright thermal anomalies (bright yellow-orange) were visible at the summit crater of Villarrica in infrared (bands B12, B11, B4) satellite images, as shown on 17 June 2023 (top left), 17 July 2023 (top right), 6 August 2023 (bottom left), and 20 September 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Av. Beauchef 1671, Santiago, Chile (URL: https://web.senapred.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Merapi, located just north of the major city of Yogyakarta in central Java, Indonesia, has had activity within the last 20 years characterized by pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome. The current eruption period began in late December 2020 and has more recently consisted of ash plumes, intermittent incandescent avalanches of material, and pyroclastic flows (BGVN 48:04). This report covers activity during April through September 2023, based on information from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data.

Activity during April through September 2023 primarily consisted of incandescent avalanches of material that mainly affected the SW and W flanks and traveled as far as 2.3 km from the summit (table 25) and white gas-and-steam emissions that rose 10-1,000 m above the crater.

Table 25. Monthly summary of avalanches and avalanche distances recorded at Merapi during April through September 2023. The number of reported avalanches does not include instances where possible avalanches were heard but could not be visually confirmed as a result of inclement weather. Data courtesy of BPPTKG (April-September 2023 daily reports).

BPPTKG reported that during April and May white gas-and-steam emissions rose 10-750 m above the crater, incandescent avalanches descended 500-2,000 m on the SW and W flanks (figure 135). Cloudy weather often prevented clear views of the summit, and sometimes avalanches could not be confirmed. According to a webcam image, a pyroclastic flow was visible on 17 April at 0531. During the week of 28 April and 4 May a pyroclastic flow was reported on the SW flank, traveling up to 2.5 km. According to a drone overflight taken on 17 May the SW lava dome volume was an estimated 2,372,800 cubic meters and the dome in the main crater was an estimated 2,337,300 cubic meters.

Figure 135. Photo showing an incandescent avalanche affecting the flank of Merapi on 8 April 2023. Courtesy of Øystein Lund Andersen.

During June and July similar activity persisted with white gas-and-steam emissions rising 10-350 m above the crater and frequent incandescent avalanches that traveled 300-2,000 m down the SW, W, and S flanks (figure 136). Based on an analysis of aerial photos taken on 24 June the volume of the SW lava dome was approximately 2.5 million cubic meters. A pyroclastic flow was observed on 5 July that traveled 2.7 km on the SW flank. According to the Darwin VAAC multiple minor ash plumes were identified in satellite images on 19 July that rose to 3.7 km altitude and drifted S and SW. During 22, 25, and 26 July a total of 17 avalanches descended as far as 1.8 km on the S flank.

Figure 136. Photo showing an incandescent avalanche descending the flank of Merapi on 23 July 2023. Courtesy of Øystein Lund Andersen.

Frequent white gas-and-steam emissions continued during August and September, rising 10-450 m above the crater. Incandescent avalanches mainly affected the SW and W flanks and traveled 400-2,300 m from the vent (figure 137). An aerial survey conducted on 10 August was analyzed and reported that estimates of the SW dome volume was 2,764,300 cubic meters and the dome in the main crater was 2,369,800 cubic meters.

Figure 137. Photo showing a strong incandescent avalanche descending the flank of Merapi on 23 September 2023. Courtesy of Øystein Lund Andersen.

Frequent and moderate-power thermal activity continued throughout the reporting period, according to a MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 138). There was an increase in the number of detected anomalies during mid-May. The MODVOLC thermal algorithm recorded a total of 47 thermal hotspots: six during April, nine during May, eight during June, 15 during July, four during August, and five during September. Some of this activity was captured in infrared satellite imagery on clear weather days, sometimes accompanied by incandescent material on the SW flank (figure 139).

Figure 138. Frequent and moderate-power thermal anomalies were detected at Merapi during April through September 2023, as shown on this MIROVA plot (Log Radiative Power). There was an increase in the number of anomalies recorded during mid-May. Courtesy of MIROVA.

Figure 139. Infrared (bands B12, B11, B4) satellite images showed a consistent thermal anomaly (bright yellow-orange) at the summit crater of Merapi on 8 April 2023 (top left), 18 May 2023 (top right), 17 June 2023 (middle left), 17 July 2023 (middle right), 11 August 2023 (bottom left), and 20 September 2023 (bottom right). Incandescent material was occasionally visible descending the SW flank, as shown in each of these images. Courtesy of Copernicus Browser.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Øystein Lund Andersen (URL: https://www.oysteinlundandersen.com/, https://twitter.com/oysteinvolcano).

Ebeko, located on the N end of Paramushir Island in Russia’s Kuril Islands just S of the Kamchatka Peninsula, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Observed eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruptive period began in June 2022, consisting of frequent explosions, ash plumes, and thermal activity (BGVN 47:10, 48:06). This report covers similar activity during June-November 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Moderate explosive activity continued during June-November 2023 (figures 50 and 51). According to visual data from Severo-Kurilsk, explosions sent ash 2-3.5 km above the summit (3-4.5 km altitude) during most days during June through mid-September. Activity after mid-September was slightly weaker, with ash usually reaching less than 2 km above the summit. According to KVERT the volcano in October and November was, with a few exceptions, either quiet or obscured by clouds that prevented satellite observations. KVERT issued Volcano Observatory Notices for Aviation (VONA) on 8 and 12 June, 13 and 22 July, 3 and 21 August, and 31 October warning of potential aviation hazards from ash plumes drifting 3-15 km from the volcano. Based on satellite data, KVERT reported a persistent thermal anomaly whenever weather clouds permitted viewing.

Figure 50. Ash explosion from the active summit crater of Ebeko on 18 July 2023; view is approximately towards the W. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Figure 51. Ash explosion from the active summit crater of Ebeko on 23 July 2023 with lightning visible in the lower part of the plume. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/).

This report summarizes behavior at Naka-dake (Nakadake) crater at Asosan (Aso, Aso-san) caldera chiefly during January 2014-February 2015. During this reporting interval Naka-dake continued to emit gas, steam, and small ash plumes. A larger eruption took place starting 25 November 2014, causing ashfall and glowing emissions. This closed a local airport, and triggered hundreds of reports on ash plumes for the aviation community by the Tokyo Volcanic Ash Advisory Center (VAAC). That eruption continued through 2014. The eruption went on into 2015 but was generally described as intermittent during late December 2014 through at least the end of February 2015. The Alert Level remained at 2 (on a scale increasing from 1 to 5) for the duration of the reporting interval. Our last report, BGVN 37:08, described the emission of ash plumes and other behavior during May-June 2011. Some remarks in this report also refer to earlier behavior, for example, a short subsection includes what JMA recorded as important in a terse summary of 2011.

Eruption details were extracted and synthesized mainly from Japan Meteorological Agency (JMA) sources. JMA frequently communicated with the Tokyo VAAC about Asosan's eruptive status. This report also discusses Volcano Ash Advisories (VAAs) issued by the JMA's Tokyo VAAC. For many of the VAAs, evidence of ash-bearing plumes reported by JMA could not be reliably detected in the satellite images. For example, the images were sometimes obscured by overlying weather-cloud cover. The plumes were also generally only rising to a few kilometers in altitude. In at least some cases, the low plumes appeared bent by high winds.

Naka-daka Crater Number 1 remained the active vent for the most part during the past eight decades. That same pattern held true during this reporting interval when myriad small eruptions, often to or below 1 km above the crater rim were documented. Visibility was sometimes impaired but monitoring instrumentation confirmed a pattern of ongoing eruption. In some cases, the eruption was not clearly seen but fresh ash was recorded. Webcameras regularly documented incandescence both in the crater and onto the crater rim. Smaller ash plumes were too numerous to mention except in occasional cases. High winds were often mentioned, which may have bearing on restricting plume heights.

Location and brief background. Asosan is located on the S of the main island of Japan (Honshu) on the island of Kyushu (figure 32).

Figure 32. Location maps showing (circular 'global' inset) Japan, and (larger map) the location of Asosan (Aso) on Kyushu island. Taken from The Daily Mail news article issued on 28 November 2014 (Malm, 2014).

The rim of Nake-dake is unusually developed for such an active volcano. Both a road and cable car carry tourists there. Shelter dugouts are provided around the crater. The Aso Volcano Museum is located nearby. Figure 33, made from radar imagery, shows Asosan's morphology.

Figure 33. Morphology of the Asosan caldera seen in shaded relief (color scale for elevations absent). N is towards the top; the N-S cross-caldera distance is ~25 km. Note the central highlands, a series of ~17 post-caldera cones that includes the active Nake-dake and its Crater Number 1. The caldera's topographic boundary is distinct on all sides but a drainage has breached the W side. This caldera vented four sets of massive eruptive deposits (pyroclastic-flows and associated ash-falls; Miyabuchi, 2013; Fujii, 2001). Source: Wikipedia (from a Nasa Shuttle Radar Topography Mission).

JMA (2013) includes a map showing the location of 12 calderas in Japan. Asosan, the largest and most active, has had many small eruptions in the past few thousand years, including many witnessed eruptions in the interval of recorded history. Spica (2013) discusses Aso in the context of other calderas in the Kyushu region. Figure 34 shows a shaded-relief map focusing on the post-caldera cones in the central highland area.

Figure 34. A shaded relief map of the elevated central area (post-caldera cones and their craters) of the Aso caldera, adding naming labels in English of some of the main features. As seen in the previous figure, the caldera floor (moat) is outside and encircling this central topographic high. Both conventional topographic and a digital elevation map (50 m grid) were used to make this map, which was published by the Geospatial Information Authority of Japan. Source: JMA (2013).

JMA's website features this summary on Asosan.

"Asosan (Aso Volcano) comprises the Aso caldera and post-caldera central cones. The Aso caldera, 25 km north-south and 18 km east-west in diameter, was formed by four gigantic pyroclastic-flow eruptions from approximately 270,000 to 90,000 years ago. Post-caldera central cones were initiated soon after the last caldera-forming eruption, producing not only local lava flows but also voluminous tephra layers which fell far beyond the caldera. Nakadake Volcano, which is the only active central cone of basaltic andesite to basalt [composition], is one of the most active volcanoes in Japan. The active crater of Nakadake Volcano is a composite of seven craterlets aligned N-S [elongate zone of depressions to the left of the label "Nakadake" and above the letter 't' in the label "Nakadake-Crater" on figure 34; see also SEAN 04:07 for a sketch map focused on this area)]. Only the northernmost [Nakadake] crater (No. 1 crater) has been active in the past 80 years, although some others were active before the 1933 eruption. The Nakadake No. 1 crater is occupied by a hyperacidic crater lake during its calm periods. During active periods, its volcanic activity is characterized by ash and strombolian eruptions and phreatic or phreatomagmatic explosions."

According to Fujii and others (2001), "Aso caldera in central Kyushu, Japan, is one of the largest calderas in the world and covers an area of 380 km₂. In late Pleistocene time, eruptions of voluminous pyroclastic flows occurred intermittently, resulting in formation of the caldera. The Aso pyroclastic-flow deposits are divided into four major units, i.e. Aso-1, Aso-2, Aso-3, and Aso-4 . . . [and] welded tuffs of these units are widely distributed in central Kyushu, and are generally well suited for paleomagnetic research . . .. K-Ar ages for Aso-1, Aso-2, Aso-3, and Aso-4 have been determined to be 266 ± 14 ka, 141 ± 5 ka, 123 ± 6 ka, and 89 ± 7 ka, respectively (Matsumoto and others, 1991)."

JMA summary for 2011 activity. JMA (2013) tabulated a summary of witnessed events (eruptions, possible eruptions, damage, significant behavior, etc.) at Asosan going back to the year 553. In the most recent behavior discussed, the authors briefly note that during 2011 (an interval they term Heisei 23) the following behavior occurred.

First, after the Mw ~9 Tohoku earthquake ~70 km off the Pacific coast on 11 March 2011, earthquakes temporarily increased roughly 10 km to the NW of the active crater. Second, very small emissions of gray-white volcanic ash occurred during 15 May to 9 June 2011. On 15 May a very small amount of tephra fall was confirmed at Sensuikyo, ~2 km to the NE of the Nakadake Number 1 crater.

2014 Activity. JMA reporting for 13 January 2014 noted the emission of a very small eruption. This came in the wake of increased tremor in late December 2013 and an increase in hazard status to Alert Level 2. (As previously mentioned, the Level remained at 2 for the duration of the reporting interval.) Further escalation in tremor took place on 2 January. On 10 January emissions reached 1,200 metric tons/day (t/d) of sulfur-dioxide (SO₂). The 13 January 2014 eruption took place at Naka-dake, which emitted a grayish-white plume that rose to 600 m that traveled S and deposited traces of ash. The resulting report from the Tokyo VAAC (a Volcanic Ash Advisory (VAA)) stating they failed to detect identifiable ash in the plume data captured on satellite images.

The small 13 January 2014 eruption triggered the first Asosan VAA in over a year. The other VAAs during January 2014 were issued on the 27th, 29th, 30th, and 31st. On one of those days, two VAAs were issued, and thus, for January there were 6 VAAs.

Bulletin editors note that the VAAs are not a linear measure of the number of eruptions. Small eruptions may not trigger a VAA at all. Several consecutive VAAs may occur associated with a single potentially larger eruption, which are issued in an effort to track an ash plume. Again, this may be an example where the number of VAAs is not reflective of the number of eruptions. Despite this, the number of VAAs are easily counted owing to new online archives. The Tokyo VAACs online presentation system is tablular in nature and is thus well suited to enable a count of reports per month.

The tally for VAAs on Asosan during 2013 was zero. The tally for 2014 involved 171 VAAs. Monthly totals for 2014 are as follows: January, 6; February, 3; March-July, 0; August, 3; September, 2; October, 0; November, 25; and December, 132. For further comparison, the tally for January and February 2015 involved 250 VAAs, with January, 132, and February, 118.

JMA reported that seismicity increased from 21 to 23 January 2014, and then decreased on 24 January. On 23 January a volcanologist observed ash plumes rising from the central vent on the crater floor. On 29 January an ash plume reported by a pilot rose to 2.7 km altitude and drifted NW. Later that day a plume rose to an altitude of 1.5 km and drifted N. JMA reported that a very small Asosan explosion occurred on 31 January. An off-white plume rose 100 m above the crater rim and drifted S.

On 5 February 2014 scientists measured decreased SO₂ emissions and fewer volcanic earthquakes.

According to the Tokyo VAAC during 30 August-1 September 2014 eruptions continuously emitted ash plumes that rose to heights of 1.2-2.1 km drifting N and NE. For example, on 1 and 6 September eruptions emitting trace amounts of ash sent plumes 600 m above the rim. (Tokyo VAAC issued VAAs stating this plume lacked identifiable ash in available satellite images.) JMA instrument surveys established SO₂ flux rates on 21 August of 1,000 t/d, and in early September of 1,200 tons/day. Counts tallying daily volcano-tectonic earthquakes (and cases of tremor) were made during 1-4 September occurring in the range 48-92 (429-500); during 5-7 September occurring the in the range 55-129 (401-463); during 8-15 September occurring in the range 394-564 (80-174).

JMA reported that during 8-16 September a persistent white plume was observed 1 km above the crater.

Preliminary counts for volcanic earthquakes (394-564 per day) and tremor (80-174 per day) were reported during 8-15 September. Field surveys conducted on 9 and 12 September yielded elevated temperatures from fumaroles and the surface of the S crater wall.

Tremor accompanied a very small eruption recorded on 22-24 October. Ashfall observed on the 24th indicated another such eruption.

During 7 September and 24 November 2014, VAAs were absent for Aso. In contrast, during 25 November 2014-31 December 2014 there were 171 VAAs issued. Multiple VAAs were issued on several different days in this later interval, for example, on 26 November, 7 VAAs were issued.

Asosan continued to erupt during the 7 September-24 November 2014 interval. Some monitored parameters such as earthquakes, tremor, and SO₂ emissions were elevated. A small eruption took place on 6 September, for example, sending a plume to 600 m above the crater. During 8-16 September JMA noted a persistent white plume 1 km above the crater. During the week 12-18 November, a steam plume rose 400 m above the crater rim.

With the start of the surge in VAAs beginning on 25 November 2014 (noted above), a stronger and comparatively sustained eruption began. During the eruption on the 25th an ash plume rose to 1.8 km above the crater rim. Ash soon fell to the E in Hanoi Aso (Kumamoto Region), Taketa (30 km NE, Oita Region), Gokase (25 km WSW, Miyazaki Region), and in Minamiaso (10 km SW, Kumamoto Region). Incandescence at night was seen with webcams.

On 26 November tephra ascended 100 m above the crater rim and an ash plume rose 1 km. Tremor began a few hours before the eruption and on the 26th, continued to be elevated. The eruption continued on 27 November; ash plumes rose 1.5 km. Volcanologists observed a strombolian eruption and found 7 cm of fresh ash that contained fist-sized scoria. Ash fell to the W, affecting the city of Kumamoto (38 km WSW). According to a news article, flights in and out of Kumamoto airport were either cancelled or diverted. On 28 November ash plumes rose 1.5 km. The eruption continued through at least 30 November; ash plumes rose at most 1.5 km and incandescent material was ejected onto the crater rim.

Although inclement weather restricted views of the crater, monitored parameters and available views indicated that the 25 November eruption continued through to at least 22 December, when it became intermittent. Ash plumes to about 1 km above the crater rim and incandescent material on the crater rim were common through the end of the year (and beyond, through this reporting interval ending in February 2015, and described as the ongoing eruption.

A news report in the 28 November 2014 issue of the Daily Mail by Sara Malm (Malm, 2014) indicated dozens of cancelled flights at Kumamoto's airport. That report included the Associated Press photo seen in figure 35. The date of the photo in that article was ambiguous, but a different article with the same photo (see caption) gave 26 November 2014 as the photo's date. The angled, bent-over character of the ash plume and location of Crater Number 1 (the active crater, at the N end of the row of craters) indicate the view was from the NW and implies strong winds roughly from the N.

Figure 35. A photo taken on 26 November 2014 of Asosan in eruption. The gray ash plume is escaping at Nake-dake Crater Number 1 blowing roughly S. The plume does not rise vertically. The plume ascends near the vent but for some distance beyond the vent the plume descends. At distance the plume appears to spread over considerable vertical extent, from near the ground surface to above the field of view. Source: Phys.Org news (crediting AP/Kyodo News).

An undated video in Malm (2014) also showed the plume. The video also showed an aerial view of the visitor area on the crater rim, which was blanketed in gray ash. Other scenes included children walking to school wearing dust masks and carrying folded umbrellas, and close up shots of what appeared to be dark colored, highly vesicular spatter.

In a 29 November 2014 MODIS image of the region, Asosan was under weather clouds but a clear view revealed a prominent ~30-km-long, beige-colored, funnel-shaped area trending SE. This was interpreted by Nasa Earth Observatory authors Jeff Schmaltz and Adam Voiland as airborne ash. Webcamera images around this time showed a glowing pit crater with extensive areas containing incandescent tephra around it. A copious plume also discharged nearby.

During a field survey on 10 December volcanologists observed 20-cm-wide blocks near the crater and 5- to 10-cm-wide blocks within 1.2 km SW of the crater. During 12-15 December the plume rose 1 km above the crater rim and ash fell to the E in Hanoi Aso (Kumamoto Region).

JMA reports for 15-30 December described the usual eruptive ash plumes that again rose 600-1,000 m above the crater and some cases of still glowing material on the crater rim. SO₂ fluxes were 2,000-3,100 t/d during 15 and 18 December.

2015 activity. As noted above, the VAAs for 2014 totaled 171, and the VAAs for the months of January and February 2015 totaled 250. This is consistent with ongoing eruption at Asosan, which was also the basic conclusion in JMA reports from monitoring and direct observations during January-February 2015, although they often described the eruption during both these months as intermittent.

JMA reported cases during January where plumes rose up to 1 km above the crater, and in some cases glowing material reached the crater rim. JMA reported SO₂ fluxes of 500-2600 tons of SO₂. Both tilt and GPS instrumentation recorded slight growth across the active crater. A pilot report on 29 January indicated an ash plume to 2.7 km altitude (~1.1 km above the rim) and drifting NW.

An image acquired on 13 January 2015 was discussed by Jesse Allen and Adam Voiland of Nasa Earth Observatory. They reported that the image was from the Operational Land Imager (OLI) on Landsat 8. They indicated that it showed ash drifting ten's of kilometers S from Aso.

For February 2015, JMA reported episodes of volcanic earthquakes, high-amplitude tremor, and infrasound data that continued to indicate ongoing intermittent eruptions. Webcamera views again documented cases of glowing material reaching the rim during the first half of the month. Plumes again rose up to 1 km above the crater rim. JMA reported intermittently detected eruptions, including during 2-6, 9-13, and 16-20 February.

References.

Fujii, J., Nakajima, T., & Kamata, H., 2001, Paleomagnetic directions of the Aso pyroclastic-flow and the Aso-4 co-ignimbrite ash-fall deposits in Japan. Earth, planets and space, 53(12), 1137-1150. (URL: http://download.springer.com/static/pdf/873/art:10.1186/BF03352409.pdf?originUrl=http://link.springer.com/article/10.1186/BF03352409&token2=exp=1434307643~acl=/static/pdf/873/art:10.1186/BF03352409.pdf?originUrl=http://link.springer.com/article/10.1186/BF03352409*~hmac=b718e24427a5900c5057d59ebb12b501c1ae870b932122de89cc3a01a5f5318f ).

JMA (Japan Meteorological Agency), 2013, National Catalog of the Active Volcanoes of Japan (4th edition; online English version), (URL: http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/menu.htm ) (accessed in June 2015)

Khin, K, 2013, Field trip to Aso volcano, Kyushu, Japan, Slideshare.net (13 annotated slides) (URL: http://www.slideshare.net/kyikyaw2/field-trip-to-aso-volcano-kyushu-japan )

Malm, S, 2014, Flights cancelled across Japanese region after Mount Aso volcano erupts for the first time in 22 years, spewing lava, smoke and a kilometre-high ash cloud, The Daily Mail 28 November 2014 (7 graphics files and a 58-second video) (accessed online June 2015) ((URL: http://www.dailymail.co.uk/news/article-2852674/Volcano-south-Japan-erupts-disrupting-flights.html#ixzz3d5POqZhu )

Matsumoto, A., K. Uto, K. Ono, and K. Watanabe, 1991, K-Ar age determinations for Aso volcanic rocks—concordance with volcano stratigraphy and application to pyroclastic flows, Abstracts to Fall Meeting in 1991, Volcanol. Soc. Japan , 73 (in Japanese).

Miyabuchi, Y, 2013, A 90,000-year tephrostratigraphic framework of Aso Volcano, Japan, Sedimentary Geology, Volume 220, Issues 3–4, 15 October 2009, Pages 169-189, ISSN 0037-0738, (URL: http://dx.doi.org/10.1016/j.sedgeo.2009.04.018 ; http://www.sciencedirect.com/science/article/pii/S0037073809001006 )

Spica, 2013, Southern Japan Calderas, Volcano Café (Volcano discussions in your living room), Wordpress.com (22 July 2013)(accessed June 2015) (URL: https://volcanocafe.wordpress.com/2013/07/22/southern-japan-calderas/ )

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

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Aso Volcano Museum (URL: http://www.asomuse.jp; and Jeff Schmaltz and Adam Voiland, NASA Earth Observatory (URL: http://earthobservatory.nasa.gov).

Our last report (BGVN 39:11) covered activity at Etna through 13 June 2014, which consisted primarily of ongoing emissions of E-directed lavas from a vent area on the lower E flank of the New South East Crater (NSEC). This report, summarizing first-hand accounts by the Istituto Nazionale di Geofisica e Vulcanologia (INGV-Catania), covers the subsequent interval from 14 June 2014 through 2 February 2015. INGV described several eruptive episodes, including strombolian eruptions, ash emissions, and the appearance of new effusive vents at the E base of the North East Cone (NEC) and on the high E flank of the NSEC cone.

Activity during June-December 2014. On 14 June a new eruptive episode began within the NSEC, with near-continuous strombolian explosions and lava fountaining. Fine ash emissions were concurrent with lava that began to overflow the edge of the South East Crater (SEC), forming a flow that continued downhill on the W wall of Valle del Bove. During the morning of 15 June the overflowing lava followed the fissure that had been formed on 28 November 2013. Explosive activity occurred from three vents inside the crater. A spatter cone also formed in the NSEC's E sector, partially filling the fissure formed on the high NE flank during eruptions of late December 2013, and January–March 2014. During 14-15 June tremor increased sharply and remained moderately high until 18 June, when it returned to normal levels.

INGV noted that this lively strombolian activity over the course of four days was similar to the episode of effusive lava emissions observed during 14-16 and 19-31 December 2013 in terms of duration and intensity.

After images of a thermal anomaly in webcam images from Monte Cagliato, located on the E flank of Etna, a new, small fissure (tens of meters long) at the E base of the North East Crater (NEC) was observed by INGV Etna observatory personnel during 5-6 July. The vent was located between 3,015 and 3,025 m elevation. Weak spattering from this vent fed a lava flow that extended ~100 m within the saddle of the NSEC and SEC cones. Weak and sporadic strombolian explosions and small ash emissions were observed during 6-7 July from NSEC, but by 11 July this activity had ceased. Activity from the new fissure continued through 11 July with frequent strombolian explosions that were audible in nearby towns. The lava flow diverged; the longer of the two branches extended ~1.5 km, reaching the bottom of Valle del Leone.

On the morning of 25 July about 1114 local time, a new eruptive vent opened near the same eruptive fissure. This new vent ("25 July vent"), located at a distance of about 150-200 m to the N of the one from 5 July, was a source of strombolian explosions, accompanied at times by modest quantities of ash. This activity continued through 31 July. The strombolian explosions occurred at intervals of about 2-5 seconds and were often accompanied by visible compression waves ("flashing arcs") and audible rumblings up to a few tens of meters away, mostly in the E and NE sectors of the volcano. As previously observed, for example during the paroxysmal episode at the NSEC during 14-16 December 2013, the rumblings were interpreted as the result of explosions of gas bubbles inside the eruptive vent. Emissions of bombs and scoria occasionally rose 200 m high and fell within a few hundred meter radius around the vent. In a few instances, the explosions were accompanied by small quantities of ash. The lava flows, which had reached ~1.8 km during the preceding week, (halting on the saddle between the Valle del Leone and the Valle del Bove), had in the recent days overlapped the earlier ones, with active fronts at least 1 km from the effusive vent.

On 9 August INGV reported a strong decrease in volcanic tremor. From the 25 July vent, there began a gradual increase in the ash emissions that formed an ash plume, which rose to 1 km above the vent area and renewed strong strombolian activity in the evening. Strombolian activity increased at NSEC and was accompanied by small emissions of black ash that remained within the crater.

With the intensification of activity at the NSEC, the eruptive activity at the E flank of the NEC diminished. At 0645 on 9 August an effusive vent opened on the high E flank of the NSEC cone, which caused a small landslide and emitted a lava flow that after an hour had reached the E base of the cone. During the first 24 hours of activity, a small pyroclastic cone in the W portion of the NSEC summit appeared, increasing the height of the NSEC structure that began to grow in 2011. On 13 August INGV reported continued strombolian explosions, accompanied by modest emissions of ash and lava from a single vent on the high E flank of the NSEC. The lava flows emitted from the effusive vent to the E had almost ceased to advance the evening before, but two new branches were overlapping the earlier flow. The longest flow changed direction to later descend about 3 km NE toward Monte Simone. INGV reported that the eruption at NSEC had ended on 15 August and that the lava flow activity had ceased by 16 August (figure 151).

Figure 151. This photo was taken on the morning of 16 August 2014 from the town of Tremestieri Etneo (from 20 km S of Etna). It shows Etna's NSEC (at right) with its new peak formed during the eruptive episode of 9-15 August. The old SEC cone (which appears lower) resides at left. Courtesy of Istituto Nazionale di Geofisica e Vulcanologia (INGV-Catania).

Beginning in the afternoon of 7 October through 16 October the NSEC produced weak and intermittent explosive activity; small ash puffs were rapidly dispersed by the wind. During some nights small strombolian explosions ejected incandescent material a few tens of meters above the crater rim.

Starting at 1850 on 28 December the NSEC produced a short but intense eruption characterized by lava fountains, lava flows, and an ash plume that drifted E, and caused ash and lapilli fall in the nearby towns of Milo, Fornazzo, Sant'Alfio, and Giarre. It was the first typically "paroxysmal" event at the NSEC since 2 December 2013. Inclement weather prevented observations of the summit area, so the erupting crater was not identifiable. Two lava flows traveled E and NE, towards the Valle del Bove. Tremor began to decrease at 2030, and indicated that the eruption was over at 2200 (figure 152).

Figure 152. Lava flows deposited associated with Etna's large 28 December 2014 eruption (red). N is towards the top. The NSEC vented on a fissure developed a few hundred meters to the SE of the SEC (NNW-trending dashed blue line). Courtesy of INGV.

On 29 December, cameras viewing Etna recorded small ash emissions from the NSEC and persistent glow from the saddle between the SEC and NSEC cones at dusk. INGV indicated that this paroxysmal episode occurred at a series of eruptive vents along a NE-SW fissure that cut across the NSEC and the southern flank of the old SEC. From the two extremities of this fissure lava flows emerged, traveling SW toward the area of Milia-Galvarina and NW toward the northern part of the Valle del Bove near Monte Simone, reaching lengths of about 4.5 and 3.3 km, respectively (figure 152).

Activity during January 2015. During the night on 1 and 2 January, cameras recorded intermittent flashes from Voragine Crater (one of four summit craters), indicating strombolian activity there for the first time in nearly two years. At 0730 on 2 January explosions at NSEC generated ash plumes that drifted SW. Emissions ejected pyroclastic materials up to ~150 m above the crater rim, which intensified during the evening of 3 January.

At night during 6-7 January the frequency of strombolian explosions at the Voragine Crater decreased; however, some of the explosions ejected incandescent pyroclastic material outside of the crater and onto the W and SW flanks. On 7 January many of the small explosions generated brown ash plumes that rose a few hundred meters above Etna's summit and quickly dissipated. Strombolian activity increased on 8 January, possibly from two vents within the crater. Pyroclastic material continued to be ejected out of the crater. Early on 9 January strombolian activity again decreased and gave way to ash emissions that rose several hundred meters. During the evening on the same day some ash emissions were accompanied by incandescent pyroclastic material that at times fell on the external flanks of the central summit. Ash emissions continued the next morning, decreased, and had almost completely ceased by late morning. Ash emissions rapidly resumed in the afternoon and were sometimes accompanied by strombolian explosions. During the morning of 13 January, new ash emissions began at the Voragine. For some hours, these emissions were continuous, but successively diminished in the afternoon to every 5-10 minutes. Marco Neri, of the INGV- Osservatorio Etneo, during a helicopter overflight on 14 January, captured a clear view of these emissions and of the summit crater area (figure 153).

Figure 153. Summit craters of Etna seen from helicopter on the morning of 14 January 2015, looking NW. In the foreground is the cone of the New South East Crater (NSEC), its summit vent being much enlarged after the 28 December 2014 paroxysm, and the old South East Crater (SEC), with an extensive fumarolic area on the saddle between the two cones. Note the two conspicuous eruptive fissures (labeled), one on the NE flank of the NSEC (in the lower right portion of the image), and the other on the S flank of the old SEC (which opened on 28 December 2014). In the background are the Bocca Nuova (at left), the Voragine (center, emitting a dense white vapor plume), and the North East Crater (NEC) (at right). The town visible in the distance at upper right is Randazzo, on the N-NW flank of Etna. Photo and caption courtesy of Marco Neri, INGV-Osservatorio Etneo.

In the evening on 14 January weak strombolian activity was recorded at the Voragine Crater and NEC. The next day, occasionally pulsating ash emissions rose from the NEC and drifted SE. Ash emissions continued through 17 January; cloud cover prevented observations of the summit area on 18 January.

A new eruptive episode began on 31 January and continued through the morning of 2 February. Poor meteorological conditions prevented views of the summit area during the first 36 hours of the eruption. During improved viewing conditions on the evening of 1 February, volcanologists observed lively strombolian activity from a single vent in the saddle between the SEC and NSEC cones. Explosions occurred every few seconds and ejected incandescent bombs 200 m high, which fell on the S flank of the SEC. At the same time, from a vent at the southern base SEC cone corresponding to the lowest part of the SE eruptive fissure from 28 December, a lava flow issued that traveled 2 km S, dividing into two branches. At dawn on 2 February the strombolian activity produced a dense ash cloud that drifted E. At about 0750 emissions stopped, and volcanic tremor suddenly decreased.

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: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/).

Piton de la Fournaise is located on Réunion island, which lies to the E of Madagascar in the Indian Ocean (figure 84). In this Bulletin, we discuss eruptions in June 2014 and February 2015. The June 2014 eruption took place on 21 June, from 0135 to 2109 local time. The February 2015 eruption occurred from 1100 local time on 4 February to 2230 local time on 15 February. In this report, all times are local unless otherwise stated (local time= UTC + 04 hours). This report represents a synthesis of available information published by Observatoire Volcanologique du Piton de la Fournaise (OVPDLF).

Our last Bulletin report on Piton de la Fournaise (BGVN 37:03) documented increased seismicity and eruptive activity from August to December 2010. Piton de la Fournaise's last eruption took place from 14 October 2010 through 10 December 2010 (BGVN 37:03).

Figure 84. Map of Reunion Island, highlighting the location of Piton de la Fournaise, a basaltic shield volcano. As shown in the inset map (top right corner), Reunion is located to the E of Madagascar. Courtesy of Observatório Vulcanológico Geotérmico Açores.

June 2014. Piton de la Fournaise erupted on 21 June 2014, ending a three and a half year period of quiescence that began on 11 December 2010.

Preceding the eruption, Piton de la Fournaise experienced a period of high activity from 7 to 20 June 2014. Table 4 details the number of volcano-tectonic (VT) earthquakes and rock-fall events recorded during this interval. The greatest number of daily VT earthquakes was recorded on 20 June, and highest number of rock-fall events occurred on 17 June. Through email correspondence, OVPDLF personnel reported that over this period (7-20 June), no deformation or significant gas emissions were detected. They also reported that observed seismicity occurred between 500 and 1,200 m above sea level (a.s.l.).

Table 4. The number of volcano-tectonic (VT) earthquakes and rock-fall events recorded at Piton de la Fournaise from 7 to 20 June 2014, which was considered a period of high activity. Source: email correspondence with OVPDLF personnel.

On 21 June 2014, at 0006, a seismic crisis began and continued for 74 minutes (email correspondence). Then at 0020, deformation began and persisted for ~3 hours (email correspondence). At 0120, tremor was detected and, at 0135, an eruption began as verified by OVPDLF cameras, which captured incandescence given off by the eruption (email correspondence). The venting took place within Enclos Fouqué on the ESE side of the central cone (figure 85) (email correspondence). OVPDLF reported that the eruptive fissures on the cone's ESE side sat between the Maillard crater and a small plateau at ~2300 m altitude (figure 85) (OVPDLF, 2014a).

During the morning of 21 June, a helicopter flyby noted (a) the presence of two eruptive fissures. From the more active fissure, small lava fountains emanated and built a spatter rampart; (b) two lava flows developed and traveled more than 1.5 km from the more active fissure. One of the flows, continued moving ~250 m E after passing the Langlois crater and the other flow continued ~500 m E-S after passing the Langlois crater (the Langlois crater is located ~2 km SE of the Dolomieu crater, figure 85); and (c) a very dilute SO₂ plume extended N (OVPDLF, 2014a).

Figure 85. Topographic map of Piton de la Fournaise, which includes the names of features associated with the volcano (the labelling on the map is in French; cratère translates to crater in English). From Gaba (2007).

During 21 June 2014, OVPDLF raised the Alert Level to 1 ("probable or imminent eruption"), and public access to the volcano was restricted. According to email correspondence with OVPDLF personnel, the eruption ended at 2109 on 21 June. OVPDLF further reported that the intensity of the detected tremor decreased during the day and disappeared at 2109 (OVPDLF, 2014a).

November and December 2014. On 2 December 2014, OVPDLF published an activity report (OVPDLF, 2014b), which indicated the following, (a) 113 VT earthquakes were recorded between 1 November and 1 December, with the highest number of earthquakes being recorded on 1 November (figure 86); (b) the majority of the earthquakes were located between 500 and 1,000 m a.s.l. at the base of Piton de la Fournaise's summit; (c) deformation registered by OVPDLF's geodetic network remained the same since September 2014; and (d) since 1 September 2014, the geochemical station at the summit detected low emissions of SO₂ that were often coupled with CO₂, H₂O and H₂S. That report also stated that on 1 November 2014, the hazard status "Vigilance Volcanic phase" was initiated due to increased geophysical activity. OVPDLF (2014b) stated that this status was lifted on 1 December 2014.

Figure 86. Histogram of the number of volcano-tectonic earthquakes recorded from 1 November 2014 to 1 December 2014. A total of 113 VT earthquakes were recorded over this interval and the highest number of earthquakes was recorded on 1 November 2014. Courtesy of OVPDLF (2014b).

February 2015. The next eruption at Piton de la Fournaise began on 4 February 2015. The information in this section was found in the reference, OVPDLF (2015), unless otherwise stated. Between 0400 and 0900 on 4 February, 180 earthquakes were recorded, five of which had magnitudes greater than 2. At 0910, a seismic crisis started and at 1050, a volcanic tremor began. Ten minutes later, at 1100, an eruption began at an eruptive fissure on the S flank of Piton de la Fournaise's cone within Enclos Fouqué. Due to the eruption, Alert Level 2-2 ('ongoing eruption') was declared.

On 5 February 2015, the eruption continued even though the intensity of the tremor had decreased since its initiation on 4 February. OVPDLF reported that the eruptive fissure formed 100 m W of Bory crater (figures 85 and 87). The fissure had a length of ~500 m and activity was reported to be concentrated at its southernmost end. The fissure emitted a lava flow that traveled S-SW, and after passing Rivals crater, it divided into several branches as it continued to spread farther S and SW (figure 87). The southernmost branch of the flow traveled passed Cornu crater (figure 85). That evening, at 1800 local time, the tremor had significantly decreased in intensity. The intensity of the tremor was about six times lower than it was at the beginning of the eruption. The eruptive fissure remained active and projected lava ~10 m high.

The eruption continued on 6 February 2015. The tremor intensity was still very low and the lava flow and its branches were still active. OVPDLF reported that during field observations, there was low levels of outgassing and material projected from eruptive vents had built small cones. On 8 February, the eruption continued and low magnitude earthquakes located in the upper part of Piton de la Fournaise reappeared. Despite poor weather, OVPDLF observed that lava continued flowing from the vents and one flow traveled farther W. By 9 February, no significant changes were noted and by late morning, the eruptive fissure was weakly active and only small splashes of lava were observed.

Figure 87. Map highlighting the margins of the lava flow and its branches in red as of 8 February 2015. For scale, the Dolomieu crater is ~1.0 km in E-W diameter. The lava that fed this flow was emitted from an eruptive fissure 100 m W of Bory crater. On the map, Bory crater is the small indent to the left of the larger Dolomieu crater. The flow length is ~2.4 km. Although the eruption continued until 15 February, there was little change to the basic pattern of the lava flow and its branches according to OVPDLF. Courtesy of OVPDLF.

The eruption continued in a similar manner until 15 February 2015. Between 10 and 15 February, OVPDLF reported that the tremor remained low and there were no significant changes in other recorded geophysical parameters. During this interval, poor weather conditions sometimes hindered observations. On the morning of 14 February, due to the absence of clouds, OVPDLF observed a clear plume rising between 2.8 and 3 km in altitude, and concluded it was probably rich in water vapor.

In the morning of 15 February, the tremor was low and stable, and equivalent to what was recorded in previous days. According to OVPDLF, at 1700 on 15 February, the tremor began to decrease in intensity. The tremor then underwent a few hours of rapid fluctuations in its intensity, before disappearing at 2230. With the disappearance of the tremor, the eruption ended. The following day, Volcano Discovery reported that the Alert Level had been lowered.

References. Gaba, E. (Wikimedia Commons user, Sting), 2007, Topographic map of the Piton de la Fournaise shield volcano on the Réunion island, Wikipedia (initial image from the NASA Shuttle Radar Topography Mission), URL: http://commons.wikimedia.org/wiki/File:Piton_Fournaise_topo_map-fr.svg#/media/File:Piton_Fournaise_topo_map-fr.svg, accessed on 27 May 2015

OVPDLF, 2014a, Actualités (News), URL: http://www.ipgp.fr/fr/ovpf/actualites-ovpf, accessed in June 2014

OVPDLF, 2014b, Bilan d'activité à la levée de la vigilance volcanique (Activity report to the lifting of the volcanic alert), URL: http://www.ipgp.fr/fr/OVPDLF/communique-de-lOVPDLF-2-decembre-2014, accessed on 10 June 2015

OVPDLF, 2015, Archive actualités (News Archive), URL: http://www.ipgp.fr/fr/OVPDLF/archive-actualites, accessed on 27 May 2015

Observatório Vulcanológico Geotérmico Açores, 2015, Notícia 1682 - Vulcão Piton de la Fournaise, Ilha da Reunião: nova erupção registada este domingo (News No. 1682 - Piton de la Fournaise volcano, Reunion Island: new recorded eruption on Sunday), URL: http://ovga.centrosciencia.azores.gov.pt/sites/default/files/Map_ide-reunion-piton-de-la-fournaise.jpg, accessed on 10 June 2015

Volcano Discovery, 2014, Piton de la Fournaise volcano (La Réunion): eruption ends, URL:

http://www.volcanodiscovery.com/piton_fournaise/news/45631/Piton-de-la-Fournaise-volcano-La-Runion-eruption-ends.html, accessed on 27 May 2015

Volcano Discovery, 2014, Piton de la Fournaise volcano (La Réunion): alert level raised, eruption warning, URL: http://www.volcanodiscovery.com/piton_fournaise/news/49537/Piton-de-la-Fournaise-volcano-La-Runion-alert-level-raised-eruption-warning.html, accessed on 27 May 2015

Volcano Discovery, 2015, Piton de la Fournaise volcano (La Réunion): eruption seems to have ended, URL: http://www.volcanodiscovery.com/piton_fournaise/news/51262/Piton-de-la-Fournaise-volcano-La-Runion-eruption-seems-to-have-ended.html, accessed on 27 May 2015.

Geologic Background. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise (OVPDLF), Institut de Physique du Globe de Paris, 14 route nationale 3, 27ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr/OVPDLF/observatoire-volcanologique-piton-de-fournaise); Nicolas Villeneuve, OVPDLF.

This report summarizes events at Popocatépetl during November 2012-December 2014. Almost all of the data discussed came from (~800) online daily reports by the Centro Nacional de Prevención de Desastres (CENAPRED). Many of those reports are issued covering a 24 hour interval (from 1000 on the stated day back to 1000 on the previous day), with occasional cases of later supplemental reports the same day. A link to those reports is provided in the "Information Contacts" section. Our previous report on Popocatépetl discussed the ongoing eruption during July-October 2012 (BGVN 37:09).

Behavior during the reporting interval included persistent emissions (often containing ash). When visibility permitted, web cameras documented nighttime emissions containing incandescent fragments, in many cases, rising hundreds of meters above the crater rim and spreading across the upper flanks. These eruptions typically deposited tephra up to ~1.5 km from the crater where it was conspicuous on the snow and ice that crowns the summit. Occasional air photos also depicted ballistics or their impacts and tracks in the summit area. Ashfall was not uncommon in villages on the volcano and it occasionally fell in parts of Mexico City and the city of Puebla. Many plumes rose on the order of 1 km, reported by CENAPRED in many cases several times a week if not more frequently. Periods of tremor occurred, some of which lasted for more than one hour. At least one volcanic-tectonic earthquake occurred on many days (maximum coda magnitudes, Mc, generally 2.0 to 2.5). Earthquakes are in general thus dismissed from detailed discussion below; however, for one sample month, November 2013, we include a larger emphasis on the record of larger earthquakes reported daily by CENAPRED. Many of the commonplace processes such as those in the above list were sufficiently common that, in order to save space, they are often omitted from this narrative.

One way CENAPRED quantifies Popocatépetl's behavior is to use daily 'exhalations' (substantive plumes inferred to contain ash) which have long been a means of monitoring and characterizing this large and tall andesitic stratovolcano. The term 'exhalation' was used extensively in Bulletin reports starting with BGVN 22:03 in 1997. Exhalations are still currently tabulated by CENAPRED. Those appear in histograms in each daily report (assessing a 24 hour interval ending at 1000 on the stated reporting date).

Wright and others (2002) explain 'exhalations' further and clarify the distinction to the larger events that they classify as 'explosions.' The authors included photos and infrared imagery to illustrate the term (omitted here).

"Exhalations are short duration (3–90 min) ash-rich gas plumes . . .. CENAPRED provide daily Web-based activity updates in which exhalations are classified as small, moderate, or large on the basis of their duration and resultant plume height. Plumes can rise as much as 5000 m above the crater rim but are generally smaller. Exhalations are common and as many as several tens can occur each day. The ash they transport may be non-juvenile in nature (possibly with a juvenile component since March 1996 when lava extrusion began), and exhalations are thought to be the result of intermittent high-pressure gas streams that scour rock fragments from the conduit walls. Thermal video images, which measure the amount of radiation emitted in the 8–14 μm region of the electromagnetic spectrum . . ., indicate that by the time the plumes have reached the altitude of the crater rim, the ash-gas mixture is generally of a very low temperature (9–12°C at the plume exterior) due to the rapid entrainment of air at ambient temperatures.

"Explosions are less frequent than exhalations. They result in larger, darker ash plumes, with bombs often thrown clear of the crater to form a high-temperature ejecta blanket on the upper slopes of the volcano . . .. The plumes most commonly reach heights of between 3000 and 5000 m above the crater rim, although several larger explosions have occurred during the recent activity. The explosion of 30 June 1997, for example, was the largest recorded since 1922 and generated a plume 13,000 m high. Although explosions during the recent activity have been most common during periods of dome growth, they have also been observed during periods when no magmatic activity has been observed on the crater floor."

Wright and others (2002) also make this comment: "Clearly, periods of prolonged and total cloud cover will prevent any useable data being acquired." They were addressing satellite observations but this also applies to visual- and webcamera-based observations. This means that during some intervals adverse meteorological factors (clouds, rain, snow, etc.) could reduce the number of reported exhalations.

From this it is reasonably clear that the vast majority (if not all) of the eruptions during the reporting interval (November 2012-December 2014) were in the category of exhalations. During this reporting interval, several plumes did reach 3-4 km above the crater rim, as is noted below (e.g., during May-July 2013) and but we know of none reported that rose to over 5 km over the crater rim (~10.4 km altitude).

The maximum number of daily exhalations in the recent past stood, since July 2012, at 211. On 23 May 2013 that record was broken when 314 daily exhalations occurred. A second increase in that maximum value occurred twice more when the daily values reached 480 exhalations on both 4 and 6 June 2014.

As discussed in other Bulletin reporting since the onset of the eruption in March 1996 (BGVN 21:01), dramatic events involved dome dynamics in the steep-walled, cylindrical, ~0.5-km-diameter summit crater. There, emissions of lava and tephra constructed the dome. Occasional energetic discharges from the vent beneath this growing dome blew out the dome's central area, leaving the dome with a ring-shaped morphology. This process has taken place many times in the intervening years since 1996 and continued in this reporting interval too.

Further discussion and references on the topic of exhalations and explosions with particular reference to Popocatépetl also appear in other studies (e.g., de la Cruz-Reyna and others, 2008; González-Mellado and de la Cruz-Reyna, 2008; and Tárraga and others, 2012).

November 2012-December 2013 activity. During the remainder of 2012, the Alert Level remained at Yellow, Phase Two (where it had been since lowered on 1 September 2012).

The usual plumes, occasionally bearing ash, rose up to ~1 km above the crater on many days during November-December 2012. For example, during 3-4 November 2012, CENAPRED daily reports noted 9 more significant eruptions and associated plumes registered at these respective times: 1100, 1450, 1548, 2346, 0157, 0240, 0532, 0835, and 0931.

On the basis of 15-day averages shown on histograms in CENAPRED daily reports, the overall November monthly average was 43. During December 2012 the overall average was 31. Lower monthly averages than December's 31 appeared during January 2013 through the first half of March 2013. During the second half of March 2013 the average daily exhalations again rose to similar levels (31). The averages dropped again after that the averages remained low well into May and early June 2013 although during these later months some daily values increased significantly. The average value for the second half of June 2013 was 33.

During the first two weeks of May 2013 there were increases in earthquakes, tremor, and emissions. During 7-8 May, CENAPRED called attention to an episode of high amplitude spasmodic tremor. It was accompanied by an explosion on 8 May that ejected an ash plume that rose 3 km above the crater and drifted SE. Ashfall was reported from the villages of San Pedro Benito Juarez (10-12 km SE), San Juan Tianguismanalco (22 km SE), and Atlixco (23 km SE), and in some areas of the City of Puebla (~50 km to the E). The main tremor episode was accompanied by incandescent fragments that reached up to 500 m distance from Popocatépetl (chiefly NE). As reported on the 8th, during the last 24 hours CENAPRED detected 40 low intensity exhalations; 2 additional stronger ones sent a small amounts of ash towards the SE. Tremor during early May generally remained below daily intervals of up to a few hours.

On 10 May 2013 CENAPRED noted that during the last 24 hours there occurred 46 generally small exhalations. In addition, two explosions occurred, of moderate magnitude, sending ash ~1 km above the crater. Tremor duration for that interval lasted ~3 hours, including some time periods with high-amplitude signals. Three small volcano-tectonic earthquakes also occurred. A second report later on 10 May indicated that during 1142-1443 a series of ash emissions and periods of spasmodic and harmonic tremor occurred with ash plumes rising as much as 1 km above the crater, again producing ashfall. Similar plume heights were seen on 11 May, and the daily report noted there were in the last 24 hours a total of 53 (chiefly small-to-moderate) exhalations.

According to CENAPRED, seismicity had intensified on the afternoon and night prior to 12 May (when the Alert Level rose to Yellow, Phase Three, stipulating a 12 km radius exclusionary zone). Additionally, the report for 12 May 2013 said that in the last 24 h, 43 exhalations of low and moderate intensity were recorded. In general, steam-and-gas plumes with small amounts of ash rose from the crater. Although foggy conditions sometimes limited visibility, sporadic ejections of incandescent tephra fell back into the crater and onto the NNE flank, 300 m from the crater rim. Tremor registered in 1-2 hour intervals, continuously or in segments. Each such interval began with an eruptive burst of moderate intensity. The most important burst took place at 1700 on the 12th and was perceived by many residents in the E and SE sectors.

On 13 May 2013 steam-and-gas plumes were observed rising from the crater during periods of good visibility. On 14 May an explosive event generated an ash plume that rose to 3 km altitude. Incandescent tephra landed up to 600 m away on the NE flank. Cloud cover again obscured summit views. Seismicity, including tremor, remained elevated. The histogram in the daily report listed 25 exhalations during the past 24 hours.

On 14 May 2013, volcanologists aboard an overflight observed a lava dome 350 m in diameter and 50 m thick, found the dome slightly deflated after an explosion. Similar dome- related events seemingly took place again during the next few days. The histogram in the daily report listed 41 exhalations during the past 24 hours.

CENAPRED noted a vigorous eruption at 0146 on 15 May that discharged an ash plume to over 3 km above the crater rim, blown NE sending tephra up to 1.5 km downslope. At 1804 that day a second blast sent a column to somewhat below 3.5 km above the crater, blown N. Both these events correlated with spasmodic tremor. The histogram in the daily report listed 56 exhalations during the past 24 hours.

On 16 May 2013, some intervals of tremor again corresponded with discharge of glowing fragments, the majority of which fell back into the crater (a process frequently mentioned throughout the reporting interval). Ash plumes rose 2 km and drifted NE. Minor ashfall was reported in Paso de Cortés, 7 km N. Incandescent tephra reached 400 m from the crater rim to the N and NE. Seismometers registered an Mc 2.2 earthquake. The histogram in the daily report listed 55 exhalations during the past 24 hours.

Two punctuated eruptions were described for the 24-hour interval ending at 1000 on the 17th (one reaching 4 km above the rim) The first took place at 2214, when the crater issued a strong explosion; the resulting incandescent fragmental material covered the flanks to 1.5 km distance and the associated gas-and-ash column rose to under 3 km above the crater, drifting NE. The second took place at 0028, generating an eruption column to 4 km above the crater and casting glowing fragments up to 1.5 km from the crater. The report for the 17th said that moderate-to-small exhalations during the past 24 hours totaled 31. On 17, 18, 19, and 20 May 2013 histograms in the respective CENAPRED reports noted that in the past 24 hours they each registered 31, 18, 24, and 54 exhalations.

During an overflight on 18 May, volcanologists observed the active crater, 200 m wide and 40 m deep, located in the dome's surface. The rest of the dome was covered with rock fragments. Tephra had landed as far as 0.5 km down the NE flank. CENAPRED inferred that the missing material forming this crater was likely excavated by explosions associated with hours of tremor that took place during 14-17 May.

On 23-27 May 2013, tremor decreased. A flight on 28 May captured several photos, one of which appears in figure 67. Note the steep crater within the ring-shaped dome and the abundance of fragmental character of some material on the dome's surface. The CENAPRED caption also drew attention to marks made by ballistic material that burrowed into the snow and ice in the summit area.

Figure 67. Aerial photo taken looking downward at the summit area of Popocatépetl on 28 May 2013. The summit hosts a deep, steep sided circular crater, within which grows a dome. The dome is frequently reamed out by powerful explosive bursts leaving the dome with a crater as seen here. Courtesy of CENAPRED (from their 1 June 2013 daily report).

During 1-7 June exhalations on the daily histograms ranged between 32 and 93. They were often described as of low intensity (steam rich and ash poor), but in some cases they were described as reaching moderate intensity. Cloud cover often prevented visual observations. Volcano-tectonic earthquakes up to Mc 2.7 took place. On 7 June 2013 the Alert Level was lowered to Yellow, Phase Two.

During the rest of June 2013, significant emissions continued. For example, during 12-17 June 2013, plumes containing ash rose as high as 4 km above the crater, and ashfall was reported in many nearby villages (figure 68). For the eruption on the 17th, perceptible ash fell as far as the SE portion of Mexico City. The eruption on the 17th was accompanied by tremor with a duration of over 2 hours and other seismicity also remained at times high.

Figure 68. Webcam image of an explosion at Popocatépetl on 17 June 2013 (at 13:26:55 local time, which corresponds to 18:26:55 UTC). The explosion generated an ash plume that reached greater than 4 km above the crater and threw incandescent fragments up to 2 km out of the crater, causing small grassland fires. Courtesy of CENAPRED.

An overflight on 25 June led to the insight that eruptions in the past few days had further altered the dome. It then had the dimensions of 250 m in diameter and 60 m deep.

According to CENAPRED's daily report on 3 July 2013, seismic activity increased again during the past 24 hours when the seismic network detected tremor for 36 minutes and two larger earthquakes (at 0407 and 0918 on the 3rd) with respective coda magnitudes, Mc 2.9 and 2.6. The daily report noted 84 exhalations on the part of the histogram for the last 24 hour interval ending at 1000 on 3 July. This was accompanied by persistent gas and ash emissions and diffuse ash plumes that rose 2-3.5 km above the crater and produced ashfall in areas as far as México City. Incandescent tephra was ejected short distances onto the N and E flanks.

This increased activity continued on 4 July 2013. According to news articles, multiple airlines canceled flights to and from the México City and Toluca (105 km WNW) airports on 4 July. The number of cancelled flights, according to the news, was 47. Flights resumed later that day.

On 5 July 2013, almost continuous tremor was recorded. Ash plumes drifted NW. Scientists employed both infrared webcamera imaging and an overflight to observe continuously ejected incandescent tephra that landed as far away as 1.5 km from the crater on almost all flanks, and an ash plume that rose 2 km. Cloud cover often obscured visual observations. A news article stated that four airlines canceled a total of 17 flights.

On 6 July 2013, low frequency, high amplitude tremor was accompanied by gas, steam, and ash emissions that rose 3 km. Three explosions were detected, but cloud cover prevented visual confirmation. News articles noted ashfall again in parts of México City. Government officials raised the Alert Level to Yellow, Phase Three, excluding the public within a 12 km radius of the crater. Later that day, the low frequency tremor amplitude decreased, followed by diminishing emissions of gas and ash.

During 7-9 July 2013, tremor was accompanied by persistent emissions of steam, gas, and small amounts of ash that drifted WSW and NW; cloud cover continued to hinder visual observations. Three explosions produced gas containing ash. Incandescence and ejected incandescent tephra were sometimes observed. During overflights on 7 and 10 July, scientists observed that a new lava dome, 250 m in diameter and 20 m thick, had recently formed in the crater.

During an overflight on 15 July 2013, scientists observed a 200-m wide and 20 to 30 m deep crater in the lava dome. The attributed the new morphology last seen on 10 July to dome destruction owing to explosions in the past few days. They also reported on M 2.3, 1.8 and 1.7 earthquakes, as well as 82 minutes of high-frequency tremor on 15 July 2013.

Emissions and occasional explosions that generated plumes with some ash continued during 10-16 July 2013 (figure 68). According to a news article, on 12 July 2013 an Alaska Airlines flight to México City's international airport was canceled and operations at a small airport in Puebla were suspended.

On 23 July 2013, the Alert Level was lowered to Yellow, Phase Two, a status that prevailed through December 2014 (the end of this reporting interval).

On 31 July 2013 a clear decrease in the size of the water vapor and gas plumes was observed; plumes blew down the NW flank and rose only 100 m above the crater rim. An explosion was detected at 2312 on 1 August, but cloud cover prevented confirmation of any ejecta. On 2 August minor amounts of ash fell in the Tepetlixpa, Atlautla, Ecatzingo, and Ozumba municipalities of Mexico State. On 4 August emissions of gas, steam, and ash drifted NW. During 5-6 August a few observed plumes rose 1-2 km and drifted WNW, W, and WSW.

On 14 August 2013 a period of tremor was accompanied by an ash emission that drifted W and fell on towns as far as ~20 km away. Gas-and-steam plumes were observed during 15-16 August. A period of tremor on 17 August was accompanied by an ash plume that rose 1.5 km and drifted WSW. Ash fell in in towns as far as 65 km SW (Cuernavaca). On 18 August tremor was accompanied by an ash emission that rose 1.2 km and drifted SW. On 19 August minor steam-and-gas emissions drifted W. During 19-20 August emissions likely contained small amounts of ash but cloud cover prevented confirmation. On 28 August ash plumes rose 200-800 m and drifted SW. Gas-and-steam plumes were observed the next day and on the 30th an ash plume rose 1 km above the crater and drifted W.

During much of September and October 2013 clouds sometimes blocked clear views of the volcano. The volcano continued to undergo seismic unrest and to emit steam and gas plumes often containing minor amounts of ash. Early September ash blew WSW to fall on settlements as far as 24 km away (including, on the 1st, at Tetela del Volcán, 20 km SW, and Ocuituco, 24 km SW, and on the 2nd, at Ecatzingo, 15 km SW. On 4 September the number of daily exhalation during the previous 15 days averaged at 5, but on that day it stood at 44 exhalations. Average values for 15 day intervals remained under ~25 during September through December 2013.

Other observational details from this interval are similar to those noted above. For example, on 24 October an explosion at 2111 produced an ash plume that rose 1 km and drifted SW. Eight low-intensity explosions on 26 October increased gas and steam emissions and produced slight amounts of ash.

Despite the low number of exhalations near year's end, during 30 October 5 November 2013, exhalations were frequently detected, varying from 30 to 97 times per day. Between 31 October and 5 November, four volcano tectonic earthquakes were recorded (Mc 2.1-2.5). Tremor was frequently detected; on 1 November, 3 hours and 21 minutes of high-frequency tremor was recorded.

During November 2013 tremor durations reached highs on the 5th, 6th, and 17th, respectively, at 55, 60, and 67 minutes. November's larger local earthquakes reported by CENAPRED included the following: on the 5th, Mc 2.1-2.5; 6th, Mc 2.7; 9th, Mc 2.3; 10th, two cases with Mc 2.1; 11th, Mc 2.3, 18th, three cases with Mc approaching 2; 20th, 6 cases with some Mc approaching 2.5; 21st; Mc 2.0; 22nd, five cases, Mc 2-3.5; 23rd, Mc 2.0; 24th, two cases with Mc 2.1; 25th, Mc 2.0; 28th, Mc 1.8; and 29th, Mc 1.2.

The Washington Volcanic Ash Advisory Center (VAAC) issued advisories for Popocatépetl every month during 2013, except for November and December 2013. The advisories were most numerous during April through July 2013. According to CENAPRED, a daily average of about 6,000 metric tons of sulfur dioxide was emitted during both 2013 and 2014.

2014 activity. During 2014, the Washington VAAC issued advisories for Popocatépetl every month, except for March. The number of advisories issued was considerably lower than that for 2013. At year's end, the Alert Level remained at Yellow, Phase Two.

Activity in 2014 was broadly similar to that in 2013, with above-mentioned frequent gas-and-steam emissions, often with minor ash content. Issues with limited visibility at times due to cloud cover also remained.

The 15-day average of the daily exhalations often stood at less than 4 during January and through 19 February 2014. Activity increased during 19-25 February 2014. At least eight explosions generated plumes (mostly ash) that rose 1-2 km above the crater. An explosion at 1233 on 21 February sent an ash plume to 4 km above the crater rim. On 26 February, scientists aboard an overflight observed that another lava dome (dome number 48) had been destroyed, leaving a funnel shaped cavity about 80 m deep. A new dome 20 30 m wide was at the bottom of the cavity. On 27 February, activity decreased considerably.

CENAPRED's 15-day average of the daily exhalations stood at 7 or below during March 2014 but it rose to 34 by 18 April 2014 and dropping to 22 by the end of that month. It rose again in late May 2014 (to 45 on 31 May). On 16 June it stood at 57; and for the last half of June it declined to below 10.

The daily value reached 480 exhalations on 4 and 6 June 2014, a new record.

Monitored and eruptive activity briefly increased in early July 2014. For example, CENAPRED reported tremor on 2 July (maximum of 80 minutes in 24 hours) and 12 July (minimum of 8 minutes). Up to 216 exhalations of low and moderate intensity were detected on 9 July. The 15-day average of the daily exhalations also rose during early July 2014, reaching over 50 during the first half of the month but dropping towards the end to 15 (on the 31st).

The first half of August had a 15-day average of 33 daily exhalations and the second half, 46 daily exhalations. Those averages (first half of the month and second half of the month) were as follows for the rest of the year: September (14 and 31); October (40 and 39); November (55 and 13); and December (41 and 72).

During 27 August and 2 September 2014, plumes reached as high as 3 km above the crater. Tremor and volcanic-tectonic earthquakes were recorded in early September.

On 17 September 2014, a day with 126 exhalations recorded by CENAPRED's monitoring system, an ash emission at 1813 resulted from a moderate explosion. The emission reached 3 km above the crater rim. It blew SE and light ash fell at villages in that direction. During the same day five other exhalations reached ~1.5 km above the crater rim. During 7-8 October 2014 ashfall was reported in Cuautla (43 km SW), Tetela del Volcán (20 km SW), Huaquechula (30 km SSW), and Morelos (60 km SW). On 12 October ash plumes rose 2 km and drifted NE. Ashfall was reported in Paso de Cortés (8 km NNW) and Tlalmanalco (30 km NW).

CENAPRED reported that during a 14 October 2014 overflight, volcanologists observed that the diameter of the inner crater (formed in July 2013) had increased to 350 m. The bottom of the inner crater floor was 100 m below the floor of the main crater, cup shaped, and covered with tephra. No sign of the lava dome (number 52) emplaced in early August 2014 was visible. Steam emissions originated from a crack in the N wall of the inner crater and ash emission came from the bottom of the crater.

Although for brevity we have generally excluded examples of explosions and ashfall for September and October 2014, which were broadly similar to previous months, a small explosion at 0317 on 25 October ejected tephra 100 m outboard onto the S flank. A steam-and-gas plume containing a small amount of ash rose 1.5 km above the crater and drifted SW. Ashfall was reported in Tetela del Volcán (20 km SW). A small explosion at 0111 on 26 October ejected a plume that rose 1.1 km above the crater rim and sent tephra 200 m onto the N flank.

Histograms in daily reports issued during 3-5 November 2014 described exhalations totaling 267, 190, and 147, respectively. These were broadly described as a continuous gas-and-steam plume, mainly with minor amounts of ash. Some more vigorous and ash rich emissions occurred and, for example, on 5 November the plume rose as high as 1 km and caused light ashfall in Paso de Cortés. That daily report also showed videos that showed incandescent fragments spreading ~800 m over the upper flanks. The 5 November report also showed a seismic record captured in the interval 2000 on the 4th to 0130 on the 5th illustrating ~190 seismic events. About an hour after those events, the same record indicated an Mc 2.1 earthquake. On 6 November, a small rockslide on the SW flank was recorded by a webcam and the seismic network. Scientists aboard an overflight observed a new dome (number 53), emplaced during 4 5 November; it was an estimated 250 m in diameter and 30 m thick.

During 7 11 November 2014, seismicity indicated continuing gas-and-steam emissions, with small amounts of ash. Incandescence from the crater was observed most nights. Explosions during 10-11 November ejected incandescent tephra and generated ash plumes that rose 1.2 km above the crater. Gas-and-steam emissions continued through the rest of November.

During December 2014, occasional explosions continued, generating ash plumes that rose as high as 3-3.5 km, resulting in minor ashfall on nearby villages. One or more rockslides were noted in addition to the usual small ash plumes, the occasional incandescence at the crater and associated with tephra. One plume on 8 December rose to 3 km above the crater. CENAPRED reported that the international airport in Puebla was temporarily closed on 17 December 2014 due to ashfall from an explosion that generated a 2 km high ash plume. The explosion also ejected incandescent tephra that landed 700 m down the N flank. During an overflight during the last half of December, volcanologists observed a lava dome at the bottom of the crater. The Alert Level remained at Yellow, Phase Two.

References: de la Cruz-Reyna, S, Yokoyama, I, Martínez-Bringas, A, and Ramos, E, 2008, Precursory seismicity of the 1994 eruption of Popocatépetl Volcano, Central Mexico. Bulletin of Volcanology, 70(6), 753-767.

González-Mellado, AO, and de la Cruz-Reyna, S, 2008, A simplified equation of state for the density of silicate hydrous magmas: an application to the Popocatépetl buoyancy-driven dome growth process. Journal of Volcanology and Geothermal Research, 171(3), 287-300.

Tárraga, M, de la Cruz-Reyna, S, Mendoza-Rosas, A, Carniel, R, Martínez-Bringas, A, García, A, and Ortiz, R, 2012, Dynamical parameter analysis of continuous seismic signals of Popocatépetl volcano (Central Mexico): A case of tectonic earthquakes influencing volcanic activity. Acta Geophysica, 60(3), 664-681.

Wright, RS, de La Cruz-Reyna, S, Harris, A, Flynn, L, and Gomez-Palacios, JJ, 2002, Infrared satellite monitoring at Popocatépetl: Explosions, exhalations, and cycles of dome growth, J. Geophys. Res., 107(B8), doi: 10.1029/2000JB000125.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Centro Nacional de Prevencion de Desastres (CENAPRED) (URL: https://www.gob.mx/cenapred/); Washington Volcanic Ash Advisory Center (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/index.html); Agence France Presse (AFP)(URL: http://www.afp.com/en/); Associated Press (URL: http://www.ap.org/); Stuff (URL: http://www.stuff.co.nz).