Ioto (Iwo-jima), located about 1,200 km S of Tokyo, lies within a 9-km-wide submarine caldera along the Izu-Bonin-Mariana volcanic arc. Previous eruptions date back to 1889 and have consisted of dominantly phreatic explosions, pumice deposits during 2001, and discolored water. A submarine eruption during July through December 2022 was characterized by discolored water, pumice deposits, and gas emissions (BGVN 48:01). This report covers a new eruption during October through December 2023, which consisted of explosions, black ejecta, discolored water, and floating pumice, based on information from the Japan Meteorological Association (JMA), the Japan Coast Guard (JCG), and satellite data.

JMA reported that an eruption had been occurring offshore of Okinahama on the SE side of the island since 21 October, which was characterized by volcanic tremor, according to the Japan Maritime Self-Defense Force (JMSDF) Iwo Jima Air Base (figure 22). According to an 18 October satellite image a plume of discolored water at the site of this new eruption extended NE (figure 23). During an overflight conducted on 30 October, a vent was identified about 1 km off the coast of Okinahama. Observers recorded explosions every few minutes that ejected dark material about 20 m above the ocean and as high as 150 m. Ejecta from the vent formed a black-colored island about 100 m in diameter, according to observations conducted from the air by the Earthquake Research Institute of the University of Tokyo in cooperation with the Mainichi newspaper (figure 24). Occasionally, large boulders measuring more than several meters in size were also ejected. Observations from the Advanced Land Observing Satellite Daichi-2 and Sentinel-2 satellite images also confirmed the formation of this island (figure 23). Brown discolored water and floating pumice were present surrounding the island.

Figure 22. Map of Ioto showing the locations of recorded eruptions from 1889 through December 2023. The most recent eruption occurred during October through December 2023 and is highlighted in red just off the SE coast of the island and E of the 2001 eruption site. A single eruption highlighted in green was detected just off the NE coast of the island on 18 November 2023. From Ukawa et al. (2002), modified by JMA.

Figure 23. Satellite images showing the formation of the new island formation (white arrow) off the SE (Okinahama) coast of Ioto on 18 October 2023 (top left), 27 November 2023 (top right), 2 December 2023 (bottom left), and 12 December 2023 (bottom right). Discolored water was visible surrounding the new island. By December, much of the island had been eroded. Courtesy of Copernicus Browser.

Figure 24. Photo showing an eruption off the SE (Okinahama) coast of Ioto around 1230 on 30 October 2023. A column of water containing black ejecta is shown, which forms a new island. Occasionally, huge boulders more than several meters in size were ejected with the jet. Dark brown discolored water surrounded the new island. Photo has been color corrected and was taken from the S by the Earthquake Research Institute, University of Tokyo in cooperation of Mainichi newspaper. Courtesy of JMA.

The eruption continued during November. During an overflight on 3 November observers photographed the island and noted that material was ejected 169 m high, according to a news source. Explosions gradually became shorter, and, by the 3rd, they occurred every few seconds; dark and incandescent material were ejected about 800 m above the vent. On 4 November eruptions were accompanied by explosive sounds. Floating, brown-colored pumice was present in the water surrounding the island. There was a brief increase in the number of volcanic earthquakes during 8-14 November and 24-25 November. The eruption temporarily paused during 9-11 November and by 12 November eruptions resumed to the W of the island. On 10 November dark brown-to-dark yellow-green discolored water and a small amount of black floating material was observed (figure 25). A small eruption was reported on 18 November off the NE coast of the island, accompanied by white gas-and-steam plumes (figure 23). Another pause was recorded during 17-19 November, which then resumed on 20 November and continued erupting intermittently. According to a field survey conducted by the National Institute for Disaster Prevention Science and Technology on 19 November, a 30-m diameter crater was visible on the NE coast where landslides, hot water, and gray volcanic ash containing clay have occurred and been distributed previously. Erupted blocks about 10 cm in diameter were distributed about 90-120 m from the crater. JCG made observations during an overflight on 23 November and reported a phreatomagmatic eruption. Explosions at the main vent generated dark gas-and-ash plumes that rose to 200 m altitude and ejected large blocks that landed on the island and in the ocean (figure 26). Discolored water also surrounded the island. The size of the new island had grown to 450 m N-S x 200 m E-W by 23 November, according to JCG.

Figure 25. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 10 November showing discolored water and a small amount of black floating material were visible surrounding the island. Photo has been color corrected. Photographed by JCG courtesy of JMA.

Figure 26. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 23 November showing a phreatomagmatic eruption that ejected intermittent pulses of ash and dark material that rose to 200 m altitude. Photo has been color corrected. Photographed by JCG courtesy of JMA.

The eruption continued through 11 December, followed by a brief pause in activity, which then resumed on 31 December, according to JMA. Intermittent explosions produced 100-m-high black plumes at intervals of several minutes to 30 minutes during 1-10 December. Overflights were conducted on 4 and 15 December and reported that the water surrounding the new island was discolored to dark brown-to-dark yellow-green (figure 27). No floating material was reported during this time. In comparison to the observations made on 23 November, the new land had extended N and part of it had eroded away. In addition, analysis by the Geospatial Information Authority of Japan using SAR data from Daichi-2 also confirmed that the area of the new island continued to decrease between 4 and 15 December. Ejected material combined with wave erosion transformed the island into a “J” shape, 500-m-long and with the curved part about 200 m offshore of Ioto. The island was covered with brown ash and blocks, and the surrounding water was discolored to greenish-brown and contained an area of floating pumice. JCG reported from an overflight on 4 December that volcanic ash-like material found around the S vent on the NE part of the island was newly deposited since 10 November (figure 28). By 15 December the N part of the “J” shaped island had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands (figure 27).

Figure 27. Photos of the new island formed off the SE (Okinahama) coast of Ioto on 4 December 2023 (left) and 15 December 2023 (right). No gas-and-ash emissions or lava flows were observed on the new land. Additionally, dark brown-to-dark yellow-green discolored water was observed surrounding the new land. During 4 and 15 December, the island had eroded to where the N part of the “J” shape had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands. Courtesy of Copernicus Browser.

Figure 28. Photo of new volcanic ash-deposits (yellow dashed lines) near the S vent on the NE coast of Ioto taken by JCG on 4 December 2023. White gas-and-steam emissions were also visible (white arrow). Photo has been color corrected. Courtesy of JMA.

References. Ukawa, M., Fujita, E., Kobayashi, T., 2002, Recent volcanic activity of Iwo Jima and the 2001 eruption, Monthly Chikyu, Extra No. 39, 157-164.

Geologic Background. Ioto, in the Volcano Islands of Japan, lies within a 9-km-wide submarine caldera. The volcano is also known as Ogasawara-Iojima to distinguish it from several other "Sulfur Island" volcanoes in Japan. The triangular, low-elevation, 8-km-long island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks that are more alkalic than those of other volcanoes in this arc. The island has undergone uplift for at least the past 700 years, accompanying resurgent doming of the caldera; a shoreline landed upon by Captain Cook's surveying crew in 1779 is now 40 m above sea level. The Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama. Numerous recorded phreatic eruptions, many from vents on the W and NW sides of the island, have accompanied the uplift.

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); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo22-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Asahi, 5-3-2, Tsukiji, Chuo Ward, Tokyo, 104-8011, Japan (URL: https://www.asahi.com/ajw/articles/15048458).

Puracé, located in Colombia, is a stratovolcano that contains a 500-m-wide summit crater. It is part of the Los Coconucos volcanic chain that is a NW-SE trending group of seven cones and craters. The most recent eruption occurred during March 2022 that was characterized by frequent seismicity and gas-and-steam emissions (BGVN 47:06). This report covers a brief eruption during November 2023 based on monthly reports from the Popayán Observatory, part of the Servicio Geologico Colombiano (SGC).

Activity during November 2022 through November 2023 primarily consisted of seismicity: VT-type events, LP-type events, HB-type events, and TR-type events (table 4). Maximum sulfur dioxide values were measured weekly and ranged from 259-5,854 tons per day (t/d) during November 2022 through April 2023. White gas-and-steam emissions were also occasionally reported.

SGC issued a report on 25 October that noted a significant increase in the number of earthquakes associated with rock fracturing. These earthquakes were located SE of the crater between Puracé and Piocollo at depths of 1-4 km. There were no reported variations in sulfur dioxide values, but SGC noted high carbon dioxide values, compared to those recorded in the first half of 2023.

SGC reported that at 1929 on 16 November the seismic network detected a signal that was possibly associated with a gas-and-ash emission, though it was not confirmed in webcam images due to limited visibility. On 17 November an observer confirmed ash deposits on the N flank. Webcam images showed an increase in degassing both inside the crater and from the NW flank, rising 700 m above the crater.

Table 4. Seismicity at Puracé during November 2022-November 2023. Volcano-tectonic (VT), long-period (LP), hybrid (HB), and tremor (TR) events are reported each month. Courtesy of SGC.

Geologic Background. Puracé is an active andesitic volcano with a 600-m-diameter summit crater at the NW end of the Los Coconucos Volcanic Chain. This volcanic complex includes nine composite and five monogenetic volcanoes, extending from the Puracé crater more than 6 km SE to the summit of Pan de Azúcar stratovolcano. The dacitic massif which the complex is built on extends about 13 km NW-SE and 10 km NE-SW. Frequent small to moderate explosive eruptions reported since 1816 CE have modified the morphology of the summit crater, with the largest eruptions in 1849, 1869, and 1885.

Information Contacts: Servicio Geologico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www.sgc.gov.co/volcanes).

Suwanosejima is an 8-km-long island that consists of a stratovolcano and two active summit craters, located in the northern Ryukyu Islands, Japan. Volcanism over the past century has been characterized by Strombolian explosions, ash plumes, and ashfall. The current eruption began in October 2004 and has more recently consisted of frequent eruption plumes, explosions, and incandescent ejecta (BGVN 48:07). This report covers similar activity of ash plumes, explosions, and crater incandescence during July through October 2023 using monthly reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity during the reporting period was relatively low; only one low-power thermal anomaly was detected during mid-July and one during early August, based on a MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph of the MODIS thermal anomaly data. On two clear weather days, a thermal anomaly was visible in infrared satellite images (figure 81).

Figure 81. Infrared (bands B12, B11, B4) satellite imagery showing a thermal anomaly (bright yellow-orange) at the Otake crater of Suwanosejima on 23 September 2023 (left) and 18 October 2023 (right). Courtesy of Copernicus Browser.

Low-level activity was reported at the Otake crater during July and no explosions were detected. Eruption plumes rose as high as 1.8 km above the crater. On 13 July an ash plume rose 1.7 km above the crater rim, based on a webcam image. During the night of the 28th crater incandescence was visible in a webcam image. An eruptive event reported on 31 July produced an eruption plume that rose 2.1 km above the crater. Seismicity consisted of 11 volcanic earthquakes on the W flank, the number of which had decreased compared to June (28) and 68 volcanic earthquakes near the Otake crater, which had decreased from 722 in the previous month. According to observations conducted by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Toshima Village, and JMA, the amount of sulfur dioxide emissions released during the month was 400-800 tons per day (t/d).

Eruptive activity in the Otake crater continued during August and no explosions were reported. An eruptive event produced a plume that rose 1 km above the crater at 1447 on 12 August. Subsequent eruptive events were recorded at 0911 on 16 August, at 1303 on 20 August, and at 0317 on 21 August, which produced ash plumes that rose 1-1.1 km above the crater and drifted SE, SW, and W. On 22 August an ash plume was captured in a webcam image rising 1.4 km above the crater (figure 82). Multiple eruptive events were detected on 25 August at 0544, 0742, 0824, 1424, and 1704, which generated ash plumes that rose 1.1-1.2 km above the crater and drifted NE, W, and SW. On 28 August a small amount of ashfall was observed as far as 1.5 km from the crater. There were 17 volcanic earthquakes recorded on the W flank of the volcano and 79 recorded at the Otake crater during the month. The amount of sulfur dioxide emissions released during the month was 400-800 t/d.

Figure 82. Webcam image of an ash plume rising 1.4 km above Suwanosejima’s Otake crater rim on 22 August 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, August 2023).

Activity continued at the Otake crater during September. Occasionally, nighttime crater incandescence was observed in webcam images and ashfall was reported. An eruptive event at 1949 on 4 September produced an ash plume that rose 1 km above the crater and drifted SW. On 9 September several eruption events were detected at 0221, 0301, and 0333, which produced ash plumes that rose 1.1-1.4 km above the crater rim and drifted W; continuous ash emissions during 0404-0740 rose to a maximum height of 2 km above the crater rim (figure 83). More eruptive events were reported at 1437 on 10 September, at 0319 on 11 September, and at 0511 and 1228 on 15 September, which generated ash plumes that rose 1-1.8 km above the crater. During 25, 27, and 30 September, ash plumes rose as high as 1.3 km above the crater rim. JMA reported that large blocks were ejected as far as 300 m from the center of the crater. There were 18 volcanic earthquakes detected beneath the W flank and 82 volcanic earthquakes detected near the Otake crater. The amount of sulfur dioxide released during the month ranged from 600 to 1,600 t/d.

Figure 83. Webcam image of an ash plume rising 2 km above Suwanosejima’s Otake crater rim on 9 September 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, September 2023).

Activity during early-to-mid-October consisted of occasional explosions, a total number of 13, and ash plumes that rose as high as 1.9 km above the Otake crater rim on 29 October (figure 84). These explosions are the first to have occurred since June 2023. Continuous ash emissions were reported during 0510-0555 on 1 October. Explosions were recorded at 0304, 2141, and 2359 on 2 October, at 0112 on 3 October, and at 1326 on 6 October, which produced ash plumes that rose as high as 1 km above the crater rim and drifted SW and W. An explosion was noted at 0428 on 3 October, but emission details were unknown. A total of eight explosions were recorded by the seismic network at 1522 on 14 October, at 0337, 0433, 0555, 1008, and 1539 on 15 October, and at 0454 and 0517 on 16 October. Ash plumes from these explosions rose as high as 900 m above the crater and drifted SE. Eruptive events during 25-27 and 29-30 October generated plumes that rose as high as 1.9 km above the crater and drifted SE, S, and SW. Ash was deposited in Toshima village (3.5 km SSW). Eruptive activity occasionally ejected large volcanic blocks as far as 600 m from the crater. Nighttime crater incandescence was visible in webcams. Intermittent ashfall was reported as far as 1.5 km from the crater. There were 43 volcanic earthquakes detected on the W flank during the month, and 184 volcanic earthquakes detected near the Otake crater. The amount of sulfur dioxide emitted ranged between 400 and 900 t/d.

Figure 84. Webcam image of an ash plume rising 1.9 km above Suwanosejima’s Otake crater on 29 October 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, October 2023).

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

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/).

Etna, located on the Italian island of Sicily, has had documented eruptions dating back to 1500 BCE. Activity typically originates from multiple cones at the summit, where several craters have formed and evolved. The currently active craters are Northeast Crater (NEC), Voragine (VOR), and Bocca Nuova (BN), and the Southeast Crater (SEC); VOR and BN were previously referred to as the “Central Crater”. The original Southeast crater formed in 1978, and a second eruptive site that opened on its SE flank in 2011 was named the New Southeast Crater (NSEC). Another eruptive site between the SEC and NSEC developed during early 2017 and was referred to as the "cono della sella" (saddle cone). The current eruption period began in November 2022 and has been characterized by intermittent Strombolian activity, lava flows, and ash plumes (BGVN 48:08). This report updates activity during July through October 2023, which includes primarily gas-and-steam emissions; during July and August Strombolian explosions, lava fountains, and lava flows were reported, based on weekly and special reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV) and satellite data.

Variable fumarolic degassing was reported at all summit craters (BN, VOR, NEC, and SEC) throughout the entire reporting period (table 15). The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on the analysis of MODIS data showed frequent low-to-moderate power thermal anomalies during the reporting period (figure 399). During mid-August there was a pulse in activity that showed an increase in the power of the anomalies due to Strombolian activity, lava fountains, and lava flows. Infrared satellite imagery captured strong thermal anomalies at the central and southeast summit crater areas (figure 400). Accompanying thermal activity were occasional sulfur dioxide plumes that exceeded 2 Dobson Units (DUs) recorded by the TROPOMI instrument on the Sentinel-5P satellite (figure 401).

Table 15. Summary of activity at the four primary crater areas at the summit of Etna during July-October 2023. Information is from INGV weekly reports.

Figure 399. Frequent thermal activity at Etna varied in strength during July through October 2023, as shown on this MIROVA plot (Log Radiative Power). There was a spike in power during mid-August, which reflected an increase in Strombolian activity. Courtesy of MIROVA.

Figure 400. Infrared (bands B12, B11, B4) satellite images showing strong thermal anomalies at Etna’s central and Southeast crater areas on 21 July 2023 (top left), 27 August 2023 (top right), 19 September 2023 (bottom left), and 29 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 401. Sulfur dioxide plumes that exceeded 2 Dobson Units (DUs) rose above Etna on 14 July 2023 (top left), 14 August 2023 (top right), 2 September 2023 (bottom left), and 7 October 2023 (bottom right). These plumes drifted NE, S, SE, and SW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during July and August was relatively low and mainly consisted of degassing at the summit craters, particularly at SEC and BN. Cloudy weather prevented clear views of the summit during early July. During the night of 2 July some crater incandescence was visible at SEC. Explosive activity resumed at SEC during 9-10 July, which was characterized by sporadic and weak ash emissions that rapidly dispersed in the summit area (figure 402). INGV reported moderate Strombolian activity began at 2034 on 14 July and was confined to the inside of the crater and fed by a vent located in the E part of SEC. An ash emission was detected at 2037. A new vent opened on 15 July in the SE part of BN and began to produce continuous gas-and-steam emissions. During an inspection carried out on 28 July pulsating degassing, along with audible booms, were reported at two active vents in BN. Vigorous gas-and-steam emissions intermittently generated rings. On rare occasions, fine, reddish ash was emitted from BN1 and resuspended by the gas-and-steam emissions.

Figure 402. Webcam image taken by the Monta Cagliato camera showing an ash emission rising above Etna’s Southeast Crater (SEC) on 10 July 2023. Photo has been color corrected. Courtesy of INGV (Report 28/2023, ETNA, Bollettino Settimanale, 03/07/2023 - 09/07/2023).

Around 2000 on 13 August INGV reported a sudden increase in volcanic tremor amplitude. Significant infrasonic activity coincided with the tremor increase. Incandescent flashes were visible through the cloud cover in webcam images of SEC (figure 403). Strombolian activity at SEC began to gradually intensify starting at 2040 as seismicity continued to increase. The Aviation Color Code (ACC) was raised to Yellow (the second lowest-level on a four-color scale) at 2126 and then to Orange (the second highest-level on a four-color scale) at 2129 due to above-background activity. The activity rapidly transitioned from Strombolian activity to lava fountains around 2333 that rose 300-400 m above the crater (figure 403). Activity was initially focused on the E vent of the crater, but then the vent located above the S flank of the cone also became active. A lava flow from this vent traveled SW into the drainage created on 10 February 2022, overlapping with previous flows from 10 and 21 February 2022 and 21 May 2023, moving between Monte Barbagallo and Monte Frumento Supino (figure 404). The lava flow was 350 m long, oriented NNE-SSW, and descended to an elevation of 2.8 km. Flows covered an area of 300,000 m² and had an estimated volume of 900,000 m³. The ACC was raised to Red at 2241 based on strong explosive activity and ashfall in Rifugio Sapienza-Piano Vetore at 1.7 km elevation on the S flank. INGV reported that pyroclastic flows accompanied this activity.

Figure 403. Webcam images of the lava fountaining event at Etna during 13-14 August 2023 taken by the Milos (EMV) camera. Images show the start of the event with increasing incandescence (a-b), varying intensity in activity (c-e), lava fountaining and pyroclastic flows (f-g), and a strong ash plume (g). Courtesy of INGV (Report 33/2023, ETNA, Bollettino Settimanale, 08/08/2023 - 14/08/2023).

Figure 404. Map of the new lava flow (yellow) and vent (red) at SEC (CSE) of Etna on 13 August 2023. The background image is a shaded model of the terrain of the summit area obtained by processing Skysat images acquired during on 18 August. The full extent of the lava flow was unable to be determined due to the presence of ash clouds. The lava flow extended more than 350 m to the SSW and reached an elevation of 2.8 km and was located W of Mt. Frumento Supino. CSE = Southeast Crater; CNE = Northeast Crater; BN = Bocca Nuova; VOR = Voragine. Courtesy of INGV (Report 34/2023, ETNA, Bollettino Settimanale, 14/08/2023 - 20/08/2023).

Activity peaked between 0240 and 0330 on 14 August, when roughly 5-6 vents erupted lava fountains from the E to SW flank of SEC. The easternmost vents produced lava fountains that ejected material strongly to the E, which caused heavy fallout of incandescent pyroclastic material on the underlying flank, triggering small pyroclastic flows. This event was also accompanied by lightning both in the ash column and in the ash clouds that were generated by the pyroclastic flows. A fracture characterized by a series of collapse craters (pit craters) opened on the upper SW flank of SEC. An ash cloud rose a few kilometers above the crater and drifted S, causing ash and lapilli falls in Rifugio Sapienza and expanding toward Nicolosi, Mascalucia, Catania, and up to Syracuse. Ashfall resulted in operational problems at the Catania airport (50 km S), which lasted from 0238 until 2000. By 0420 the volcanic tremor amplitude values declined to background levels. After 0500 activity sharply decreased, although the ash cloud remained for several hours and drifted S. By late morning, activity had completely stopped. The ACC was lowered to Orange as volcanic ash was confined to the summit area. Sporadic, minor ash emissions continued throughout the day. At 1415 the ACC was lowered to Yellow and then to Green at 1417.

During the night of 14-15 August only occasional flashes were observed, which were more intense during avalanches of material inside the eruptive vents. Small explosions were detected at SEC at 2346 on 14 August and at 0900 on 26 August that each produced ash clouds which rapidly dispersed into the atmosphere (figure 405). According to a webcam image, an explosive event detected at 2344 at SEC generated a modest ash cloud that was rapidly dispersed by winds. The ACC was raised to Yellow at 2355 on 14 August due to increasing unrest and was lowered to Green at 0954 on 15 August.

Figure 405. Webcam image of an ash plume rising above Etna’s SEC at 0902 (local time) on 26 August taken by the Montagnola EMOV camera. Photo has been color corrected. Courtesy of INGV (Report 35/2023, ETNA, Bollettino Settimanale, 21/08/2023 - 27/08/2023).

Activity during September and October was relatively low and mainly characterized by variable degassing from BN and SEC. Intense, continuous, and pulsating degassing was accompanied by roaring sounds and flashes of incandescence at BN both from BN1 and the new pit crater that formed during late July (figure 406). The degassing from the new pit crater sometimes emitted vapor rings. Cloudy weather during 6-8 September prevented observations of the summit craters .

Figure 406. Webcam image (top) showing degassing from Etna’s Bocca Nuova (BN) crater accompanied by nighttime crater incandescence at 0300 (local time) on 2 September 2023 by the Piedimonte Etneo (EPVH) camera and a photo of incandescence at BN1 and the new pit crater (bottom) taken by an observatory scientist from the E rim of BN during a survey on 2 September 2023. Courtesy of INGV (Report 36/2023, ETNA, Bollettino Settimanale, 28/08/2023 - 03/09/2023).

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: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); 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 MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Aira caldera, located in the northern half of Kagoshima Bay, Japan, contains the post-caldera Sakurajima volcano. Eruptions typically originate from the Minamidake crater, and since the 8th century, ash deposits have been recorded in the city of Kagoshima (10 km W), one of Kyushu’s largest cities. The Minamidake summit cone and crater has had persistent activity since 1955; the Showa crater on the E flank has also been intermittently active since 2006. The current eruption period began during March 2017 and has recently been characterized by intermittent explosions, eruption plumes, and ashfall (BGVN 48:07). This report updates activity during July through October 2023 and describes explosive events, ash plumes, nighttime crater incandescence, and ashfall, according to monthly activity reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity remained at low levels during this reporting period, according to the MIROVA (Middle InfraRed Observation of Volcanic Activity) system (figure 149). There was a slight increase in the number of anomalies during September through October. Occasional thermal anomalies were visible in infrared satellite images mainly at the Minamidake crater (Vent A is located to the left and Vent B is located to the right) (figure 150).

Table 30. Number of monthly explosive events, days of ashfall, area of ash covered, and sulfur dioxide emissions from Sakurajima’s Minamidake crater at Aira during July-October 2023. Note that smaller ash events are not listed. Ashfall days were measured at Kagoshima Local Meteorological Observatory and ashfall amounts represent material covering all the Kagoshima Prefecture. Data courtesy of JMA monthly reports.

Figure 149. Thermal activity at Sakurajima in the Aira caldera was relatively low during July through October 2023, based on this MIROVA graph (Log Radiative Power). There was an increase in the number of detected anomalies during September through October. Courtesy of MIROVA.

Figure 150. Infrared (bands B12, B11, B4) satellite images show a persistently strong thermal anomaly (bright yellow-orange) at the Minamidake crater at Aira’s Sakurajima volcano on 28 September 2023 (top left), 3 October 2023 (top right), 23 October 2023 (bottom left), and 28 October 2023 (bottom right). Vent A is located to the left and Vent B is to the right of Vent A; both vents are part of the Minamidake crater. Courtesy of Copernicus Browser.

JMA reported that during July, there were eight eruptions, three of which were explosion events in the Showa crater. Large blocks were ejected as far as 600 m from the Showa crater. Very small eruptions were occasionally reported at the Minamidake crater. Nighttime incandescence was observed in both the Showa and Minamidake crater. Explosions were reported on 16 July at 2314 and on 17 July at 1224 and at 1232 (figure 151). Resulting eruption plumes rose 700-2,500 m above the crater and drifted N. On 23 July the number of volcanic earthquakes on the SW flank of the volcano increased. A strong Mw 3.1 volcanic earthquake was detected at 1054 on 26 July. The number of earthquakes recorded throughout the month was 545, which markedly increased from 73 in June. No ashfall was observed at the Kagoshima Regional Meteorological Observatory during July. According to a field survey conducted during the month, the daily amount of sulfur dioxide emissions was 1,600-3,200 tons per day (t/d).

Figure 151. Webcam image showing a strong, gray ash plume that rose 2.5 km above the crater rim of Aira’s Showa crater at 1232 on 17 July 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, July 2023).

There were three eruptions reported at the Minamidake crater during August, each of which were explosive. The explosions occurred on 9 August at 0345, on 13 August at 2205, and on 31 August at 0640, which generated ash plumes that rose 800-2,000 m above the crater and drifted W. There were two eruptions detected at Showa crater; on 4 August at 2150 ejecta traveled 800 m from the Showa crater and associated eruption plumes rose 2.3 km above the crater. The explosion at 2205 on 13 August generated an ash plume that rose 2 km above the crater and was accompanied by large blocks that were ejected 600 m from the Minamidake crater (figure 152). Nighttime crater incandescence was visible in a high-sensitivity surveillance camera at both craters. Seismicity consisted of 163 volcanic earthquakes, 84 of which were detected on the SW flank. According to the Kagoshima Regional Meteorological Observatory there was a total of 7 g/m² of ashfall over the course of 10 days during the month. According to a field survey, the daily amount of sulfur dioxide emitted was 1,800-3,300 t/d.

Figure 152. Webcam image showing an eruption plume rising 2 km above the Minamidake crater at Aira at 2209 on 13 August 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, August 2023).

During September, four eruptions were reported, three of which were explosion events. These events occurred at 1512 on 9 September, at 0018 on 11 September, and at 2211 on 13 September. Resulting ash plumes generally rose 800-1,100 m above the crater. An explosion produced an ash plume at 2211 on 13 September that rose as high as 1.7 km above the crater. Large volcanic blocks were ejected 600 m from the Minamidake crater. Smaller eruptions were occasionally observed at the Showa crater. Nighttime crater incandescence was visible at the Minamidake crater. Seismicity was characterized by 68 volcanic earthquakes, 28 of which were detected beneath the SW flank. According to the Kagoshima Regional Meteorological Observatory there was a total of 3 g/m² of ashfall over the course of seven days during the month. A field survey reported that the daily amount of sulfur dioxide emitted was 1,600-2,300 t/d.

Eruptive activity during October consisted of 69 eruptions, 33 of which were described as explosive. These explosions occurred during 4 and 11-21 October and generated ash plumes that rose 500-3,600 m above the crater and drifted S, E, SE, and N. On 19 October at 1648 an explosion generated an ash plume that rose 3.6 km above the crater (figure 153). No eruptions were reported in the Showa crater; white gas-and-steam emissions rose 100 m above the crater from a vent on the N flank. Nighttime incandescence was observed at the Minamidake crater. On 24 October an eruption was reported from 0346 through 0430, which included an ash plume that rose 3.4 km above the crater. Ejected blocks traveled 1.2 km from the Minamidake crater. Following this eruption, small amounts of ashfall were observed from Arimura (4.5 km SE) and a varying amount in Kurokami (4 km E) (figure 154). The number of recorded volcanic earthquakes during the month was 190, of which 14 were located beneath the SW flank. Approximately 61 g/m² of ashfall was reported over eight days of the month. According to a field survey, the daily amount of sulfur dioxide emitted was 2,200-4,200 t/d.

Figure 153. Webcam image showing an ash plume rising 3.6 km above the Minamidake crater at Aira at 1648 on 19 October 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, October 2023).

Figure 154. Photo showing ashfall (light gray) in Kurokami-cho, Sakurajima on 24 October 2023 taken at 1148 following an eruption at Aira earlier that day. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, October 2023).

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

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); 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/).

Nishinoshima is a small island in the Ogasawara Arc, about 1,000 km S of Tokyo, Japan. It contains prominent submarine peaks to the S, W, and NE. Recorded eruptions date back to 1973, with the current eruption period beginning in October 2022. Eruption plumes and fumarolic activity characterize recent activity (BGVN 48:10). This report covers the end of the eruption for September through October 2023, based on information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports, and satellite data.

No eruptive activity was reported during September 2023, although JMA noted that the surface temperature was slightly elevated compared to the surrounding area since early March 2023. The Japan Coast Guard (JCG) conducted an overflight on 20 September and reported white gas-and-steam plumes rising 3 km above the central crater of the pyroclastic cone, as well as multiple white gas-and-steam emissions emanating from the N, E, and S flanks of the crater to the coastline. In addition, dark reddish brown-to-green discolored water was distributed around almost the entire circumference of the island.

Similar low-level activity was reported during October. Multiple white gas-and-steam emissions rose from the N, E, and S flanks of the central crater of the pyroclastic cone and along the coastline; these emissions were more intense compared to the previous overflight observations. Dark reddish brown-to-green discolored water remained visible around the circumference of the island. On 4 October aerial observations by JCG showed a small eruption consisting of continuous gas-and-steam emissions emanating from the central crater, with gray emissions rising to 1.5 km altitude (figure 129). According to observations from the marine weather observation vessel Keifu Maru on 26 October, white gas-and-steam emissions persisted from the center of the pyroclastic cone, as well as from the NW, SW, and SE coasts of the island for about five minutes. Slightly discolored water was visible up to about 1 km.

Figure 129. Aerial photos of gray emissions rising from the central crater of Nishinoshima’s pyroclastic cone to an altitude of 1.5 km on 4 October 2023 taken at 1434 (left) and 1436 (right). Several white gas-and-steam emissions also rose from the N, E, and S flanks of the central crater. Both photos have been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, October, 2023).

Frequent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during September (figure 130). Occasional anomalies were detected during October, and fewer during November through December. A thermal anomaly was visible in the crater using infrared satellite imagery on 6, 8, 11, 16, 18, 21, and 23 September and 8, 13, 21, 26, and 28 October (figure 131).

Figure 130. Low-to-moderate power thermal anomalies were detected at Nishinoshima during September through December 2023, showing a decrease in the frequency of anomalies after September, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 131. Infrared (bands B12, B11, B4) satellite images showing a strong thermal anomaly at the crater of Nishinoshima on 21 September 2023 (left) and 13 October 2023 (right). A strong gas-and-steam plume accompanied the thermal activity, extending NW. 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); 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/).

Kīlauea is on the island of Hawai’i 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 been characterized by low-level lava effusions in the active Halema’uma’u lava lake (BGVN 48:01). This report covers three notable eruption periods during February, June, and September 2023 consisting of lava fountaining, lava flows, and spatter during January through September 2023 using information from daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Activity during January 2023. Small earthquake swarms were recorded on 2 January 2023; increased seismicity and changes in the pattern of deformation were noted on the morning of 5 January. At around 1500 both the rate of deformation and seismicity drastically increased, which suggested magma movement toward the surface. HVO raised the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale) and the Aviation Color Code (ACC) to Orange (the second highest color on a four-color scale) at 1520.

Multiple lava fountains and lava effusions from vents in the central eastern portion of the Halema’uma’u crater began on 5 January around 0434; activity was confined to the eastern half of the crater and within the basin of the western half of the crater, which was the focus of the eruption in 2021-2022 (figure 525). Incandescence was visible in webcam images at 1634 on 5 January, prompting HVO to raise the VAL to Warning (the highest level on a four-level scale) and the ACC to Red (the highest color on a four-color scale). Lava fountains initially rose as high as 50 m above the vent at the onset of the eruption (figure 526) but then declined to a more consistent 5-6 m height in the proceeding days. By 1930 that same day, lava had covered most of the crater floor (an area of about 1,200,000 m²) and the lava lake had a depth of 10 m. A higher-elevation island that formed during the initial phase of the December 2020 eruption remained exposed, appearing darker in images, along with a ring of older lava around the lava lake that was active prior to December 2022. Overnight during 5-6 January the lava fountains continued to rise 5 m high, and the lava effusion rate had slowed.

Figure 525. A reference map of Kīlauea showing activity on 6 January 2023, based on measurements taken from the crater rim at approximately 0900. Multiple eruptive vents (orange color) are on the E floor of Halema’uma’u crater effusing into a lava lake (red color). Lava from these vents flowed laterally across the crater floorcovering an area of 880,000 m2. The full extent of new lava from this eruption (red and pink colors) is approximately 1,120,000 m2. An elevated part of the lake (yellow color) that is higher in elevation compared to the rest of the crater floor was not covered in lava flows. Courtesy of USGS, HVO.

Figure 526. Image of the initial lava fountain at the onset of Kīlauea’s eruption on 5 January 2023 from a newly opened vent in the Halema’uma’u crater at 0449. This lava fountain rose as high as 50 m and ejected lava across the crater floor. Courtesy of USGS, HVO.

On 6 January at 0815 HVO lowered the VAL to Watch and the ACC to Orange due to the declining effusion rates. Sulfur dioxide emission rates ranged from 3,000-12,500 tonnes per day (t/d), the highest value of which was recorded on 6 January. Lava continued to erupt from the vents during 6-8 January, although the footprint of the active area had shrunk; a similar progression has been commonly observed during the early stages of recent eruptions at Halema’uma’u. On 9 January HVO reported one dominant lava fountain rising 6-7 m high in the E half of the crater. Lava flows built up the margins of the lake, causing the lake to be perched. On 10 January the eastern lava lake had an area of approximately 120,000 m² that increased to 250,000 m² by 17 January. During 13-31 January several small overflows occurred along the margins of the E lake. A smaller area of lava was active within the basin in the W half of the crater that had been the focus of activity during 2021-2022. On 19 January just after 0200 a small ooze-out was observed on the crater’s W edge.

Activity during February 2023. Activity continued in the E part of Halema’uma’u crater, as well as in a smaller basin in the W part of the 2021-2022 lava lake (figure 527). The E lava lake contained a single lava fountain and frequent overflows. HVO reported that during the morning of 1 February the large E lava lake began to cool and crust over in the center of the lake; two smaller areas of lava were observed on the N and S sides by the afternoon. The dominant lava fountain located in the S part of the lava lake paused for roughly 45 minutes at 2315 and resumed by midnight, rising 1-2 m. At 0100 on 2 February lava from the S part was effusing across the entire E lava lake area, covering the crusted over portion in the center of the lake and continuing across the majority of the previously measured 250,000 m² by 0400. A small lava pond near the E lake produced an overflow around 0716 on 2 February. On 3 February some lava crust began to form against the N and E levees, which defined the 250,000 m² eastern lava lake. The small S lava fountain remained active, rising 1-6 m high during 3-9 February; around 0400 on 5 February occasional bursts doubled the height of the lava fountain.

Figure 527. An aerial visual and thermal image taken of Kīlauea’s Halema’uma’u crater on 2 February 2023. The largest lava lake is in the E part of the crater, although lava has also filled areas that were previously active in the W part of the crater. The colors of the map indicate temperature, with blues indicative of cooler temperatures and reds indicative of warmer temperatures. Courtesy of USGS, HVO.

A large breakout occurred overnight during 2100 on 4 February to 0900 on 5 February on the N part of the crater floor, equal to or slightly larger in size than the E lava lake. A second, smaller lava fountain appeared in the same area of the E lava lake between 0300 and 0700 on 5 February and was temporarily active. This large breakout continued until 7 February. A small, brief breakout was reported in the S of the E lava lake around midnight on 7 February. In the W lake, as well as the smaller lava pond in the central portion of the crater floor, contained several overflows during 7-10 February and intermittent fountaining. Activity at the S small lava pond and the small S lava fountain within the E lake declined during 9-10 February. The lava pond in the central portion of the crater floor had nearly continuous, expansive flows during 10-13 February; channels from the small central lava pond seemed to flow into the larger E lake. During 13-18 February a small lava fountain was observed in the small lava pond in the central portion of the crater floor. Continuous overflows persisted during this time.

Activity in the eastern and central lakes began to decline in the late afternoon of 17 February. By 18 February HVO reported that the lava effusions had significantly declined, and that the eastern and central lakes were no longer erupting. The W lake in the basin remained active but at a greatly reduced level that continued to decline. HVO reported that this decrease in activity is attributed to notable deflationary tilt that began early on the morning of 17 February and lasted until early 19 February. By 19 February the W lake was mostly crusted over although some weak lava flows remained, which continued through 28 February. The sulfur dioxide emission rates ranged 250-2,800 t/d, the highest value of which was recorded on 6 February.

Activity during March 2023. The summit eruption at Halema’uma’u crater continued at greatly reduced levels compared to the previous two months. The E and central vents stopped effusing lava, and the W lava lake remained active with weak lava flows; the lake was mostly crusted over, although slowly circulating lava intermittently overturned the crust. By 6 March the lava lake in the W basin had stopped because the entire surface was crusted over. The only apparent surface eruptive activity during 5-6 March was minor ooze-outs of lava onto the crater floor, which had stopped by 7 March. Several hornitos on the crater floor still glowed through 12 March according to overnight webcam images, but they did not erupt any lava. A small ooze-out of lava was observed just after 1830 in the W lava lake on 8 March, which diminished overnight. The sulfur dioxide emission rate ranged from 155-321 t/d on 21 March. The VAL was lowered to Advisory, and the ACC was lowered to Yellow (the second lowest on a four-color scale) on 23 March due to a pause in the eruption since 7 March.

Activity during April-May 2023. The eruption at Halema’uma’u crater was paused; no lava effusions were visible on the crater floor. Sulfur dioxide emission rates ranged from 75-185 t/d, the highest of which was measured on 22 April. During May and June summit seismicity was elevated compared to seismicity that preceded the activity during January.

Activity during June 2023. Earthquake activity and changes in the patterns of ground deformation beneath the summit began during the evening of 6 June. The data indicated magma movement toward the surface, prompting HVO to raise the VAL to Watch and the ACC to Orange. At about 0444 on 7 June incandescence in Halema’uma’u crater was visible in webcam images, indicating that a new eruption had begun. HVO raised the VAL to Warning and the ACC to Red (the highest color on a four-color scale). Lava flowed from fissures that had opened on the crater floor. Multiple minor lava fountains were active in the central E portion of the Halema’uma’u crater, and one vent opened on the W wall of the caldera (figure 528). The eruptive vent on the SW wall of the crater continued to effuse into the lava lake in the far SW part of the crater (figure 529). The largest lava fountain consistently rose 15 m high; during the early phase of the eruption, fountain bursts rose as high as 60 m. Lava flows inundated much of the crater floor and added about 6 m depth of new lava within a few hours, covering approximately 10,000 m². By 0800 on 7 June lava filled the crater floor to a depth of about 10 m. During 0800-0900 the sulfur dioxide emission rate was about 65,000 t/d. Residents of Pahala (30 km downwind of the summit) reported minor deposits of fine, gritty ash and Pele’s hair. A small spatter cone had formed at the vent on the SW wall by midday, and lava from the cone was flowing into the active lava lake. Fountain heights had decreased from the onset of the eruption and were 4-9 m high by 1600, with occasional higher bursts. Inflation switched to deflation and summit earthquake activity greatly diminished shortly after the eruption onset.

Figure 528. Photo of renewed activity at Kīlauea’s Halema’uma’u crater that began at 0444 on 7 June 2023. Lava flows cover the crater floor and there are several active source vents exhibiting lava fountaining. Courtesy of USGS, HVO.

Figure 529. Photo of a lava fountain on the SW wall of Kīlauea’s Halema’uma’u crater on 7 June 2023. By midday a small cone structure had been built up. The fissure was intermittently obscured by gas-and-steam plumes. Courtesy of USGS, HVO.

At 0837 on 8 June HVO lowered the VAL to Watch and the ACC to Orange because the initial high effusion rates had declined, and no infrastructure was threatened. The surface of the lava lake had dropped by about 2 m, likely due to gas loss by the morning of 8 June. The drop left a wall of cooled lava around the margins of the crater floor. Lava fountain heights decreased during 8-9 June but continued to rise to 10 m high. Active lava and vents covered much of the W half of Halema’uma’u crater in a broad, horseshoe-shape around a central, uplifted area (figure 530). The preliminary average effusion rate for the first 24 hours of the eruption was about 150 cubic meters per second, though the estimate did not account for vesiculated lava and variations in crater floor topography. The effusion rate during the very earliest phases of the eruption appeared significantly higher than the previous three summit eruptions based on the rapid coverage of the entire crater floor. An active lava lake, also referred to as the “western lava lake” was centered within the uplifted area and was fed by a vent in the NE corner. Two small active lava lakes were located just SE from the W lava lake and in the E portion of the crater floor.

Figure 530. A compilation of thermal images taken of Kīlauea’s Halema’uma’u crater on 7 June 2023 (top left), 8 June 2023 (top right), 12 June 2023 (bottom left), and 16 June 2023 (bottom right). The initial high effusion rates that consisted of numerous lava fountains and lava flows that covered the entire crater floor began to decline and stabilize. A smaller area of active lava was detected in the SW part of the crater by 12 June. The colors of the thermal map represent temperature, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

During 8-9 June the lava in the central lava lake had a thickness of approximately 1.5 m, based on measurements from a laser rangefinder. During 9-12 June the height of the lava fountains decreased to 9 m high. HVO reported that the previously active lava lake in the E part of the crater appeared stagnant during 10-11 June. The surface of the W lake rose approximately 1 m overnight during 11-12 June, likely due to the construction of a levee around it. Only a few small fountains were active during 12-13 June; the extent of the active lava had retreated so that all activity was concentrated in the SW and central parts of Halema’uma’u crater. Intermittent spattering from the vent on the SW wall was visible in overnight webcam images during 13-18 June. On the morning of 14 June a weak lava effusion originated from near the western eruptive vent, but by 15 June there were no signs of continued activity. HVO reported that other eruptive vents in the SW lava lake had stopped during this time, following several days of waning activity; lava filled the lake by about 0.5 m. Lava circulation continued in the central lake and no active lava was reported in the northern or eastern parts of the crater. Around 0800 on 15 June the top of the SW wall spatter cone collapsed, which was followed by renewed and constant spattering from the top vent and a change in activity from the base vent; several new lava flows effused from the top of the cone, as well as from the pre-existing tube-fed flow from its base. Accumulation of lava on the floor resulted in a drop of the central basin relative to the crater floor, allowing several overflows from the SW lava lake to cascade into the basin during the night of 15 June into the morning of 16 June.

Renewed lava fountaining was reported at the eruptive vent on the SW side of the crater during 16-19 June, which effused lava into the far SW part of the crater. This activity was described as vigorous during midday on 16 June; a group of observatory geologists estimated that the lava was consistently ejected at least 10 m high, with some spatter ejected even higher and farther. Deposits from the fountain further heightened and widened the spatter cone built around the original eruptive vent in the lower section of the crater wall. Multiple lava flows from the base of the cone were fed into the SW lava lake and onto the southwestern-most block from the 2018 collapse within Halema’uma’u on 17 June (figure 531); by 18 June they focused into a single flow feeding into the SW lava lake. On the morning of 19 June a second lava flow from the base of the eruptive cone advanced into the SW lava lake.

Figure 531. Nighttime photo of the upwelling area at the base of the spatter cone at Kīlauea’s Halema’uma’u crater on 17 June 2023. This upwelling feeds a lava flow that spreads out to the E of the spatter cone. Courtesy of M. Cappos, USGS.

Around 1600 on 19 June there was a rapid decline in lava fountaining and effusion at the eruptive vent on the SW side of the crater; vent activity had been vigorous up to that point (figure 532). Circulation in the lava lake also slowed, and the lava lake surface dropped by several meters. Overnight webcam images showed some previously eruptive lava still flowing onto the crater floor, which continued until those flows began to cool. By 21 June no lava was erupting in Halema’uma’u crater. Overnight webcam images during 29-30 June showed some incandescence from previously erupted lava flows as they continued to cool. Seismicity in the crater declined to low levels. Sulfur dioxide emission rates ranged 160-21,000 t/d throughout the month, the highest measurement of which was recorded on 8 June. On 30 June the VAL was lowered to Advisory (the second level on a four-level scale) and the ACC was lowered to Yellow. Gradual inflation was detected at summit tiltmeters during 19-30 June.

Figure 532. Photos showing vigorous lava fountaining and lava flows at Kīlauea’s Halema’uma’u crater at the SW wall eruptive vent on 18 June 2023 at 1330 (left). The eruption stopped abruptly around 1600 on 19 June 2023 and no more lava effusions were visible, as seen from the SW wall eruptive vent at 1830 on 19 June 2023 (right). Courtesy of M. Patrick, USGS.

Activity during July-August 2023. During July, the eruption paused; no lava was erupting in Halema’uma’u crater. Nighttime webcam images showed some incandescence from previously erupted lava as it continued to cool on the crater floor. During the week of 14 August HVO reported that the rate in seismicity increased, with 467 earthquakes of Mw 3.2 and smaller occurring. Sulfur dioxide emission rates remained low, ranging from 75-86 t/d, the highest of which was recorded on 10 and 15 August. On 15 August beginning at 0730 and lasting for several hours, a swarm of approximately 50 earthquakes were detected at a depth of 2-3 km below the surface and about 2 km long directly S of Halema’uma’u crater. HVO reported that this was likely due to magma movement in the S caldera region. During 0130-0500 and 1700-2100 on 21 August two small earthquake swarms of approximately 20 and 25 earthquakes, respectively, occurred at the same location and at similar depths. Another swarm of 50 earthquakes were recorded during 0430-0830 on 23 August. Elevated seismicity continued in the S area through the end of the month.

Activity during September 2023. Elevated seismicity persisted in the S summit with occasional small, brief seismic swarms. Sulfur dioxide measurements were relatively low and were 70 t/d on 8 September. About 150 earthquakes occurred during 9-10 September, and tiltmeter and Global Positioning System (GPS) data showed inflation in the S portion of the crater.

At 0252 on 10 September HVO raised the VAL to Watch and the ACC to Orange due to increased earthquake activity and changes in ground deformation that indicated magma moving toward the surface. At 1515 the summit eruption resumed in the E part of the caldera based on field reports and webcam images. Fissures opened on the crater floor and produced multiple minor lava fountains and flows (figure 533). The VAL and ACC were raised to Warning and Red, respectively. Gas-and-steam plumes rose from the fissures and drifted downwind. A line of eruptive vents stretched approximately 1.4 km from the E part of the crater into the E wall of the down dropped block by 1900. The lava fountains at the onset of the eruption had an estimated 50 m height, which later rose 20-25 m high. Lava erupted from fissures on the down dropped block and expanded W toward Halema’uma’u crater. Data from a laser rangefinder recorded about 2.5 m thick of new lava added to the W part of the crater. Sulfur dioxide emissions were elevated in the eruptive area during 1600-1500 on 10 September, measuring at least 100,000 t/d.

Figure 533. Photo of resumed lava fountain activity at Kīlauea’s Halema’uma’u crater on 10 September 2023. The main lava fountain rises approximately 50 m high and is on the E crater margin. Courtesy of USGS, HVO.

At 0810 on 11 September HVO lowered the VAL and ACC back to Watch and Orange due to the style of eruption and the fissure location had stabilized. The initial extremely high effusion rates had declined (but remained at high levels) and no infrastructure was threatened. An eruption plume, mainly comprised of sulfur dioxide and particulates, rose as high as 3 km altitude. Several lava fountains were active on the W side of the down dropped block during 11-15 September, while the easternmost vents on the down dropped block and the westernmost vents in the crater became inactive on 11 September (figure 534). The remaining vents spanned approximately 750 m and trended roughly E-W. The fed channelized lava effusions flowed N and W into Halema’uma’u. The E rim of the crater was buried by new lava flows; pahoehoe lava flows covered most of the crater floor except areas of higher elevation in the SW part of the crater. The W part of the crater filled about 5 m since the start of the eruption, according to data from a laser rangefinder during 11-12 September. Lava fountaining continued, rising as high as 15 m by the morning of 12 September. During the morning of 13 September active lava flows were moving on the N and E parts of the crater. The area N of the eruptive vents that had active lava on its surface became perched and was about 3 m higher than the surrounding ground surface. By the morning of 14 September active lava was flowing on the W part of the down dropped block and the NE parts of the crater. The distances of the active flows progressively decreased. Spatter had accumulated on the S (downwind) side of the vents, forming ramparts about 20 m high.

Figure 534. Photo of a strong lava fountain in the E part of Kīlauea’s Halema’uma’u crater taken on the morning of 11 September 2023. The lava fountains rise as high as 10-15 m. Courtesy of J. Schmith, USGS.

Vigorous spattering was restricted to the westernmost large spatter cone with fountains rising 10-15 m high. Minor spattering occurred within the cone to the E of the main cone, but HVO noted that the fountains remained mostly below the rim of the cone. Lava continued to effuse from these cones and likely from several others as well, traveled N and W, confined to the W part of the down-dropped block and the NE parts of Halema’uma’u. Numerous ooze-outs of lava were visible over other parts of the crater floor at night. Laser range-finder measurements taken of the W part of the crater during 14-15 September showed that lava filled the crater by 10 m since the start of the eruption. Sulfur dioxide emissions remained elevated after the onset of the eruption, ranging 20,000-190,000 t/d during the eruption activity, the highest of which occurred on 10 September.

Field crews observed the eruptive activity on 15 September; they reported a notable decrease or stop in activity at several vents. Webcam images showed little to no fountaining since 0700 on 16 September, though intermittent spattering continued from the westernmost large cone throughout the night of 15-16 September. Thermal images showed that lava continued to flow onto the crater floor. On 16 September HVO reported that the eruption stopped around 1200 and that there was no observable activity anywhere overnight or on the morning of 17 September. HVO field crews reported that active lava was no longer flowing onto Halema’uma’u crater floor and was restricted to a ponded area N of the vents on the down dropped block. They reported that spattering stopped around 1115 on 16 September. Nighttime webcam images showed some incandescence on the crater floor as lava continued to cool. Field observations supported by geophysical data showed that eruptive tremor in the summit region decreased over 15-16 September and returned to pre-eruption levels by 1700 on 16 September. Sulfur dioxide emissions were measured at a rate of 800 t/d on 16 September while the eruption was waning, and 200 t/d on 17 September, which were markedly lower compared to measurements taken the previous week of 20,000-190,000 t/d.

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/).

Tinakula is a remote 3.5 km-wide island in the Solomon Islands, located 640 km ESE of the capital, Honiara. The current eruption period began in December 2018 and has more recently been characterized by intermittent lava flows and thermal activity (BGVN 48:06). This report covers similar activity during June through November 2023 using satellite data.

During clear weather days (20 July, 23 September, 23 October, and 12 November), infrared satellite imagery showed lava flows that mainly affected the W side of the island and were sometimes accompanied by gas-and-steam emissions (figure 54). The flow appeared more intense during July and September compared to October and November. According to the MODVOLC thermal alerts, there were a total of eight anomalies detected on 19 and 21 July, 28 and 30 October, and 16 November. Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) detected a small cluster of thermal activity occurring during late July, followed by two anomalies during August, two during September, five during October, and five during November (figure 55).

Figure 54. Infrared (bands B12, B11, B4) satellite images showed lava flows mainly affecting the W flank of Tinakula on 20 July 2023 (top left), 23 September 2023 (top right), 23 October 2023 (bottom left), and 12 November 2023 (bottom right). Some gas-and-steam emissions accompanied this activity. Courtesy of Copernicus Browser.

Figure 55. Low-power thermal anomalies were sometimes detected at Tinakula during July through November 2023, as shown on this MIROVA plot (Log Radiative Power). A small cluster of thermal anomalies were detected during late July. Then, only two anomalies were detected during August, two during September, five during October, and five during November. Courtesy of MIROVA.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. It has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The Mendana cone is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Recorded eruptions have frequently originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

Information Contacts: 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/).

Fuego is one of three large stratovolcanoes overlooking the city of Antigua, Guatemala. It has been erupting since January 2002, with observed eruptions dating back to 1531 CE. Typical activity is characterized by ashfall, pyroclastic flows, lava flows, and lahars. Frequent explosions with ash emissions, block avalanches, and lava flows have been reported since 2018. More recently, activity has been characterized by multiple explosions and ash plumes each day, ashfall, block avalanches, and pyroclastic flows (BGVN 48:09). This report describes similar activity of explosions, gas-and-ash plumes, and block avalanches during August through November 2023 based on daily reports from the Instituto Nacional de Sismologia, Vulcanología, Meteorología e Hidrologia (INSIVUMEH) and various satellite data.

Multiple explosions each day were reported during August through November 2023 that produced ash plumes that rose to 4.9 km altitude and drifted as far as 30 km in different directions. The explosions also caused rumbling sounds of varying intensities, with shock waves that vibrated the roofs and windows of homes near the volcano. Incandescent pulses of material rose as high as 350 m above the crater, accompanied by block avalanches that descended multiple drainages. Light ashfall was often reported in nearby communities (table 29). MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent low-to-moderate power thermal activity during the reporting period (figure 175). A total of seven MODVOLC thermal alerts were issued on 11 August, 1, 13, and 23 September, and 10, 17, and 18 November. On clear weather days thermal anomalies were also visible in infrared satellite imagery in the summit crater (figure 176).

Table 29. Activity at Fuego during August through November 2023 included multiple explosions every hour. Ash emissions rose as high as 4.9 km altitude and drifted in multiple directions as far as 30 km, causing ashfall in many communities around the volcano. Data from daily INSIVUMEH reports.

Figure 175. Intermittent low-to-moderate power thermal activity was detected at Fuego during August through November 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 176. Infrared (bands B12, B11, B4) satellite images showing a persistent thermal anomaly at the summit crater of Fuego on 27 August 2023 (top left), 1 September 2023 (top right), 16 October 2023 (bottom left), and 30 November 2023 (bottom right). Courtesy of Copernicus Browser.

Activity during August consisted of 1-11 explosions each day, which generated ash plumes that rose to 4-4.8 km altitude and drifted 8-30 km W, NW, SW, N, NE, and E. Fine ashfall was reported in Panimaché I and II (8 km SW), Morelia (9 km SW), Santa Sofía (12 km SW), Yepocapa (8 km NW), Finca Palo Verde (10 km WSW), Sangre de Cristo (8 km WSW), Acatenango (8 km E), Aldeas, El Porvenir (11 km SW), La Reunión (7 km SE), San Miguel Dueñas (10 km NE), Ciudad Vieja (13.5 km NE), Antigua (18 km NE), Quisaché (8 km NW), and El Sendero. The explosions sometimes ejected incandescent material 50-250 m above the crater and generated weak-to-moderate block avalanches that descended the Santa Teresa (W), Seca (W), Taniluyá (SW), Ceniza (SSW), Las Lajas (SE), El Jute (ESE), Trinidad (S), and Honda (E) drainages. Lahars were reported in the Ceniza drainage on 8-9, 16, 26-27, and 29 August, carrying fine and hot volcanic material, branches, tree trunks, and blocks measured 30 cm up to 1.5 m in diameter. Similar lahars affected the Las Lajas, El Jute, Seca, and El Mineral (W) drainages on 27 August.

Daily explosions ranged from 3-11 during September, which produced ash plumes that rose to 4-4.8 km altitude and drifted 10-30 km SW, W, NW, S, and SE. The explosions were accompanied by block avalanches that affected the Seca, Taniluyá, Ceniza, Las Lajas, Honda, Santa Teresa, Trinidad, and El Jute drainages and occasional incandescent ejecta rose 50-300 m above the crater. Fine ashfall was reported in Panimaché I and II, Morelia, Palo Verde, Sangre de Cristo, Yepocapa, El Porvenir, Aldeas, Santa Sofía, Montellano, El Socorro, La Rochela (8 km SSW), La Asunción (12 km SW), San Andrés Osuna (11 km SSW), Guadalupe, La Trinidad (S). Lahars triggered by rainfall were detected in the Ceniza drainage on 3-4, 8, 13-14, 17, 20-21, 24, 26, 29-30 September, which carried fine and hot volcanic material, branches, tree trunks, and blocks measuring 30 cm to 3 m in diameter. Similar lahars were also detected in the Seca, El Mineral, Las Lajas, and El Jute drainages on 27 September.

There were 2-10 explosions recorded each day during October, which produced ash plumes that rose to 4-4.9 km altitude and drifted 10-30 km W, SW, S, NW, N, NE, and SE. Incandescent pulses of material rose 50-350 m above the crater. Many of the explosions generated avalanches that descended the Ceniza, Santa Teresa, Taniluyá, Trinidad, Seca, El Jute, Las Lajas, and Honda drainages. Ashfall was reported in Aldeas, Panimaché I and II, Morelia, Santa Sofía, El Porvenir, Sangre de Cristo, Yepocapa, Yucales, Palo Verde, Acatenango, Patzicía, Alotenango, La Soledad (11 km N), El Campamento, La Rochela, Las Palmas, and Quisaché. Lahars continued to be observed on 2-5, 7, 9, 11, and 21-22 October, carrying fine and hot volcanic material, branches, tree trunks, and blocks measuring 30 cm to 3 m in diameter. Similar lahars were also reported in the Seca and Las Lajas drainage on 2 October and in the Las Lajas drainage on 4 October. On 4 October lahars overflowed the Ceniza drainage toward the Zarco and Mazate drainages, which flow from Las Palmas toward the center of Siquinalá, resulting from intense rainfall and the large volume of pyroclastic material in the upper part of the drainage. On 9 October a lahar was reported in the Seca and Las Lajas drainages, and lahars in the Las Lajas and El Jute drainages were reported on 11 October. A lahar on 22 October was observed in the Seca drainage, which interrupted transportation between San Pedro Yepocapa and the communities in Santa Sofía, Morelia, and Panimaché.

During November, 1-10 daily explosions were recorded, sometimes accompanied by avalanches, rumbling sounds, and shock waves. Gas-and-ash plumes rose 4.5-4.8 km altitude and extended 10-30 km W, SW, S, E, SE, NW, and N. Incandescent pulses of material rose 50-200 m above the crater. Fine ashfall was reported in Panimaché I and II, Morelia, Yepocapa, El Porvenir, Palo Verde, Santa Sofía, Aldeas, Sangre de Cristo, Yucales, La Rochela, San Andrés Osuna, Ceilán (9 km S), Quisaché, Acatenango, La Soledad. Avalanches of material descended the Seca, Taniluyá, Ceniza, Las Lajas, El Jute, Honda, Santa Teresa, and Trinidad drainages.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/ ); 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/).

The Santiaguito lava dome complex of Guatemala’s Santa María volcano has been actively erupting since 1922. The lava dome complex lies within a large crater on the SW flank of Santa María that was formed during the 1902 eruption. Ash explosions, pyroclastic flows, and lava flows have emerged from Caliente, the youngest of the four vents in the complex for more than 40 years. A lava dome that appeared within Caliente’s summit crater in October 2016 has continued to grow, producing frequent block avalanches down the flanks. More recently, activity has been characterized by frequent explosions, lava flows, ash plumes, and pyroclastic flows (BGVN 48:09). This report covers activity during August through November 2023 based on information from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) and various satellite data.

Activity during August consisted of weak-to-moderate explosions, avalanches of material, gas-and-ash plumes, and incandescence observed at night and in the early morning. Weak degassing plumes rose 300-600 m above the crater. Frequent explosions were detected at a rate of 1-2 per hour, which produced gas-and-ash plumes that rose 200-1,000 m above the crater and drifted W, NW, SW, S, E, and NE. Two active lava flows continued mainly in the Zanjón Seco (SW) and San Isidro (W) drainages. Incandescent block avalanches and occasional block-and-ash flows were reported on the W, S, E, SE, and SW flanks, as well as on the lava flows. On 26 and 29 August, fine ash plumes rose to 3.5 km altitude and drifted E and NE, causing ashfall in Belén (10 km S) and Calaguache (9 km S), as well as Santa María de Jesús (5 km SE) on 29 August.

Daily degassing, weak-to-moderate explosions, gas-and-ash plumes, and nighttime and early morning incandescence in the upper part of the dome continued during September. Explosions occurred at a rate of 1-2 per hour. Gas-and-ash plumes rose 200-1,000 m above the crater and drifted SW, W, SE, and NW. Block avalanches descended the SW, S, SE, and E flanks, often reaching the base of the Caliente dome. These avalanches were sometimes accompanied by short pyroclastic flows, resulting in fires in some vegetated areas. Block-and-ash flows descended all flanks of the Caliente dome on 16 and 24 September following the eruption of gas-and-ash plumes that rose 700-1,000 m above the crater. Gray ash was primarily deposited in the drainages.

Continuous gas-and-steam emissions occurred in October, along with weak-to-moderate explosions, block avalanches, crater incandescence, and an active lava flow on the WSW flank. Explosions occurred at a rate of 1-4 per hour, that generated gas-and-ash plumes rose 200-1,000 m above the crater and drifted in different directions. Block avalanches traveled down the SW, S, SE, and E flanks, sometimes accompanied by small pyroclastic flows. On 21 and 25 October as many as 50 explosions occurred over the course of 24 hours.

Similar activity persisted during November, with frequent explosions, crater incandescence, and block avalanches. The active lava flow persisted on the WSW flank. Weak-to-moderate explosions occurred at a rate of 1-4 per hour. Incandescence was observed at night and in the early morning. Gas-and-ash emissions rose 700-900 m above the crater and drifted W, SW, S, and NW. Block avalanches were reported on the SW, W, S, SE, and E flanks, which deposited gray ash material in the drainages, sometimes reaching the base of the Caliente dome. Those avalanches were sometimes accompanied by small pyroclastic flows that reached the base of the dome on the W, SW, and S flanks. Ashfall was reported in Las Marías (10 km S), El Viejo Palmar (12 km SSW), El Patrocinio, and San Marcos (8 km SW) on 18 and 22 November. On 26 and 30 November ashfall was reported in San Marcos and Loma Linda Palajunoj (7 km SW).

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph showed frequent moderate-power thermal anomalies during the reporting period (figure 140). A total of 26 MODVOLC thermal alerts were issued on 6, 7, 7, 15, 16, and 21 August, 15 and 23 September, 19, 26, 27, and 29 October, and 2, 7, 11, 27, 28, and 29 November. Clouds covered the summit of the volcano on most days, so thermal anomalies could not be identified in most Sentinel infrared satellite images.

Figure 140. Moderate-power thermal anomalies were frequently detected at Santa María during August through November 2023, as shown on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

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

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); 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/).

Karangetang (also known as Api Siau), at the northern end of the island of Siau, Indonesia, contains five summit craters along a N-S line. More than 40 eruptions have been recorded since 1675; recent eruptions have included frequent explosive activity, sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters and collapses of lava flow fronts have also produced pyroclastic flows. The two active summit craters are Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). The most recent eruption began in early February 2023 and was characterized by lava flows, incandescent avalanches, and ash plumes (BGVN 48:07). This report covers similar activity through the end of the eruption during July through September 2023 using reports from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin VAAC (Volcano Ash Advisory Center), and satellite data.

Webcam images occasionally showed crater incandescence and lava flows on the flanks of Main Crater during July. Near daily white gas-and-steam plumes rose 50-400 m above the crater and drifted in multiple directions. A webcam image taken at 1732 on 1 July suggested that a pyroclastic flow descended the SE flank, as evident from a linear plume of gas-and-ash rising along its path (figure 66). Incandescent material extended about 1 km down the S flank and about 600 m down the SSW and SW flank, based on a Sentinel satellite image taken on 2 July (figure 67). During the evening of 3 July a lava avalanche descended the Kahetang drainage (SE), extending 1-1.8 km, and the Timbelang and Beha drainages, extending 700-1,000 m. There were 53 earthquakes also detected that day. According to a news article from 6 July the lava avalanche from 2 July continued down the SW flank of Main Crater toward the Batang, Timbelang, and Beha Barat drainages for 1.5 km. An avalanche was also visible on the S flank, affecting the Batuawang and Kahetang drainages, and extending 1.8 km. Incandescent avalanches were reported during 8-9 July, traveling 1.8 km toward the Kahetang, Batuawang (S), and Timbelang drainages (figure 68). PVMBG issued two VONAs (Volcano Observatory Notices for Aviation) at 0759 and 0850 on 10 July, which reported two pyroclastic flows that traveled about 2 km toward the Kahetang drainage (figure 69). There were also 55 earthquakes detected on 10 July. As a result, 17 residents from Bolo Hamlet, Tarorane Village, East Siau District, Sitaro Islands Regency, North Sulawesi were evacuated.

Figure 66. Webcam image showing a possible pyroclastic flow descending the SE flank of Karangetang at 1732 on 1 July 2023. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Figure 67. Incandescent avalanches of material and summit crater incandescence was visible in infrared (bands B12, B11, B4) satellite images at both the N and S summit craters of Karangetang on 2 July 2023 (top left), 16 August 2023 (top right), 25 September 2023 (bottom left), and 25 October 2023 (bottom right). The incandescent avalanches mainly affected the S flank and gas-and-steam plumes (blue color) were also sometimes visible. Courtesy of Copernicus Browser.

Figure 68. Webcam image showing crater incandescence and lava flows from Main Crater descending Karangetang at 1936 on 8 July 2023. Courtesy of MAGMA Indonesia.

Figure 69. Webcam image showing a pyroclastic flow descending the SE flank of Karangetang at 0850 on 10 July 2023. Courtesy of MAGMA Indonesia.

An incandescent avalanche of material descended 1-1.8 km down the Kahetang drainage and 1 km down the Batang drainage on 14 July. During 18-29 July lava avalanches continued to move 1-1.8 km toward the Kahetang drainage, 700-1,000 m toward the Batuawang and Batang drainages, 700-1,000 m toward the Timbelang and Beha Barat drainage, and 1.5 km toward the West Beha drainage. Gray-and-white plumes accompanied the lava avalanches. During 20 July crater incandescence was visible in the gas-and-steam column 10-25 m above the crater. The Darwin VAAC reported that ash plumes rose to 2.4 km altitude at 1710 on 21 July, at 1530 on 22 July, and at 0850 on 23 July, which drifted NE and E. According to a news article, there were 1,189 earthquakes associated with lava avalanches recorded during 24-31 July.

Incandescent avalanches originating from Main Crater and extending SW, S, and SE persisted during August. Frequent white gas-and-steam plumes rose 25-350 m above the crater and drifted in different directions during August. Incandescent avalanches of material traveled S as far as 1.5 km down the Batuawang drainage, 1.8-1.9 km down the Kahetang drainage, and 2-2.1 km down the Keting drainage and SW 800-1,500 m down the Batang, Timbelang, and Beha Barat drainages. Occasional gray plumes accompanied this activity. According to a news article, 1,899 earthquakes associated with lava avalanches were recorded during 1-7 August. Incandescent ejecta from Main Crater was visible up to 10-25 m above the crater. Nighttime crater incandescence was visible in the N summit crater. There were 104 people evacuated from Tatahadeng and Tarorane during the first week of August, based on information from a news article that was published on 9 August. According to a news article published on 14 August the frequency of both earthquakes and lava avalanches decreased compared to the previous week; there were 731 earthquakes associated with avalanches detected during 8-15 August, and 215 during 24-31 August . Lava avalanches descending the Batang and Timbelang drainages continued through 24 August and the Batuawang, Kahetang, and Keting through 30 August. A news article published on 17 August reported pyroclastic flows due to collapsing accumulated material from lava flows.

Near-daily white gas-and-steam plumes rose 25-300 m above the crater and drifted in multiple directions during September. According to news articles, lava avalanches from Main Crater continued toward the Batuawang, Kahetang, and Keting drainages, reaching distances of 1-1.8 km. Lava avalanches also descended the Batang, Timbelang, and Beha Barat drainages as far as 1 km from Main Crater. Main Crater and N Crater incandescence were visible as high as 10 m above the crater. During 1-7 September the number of earthquakes associated with avalanches declined, although effusive activity continued. During 8-15 September lava effusion at Main Crater was not visible, although sounds of avalanches were sometimes intense, and rumbling was also occasionally heard. According to a news article published on 26 September, avalanches were no longer observed.

On 29 November PVMG lowered the Volcano Alert Level (VAL) to 2 (the second lowest level on a scale of 1-4) due to declining activity. Seismic data and visual observations indicated that effusion had decreased or stopped, and lava avalanches were no longer observed.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed strong thermal activity during July through August 2023, which was mainly characterized by incandescent avalanches of material and lava flows (figure 70). During August, the frequency and intensity of the thermal anomalies declined and remained relatively low through December. There was a brief gap in activity in late September. According to data recorded by the MODVOLC thermal algorithm, there were a total of 22 during July and 19 during August. Infrared satellite images showed summit crater incandescence at both the N and S craters and occasional incandescent avalanches of material affecting mainly the S flank (figure 67).

Figure 70. Strong thermal activity was detected at Karangetang during July through August 2023, as recorded by this MIROVA graph (Log Radiative Power). The frequency and intensity of the thermal anomalies declined during August and remained relatively low through December. A brief gap in activity was visible in late September. Courtesy of MIROVA.

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

Information Contacts: 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); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.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/); Antara News, Jalan Antara Kav. 53-61 Pasar Baru, Jakarta Pusat 10710, Indonesia (URL: antaranews.com).

Langila consists of a group of four small overlapping composite cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain, Papua New Guinea. It was constructed NE of the breached crater of Talawe. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m. The current eruption period began in October 2015 and recent activity has consisted of small thermal anomalies and an ash plume (BGVN 48:04). This report covers similar low-level activity during April through October 2023, based on information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite images.

Activity was relatively low during the reporting period and primarily consisted of thermal activity. The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph showed intermittent low-power thermal anomalies: three anomalies were detected during late April, one during May, one during late June, four during mid-July, two during mid-August, one during mid-September, and seven during October (figure 33). A total of two thermal hotspots were detected by the MODVOLC thermal alerts algorithm on 20 July and 18 August. Some of this activity was also visible as a small thermal anomaly on clear weather days in infrared satellite images in the SE crater (figure 34). Small sulfur dioxide plumes, some of which had column densities exceeding 2 Dobson Units (DU), drifted in different directions, based on data from the TROPOMI instrument on the Sentinel-5P satellite (figure 35).

Figure 33. Intermittent low-power thermal anomalies were detected at Langila during April through October 2023, based on this MIROVA graph (Log Radiative Power). Three anomalies were detected during late April, one during May, one during late June, four during mid-July, two during mid-August, one during mid-September, and seven during October. Courtesy of MIROVA.

Figure 34. Infrared (bands B12, B11, B4) satellite images showing a continuous but small thermal anomaly (bright yellow-orange) in the SE crater on 6 May 2023 (top left), 12 June 2023 (top right), 21 June 2023 (bottom left), and 20 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 35. Small sulfur dioxide plumes were detected above Langila based on data from the TROPOMI instrument on the Sentinel-5P satellite. Plumes drifted SW on 11 May 2023 (top left), SE on 19 July 2023 (top right), NW on 14 October 2023 (bottom left), and N on 18 October 2023 (bottom right). Weak plumes were also occasionally visible from Manam (to the W). Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

The Darwin VAAC reported that diffuse ash plumes were visible in satellite images at 1440 on 14 July that rose to 1.8 km altitude and drifted N. Diffuse ash emissions continued into most of the next day. By 1500 on 15 July the ash emissions dissipated, but gas-and-steam emissions continued. On 19 July the Darwin VAAC reported ash plumes that were visible in satellite images that rose to 1.8-2.4 km altitude and drifted SE.

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

Information Contacts: 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/); 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/br

The current lava dome in Colima's crater, first observed around early February 2007 (BGVN 32:10 and 33:04), continued slow growth through March 2010, as reported by Vyacheslav Zobin of the Observatorio Vulcanológico de la Universidad de Colima. As previously noted (BGVN 33:04), in March 2009 the dome had a volume of ~ 1,400,000 m³, about 80% of the total crater volume. The mean effusion rate had stood at ~ 0.03 m³/s in October 2007 (BGVN 34:04). The growth rate slowed further during 2009 and through March 2010 to ~ 0.02 m³/s.

In fall 2009, the new dome growth reached the western rim of the crater. Claus Siebe (UNAM) sent us a photo taken during an overflight on 7 February 2010, showing the lava dome (figure 93). As expected, based on the lack of barrier on the dome's W side, rockfalls became prominent in February 2010 (figure 94).

Figure 93. Aerial photo of Colima's lava dome taken looking from W to E at 1030 on 7 February 2010. Courtesy of Claus Siebe.

Figure 94. Variations in the daily number of small explosions and rockfalls at Colima during 2009 and 2010, obtained from seismic station EZV4, 1.7 km from the crater. Note the increase in the number of rockfalls starting in January-February 2010. Courtesy of Observatorio Vulcanológico de la Universidad de Colima.

The rockfalls began after heavy rains and the emergence of a small new lobe of lava in the W sector. That lobe had perched on the dome's margin, traversed across the buried crater rim, and began descent down the outer flank (figures 95 and 96). Figure 96 shows the new western lobe in an aerial view. That lobe was ~ 50 m long by March 2010.

Figure 95. A 24 February 2010 photo showing lava dome and new lava lobe at Colima. Courtesy of Civil Protection of Jalisco state.

Figure 96. View of dome on 29 March 2010 showing Colima's western lobe (left) and the overspilling of the crater to the S (right). Courtesy of Faculty of Science, Colima University.

Ash plumes rising to ~ 4 km altitude were observed during 24 July-4 August, 22 October, and 1-2 December 2009. On 2 December incandescent tephra was also ejected 50 m above crater to the SE. Seismic data indicated that small explosions continued through April 2010 (figure 94).

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

Information Contacts: Vyacheslav Zobin, Observatorio Vulcanológico de la Universidad de Colima, Colima, Col., 28045, México (URL: https://portal.ucol.mx/cueiv/); Claus Siebe, Instituto de Geofísica UNAM, Ciudad Universitaria, Delegación Coyoacán, C.P. México D.F 04510 (URL: http://www.geofisica.unam.mx/); Gobierno del Estado de Colima (URL: http://www.colima-estado.gob.mx/seguridad/indvolcan.php).

Following a phreatic eruption on 15 April 2008, Egon was relatively quiet for a year, prompting the Center of Volcanology and Geological Hazard Mitigation (CVGHM) to lower the Alert Level from 2 to 1 on 16 July 2009 (BGVN 34:10). This report notes seismicity and steaming between July 2009 and June 2010.

Egon (figure 1) apparently remained quiet until late March 2010, with an average of one or two daily volcanic earthquakes during January-March. Instruments then recorded a volcanic swarm, with as many as 122 events (amplitudes up to 12 mm, durations up to 45 seconds) during the last few days of March. A volcanic swarm also occurred during the week 31 March-6 April, with 26 events (amplitudes up to 4 mm, durations up to 72 seconds). Since 6 April 2010, small steam plumes rose 10 m above the crater and drifted W.

Figure 1. A shaded-relief map showing the location of Egon on central Flores Island (upper box) and a more detailed view of the elongate island and some active volcanoes (solid trianges). Courtesy of CVGHM.

Egon is monitored from the village of Nangatobong, near the island's N coast ~ 6 km NNW of the summit.During the last year and as recent as mid-2010, MODVOLC thermal alerts had been absent.

Geologic Background. Gunung Egon, also known as Namang, sits within the narrow section of eastern Flores Island. The barren, sparsely vegetated summit region has a 350-m-wide, 200-m-deep crater that sometimes contains a lake. Other small crater lakes occur on the flanks. A lava dome forms the southern summit. Solfataric activity occurs on the crater wall and rim and on the upper S flank. Reports of eruptive activity prior to explosive eruptions beginning in 2004 are unconfirmed. Emissions were often observed above the summit during 1888-1892. Strong emissions in 1907 reported by Sapper (1917) was considered by the Catalog of Active Volcanoes of the World (Neumann van Padang, 1951) to be an historical eruption, but Kemmerling (1929) noted that this was likely confused with an eruption on the same date and time from Lewotobi Lakilaki.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Saut Simatupang, 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Beginning in September 2009, eruptions at Gaua included emissions with heavy ashfall, a small pyroclastic flow, and strong Strombolian activity. Eruptions were also accompanied by strong and continuous degassing, and elevated SO₂ levels (BGVN 34:10 and 34:12).

The previous report discussed activity through 1 February 2010. This report covers activity between 2 February 2010 and 19 June 2010. However, no information was available for February or March from the Vanuatu Geohazards Observatory (VGO). Satellite thermal alerts from the MODVOLC system occurred on 28 March and 6 April (both one pixel).

In early April 2010 activity increased. The VGO reported that the ash plumes became thicker and higher (figure 17) and rose daily to altitudes of 2.1-3 km. Starting on 3 April, villagers living in the N and S parts of the island reported ashfall and frequently saw volcanic bombs ejected. Explosions were heard in nearby villages. Tremor counts after 1 January 2010 increased with time at least through 7 May (figure 18).

Figure 17. Satellite image of a dense steam-rich ash plume from Guau blowing directly NE on 24 April 2010. The steam is brighter white than the surrounding lower-altitude clouds. Vegetation is green, as is Lake Letas. Vegetation to the S and W of the volcano, damaged by ash and acidic volcanic gases, is dark gray-brown. Image taken by the Advanced Land Imager aboard NASA's Earth Observing-1 (EO-1) spacecraft. Courtesy NASA Earth Observatory.

Figure 18. Tremor counts at Gaua between 1 January 2010 and about 22 April 2010. Courtesy of VGO.

According to a 21 April news account the Red Cross disaster management officer in Vanuatu, Augustin Garae, told Radio New Zealand International that the fumes and ashfall had become a health hazard, with ash and acid rain affecting food crops and contaminating the water. News media on 19 April reported that Vanuatu government officials had made preparations to evacuate the approximately 3,000 residents of Gaua Island, who were threatened by ash, poisonous gases, and landslides.

On 11 May, the VGO reported that field observations revealed continued activity during April through the beginning of May. Significant emissions of gas and ash caused damage to vegetation around the crater and in areas on the NW, W, and SW parts of the island, the dominant wind directions. Lahars on the W part of the island were seen in April. Seismic data revealed that tremors had become more frequent since the beginning of the year. The VGO has advised visitors to stay away from the restricted area, especially within 3 km zone around the volcano.

Based on information from the VGO and satellite imagery, the Wellington VAAC reported ash plumes from Gaua on 8-16 April, 19-21 April, 29 April, 2 May, 11 May, 18-19 May, 21-22 May, 26 May, 7 June, and 16-19 June that all rose to an altitude of 3 km. On 19 June, the plume drifted more than 90 km W. The advisories on 14-15 April noted that the plumes were mostly steam. A spokesman for the Vanuatu Disaster Management Office described the activity as "huge, dark plumes" in a news report.

Geologic Background. The roughly 20-km-diameter Gaua Island, also known as Santa Maria, consists of a basaltic-to-andesitic stratovolcano with an 6 x 9 km summit caldera. Small vents near the caldera rim fed Pleistocene lava flows that reached the coast on several sides of the island; littoral cones were formed where these lava flows reached the ocean. Quiet collapse that formed the roughly 700-m-deep caldera was followed by extensive ash eruptions. The active Mount Garet (or Garat) cone in the SW part of the caldera has three pit craters across the summit area. Construction of Garet and other small cinder cones has left a crescent-shaped lake. The onset of eruptive activity from a vent high on the SE flank in 1962 ended a long period of dormancy.

Information Contacts: E. Garaebiti, S. Todman, C. Haruel, D. Charley, D. Nakedau, J. Cevuard, and A. Worwor, Vanuatu Geohazards Observatory (VGO), Department of Geology, Mines and Water Resources (DGMWR), Vanuatu (URL: http://www.vmgd.gov.vu/vmgd/); Wellington Volcanic Ash Advisory Centre, Meteorological Service of New Zealand Limited, 30 Salamanca Road, Kelburn, PO Box 722, Wellington, New Zealand (URL: http://vaac.metservice.com/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sydney Morning Herald (URL: http://news.smh.com.au/); Radio New Zealand International (URL: http://www.rnzi.com/).

Activity at Ibu (figure 3) has been frequent since December 1998; the current eruption began in April 2008. Thermal alerts (MODIS/MODVOLC) continued into June 2009, and incandescent ejections were seen in early August (BGVN 34:10).

Figure 3. Location map of Ibu volcano on Halmahera Island. Insert map shows the locations of other active Indonesian volcanoes with triangles. Courtesy of Agustan and others (2010).

Between July 2009 and June 2010, thermal alerts at Ibu were detected by MODVOLC on an average of about five or more days every month (normally one pixel per pass?at least one alert each 55 days during the 1-year period), about the same frequency as those measured during the first half of 2009 and far more frequent than that for the 4-year period October 2004-October 2008 (BGVN 33:09). Ground observations reported below in August-September 2009 and January 2010 confirm continued doem growth.

The Center of Volcanology and Geological Hazard Mitigation (CVGHM) reported that white and gray plumes rose 300-400 m above the crater rim during 15 July-4 August 2009 and that earthquakes became more frequent during 27 July-4 August 2009. Lava flows were seen on 2 August. Later that day, a thunderous sound was followed by incandescence at the summit. On 3 August, incandescent material was ejected about 20 m above the crater. The lava dome had reached a height sufficient to be seen from Duono Village, about 5 km NW.

Martin Rietze and Alain de Toffoli took some photographs of Ibu during a 31 August-7 September 2009 visit (figures 4-7), posted on the website of Th. Boeckel. The travelers noted that during the last 10 years the crater has been filled with a lava dome. The volcano has several vents that discharge ash and gas, along with Strombolian ejections. Only two vents were active during the visit.

Figure 4. Photo of the edge of the Ibu crater, taken during 25 August-8 September 2009. The vegetation near the rim was partially charred, and lava from the dome caused small bush fires. Photo taken from the W rim looking toward the E rim. Courtesy of Alain de Toffoli and M. Rietze.

Figure 5. Gas and ash emissions from the growing dome at Ibu, taken during 25 August-8 September 2009. Photo taken from the W rim looking E. Courtesy of Alain de Toffoli and M. Rietze.

Figure 6. Two active vents in Ibu's crater, taken during 25 August-8 September 2009. Photo taken from the W rim looking E. Courtesy of Alain de Toffoli and M. Rietze.

Figure 7. Dome growth in Ibu's crater with Strombolian ejections, taken during 25 August-8 September 2009. Photo taken from the W rim looking E. Courtesy of Alain de Toffoli and M. Rietze.

Arnold Binas posted some photos that demonstrated rapid dome growth (figure 8). Before 1998, the crater was vegetated down to the bottom. The dome was distinctly lower in 2007 (BGVN 34:05). Binas commented that "After a period of no steam emissions at all, [the dome vent] would start steaming slightly to announce it's next eruption. After being done emitting rocks and ash, it would steam vigorously for a while before quieting down again." Binas was also told that material had started to go through a breach in the N side of the crater.

Figure 8. Growth in a single day at Ibu's dome from one of the two vents erupting around 21 January 2010. The top photo was taken during a moment of quiet, and is the older image. Both photos taken from the W rim looking E. Courtesy of Arnold Binas.

A recent article by Agustan and others (2010) used radar to measure ground deformation related to volcanic activity on Ibu. Specifically, they used longer wavelength (L-band) differential interferometry to identify an inflation-deflation region, especially in the crater area before the March-April 2008 eruption (BGVN 33:09).

Reference. Agustan, Kimata, F., Abidin, H.Z., and Pamitro, Y.E., 2010, Measuring ground deformation of the tropical volcano, Ibu, using ALOS-PALSAR data, Remote Sensing Letters, v. 1, issue 1, p. 37-44, DOI: 1080/01431160903246717.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Saut Simatupang, 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Martin Rietze (URL: http://www.mrietze.com/info.htm); Arnold Binas, Toronto, Canada (URL: http://www.flickr.com/photos/hshdude/).

Plumes were reported from Karkar on 25-26 November 2009 (BGVN 34:11). Additional plumes were recognized in early December 2009 and during February 2010, but it was ambiguous whether these were ash-bearing plumes.

Rabaul Volcano Observatory (RVO) was unable to confirm that an eruption during 25 November to early December had occurred. In later fieldwork RVO found dead vegetation and noted that a few "holes" (small depressions) had developed in the crater area.

According to RVO, on 25-26 February local residents observed steam rising from inside the caldera and heard low roaring noises. Lightning was also observed above the summit area. The Darwin Volcanic Ash Advisory Center (VAAC) issued aviation reports on those dates noting that an ash plume rose to ~ 2.4 km and was blown as far as ~ 74 km NE and SE. Ultimately the plume was hidden by meteorological clouds.

Inferred debris-avalanche deposits and tsunami hazards. In a recent publication by Silver and others (2009), 12 debris avalanches were mapped in the Bismarck volcanic arc, including at Karkar (figure 8).

Figure 8. Location of Karkar and the broad-scale bathymetry of the Papua New Guinea region, showing the New Britain trench and its impingement on the main island of New Guinea as the Ramu-Markham fault. Index map appears at lower left. Map created using ERMapper.

The authors used sidescan sonar (figure 9) and around Karkar found very high-amplitude reflectivity to the SW, indicating young lava flows. Adjacent to these flows are small regions of blocky backscatter, which the authors suggested were debris-avalanche deposits. Although these deposits are small, it is possible that they coalesce beneath the lava flows and have been buried by the younger activity. High-amplitude reflectivity also occurs on the N side of the volcano. Individual blocks are scattered, and the reflectivity pattern appears to result from both lava flows and small debris avalanches.

Figure 9. Detailed side-scan imagery of the seafloor around Karkar and nearby Bagabag. Dotted line surrounds the author's inferred debris avalanche deposit. 5' of latitude = 9.3 km. Bagabag, a Pleistocene-age volcano that lacks documented Holocene eruptions, sits ~ 20 km SE of Karkar. After Silver and others (2009).

A subaerial landslide scar lies on Karkar's S side (figure 9). It could be associated with some of the debris imaged S of the volcano, although the available data came largely from transit lines and is incomplete. The authors state that, although the mapped extent of the debris avalanche deposit is small, it is still large enough to have produced a potentially significant tsunami in the Madang area.

Reference. Silver, E., Day, S., Ward, S., Hoffmann, G., Llanes, P., Driscoll, N., Appelgate, B., and Saunders, S., 2009, Volcano collapse and tsunami generation in the Bismarck Volcanic Arc, Papua New Guinea, J. of Volcanology and Geothermal Research, 186, 3-4, 210-222.

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

Information Contacts: Rabaul Volcano Observatory (RVO), PO Box 386, Rabaul, Papua New Guinea; Department of Mineral Policy & Geohazards Management, P.O. Box 3386, Kokopo, East New Britain Province, Papua New Guinea, Volcanological Observatory Geohazards Management Division; 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/).

Visitors who climbed Ol Doinyo Lengai (figure 138) in February 2010 observed periodic eruptions from a small fissure, steam emissions from an area of the crater rim, and fresh lava flows. Another group visited the volcano in March 2010 and saw lesser activity, but documented some features (figure 139). Table 24 gives an updated summary of brief observations from visitors through March 2010, reported primarily on Frederick Belton's website (a continuation of observations reported in BGVN 34:05 and 34:08).

Figure 138. View in late August 2009 of Ol Doinyo Lengai's N crater looking N from the rim. The collapse area on the NE crater rim is apparent on the far side. Courtesy of Thomas Holden.

Figure 139. Topographic map made from the Ol Doinyo Lengai quadrangle of 1990 and employing a contour interval of 20 m. The map was modified in the N Crater area from observations made on 12 March 2010. Courtesy of Sherrod and others (2010).

Table 24. Summary of selected observations of Ol Doinyo Lengai (from a climb, aerial overflight, flank, or satellite) from October 2009 through March 2010. Courtesy of Frederick Belton.

Belton summarized the reports and photos of climbers over the last few months by noting that it appeared that there had been a return to the small eruptions of natrocarbonatite lava and cone building that were typical prior to the 2007-2008 explosive eruption. Although the new lava had not been sampled and its composition is unknown, the crater floor vents were typical of lava pond basins and spatter cones that characterize eruptions of low-viscosity natrocarbonatite lava.

Observations during 14-15 February 2010. During the night of 14-15 February 2010, Frank Möckel, Kerstin Nindel, Thomas Seifert, and Peter Slesaczeck, German geologists, climbed via the W route to investigate the volcano and numerous small cones and craters (such as Sinja Lalarasi cone, Armykon hill, Black Belly, Dorobo cone, Nasira cones; figure 140). Möckel noted that the route was covered with gray ash and lapilli of the 2007-2008 eruptions, and the vegetation was totally destroyed.

Figure 140. Map indicating key craters and cones in the Natron-Ol Doinyo Lengai area. Contour interval 200 m; topography adapted from Sheet 39/4 Topographic Map series 1:50,000. From Keller and others (2006).

The upper part of the volcano had many erosional gullies. The surface of the ash was strongly cemented, to a thickness of ~0.5-1 cm. Under this solid layer the ash was still loose. About 500 m below the S crater rim, the surface was also cemented, but many large bombs, stones and blocks were embedded, forming a very rough surface.

The rim of the new ash cone was only ~1 m wide, and many small bombs were lying on it. The group saw new, small black hornitos and cones (figure 141) in addition to a new black lava flow on the S part of the crater floor. The three new hornitos in the western-most part of the crater floor looked very fresh. In the middle of the crater floor was a gray hornito. The E part of the crater floor was collapsed, containing older lava that was light gray and slightly altered. The amount of new lava inside of the cone was small. In and around the crater many fumaroles were observed, emitting sulfur odors. No seismic activity was noticeable, and the group heard no noise from the crater.

Figure 141. Detail of a vent on the floor of Ol Doinyo Lengai's N crater on 14 February 2010. Note the cone's rim in the left foreground. Courtesy of Frank Möckel.

Observations during 12-13 March 2010. David Sherrod, Kisana Mollel, and Olemelok Nantatwa (2010) visited the N crater on 12 March 2010, finding it quiescent. They made a map (figure 138) and recorded measurements of crater depth, diameter of the crater rim, and dips of strata, using compass, inclinometer, laser rangefinder, and a handheld GPS receiver. According to their terminology, the 'throat' refers to the steep-sided inner crater and 'mouth' to the total opening of the crater (figure 142).

Figure 142. Schematic cross section depicting the N crater on 12 March 2010. Courtesy of Sherrod and others (2010).

The crater rim, approximately circular, ranged in diameter from 270-310 m (figure 143). They measured the crater depth to be 110-122 m, two-thirds or more of that was the height of cliffs that rise directly from the crater floor to its throat, and the remainder was the height of the sloping crater rim. If the crater had not deepened since June 2009, then previously reported depths of 80 m were underestimates. From its rim, the crater mouth sloped inward 30°, descending at that pitch for 15-30 m before ending abruptly at the cliffs of the crater throat.

Figure 143. View looking NE on 12 March 2010 showing large-block rockfall on the Ol Doinyo Lengai crater floor and resulting scar on NE rim of the crater. Courtesy of Sherrod and others (2010).

Fine-grained pale-gray or light-brownish gray ash mantled the upper slopes. A rockfall that occurred before April 2009 (A. Daneel's photos in BGVN 34:05) had scalloped the NE crater slope, leaving a shallow scar 70 m wide (figure 143). Outward-dipping beds of vent-building tephra, exposed by the scar, provide visual evidence that the mouth-coating ash layers are thin, only 30 cm or less in most places. The throat is ~200 m diameter, and the crater floor has an area of 3.26 ha (3.26 x 10⁴ m²).

The crater as viewed on 12 March appeared unchanged from its appearance in photos taken a month earlier by Möckel; at that time, pahoehoe had covered nearly half of the crater floor. Sulfur odors were weak, and steam escaped from the few cracks that cut across the crater slopes. Neither steam nor fume were notable in the deeper part of the crater. Several volcanic and erosional events (1-3 below) could be surmised from geological relations on the crater floor, although none of the events is dated except by observations during previous visits. The recent lava flows and hornitos presumably are carbonatite, but none was sampled because they are inaccessible.1) Globs of lava were spattered on the throat walls. The spatter and rock alteration obscure much of the layering of strata exposed in the cliff faces.

2) Rubble from wall collapses formed talus cones over more than half the floor. The oldest deposits comprise large blocks from the pre-April 2009 rockfall that carved the E side of the crater (figure 144). This large-block rockfall has light brown ash on it, erupted from a 3-4-m-wide pit blasted through the talus debris. Several smaller ash-rimmed craters seen in March and April 2009 photos (S. L?bben and B. Wilhelmi, respectively, on Belton's website) were muted. A small hornito nestled in the talus with short pahoehoe flows was covered by the same brown ash. If the hornito is a product of June 2009 activity, then the brown ash must post-date June 2009.

Figure 144. Another view of Ol Doinyo Lengai's crater and floor, looking N across to the scar on the NE rim on 12 March 2010. Note the left (N) portion of the crater floor, which is lava covered and contains several open bocas and, barely visible towards the viewer, a hornito (labeled). Courtesy of Sherrod and others (2010).

3) Young gray pahoehoe, lacking any overlying ash, covered about 1.37 ha (1.37 x 10⁴ m²), or 40 % of the crater floor (figure 144, left side of crater floor). It issued from three boccas near the N crater wall (also photographed during 12-14 March 2010). Within this field of pahoehoe is the second of the two hornitos on the crater floor.

The faintest of noise was heard from the area of the three boccas in the N crater. The sound was reminiscent of sloshing heard deep in pits on Kīlauea, although with more of a drumming percussion, like that of periodic gas release occurring every 3-10 seconds, but wind across the crater rim made it extremely difficult to resolve the crater sounds or their origin. When a nighttime visit to the rim was made on 13 March no incandescence was seen.

The S crater, where Sherrod, Mollel, and Nantatwa camped during 12-13 March, was dotted by over 200 ballistic craters, features presumably resulting from the eruptive events of 2007-08. The shallow impact craters, as wide as 1.5 m diameter, chiefly or entirely postdate the ashfall that accompanied the 2007-08 eruptions, judging from crater preservation and the deformation of surface beds.

Hazards. Sherrod commented that under the quiescent conditions encountered in mid-March 2010, the chief hazard at the N crater was rim stability. Rim-collapse hazard may diminish with time as the rim materials become better cemented; or the hazard may increase as ongoing alteration weakens otherwise stable rock. The NE rim seemed especially unstable.

No perceptible gas hazards were found. The summit area was well ventilated, and the smell of sulfide gas was weak. Measurements made by inserting a handheld device into low-temperature steaming cracks at the summit detected no CO₂. Null values were also obtained when placing the CO₂ monitor into shallow pits in the S crater where soil gas might percolate, but the holes were so shallow (< 60 cm) that breezes probably prevent gas accumulation. Concentration of ground CO₂ was unlikely to pose a hazard for those in closed areas (such as tents), given the wind. The N crater's cliff-lined walls precluded access to the only obvious topographic trap for CO₂.

Recent publications. Abstracts of several papers given at the Cities on Volcanoes 6th (COV6) Conference at Tenerife, 31 May-4 June 2010, described the behavior, volatiles, minerals (including gem-quality ha?yne?Zaitsev and others, 2009), and petrology at Ol Doinyo Lengai. Wiedenmann and others (2010) described melilite minerals that allow tracing the volcano's evolution of rock types. Stoppa (2010) compared Italian carbonatites with those from Ol Doinyo Lengai.

References. Keller, J., Zaitsev, A.N., and Wiedenmann, D., 2006, Primary magmas at at Oldoinyo Lengai: The role of olivine melilitites: Lithos, v.91, no. 1-4, p. 150-172; doi:10.1016/j.lithos.2006.03.014.

Sherrod, D., Mollel, K., and Nantatwa, O., 2010, Oldonyo Lengai: Trip Report, March 12-14, 2010, informal report (URL: http:/Sherrod_OldonyoLengai_March12_20106-1.pdf).

Stoppa, F., 2010, CO₂ Discharge and Volcanic Risk in Italy, Cities on Volcanoes 6th (COV6) Conference, Tenerife, Canary Islands, Spain, 31 May-4 June 2010, Abstract Volume, p. 57-58 (URL: http://www.citiesonvolcanoes6.com/).

Wiedenmann, D., Keller, J., and Zaitsev. A.N., 2010, Melilite-Group Minerals at Oldoinyo Lengai, Tanzania, Cities on Volcanoes 6th (COV6) Conference, Tenerife, Canary Islands, Spain, 31 May-4 June 2010, Abstract Volume, p. 24 (URL: http://www.citiesonvolcanoes6.com/).

Zaitsev, A.N., Zaitseva, O.A., Buyko, A.K., Keller, J., Klaudius, J., and Zolotarev, A.A., 2009, Gem-Quality Yellow-Green Ha?yne from Oldoinyo Lengai Volcano, Northern Tanzania: Gems & Gemology, v. 45, no. 3, p. 200-203.

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

Information Contacts: Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA(URL: http://www.mtsu.edu/~fbelton/ and http://oldoinyolengai.pbworks.com/); Tony van Marken, XVenture Consulting (Pty) Ltd., Po Box 581, Franschhoek, 7690, South Africa (URL: http://www.humanedgetech.com/expedition/OlDoinyoLengai2009); David Sherrod, Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

According to a weekly USGS/CNMI Emergency Management Office (EMO) update on 4 June 2010, an explosive eruption from a submarine vent S of Sarigan Island occurred during 28-29 May 2010. The most energetic phase of the eruption, at 2147 on 29 May, lasted about 10 minutes based on seismic records from nearby Sarigan and Anatahan islands. The eruption sent an ash and gas cloud perhaps as high as 12.2 km altitude. Precursors included two moderate earthquakes and many smaller shocks; an earthquake of ~ M 4.8 accompanied the eruption. The plume drifted S, eventually passing over Guam (350 km SSW), but no fallout was reported there or on Saipan (165 km S).

Evidence of submarine volcanic activity was first observed late on 28 May during aerial observations by Juan Camacho, an EMO technician, who noted an elongate patch of discolored ocean water and possible light-colored floating debris about 8 km S of Sarigan Island. Alaska Volcano Observatory seismologists noted tremor on nearby seismic stations about the same time. Satellite image analysis by the National Weather Service in Guam indicated that the area of disturbance had grown significantly in size prior to the 29 May explosion. Within hours after the eruption, seismic activity began to decline, and after 31 May only a few scattered events were recorded.

The vent's exact location and depth remained uncertain. According to the USGS, the most likely source is the summit area of the South Sarigan seamount ~ 11 km S of Sarigan Island (figure 1). The summit of the conical structure lies at depths less than ~ 500 m below the surface.

Figure 1. Location map showing the volcanic islands of Sarigan and Anatahan, as well as the South and West Sarigan volcanic seamounts. Modified from Embley and Chadwick (2003).

A U.S. Fish and Wildlife Service research crew camped on Sarigan Island reported many felt earthquakes prior to the eruption. An explosion that sounded like thunder accompanied the event. They also heard increasing ocean wave activity during the night. Subsequently, a minor ashfall occurred.

The NOAA West Coast and Alaska Tsunami Warning Center reported that a 5-cm-high tsunami related to this event may have been recorded on a tide gauge in Saipan. According to a news article in the Saipan Tribune, the U.S. Fish and Wildlife evacuated 16 people, mostly scientists, from nearby islands, including eight people from Sarigan.

The USGS reported no evidence of volcanism after the 29 May eruption. Seismicity recorded by the nearest seismometer, on Sarigan Island, had returned to background levels. Nothing unusual was observed in satellite data.

Reference. Embley, B., and Chadwick, B., 2003, MR1 sonar provides new view of sea floor around Mariana Islands: NOAA Ocean Explorer, Submarine Ring of Fire 2003, 17 February 2003 (http://oceanexplorer.noaa.gov/explorations/03fire/logs/feb17/feb17.html).

Geologic Background. South Sarigan seamount, rising to within about 184 m of the ocean surface 12 km S of Sarigan Island, was the site of a short explosive submarine eruption in May 2010 that produced a plume of ash and steam to 12 km altitude. Sidescan sonar imagery taken in 2003 shows an irregular summit with multiple peaks, including a possibly young cone at about 350 m depth, and flank morphology suggests it is a frequently active volcano.

Information Contacts: Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO-CNMI) and USGS Hawaii Volcano Observatory, PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/ and https://volcanoes.usgs.gov/nmi/activity/); NOAA Ocean Explorer, National Oceanic and Atmospheric Administration, U.S. Department of Commerce (URL: http://oceanexplorer.noaa.gov/explorations/03fire/logs/feb17/feb17.html).