Mount Agung, located on the E end of the island of Bali, Indonesia, rises above the SE rim of the Batur caldera. The summit area extends 1.5 km E-W, with the highest point on the W and a steep-walled 800-m-wide crater on the E. Recorded eruptions date back to the early 19th century. A large and deadly explosive and effusive eruption occurred during 1963-64, which was characterized by voluminous ashfall, pyroclastic flows, and lahars that caused extensive damage and many fatalities. More recent activity was documented during November 2017-June 2019 that consisted of multiple explosions, significant ash plumes, lava flows at the summit crater, and incandescent ejecta. This report covers activity reported during April-May 2022 and December 2022 based on data from the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during 2022 was relatively low and mainly consisted of a few ash plumes during April-May and December. An ash plume on 3 April rising to 3.7 km altitude (700 m above the summit) and drifting N was reported in a Darwin VAAC notice based on a ground report, with ash seen in HIMAWARI-8 visible imagery. Another ash plume was reported at 1120 on 27 May that rose to 5.5 km altitude (2.5 m above the summit); the plume was not visible in satellite or webcam images due to weather clouds. An eruption was reported based on seismic data at 0840 on 13 December, with an estimated plume altitude of 3.7 km; however, no ash was seen using satellite imagery in clear conditions before weather clouds obscured the summit.

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

Information Contacts: 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/).

Tengger Caldera, located at the N end of a volcanic massif in Indonesia’s East Java, consists of five overlapping stratovolcanoes. The youngest and only active cone in the 16-km-wide caldera is Bromo, which typically produces gas-and-steam plumes, occasional ash plumes and explosions, and weak thermal signals (BGVN 44:05, 47:01). This report covers activity during January 2022-December 2023, consisting of mostly white gas-and-steam emissions and persistent weak thermal anomalies. Information was provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and satellite imagery. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay at least 1 km from the crater.

Activity was generally low during the reporting period, similar to that in 2021. According to almost daily images from MAGMA Indonesia (a platform developed by PVMBG), white emissions and plumes rose from 50 to 900 m above the main crater during this period (figure 24). During several days in March and June 2022, white plumes reached heights of 1-1.2 km above the crater.

Figure 24. Webcam image showing a gas-and-steam plume from the Bromo cone in the Tengger Caldera on 2 April 2023. Courtesy of MAGMA Indonesia.

After an increase in activity at 2114 on 3 February 2023, a PVMBG team that was sent to observe white emissions rising as high as 300 m during 9-12 February and heard rumbling noises. A sulfur dioxide odor was also strong near the crater and measurements indicated that levels were above the healthy (non-hazardous) threshold of 5 parts per million; differential optical absorption spectroscopy (DOAS) measurements indicated an average flux of 190 metric tons per day on 11 February. Incandescence originating from a large fumarole in the NNW part of the crater was visible at night. The team observed that vegetation on the E caldera wall was yellow and withered. The seismic network recorded continuous tremor and deep and shallow volcanic earthquakes.

According to a PVMBG press release, activity increased on 13 December 2023 with white, gray, and brown emissions rising as high as 900 m above Bromo’s crater rim and drifting in multiple directions (figure 25). The report noted that tremor was continuous and was accompanied in December by three volcanic earthquakes. Deformation data indicated inflation in December. There was no observable difference in the persistent thermal anomaly in the crater between 11 and 16 December 2023.

Figure 25. Webcam image showing a dark plume that rose 900 m above the summit of the Bromo cone in the Tengger Caldera on 13 December 2023. Courtesy of MAGMA Indonesia.

All clear views of the Bromo crater throughout this time, using Sentinel-2 infrared satellite images, showed a weak persistent thermal anomaly; none of the anomalies were strong enough to cause MODVOLC Thermal Alerts. A fire in the SE part of the caldera in early September 2023 resulted in a brief period of strong thermal anomalies.

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

Information Contacts: 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); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).

Saunders is one of eleven islands that comprise the South Sandwich Islands in the South Atlantic. The active Mount Michael volcano has been in almost continuous eruption since November 2014 (BGVN 48:02). Recent activity has resulted in intermittent thermal anomalies and gas-and-steam emissions (BGVN 47:03, 48:02). Visits are infrequent due to its remote location, and cloud cover often prevents satellite observations. Satellite thermal imagery and visual observation of incandescence during a research expedition in 2019 (BGVN 28:02 and 44:08) and a finding confirmed by a National Geographic Society research team that summited Michael in November 2022 reported the presence of a lava lake.

Although nearly constant cloud cover during February 2023 through January 2024 greatly limited satellite observations, thermal anomalies from the lava lake in the summit crater were detected on clear days, especially around 20-23 August 2023. Anomalies similar to previous years (eg. BGVN 48:02) were seen in both MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS instruments and in Sentinel 2 infrared imagery. The only notable sulfur dioxide plume detected near Saunders was on 25 September 2023, with the TROPOMI instrument aboard the Sentinel-5P satellite.

Geologic Background. Saunders Island consists of a large central volcanic edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weather conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

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/); 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 (URL: https://dataspace.copernicus.eu/browser).

Shishaldin is located on the eastern half of Unimak Island, one of the Aleutian Islands. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. The previous eruption ended in May 2020 and was characterized by intermittent thermal activity, increased seismicity and surface temperatures, ash plumes, and ash deposits (BGVN 45:06). This report covers a new eruption during July through November 2023, which consisted of significant explosions, ash plumes, ashfall, and lava fountaining. Information comes from daily, weekly, and special reports from the Alaska Volcano Observatory (AVO) and various satellite data. AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

AVO reported that intermittent tremor and low-frequency earthquakes had gradually become more regular and consistent during 10-13 July. Strongly elevated surface temperatures at the summit were identified in satellite images during 10-13 July. On 11 July AVO raised the Aviation Color Code (ACC) to Yellow (the second color on a four-color scale) and Volcano Alert Level (VAL) to Advisory (the second level on a four-level scale) at 1439. Later in the day on 11 July summit crater incandescence was observed in webcam images. Observations of the summit suggested that lava was likely present at the crater, which prompted AVO to raise the ACC to Orange (the second highest color on a four-color scale) and the VAL to Watch (the second highest level on a four-level scale). The US Coast Guard conducted an overflight on 12 July and confirmed that lava was erupting from the summit. That same day, sulfur dioxide emissions were detected in satellite images.

A significant explosion began at 0109 on 14 July that produced an ash plume that rose to 9-12 km altitude and drifted S over the Pacific Ocean (figure 43). Webcam images and photos taken around 0700 from a ship SW off Unimak Island showed small lahar deposits, which were the result of the interaction of hot pyroclastic material and snow and ice on the flanks. There was also ashfall on the SW and N flanks. A smaller explosion at 0710 generated an ash plume that rose to 4.5 km altitude. Webcam images and pilot reports showed continued low-level ash emissions during the morning, rising to less than 4.6 km altitude; those emissions included a small ash plume near the summit around 1030 resulting from a small explosion.

Figure 43. Photo of a strong ash plume that rose to 9-12 km altitude on the morning of 14 July 2023. Lahar deposits were visible on the SW flank (white arrows). Photo has been color corrected. Courtesy of Christopher Waythomas, AVO.

Seismic tremor amplitude began increasing at around 1700 on 15 July; strongly elevated surface temperatures were also reported. An ash plume rose to 4.6 km altitude and drifted SSE at 2100, based on a satellite image. A continuous ash plume during 2150 through 2330 rose to 5 km altitude and extended 125 km S. At 2357 AVO raised the ACC to Red (the highest color on a four-color scale) and the VAL to Warning (the highest level on a four-level scale), noting that seismicity remained elevated for more than six hours and explosion signals were frequently detected by regional infrasound (pressure sensor) networks. Explosions generated an ash plume that rose to 4.9 km altitude and drifted as far as 500 km SE. Activity throughout the night declined and by 0735 the ACC was lowered to Orange and the VAL to Watch. High-resolution satellite images taken on 16 July showed pyroclastic deposits extending as far as 3 km from the vent; these deposits generated lahars that extended further down the drainages on the flanks. Ash deposits were mainly observed on the SSE flank and extended to the shore of Unimak Island. During 16-17 July lava continued to erupt at the summit, which caused strongly elevated surface temperatures that were visible in satellite imagery.

Lava effusion increased at 0100 on 18 July, as noted in elevated surface temperatures identified in satellite data, increasing seismic tremor, and activity detected on regional infrasound arrays. A significant ash plume at 0700 rose to 7 km altitude and continued until 0830, eventually reaching 9.1 km altitude and drifting SSE (figure 44). As a result, the ACC was raised to Red and the VAL to Warning. By 0930 the main plume detached, but residual low-level ash emissions continued for several hours, remaining below 3 km altitude and drifting S. The eruption gradually declined and by 1208 the ACC was lowered to Orange and the VAL was lowered to Watch. High-resolution satellite images showed ash deposits on the SW flank and pyroclastic deposits on the N, E, and S flanks, extending as far as 3 km from the vent; lahars triggered by the eruption extended farther down the flanks (figure 45). Lava continued to erupt from the summit crater on 19 July.

Figure 44. Photo of an ash-rich plume rising above Shishaldin to 9.1 km altitude on 18 July 2023 that drifted SE. View is from the N of the volcano and Isanotski volcano is visible on the left-hand side of the image. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Figure 45. Near-infrared false-color satellite image of Shishaldin taken on 18 July 2023 showing ash deposits on the N, E, and S flanks extending as far as 3 km from the vent due to recent eruption events. Courtesy of Matthew Loewen, AVO.

Elevated surface temperatures were detected in satellite images during 19-25 July, despite occasional weather cloud cover, which was consistent with increased lava effusion. During 22-23 July satellite observations acquired after the eruption from 18 July showed pyroclastic flow and lahar deposits extending as far as 3 km down the N, NW, and NE flanks and as far as 1.5 km down the S and SE flanks. Ash deposits covered the SW and NE flanks. No lava flows were observed outside the crater. On 22 July a sulfur dioxide plume was detected in satellite data midday that had an estimated mass of 10 kt. In a special notice issued at 1653 on 22 July AVO noted that eruptive activity had intensified over the previous six hours, which was characterized by an hours-long steady increase in seismic tremor, intermittent infrasound signals consistent with small explosions, and an increase in surface temperatures that were visible in satellite data. Pilots first reported low-level ash plumes at around 1900. At 2320 an ash plume had risen to 9 km altitude based on additional pilot reports and satellite images. The ACC was increased to Red and the VAL to Warning at 2343. Satellite images indicated growth of a significantly higher ash plume that rose to 11 km altitude continued until 0030 and drifted NE. During the early morning hours of 23 July ash plumes had declined to 4.6 k altitude. Seismic tremor peaked at 0030 on 23 July and began to rapidly decline at 0109; active ash emissions were no longer visible in satellite data by 0130. The ACC was lowered to Orange and the VAL to Watch at 0418; bursts of increased seismicity were recorded throughout the morning, but seismicity generally remained at low levels. Elevated surface temperatures were visible in satellite data until about 0600. On 24 July pilots reported seeing vigorous gas-and-steam plumes rising to about 3 km altitude; the plumes may have contained minor amounts of ash.

During 24-25 July low level seismicity and volcanic tremor were detected at low levels following the previous explosion on 23 July. Strongly elevated surface temperatures were observed at the summit crater in satellite data. Around 2200 on 25 July seismicity began to increase, followed by infrasound signals of explosions after 0200 on 26 July. An ash plume rose to 3 km altitude at 0500 and drifted ENE, along with an associated sulfur dioxide plume that drifted NE and had an estimated mass of 22 kt. Diffuse ash emissions were visible in satellite data and rose to 6.1-7.6 km altitude and extended 125 km from the volcano starting around 1130. These ash events were preceded by about seven hours of seismic tremor, infrasound detections of explosions, and five hours of increased surface temperatures visible in satellite data. Activity began to decline around 1327, which included low-frequency earthquakes and decreased volcanic tremor, and infrasound data no longer detected significant explosions. Surface temperatures remained elevated through the end of the month.

Seismicity, volcanic tremor, and ash emissions remained at low levels during early August. Satellite images on 1 August showed that some slumping had occurred on the E crater wall due to the recent explosive activity. Elevated surface temperatures continued, which was consistent with cooling lava. On 2 August small explosive events were detected, consistent with low-level Strombolian activity. Some episodes of volcanic tremor were reported, which reflected low-level ash emissions. Those ash emissions rose to less than 3 km altitude and drifted as far as 92.6 km N. Pilots that were located N of the volcano observed an ash plume that rose to 2.7 km altitude. Seismicity began to increase in intensity around 0900 on 3 August. Seismicity continued to increase throughout the day and through the night with strongly elevated surface temperatures, which suggested that lava was active at the surface.

An ash cloud that rose to 7.6-7.9 km altitude and drifted 60-75 km NE was visible in a satellite image at 0520 on 4 August. Pilots saw and reported the plume at 0836 (figure 46). By 0900 the plume had risen to 9.1 km altitude and extended over 100 km NE. AVO raised the ACC to Red and the VAL to Warning as a result. Seismic tremor levels peaked at 1400 and then sharply declined at 1500 to slightly elevated levels; the plume was sustained during the period of high tremor and drifted N and NE. The ACC was lowered to Orange and the VAL to Watch at 2055. During 5-14 August seismicity remained low and surface temperatures were elevated based on satellite data due to cooling lava. On 9 August a small lava flow was observed that extended from the crater rim to the upper NE flank. It had advanced to 55 m in length and appeared in satellite imagery on 11 August. Occasional gas-and-steam plumes were noted in webcam images. At 1827 AVO noted that seismic tremor had steadily increased during the afternoon and erupting lava was visible at the summit in satellite images.

Figure 46. Photo showing an ash plume rising above Shishaldin during the morning of 4 August 2023 taken by a passing aircraft. The view is from the N showing a higher gas-rich plume and a lower gray ash-rich plume and dark tephra deposits on the volcano’s flank. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Strong explosion signals were detected at 0200 on 15 August. An ash cloud that was visible in satellite data extended 100 km NE and may have risen as high as 11 km altitude around 0240. By 0335 satellite images showed the ash cloud rising to 7.6 km altitude and drifting NE. Significant seismicity and explosions were detected by the local AVO seismic and infrasound networks, and volcanic lightning was detected by the World Wide Lightning Location Network (WWLLN). A sulfur dioxide plume associated with the eruption drifted over the S Bering Sea and parts of Alaska and western Canada. Seismicity was significantly elevated during the eruption but had declined by 1322. A pilot reported that ash emissions continued, rising as high as 4.9 km altitude. Elevated surface temperatures detected in satellite data were caused by hot, eruptive material (pyroclastic debris and lava) that accumulated around the summit. Eruptive activity declined by 16 August and the associated sulfur dioxide plume had mostly dissipated; remnants continued to be identified in satellite images at least through 18 August. Surface temperatures remained elevated based on satellite images, indicating hot material on the upper parts of the volcano. Small explosions were detected in infrasound data on the morning of 19 August and were consistent with pilot reports of small, short-lived ash plumes that rose to about 4.3 km altitude. Low-level explosive activity was reported during 20-24 August, according to seismic and infrasound data, and weather clouds sometimes prevented views. Elevated surface temperatures were observed in satellite images, which indicated continued hot material on the upper parts of the volcano.

Seismic tremor began to increase at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with erupting lava. Small explosions were recorded in infrasound data. The ACC was raised to Red and the VAL to Warning at 1204 after a pilot reported an ash plume that rose to 9.1 km altitude. Seismicity peaked at 1630 and began to rapidly decline at around 1730. Ash plumes rose as high as 10 km altitude and drifted as far as 400 km NE. By 2020 the ash plumes had declined to 6.4 km altitude and continued to drift NE. Ash emissions were visible in satellite data until 0000 on 26 August and seismicity was at low levels. AVO lowered the ACC to Orange and the VAL to Watch at 0030. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures were still visible in satellite imagery through the rest of the month. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to 4.5 km altitude and rapidly dissipated. The mass flow was likely caused by the collapse of spatter that accumulated on the summit crater rim.

Similar variable explosive activity was reported in September, although weather observations sometimes prevented observations. A moderate resolution satellite image from the afternoon of 1 September showed gas-and-steam emissions filling the summit crater and obscuring views of the vent. In addition, hot deposits from the previous 25-26 August explosive event were visible on the NE flank near the summit, based on a 1 September satellite image. On 2 and 4 September seismic and infrasound data showed signals of small, repetitive explosions. Variable gas-and-steam emissions from the summit were visible but there was no evidence of ash. Possible summit crater incandescence was visible in nighttime webcam images during 3-4 September.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated at around 0830. A pilot reported an ash plume that rose to 7.6 km altitude at 0842 and continued to rise as high as possibly 9.7 km altitude and drifted SSE based on satellite images (figure 47). The ACC was raised to Red and the VAL to Warning at 0900. In addition to strong tremor and sustained explosions, the eruption produced volcanic lightning that was detected by the WWLLN. Around 1100 seismicity decreased and satellite data confirmed that the altitude of the ash emissions had declined to 7.6 km altitude. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. The ACC was lowered to Orange and the VAL to Watch at 1440. Satellite images showed extensive pyroclastic debris flows on most of the flanks that extended 1.2-3.3 km from the crater rim.

Figure 47. Webcam image taken from the S of Shishaldin showing a vertical ash plume on 5 September 2023. Courtesy of AVO.

During 6-13 September elevated surface temperatures continued to be observed in satellite data, seismicity remained elevated with weak but steady tremor, and small, low-frequency earthquakes and small explosions were reported, except on 12 September. On 6 September a low-level ash plume rose to 1.5-1.8 km altitude and drifted SSE. Occasional small and diffuse gas-and-steam emissions at the summit were visible in webcam images. Around 1800 on 13 September seismic tremor amplitudes began to increase, and small explosions were detected in seismic and infrasound data. Incandescent lava at the summit was seen in a webcam image taken at 0134 on 14 September during a period of elevated tremor. No ash emissions were reported during the period of elevated seismicity. Lava fountaining began around 0200, based on webcam images. Satellite-based radar observations showed that the lava fountaining activity led to the growth of a cone in the summit crater, which refilled most of the crater. By 0730 seismicity significantly declined and remained at low levels.

Seismic tremor began to increase around 0900 on 15 September and rapidly intensified. An explosive eruption began at around 1710, which prompted AVO to raise the ACC to Red and the VAL to Warning. Within about 30 minutes ash plumes drifted E below a weather cloud at 8.2 km altitude. The National Weather Service estimated that an ash-rich plume rose as high as 12.8 km altitude and produced volcanic lightning. The upper part of the ash plume detached from the vent around 1830 and drifted E, and was observed over the Gulf of Alaska. Around the same time, seismicity dramatically decreased. Trace ashfall was reported in the community of False Pass (38 km ENE) between 1800-2030 and also in King Cove and nearby marine waters. Activity declined at around 1830 although seismicity remained elevated, ash emissions, and ashfall continued until 2100. Lightning was again detected beginning around 1930, which suggested that ash emissions continued. Ongoing explosions were detected in infrasound data, at a lower level than during the most energetic phase of this event. Lightning was last detected at 2048. By 2124 the intensity of the eruption had decreased, and ash emissions were likely rising to less than 6.7 km altitude. Seismicity returned to pre-eruption levels. On 16 September the ACC was lowered to Orange and the VAL to Watch at 1244; the sulfur dioxide plume that was emitted from the previous eruption event was still visible over the northern Pacific Ocean. Elevated surface temperatures, gas-and-steam emissions from the vent, and new, small lahars were reported on the upper flanks based on satellite and webcam images. Minor deposits were reported on the flanks which were likely the result of collapse of previously accumulated lava near the summit crater.

Elevated seismicity with tremor, small earthquakes, and elevated surface temperatures were detected during 17-23 September. Minor gas-and-steam emissions were visible in webcam images. On 20 September small volcanic debris flows were reported on the upper flanks. On 21 September a small ash deposit was observed on the upper flanks extending to the NE based on webcam images. Seismic tremor increased significantly during 22-23 September. Regional infrasound sensors suggested that low-level eruptive activity was occurring within the summit crater by around 1800 on 23 September. Even though seismicity was at high levels, strongly elevated surface temperatures indicating lava at the surface were absent and no ash emissions were detected; weather clouds at 0.6-4.6 km altitude obscured views. At 0025 on 24 September AVO noted that seismicity continued at high levels and nearly continuous small infrasound signals began, likely from low-level eruptive activity. Strongly elevated surface temperatures were identified in satellite images by 0900 and persisted throughout the day; the higher temperatures along with infrasound and seismic data were consistent with lava erupting at the summit. Around 1700 similarly elevated surface temperatures were detected from the summit in satellite data, which suggested that more vigorous lava fountaining had started. Starting around 1800 low-level ash emissions rose to altitudes less than 4.6 km altitude and quickly dissipated.

Beginning at midnight on 25 September, a series of seismic signals consistent with volcanic flows were recorded on the N side of the volcano. A change in seismicity and infrasound signals occurred around 0535 and at 0540 a significant ash cloud formed and quickly reached 14 km altitude and drifted E along the Alaska Peninsula. The cloud generated at least 150 lightning strokes with thunder that could be heard by people in False Pass. Seismicity rapidly declined to near background levels around 0600. AVO increased the ACC to Red and the VAL to Warning at 0602. The ash cloud detached from the volcano at around 0700, rose to 11.6 km altitude, and drifted ESE. Trace to minor amounts of ashfall were reported by the communities of False Pass, King Cove, Cold Bay, and Sand Point around 0700. Ash emissions continued at lower altitudes of 6-7.6 km altitude at 0820. Small explosions at the vent area continued to be detected in infrasound data and likely represented low-level eruptive activity near the vent. Due to the significant decrease in seismicity and ash emissions the ACC was lowered to Orange and the VAL to Watch at 1234. Radar data showed significant collapses of the crater that occurred on 25 September. Satellite data also showed significant hot, degassing pyroclastic and lahar deposits on all flanks, including more extensive flows on the ENE and WSW sections below two new collapse scarps. Following the significant activity during 24-25 September, only low-level activity was observed. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease through the end of the month, though tremor and small earthquakes were still reported. No explosive activity was detected in infrasound data through 2 October. Gas-and-steam emissions rose to 3.7 km altitude, as reported by pilots and seen in satellite images. Satellite data from 26 September showed that significant collapses had occurred at the summit crater and hot, steaming deposits from pyroclastic flows and lahars were present on all the flanks, particularly to the ENE and WSW. A small ash cloud was visible in webcam images on 27 September, likely from a collapse at the summit cone. High elevated surface temperatures were observed in satellite imagery during 27-28 September, which were likely the result of hot deposits on the flanks erupted on 25 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September.

During October, explosion events continued between periods of low activity. Seismicity significantly increased starting at around 2100 on 2 October; around the same time satellite images showed an increase in surface temperatures consistent with lava fountaining. Small, hot avalanches of rock and lava descended an unspecified flank. In addition, a distinct increase in infrasound, seismicity, and lightning detections was followed by an ash plume that rose to 12.2 km altitude and drifted S and E at 0520 on 3 October, based on satellite images. Nighttime webcam images showed incandescence due to lava fountaining at the summit and pyroclastic flows descending the NE flank. AVO reported that a notable explosive eruption started at 0547 and lasted until 0900 on 3 October, which prompted a rise in the ACC to Red and the VAL to Warning. Subsequent ash plumes rose to 6-7.6 km altitude by 0931. At 1036 the ACC was lowered back to Orange and the VAL to Watch since both seismic and infrasound data quieted substantially and were slightly above background levels. Gas-and-steam emissions were observed at the summit, based on webcam images. Trace amounts of ashfall were observed in Cold Bay. Resuspended ash was present at several kilometers altitude near the volcano. During the afternoon, low-level ash plumes were visible at the flanks, which appeared to be largely generated by rock avalanches off the summit crater following the explosive activity. These ash plumes rose to 3 km altitude and drifted W. Trace amounts of ashfall were reported by observers in Cold Bay and Unalaska and flights to these communities were disrupted by the ash cloud. Satellite images taken after the eruption showed evidence of pyroclastic flows and lahar deposits in drainages 2 km down the SW flank and about 3.2 km down the NE flank, and continued erosion of the crater rim. Small explosion craters at the end of the pyroclastic flows on the NE flank were noted for the first time, which may have resulted from gas-and-steam explosions when hot deposits interact with underlying ice.

During 4 October seismicity, including frequent small earthquakes, remained elevated, but was gradually declining. Ash plumes were produced for over eight hours until around 1400 that rose to below 3.7 km altitude. These ash plumes were primarily generated off the sides of the volcano where hot rock avalanches from the crater rim had entered drainages to the SW and NE. Two explosion craters were observed at the base of the NE deposits about 3.2 km from the crater rim. Webcam images showed the explosion craters were a source of persistent ash emissions; occasional collapse events also generated ash. Seismicity remained elevated with sulfur dioxide emissions that had a daily average of more than 1,000 tons per day, and frequent small earthquakes through the end of the month. Frequent elevated surface temperatures were identified in satellite images and gas-and-steam plumes were observed in webcam images, although weather conditions occasionally prevented clear views of the summit. Emissions were robust during 14-16 October and were likely generated by the interaction of hot material and snow and ice. During the afternoon of 21 October a strong gas-and-steam plume rose to 3-4.6 km altitude and extended 40 km WSW, based on satellite images and reports from pilots. On 31 October the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Activity in November was characterized by elevated seismicity with ongoing seismic tremor and small, low-frequency earthquakes, elevated surface temperatures, and gas-and-steam emissions. There was an increase in seismic and infrasound tremor amplitudes starting at 1940 on 2 November. As a result, the ACC was again raised to Orange and the VAL was increased to Watch, although ash was not identified in satellite data. An ash cloud rose to 6.1 km altitude and drifted W according to satellite data at 2000. By 0831 on 3 November ash emissions were no longer visible in satellite images. On 6 and 9 November air pressure sensors detected signals consistent with small explosions. Small explosions were detected in infrasound data consistent with weak Strombolian activity on 19 and 21 November. Seismicity started to decrease on 21 November. On 25 November gas-and-steam emissions were emitted from the vent as well as from a scarp on the NE side of the volcano near the summit. A gas-and-steam plume extended about 50 km SSE and was observed in satellite and webcam images on 26 November. On 28 November small explosions were observed in seismic and local infrasound data and gas-and-steam emissions were visible from the summit and from the upper NE collapse scarp based on webcam images. Possible small explosions were observed in infrasound data on 30 November. Weakly elevated surface temperatures and a persistent gas-and-steam plume from the summit and collapse scarps on the upper flanks. A passing aircraft reported the gas-and-steam plume rose to 3-3.4 km altitude on 30 November, but no significant ash emissions were detected.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed a strong pulse of thermal activity beginning in July 2023 that continued through November 2023 (figure 48). This strong activity was due to Strombolian explosions and lava fountaining events at the summit crater. According to data from MODVOLC thermal alerts, a total of 101 hotspots were detected near the summit crater in July (11-14, 16-19, 23-24 and 26), August (4, 25-26, and 29), September (5, 12, and 17), and October (3, 4, and 8). Infrared satellite data showed large lava flows descending primarily the northern and SE flanks during the reporting period (figure 49). Sulfur dioxide plumes often exceeded two Dobson Units (DUs) and drifted in different directions throughout the reporting period, based on satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 50).

Figure 48. Graph of Landsat 8 and 9 OLI thermal data from 1 June 2024 showing a strong surge in thermal activity during July through November 2023. During mid-October, the intensity of the hotspots gradually declined. Courtesy of MIROVA.

Figure 49. Infrared (bands B12, B11, B4) satellite images show several strong lava flows (bright yellow-orange) affecting the northern and SE flanks of Shishaldin on 18 July 2023 (top left), 4 June 2023 (top right), 26 September 2023 (bottom left), and 3 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 50. Strong sulfur dioxide plumes were detected at Shishaldin and drifted in different directions on 15 August 2023 (top left), 5 September 2023 (top right), 25 September 2023 (bottom left), and 6 October 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

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).

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/).

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/).

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/).

Ambrym Island was investigated by John Seach and Perry Judd during a climb into the caldera 1-8 January 1999. A lava lake in Benbow cone was present during 1-3 January but was covered by deposits from an avalanche that occurred overnight 4-5 January. Fumarolic and Strombolian activity was observed at other craters.

Activity at Benbow. Benbow crater was climbed from the S, after which observers lowered themselves using ropes 200 m down from the crater rim to a point where they could observe the crater interior. In the center of the crater, an active lava lake was seen 220 m below the observation point. The lava lake was ~40 m in diameter and constantly in motion. Large explosions caused lava fountains that reached 100 m high. Bombs glowed for up to one minute in daylight and radiated great heat. Bombs could be heard landing on the side of the pit where they caused glowing avalanches. At night a strong glow from the lava lake was visible in the sky over Benbow.

Elsewhere inside Benbow crater, Pele's hair covered the ground and fumaroles were active on the NE crater wall. Acid rain burned eyes and skin. Heavy rainfall caused many waterfalls to form inside the crater rim and a shallow brown pond formed on the floor of the first level.

During 4-5 January violent Strombolian explosions could be heard almost hourly. Each series of explosions lasted 5-10 minutes and produced dark ash columns above the crater. At some time during these explosions an avalanche on the W side of the lava lake crater completely covered the lava lake. No night glow was visible above the crater after the night of 5 January.

On 6 January Benbow crater was entered again. The wall collapse that covered the lava lake was confirmed visually. In the location of the former lava lake was a depression of rubble with two small, glowing vents nearby. The entire crater was clear of magmatic gases. Three violent Strombolian eruptions were viewed from the crater rim in the afternoon. Bombs were thrown 300 m into the air and dark ash clouds were emitted.

Activity at Niri Mbwelesu Taten. This small collapse pit continuously emitted white, brown, and blue vapors. Red deposits covered the crater walls. A small amount of yellow deposits covered the S wall. Fumarole temperatures were 66 to 69°C at a point 40 m SE of the pit. On 6-7 January numerous deep, loud degassings were heard from a distance of 4 km.

Activity at Niri Mbwelesu. Pungent, sulfurous-smelling white vapor was emitted from this crater. Periods of good visibility enabled views 200 m down from the crater rim, but the bottom could not be seen. Rockfalls were heard inside the crater.

Activity at Mbwelesu. Excellent visibility to the bottom of this crater enabled detailed observations of the lava lake. Night observations were also obtained. The lava lake was in constant motion and splashing lava out over the sides of the pit. The lake was at a lower level than during observations made three months earlier (BGVN 23:09). Large explosions sent lava fountains up to 100 m in height and threw lava onto the sides of the pit causing glowing avalanches. During one night observation a 20 x 5 m section of the crater wall broke off and fell into the lava lake. The 60-m-wide lake radiated heat that could be felt from the viewing area 380 m away. North of the lava lake was a circular vent 20 m in diameter that glowed brilliantly from magma inside and huffed out burning gasses every 20 seconds. Foul gas, smelling of rotten fish, was emitted from the crater. South of the lava lake was an elongated vent (40 x 10 m) that spattered lava every 5-10 seconds and sent showers of glowing orange lava spray 150 m high.

On the S side of Mbwelesu, fumarole temperatures averaged 43°C at 10 m from the crater edge. On the SE side, 40 m from the crater edge, fumaroles measured 57°C. On 4 January ashfall occurred on the S side of the caldera.

Geologic Background. Ambrym is a large basaltic volcano with a 12-km-wide caldera formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have been frequently reported since 1774, though mostly limited to extra-caldera eruptions that would have affected local populations. Since 1950 observations of eruptive activity from cones within the caldera or from flank vents have occurred almost yearly.

Information Contacts: John Seach, P.O. Box 16, Chatsworth Island, NSW, 2469, Australia.

During February, seismic and volcanic activity at Bezymianny increased in intensity, causing the hazard status to be raised from Green to Yellow on 16 February and then to Orange on 25 February. The activity decreased on the 26th and the "Level of Concern Color Code" was reduced to Yellow. In the first two weeks of the month, numerous weak earthquakes were registered under the volcano, and fumarolic plumes rising up to a few hundred meters above the summit occurred frequently.

Starting on 15 February and continuing the following week, seismicity rose above background levels and 20-40 shallow earthquakes were registered every day. The hazard status was raised to Yellow. Fumarolic plumes continued to rise to a few hundred meters above the summit, and could be seen when not obscured by clouds. Satellite images during the week indicated a persistent thermal anomaly possibly caused by rock avalanches from the summit dome.

The hazard status was raised to Orange on 25 February after volcanic tremor began under the volcano and continued for ~6 hours. Two large explosions during that period each lasted several minutes and a gas-and-ash plume rose 5 km above the summit. Satellite images that morning showed an ash-rich plume heading SE. Over the next few days, using satellite imagery, the ash cloud was tracked for 1,500 km to the SE, but by early on the 27th the cloud had dissipated. Activity declined after the 25th and the hazard status was reduced to Yellow.

On 27-28 February the seismicity was above background levels. Low-level spasmodic tremor continued to be recorded. On the morning of 28 February a steam-and-gas plume rose 300 m. The volcano was obscured by clouds after 28 February.

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

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

The unusually large 10 February explosion was followed by collateral reports by (a) F. Núñez-Cornú, G. Réyes-Davila, and C. Suárez-Plascenia and (b) John B. Murray. In addition, this summary of the interval 26 February to 16 March benefitted from press releases from the Colima Volcano Observatory. These three sources are discussed in separate sections below.

Geophysical signature of the 10 February explosion. F. Núñez-Cornú, G. Réyes-Davila, and C. Suárez-Plascencia provided the following report.

"On 10 February at 0145 an explosive event occurred at Colima's summit dome; this generated a shock wave that broke windows and opened gates in the small town of Juan Barragan, 8.75 km SE of the summit. The sonic wave was also heard in the towns of Tonila, Quesería, San Marcos, Atenquique, El Fresnito, Ejido de Atenquique, and up to 28 km NE of the volcano at Ciudad Guzman.

"This was the biggest explosion reported for the volcano in the last 80 years; the resulting exhalation emitted both ash and lava blocks (bombs made up of both fresh and altered components). A substantial amount of incandescent tephra fell and started fires on both the volcano's upper slopes and on Nevado de Colima's S slopes; most of the fires were extinguished by snow and rain storms during the subsequent 48 hours.

"As summarized in table 8, a seismic event took place hours before the explosion, at 2231 of 9 February; it was followed by other volcanic and tremor signals at about 0100; some of these precursory events saturated the amplitude response of analog instruments at stations EZV4 (Somma) and EZV7 (Volcancito). Four additional large, post-eruptive seismic events also occurred. These strong events were observed clearly at farther stations EZV3 (Nevado, 5.8 km from the summit), and EZV2 (Cerro Grande, 25 km from the summit)."

Table 8. Noteworthy seismic events around the time of the 10 February 1999 explosion at two Colima seismic stations (EZV3 and EZV2); the earliest reading (on the top line) took place the night before the explosion. See text for station locations. Courtesy of F. Nunez-Cornu, G. Reyes-Davila, and C. Suarez-Plascencia.

"Currently the Jalisco civil defense operates an observational base called Nevado located 900 m NW from the summit of Nevado de Colima.

"Since the end of November 1998, three seismic instruments (MarsLite with LE3d (1 Hz) sensors) were deployed to complement the RESCO network at the volcano. To improve spatial resolution the authors moved one of these instruments to El Playon on 11 February. On the way to El Playon we observed fires on the southern slopes of Nevado out to a maximum distance of 4.5 km from the volcano's summit.

"On the road at a spot 2.9 km NE of the summit and at 3,120 m elevation we found several impact craters. The first one contained an andesite block with dimensions of 0.37 x 0.44 x 0.43 m. Several small impacts occurred nearby. We found another impact pit near the road, 100 m away from the first site but at similar distance and direction from the summit. This pit measured 1.94 x 0.70 m on the surface and had a depth of 0.60 m. It contained a partially buried andesite block (identified as R3) that measured 0.60 x 0.41 x 0.70 m. The block's temperature was 40°C. The pit sat in a spot surrounded by 10- to 15-m-tall trees; their lack of visible damage suggested a near vertical angle of impact, which we estimated as 80-85°.

"At 70 m away from block R3 we found a volcanic bomb that struck the middle of the road. The bomb consisted of hydrothermally altered volcanic breccia (identified as R4, figure 34), which had shattered on the road over an area 1.73 x 1.64 m; the bomb failed to excavate a crater.

Figure 34. Impact crater R4, created by Colima's 10 February 1999 explosion. Courtesy of F. Nunez-Cornu, G. Reyes-Davila, and C. Suarez-Plascencia.

"In traveling across El Playon we observed dozens of impacts, but elected to stay the minimum time possible in order to reduce exposure to hazards. Most of the bombs seen and sampled consisted of either andesite resembling the new dome or hydrothermally altered andesite, perhaps from the 1987 crater wall. When visiting the same area on 26 February, we found the small and medium impact craters difficult to identify; most of the impacts below trees were covered by newly fallen leaves."

Leveling survey and field examination of the 10 February bombs. On 28 February, John B. Murray, assisted by members of the Colima fire department (Mitchell Ventura, Filiberto de la Mora, and Juan Carlos Martinez) measured two branches of a N-flank leveling traverse last surveyed in January 1997. The first branch, which was 740 m long, left the Playon vehicle track and followed the path up Volcancito passing through stations Porte de Colima (1.3 km from the volcano's summit) and Albergue (1.9 km from the summit). The movement measured since 1997 showed subsidence at stations nearest the volcano totaling 13 mm for the entire section. This was nearly double the subsidence measured during 1995-97, an interval without any lava emission. There was also 13 mm of subsidence seen during 1990-92, an interval which included lava emission (in 1991).

The second branch of the leveling traverse began at Albergue station and ended at Voltaire station, a spot 2.3 km from the summit. Compared to 1997, the Albergue station had subsided just over 8 mm relative to the Voltaire station. Little significant change occurred here during 1995-97 (1 mm rise) and 1990-92 (0.4 mm rise). During a 15-year interval (1982-97) these two stations subsided a total of only 6 mm, and thus looks like a small though significant change in movement. Most of the change (5.6 mm) was measured between two stations 160 m apart at a distance of 2 km from the summit. The possibility of a small error cannot be ruled out, although the movement does follow the same sense throughout this section of the leveling traverse.

The total subsidence between the farthest (2.3 km) and the nearest (1.3 km) station to the summit was 22 mm. This is rather larger than during the 1991 crisis, when the subsidence between the same two stations was 13 mm. Viewing this movement as deflation of a magma chamber (Murray, 1993), this may simply be a reflection of the rather larger output of the volcano in 1998-99 compared to 1991. However, equally tenable is the hypothesis that the movement is due to volcano spreading, or even to Colima's slow slipping down the southern flanks of the larger Nevado volcano, on whose southern slopes Colima is situated. Increases in the rate of subsidence were also observed following the Mexican earthquake of 1985, as well as during the 1991 crisis described above. Although the subsidence during 1997-99 is greater than previously measured, there is nothing in the measurements to suggest that the volcano is building up to a bigger eruption, or to distinguish between the Mogi deflation or downslope slipping models.

The distribution of volcanic bombs from the 10 February explosion was noted at sites along the leveling traverse. Table 9 lists the estimated average distance between impact craters at the various sites where measurements were made. Murray and co-worker identified fragments that varied in size between 10 and 70 cm in diameter, there being no noticeable trend in size between bombs found in the region 1.3 to 2.8 km from the summit. The largest bomb crater found had taken away one third of the road on the north edge of the 1869 lava flow near station Hector, a spot 2.1 km from the summit. This crater was at least 2 m in diameter. However, the numbers of impacts per unit area decreased as distance from the volcano increased.

There is also some evidence of directed blast in table 9, there being distinctly higher concentrations of bombs NNE of the volcano (station Esteban) than at similar distances NE (station C15). Bombs appeared to be of two distinct types: 1) solid, dark, fresh-looking andesitic rocks with high density and no sign of vesiculation, and 2) crumbly, light-colored, altered, vesicular, pumice-like ejecta with low density (guessed at around 1,000 kg/m³) There did not appear to be any predominance of one type or the other with distance from the volcano.

Table 9. Average spacing of N-flank bomb strikes that were found after Colima's 10 February 1999 explosion. Courtesy of John B. Murray.

A bomb found near the campsite, 1.75 km from the summit, left evidence of its trajectory as it had smashed a 10 cm branch of a tree just before landing. The bomb itself was of solid andesite, and had fractured into several pieces on landing, but it appeared to have had an original diameter of about 40 cm. It had made an impact crater ~1 m in diameter and 50 cm deep. Using the level as a horizontal marker, three measurements of the angle between the broken branch and the crater bottom gave 44 ± 3° from the horizontal.

Six fire sites were inspected and described; usually these were associated with a bomb, but not always. At first, these fire sites went unnoticed because they chiefly consumed low-growing vegetation, and in no case was a completely burned tree to be found. The view towards the volcano from the Playon was unaffected, as green bushes and trees were seen as usual.

For example, at fire site 3, located 2 km NNE of the summit (N side of road, just past bend near station Esteban) we found an isolated pumice bomb 20 cm across, but without burnt vegetation in contact. However, the bomb ignited grass clumps 2 and 3.5 m away; none of the grass between the bomb and the clumps had been affected.

Most fire sites were close to bombs, usually burning on the side away from the volcano. However, most were not in direct contact with the bomb in question, but centered around dry vegetation, particularly tall grass clumps, succulents, small bushes, and (occasionally) trees. The grass and succulents were not dead, but had fresh green shoots sprouting from the top. Presumably because of the high water content, only the dry, dead leaves at the base of the succulents were burned, but there were large areas where succulents were affected in this way, the adjacent vegetation being quite unaffected. There was often no obvious associated bomb in the vicinity. Similarly with grass clumps, there would be gaps of 2 or 3 m between burned clumps, from which the fire had apparently spread radially for a short distance before going out, with no sign of burning of the dry, low grass cover in between. However, not all bombs in the same area had the same effect. In some cases, the only sign of burning was directly beneath the bomb itself, where the grass was singed black but still fairly intact. Yet in places nearby, the landscape had clearly been very slowly burned over an extensive area 10 to 30 m wide, and in one case discussed below, it was still burning.

Murray goes on to comment: "The odd characteristics of these fire sites suggests the possibility of an abnormal ignition mechanism. It seems that ignition depended in many cases not on the proximity to the source of heat (bombs) but rather on the characteristics of the ignited vegetation. It was as if in certain (sometimes quite extensive) areas those low-growing plants below a certain water content, or containing appropriate oils would ignite, and the rest would not. This implies a very high air temperature close to the ground over areas in some cases tens of meters across. The most obvious source of these high temperatures would seem to be hot gas, usually emanating from bombs but not always so. Where associated with bombs, the isolated fire sites would always be on the side facing away from the summit. In other words, there is evidence that extensive degassing took place from bombs upon impact; and that there might also have been some local associated ground-hugging nuees of a weak and intermittent type."

Explosion on 28 February 1999. Murray also noted that "At 1715 on 28 February, while examining the distant bombs and impact craters 2.8 km NE of the summit on the forest road outside the caldera, we heard a distant, faint rushing sound coming from the summit, resembling a large rockfall or an aircraft. On looking up, a large whitish-grey convective cloud, like a cumulus cloud, could be seen rising from the summit and blowing in our direction. It had clearly started some time previously and was already stretching some distance towards us. A heavy rain of ash began nine minutes later, at 1724, ceasing at ~1731. The ashfall, which was sampled, sounded like large raindrops hitting the leaves in the nearby forest but on spreading out a sheet of paper on the ground, only sand-sized ash particles could be seen accumulating on it. At the end of the shower, there was one particle every centimeter approximately, the largest particle being ~ 2 mm across, and the smallest just under 0.5 mm. From the sound of the particles falling in the trees round about, it sounded as if much larger particles were involved in the shower, but none of these fell on the spread-out paper."

Official press releases. A 26 February update by the Colima Volcano Observatory stated that chemical analysis of Colima's water and ash had indicated insignificant risk to human health. At this time the established security limit was set at 10-10.5 km from the summit. Evacuated settlements included Yerbabuena, Causenta, Atenguillo, El Fresnal, La Cofradía, Juan Barragán, El Agostadero, Los Machos, El Alpizahue, El Saucillo, and El Borbollón. The local populations were advised to avoid a long list of drainages, as well as to hand-carry important documents, and to advise authorities of those requiring help in order to secure transport in case of more extensive evacuations. Meanwhile, during the previous 24 hours the monitored parameters indicated relative quiet, suggesting possible voluntary return to evacuated areas at noon on 2 March if these conditions persisted. The 5 March update noted degassing events during the previous 24 hours, the majority of these around 1400 on 5 March. The 16 March update mentioned the recent occurrence of both degassing and minor ash emissions.

Reference. Murray, J.B., 1993, Ground deformation at Colima Volcano, Mexico, 1982 to 1991: Geofisica Internacional, v. 32, no. 4, p. 659-669.

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: F. Nunez-Cornu^1,4, G. Reyes-Davila², and C. Suarez-Plascencia^3,4; 1) Laboratoria Sismologia, University of Guadelajara, Guadelajara, Mexico; 2) RESCO, University of Colima, Colima, Mexico; 3) Department of Geology, University of Guadelajara, Guadelajara, Mexico; 4) U. Est. Proteccion Civil Jalisco; Colima Volcano Observatory, Universidad de Colima, Av. Gonzalo de Sandoval 444, Colima, Colima 28045, Mexico (URL: https://portal.ucol.mx/cueiv/); J.B. Murray, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, England.

The following report summarizes activity observed at Etna from January through February 1999. Bocca Nuova exhibited minor explosive activity through early February, but Northeast Crater and Voragine were quiet. Southeast Crater had seven distinct eruptive episodes between 5 January and 4 February; the latest was accompanied by the opening of a new eruptive fissure at its southeastern base. The information for this report was compiled by Boris Behncke at the Istituto di Geologia e Geofisica, University of Catania (IGGUC), and posted on his internet web site. The compilation was based on personal summit visits, observations from Catania, and other sources cited in the text.

Activity at Southeast Crater (SEC) until 23 January. After one week of relative quiet, the sixteenth eruptive episode of SEC since 15 September occurred shortly before noon on 5 January; this was preceded by weak Strombolian activity that started around midnight. The paroxysmal phase was characterized by vigorous fountaining, and lava flowed towards the northeast while tephra was driven southwest by the strong wind. Loud detonations were audible in towns on the flanks of Etna.

Episode 17, during the night of 9-10 January, was preceded by mild Strombolian activity; the paroxysmal phase occurred shortly after midnight. Lava presumably flowed NE again and tephra fell NE; Fiumefreddo, ~8 km SW of Taormina, received a light showering of ash. Loud detonations during the final phase were audible over a wide area, and clear weather conditions permitted many in the Catania area to watch the spectacular display.

After the shortest repose interval observed since early in the current eruptive sequence in September, episode 18 took place on the morning of 13 January, between about 0630 and 0930. Visibiliby was hampered by clouds, but loud detonations were audible in a wide area around the volcano. Ash fell as far as Giarre, ~15 km E.

The next eruptive episode occurred on 18 January, shortly after 0800, and lasted ~ 45 minutes. Minor Strombolian and effusive activity had occurred earlier during the night. As in preceding episodes, the culminating phase was characterized by initial strong lava fountaining which gradually became more ash-rich, generating a dense eruption column. Due to calm conditions, the column rose several kilometers above the summit (3 km as estimated from Catania) and attained a spectacular mushroom shape visible in the morning sky from all around the volcano. At the SEC cone itself, the heavy fallout and rapid accumulation of pyroclastics led to frequent avalanches, especially on the steep eastern side. After 0830, dull explosion sounds were audible to as far as Catania, accompanying the rhythmic uprush of dark ash. The activity declined rapidly at 0845, but ash emissions became again more forceful after 0900 and continued sporadically for several hours, accompanied by sliding of hot pyroclastics from the steep E side of the cone. No information was available about lava flows although it is likely that they occurred, possibly on the NE side of SEC.

SEC erupted again after only two days and four hours of inactivity, shortly after noon on 20 January. Increased gas emission began at ~ 1215, and by 1240 a lava fountain appeared at the vent of the SE Crater cone. This fountain rapidly rose to a height of several hundred meters, and the column which rose above it became more and more ash-rich. Less than 15 minutes after the onset of the eruption there occurred the first slides of hot pyroclastics from the upper part of the cone, and five minutes later the whole cone and part of Etna's main summit cone were veiled by a black curtain of falling bombs and scoriae. By 1300, the vertical eruption column had risen several kilometers above Etna's summit. Ten minutes later the activity began to decline rapidly, and by 1315 the eruptive episode was essentially over, with only a few ash puffs being emitted during the following 30 minutes.

During a summit visit by Boris Behncke and Giovanni Sturiale (IGGUC) on 21 January, the crater was completely quiet, and only a few weak fumaroles played on the SW and E crater rims. The cone at SEC had grown higher than 3,250 m, about as high as the rim of the former Central Crater (filled by lavas and pyroclastics in the 1950's and 60's). While its flanks were steep and regular on most sides, obliterating any trace of the pre-1998 crater rim, a deep V-shaped notch was present in the northern crater rim through which lava had spilled onto the cone's flanks during recent eruptive episodes. These lavas had formed a fan-shaped lava field on the northeastern base of the cone, extending to the rim of Valle del Bove.

Behncke and Sturiale also investigated the pyroclastic deposits of the recent eruptive episodes which extended in relatively narrow fans from SEC in various directions. During the 18 and 20 January epidsodes, most fallout had occurred in a radius of <1 km from the cone, mainly on the SE side of the former Central Crater where 0.5-1 m of pyroclastics had accumulated since late 1998. Meter-sized bombs had fallen up to 500 m from SEC, creating spectacular impact craters. Among the most peculiar features of the recent eruptive products was a small lahar on the southwestern side of SEC which extended ~300 m from the base of its cone; this was probably produced during the 5 January episode. Records of lahars are relatively rare in the recent history of Etna, the most notable occurring in 1755.

On the morning of 23 January, SEC was the site of yet another eruptive episode that began at about 0630 and probably lasted less than one hour. Due to the absence of wind, an eruption column rose several kilometers above the summit then drifted slowly SE. In Catania, the ashfall was not dense, but people in the streets felt particles entering in the eyes; these particles were less than 1 mm in diameter and left a thin, discontinuous film on the ground. More serious effects were caused by the fallout in the upper southern parts of the mountain where skiing was rendered impossible by scoria on the snow. The repose period between this and the previous eruptive episode was two days and 18 hours.

There appears to have been no significant seismic or eruptive activity between 23 January and 4 February; the few clear views during that period revealed no morphological changes.

The January eruptive episodes continued to build the SEC cone, which has changed beyond recognition from its mid-1998 appearance. The large crater formed in 1990 at the summit of the SEC cone was completely filled, and a new, tall summit grew over it, burying any trace of the 1990 crater and much of the lava flows erupted from mid-1997 to late July 1998. After the 23 January episode the cone's new summit was at ~ 3,270 m elevation, almost 90 m higher than the highest point of the 1990 crater rim in 1997.

New eruptive fissure opens on 4 February. A new eruptive episode from SEC began at 1600, producing a spectacular eruption column visible from Catania and all around the mountain. Like previous episodes, this event was characterized by vigorous fire-fountaining, tephra emission, and lava, and was preceded by a gradual increase in gas emissions and then mild Strombolian activity. The activity began to culminate at around 1600 when a tall fountain jetted from the summit crater of the cone, and lava spilled through the breach in the N crater rim.

Sometime around 1630, the SE side of the cone fractured, and a new vent opened about halfway down the cone's flank, producing a tall lava fountain 250-350 m high and feeding a dense, ash-laden eruption column. An eruption column rose ~ 2-3 km above the summit before being driven SE, dropping fine ash on the flanks. Lava soon began to flow SE from this vent (figure 75). At about 1640, a row of incandescent spots appeared below the newly formed vent, indicating that a fissure had begun to propagate downslope from the base of the SEC cone. Vigorous lava fountaining and tephra emission from the new vent on the SE flank of SEC diminished rapidly shortly after 1700, but activity continued at the smaller vents on the fissure below that vent, at ~ 2,950 m elevation, and lava advanced rapidly towards the rim of Valle del Bove. At nightfall, both this lava flow and the lava erupted at the beginning of the episode onto the northern side of SEC were brightly incandescent and well visible from towns on the eastern side of the volcano, causing rumors of the opening of fractures on both sides of the cone. However, the northern flow soon stagnated and cooled, and no further lava emission occurred on that side for the remainder of February.

Figure 75. Sketch map showing Etna's summit craters SEC, Voragine (V), and Bocca Nuova (BN). The approximate extent of lava flows emitted during the 4 February eruption are in medium gray and those following the 4 February eruption are in black. Flows erupted from 1971 to 1993 are shown in light gray. Courtesy of Boris Behncke.

On 5 February, lava had begun to spill into Valle del Bove, forming a cascade on its steep western wall. The flow advanced very slowly, and had not yet reached the valley floor (at ~2,000 m elevation) on the next day when the new eruptive fissure was visited by Behncke and Giuseppe Scarpinati (L'Association Volcanologique Européenne, LAVE). Mild explosive activity was building several hornitos in the upper part of the ~100-m-long, SE-trending fissure at the base of the SEC cone while lava was issuing from numerous vents along the whole length of the fissure, feeding several channellized flows and some minor a`a flows. The effusion rate was estimated at 5 m³/s or more, significantly higher than during previous mainly effusive eruptions near Etna's summit craters (mainly at NE Crater in the 1970's) and similar to the effusion rates of some of Etna's flank eruptions. Pahoehoe lava was abundant around the effusive vents. The cone of SEC was found to be fractured from its summit down to its base, but only the main 4 February vent appeared to have produced significant eruptive activity while only minor spatter and scoriae were found in the part of the fracture between that vent and the still-active fissure.

On 15 February, Behncke and Scarpinati again visited the eruptive fissure and observed its activity for about 4 hours. By that day the lava spilling into the Valle del Bove had reached ~ 2,000 m elevation. There was no sign that the activity was diminishing, and the effusion rate remained perhaps as high as 5 m³/s.

Lava continued to issue from a number of effusive vents on the active fissure, forming at least two main rivers and several smaller and short-lived flows. In the course of a few hours Behncke and Scarpinati saw some of the lesser flows cease and others reactivate, forming blocky a`a while the more vigorous and long-lived flows moved in well-defined channels and showed no significant flux variations. Numerous short lava tubes, well-developed flow channels, and secondary vents had formed. Most effusive activity occurred ~50-100 m downslope from the upper end of the fissure, but several vents were also higher upslope. In the uppermost part of the fissure, numerous hornitos had formed, most of them concentrated in three clusters, and this area had countless incandescent vents producing high-pressure gas emission accompanied by a persistent hissing noise. The largest hornitos formed thin, vertical spires up to 3 m high while others were small humps a few tens of centimeters high. There was little explosive activity; only one vent in the uppermost hornito cluster rarely ejected incandescent pyroclastics.

Similar activity continued through the end of February. Lava flowed into the Valle del Bove, forming numerous lobes that moved on top or adjacent to earlier flows, and the farthest flow fronts did not extend much beyond 2,000 m elevation, remaining above the Monti Centenari, a cluster of cones formed during the 1852-53 eruption on the floor of Valle del Bove. The flow field gradually widened to ~500 m on the rim, and flows were issuing from numerous ephemeral vents on the W slope of the Valle.

Activity at Bocca Nuova (BN), Voragine, and Northeast Crater (NEC). Little significant activity occurred at these craters during January-February 1999 except for a brief resurgence of activity at BN during the week preceding the 4 February SEC events. During the 21 January visit by Behncke and Sturiale, spattering and Strombolian activity occurred deep within the large crater in the southeastern part of BN, accompanied by dense gas emission.

The cone in the northwestern part of BN produced violent noisy explosions every few minutes which ejected fountains of bombs high above the crater rim; ejecta frequently fell outside the crater, mostly to the W but in a few cases also SW and S. Between the explosions, deep-seated minor activity occurred within the 50-80-m-wide crater of the cone. No effusive activity had taken place in BN since it was invaded by lava from Voragine on 22 July 1998.

Bright crater glow was visible above BN in the first nights of February, the first time in about five months. This glow persisted during the night of 3-4 February but was much weaker on the evening of 4 February, indicating a drop of the magma level, probably related to the opening of the eruptive fissure on the SE base of SEC earlier that day. During the following week, only infrequent weak glows were visible above BN and then vanished altogether.

Very little activity except profuse steaming was observed within the Voragine during the 21 January visit by Behncke and Sturiale, who were able to descend into this crater and arrived at the "diaframma," the septum that separates the Voragine from Bocca Nuova. The floor of the crater was very flat in its eastern part, while a cluster of four craters with low cones occupied its central-western portion. The central crater, ~50 m wide and 30 m deep, was completely quiet; on its W side a much shallower, ~20-m-wide crater contained a 2-m-wide degassing hole with overhanging walls on whose floor numerous incandescent spots could be seen. A small crater with a diameter of less than 20 m, and ~ 10 m deep, lay on the SE side of the central crater. The largest crater in the Voragine was in the SW part of the Voragine and was between 70 and 100 m wide and more than 50 m deep with very steep and unstable walls, so that its floor could not be seen. Eruptive activity occurred at depth; as could be judged from the noises this was similar to the activity observed in the southeastern BN vents on the same day. A fifth vent that was active in August and early September 1998 on the crest of the "diaframma" appeared to have collapsed into the large SW vent, and only a part of its cone remained standing.

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

Information Contacts: Boris Behncke, Istituto di Geologia e Geofisica (IGGUC), Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy.

Seismicity remained low during January and February 1999. Volcano-tectonic (VT) earthquakes were common from two sources at depths of 0.2-18.8 km and had a coda magnitude range between -0.6 and 3. The first area was below the active cone, and the second was NNE of Galeras. The most significant VT event registered on 3 January at 0714 with a coda magnitude of 3, an epicenter ~14 km NNE of the volcano, and felt earthquakes in Pasto. Other types of VT events located toward the E flank have been called "trenes" (trains) because they are recorded consecutively, to make up packets of 2-5 events. They were small events, recorded at only four of the nine stations in the Galeras network. Those events had a depth range of 3.3-7.3 km and a coda magnitude range between -0.6 and 0.9.

Previous VT events at times have preceded seismic sequences, such as those during November-December 1993 and March 1995, as well as a small seismic sequence in July 1997. However, events have also been recorded in periods of no seismic sequences.

Quasi-monochromatic volcanic tremor episodes were recorded during 4-6 January. The maximum amplitudes were obtained on the E-W components of the broadband stations whereas the minimal amplitudes were recorded on the vertical components of those stations. The spectral frequencies show stable values with small variations of 0.5 Hz. Analysis of the tremor episodes suggested that the source directions of these events were toward the active cone of the volcano.

The electronic tiltmeter Peladitos, on the E flank of Galeras, showed stable behavior with small variations (<1 µrad) in both radial and tangential components. The Chorrillo and Huairatola portable tiltmeters showed stable behavior in the tangential components whereas the radial components continued a descending trend that began at the end of September 1998. Through 26 January, the cumulative decline in the Chorrillo radial component was ~35 µrad, and the Huairatola radial component decline was ~600 µrad.

Most of the radon stations showed stable behavior of the Rn-222 gas emission with changes <200 pCi/l. In contrast, the Meneses-1 station showed variations of ~ 3,300 pCi/l on an ascending trend; the Meneses-3 stations, ~2,700 pCi/l on a descending trend.

When the Alfa Deformes fumarole was measured in December 1998, it had a pH of 0.6. The next measurement, in May 1998, revealed a pH of 2.3, followed by a gradual decline to a value of 0.3 on 25 February. Measured fumarole temperatures generally remained stable, although the La Joya fumarole had increased to 181°C on 6 March from 148°C on 25 February. Scientists observed numerous fissures emitting gas during a summit visit, as well as cracks that could generate small landslides on the main cone.

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

Information Contacts: Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

The Instituto Geofísico (IG-EPN) monitors seismic events, crustal deformation, geochemistry, and records visual observations at Guagua Pichincha. This volcano consists of a 2-km-wide caldera, breached to the west, on whose floor lies a dome complex and the present explosion craters. The following report summarizes their daily observations from 1 January to 31 March 1999. During this period, a Yellow alert status persisted.

Bad weather often prevented or hindered visual observations. Guards at the refuge station and visiting scientists frequently reported noises and the strong smell of sulfur from the fumaroles. COSPEC data from 16 January and 13 March showed only background concentrations of SO₂ from the fumaroles, following the maximum concentrations yet recorded (170 t/day) on 10 December. Ash-and-steam plumes from dome fumaroles, when visible, ranged from 100 to 800 m in height, while explosion plumes reached 3 km. The 1981 explosion crater had increased in diameter and almost absorbed the September 1998 crater.

People living along the Cristal river (W flank) confirmed the seismic detection of small debris flows and floods that were generated on 7 and 27 January, 2, 16, and 21 February, and 1 March, all related to intense rainfalls; these traveled down the Rio Cristal at least 10-15 km. Estimated volumes are between 0.3 and 1 x 10^-6 m³ with estimated peak discharges of 100-250 m³/s.

Phreatic explosions covered the dome and the interior of the caldera with ash and rocks. A guard at the refuge station and Civil Defense personnel found 2-5 mm of new ash and new impact craters in the Terraza area following the explosions of 21 and 23 January. Analysis of the ash showed no juvenile material, suggesting that magma had not ascended. Ballistically ejected rock fragments up to 30 cm in diameter were found 1-1.5 km S and SE of the dome, the result of phreatic explosions in this time period.

Volcano-tectonic (VT), long-period (LP), and hybrid earthquakes, sometimes in multiples, occurred almost daily throughout January, February, and March. Phreatic explosions were frequent during that period, occurring on average once per day in February and March. Daily LP event counts varied between 1 and 40, but many days had few VT or LP events. Still, 24 VT events occurred on 28 February and 1 March. .High-frequency tremor episodes of a few minutes to as much as four hours (9 February) duration were recorded, but possible associated effects in at the caldera summit could not be confirmed due to bad weather. Some rockfalls in the caldera were heard by the refuge guards while tremor episodes were occurring.

On 9 February and 14 March instruments detected 16 and 70 tectonic earthquakes along the N part of the Quito fault. The largest events had magnitudes of 3.7 and 4.0, respectively. It had been speculated that these events represented sympathetic responses to stresses produced by the volcano's magma chamber. This idea came from an earlier observation of an "on-off scenario" where the presence earthquakes in the N Quito area correlated with little seismicity registering under the caldera, and vice versa.

Reduced displacement measurements (RDs) of phreatic explosions ranged from those too small to measure to several that were 20 cm² or greater. Some of these larger RDs, such as those on 18 and 28 January, and 13, 19, and 28 February, were the largest since October 1998. The one on 28 February was the largest yet recorded. A summary of seismic events since August 1998 is presented in table 2.

Table 2. Monthly summaries of explosions and seismic events at Guagua Pichincha, August 1998-March 1999. Courtesy IG-EPN.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately W of Ecuador's capital city, Quito. A lava dome grew at the head of a 6-km-wide scarp formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the lava dome. Many minor eruptions have been recorded since the mid-1500's; the largest took place in 1660, when ash fell over a 1,000 km radius and accumulated to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

Local residents first noticed thick gray ash emissions from the summit on 18 December 1998 (corrected from BGVN 24:01); this information reached the Volcanological Survey of Indonesia (VSI) Gamkonora volcano observatory on the 31st. On 2 January personnel from VSI who went to the island to take COSPEC measurements of the SO₂ release observed a loud eruption that caused up to 3 mm of ashfall in and around Tugure Batu Village. The eruption lasted 35 minutes and generated a plume 1,000 m high. Another eruption observed on 5 January 1999 lasted for 60 minutes. Thunderclaps from the summit were heard on 16 January and a night glow from ejecta was evident above the summit area. Residents also reportedly saw lava at the crater rim. The seismometer from Gamkonora (an RTS PS-2) was installed ~2 km from the summit of Ibu on 3 February along with an ARGOS satellite system tiltmeter.

Field observations on 11 March revealed continuing eruptions and rumbling noises, but the larger eruptions (accompanied by booming and thick ash ejection) had decreased to a rate of one every 15-20 minutes. When observed on 2 February larger eruptions occurred every 5 minutes. Seismograph records are still dominated by explosion events; during 9-15 March there were 779 events, increased from 673 events the previous week.

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: R. Sukhyar and Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

During fieldwork on Santa Ana volcano in February, increased steaming was observed at the summit of Izalco relative to levels of previous years. Strong fumarolic activity occurred along the entire circumference of the 250-m-wide summit crater, with the exception of the NE side facing Cerro Verde. Activity was most vigorous at a vent on the N side of the crater floor, but was also strong along much of the inner rim of the crater and along its outer flanks. Steaming was observed over broad areas on the outer southern flanks to ~50 m below the rim, and on the W flank immediately N of a shoulder of the cone at ~1,800 m elevation, roughly 150 m below the summit. Activity had earlier been noticed to have increased in November 1998 following Hurricane Mitch. Most of the steaming was water vapor, and the increased activity was attributed to saturation of the still-warm cone by heavy rains accompanying the hurricane.

Geologic Background. Volcán de Izalco began growing in 1770 CE on the southern flank of Santa Ana volcano, eventually building a steep 650-m-high stratovolcano truncated by a 250-m-wide summit crater. Frequent Strombolian eruptions during the two centuries prior to the cessation of activity in 1966 provided a night-time beacon for ships, causing it to be known as El Faro, the "Lighthouse of the Pacific." The dominantly basaltic andesite tephra and lava flows are geochemically distinct from those of both Santa Ana and its fissure-controlled flank vents. Lava flows have primarily erupted from flank vents, traveling as far as about 7 km south down the slopes of Santa Ana.

Information Contacts: Carlos Pullinger, Calle Padres Aguilar 448, Colonia Escalon, San Salvador, El Salvador; Demetrio Escobar, Centro de Investigaciones Geotecnicas (CIG), Final Blvd. Venezuela y calle a La Chacra, Apdo. Postal 109, San Salvador, El Salvador; Lee Siebert and Paul Kimberly, Global Volcanism Program, Smithsonian Institution.

Krakatau erupted on 5 February 1999 accompanied by thunderclaps and an ash plume that reached a height of ~1,000 m above the summit. The activity continued until 10 February with ash plumes reaching ~100-300 m above the summit. The continuing sporadic eruptions deposited small amounts of ash over most of the island; a deposit of ~0.3 mm was measured near the observatory. On 11 February, the glow of ejecta was observed reaching ~25 m above the summit and continued during the night.

Activity decreased early during the week of 9-15 March. Weak booming noises were heard twice on 9 and 10 March, but plumes were not observed. At the end of the week booming noises were rare, and a white-gray ash plume was seen on 14 March that rose 100-300 m above the summit. The current activity is a continuation of eruptions that began in 1992.

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

Information Contacts: R. Sukhyar and Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

The following report is based on photos taken between September and November 1998. Most of the photos were taken by local mountain guide Burra Ami Gadiye. Sketches and descriptions of the photos were provided by Celia Nyamweru of St. Lawrence University.

Lava from within the crater breached the rim, causing small lava flows down the outer crater wall; the breach on the NW probably occurred in late October, and the breach on the E began in early November. Small, narrow tongues of pahoehoe lava erupted continuously from vents around the upper slopes of cones T37S, T37N, and T40 (figure 55). Most of these flows moved E or NE, although a few moved W. The tops of T37S and T37N were built up into broad cones with jagged crowns. Some growth also occurred at T40. Little change was apparent on any of the other cones that were in existence in August (BGVN 23:10). In mid-November a new cone, which has been numbered T50, formed at the base of the SE wall.

Figure 55. View of Ol Doinyo Lengai looking N from the summit on 29 September 1998. Traced by Celia Nyamweru from a photo by B.A. Gadiye.

Activity during September and October. Narrow flows of pahoehoe lava emerged in late September from vents close to the summit of T37S and flowed E and W. The westward-flowing lava reached the center of the crater; the eastward-flowing lava reached the rim of T24 and the base of the crater wall. These flows were very dark in color suggesting they were still fluid or only very recently formed. The summit of T37S had a jagged profile (figure 56), replacing the broad dome seen in August.

Figure 56. View of Ol Doinyo Lengai looking NW from SE crater rim as seen on 29 September 1998. Traced by C. Nyamweru from photographs by B. A. Gadiye.

Small, narrow, very dark colored pahoehoe flows emerged in early October from vents close to the summits of T37S and T40 (figure 57). Behind T40 and to the right of T45, the T37 cluster showed some dark lava extending westwards from its summit past T47, the very tall narrow cone in front of the south wall. Cone T40 had fresh lava extending from the summit onto its lower slopes.

Figure 57. Photograph of Ol Doinyo Lengai taken on 3 October 1998 of the view S from the N crater rim. Courtesy B.A. Gadiye.

In another photo on 7 October (figure 58), the top of T37S was dark brown, in striking contrast with the very pale brown lower slopes. Surrounding cones were pale brown. A large dark brown flow from a source between T45 and T37 extended around the eastern slope of T45. The flow showed no sign of whitening along the edges of the slabs, unlike the flow in front of it, and, therefore, might have been only a few hours old. The E crater wall was estimated to be 5 m high based on the appearance of a person in one photo. This was not an estimate of the lowest point on the crater wall.

Figure 58. Photograph of Ol Doinyo Lengai taken on 7 October 1998 of the view SW from the E crater rim. Courtesy B.A. Gadiye.

Activity during November. In early November fresh, black, shiny, pahoehoe lava flowed from a vent between T45 and T37S. Gadiye noted the source of the flow as the cone T5T9. Only the very top of T5T9 remained visible, since the remainder was covered by 20 m of lava. Another lava flow originated from a vent on the S slope of T40 and flowed around the E side of this cone. According to Gadiye the crater had filled and lava was pouring over the NW rim. A few weeks later he took two photographs, noting that the lava was spilling over the crater rim on the E and had burned the grass on the slope. The lava in one of these photos (taken just outside the rim) consisted of brown and gray smooth pahoehoe flows that did not seem to be more than 10 to 20 cm thick. Judging from the pale color, it had probably undergone weathering during the weeks since it flowed.

Aerial photographs taken late in November showed several narrow tongues of very dark lava over an older surface of white and pale brown lava. These dark flows originated from the slopes of T37S and from the cluster of cones around T37N1. A narrow white streak that overflowed the rim on the NW side was probably recent lava. A few days later fresh pahoehoe flows effused from T37S and T37N and flowed E toward the crater wall and the remains of the rim of T24 (figure 59). In this area was a new cone near the base of the S wall: a low circular feature, just out of view in figure 59, which Gadiye described as "a new cone near the SE rim that is boiling and giving out a lot of steam." This has been designated T50. Lava was seen to be overflowing the NW rim. T37S had a very jagged appearance and there also seemed to have been considerable growth at T37N1, between T37S and T45. Some fresh pahoehoe, very dark over the white older flows, was also visible farther west on the crater floor, near the T44/T48/T49 cone cluster.

Figure 59. Photograph of Ol Doinyo Lengai taken on 24 November 1998 looking SW from the crater floor. Courtesy of B.A. Gadiye.

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: Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617 USA (URL: http://blogs.stlawu.edu/lengai/).

During 1963-82 ash emissions, lava flows, lava fountains, and Strombolian explosions occurred intermittently at Lopevi. In 1968-69 activity mainly affected the SE flank (figure 1), where two lava flows from the summit reached the sea. The twenty-year pattern of activity ended with emission of a major plume that rose to 6,000 m on 24 October 1982 (SEAN 07:010).

Figure 1. View of the SE flank of Lopevi volcano, looking toward the NW in May 1995. Paama Island, from which recent observations were made, and Ambrym Island, a currently active volcano, are in the background (to the N). Courtesy IRD; photo by P. Evin, IRD.

Since then, activity had been generally fumarolic. Eruptive activity resumed in July 1998. A series of Strombolian explosions in the main 1963 crater (just NW of the central crater) was observed during November 1998. On 29, 30, and 31 December 1998, Strombolian explosions and Vulcanian emissions were observed from the island of Paama every 4-5 minutes.

Sporadic eruptive activity observed between the end of December 1998 and March 1999 was confined to the 1963 crater on the NW flank (figure 2). The appearance of this large crater, at ~900 m elevation, ruined the perfect conic profile of Lopevi, a rare volcano of the archipelago without a caldera.

Figure 2. View of the active crater on Lopevi's NW flank as seen in January 1999. Courtesy IRD; photo by J-M. Bore, IRD.

Lopevi, an island ~6 km in diameter, 1,450 m high, and 3,500 m above the seafloor, is one of the most active of the Vanuatu archipelago. The first written description came from Captain Cook, who in 1774 entered in his ship's log that the volcano was "seemingly without activity." Volcanic crises reported since 1863 appear to have occurred in cycles of ~15-20 years. In 1960, following a significant Plinian eruption from the NW flank, a series of pyroclastic flows, lava flows, Strombolian activity, and fumarolic emissions were observed during one month. In 1963, over a period of several months, large quantities of flowing lava and ash spread through ~ 1,000 ha in the NW part of the island.

Geologic Background. The small 7-km-wide conical island of Lopevi, known locally as Vanei Vollohulu, is one of Vanuatu's most active volcanoes. A small summit crater containing a cinder cone is breached to the NW and tops an older cone that is rimmed by the remnant of a larger crater. The basaltic-to-andesitic volcano has been active during historical time at both summit and flank vents, primarily along a NW-SE-trending fissure that cuts across the island, producing moderate explosive eruptions and lava flows that reached the coast. Historical eruptions at the 1413-m-high volcano date back to the mid-19th century. The island was evacuated following major eruptions in 1939 and 1960. The latter eruption, from a NW-flank fissure vent, produced a pyroclastic flow that swept to the sea and a lava flow that formed a new peninsula on the western coast.

Information Contacts: Michel Lardy, Institut de recherche pour le développement (IRD), B.P. 76, Port Vila, Vanuatu; Douglas Charley and Roland Priam, Department of Geology, Mines and Water Resources, PMB 01, Port Vila, Vanuatu.

Explosive activity resumed on 2 January 1999 at Pacaya for the first time since the end of a major eruptive episode on 19 September 1998. Current activity has consisted of small explosions that ejected ash without incandescent material. Beginning on 8 January, the number of explosions increased from 100-200/day to more than 400/day, reaching a peak of ~ 550 on 21 January (figure 19). Explosion counts declined to ~200/day by the end of the month. Volcanologists from INSIVUMEH and the Smithsonian Institution observed frequent small ash eruptions during a 1 February visit. The explosions were not accompanied by detonations, and produced billowing gray-to-brown ash columns that rose ~100 m above the vent. They observed that two vents produced explosions; the largest explosions originated from the westernmost and lower of two vents in the breached crater. Intense fumarolic activity occurred from the inclined floor of the summit crater, its rim, and the outer flanks.

Figure 19. Daily explosion counts at Pacaya during January 1999. Courtesy of INSIVUMEH.

Significant changes to the morphology of MacKenney cone had occurred since a strong explosive eruption on 18-19 September 1998. That eruption left a major breach 20-25 m wide that extended SW. By the time of the 1 February visit, erosion had widened the breach to 70-80 m. At its head, the breach had nearly vertical walls more than 50 m deep, and formed a gully that extended more than 1 km down to ~1,800 m elevation. The NE side of the crater was also notched, but not nearly as deeply. Fractures and down-dropped blocks of summit agglutinate material along the crater rim also showed this SW-NE orientation in line with the location of two flank vents active during September 1998. The breach gives MacKenney cone a twin-peaked appearance when viewed from the W flank (figure 20). The present form of the crater increases the possibility of future eruptive or collapse events being directed toward the W-flank village of El Patrocinio (figure 21).

Figure 20. A prominent gully extends more than 1 km down the SW flank of Pacaya from the twin-peaked summit of MacKenney cone, 1 February 1999. The dark lava flow at the lower right was one of two emplaced from flank vents at the end of the 18-19 September 1998 eruption. Photograph courtesy of Lee Siebert.

Figure 21. Sketch map of Pacaya and nearby towns. Hachured arcuate line indicates the caldera rim. Contour interval 100 m; contour intervals around MacKenney crater are approximate. Courtesy of INSIVUMEH.

The accumulation of spatter and ejecta from the September 1998 explosions had built MacKenney cone to a height about 30-35 m above an older cone immediately SE of MacKenney crater. The older cone, the previous vantage point for observing explosive activity from Pacaya, had itself grown about 10 m in the past decade from the accumulation of ejecta from MacKenney crater. The height of MacKenney cone now exceeds that of Cerro Grande, a vegetated ~2,560-m-high prehistorical cone of Pacaya located 2 km NE of MacKenney.

September 1998 eruption. A major explosive and effusive eruption took place on 18-19 September (table 3). During the first 17 hours of the eruption, a 1.2-km-long lava flow descended WNW into the caldera moat and down the flank of the volcano to the Montanas las Granadillas area SW of Cerro Chino. From 1700-2200 an explosive eruption ejected ash columns to 5 km above the crater, producing ashfall to the SW and NNW. Fine ashfall caused the closing of the international airport in Guatemala City for 35 hours. About 1 m of volcanic bombs were deposited on the caldera rim. Pyroclastic avalanches of incandescent ejecta mantled the upper half of the cone. One 3-m-wide impact crater was formed at the base of the lava flow near El Patrocinio, and 1-m-wide impact craters were found as far as 5 km from the vent. During the final explosive phase, the SW rim of MacKenney crater collapsed, forming a debris avalanche that traveled 2 km down the SW flank to ~1,500 m elevation. Coarse blocks littered the surface of the deposit, whose light color contrasted with that of adjacent dark-colored lava flows.

Table 3. Summary of major eruptive events at Pacaya volcano from January 1987 to September 1998.

Several lava flows accompanied the explosive activity (figure 22). The longest of these traveled ~4 km from a notch in the NE crater rim. The flow initially descended northward into the caldera moat where it was deflected by the caldera wall, flowed across the moat, and then down the SW flank to 1,760 m elevation before diverging around a small kipuka and scorching trees at its northern margin below Cerro Chino. Much of the caldera moat was covered by lava flows of the September eruption, and the prominent 1984 spatter cone low on the N flank was nearly buried.

Figure 22. Photograph of the lava flow (foreground) that descended from Pacaya's caldera moat down the W flank. This flow and the two dark lobes above it originated from MacKenney cone during the 18-19 September 1998 eruption. Light-colored tephra deposits between the flows mantle previous lava flows. Photograph taken on 1 February 1999. Courtesy of Paul Kimberly, SI.

At the end of the eruption, two small lava flows took place from flank vents on opposite sides of the cone. A vent on the upper NE flank at ~2,450 m elevation produced a short lava flow that reached the caldera moat. A vent on the lower SW flank at ~1,800 m elevation (figure 22) produced a short lava flow that divided into two lobes, one traveling to the SW and the other to the south.

Summary of 1987-1998 activity. Routine explosive activity characteristic of Pacaya occurred through much of the period from 1987 to the present but is not listed in table 3. Strong explosive eruptions in January 1987 and June 1991 destroyed the upper part of MacKenney cone, deepening and widening the crater, after which renewed eruptions reconstructed the cone. Major eruptions on 7 and 14 June 1995 destroyed the WNW side of the crater, leaving two notches at the summit. Debris from the 7 June collapse slammed into the caldera wall at Cerro Chino, 1 km NW of the summit, and produced a secondary hot cloud that swept over Cerro Chino, destroyed a radio antenna, and affected houses within 2 km of the active vent. The shockwave threw INSIVUMEH observer Pastor Alfaro down a slope, fracturing his leg. The 7 June event produced a 2.5-km-high plume. The second collapse on 14 June produced an avalanche that traveled SW toward El Rodeo and was accompanied by a 4-km-high plume. Lava flows subsequently traveled 2 km. Figure 23 shows RSAM plots for 1995-98.

Figure 23. Plot of seismic activity at Pacaya as represented by Real-time Seismic Amplitude Measurement (RSAM) counts during January 1995-December 1998. Courtesy of INSIVUMEH.

A strong explosive eruption on 20 May 1998 produced a 4-km-high ash column. Incandescent bombs burned trees on the SSW flank of Cerro Grande, 2 km N of the crater, and scoria fall damaged vegetation and crops. Two persons in the settlement of San Francisco de Sales, 2.5 km NE of the crater, were injured by falling scoria blocks. The ash plume was primarily blown to the NE, with a lesser plume to the SW (figure 24). Ash fell from 1300-1600 in the villages and towns within 5 km of the volcano. During 1400-1830 ash fell in the capital city of Guatemala, causing closure of the international airport. Ashfall covered an area of 800 km², and had an estimated volume of ~2.3 x 10⁶ m³. The eruption caused the evacuation of 254 residents from surrounding villages to the town of San Vicente de Pacaya. Lava flows during the 20 May eruption traveled down the N, W, and SW flanks and had a volume of 6.3 x 10⁵ m³.

Figure 24. Isopachs of the 20 May 1998 explosive eruption from Pacaya volcano. Courtesy of Otoniel Matias, INSIVUMEH.

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

Information Contacts: Otoniel Matias, Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Ministerio de Communicaciones, Transporte y Obras Publicas, 7A Avenida 14-57, Zona 13, Guatemala City, Guatemala; Lee Siebert and Paul Kimberly, Global Volcanism Program, National Museum of Natural History, Room E-442, Smithsonian Institution, Washington DC 20560-0119.

Seismicity under the volcano was about at background levels from December 1998 through February 1999. On 2 February a M 2 earthquake was located at 23 km depth. Weak volcanic tremor and small earthquakes were registered during the first half of February, and on 21 February a 6-minutes series of shallow earthquakes was detected. The Level of Concern Color Code remained Green.

The volcano was frequently obscured by clouds, making observations only intermittently possible. Fumarolic plumes rising 50-400 m were noted on 10 December, 8, 13-14, and 20 January, 6-7, 13, 16-18, and 22 February. Higher plumes, in the range of 700-800 m above the summit, were observed on 21 and 23 January, and 5 February. On 10 and 15 February fumarolic plumes rose 1,000 m.

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

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

During the first week of February, National Weather Service personnel in Cold Bay, 93 km ENE of Shishaldin, observed anomalous steaming. On 9 February a vigorous steam plume rose as high as 1,830 m above the vent and a long plume drifted downwind. Satellite imagery taken that day showed a thermal anomaly at the vent in addition to the steam plume. The steam activity decreased during the week, becoming only light puffs rising a few meters above the vent; however, the thermal anomaly at the vent persisted. A newly installed seismic net recorded slightly elevated seismicity beginning at the end of January.

The hazard status was raised to Yellow on 18 February due to the persistence of the thermal anomaly and the identification of low-level seismic tremor. Pilots and ground observers reported a large steam plume rising to 5,800 m on 18 February. No ash was detected on satellite imagery. Cloudy weather precluded ground observations for most of the following week.

Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetrical cone with a basal diameter of approximately 16 km. A small summit crater typically emits a noticeable steam plume with occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, situated near that part of the arc where the maximum rate of subduction occurs. It has erupted at least 27 times since 1775. Major explosive eruptions occurred in 1830 and 1932, and eight historical eruptions have produced lava flows. Steam and minor ash emission began in March 1986 and continued intermittently through mid-February, 1987. A poorly documented short-lived eruption of steam and ash, perhaps as high as 10 km, occurred in December 1995 (BGVN 21:01). Fresh ash was noted on the upper flanks and crater rim but no specific eruptive event was identified for the deposits.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory, a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Several small dome collapses, some that were initially explosive, generated pyroclastic flows in December. Episodes of ash venting occurred almost daily and seismicity was dominated by volcano-tectonic earthquakes and rockfalls. The number of volcano-tectonic earthquakes declined toward the end of December but the number of long-period signals, corresponding to ash venting, increased slightly. Some explosive eruptions during early- to mid-January generated substantial ash clouds. Brief episodes of ash venting, correlating with seismic tremor, became shorter and weaker toward the end of January. Small-volume pyroclastic flows were generated by dome collapse, but some flows may have been generated by fountain collapse during small explosive eruptions. The average SO₂ flux was elevated throughout December and January. Eastward movement of the Long Ground and Tar River GPS sites continued.

Visual observations.Daily periods of volcanic tremor during December coincided with steam-and-ash venting. On 8 December mudflows occurred all around the volcano.

A pyroclastic flow generated by dome collapse on 14 December reached the sea at the Tar River delta. Deposits were fluidized, fine-grained material with very few blocks. A large ash cloud was generated that rose rapidly to ~6,100 m. Ash fell W and NW of the volcano, attaining a thickness of 2 mm in Salem and containing accretionary lapilli up to 2 mm in diameter. On 19 December a pyroclastic flow reached the Tar River delta in less than five minutes. Powerful black jets of ash and rock burst from the dome at the onset of the event but it is unclear if this explosive activity preceded or followed the dome collapse. The small deposit was almost entirely confined to the incised channel in the Tar River valley on top of the 14 December deposits. On 21 December, at the onset of a sudden large seismic signal, dense black jets of ash and vigorously convecting ash clouds escaped from the main vent in the 3 July scar. Ballistic blocks rose 80 m above the vent. Very vigorous ash venting continued for more than 30 minutes after the initial explosion. A minor dome collapse on 27 December resulted in a small-volume pyroclastic flow reaching the Tar River delta. Poor visibility hampered observations, but a significant ash cloud was generated.

Minor ash venting took place on 1 and 5 January. At 0358 on 7 January, a large long-period seismic signal immediately preceded a 30-minute episode of tremor (usually associated with vigorous ash venting). Later the same day, a small dome collapse generated a pyroclastic flow that traveled half-way down the Tar River valley and a low-level ash cloud that moved W over Plymouth. On 13 January an explosive event generated an ash cloud to 6,100 m and a pyroclastic flow. The onset of the seismic signal had a long-period component, and a pressure wave was recorded at Long Ground. A booming sound was reported by many. The pyroclastic-flow deposit in the Tar River valley was small in volume but its extent suggested that the flow had been very mobile. Narrow small-volume pyroclastic-flow deposits were observed S of the dome as far as the former position of Galway's Soufriere. Two small dome-collapse pyroclastic flows occurred on 14 January. At 0827 on 15 January a small explosive event generated an ash cloud that rose to 4,600 m. The cloud moved NW and light ashfall affected Salem and Old Towne. Ash venting continued in pulses for 15 minutes. Another small explosion on 16 January generated an ash cloud to 3,000 m. Rockfalls were triggered on the inner walls of the 3 July scar and on the outer SE and NE flanks of the dome. A minor dome-collapse pyroclastic flow on 20 January almost reached the sea at the Tar River delta. The resulting steam-rich plume dissipated rapidly. Several brief (20 minute) episodes of tremor preceded by a rockfall corresponded to weak ash venting on 24 January. Further short episodes of ash venting occurred on 25 and 27 January.

Clear conditions on 26 and 27 January enabled MVO staff to survey the dome (figure 44). The canyon, which had been incised through the dome, was clearly visible. It bisected the dome in a NW-SE direction from the top of Tar River Valley to the top of Gages Valley. The inner walls of the canyon were vertical and surfaces looked fresh because of repeated small rockfalls.

Figure 44. Photograph of the dome area at Soufriere Hills taken in late January 1999. This was used to calculate the dome volume and shows an exceptionally clear view of the gully running through the dome. Courtesy MVO; photograph by Richard Herd and Chloe Harford.

Seismicity. Seismicity in December consisted chiefly of volcano-tectonic earthquakes and rockfall signals. Many of the latter were associated with small pyroclastic flows or venting. Small clusters of earthquakes were located under George's Hill to the NW of the dome, under Roaches Yard to the SE, and under Hermitage Estate to the NE.

Overall, January was quiet seismically. Pyroclastic-flow signals had low-frequency precursors. These events were associated with booming noises and were followed by periods of vigorous ash venting, suggesting the collapses were caused by violent degassing of the dome.

Ground deformation. The only area where significant deformation took place in December was on the E flank. The vectors for Long Ground showed eastward movement of these two sites amounting to 5 cm since lava stopped erupting. Most of this movement occurred during the last three months (a time of increased surface activity). The differential movement between Whites and Long Ground since June 1996 is more than 10 cm. The two sites are 733 m apart and the movement between them cannot be fit elastically. A ground inspection on 30 December revealed a possible fault between the two sites. The only surface expression is a linear break in the road and it is not currently known whether this is related to volcanic deformation or to surficial movements. The Tar River GPS pin has followed a similar movement to Long Ground throughout the eruption. The Perches site, until it was destroyed in July, followed a similar path. One possible interpretation is that a sector of the volcano including Long Ground, Perches, and Tar River is moving as a block along faults in a NE direction.

Eastward movement of Long Ground and Tar River continued in January but at a reduced rate. A local EDM network of five pins was set up on 27 January to learn whether the surface feature is a fault.

Environmental monitoring. The miniCOSPEC was used several times in December. The SO₂ flux was elevated and on 22 December and reached a peak average flux of 1,700 metric tons per day (figure 45). Sulfur-dioxide flux decreased throughout January, but generally remained elevated. Concentrations were also measured at ground level by using diffusion tubes around the island.

Figure 45. Average daily SO2 fluxes at Soufriere Hills measured by miniCOSPEC, December 1998-January 1999. The lines connecting measured points are guidelines only; the actual measured levels varied. The measurements made on 19 January showed a very low flux: observations suggested that at least part of the plume was at a very low altitude and may have been found partly below the elevation of the traversing helicopter. Data courtesy of MVO.

Ash and rainwater collection continued throughout January. Ash samples from the small explosive events tended to very coarse, with lithic and crystal fragments up to 6 mm in size in the Richmond Hill-St. Georges area. In contrast, ash generated by dome-collapse pyroclastic flows was very fine-grained.

Volume measurements. A detailed photographic and theodolite survey was conducted from twelve sites around the volcano at the end of January. A photographic survey was also conducted from the helicopter with the GPS onboard. The information has been processed to produce a detailed dome map and volume measurement. The dome had a volume of 76.8 x 10⁶ m³ and its highest point was 977 m at the top of the White River Valley. The dome was split deeply by the collapse on 3 July 1998 and by subsequent events. The N part of the dome, which comprises three main buttresses above Gages, the N flank, and Tar River, contains two-thirds of the total dome volume. The scar cuts up to 100 m into the pre-1995 crater floor and has removed a minimum of 5.4 x 10⁶ m³ of old rock from this area.

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

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

On 18 February, a gas-and-steam explosion generated a plume to 600 m above the volcano. Small (magnitudes near zero) shallow earthquakes were registered under the volcano and continued through the month, coincident with M 1.5 events at 15-30 km depth. No further unusual seismicity was reported as of mid-March.

The massive Tolbachik basaltic volcano is located at the southern end of the dominantly andesitic Kliuchevskaya volcano group. The Tolbachik massif is composed of two overlapping, but morphologically dissimilar volcanoes. The flat-topped Plosky Tolbachik shield volcano with its nested Holocene Hawaiian-type calderas up to 3 km in diameter is located east of the older and higher sharp-topped Ostry Tolbachik stratovolcano. Lengthy rift zones extending NE and SSW of the volcano have erupted voluminous basaltic lava flows during the Holocene, with activity during the past two thousand years being confined to the narrow axial zone of the rifts. The last eruptive activity, in 1975-76, vented from both the summit and SSW-flank fissures; it was the largest historical basaltic eruption in Kamchatka.

Geologic Background. The massive Tolbachik volcano is located at the southern end of the Kliuchevskaya volcano group. The massif is composed of two overlapping, but morphologically distinct, volcanoes. The flat-topped Plosky Tolbachik shield volcano with its nested Holocene calderas up to 3 km in diameter is located east of the older and higher sharp-topped Ostry Tolbachik stratovolcano. The summit caldera at Plosky Tolbachik was formed in association with major lava effusion about 6,500 years ago and simultaneously with a major southward-directed sector collapse of Ostry Tolbachik. Long rift zones extending NE and SSW of the volcano have erupted voluminous basaltic lava flows during the Holocene, with activity during the past two thousand years being confined to the narrow axial zone of the rifts. The 1975-76 eruption originating from the SSW-flank fissure system and the summit was the largest historical basaltic eruption in Kamchatka.

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

Volcanic-tremor levels on White Island (BGVN 23:10-23:12 and 24:01) have remained low since 22 January and low-level eruptive activity continued through mid-March. On 12 February, the low-energy hydrothermal activity within Metra Crater was dominated by gas-and-steam emissions from small fumaroles on the N and W sides of the crater. Four small ponds had formed on the crater floor. A weak gas (SO₂) and steam plume from PeeJay Vent rose 400-500 m, forming haze visible 40-50 km away.

During a visit by C.P. Wood on 13 March activity was generally constant with the ash-and-steam column rising to ~ 1,060 m and drifting many kilometers downwind, with sea discoloration from fall-out evident to 1 km from the island. PeeJay Vent was continuously emitting ash-charged gray-brown steam, but with varying intensity. During peak discharges, observers standing on the 1978/90 Crater Complex edge noted a rumbling noise from PeeJay, but no block ejection was seen. The vent diameter appeared to have increased and was an obvious funnel shape lined with whitish sublimate deposits. Ash could not be collected because of the wind direction. Metra Crater was occupied by a lurid lime-green lake, which largely filled the original crater and peripheral scallops to ~ 1 m below the rim (the old lake floor). There was no sign of thermal disturbance in the Metra lakelet. The ash surface throughout Main Crater was rain-washed and smooth (except for the route used by tourist operators), with no sign of recent impact craters near the 1978/90 Crater Complex edge.

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

Information Contacts: Brad Scott, Wairakei Research Centre, Institute of Geological and Nuclear Sciences (IGNS) Limited, Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).