The Weekly Volcanic Activity Report: November 29 – December 5, 2023

the weekly volcanic activity report

New activity/unrest was reported for 2 volcanoes from November 29 to December 5, 2023. During the same period, ongoing activity was reported for 14 volcanoes.

New activity/unrest: Etna, Sicily (Italy) | Marapi, Central Sumatra.

Ongoing activity: Aira, Kyushu (Japan) | Bagana, Bougainville (Papua New Guinea) | Dukono, Halmahera | Fuego, South-Central Guatemala | Great Sitkin, Andreanof Islands (USA) | Krakatau, Sunda Strait | Lewotolok, Lembata Island | Mayon, Luzon (Philippines) | Merapi, Central Java | Santa Maria, Southwestern Guatemala | Semeru, Eastern Java | Sheveluch, Central Kamchatka (Russia) | Shishaldin, Fox Islands (USA) | Suwanosejima, Ryukyu Islands (Japan).

New activity/unrest

Etna, Sicily (Italy)

37.748°N, 14.999°E | Summit elev. 3357 m

INGV reported that Strombolian activity at Etna’s SE Crater (SEC) continued during 29 November-5 December. Over the longer period of 19 November-1 December there were over 250 Strombolian explosions counted. Eruptive episodes were quite regular, with episodes lasting 5-15 minutes separated by 60-70 minutes of quiet. At peak intensity there were strong explosions and incandescent jets as high as 200 m above the crater rim. Activity primarily took place at the western-most vent within the saddle of the SEC, but the eastern-most vent was also active. On 29 November eruptive activity began to intensify, and by 1 December the time between episodes was only about 5 minutes. Low pulsating lava fountains and small lava flows on the SW flank of the SEC were observed. A VONA was issued at 1708 and the Aviation Color Code was Red. A new paroxysmal episode began at approximately 1800 with sustained lava fountains sending material 400 m above the crater and producing lava flows down the SSW and ESE flanks to the base of the SEC. Lava flows reached about 2,860 m elevation on the SSW flank and 2,300 m on the SE flank. The Strombolian activity produced ash plumes that rose as high as 6 km a.s.l. and drifted ESE. Ashfall was reported in the towns of Fiumefreddo (20 km ENE), Calatabiano (24 km NE), and Naxos Gardens (25 km NE). The strong activity lasted around three hours, and started to gradually decrease around 2050. A VONA was issued at 2238 lowering the Aviation Color Code to Orange. On 2 December at 1136 and 1254 the Aviation Color Code was lowered to Yellow then Green respectively. During 2-5 December no explosive activity was observed, but degassing was observed from both the SEC and the Bocca Nuova craters.

Geological summary: 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.

Marapi, Central Sumatra

0.38°S, 100.474°E | Summit elev. 2885 m

An explosive eruption from Marapi (on Sumatra) began at 1454 local time (Western Indonesian Time, or WIB; 0754 UTC) on 3 December, as reported by PVMBG and the National Disaster Management Agency’s (BNPB) Operations and Control Center (Pusdalops). At that time 75 people were known to be hiking on the volcano, having registered at either the Batu Palano entrance in Agam Regency or the Koto Baru entrance in Tanah Dara Regency. News reports late on the evening of 5 December indicated that rescue teams had accounted for all of the climbers, with officials reporting 23 deaths and 52 survivors, most of whom had suffered burns along with other injuries.

PVMBG reports and Volcano Observatory Notices for Aviation (VONAs) stated that the initial eruption lasted 4 minutes and 41 seconds, producing a dense dark gray ash plume that rose 3 km above the vent (5.9 km a.s.l.) and drifted E. The eruption was also accompanied by a roaring sound and a pyroclastic flow near the N flank that descended as far as 3 km. The Aviation Color Code was raised from Orange to Red. Tephra fall was reported within 6 km from the vent, and ash as far as 13 km away. Thick ashfall was reported as of 1742 WIB in the Nagari Lasi area within the Canduang District of Agam Regency, and less intense ashfall was reported in the Sungai Pua District. By 0728 WIB on 4 December, tephra consisting of mainly ash and some larger clasts described as “gravel or stones” had been reported in the Canduang, Sungai Pua, Ampek-Ampek, Malalak, Banuhampu, Tilatang Kamang, Baso, Tanjung Raya, Lubuk Basung, IV Koto, Matur, Tanjung Mutiara, Palembayan, and Kamang Magek subdistricts.

Prior to the eruption, during 29 November-2 December, summit views were blocked by weather clouds and recorded seismicity consisted of typical volcano-tectonic (VT) earthquakes; the eruption was not preceded by increased seismicity. Overall on 3 December the seismic network recorded 36 eruption events starting with the 1454 WIB activity. Another 10 eruption events were recorded on 4 December, including an explosion at 0822 that produced a dense gray-to-black ash plume that rose as high as 800 m above the vent and drifted SW. The Aviation Color Code was lowered from Red to Orange. The seismic network recorded six additional eruption signals during 5 December, though summit views were often blocked by weather clouds.

The Alert Level remained at 2 (on a scale of 1-4) and the public was warned to stay at least 3 km away from the crater in all directions. There were no evacuations, but people within the Canduang, Sungai Pua, Ampek Angkek, and Malalak districts were advised to avoid outdoor activities and wear masks. With support from the West Sumatra Provincial Government, the Agam Regency government activated health centers to help those affected by ashfall.

Geological summary: Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra’s most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in historical time.

Ongoing activity

Aira, Kyushu (Japan)

31.5772°N, 130.6589°E | Summit elev. 1117 m

JMA reported ongoing activity at Minamidake Crater (Aira Caldera’s Sakurajima volcano) during 29 November-5 December, with very small eruptions and crater incandescence observed nightly. One volcanic earthquake was recorded on 3 November, and another on 4 November. A field survey conducted on 30 November detected an extremely high SO2 rate of 4,200 tonnes per day. The Alert Level remained at 3 (on a 5-level scale), and the public was warned to stay 2 km away from both craters.

Geological summary: 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.

Bagana, Bougainville (Papua New Guinea)

6.137°S, 155.196°E | Summit elev. 1855 m

The Darwin VAAC reported that an ash plume was detected in satellite data acquired by the Himawari-9 satellite on 29 November at 2030 UTC. The plume rose as high as 2.1 km a.s.l. and drifted N and NE. The plume dissipated by 0250 UTC on 30 November.

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

Dukono, Halmahera

1.6992°N, 127.8783°E | Summit elev. 1273 m

PVMBG reported that the eruption at Dukono was ongoing during 29 November-5 December. Daily, dense ash-and-gas plumes rose 50-2,200 m above the summit and drifted E and W. The greatest plumes were observed on 30 November and 1 December and rose 2 km and 2.2 km above the summit respectively. The Alert Level remained at Level 2 (on a scale of 1-4), and the public was warned to remain outside of the 2-km exclusion zone.

Geological summary: Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia’s most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the N-flank Gunung Mamuya cone. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

Fuego, South-Central Guatemala

14.473°N, 90.88°W | Summit elev. 3763 m

INSIVUMEH reported that eruptive activity continued at Fuego during 29 November-5 December. Weak and moderate explosions were recorded daily; the number of explosive events averaged 1-12 per hour. Daily explosions produced gas-and-ash plumes that rose to 4.3-4.8 km a.s.l. (600-1,100 m above the summit) and drifted as far as 30 km SW, W, and NW. Fine ashfall was reported in areas downwind such as Panimaché I and II (8 km SW), Morelia (9 km SW), Santa Sofía, Yucales (12 km SW), Quisache, and Yepocapa (12 km SW) on 30 November, in Morelia, Finca Palo Verde (10 km WSW), Sangre de Cristo (8 km W), and Panimache (8 km SW) on 2 December, and in Panimaché I and II, Morelia, Santa Sofía, Yucales (12 km SW), and Yepocapa on 4 December. Additionally, explosions produced faint rumbles, shockwaves, pulses of incandescent ejecta 100-300 m above the crater, and weak and moderate avalanches that descended the Las Lajas (SE), Ceniza (SSW), Taniluyá (SW), Santa Teresa (W), and Seca (W) drainages.

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

Great Sitkin, Andreanof Islands (USA)

52.076°N, 176.13°W | Summit elev. 1740 m

AVO reported that slow lava effusion continued at the summit crater of Great Sitkin during 29 November-5 December. Seismicity was low. Satellite radar data during 29-30 November showed radial cracks on the lava flow surface due to local uplift of lava above the vent and indicated that lava continued to flow E towards the intra-crater glacier. Weakly elevated surface temperatures and steam emissions were possibly observed in satellite data and webcam images, respectively, during 1-2 December. Weather clouds on other days often blocked views of the summit. The Volcano Alert Level remained at Watch (the third level on a four-level scale) and the Aviation Color Code remained at Orange (the third color on a four-color scale).

Geological summary: The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger parasitic volcano capped by a small, 0.8 x 1.2 km ice-filled summit caldera was constructed within a large late-Pleistocene or early Holocene scarp formed by massive edifice failure that truncated an ancestral volcano and produced a submarine debris avalanche. Deposits from this and an older debris avalanche from a source to the south cover a broad area of the ocean floor north of the volcano. The summit lies along the eastern rim of the younger collapse scarp. Deposits from an earlier caldera-forming eruption of unknown age cover the flanks of the island to a depth up to 6 m. The small younger caldera was partially filled by lava domes emplaced in 1945 and 1974, and five small older flank lava domes, two of which lie on the coastline, were constructed along northwest- and NNW-trending lines. Hot springs, mud pots, and fumaroles occur near the head of Big Fox Creek, south of the volcano. Historical eruptions have been recorded since the late-19th century.

Krakatau, Sunda Strait

6.1009°S, 105.4233°E | Summit elev. 285 m

PVMBG reported that eruptive activity continued at Krakatau during 29 November-5 December. The seismic network recorded eruption events, low-frequency earthquakes, hybrid earthquakes, volcanic earthquakes, harmonic tremor events, and continuous tremor events almost daily. During 29 November-1 December, white and gray plumes rose as high as 300 m above the vent. Volcano Observatory Notices for Aviation (VONAs) and Volcanic Eruption Notices (VENs) were issued during 2-5 December. Incandescent ejecta often accompanied the eruptive events. Eruptions described in three VONAs on 2 December produced dense ash plumes that rose 500-800 m above the vent and drifted NE and NW. On 3 December, six VONAs were issued for eruptions that produced dense gray-to-black and black ash plumes that rose 600-1500 m above the vent and drifted W and NW. Eleven VONAs were issued on 4 December for eruptions that produced dense black ash plumes that rose 700-2,000 m above the vent and drifted SE and NW. Another eight VONAs and VENs were issued during 5 December for eruptions that produced dense gray-to-black and black ash plumes that rose 500-1,500 m above the vent and drifted NW. The Alert Level remained at 3 (on a scale of 1-4), and the public was warned to stay at least 5 km away from the crater in all directions.

Geological summary: 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.

Lewotolok, Lembata Island

8.274°S, 123.508°E | Summit elev. 1431 m

PVMBG reported that eruptive activity continued at Lewotolok during 29 November-5 December. Daily white and sometimes white-to-gray emissions rose 25-600 m above the active vent. Seismic signals associated with both eruption and gas emission events were recorded daily. Volcano Observatory Notices for Aviation (VONAs) and Volcanic Eruption Notices (VENs) were issued during 30 November through 5 December for eruptions that produced white, gray, and white-to-gray ash plumes that rose as high as 550 m above the active vent and drifted W and NW. The Alert Level remained at 2 (on a scale of 1-4) and the public was warned to stay at least 2 km away from the summit crater.

Geological summary: The Lewotolok (or Lewotolo) stratovolcano occupies the eastern end of an elongated peninsula extending north into the Flores Sea, connected to Lembata (formerly Lomblen) Island by a narrow isthmus. It is symmetrical when viewed from the north and east. A small cone with a 130-m-wide crater constructed at the SE side of a larger crater forms the volcano’s high point. Many lava flows have reached the coastline. Eruptions recorded since 1660 have consisted of explosive activity from the summit crater.

Mayon, Luzon (Philippines)

13.257°N, 123.685°E | Summit elev. 2462 m

PHIVOLCS reported that slow lava effusion at Mayon’s summit crater continued during 29 November-5 December. The length of the lava flows in the Mi-Isi (S), Bonga (SE), and Basud (E) drainages remained at 2.8 km, 3.4 km, and 1.1 km, respectively. Collapses at the lava dome produced rockfalls and occasional pyroclastic density currents (PDCs, or pyroclastic flows) that descended the flanks as far as 4 km. Each day seismic stations recorded 0-4 rockfall events and 1-10 volcanic earthquakes. Sulfur dioxide emissions averaged 1,202-1,279 tonnes per day. The Alert Level remained at 3 (on a 0-5 scale) and residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ). PHIVOLCS recommended that civil aviation authorities advise pilots to avoid flying close to the summit.

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

Merapi, Central Java

7.54°S, 110.446°E | Summit elev. 2910 m

BPPTKG reported that the eruption at Merapi (on Java) continued during 24-30 November. The SW lava dome produced a total of 97 lava avalanches that descended the S and SW flanks; 20 traveled S as far as 1.5 km down the upper part of the Boyong drainage and 77 traveled SW as far as 1.8 km down the upper part of the Bebeng drainage. Two pyroclastic density currents (PDC’s, or pyroclastic flows) traveled down the Boyong drainage as far as 1.5 km and down the Bebeng drainage as far as 2 km. Variable white emissions rose 75 m above the summit. Minor morphological changes to the SW lava dome were identified in webcam images due to continuing lava effusion and collapses of material. The Alert Level remained at 3 (on a scale of 1-4), and the public was warned to stay 3-7 km away from the summit, based on location.

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

Santa Maria, Southwestern Guatemala

14.757°N, 91.552°W | Summit elev. 3745 m

INSIVUMEH reported that eruptive activity continued at Santa Maria’s Santiaguito lava dome complex during 29 November-5 December. Extrusion at the El Caliente dome continued, with growth concentrated toward the WSW. Weak to moderate degassing was observed daily; gas-and-steam plumes rose to 300 m above the lava dome and drifted SW and W. Daily explosions produced gas-and-ash plumes to 3.2-3.5 km a.s.l. (700-1,000 m above the dome complex) that drifted S, SW, and NW; sometimes areas around the volcano appeared hazy due to ashfall. Ashfall was reported in Loma Linda (6 km WSW), San Marcos (8 km SW), and nearby farms on 30 November and 4 December. Block avalanches descended the SE, S, and SW flanks, and some were accompanied by pyroclastic flows (PDCs). On 30 November and 5 December PDCs nearly reached the base of the edifice. Incandescence from the lava dome and flow was observed in the crater and along the flanks during most nights and early mornings.

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

Semeru, Eastern Java

8.108°S, 112.922°E | Summit elev. 3657 m

PVMBG reported that eruptive activity continued at Semeru during 29 November-5 December. Ash emissions were observed each day except for 2 December. Ash plumes rose 500-700 m above the summit and drifted S, SW, and W. On 1 November white emissions rose 100 m above the summit and drifted NW and NE. The Alert Level remained at 3 (third highest on a scale of 1-4). The public was warned to stay at least 5 km away from the summit in all directions, 13 km from the summit to the SE, 500 m from the banks of the Kobokan drainage as far as 17 km from the summit, and to avoid other drainages including the Bang, Kembar, and Sat, due to lahar, avalanche, and pyroclastic flow hazards.

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

Sheveluch, Central Kamchatka (Russia)

56.653°N, 161.36°E | Summit elev. 3283 m

KVERT reported that the eruption at Sheveluch continued during 23-30 November. Thermal anomalies were identified in satellite images on 23, 26, and 28-29 November. Strong steam-and-gas emissions were observed in the area of the Karan Dome. The Aviation Color Code remained at Orange (the third level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

Geological summary: 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.

Shishaldin, Fox Islands (USA)

54.756°N, 163.97°W | Summit elev. 2857 m

AVO reported that unrest continued at Shishaldin during 29 November-5 December. Seismicity remained elevated, with frequent small low-frequency earthquakes. Weak explosions were identified in infrasound data during 29 November-3 December. Activity observed at the summit and upper NE flank collapse scarp consisted of weakly elevated surface temperatures detected in satellite images and gas-and-steam plumes seen in webcam images. Persistent degassing activity was observed during 29 November-3 December. Weak degassing activity produced steam plumes that drifted S of Unimak Island during 4-5 December. The Volcano Alert Level remained at Watch (the third level on a four-level scale) and the Aviation Color Code remained at Orange (the third color on a four-color scale).

Geological summary: The symmetrical glacier-covered Shishaldin is the highest and one of the most active volcanoes of the Aleutian Islands. It 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 ancestral volcano 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 blanketed 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.

Suwanosejima, Ryukyu Islands (Japan)

29.638°N, 129.714°E | Summit elev. 796 m

JMA reported that eruptive activity continued at Suwanosejima’s Ontake Crater during 27 November-4 December. Crater incandescence was observed nightly in webcam images. Eruption plumes rose as high as 900 m above the vent. Seismicity consisted of volcanic tremors associated with eruptions, and a few volcanic earthquakes detected in the W area of Suwanosejima. The Alert Level remained at 2 (on a 5-level scale) and the public was warned to stay at least 1 km away from the crater in all directions.

Geological summary: The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east 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 blanketed 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 the open Sakuchi caldera, which extends 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.

References:

Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report, November 29 – December 5, 2023 – Managing Editor: Sally Sennert.

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