The Weekly Volcanic Activity Report: September 27 – October 3, 2023

the weekly volcanic activity report

New activity/unrest was reported for 1 volcano from September 27 to October 3, 2023. During the same period, ongoing activity was reported for 21 volcanoes.

New activity/unrest: Villarrica, Central Chile.

Ongoing activity: Aira, Kyushu (Japan) | Ambae, Vanuatu | Ebeko, Paramushir Island (Russia) | Epi, Vanuatu | Fuego, South-Central Guatemala | Great Sitkin, Andreanof Islands (USA) | Klyuchevskoy, Central Kamchatka (Russia) | Lewotolok, Lembata Island | Mayon, Luzon (Philippines) | Merapi, Central Java | Nevado del Ruiz, Colombia | Sabancaya, Peru | Santa Maria, Southwestern Guatemala | Semeru, Eastern Java | Sheveluch, Central Kamchatka (Russia) | Shishaldin, Fox Islands (USA) | Slamet, Central Java | Stromboli, Aeolian Islands (Italy) | Suwanosejima, Ryukyu Islands (Japan) | Ubinas, Peru | Yasur, Vanuatu.

New activity/unrest

Villarrica, Central Chile

39.42°S, 71.93°W | Summit elev. 2847 m

SERNAGEOMIN reported that the eruption in Villarrica’s summit crater continued during 26 September-3 October. Strombolian activity was observed almost nightly and largely confined to the crater, though sometimes material was ejected beyond the crater onto the upper flanks; weather conditions sometimes prevented visual observations of the summit. Satellite images from 26 September showed a spatter cone on the crater floor with one vent measuring 10 x 14 m, and a smaller vent about 35 m NE of the cone. During 26-27 September there were deposits of bombs around the upper flanks within 150 of the crater rim and several impact craters on the snow from explosive activity. Discrete emissions with low ash content were visible. Steam-and-gas emissions were visible during 27-28 September and tephra was ejected onto the upper NW flank. Steam-and-gas emissions sometimes contained ash during 28-29 September; a period of continuous ash emissions recorded during 1020-1250 on 29 September rose 60 m above the crater rim and drifted NW.

During an overflight on 29 September scientists observed lava in the vent and deposits of blocks in the crater. A satellite image showed ash deposits on the WNW flank as far as 3 km from the crater. Material was ejected from the crater several times during 29-30 September, with the emissions varying in duration and tephra content; notably, at 0740 on 30 September a pulsating ash plume rose 1.1 km above the crater rim and drifted NNW. Deposits on the S flank extended as far as 4.5 km from the crater rim as seen in 30 September satellite images. Steam-and-gas emissions with no or low ash content rose to lower heights on 1 October. Incandescence lit up the bases of two gas plumes, rising from the two vents, on 2 October, and dense white gas plumes rose as high as 300 m on 3 October. The Volcanic Alert Level remained at Orange (the third level on a four-level scale) and the public was warned to stay 8 km away from the crater. SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

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

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 25 September-2 October and incandescence at the crater was observed nightly. Very small eruptive events were recorded during the week. Sulfur dioxide emissions were high, averaging 2,300 tons per day on 25 September. The geothermal areas on the SE flank of Minamidake and near the Showa Crater were observed during a field survey on 27 September. A very small eruptive event occurred at Showa on 28 September. 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. The eruption of the voluminous Ito pyroclastic flow accompanied the 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.

Ambae, Vanuatu

15.389°S, 167.835°E | Summit elev. 1496 m

On 28 September the Vanuatu Meteorology and Geo-Hazards Department (VMGD) reported that steam and gas emissions at Ambae were ongoing based on satellite images. The Alert Level remained at 2 (on a scale of 0-5) and the public was warned to stay outside of the Danger Zone, defined as a 2-km radius around the active vents in Lake Voui, and away from drainages during heavy rains.

Geological summary: The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called the Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Ebeko, Paramushir Island (Russia)

50.686°N, 156.014°E | Summit elev. 1103 m

KVERT reported that moderate explosive activity at Ebeko was ongoing during 21-28 September. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E), explosions during 22, 24, and 27-28 September generated ash plumes that rose as high as 3 km (10,000 ft) a.s.l and drifted to the E. A thermal anomaly was identified in satellite images on 22 and 28 September; weather clouds obscured views on other days. 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 flat-topped summit of the central cone of the Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters are located along an SSW-NNE line from Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Epi, Vanuatu

16.68°S, 168.37°E | Summit elev. 833 m

On 28 September the Vanuatu Meteorology and Geo-Hazards Department (VMGD) reported that minor unrest continued at Epi. Volcanic seismicity was sustained, though no surficial activity was observed. The Alert Level remained at 1 (on a scale of 0-5) and the public was warned to stay outside of the Danger Zone, defined as a 2 km radius around the active vent.

Geological summary: A large caldera, with submarine post-caldera cones active in historical time, lies off the eastern coast of Epi Island. Epi Island itself, located slightly west of the main New Hebrides volcanic arc, largely consists of two Quaternary volcanoes, Mount Allombei on the west and Pomare (Tavani Kutali) on the east. Tavani Ruro, which forms an elongated eastern extension of Epi Island across a narrow isthmus, is related to Kuwae caldera to the east. Pomare volcano is the highest point on the island and has three well-preserved subsidiary cones to the east with youthful summit craters. Pomare volcano is truncated on its eastern side by the largely submarine East Epi caldera, which has been the source of all historical eruptions. Three small submarine basaltic and dacitic cones, known as Epi A, Epi B, and Epi C, are located along the northern rim of the breached caldera. Ephemeral islands were formed during eruptions in 1920 and 1953, and the summit of the shallowest cone, Epi B, was 34 m below sea level at the time of a 2001 survey.

Fuego, South-Central Guatemala

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

INSIVUMEH reported that in general 3-8 explosions per hour were recorded at Fuego during 26 September-3 October, though the rate was not determined on some of the days. The explosions generated ash-and-gas plumes that rose as high as 1.1 km above the crater rim and drifted as far as 30 km NW, W, and SW. Minor ashfall was reported in areas downwind including Morelia (9 km SW), Panimaché I and II (8 km SW), Santa Sofía (12 km SW), El Porvenir (8 km SE), Aldeas, and San Pedro Yepocapa (8 km NW). The rumbling was heard daily, and shock waves were occasionally detected. Explosions caused daily block avalanches to descend various drainages including the Ceniza (SSW), Seca (W), Trinidad (S), Taniluyá (SW), Honda (E), and El Jute (ESE), and Las Lajas (SE). The explosions ejected incandescent material as high as 300 m above the summit on most days. In the early afternoon of 27 September lahars descended the Mineral and Seca drainages, carrying tree branches, trunks, and blocks as large as 1 m in diameter. In the early evening of that same day, lahars descended the Las Lajas and Jute drainages, carrying tree branches, trunks, and blocks as large as 1.5 m in diameter. In the afternoon of 29 September a hot lahar traveled down the Ceniza, carrying blocks as large as 1.5 m as well as tree trunks and branches. On 30 September a lahar in the Ceniza transported trunks and branches and blocks up to 3 m in diameter.

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. The 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 likely continued at Great Sitkin during 27 September-3 October, producing a thick flow in the summit crater that mainly expanded E. Seismicity was characterized as low with only a few daily earthquakes recorded by the seismic network. Weakly elevated surface temperatures were identified in satellite data from 2-3 October. Weather clouds sometimes obscured views. 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 of 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.

Klyuchevskoy, Central Kamchatka (Russia)

56.056°N, 160.642°E | Summit elev. 4754 m

KVERT reported that the explosive Strombolian eruption at Klyuchevskoy continued from 21-28 September. The explosions produced variable amounts of ash. A daily bright thermal anomaly was identified in satellite images. Lava fountaining fed flows that advanced down the Apakhonchichsky and Kozyrevsky drainages on the SE flank. The Aviation Color Code remained at Yellow (the second level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

Geological summary: Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka’s highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of the sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-meter summit crater has been frequently modified by historical eruptions, which have been recorded since the late 17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Lewotolok, Lembata Island

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

PVMBG reported that the eruption at Lewotolok continued from 27 September-3 October. Daily white steam-and-gas plumes rose as high as 500 m above the summit and drifted W, NW, and E. Dense gray or white-and-gray ash plumes 600-700 m above the summit and drifted W during 28-29 September and 1 and 3 October. 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-meter 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 the eruption at Mayon continued from 26 September-3 October. The lengths of the lava flow 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 and from the margins of the lava flows produced rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) that descended the flanks as far as 4 km. Each day seismic stations recorded 68-144 rockfall events, 1-3 PDC events, and 1-34 daily volcanic earthquakes. Two tremor events, each lasting one minute, were recorded during 27-28 September and 1-2 October. Sulfur dioxide emissions measured almost daily averaged between 716 and 1,593 tonnes per day, with the highest value recorded on 1 October. 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 in 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 from 22-28 September and seismicity remained at elevated levels. The SW lava dome produced a total of 171 lava avalanches that descended the S and SW flanks; 13 traveled as far as 1.5 km down the upper part of the Boyong drainage, 155 traveled as far as 2 km down the upper Bebeng drainage, and two traveled 1.6 km down the Sat/Putih drainage. One lava avalanche traveled 600 m down the Gendol drainage on the SE flank. Morphological changes to the SW lava dome were due to continuing collapses of material; based on thermal photos from 27 September and a drone overflight on 28 September the SW dome had grown slightly taller while the dome in the summit crater remained unchanged. The highest temperature measured at the SW dome was 409 degrees Celsius, lower than the previous measurement. At the central dome, the highest temperature was 236 degrees Celsius, or near the temperatures of the surrounding rocks. 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 the 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 the 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.

Nevado del Ruiz, Colombia

4.892°N, 75.324°W | Summit elev. 5279 m

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that the eruption at Nevado del Ruiz continued at low-to-moderate levels during 26 September-2 October. Seismicity indicating the movement of fluids was similar to the week before. The number of signals indicating rock fracturing significantly increased during 28-29 September. These events were located in areas 1-4 km S and SSE of Arenas Crater at depths of 2-5 km. Earthquakes recorded at 0512 (M 3.5) and 0614 (M 3.8) on 28 September were felt by residents. Rock-fracturing earthquakes were at low levels on the other days. Seismicity was generally low. Several thermal anomalies in the crater were identified in satellite images. Ash-and-gas emissions continued during the week, with the highest plumes rising to 2 km above the crater rim on 30 September. Plumes drifted NW, W, and SW; ashfall was reported in Manizales (27 km NW) on 30 September. The Alert Level remained at Yellow, Level III (the second level on a four-level scale).

Geological summary: Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical times. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America’s deadliest eruption.

Sabancaya, Peru

15.787°S, 71.857°W | Summit elev. 5960 m

Instituto Geofísico del Perú (IGP) reported that the eruption at Sabancaya continued from 25 September-1 October with a daily average of 39 explosions. Gas-and-ash plumes rose as high as 3 km above the summit and drifted E, SE, S, and SW. A total of 12 thermal anomalies from the lava dome in the summit crater were detected using satellite data. Minor inflation was detected near the Hualca Hualca sector (4 km N). Sulfur dioxide emissions increased, averaging 2,039 tons per day on 1 October. The Alert Level remained at Orange (the second highest level on a four-color scale) and the public was warned to stay outside of a 12 km radius.

Geological summary: Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning “tongue of fire” in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

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 from 26 September-3 October. Incandescence from the Caliente dome was visible during most nights and early mornings, and occasionally from the lava flow on the SW flank. Lava extrusion continued and generated block avalanches on the SW, S, and E flanks. Daily weak-to-moderate explosions generated gas-and-ash plumes that rose 700-1,000 m above the dome and drifted mainly W and SW. Explosions triggered incandescent avalanches that descended the dome’s flanks in all directions. A special bulletin issued at 1300 on 27 September described a hot lahar with a sulfur odor that descended the Zanjón Seco drainage on the SW flank carrying blocks 1-2 m in diameter. About 40 minutes later a viscous lahar descended the Cabello de Ángel River, a tributary of the Nimá I, on the E flank, carrying volcanic blocks up to 1 m in diameter, tree trunks, and branches. During 29-30 September explosions occurred at a rate of 1-2 per hour and produced block-and-ash flows that descended the W and SW flanks and left deposits of gray ash. Block avalanches descended the SW, S, and SE flanks during 30 September-1 October and occasionally produced small pyroclastic flows.

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 the 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 W 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 from 27 September-3 October. White steam-and-gas plumes were visible rising as high as 200 m above the summit and drifting in multiple directions during 28-29 September and 1-3 October. Dense white-and-gray or brown-and-gray ash plumes rose generally 600-800 m above the summit and drifted SW, W, N, and NE, though from 30 September-1 October the plumes rose as high as 1.5 km. 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 21-28 September. Thermal anomalies were identified in satellite images on 22 and 27-28 September; observations on other days were obscured by weather clouds. 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 the 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 the significant explosive eruption at Shishaldin that occurred on 25 September was the eleventh that had occurred since 14 July and among the most impactful. Ashfall was reported in several communities downwind and flight cancellations occurred in the region. A large ash cloud quickly rose to 14 km (45,000 ft) a.s.l., produced at least 150 lightning strokes with thunder heard in False Pass, and drifted E along the Alaska Peninsula. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease from 26 September-3 October. 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 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. Elevated surface temperatures were intense during 27-28 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September. Weakly elevated surface temperatures were identified in satellite data during 30 September-1 October and a pilot reported steaming from the vent on 1 October.

Seismicity significantly increased starting at around 2100 on 2 October and around the same time, satellite images showed an increase in surface temperature consistent with lava fountaining. Small hot avalanches of rock and lava descended the flank. A distinct increase in infrasound, seismicity, and lighting detections was followed by the identification of an ash plume at 12 km (40,000 ft) a.s.l. at 0520 on 3 October in satellite images. The Aviation Color Code was raised to Red (the highest color on a four-color scale) and the Volcano Alert Level was raised to Warning (the highest level on a four-level scale). By 0931 ash plumes were only rising to altitudes of 6.1-7.6 km (20,000-25,000 ft) a.s.l. and both seismicity and infrasound levels had decreased. At 1036 the Aviation Color Code was lowered to Orange and the Volcano Alert Level was lowered to Watch; seismic and infrasound activity were slightly above background levels and steaming at the summit was observed in webcam images.

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.

Slamet, Central Java

7.242°S, 109.208°E | Summit elev. 3428 m

In a 2 October press release, PVMBG noted recent inflation and increased seismicity at Slamet. In 2023 seismicity was dominated by earthquake signals indicating movement of fluids around the surface and gas emissions, and these signals averaged 168 per day. Gas emissions rose 25-300 m above the summit. During 24 September-1 October, the number of local tectonic earthquakes increased followed by an increase in tremor amplitude. Tiltmeter and Electronic Distance Measurement data from the Cilik station located at an elevation of 1,500 m showed inflation while another station (Buncis) showed deflation; the Jurangmangu station located at a lower elevation than Cilik showed no significant deformation pattern. The Alert Level remained at 1 (on a scale of 1-4) and the public was warned to stay outside a 1 km radius.

Geological summary: Slamet, Java’s second-highest volcano at 3428 m and one of its most active has a cluster of about three dozen cinder cones on its lower SE-NE flanks and a single cinder cone on the western flank. It is composed of two overlapping edifices, an older basaltic-andesite-to-andesitic volcano on the west and a younger basaltic-to-basaltic-andesite one on the east. Gunung Malang II cinder cone on the upper E flank on the younger edifice fed a lava flow that extends 6 km E. Four craters occur at the summit of Gunung Slamet, with activity migrating to the SW over time. Historical eruptions, recorded since the 18th century, have originated from a 150-m-deep, 450-m-wide, steep-walled crater at the western part of the summit and have consisted of explosive eruptions generally lasting a few days to a few weeks.

Stromboli, Aeolian Islands (Italy)

38.789°N, 15.213°E | Summit elev. 924 m

INGV reported that eruptive activity continued at Stromboli from 25 September-1 October. Webcam images showed Strombolian activity at three vents in Area N (two at N1 and one at N2), within the upper part of the Sciara del Fuoco, and from three vents in Area C-S (South-Central Crater) in the crater terrace. Explosions of variable intensities occurred at a rate of 7-13 per hour at Area N. Explosions at N2 ejected mainly coarse material (bombs and lapilli), sometimes mixed with ash, up to 150 m above the vents. Spattering occurred at N1 and was intense during 27-28 September, coinciding with the effusion of a lava flow. The flow was first visible at 1428 on 27 September as it moved out of Area N and onto the upper part of the Sciara del Fuoco. By 2107 on 28 September spattering had decreased and the flow was no longer being fed. Spattering again increased and at about 0833 on 3 October lava from Area N flowed onto the upper part of the Sciara del Fuoco.

High-intensity explosions in sector S2 (Area C-S) averaged 6-8 per hour from the vents, ejecting a mix of coarse material as high as 150 m. The material was deposited in a wide area along the crater terrace. Intense spattering was visible on 26 September. The Dipartimento della Protezione Civile maintained the Alert Level at Yellow (the second highest level on a four-level scale).

Geological summary: Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the “Lighthouse of the Mediterranean” in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Suwanosejima, Ryukyu Islands (Japan)

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

JMA reported that the eruption at Suwanosejima’s Ontake Crater continued during 25 September-2 October. Eruptive events on 25, 27, and 30 September, and 1-2 October, produced ash plumes that rose as high as 1.3 km above the crater rim and ejected blocks as far as 300 m from the crater. Crater incandescence was visible nightly. Ash emissions were continuous from 0510 to 0555 on 1 October. 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.

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.

Ubinas, Peru

16.355°S, 70.903°W | Summit elev. 5672 m

Instituto Geofísico del Perú (IGP) reported that the eruption at Ubinas continued during 25 September – 1 October at low-to-moderate levels. There were daily averages of 129 volcano-tectonic earthquakes indicating rock fracturing and 52 long-period earthquakes signifying the movement of gas and magma. A period of continuous ash emissions began at 1656 on 1 October and lasted about 90 minutes; the ash plumes rose as high as 1 km above the crater rim and drifted more than 10 km E, S, and SW. The Alert Level remained at Orange (the third level on a four-color scale) and the public was warned to stay 4 km away from the crater.

Geological summary: A small, 1.4-km-wide caldera cuts the top of Ubinas, Perú’s most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by the construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45 degrees. The steep-walled, 150-m-deep summit caldera contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one of Holocene age about 1,000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

Yasur, Vanuatu

19.532°S, 169.447°E | Summit elev. 361 m

On 28 September the Vanuatu Meteorology and Geohazards Department (VMGD) reported that activity at Yasur continued at a high level of “major unrest,” as defined by the Alert Level 2 status (the middle level on a scale of 0-4). Recent satellite observations indicated an increase in steam, gas, and ash emissions from the summit crater. Explosions continued, with some ejecting bombs that landed back in and around the crater. The public was reminded to not enter the restricted area within 600 m around the boundaries of the Permanent Exclusion Zone, defined by Danger Zone A on the hazard map.

Geological summary: Yasur has exhibited essentially continuous Strombolian and Vulcanian activity at least since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island in Vanuatu, this pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide open feature associated with the eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.


Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report – September 27 – October 3, 2023 – Managing Editor: Sally Sennert.

Featured image credit: The Watchers

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