The Weekly Volcanic Activity Report: December 6 - 12, 2023

New activity/unrest was reported for 4 volcanoes from December 6 – 12, 2023. During the same period, ongoing activity was reported for 18 volcanoes.

New activity/unrest: Ambae, Vanuatu | Ioto, Volcano Islands | Marapi, Central Sumatra | Ulawun, New Britain (Papua New Guinea).

New activity/unrest

Ambae, Vanuatu

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

On 4 December the Vanuatu Meteorology and Geohazards Department (VMGD) reported that steam-and-gas emissions at Ambae were ongoing based on satellite images. According to the Wellington VAAC an eruption produced an ash plume visible in satellite images at 1240 on 11 December that rose to 4.3 km (14,000 ft) a.s.l. and drifted NE. 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 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.

Ioto, Volcano Islands

24.751°N, 141.289°E | Summit elev. 169 m

According to a news article the eruption at Ioto (Iwo-jima), from a submarine vent about 1 km off the SE coast at Okinahama, continued on 4 December. During an overflight of the volcano passengers observed explosions that produced 100-m-high black plumes every few minutes. Ejected material combined with wave erosion transformed the shape of the island into a “J” shape, 500 m long and with the curved part about 200 m offshore of the main island. The island was covered with brown ash and blocks, and the surrounding water was green to brown in color and there were areas of floating pumice.

Geological summary: Ioto in the central Volcano Islands portion of the Izu-Bonin-Mariana arc lies within a 9-km-wide submarine caldera. Ioto, Iwojima, and Iojima are among many transliterations of the name. 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.

Marapi, Central Sumatra

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

PVMBG reported that eruptive activity at Marapi was ongoing during 6-12 December, though foggy and raining weather conditions often prevented visual observations of the summit. White-and-gray ash plumes rose 400-500 m and drifted NE, E, and W on 6 December. Eruptive events were recorded by the seismic network at 1009 on 6 December and at 0141 on 7 December though no ash emissions were observed. On 11 December white-and-gray ash plumes rose 200-400 m above the summit and drifted E and S. At 0805 that same day a dense gray ash plume rose 400 m above the summit and drifted N. The Alert Level remained at 2 (on a scale of 1-4), and the public was warned to stay 3 km away from the summit crater.

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.

Ulawun, New Britain (Papua New Guinea)

5.05°S, 151.33°E | Summit elev. 2334 m

According to ReliefWeb, the U.S. Agency for International Development (USAID) reported that the 20 November eruption at Ulawun resulted in five deaths and the evacuation of more than 16,000 people, with many more affected, as of 9 December. Ashfall from the eruption impacted oil palm trees, water sources, household gardens, and nearby properties.

Geological summary: The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea’s most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

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 4-11 December, with incandescence at the crater observed nightly. Small eruptive events were recorded during 4-8 December. Sulfur dioxide emissions were high, averaging 2,900 tons per day on 8 December. Explosions at 1028 and 1533 on 10 December produced ash plumes that rose 1.5-1.8 km above the crater rim and drifted N. An eruptive event at 1748 on that same day produced an ash plume that rose 1.2 km and drifted N. 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.

East Epi, Vanuatu

16.6797°S, 168.3893°E | Summit elev. 833 m

On 4 December the Vanuatu Meteorology and Geohazards Department (VMGD) reported that minor unrest continued at East Epi during November. Volcanic seismicity was sustained, though no activity was observed above the ocean surface. 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 submarine vent.

Geological summary: The submarine East Epi group of basaltic and dacitic cones are located within a possible 10-km-diameter caldera off the NE coast of southern Epi Island in Vanuatu. Three cones (1-1.5 basal diameter), known as Epi A, Epi B (or Cioan), and Epi C, are located along the northern rim of the inferred caldera, though Beier et al. (2018) suggest an alternate to the post-caldera formation model. A few smaller cones are south of Epi B. Ephemeral islands were formed during eruptions at Epi B in 1920 and 1953. Explosive activity was reported in 1958 and 1960, discolored water was often seen during 1971-1974 and 1988, a new vent was detected in 1979, and explosive activity occurred in 1999, 2002, 2004, and 2023. The summit was at 34 m below sea level at the time of a 2001 survey, and a research cruise in 2013 (R/V Sonne SO-229; Haase et al, 2013) sampled six cones, including “very fresh pumice and lava bombs” from Epi B and “relatively old-looking” material from the others. The SO-229 cruise also recovered rhyodacitic pumice with mafic streaks, similar to that sampled by previous cruises, and described Epi B as “covered by massive blocks of pumice.”

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 1-7 December. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E), explosions during the week generated ash plumes that rose as high as 3 km (10,000 ft) a.s.l and drifted E, NE, and N. The Aviation Color Code remained at Orange (the third level on a four-color scale). Dates are UTC; specific events are in local time where noted.

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

Great Sitkin, Andreanof Islands (USA)

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

AVO reported that slow lava effusion continued at Great Sitkin during 6-12 December with a thick flow in the summit crater mainly expanding E. Seismicity was low. Weather clouds obscured views during most of the week. 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.

Karangetang, Sangihe Islands

2.781°N, 125.407°E | Summit elev. 1797 m

PVMBG lowered the Alert Level for Karangetang to 2 (on a scale of 1-4) on 29 November due to declining activity. According to a news article seismic data and visual observations indicated that effusion had decreased or ceased and that lava avalanches were no longer observed. Seismicity decreased and deformation data indicated deflation. The public was warned to stay 1.5 km away from both Main Crater and North Crater with an extension to 2.5 km on the S and SE flanks.

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

Kirishimayama, Kyushu (Japan)

31.934°N, 130.862°E | Summit elev. 1700 m

JMA lowered the Alert Level for Kirishimayama to 1 (on a 5-level scale) on 6 December. The number of volcanic earthquakes had increased during July-August in an area around the NE side of Karakunidake and on 15 September and had remained low since then. Minor inflation detected in May slowed in October and was no longer detected in November. Fumarolic areas remained at normal levels.

Geological summary: Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Krakatau, Sunda Strait

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

PVMBG reported that the eruption at Krakatau continued during 6-12 December. White-and-gray ash plumes that were sometimes dense and dark rose as high as 1.2 km above the summit and drifted NE, N, and NW during 6-8 and 10 December. White plumes that rose as high as 150 m and drifted N and NE were visible on the other days. Webcam images posted with the daily reports showed incandescence at the vent. 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.

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 the eruption at Lewotolok continued during 6-12 December. White steam-and-gas plumes rose 300-500 m above the summit and drifted in multiple directions on 6, 9, and 12 December. White-and-gray ash plumes rose 300-400 m during 7-8 and 10-11 December and drifted E, N, and W. Ejections of incandescence lava at the summit were visible on 6 and 12 December. 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 lowered the Alert Level for Mayon to 2 (on a 0-5 scale) at 0800 on 8 December due to a continuing decline in activity during the previous several weeks. Volcanic earthquakes indicating lava extrusion, degassing, and occasional fracturing beneath the edifice decreased from an average of 11 events per day in November to almost zero events per day in the first week of December. Similarly, the average number of daily rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) decreased from monthly averages of 122 to 87 and 5 to 2 events per day, respectively, from October to November; there were almost zero events per day in the first week of December. Incandescence at the summit crater and from lava flows had declined starting in the last week of November. The length of the lava flows in the Mi-Isi (S), Bonga (SE), and Basud (E) drainages had remained at 2.8 km, 3.4 km, and 1.1 km, respectively, since July and August; the newest lava deposits observed since 23 November were in the Bonga within 400 m of the summit. Sulfur dioxide gas emissions peaked at 4,756 tonnes per day (t/d) on 16 August and had been variable but declining overall since; a monthly average of 1,417 t/d in November decreased to an average of 1,095 t/d in December. Electronic Distance Measuring (EDM), continuous GPS, and electronic tilt monitoring data showed continuing deformation. Deflation was detected in GPS data beginning around August-September and in tilt data starting in November. Net inflation in the longer-term was indicated in electronic tilt data as far back as June and in EDM data starting in February. Residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ) and pilots were advised 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 1-8 December. Two pyroclastic flows traveled S as far as 1.3 km down the upper part of the Boyong drainage and four pyroclastic flows traveled SW as far as 3 km down the upper parts of the Bebeng and Krasak drainages during 1-7 December. Minor ashfall occurred in the Sawangan District (15 km W), Magelang, and the Selo District (5 km NNE), Boyolali. The SW lava dome produced a total of 192 lava avalanches; 23 traveled as far as 1.5 km down the Boyong drainage and 169 traveled as far as 1.7 km down the Bebeng drainage. According to BNPB several dark gray pyroclastic flows were detected by the seismic network starting at 1449 on 8 December and traveled as far as 3.5 km down the Krasak drainage on the SW flank. Ash mixed with rain fell in Krinjing (5 km WNW) and Paten (9 km WNW) villages, Dukun District in the Magelang Regency, as well as in the Stabelan (4 km NW), Klakah (4 km NW), and Tlogolele (5 km NW) villages in the Selo District, Boyolali Regency. BPPTKG noted that minor morphological changes to the SW lava dome were identified in webcam images due to continuing lava effusion and collapses of material. Seismicity remained at high levels during the week. 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.

Semeru, Eastern Java

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

PVMBG reported that eruptive activity continued at Semeru during 6-12 December. Daily white-and-gray ash plumes that were often dense rose as high as 1 km above the summit and drifted in multiple directions during 8-12 December. 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 1-7 December. A daily thermal anomaly was identified in satellite images. During 3-6 December plumes of resuspended ash drifted about 230 km E and SE. 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 6-12 December. Seismicity remained low with small, frequent, low-frequency earthquakes recorded daily. Weak explosions were detected in infrasound data during 6 and 8-9 December, though none produced ash. Elevated surface temperatures were identified in a clear satellite image during 8-9 December, and minor steaming at the summit was observed in webcam images during 8-10 December. Cloudy weather sometimes prevented 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 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.

Stromboli, Aeolian Islands (Italy)

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

INGV reported that eruptive activity continued at Stromboli during 4-10 December. 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 two vents in Area C-S (South-Central Crater) in the crater terrace. Explosions of low-to-medium intensities occurred at a rate of 5-6 per hour at Area N2 and ejected mainly coarse material (bombs and lapilli), sometimes mixed with ash, lower than 150 m above the vents. Variable-intensity explosions in sector S2 (Area C-S) averaged 5-9 per hour from the vents, ejecting a mix of coarse material higher than 150 m. 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 which extends 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 4-11 December and crater incandescence was visible nightly. No explosions were detected, though ash plumes rose as high as 900 m above the crater rim. 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

According to the Washington VAAC an ash puff from Ubinas was identified in a satellite image at 0040 on 11 December rising 5.5 km (18,000 ft) a.s.l. and drifting NW. Webcam images at 0620 and 1220 showed continuous steam emissions possibly containing diffuse ash rising as high as 7 km (23,000 ft) a.s.l. Steam emissions containing small amounts of ash were visible at 1810. Emissions were no longer visible in satellite and webcam images at 0010 on 12 December.

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 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.

Villarrica, Central Chile

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

POVI reported that vigorous Strombolian activity at Villarrica was visible overnight during 2-3 December. Incandescent material was ejected as high as 85 m above the crater rim. SERNAGEOMIN reported that at 2030 on 9 December a long-period (LP) earthquake associated with fluid movement was accompanied by an emission that rose 120 m above the vent and drifted ENE. The Volcanic Alert level remained at Yellow (the third level on a four-level scale) according to SERNAGEOMIN.

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 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.

Yasur, Vanuatu

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

On 4 December the Vanuatu Meteorology and Geohazards Department (VMGD) reported that activity at Yasur continued at a level of “major unrest,” as defined by the Alert Level 2 status (the middle level on a scale of 0-4) during November. 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 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.

References:

1 Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report – December 6 – 12, 2023 – Managing Editor: Sally Sennert

Featured image credit: The Watchers

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