The Weekly Volcanic Activity Report: November 22 – 28, 2023 - Thursday, November 30, 2023
https://watchers.news/2023/11/30/the-weekly-volcanic-activity-report-november-22-28-2023/
New activity/unrest: Bogoslof, Fox Islands (USA) | Brava, Cape Verde | Ioto, Volcano Islands | Purace, Colombia | Reykjanes, Reykjanes Peninsula | Ulawun, New Britain (Papua New Guinea).
Ongoing activity: Aira, Kyushu (Japan) | Bagana, Bougainville (Papua New Guinea) | Dukono, Halmahera | Etna, Sicily (Italy) | Great Sitkin, Andreanof Islands (USA) | Krakatau, Sunda Strait | Lewotolok, Lembata Island | Mayon, Luzon (Philippines) | Merapi, Central Java | Nevado del Ruiz, Colombia | Reventador, Ecuador | Ruapehu, North Island (New Zealand) | Sabancaya, Peru | Sangay, Ecuador | Semeru, Eastern Java | Sheveluch, Central Kamchatka (Russia) | Shishaldin, Fox Islands (USA) | Suwanosejima, Ryukyu Islands (Japan) | Taal, Luzon (Philippines).
New activity/unrest
Bogoslof, Fox Islands (USA)
53.93°N, 168.03°W | Summit elev. 150 m
AVO reported that seismicity at Bogoslof had declined during the previous three weeks to background levels and the last moderate earthquake, a M 2.7, was recorded on 9 November. No other signs of unrest were detected. Both the Volcano Alert Level and the Aviation Color Code were changed to Unassigned on 24 November. A seismic swarm had started around 22 October and was characterized by 5-10 events per hour and a total of around 1,100 earthquakes per week at the peak. The volcano was monitored by a single local seismic station, distant seismic and infrasound instruments, satellite data, and lightning networks.
Geological summary: Bogoslof is the emergent summit of a submarine volcano that lies 40 km N of the main Aleutian arc. It rises 1,500 m above the Bering Sea floor. Repeated construction and destruction of lava domes at different locations during historical time has greatly modified the appearance of this “Jack-in-the-Box” volcano and has introduced a confusing nomenclature applied during frequent visits by exploring expeditions. The present triangular-shaped, 0.75 x 2 km island consists of remnants of lava domes emplaced from 1796 to 1992. Castle Rock (Old Bogoslof) is a steep-sided pinnacle that is a remnant of a spine from the 1796 eruption. The small Fire Island (New Bogoslof), about 600 m NW of Bogoslof Island, is a remnant of a lava dome formed in 1883.
Brava, Cape Verde
14.8514°N, 24.7041°W | Summit elev. 956 m
According to the Cape Verde National Institute of Meteorology and Geophysics (INMG), seismicity at Brava increased on 30 October and remained elevated. A M 3.6 earthquake was recorded at 1819 on 30 October and was followed by eight more events of similar magnitudes that were felt by residents. A M 4.8 was recorded at 2100 and was followed by a 48-hour-long seismic swarm mostly consisting of low-magnitude events that were unable to be located. The Alert Level was raised to 2 (the second level on a five-level scale) on 30 October.
The rate of seismicity was variable, with two events every 1-10 minutes during 30 October-1 November. Most of the events were located at depths of 3-4 km; the hypocenters were initially located in the Praia de Águada area but then they migrated towards the center of the island. Continuous harmonic tremor emerged on 9 November and was interpreted as indicating the movement of magmatic gases. On 15 November seismicity increased and was characterized by more intense periods of volcanic tremor, long-period events, and “burst” events defined as identical events separated by a few seconds. In addition, residents began to feel earthquakes more often. The Alert Level was raised to 3. Three earthquakes all with magnitudes greater than three occurred during 18-19 November, and harmonic tremor again intensified on 19 November.
Geological summary: Brava Island, 20 km W of Fogo, is the westernmost of the southern Cape Verde islands. The 10-km-wide island contains 15 morphologically youthful craters located along two or three lineaments intersecting along the crest of the island. Most of the younger eruptions originated from the interaction of phonolitic magmas with a large groundwater reservoir contained within an older volcanic series characterized by thick welded ignimbrites and block-and-ash flow deposits. Carbonatitic lavas are also found.
Ioto, Volcano Islands
24.751°N, 141.289°E | Summit elev. 169 m
The Japan Coast Guard made observations during an overflight of Ioto (Iwo-jima) on 23 November. They posted video, photos, and infrared photos that showed explosions at the main vent producing dark, dense ash-and-steam plumes and ejecting large blocks that landed on the island and in the ocean.
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.
Purace | Colombia
2.32°N, 76.4°W | Summit elev. 4650 m
Observatorio Vulcanológico y Sismológico de Popayán, Servicio Geologico Colombiano (SGC) reported that at 1929 on 16 November the seismic network recorded a signal at Puracé associated with an ash-and-gas emission. Ash deposits on the N flank were confirmed by an observer the next day. SGC noted that localized deformation between Puracé and Curiquinga volcanoes continued to be recorded. Sulfur dioxide gas emissions had stable values, but carbon dioxide concentrations were high compared to values recorded so far in 2023. The Alert Level remained at Yellow (the second lowest on a four-color scale).
Geological summary: Puracé in Colombia consists of an andesitic stratovolcano with a 500-m-wide summit crater constructed over a dacitic shield volcano. It lies at the NW end of a volcanic massif opposite Pan de Azúcar stratovolcano, 6 km SE. A NW-SE-trending group of seven cones and craters, Los Coconucos, lies between the two larger edifices. Frequent explosive eruptions in the 19th and 20th centuries have modified the morphology of the summit crater. The largest eruptions occurred in 1849, 1869, and 1885.
Reykjanes, Reykjanes Peninsula
63.817°N, 22.717°W | Summit elev. 140 m
On 22 November the Icelandic Meteorological Office (IMO) lowered the Aviation Color Code for Reykjanes to Yellow (the second level on a four-color scale), noting that seismicity associated with the magmatic dike intrusion had decreased during the previous week. Although inflation continued to be detected at Svartsengi, they determined that the likelihood of an eruption had decreased. During 22-27 November seismic activity was relatively stable at a rate of about 500 earthquakes per day, with most events concentrated near Sýlingarfell and Hagafell. Sometimes around midnight on 27 November an hour-long seismic swarm occurred in the vicinity of Sýlingarfell. A total of 170 earthquakes were recorded and located at depths of 3-5 km; the largest event was an M 3. Seismicity slowly decreased during 28-29 November and most of the events were small, below M 1. The rate of deformation also declined, though uplift at Svartsengi continued at around 1 cm per day. The seismic and deformation data suggested that magma continued to flow into the middle portion of the dike.
Geological summary: The Reykjanes volcanic system at the SW tip of the Reykjanes Peninsula, where the Mid-Atlantic Ridge rises above sea level, comprises a broad area of postglacial basaltic crater rows and small shield volcanoes. The submarine Reykjaneshryggur volcanic system is contiguous with and is considered part of the Reykjanes volcanic system, which is the westernmost of a series of four closely-spaced en-echelon fissure systems that extend diagonally across the Reykjanes Peninsula. Most of the subaerial part of the system (also known as the Reykjanes/Svartsengi volcanic system) is covered by Holocene lavas. Subaerial eruptions have occurred in historical time during the 13th century at several locations on the NE-SW-trending fissure system, and numerous submarine eruptions dating back to the 12th century have been observed during historical time, some of which have formed ephemeral islands. Basaltic rocks of probable Holocene age have been recovered during dredging operations, and tephra deposits from earlier Holocene eruptions are preserved on the nearby Reykjanes Peninsula.
Ulawun, New Britain (Papua New Guinea)
5.05°S, 151.33°E | Summit elev. 2334 m
Rabaul Volcano Observatory (RVO) reported that white steam plumes and occasional puffs of ash rose from Ulawun’s summit crater during 22-24 November, though weather clouds hindered views during 23-24 November. Lava continued to flow from a new fissure vent that had opened on the SW flank, near the vent that had formed in 2019. The intensity of the incandescence from the flow decreased during 23-24 November, suggesting that effusion may have slowed. Seismicity remained at background levels. The Alert Level remained at Stage 2 (on the four-level scale).
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 20-27 November, with incandescence at the crater observed nightly. Explosions at 1905 on 20 November and 0226 on 21 November produced ash plumes that rose 1.2 km above the crater rim and drifted SW. Eruptive events on 22 November produced emissions that rose 1 km and drifted N and E. During an overflight of the crater on 24 November plumes obscured views of Minamidake Crater, though observers noted no changes at the geothermal area at Showa Crater. 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 from Bagana was identified in satellite images rising to 3 km (10,000 ft) a.s.l. and drifting SW at 1300 on 24 November. The plume was continuously emitted for several hours. The plume had dissipated by 0710 on 25 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 15-21 November. Although seismicity had returned to normal eruptive levels the previous week, ash-and-gas plumes continued to rise to higher-than-normal heights. Daily dense white-and-gray or gray-to-black ash plumes rose as high as 2.5 km above the summit and drifted NW, W, and S. 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.
Etna, Sicily (Italy)
37.748°N, 14.999°E | Summit elev. 3357 m
INGV reported that Strombolian activity at Etna’s SE Crater (SEC) was periodically visible during 20-26 November; weather clouds often prevented visual observations. The frequency of the eruptive activity was on the scale of hours, and explosions were most intense during 25-26 November. The explosions ejected material that fell within the crater or nearby on the flanks and produced ash emissions that rapidly dispersed near the summit. Lava overflowed the crater starting at 1810 on 24 November and produced a slow-moving lava flow that descended the S flank to the base of the cone. The lava flowed down the same ravine as one of the three flows emplaced on 12 November and was no longer being fed by 0450 on 25 November. During periods of more intense Strombolian activity on 26 November ejected lava that accumulated on the upper S flank and was visible in thermal webcam images. Activity at Bocca Nuova Crater was characterized by pulsating gas emissions and flashes of incandescence generated by the hot gases.
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.
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 22-28 November characterized by a thick flow in the summit crater that mainly expanded E. Seismicity was low. Weakly elevated surface temperatures were identified in satellite imagery during 24-25 November. 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 issued four Volcano Observatory Notices for Aviation (VONAs) on 26 November describing ash plumes from Krakatau rising as high as 1 km above the summit and drifting NW and NE. Webcam images showed incandescent material being ejected above the vent. There were 12 VONAs issued on 27 November; white-and-gray ash plumes that were sometimes dense rose as high as 1.5 km and drifted NE and NW. Webcam images continued to show incandescence at the vent and material being ejected from the vent. At least nine VONAs on 28 November described ash plumes rising as high as 2 km and drifting NE, N, and 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.
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 22-28 November. White steam-and-gas plumes rose 300-400 m above the summit and drifted W and NW during 22 and 24-26 November. White-and-gray ash plumes rose as high as 1.9 km during 23 and 27-28 November and drifted W and NW. Incandescence at the summit was visible on 22 and 26 November. 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 22-28 November. The lengths 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. Seismic stations recorded 7-111 daily rockfall events and 0-7 daily volcanic earthquakes. Two earthquakes indicated Strombolian explosions during 26-27 November. A total of three PDC events were recorded during the week. Sulfur dioxide emissions averaged 857-1,992 tonnes per day. Lahars descended the Buyoan-Padang and Mi-Isi drainages during 25-26 November. 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 17-23 November. The SW lava dome produced a total of 91 lava avalanches that descended the flanks; three traveled as far as 1.3 km down the upper part of the Boyong drainage and 88 traveled as far as 1.8 km down the upper Bebeng drainage. 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.
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 21-27 November. Seismic events indicating the movement of fluids increased in number and intensity compared to the previous week, especially with respect to the signals indicating emissions. The number of signals indicating rock fracturing decreased in both number an intensity. These events were located in areas up to 6 km in various directions from Arenas Crater, at depths of 1-8 km. The largest earthquake, a M 2, was recorded at 1949 on 25 November. Thermal anomalies on the crater floor were identified in satellite images. Ash-and-gas plumes rose as high as 1.3 km above the summit on 21 November and up to 1.7 km on 27 November. The plumes drifted WSW and SW. Sulfur dioxide emissions increased through the week. 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 time. 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.
Reventador, Ecuador
0.077°S, 77.656°W | Summit elev. 3562 m
IG-EPN reported that the eruption at Reventador was ongoing during 21-28 November. Seismicity was characterized by 31-60 daily explosions, long-period earthquakes, harmonic tremor, and tremor associated with emissions. Several daily ash-and-gas plumes rose as high as 1.2 km above the crater rim and drifted in multiple directions. Daily crater incandescence was visible during both overnight and morning hours. Incandescent material was ejected 200 m above the crater rim during 21-23 November, and avalanches of incandescent material descended the flanks daily, traveling as far as 800 m from the summit. Weather conditions sometimes prevented views of the volcano. Secretaría de Gestión de Riesgos maintained the Alert Level at Orange (the second highest level on a four-color scale).
Geological summary: Volcán El Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have left extensive deposits on the scarp slope. The largest recorded eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.
Ruapehu, North Island (New Zealand)
39.28°S, 175.57°E | Summit elev. 2797 m
GeoNet reported that 40 small earthquakes at Ruapehu was recorded by the seismic network during 21-26 November. The earthquakes were 0.3-1.8 in magnitude and located at depths of 3-6 km, though most clustered at depths of 4-5 km. Volcanic tremor levels were low during 2023 and did not vary in response to the sequence. The temperature of the crater lake water was slowly rising, starting in mid-October. During an overflight on 22 November, scientists observed that the color of the lake had changed to blue-green instead of the typical gray color, consistent with less gas flux disturbing lake-bottom sediment. No upwelling was observed and sulfur slicks on the water’s surface were visible. Water was overflowing at the lake’s outlet. Overall, activity was low. The Volcanic Alert Level remained at 1 (on a scale from 0-5) and the Aviation Color Code remained at Green (the lowest level on a four-color scale).
Geological summary: Ruapehu, one of New Zealand’s most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.
Sabancaya, Peru
15.787°S, 71.857°W | Summit elev. 5960 m
Instituto Geofísico del Perú (IGP) reported that the eruption at Sabancaya continued at moderate levels during 20-26 November with a daily average of 31 explosions. Gas-and-ash plumes rose as high as 2.9 km above the summit and drifted NE and SW. Thermal anomalies over the lava dome in the summit crater were identified in satellite data. Minor inflation was detected near the Hualca Hualca sector (4 km N). The Alert Level remained at Orange (the second highest level on a four-color scale) and the public were 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.
Sangay, Ecuador
2.005°S, 78.341°W | Summit elev. 5286 m
IG-EPN reported a high level of eruptive activity at Sangay during 21-28 November, with seismic stations recording 232-463 daily explosions. Ash-and-gas plumes visible in webcam and satellite images during 21, 23, and 27-28 November rose as high as 2 km above the crater rim and drifted N, NE, SE, and SW. Webcam images showed incandescent material at the summit vent and descending the SE flank as far as 1.8 km from the crater during 21-22 November. Weather clouds prevented observations during the rest of the week. Secretaría de Gestión de Riesgos maintained the Alert Level at Yellow (the second highest level on a four-color scale).
Geological summary: The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador’s volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within the open calderas of two previous edifices which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been eroded by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of an eruption was in 1628. Almost continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.
Semeru, Eastern Java
8.108°S, 112.922°E | Summit elev. 3657 m
PVMBG reported that eruptive activity continued at Semeru during 22-28 November. No emissions were observed on 22 November. White-and-gray ash plumes that were often dense rose 300-800 m above the summit and drifted in multiple directions during 23-28 November. 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 16-23 November. A daily thermal anomaly was identified in satellite images. On 17 November plumes of resuspended ash drifted about 116 km E. 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 22-28 November. Seismicity began to decrease on 21 November, though remained elevated with ongoing seismic tremor and small, low-frequency earthquakes recorded during the week. Elevated surface temperatures were identified in satellite images during 22 and 25-26 November. Robust steam emissions rising from the summit vent as well as from a scarp on the upper NE flank, near the summit were visible in satellite and webcam images drifting 50 km SSE during 25-26 November. During 27-28 November steam plumes were visible in webcam images and small explosions were observed in seismic and local infrasound data. Weather clouds often prevented views of the volcano. 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 the eruption at Suwanosejima’s Ontake Crater continued during 20-27 November and crater incandescence was visible nightly. No explosions were detected, though ash plumes rose as high as 1.2 km above the crater rim and drifted mainly SE and W during 20-21 and 25 November. 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.
Taal, Luzon (Philippines)
14.0106°N, 120.9975°E | Summit elev. 311 m
PHIVOLCS reported that unrest at Taal continued during 22-28 November. Daily pronounced upwelling of gases and hot fluids in the lake generated steam-and-gas plumes that rose as high as 1.2 km above the crater and drifted SW. Sulfur dioxide emissions averaged 7,608 and 11,962 tonnes per day on 23 and 27 November, respectively. The seismic network recorded 6-66 daily periods of volcanic tremor each lasting 1-7 minutes long. The Alert Level remained at 1 (on a scale of 0-5), and PHIVOLCS reminded the public that the entire Taal Volcano Island was a Permanent Danger Zone (PDZ).
Geological summary: Taal is one of the most active volcanoes in the Philippines and has produced some powerful eruptions. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, with several submerged eruptive centers. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all observed eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones. Powerful pyroclastic flows and surges have caused many fatalities.
References:
Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report, November 22 – 28, 2023 – Managing Editor: Sally Sennert.
https://watchers.news/2023/11/30/the-weekly-volcanic-activity-report-november-22-28-2023/
New activity/unrest: Bogoslof, Fox Islands (USA) | Brava, Cape Verde | Ioto, Volcano Islands | Purace, Colombia | Reykjanes, Reykjanes Peninsula | Ulawun, New Britain (Papua New Guinea).
Ongoing activity: Aira, Kyushu (Japan) | Bagana, Bougainville (Papua New Guinea) | Dukono, Halmahera | Etna, Sicily (Italy) | Great Sitkin, Andreanof Islands (USA) | Krakatau, Sunda Strait | Lewotolok, Lembata Island | Mayon, Luzon (Philippines) | Merapi, Central Java | Nevado del Ruiz, Colombia | Reventador, Ecuador | Ruapehu, North Island (New Zealand) | Sabancaya, Peru | Sangay, Ecuador | Semeru, Eastern Java | Sheveluch, Central Kamchatka (Russia) | Shishaldin, Fox Islands (USA) | Suwanosejima, Ryukyu Islands (Japan) | Taal, Luzon (Philippines).
New activity/unrest
Bogoslof, Fox Islands (USA)
53.93°N, 168.03°W | Summit elev. 150 m
AVO reported that seismicity at Bogoslof had declined during the previous three weeks to background levels and the last moderate earthquake, a M 2.7, was recorded on 9 November. No other signs of unrest were detected. Both the Volcano Alert Level and the Aviation Color Code were changed to Unassigned on 24 November. A seismic swarm had started around 22 October and was characterized by 5-10 events per hour and a total of around 1,100 earthquakes per week at the peak. The volcano was monitored by a single local seismic station, distant seismic and infrasound instruments, satellite data, and lightning networks.
Geological summary: Bogoslof is the emergent summit of a submarine volcano that lies 40 km N of the main Aleutian arc. It rises 1,500 m above the Bering Sea floor. Repeated construction and destruction of lava domes at different locations during historical time has greatly modified the appearance of this “Jack-in-the-Box” volcano and has introduced a confusing nomenclature applied during frequent visits by exploring expeditions. The present triangular-shaped, 0.75 x 2 km island consists of remnants of lava domes emplaced from 1796 to 1992. Castle Rock (Old Bogoslof) is a steep-sided pinnacle that is a remnant of a spine from the 1796 eruption. The small Fire Island (New Bogoslof), about 600 m NW of Bogoslof Island, is a remnant of a lava dome formed in 1883.
Brava, Cape Verde
14.8514°N, 24.7041°W | Summit elev. 956 m
According to the Cape Verde National Institute of Meteorology and Geophysics (INMG), seismicity at Brava increased on 30 October and remained elevated. A M 3.6 earthquake was recorded at 1819 on 30 October and was followed by eight more events of similar magnitudes that were felt by residents. A M 4.8 was recorded at 2100 and was followed by a 48-hour-long seismic swarm mostly consisting of low-magnitude events that were unable to be located. The Alert Level was raised to 2 (the second level on a five-level scale) on 30 October.
The rate of seismicity was variable, with two events every 1-10 minutes during 30 October-1 November. Most of the events were located at depths of 3-4 km; the hypocenters were initially located in the Praia de Águada area but then they migrated towards the center of the island. Continuous harmonic tremor emerged on 9 November and was interpreted as indicating the movement of magmatic gases. On 15 November seismicity increased and was characterized by more intense periods of volcanic tremor, long-period events, and “burst” events defined as identical events separated by a few seconds. In addition, residents began to feel earthquakes more often. The Alert Level was raised to 3. Three earthquakes all with magnitudes greater than three occurred during 18-19 November, and harmonic tremor again intensified on 19 November.
Geological summary: Brava Island, 20 km W of Fogo, is the westernmost of the southern Cape Verde islands. The 10-km-wide island contains 15 morphologically youthful craters located along two or three lineaments intersecting along the crest of the island. Most of the younger eruptions originated from the interaction of phonolitic magmas with a large groundwater reservoir contained within an older volcanic series characterized by thick welded ignimbrites and block-and-ash flow deposits. Carbonatitic lavas are also found.
Ioto, Volcano Islands
24.751°N, 141.289°E | Summit elev. 169 m
The Japan Coast Guard made observations during an overflight of Ioto (Iwo-jima) on 23 November. They posted video, photos, and infrared photos that showed explosions at the main vent producing dark, dense ash-and-steam plumes and ejecting large blocks that landed on the island and in the ocean.
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.
Purace | Colombia
2.32°N, 76.4°W | Summit elev. 4650 m
Observatorio Vulcanológico y Sismológico de Popayán, Servicio Geologico Colombiano (SGC) reported that at 1929 on 16 November the seismic network recorded a signal at Puracé associated with an ash-and-gas emission. Ash deposits on the N flank were confirmed by an observer the next day. SGC noted that localized deformation between Puracé and Curiquinga volcanoes continued to be recorded. Sulfur dioxide gas emissions had stable values, but carbon dioxide concentrations were high compared to values recorded so far in 2023. The Alert Level remained at Yellow (the second lowest on a four-color scale).
Geological summary: Puracé in Colombia consists of an andesitic stratovolcano with a 500-m-wide summit crater constructed over a dacitic shield volcano. It lies at the NW end of a volcanic massif opposite Pan de Azúcar stratovolcano, 6 km SE. A NW-SE-trending group of seven cones and craters, Los Coconucos, lies between the two larger edifices. Frequent explosive eruptions in the 19th and 20th centuries have modified the morphology of the summit crater. The largest eruptions occurred in 1849, 1869, and 1885.
Reykjanes, Reykjanes Peninsula
63.817°N, 22.717°W | Summit elev. 140 m
On 22 November the Icelandic Meteorological Office (IMO) lowered the Aviation Color Code for Reykjanes to Yellow (the second level on a four-color scale), noting that seismicity associated with the magmatic dike intrusion had decreased during the previous week. Although inflation continued to be detected at Svartsengi, they determined that the likelihood of an eruption had decreased. During 22-27 November seismic activity was relatively stable at a rate of about 500 earthquakes per day, with most events concentrated near Sýlingarfell and Hagafell. Sometimes around midnight on 27 November an hour-long seismic swarm occurred in the vicinity of Sýlingarfell. A total of 170 earthquakes were recorded and located at depths of 3-5 km; the largest event was an M 3. Seismicity slowly decreased during 28-29 November and most of the events were small, below M 1. The rate of deformation also declined, though uplift at Svartsengi continued at around 1 cm per day. The seismic and deformation data suggested that magma continued to flow into the middle portion of the dike.
Geological summary: The Reykjanes volcanic system at the SW tip of the Reykjanes Peninsula, where the Mid-Atlantic Ridge rises above sea level, comprises a broad area of postglacial basaltic crater rows and small shield volcanoes. The submarine Reykjaneshryggur volcanic system is contiguous with and is considered part of the Reykjanes volcanic system, which is the westernmost of a series of four closely-spaced en-echelon fissure systems that extend diagonally across the Reykjanes Peninsula. Most of the subaerial part of the system (also known as the Reykjanes/Svartsengi volcanic system) is covered by Holocene lavas. Subaerial eruptions have occurred in historical time during the 13th century at several locations on the NE-SW-trending fissure system, and numerous submarine eruptions dating back to the 12th century have been observed during historical time, some of which have formed ephemeral islands. Basaltic rocks of probable Holocene age have been recovered during dredging operations, and tephra deposits from earlier Holocene eruptions are preserved on the nearby Reykjanes Peninsula.
Ulawun, New Britain (Papua New Guinea)
5.05°S, 151.33°E | Summit elev. 2334 m
Rabaul Volcano Observatory (RVO) reported that white steam plumes and occasional puffs of ash rose from Ulawun’s summit crater during 22-24 November, though weather clouds hindered views during 23-24 November. Lava continued to flow from a new fissure vent that had opened on the SW flank, near the vent that had formed in 2019. The intensity of the incandescence from the flow decreased during 23-24 November, suggesting that effusion may have slowed. Seismicity remained at background levels. The Alert Level remained at Stage 2 (on the four-level scale).
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 20-27 November, with incandescence at the crater observed nightly. Explosions at 1905 on 20 November and 0226 on 21 November produced ash plumes that rose 1.2 km above the crater rim and drifted SW. Eruptive events on 22 November produced emissions that rose 1 km and drifted N and E. During an overflight of the crater on 24 November plumes obscured views of Minamidake Crater, though observers noted no changes at the geothermal area at Showa Crater. 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 from Bagana was identified in satellite images rising to 3 km (10,000 ft) a.s.l. and drifting SW at 1300 on 24 November. The plume was continuously emitted for several hours. The plume had dissipated by 0710 on 25 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 15-21 November. Although seismicity had returned to normal eruptive levels the previous week, ash-and-gas plumes continued to rise to higher-than-normal heights. Daily dense white-and-gray or gray-to-black ash plumes rose as high as 2.5 km above the summit and drifted NW, W, and S. 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.
Etna, Sicily (Italy)
37.748°N, 14.999°E | Summit elev. 3357 m
INGV reported that Strombolian activity at Etna’s SE Crater (SEC) was periodically visible during 20-26 November; weather clouds often prevented visual observations. The frequency of the eruptive activity was on the scale of hours, and explosions were most intense during 25-26 November. The explosions ejected material that fell within the crater or nearby on the flanks and produced ash emissions that rapidly dispersed near the summit. Lava overflowed the crater starting at 1810 on 24 November and produced a slow-moving lava flow that descended the S flank to the base of the cone. The lava flowed down the same ravine as one of the three flows emplaced on 12 November and was no longer being fed by 0450 on 25 November. During periods of more intense Strombolian activity on 26 November ejected lava that accumulated on the upper S flank and was visible in thermal webcam images. Activity at Bocca Nuova Crater was characterized by pulsating gas emissions and flashes of incandescence generated by the hot gases.
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.
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 22-28 November characterized by a thick flow in the summit crater that mainly expanded E. Seismicity was low. Weakly elevated surface temperatures were identified in satellite imagery during 24-25 November. 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 issued four Volcano Observatory Notices for Aviation (VONAs) on 26 November describing ash plumes from Krakatau rising as high as 1 km above the summit and drifting NW and NE. Webcam images showed incandescent material being ejected above the vent. There were 12 VONAs issued on 27 November; white-and-gray ash plumes that were sometimes dense rose as high as 1.5 km and drifted NE and NW. Webcam images continued to show incandescence at the vent and material being ejected from the vent. At least nine VONAs on 28 November described ash plumes rising as high as 2 km and drifting NE, N, and 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.
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 22-28 November. White steam-and-gas plumes rose 300-400 m above the summit and drifted W and NW during 22 and 24-26 November. White-and-gray ash plumes rose as high as 1.9 km during 23 and 27-28 November and drifted W and NW. Incandescence at the summit was visible on 22 and 26 November. 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 22-28 November. The lengths 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. Seismic stations recorded 7-111 daily rockfall events and 0-7 daily volcanic earthquakes. Two earthquakes indicated Strombolian explosions during 26-27 November. A total of three PDC events were recorded during the week. Sulfur dioxide emissions averaged 857-1,992 tonnes per day. Lahars descended the Buyoan-Padang and Mi-Isi drainages during 25-26 November. 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 17-23 November. The SW lava dome produced a total of 91 lava avalanches that descended the flanks; three traveled as far as 1.3 km down the upper part of the Boyong drainage and 88 traveled as far as 1.8 km down the upper Bebeng drainage. 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.
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 21-27 November. Seismic events indicating the movement of fluids increased in number and intensity compared to the previous week, especially with respect to the signals indicating emissions. The number of signals indicating rock fracturing decreased in both number an intensity. These events were located in areas up to 6 km in various directions from Arenas Crater, at depths of 1-8 km. The largest earthquake, a M 2, was recorded at 1949 on 25 November. Thermal anomalies on the crater floor were identified in satellite images. Ash-and-gas plumes rose as high as 1.3 km above the summit on 21 November and up to 1.7 km on 27 November. The plumes drifted WSW and SW. Sulfur dioxide emissions increased through the week. 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 time. 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.
Reventador, Ecuador
0.077°S, 77.656°W | Summit elev. 3562 m
IG-EPN reported that the eruption at Reventador was ongoing during 21-28 November. Seismicity was characterized by 31-60 daily explosions, long-period earthquakes, harmonic tremor, and tremor associated with emissions. Several daily ash-and-gas plumes rose as high as 1.2 km above the crater rim and drifted in multiple directions. Daily crater incandescence was visible during both overnight and morning hours. Incandescent material was ejected 200 m above the crater rim during 21-23 November, and avalanches of incandescent material descended the flanks daily, traveling as far as 800 m from the summit. Weather conditions sometimes prevented views of the volcano. Secretaría de Gestión de Riesgos maintained the Alert Level at Orange (the second highest level on a four-color scale).
Geological summary: Volcán El Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have left extensive deposits on the scarp slope. The largest recorded eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.
Ruapehu, North Island (New Zealand)
39.28°S, 175.57°E | Summit elev. 2797 m
GeoNet reported that 40 small earthquakes at Ruapehu was recorded by the seismic network during 21-26 November. The earthquakes were 0.3-1.8 in magnitude and located at depths of 3-6 km, though most clustered at depths of 4-5 km. Volcanic tremor levels were low during 2023 and did not vary in response to the sequence. The temperature of the crater lake water was slowly rising, starting in mid-October. During an overflight on 22 November, scientists observed that the color of the lake had changed to blue-green instead of the typical gray color, consistent with less gas flux disturbing lake-bottom sediment. No upwelling was observed and sulfur slicks on the water’s surface were visible. Water was overflowing at the lake’s outlet. Overall, activity was low. The Volcanic Alert Level remained at 1 (on a scale from 0-5) and the Aviation Color Code remained at Green (the lowest level on a four-color scale).
Geological summary: Ruapehu, one of New Zealand’s most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.
Sabancaya, Peru
15.787°S, 71.857°W | Summit elev. 5960 m
Instituto Geofísico del Perú (IGP) reported that the eruption at Sabancaya continued at moderate levels during 20-26 November with a daily average of 31 explosions. Gas-and-ash plumes rose as high as 2.9 km above the summit and drifted NE and SW. Thermal anomalies over the lava dome in the summit crater were identified in satellite data. Minor inflation was detected near the Hualca Hualca sector (4 km N). The Alert Level remained at Orange (the second highest level on a four-color scale) and the public were 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.
Sangay, Ecuador
2.005°S, 78.341°W | Summit elev. 5286 m
IG-EPN reported a high level of eruptive activity at Sangay during 21-28 November, with seismic stations recording 232-463 daily explosions. Ash-and-gas plumes visible in webcam and satellite images during 21, 23, and 27-28 November rose as high as 2 km above the crater rim and drifted N, NE, SE, and SW. Webcam images showed incandescent material at the summit vent and descending the SE flank as far as 1.8 km from the crater during 21-22 November. Weather clouds prevented observations during the rest of the week. Secretaría de Gestión de Riesgos maintained the Alert Level at Yellow (the second highest level on a four-color scale).
Geological summary: The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador’s volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within the open calderas of two previous edifices which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been eroded by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of an eruption was in 1628. Almost continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.
Semeru, Eastern Java
8.108°S, 112.922°E | Summit elev. 3657 m
PVMBG reported that eruptive activity continued at Semeru during 22-28 November. No emissions were observed on 22 November. White-and-gray ash plumes that were often dense rose 300-800 m above the summit and drifted in multiple directions during 23-28 November. 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 16-23 November. A daily thermal anomaly was identified in satellite images. On 17 November plumes of resuspended ash drifted about 116 km E. 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 22-28 November. Seismicity began to decrease on 21 November, though remained elevated with ongoing seismic tremor and small, low-frequency earthquakes recorded during the week. Elevated surface temperatures were identified in satellite images during 22 and 25-26 November. Robust steam emissions rising from the summit vent as well as from a scarp on the upper NE flank, near the summit were visible in satellite and webcam images drifting 50 km SSE during 25-26 November. During 27-28 November steam plumes were visible in webcam images and small explosions were observed in seismic and local infrasound data. Weather clouds often prevented views of the volcano. 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 the eruption at Suwanosejima’s Ontake Crater continued during 20-27 November and crater incandescence was visible nightly. No explosions were detected, though ash plumes rose as high as 1.2 km above the crater rim and drifted mainly SE and W during 20-21 and 25 November. 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.
Taal, Luzon (Philippines)
14.0106°N, 120.9975°E | Summit elev. 311 m
PHIVOLCS reported that unrest at Taal continued during 22-28 November. Daily pronounced upwelling of gases and hot fluids in the lake generated steam-and-gas plumes that rose as high as 1.2 km above the crater and drifted SW. Sulfur dioxide emissions averaged 7,608 and 11,962 tonnes per day on 23 and 27 November, respectively. The seismic network recorded 6-66 daily periods of volcanic tremor each lasting 1-7 minutes long. The Alert Level remained at 1 (on a scale of 0-5), and PHIVOLCS reminded the public that the entire Taal Volcano Island was a Permanent Danger Zone (PDZ).
Geological summary: Taal is one of the most active volcanoes in the Philippines and has produced some powerful eruptions. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, with several submerged eruptive centers. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all observed eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones. Powerful pyroclastic flows and surges have caused many fatalities.
References:
Smithsonian Institution / US Geological Survey – Weekly Volcanic Activity Report, November 22 – 28, 2023 – Managing Editor: Sally Sennert.
