The Weekly Volcanic Activity Report: September 6 – 12, 2023

the weekly volcanic activity report

New activity/unrest was reported for 1 volcano from September 6 to 12, 2023. During the same period, ongoing activity was reported for 19 volcanoes.

New activity/unrest: Kilauea, Hawaiian Islands (USA).

Ongoing activity: Aira, Kyushu (Japan) | Ebeko, Paramushir Island (Russia) | Great Sitkin, Andreanof Islands (USA) | Kanlaon, Philippines | Klyuchevskoy, Central Kamchatka (Russia) | Krakatau, Sunda Strait | Lewotolok, Lembata Island | Lokon-Empung, Sulawesi | Mayon, Luzon (Philippines) | Merapi, Central Java | Rincon de la Vieja, Costa Rica | Sabancaya, Peru | Sheveluch, Central Kamchatka (Russia) | Shishaldin, Fox Islands (USA) | Soufriere Hills, Montserrat | Stromboli, Aeolian Islands (Italy) | Suwanosejima, Ryukyu Islands (Japan) | Tungurahua, Ecuador | Ubinas, Peru.

New activity/unrest

Kilauea, Hawaiian Islands (USA)

19.421°N, 155.287°W | Summit elev. 1222 m

HVO reported that a new eruption began in Kilauea’s Halema`uma`u Crater on 10 September following a period of increased seismicity. Seismicity increased on 22 August; most of the earthquakes were located at depths of 2-3 km and were all smaller than M2. About 150 occurred during 9-10 September. Tiltmeter and Global Positioning System (GPS) data showed inflation in the S portion of the crater.

At 0252 on 10 September HVO raised the Volcano Alert Level to Watch (the third level on a four-level scale) and the Aviation Color Code to Orange (the third level on a four-color scale) due to increased earthquake activity and changes in ground deformation that indicated magma moving towards the surface. An eruption commenced at about 1515 in the E part of the caldera based on field reports and webcam images. Fissures opened on the crater floor and produced lava fountains and flows. The Volcano Alert Level and Aviation Color Code were raised to Warning and Red, respectively. Gas-and-steam plumes rose from the fissures and drifted downwind. By 1900 the line of fissures was about 1.4 km long and extended into the E wall of the down-dropped block. Multiple active fountains were about 20-25 m high; fountains at the initial eruption onset were an estimated 50 m.

At 0810 on 11 September the Volcano Alert Level was lowered back to Watch and the Aviation Color Code was lowered back to Orange because the style of eruption and fissure location had stabilized, the initial extremely high effusion rates had declined (but remained at high levels), and no infrastructure was threatened. The eruption plume, mainly comprised of sulfur dioxide and particulates, rose as high as 3 km (10,000 ft) a.s.l. and had become less dense. Hawai’i Volcanoes National Park had closed to visitors. Lava erupted from fissures on the down-dropped block flowed W towards Halema`uma`u, covering much of the surface with active lava as deep as about 2.5 m. During 11-12 September easternmost vents on the down-dropped block and the westernmost vents in Halema`uma`u became inactive; the active vents were E-W-trending and spanned a distance of about 750 m. Channelized lava flows traveled N and W onto the Halema`uma`u Crater floor, burying the E rim of the crater and most of the crater floor; higher older lava flows prevented movement onto the SW part of the floor. Lava fountaining continued, rising as high as 15 m by the morning of 12 September. A laser rangefinder pointed at the W portion of the crater recorded almost 5 m of uplift from the magmatic intrusion beneath the caldera since the onset of the eruption.

Geological summary: Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Ongoing activity

Aira, Kyushu (Japan)

31.593°N, 130.657°E | Summit elev. 1117 m

JMA reported ongoing activity at both Minamidake Crater and Showa Crater (Aira Caldera’s Sakurajima volcano) during 4-11 September. Incandescence at Minamidake was observed nightly. Very small eruptive events occurred at both craters on 4 September. Sulfur dioxide emissions were slightly high at 1,600 tons per day on that same day. An explosion on 1512 on 9 September produced an ash plume that rose 800 m above the crater rim and ejected large blocks 500-700 m from the crater. Another explosion at 0018 on 11 September generated an ash plume that rose 1.1 km and ejected large blocks 300-500 m; an ash plume rose 1.3 km from an explosion at 1642 on that same day. The Alert Level remained at 3 (on a 5-level scale), and the public was warned to stay 2 km away from both craters.

Geological summary: The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan’s most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu’s largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

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 31 August-7 September. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E), explosions during 31 August and 2-5 September generated ash plumes that rose as high as 3.5 km (11,500 ft) a.s.l and drifted to the E. A thermal anomaly was identified in satellite images during 4-5 September; weather clouds obscured views on other days. The Aviation Color Code remained at Orange (the third level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

Geological summary: The flat-topped summit of the central cone of 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 likely continued at Great Sitkin during 6-12 September, producing a thick flow in the summit crater that mainly expanded E. Seismicity was low; earthquakes were detected daily and were numerous during 6-7 September. Web camera images showed diffuse gas emissions rising from the summit during 6-7 September and weakly elevated surface temperatures were identified in satellite data during 6-7 and 10-11 September. Weather clouds often prevented satellite and webcam observations. 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.

Kanlaon, Philippines

10.412°N, 123.132°E | Summit elev. 2435 m

PHIVOLCS issued a special notice for Kanlaon at 0900 on 6 September, noting increased seismicity. The seismic network detected 36 volcanic earthquakes including 34 volcano-tectonic earthquakes between 0500 on 4 September and 0750 on 6 September at depths of 0-9 km beneath the NE flank. The earthquakes had local magnitudes of 0.8-3.4. Ground deformation data from continuous GPS and electronic tilt data had been recording inflation at the mid-flanks of the volcano since March. Sulfur dioxide emissions at the summit crater averaged 788 and 301 tonnes per day (t/d) on 7 and 11 September, respectively. The number of volcano-tectonic earthquakes decreased during the rest of the week; there were 23 recorded during 6-7 September and 2-9 daily earthquakes during 8-12 September. The Alert Level remained at 1 (on a scale of 0-5) and PHIVOLCS reminded the public to remain outside of the 4-km-radius Permanent Danger Zone.

Geological summary: Kanlaon volcano (also spelled Canlaon), the most active of the central Philippines, forms the highest point on the island of Negros. The massive andesitic stratovolcano is dotted with fissure-controlled pyroclastic cones and craters, many of which are filled by lakes. The largest debris avalanche known in the Philippines traveled 33 km SW from Kanlaon. The summit contains a 2-km-wide, elongated northern caldera with a crater lake and a smaller, but higher, historically active vent, Lugud crater, to the south. Historical eruptions, recorded since 1866, have typically consisted of phreatic explosions of small-to-moderate size that produce minor ashfalls near the volcano.

Klyuchevskoy, Central Kamchatka (Russia)

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

KVERT reported that the explosive Strombolian eruption at Klyuchevskoy continued 31 August-7 September. A bright thermal anomaly was identified in satellite images every day, except on 7 September due to weather clouds. Lava fountaining fed flows that advanced down the Apakhonchichsky and Kozyrevsky drainages. Plumes of resuspended ash drifted 120 km E during 3-5 September. The Aviation Color Code remained at Yellow (the second level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

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

Krakatau, Sunda Strait

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

PVMBG reported that white gas-and-steam plumes rose as high as 300 m above Krakatau’s summit on most days during 6-12 September and drifted NW and NE. White, gray, and black ash plumes of variable densities rose as high as 1 km and drifted N, NE, and E during 10-11 September. White-and-black ash plumes rose as high as 100 m and drifted N on 12 September. 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 September. White-and-gray plumes rose 100-500 m above the summit and drifted W and NW during 4-6 September. White steam-and-gas plumes rose as high as 400 m and drifted W and NW each day during the rest of the week. 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.

Lokon-Empung, Sulawesi

1.358°N, 124.792°E | Summit elev. 1580 m

PVMBG reported that activity at Lokon Empung had decreased during the previous few weeks. Seismicity notably decreased during 21-22 July; volcanic earthquakes started to become less frequent and tremor amplitude significantly decreased. Gas-and-steam emissions had variable densities but decreased from a maximum height of 400 m above the crater during 18-31 July to a maximum height of 200 m above the crater during 1-20 August. At 1200 on 21 August the Alert Level was lowered to 2 (on a sale of 1-4) and the public was reminded not to approach Tompaluan Crater within a radius of 1.5 km.

Geological summary: The Lokong-Empung volcanic complex, rising above the plain of Tondano in North Sulawesi, includes four peaks and an active crater. Lokon, the highest peak, has a flat craterless top. The morphologically younger Empung cone 2 km NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century. A ridge extending 3 km WNW from Lokon includes the Tatawiran and Tetempangan peaks. All eruptions since 1829 have originated from Tompaluan, a 150 x 250 m crater in the saddle between Lokon and Empung. These eruptions have primarily produced small-to-moderate ash plumes that sometimes damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred.

Mayon, Luzon (Philippines)

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

PHIVOLCS reported that the eruption at Mayon continued during 6-12 September. The lengths of the lava flow in the Mi-Isi (S), Bonga (SE), and Basud (E) drainages remained at 2.8 km, 3.4 km, and 1.1 km, respectively. Collapses at the lava dome and from the lava flows produced incandescent rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) that descended the flanks as far as 4 km. Each day seismic stations recorded 118-239 rockfall events, 0-3 PDC events, and 3-35 daily volcanic earthquakes. Daily sulfur dioxide emissions averaged between 609 and 2,252 tonnes per day, with the highest value recorded on 8 September. One ash emission was recorded during 11-12 September. 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 1-7 September and seismicity remained at elevated levels. The SW lava dome produced a total of 154 lava avalanches that descended the S and SW flanks; five traveled as far as 1.5 km down the upper part of the Boyong drainage, 146 traveled as far as 2 km down the upper Bebeng drainage, and three traveled 1.5 km down the Senowo drainage. Morphological changes to the SW lava dome were due to continuing 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.

Rincon de la Vieja, Costa Rica

10.83°N, 85.324°W | Summit elev. 1916 m

OVSICORI-UNA reported that daily gas-and-steam emissions continued at Rincón de la Vieja during 6-12 September rising as high as 1 km above the crater rim. Some of the emissions were produced by small phreatic events. The Alert Level remained at Level 3, Orange, the third level on a four-level scale.

Geological summary: Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the “Colossus of Guanacaste,” it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Sabancaya, Peru

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

Instituto Geofísico del Perú (IGP) reported that the eruption at Sabancaya continued during 4-10 September with a daily average of 13 explosions. Gas-and-ash plumes rose as high as 2.3 km above the summit and drifted SE, E, NE, and NW. A total of 10 thermal anomalies from the lava dome in the summit crater were detected using satellite data. Minor inflation was detected near the Hualca Hualca sector (4 km N). 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.

Sheveluch, Central Kamchatka (Russia)

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

KVERT reported that the eruption at Sheveluch continued during 31 August-7 September. Intense fumarolic activity was visible at the active dome, and thermal anomalies were identified in satellite images on all days except 7 September (due to weather clouds). Plumes of resuspended ash drifted 650 km SE and E during 31 August and 3-4 September. 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 the eruption at Shishaldin continued during 6-12 September. Seismicity remains elevated and characterized by weak but steady tremor, small low-frequency earthquakes, and small explosions; explosions were not reported on 12 September. Elevated surface temperatures were identified daily in satellite images and were sometimes intense. A Significant Meteorological Information (SIGMET) notice issued on 6 September advised aircraft of a low-level ash plume drifting SSE at 1.5-1.8 km (5,000-6,000 ft) a.s.l. Small plumes visible in webcam images drifted ESE and SSE during 6-7 September. Additional small diffuse steam-and-gas plumes from the summit were observed in web camera images during 9-11 September. 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.

Soufriere Hills, Montserrat

16.72°N, 62.18°W | Summit elev. 915 m

MVO reported that a very small increase in activity at Soufrière Hills during 1-8 September was characterized by small rockfalls coincident with volcano-tectonic earthquake activity and occasional small, low-frequency, volcanic earthquakes. The seismic network recorded 27 volcano-tectonic earthquakes spilt between two swarms recorded on 6 and 8 September. Minor rockfall activity and volcano-tectonic seismicity began at about 2100 on 7 September and was ongoing. Rockfalls took place on the lava dome based on seismic data, though their locations could not be visually confirmed. The swarm on 8 September was followed by a long-period earthquake at 0742, and a few additional smaller events. Access to the Upper Belham valley and Plymouth were suspended for the day out of an abundance of caution. Sulfur dioxide emissions averaged 436 tonnes per day on 1 September (measured from a boat) and 367 tonnes per day on 6 September (measured by helicopter). The Hazard Level remained at 1 (on a scale of 1-5).

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

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 September. Webcam images showed Strombolian activity at three vents in Area N (one at N1 and two 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. Low- and medium-intensity explosions at a rate of 6-9 per hour from Area N2 ejected mainly coarse material (bombs and lapilli), sometimes mixed with ash, up to 200 m above the vents. Intense spattering occurred at N1 on 4 September. Low- to medium-intensity explosions averaged 5-14 per hour from the two vents in sector S2 (Area C-S), ejecting a mix of coarse material and ash as high as 200 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 September. Eruptive events at 0221, 0301, and 0333 on 9 September produced ash plumes that rose 1.1-1.4 km above the crater rim and drifted W. Ash emissions were continuous during 0404-0740 and rose as high as 2 km above the crater rim. More eruptive events at 1437 on 10 September and at 0319 on 11 September produced ash plumes that rose 1.7-1.8 km. 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.

Tungurahua, Ecuador

1.467°S, 78.442°W | Summit elev. 5023 m

The Washington VAAC reported that at 0820 on 7 September a plume of resuspended ash at Tungurahua drifted W at an altitude of 4.9 km (16,000 ft) a.s.l., based on weather models and satellite images. The plume had dissipated by 1440.

Geological summary: Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador’s most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano’s base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Ubinas, Peru

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

Instituto Geofísico del Perú (IGP) reported that the eruption at Ubinas continued at low-to-moderate levels during 4-10 September. There were daily averages of 183 volcano-tectonic earthquakes indicating rock fracturing and 27 long-period earthquakes signifying the movement of gas and magma. Gas-and-steam emissions rose as high as 1 km above the crater rim and drifted as far as 5 km NE, E, and SE; no explosions or ash plumes were recorded during the week. The Alert Level remained at Orange (the third level on a four-color scale) and the public was warned to stay 4 km away from the crater.

Geological summary: A small, 1.4-km-wide caldera cuts the top of Ubinas, Perú’s most active volcano, giving it a truncated appearance. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by 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.


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

Featured image credit: TW


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