The Weekly Volcanic Activity Report: April 5 – 11, 2023

the weekly volcanic activity report

New activity/unrest was reported for 4 volcanoes from April 5 to 11, 2023. During the same period, ongoing activity was reported for 16 volcanoes.

New activity/unrest: Ambae, Vanuatu | Bezymianny, Central Kamchatka (Russia) | Nevado del Ruiz, Colombia | Sheveluch, Central Kamchatka (Russia).

Ongoing activity: Aira, Kyushu (Japan) | Cotopaxi, Ecuador | Ebeko, Paramushir Island (Russia) | Fuego, South-Central Guatemala | Great Sitkin, Andreanof Islands (USA) | Karangetang, Sangihe Islands | Katmai, Alaska | Krakatau, Sunda Strait | Lascar, Northern Chile | Lewotolok, Lembata Island | Merapi, Central Java | Nishinoshima, Izu Islands | Santa Maria, Southwestern Guatemala | Semeru, Eastern Java | Semisopochnoi, Aleutian Islands (USA) | Suwanosejima, Ryukyu Islands (Japan).

New activity/unrest

Ambae, Vanuatu

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

The cone in Ambae’s Lake Voui continued to produce emissions consisting of steam, volcanic gases, and ash that drifted downwind during 5-7 April, based on reports from Vanuatu Meteorology and Geo-Hazards Department (VMGD) and Wellington VAAC notices. On 5 April low-level intermittent plumes of gas and ash rose as high as 2.5 km above the crater rim and drifted W and SW. A loud explosion followed by a dark ash emission was possibly reported. Webcam images from 2100 showed incandescence above the crater and reflected in the clouds. Intermittent emissions of gas-and-steam and gas-and-ash were visible on 7 April. Plumes rose an estimated 3 km above the crater rim and drifted E. Webcam images during 0107-0730 on 7 April showed continuing ash emissions. The Alert Level remained at 2 (on a scale of 0-5) and the public was warned to stay outside of the Danger Zone, defined as a 2-km radius around the active vents in Lake Voui, and away from drainages during heavy rains.

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

Bezymianny, Central Kamchatka (Russia)

55.972°N, 160.595°E | Summit elev. 2882 m

KVERT reported an ongoing eruption at Bezymianny generally characterized by gas-and-steam emissions and occasional collapses of hot material from the flanks of the summit lava dome. A thermal anomaly over the summit persisted in satellite images during 31 March-6 April. Diffuse ash plumes drifted as far as 550 km E during 2 and 5-6 April. On 6 April volcanologists at the Kamchatka Volcanological Station (KVS) traveled to the Apakhonchich station to assess Bezymianny, take aerial photos, and install monitoring equipment. They observed frequent collapses of the crater rim and ash plumes that drifted NE towards Apakhonchich.

KVERT stated that at 1326 local time on 7 April a satellite image showed an ash plume drifting 150 km E at altitudes of 5.5-6 km (18,000-19,700 ft) a.s.l. A satellite image from 1600 local time that same day showed an ash plume extending as far as 230 km ESE. KVERT noted that ash emissions were intensifying, likely caused by hot avalanches from a growing lava dome. The Aviation Color Code was raised to Red (the highest level on a four-color scale). On 7 April KVS volcanologists traveled to Ambon to collect ash. They reported that a notable eruption began at 1730 local time, and within 20 minutes a large ash plume had risen to 10 km and drifted NW. KVERT reported that the strong explosive phase began at 1738 local time. Based on webcam and satellite data ash plumes rose 10-12 km (32,800-39,400 ft) a.s.l. and drifted SE. Explosions were clearly audible at 20 km distance and were heard for 90 minutes, according to KVS.

Significant amounts of ash fell at the Apakhonchich station, and the snow turned gray. The volcanologists had returned to KVS by the evening, and they were covered in ash. Ashfall continued until the morning of 8 April. In a Volcano Observatory Notice for Aviation (VONA) issued at 0906 on 8 April, KVERT stated that the strong eruptive phase was over, and the Aviation Color Code was lowered to Orange. Ash plumes had drifted about 2,000 km E. The KVS team saw a lava flow on the active dome once the conditions were clear.

Geological summary: The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an ancestral edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by the collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Nevado del Ruiz, Colombia

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

Servicio Geológico Colombiano’s (SGC) reported that the number of daily events detected by the seismic network at Nevado del Ruiz fluctuated during 3-8 April but declined overall. Earthquakes were generally located 2-5 km SW of Arenas Crater at depths of 3-4 km. On 9 April SGC noted that daily counts of events were no longer going to be posted in their daily reports and instead the focus will be on the location of the earthquakes. Gas emissions had also decreased in density. That same day Gestión del Riesgo de Desastres (UNGRD) reported that 2,000-2,500 families had been identified as living in high-risk areas and requested that local authorities expedite a preventive evacuation; families that had evacuated were staying with family and friends and not in evacuation shelters.

During 3-11 April the earthquakes were associated with daily emissions of gas-and-steam and gas-and-ash plumes. Ash-and-gas plumes rose as high as 1.5 km above the crater rim on 3 April and drifted NW and SW. During 6-7 April several pulsating ash emissions were observed in webcam images and by residents of La Cabaña (Murillo, Tolima). The emissions rose as high as 1.5 km and drifted SE and NE. Webcam images and La Cabaña residents again saw ash emissions on 10 April, rising as high as 1.8 km and drifting E and NE. Thermal anomalies from Arenas Crater were periodically visible throughout the week. The Alert Level remained at Orange, Level II (the second highest 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.

Sheveluch, Central Kamchatka (Russia)

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

On 28 March the Kamchatka Volcanological Station (KVS) reported that activity had increased at Sheveluch during the previous few days. Incandescence at the summit of the lava dome was constant and the focus of activity shifted from the E side to the NE side. KVERT reported that the ongoing eruption was generally characterized by explosions, hot avalanches, lava-dome extrusion, and strong fumarolic activity. A daily thermal anomaly was identified in satellite images during 30 March-6 April. Satellite images showed an ash plume drifting 250 km E and SE.

Seismic data around 0054 local time on 11 April indicated a significant increase in activity, as reported by the Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS). According to the Tokyo VAAC the ash plume had risen to 15.8 km (52,000 ft) a.s.l. by 0110 and was drifting NW. By 0158 the plume extended over a 75 x 100 km area. KVS reported that significant pulses of activity occurred at around 0200, 0320, and then a stronger phase started around 0600. Video of the rising plume was taken at around 0600 from near Békés (3 km away) by Levin Dmitry, who reported that a pyroclastic flow traveled across the road behind him as he left the area. Ashfall began in Klyuchi (45 km SW) at 0630, and the large black ash plume had blocked the daylight by 0700. At 0729 KVERT issued a Volcano Observatory Notice for Aviation (VONA) raising the Aviation Color Code to Red (the highest level on a four-color scale). They stated that a large ash plume had risen to 10 km (32,800 ft) a.s.l. and drifted 100 km W. According to IVS FEB RAS the cloud was 200 km long and 76 km wide by 0830, and was spreading W at altitudes of 6-12 km (19,700-39,400 ft) a.s.l.

KVS reported that at about 0930 the plume drifted over Kozyrevsk (112 km SW) and turned the day to night. Almost constant lightning strikes in the plume were visible and sounds like thunderclaps were heard until about 1000. The sky lightened up in Kozyrevsk at about 1030; residents in Klyuchi reported continuing darkness and ashfall at 1100. As the day went on the light had a reddish-brown hue due to the ash in the atmosphere. In some areas, ashfall was 6 cm deep and some residents reported dirty water coming from their plumbing. At 1150 an ash cloud 400 km long and 250 km wide was spreading W at altitudes of 5-20 km (16,400-65,600 ft) a.s.l., according to IVS FEB RAS. KVERT issued a VONA at 1155 noting that ash had risen to 10 km and that it had extended 340 km NNW and 240 km WSW. According to Simon Carn about 0.2 Tg of sulfur dioxide in the plume was measured in a satellite image acquired at 1343. A satellite image at 1748 showed ash plumes rising to 8 km (26,200 ft) a.s.l. and drifting 430 km WSW and S, based on a VONA. Residents of Klyuchi measured ashfall as thick as 8.5 cm, according to the Kamchatka Branch of Geophysical Services (KBGS; Russian Academy of Sciences). In a VONA issued at 0748 on 12 April KVERT stated that strong explosions were continuing. Ash plumes from explosions rose to 8 km and drifted ESE. The larger ash cloud continued to drift and had extended 600 km SW and 1,050 km ESE. IVS FEB RAS scientists photographed the terminal part of a pyroclastic flow that had traveled 19 km SSE the day before, which had stopped a few hundred meters from a bridge on the road between Kliuchi and Petropavlovsk-Kamchatsky.

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.

Ongoing activity

Aira, Kyushu (Japan)

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

JMA reported ongoing eruptive activity at Minamidake Crater (at Aira Caldera’s Sakurajima volcano) during 3-10 April, with crater incandescence visible nightly. Very small eruptive events occasionally occurred. Sulfur dioxide emissions were high at 2,700 tons per day on 4 March. The Alert Level remained at 3 (on a 5-level scale), and residents were 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 the formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the 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.

Cotopaxi, Ecuador

0.677°S, 78.436°W | Summit elev. 5911 m

IG reported that eruptive activity at Cotopaxi was ongoing during 4-11 April. Gas, steam, and ash plumes visible in webcam images and reported by the Washington VAAC during 4-6 and 8 April rose 200-800 m above the summit and drifted E, SE, S, and SW. Minor ashfall was reported in Mulaló (9.5 km WSW) and San Agustín (10 km W). Gas-and-steam plumes rose 300 m and drifted S and SE on 7 April. On 10 April ash plumes rose 1-1.5 km and drifted W, SW, and SE. Weather clouds prevented visual observations on the other days. Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE) maintained the Alert Level at Yellow (the second lowest level on a four-color scale).

Geological summary: The symmetrical, glacier-covered, Cotopaxi stratovolcano is Ecuador’s most well-known volcano and one of its most active. The steep-sided cone is capped by nested summit craters, the largest of which is about 550 x 800 m in diameter. Deep valleys scoured by lahars radiate from the summit of the andesitic volcano, and large andesitic lava flows extend to its base. The modern edifice has been constructed since a major collapse sometime prior to about 5,000 years ago. Pyroclastic flows (often confused in historical accounts with lava flows) have accompanied many explosive eruptions, and lahars have frequently devastated adjacent valleys. Strong eruptions took place in 1744, 1768, and 1877. Pyroclastic flows descended all sides of the volcano in 1877, and lahars traveled more than 100 km into the Pacific Ocean and the western Amazon basin. Smaller eruptions have been frequent since that time.

Ebeko, Paramushir Island (Russia)

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

KVERT reported that moderate activity at Ebeko was ongoing from 30 March-6 April and a daily thermal anomaly was identified in satellite images. According to volcanologists in Severo-Kurilsk (Paramushir Island, about 7 km E) explosions during the week generated ash plumes that rose as high as 4 km (13,100 ft) a.s.l. and drifted in various directions. Ash plumes drifted 12 km E and SW on 3 and 6 April. The Aviation Color Code remained at Orange (the second highest level on a four-color scale). Dates are based on UTC times; specific events are in local time where noted.

Geological summary: The flat-topped summit of the central cone of the Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along an 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.

Fuego, South-Central Guatemala

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

INSIVUMEH reported that 4-12 explosions per hour were generally recorded at Fuego during 4-11 April, generating ash plumes that rose as high as 1.1 km above the crater rim and drifted as far as 20 km W, SW, and S. Weather clouds sometimes prevented views. Daily ashfall was recorded in areas downwind including Morelia (9 km SW), Panimaché I and II (8 km SW), Santa Sofía (12 km SW), El Porvenir (8 km ENE), La Rochela, San Andrés Osuna, Siquinala, Ceylon, Finca La Asunción, and Finca Palo Verde. Daily block avalanches descended multiple drainages including the Ceniza (SSW), Seca (W), Trinidad (S), Taniluyá (SW), Honda, Las Lajas (SE), and El Jute (ESE), and often reached vegetated areas. Daily shock waves rattled structures in communities around the volcano and rumbling was often heard. Explosions ejected incandescent material as high as 350 m above the summit each day.

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

Great Sitkin, Andreanof Islands (USA)

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

AVO reported that lava likely continued to slowly effuse at the summit of Great Sitkin during 5-11 April, producing a thick lava flow. Seismicity was low, and during 9-10 April only a few events were detected. A satellite radar image on 2 April showed that the lava flow was mostly expanding to the E and slowly to the S into the summit crater ice field. Elevated surface temperatures were identified in satellite images during 4-6 April. Weather clouds obscured views during 7-10 April. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

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

Karangetang, Sangihe Islands

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

Webcam images of Karangetang captured in the PVMBG daily reports periodically showed small areas of incandescence at the summit Main Crater (S crater) and on the flanks during 4-10 April. The Alert Level remained at 3 (on a scale of 1-4) and the public was advised to stay 2.5 km away from Main Crater with an extension to 3.5 km on the S and SE flanks.

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

Katmai, Alaska

58.28°N, 154.963°W | Summit elev. 2047 m

AVO reported that on 8 April strong winds in the vicinity of Katmai and the Valley of Ten Thousand Smokes blew unconsolidated ash SE across Shelikof Strait to Kodiak Island at an altitude up to 2.4 km 8,000 ft) a.s.l. The ash was originally deposited during the Novarupta eruption in 1912. The Volcano Alert Level remained at Normal (the lowest level on a four-level scale) and the Aviation Color Code remained at Green (the lowest level on a four-color scale).

Geological summary: Katmai was initially considered to be the source of the Valley of Ten Thousand Smokes ash flow in 1912. However, the 3 x 4 km caldera of 1912 is now known to have formed as a result of the voluminous eruption at the nearby Novarupta volcano. Prior to 1912, this compound stratovolcano had four NE-SW-trending summits, most of which were truncated by caldera collapse in that year. Two or more large explosive eruptions took place during the late Pleistocene. Most of the two overlapping pre-1912 Katmai volcanoes are Pleistocene, but Holocene lava flows from a flank vent descend the SE flank of the SW edifice into the Katmai River canyon. The steep-walled young caldera has a jagged rim that rises 500-1,000 m above the caldera floor and contains a 250-m-deep, still-rising lake. Lake waters have covered a small post-collapse lava dome (Horseshoe Island) that was seen on the caldera floor at the time of the initial ascent to the caldera rim in 1916.

Krakatau, Sunda Strait

6.102°S, 105.423°E | Summit elev. 155 m

PVMBG reported that daily white steam-and-gas plumes rose as high as 200 m above Anak Krakatau’s summit during 5-11 April. White-and-black plumes rose as high as 300 m above the summit and drifted NE on 9 April. 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 volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral 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, Rakata, Danan, and Perbuwatan volcanoes 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 cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Lascar, Northern Chile

23.37°S, 67.73°W | Summit elev. 5592 m

SERNAGEOMIN reported that during the last two months activity at Láscar had declined to low levels, based on seismological, geodetic, geochemical, and remote sensing data, though remained above baseline. The volume of the lava dome remained unchanged, seismicity was low including small numbers of volcano-tectonic and tornillo-type events, sulfur dioxide gas emissions were low, and tephra was absent from emissions. On 6 April the Alert Level was lowered to Yellow (the second lowest level on a four-color scale) and the public was warned to stay at least 5 km away from the crater. SENAPRED updated the Alert Level Yellow (the middle level on a three-color scale) for San Pedro de Atacama (70 km NW) and maintained a security perimeter of 10 km around the volcano.

Geological summary: Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Lewotolok, Lembata Island

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

PVMBG reported that the eruption at Lewotolok was ongoing during 5-11 April. White steam and gas plumes were seen on most days rising as high as 500 m above the summit and drifting in multiple directions. According to the Darwin VAAC an ash plume rose to 1.8 km (6,000 ft) a.s.l., or about 370 m above the summit, on 6 April and drifted N. On 7 April white-and-gray plumes of variable densities rose 100-400 m and drifted SE and W; similar plumes on 9 April rose 200-350 m and drifted NE and E. The Alert Level remained at 2 (on a scale of 1-4) and the public was warned to stay 2 km 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.

Merapi, Central Java

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

BPPTKG reported that the eruption at Merapi (on Java) continued from 31 March-6 April and seismicity remained at high levels. The SW lava dome produced 79 lava avalanches that traveled as far as 1.8 km down the SW flank (upstream in the Bebeng and Boyong drainages). One pyroclastic flow traveled 1.1 km down the SW flank, upstream of the Bebeng drainage. Morphological changes to the SW lava dome were evident in webcam images 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 the Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with a major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying the growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Nishinoshima, Izu Islands

27.247°N, 140.874°E | Summit elev. 25 m

JMA reported that at 1050 and 1420 on 11 April ash plumes from Nishinoshima rose 1.9 km above the crater rim and drifted NW and W.

Geological summary: The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Another eruption that began offshore in 2013 completely covered the previous exposed surface and enlarged the island again. Water discoloration has been observed on several occasions since. The island is the summit of a massive submarine volcano that has prominent satellitic peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the sea surface 9 km SSE.

Santa Maria, Southwestern Guatemala

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

INSIVUMEH reported that the eruption at Santa María’s Santiaguito lava-dome complex continued during 4-11 April. Daily explosions produced gas, steam, and ash plumes that rose as high as 1 km above the complex and drifted sometimes as far as 8 km W, SW, and S; during 8-9 April the plumes drifted W and rose as high as 1.8 km. Rumbling was barely audible on nearby farms during 8-9 April. Effusion from the Caliente dome fed lava flows that slowly descended the San Isidro and Zanjón Seco drainages on the W and SW flanks. Incandescence from the dome and the lava flows was visible nightly and some early mornings. Block avalanches from the dome, and from both the ends and sides of the flows, descended the S, SW, and W flanks were reported almost daily. Residents were reminded to stay at least 6 km away from the complex.

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

Semeru, Eastern Java

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

PVMBG reported that the eruption at Semeru continued during 5-11 April and a few Volcano Observatory Notices for Aviation (VONAs) were issued throughout the week. At 0559 on 8 April, a dense white-to-brown plume rose 800 m above the summit and drifted NE. About an hour and a half later, at 0732, a white-to-gray plume rose 600 m and drifted N. At 0635 and 2218 on 10 April variable white-and-gray plumes rose 500-800 m and drifted N and NE. At 0535 on 11 April a dense white-and-gray plume rose 500 m and drifted N. The Alert Level remained at 3 (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, 100 m from the banks of the Kobokan drainage as far as 17 km from the summit, and to avoid other drainages originating on Semeru, 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.

Semisopochnoi, Aleutian Islands (USA)

51.93°N, 179.58°E | Summit elev. 1221 m

AVO reported that low-level unrest continued at Semisopochnoi during 5-11 April. An extensive low-altitude steam plume drifted more than 400 km during 4-6 April and was likely related to volcanic emissions but did not appear to contain ash. Several small explosions were detected on 5 April, and periods of seismic tremor and local earthquakes were recorded during 5-6 April, but seismicity was quiet during the rest of the week. Clouds obscured webcam and satellite views during 7-10 April. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Geological summary: Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of the collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island’s northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

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 3-10 April. The eruptive activity produced ash plumes that rose as high as 1.6 km above the crater rim and drifted mainly SE, E, and N. Large blocks were ejected as far as 300 m from the vent. Crater incandescence was visible at night during 3-7 April and one explosion was detected during 7-10 April. The Alert Level remained at 3 (on a 5-level scale) and residents were warned to stay 2 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.


Smithsonian / US Geological Survey Weekly Volcanic Activity Report – April 5 – 11, 2023 – Managing Editor: Sally Kuhn Sennert


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