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Report on Semisopochnoi (United States) — 4 August-10 August 2021


Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 4 August-10 August 2021
Managing Editor: Sally Sennert.

Please cite this report as:

Global Volcanism Program, 2021. Report on Semisopochnoi (United States) (Sennert, S, ed.). Weekly Volcanic Activity Report, 4 August-10 August 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (4 August-10 August 2021)


United States

51.93°N, 179.58°E; summit elev. 1221 m

All times are local (unless otherwise noted)

AVO reported that an eruption at Semisopochnoi’s North Cerberus crater was ongoing during 4-10 August, characterized by daily explosions, ash plumes, and sulfur dioxide emissions identified in satellite images. Multiple energetic explosions were detected almost daily in seismic and infrasound data. An ash cloud rose to 1.5-3 km (5,000-10,000 ft) a.s.l. and drifted 33 km NE during 3-4 August. Ash emissions were also visible in webcam images and photographed by an observer S of the island. An ash cloud was generated at 2123 on 4 August and was visible in satellite data until 0250 on 5 August, drifting 180 km SE over Gareloi and southern Tanaga Island. A small ash cloud at 2040 on 6 August drifted 50 km beyond the N crater vent and quickly dissipated. Another small ash cloud at 2103 caused ashfall on the webcam located 5 km NE about 30 minutes later. Ash-and-steam emissions were visible in webcam images until 2333. A large infrasound signal was recorded at 0653 on 8 August, though weather conditions prevented visual confirmation of emissions. A possible small ash cloud was visible in a satellite image at 0700. At least three separate explosions were recorded at 1714 on 8 August and 0339 and 0417 on 9 August. A small ash cloud at 900 m (3,000 ft) a.s.l. was visible in satellite images on the morning of 9 August. There were 10 explosions recorded during 9-10 August; small ash-and-steam emissions were visible in the webcam and small low-level plumes were occasionally identified in satellite data. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

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 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.

Source: US Geological Survey Alaska Volcano Observatory (AVO)