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Report on Semisopochnoi (United States) — 3 February-9 February 2021


Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 3 February-9 February 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, 3 February-9 February 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (3 February-9 February 2021)


United States

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

All times are local (unless otherwise noted)

AVO reported that a 6 February satellite image of Semisopochnoi showed small ash deposits extending in a narrow strip less than 3 km N of North Cerberus Crater. The deposits were likely the result of a small explosions that occurred during the previous week. Steam emission obscured view of the summit crater. The Aviation Color Code and the Volcano Alert Level were raised to Yellow/Advisory. A second ash deposit, similar to the first, was visible in a satellite image on 7 February. This deposit extended at least 3 km NE of North Cerberus Crater. Weather clouds obscured views of the S side of the volcano. The report noted that the ash plumes associated with the deposits had not been observed; they likely rose less than 3 km (10,000 ft) a.s.l. and were short lived. The Aviation Color Code and the Volcano Alert Level were raised to Orange/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)