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Report on Semisopochnoi (United States) — 18 August-24 August 2021


Semisopochnoi

Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
18 August-24 August 2021
Managing Editor: Sally Kuhn Sennert

Please cite this report as:

Global Volcanism Program, 2021. Report on Semisopochnoi (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 18 August-24 August 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (18 August-24 August 2021)

Semisopochnoi

United States

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

All times are local (unless otherwise noted)


AVO reported that multiple explosions and seismicity at Semisopochnoi's North Cerberus crater continued during 18-24 August. Low-level gas-and-ash emissions, including occasional sulfur dioxide emissions were detected in satellite and webcam data during 18-24 August and rose no higher than 3 km (10,000 ft) a.s.l.; views were often obscured due to weather. On 20 August minor ashfall deposits were reported; ashfall may have continued following explosive events during the rest of the week. 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 was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank of Cerberus 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 Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Source: US Geological Survey Alaska Volcano Observatory (AVO)