Report on Semisopochnoi (United States) — 18 November-24 November 2020
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 18 November-24 November 2020
Managing Editor: Sally Sennert.
Please cite this report as:
Global Volcanism Program, 2020. Report on Semisopochnoi (United States) (Sennert, S, ed.). Weekly Volcanic Activity Report, 18 November-24 November 2020. Smithsonian Institution and US Geological Survey.
51.93°N, 179.58°E; summit elev. 1221 m
All times are local (unless otherwise noted)
AVO reported that no signs of eruptive activity at Semisopochnoi had been recorded since mid-June. Seismicity had been elevated above background levels but declined to very low levels before 11 November, when the satellite the link to transmit the data failed. Both the Aviation Color Code and the Volcano Alert Level were lowered to Unassigned, reflecting the lack of available seismic data to detect unrest.
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.