Report on Semisopochnoi (United States) — 19 April-25 April 2023
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 19 April-25 April 2023
Managing Editor: Sally Sennert.
Please cite this report as:
Global Volcanism Program, 2023. Report on Semisopochnoi (United States) (Sennert, S, ed.). Weekly Volcanic Activity Report, 19 April-25 April 2023. 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 low-level unrest continued at Semisopochnoi during 18-25 April. Daily periods or bursts of tremor and occasional low-frequency earthquakes were detected during the week. Small explosions were detected in seismic and infrasound data during 18-19 and 24-25 April. Cloudy weather prevented webcam and satellite views on most days. Possible recent ash deposits on Mount Young’s crater rim were visible in clear webcam images during 22-23 April, and steam emissions from the active N crater were visible during 22-25 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 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.