Report on Semisopochnoi (United States) — 21 July-27 July 2021
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 21 July-27 July 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, 21 July-27 July 2021. Smithsonian Institution and US Geological Survey.
51.93°N, 179.58°E; summit elev. 1221 m
All times are local (unless otherwise noted)
AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory for Semisopochnoi on 22 July, noting that explosive activity had not been detected since 12 July. Seismicity continued to be elevated during 23-27 July. Robust steam emissions were visible in webcam views during 22-24 July and sulfur dioxide plumes were identified in satellite images on 23 July. Weather clouds obscured views during 24-27 July.
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.