Report on Semisopochnoi (United States) — 7 July-13 July 2021
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 7 July-13 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, 7 July-13 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 reported that continuous volcanic tremor at Semisopochnoi began at 1200 on 12 July and explosive activity was recorded by the infrasound network. Emissions began at 1300 and lasted tens of minutes; a sulfur dioxide gas plume possibly containing ash was identified in satellite images drifting S at an altitude less than 1.5 km (5,000 ft) a.s.l. The Aviation Color Code was raised to Orange and the Volcano Alert Level was raised to Watch. Volcanic tremor decreased to low levels after several hours.
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.