Report on Semisopochnoi (United States) — 29 September-5 October 2021
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 29 September-5 October 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, 29 September-5 October 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 eruptive activity at Semisopochnoi's North Cerberus crater continued during 29 September-5 October. Seismicity remained elevated with intermittent explosion signals or bursts of activity likely from explosions. A few explosions were also detected in regional infrasound data during 2-5 October. Small ash clouds were observed almost daily in either webcam or satellite images rising to altitudes below 3 km (10,000 ft) a.s.l.; plumes drifted ENE on 2 October, then N and NW on 3 October. Sulfur dioxide plumes were possibly observed on a few days, though definitely on 3 October. Webcams located 5-6 km from the active vent recorded ashfall during 4-5 October. 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 (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.