Report on Semisopochnoi (United States) — 11 August-17 August 2021
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
11 August-17 August 2021
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2021. Report on Semisopochnoi (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 11 August-17 August 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 an eruption at Semisopochnoi’s North Cerberus crater was ongoing during 11-17 August, characterized by daily explosions, ash plumes, and occasional sulfur dioxide emissions identified in satellite images. Multiple energetic explosions were detected daily in seismic and infrasound data, and strong seismic tremor was occasionally recorded. Daily steam-and-ash plumes identified in webcam and satellite data rose no higher than 3 km (10,000 ft) a.s.l. and drifted 200-300 km W. Daily ashfall likely occurred on the island. 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.
Source: US Geological Survey Alaska Volcano Observatory (AVO)