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Report on Semisopochnoi (United States) — 21 August-27 August 2019

Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 21 August-27 August 2019
Managing Editor: Sally Kuhn Sennert

Please cite this report as:

Global Volcanism Program, 2019. Report on Semisopochnoi (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 21 August-27 August 2019. Smithsonian Institution and US Geological Survey.

Volcano Profile |  Weekly Report (21 August-27 August 2019)


Semisopochnoi

United States

51.93°N, 179.58°E; summit elev. 1221 m

All times are local (unless otherwise noted)


AVO reported that during 17-23 August seismicity at Semisopochnoi remained elevated and was characterized by periods of continuous tremor and discrete low-frequency earthquakes; seismic data went offline starting sometime on 17 August, though was available by around 22 August. Ground-coupled airwaves, indicative of explosive activity, were sometimes recorded in seismic data; an infrasound signal was recorded during 23-24 August. Cloudy weather often prevented satellite views of the volcano, though a steam plume was visible on 18 August and sulfur dioxide emissions were detected during 21-22 August. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Geologic Background. 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 1221-m-high Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked 774-m-high Mount Cerberus volcano was constructed during the Holocene within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side. Other post-caldera volcanoes include the symmetrical 855-m-high 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 historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time.

Source: US Geological Survey Alaska Volcano Observatory (AVO)