Semisopochnoi (United States)

Status Unrest Eruption 2018 1221m
Stratovolcano (Subduction zone / Intermediate crust (15-25 km))

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.

The western slopes of symmetrical Sugarloaf Peak (left) rise above tundra-covered Semisopochnoi Island. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. Three-peaked Mount Cerberus volcano was constructed within the caldera during the Holocene, along with symmetrical Sugarloaf Peak volcano outside of the caldera to the SSE. Most documented historical eruptions have originated from Cerberus.

Photo by Steve Ebbert, 1997 (U.S. Fish and Wildlife Service).

Last updated 2022-06-11 05:00:02