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Semisopochnoi Island description and information




Official Name: Semisopochnoi Island
Type:Intracaldera stratocones
Most Recent Activity:February 6, 2021
Seismically Monitored: Yes
Color Code:GREEN
Alert Level:NORMAL
Elevation: 2625 ft (800 m)
Latitude: 51.9288° N
Longitude:179.5977° E
Quadrangle:Rat Islands
CAVW Number:311060
Pronunciation: Sound file
Associated Features:Sugarloaf Peak
Lakeshore Cone
Mount Young
Anvil Peak
Ragged Top
Three-quarter cone
Nearby towns:Adak 161 mi (259 km) SE
Shemya Station 238 mi (383 km) NW
Atka 264 mi (425 km) NE
Attu Station 277 mi (445 km) NW
Anchorage 1309 mi (2107 km) NE
From Wood and Kienle (1990) [1]: "Semisopochnoi is the largest young volcanic island in the western Aleutians and is composed of a variety of volcanic landforms. Basaltic pyroclastic material built a shield of ~20 km wide (at sea level) which culminated in a post-glacial pumice and ash eruption of dacite and andesite, producing an 8-km-wide caldera. Smaller composite cones are both pre- and post-glacial. Mount Cerberus is the most active of the three younger cones within the caldera. These young cones are dominantly two-pyroxene, high-alumina basalt, and andesite. One young composite cone (Sugarloaf) has olivine basalt. Dacite and andesite are found among the eruptive products of the pre caldera shield. Much of the island is covered by basaltic to andesitic ash derived from the younger cones. Semisopochnoi's tholeiitic differentiation trend (iron is enriched as silica increases) and relatively large volume are common in volcanoes near segment boundaries. Semisopochnoi is also on a small submarine ridge that extends northward as a part of the scorpion-tail-shaped Bower's Ridge; it is unclear if this setting influences its volcanism.

"An historic eruption of Semisopochnoi was reported in 1873, and at least four others may have occurred in the previous hundred years, but documentation is scanty. These eruptions apparently emanated from the flanks of Mount Cerberus; the most recent flow appears to be less than a century old."

Volcanoes of North America: United States and Canada, 1990
Wood, C. A., and Kienle, Juergen, (eds.), 1990, Volcanoes of North America: United States and Canada: New York, Cambridge University Press, 354 p.

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Page modified: September 29, 2020 12:19
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