• Official Name: Semisopochnoi Island
  • Seismically Monitored: Yes
  • Color Code: GREEN
  • Alert Level: NORMAL
  • Elevation: 800m (2624ft)
  • Latitude: 51.9288
  • Longitude: 179.5977
  • Smithsonian VNum: 311060
  • Pronunciation:
  • 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
    • Nikolski 490 mi (789 km) NE
  • Subfeatures:
    • Sugarloaf Peak
    • Lakeshore Cone
    • Mount Young
    • Anvil Peak
    • Pochnoi
    • Ragged Top
    • Three-quarter cone


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."

Name Origin

"Semisopochnoi Island" was published by Lieutenant Sarichev (1826) as "O[strov] Semisopochnyy" (Semisopochnoi Island) (Orth, 1971). While there are seven prominent peaks on Semisopochnoi, there are numerous active volcanic vents.

References Cited

[1] 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.

Current Activity

No new updates for Semisopochnoi volcano since August 4, 2023, 2:57 pm.



Color Code Timeline


Loading Past Activity...

Loading Images...

Loading Maps...

Loading Bibliography...