Semisopochnoi
Stratovolcano in United States
Key Facts
Elevation
1,221 m (4,006 ft)
Type
Stratovolcano
Location
51.930°, 179.580°
Region
Aleutian Ridge Volcanic Arc
Total Eruptions
6
Max VEI
VEI 2
Rock Type
Basalt / Picro-Basalt
Tectonic Setting
Subduction zone
Location
Loading map...
Eruption Timeline
Most recent confirmed eruption
Historical eruption (estimated)
Historical eruption (estimated)
Historical eruption (estimated)
Historical eruption (estimated)
Historical eruption (estimated)
Overview
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.
Volcanic Hazards & Risk Assessment
Primary Hazards
- Pyroclastic flows
- Lava flows
- Volcanic bombs and ballistics
- Lahars and mudflows
Risk Level
Geological Composition & Structure
Rock Types
Tectonic Setting
Age & Formation
Eruption Statistics & Analysis
| Metric | Value | Global Ranking | Significance |
|---|---|---|---|
| Total Recorded Eruptions | 6 | Low | Moderately active volcano |
| Maximum VEI | VEI 2 | Minor | Local impact potential |
| Recent Activity | 3 years ago | Very Recent | Currently active |
Monitoring & Alert Status
Monitoring Networks
Current Status
Nearby Volcanoes in North America Volcanic Regions
Quick Info
- •Smithsonian ID: 311060
- •Evidence: Eruption Observed
- •Epoch: Holocene
About the Photo
The western slopes of Sugarloaf Peak (left) are seen here on Semisopochnoi Island. One of the largest subaerial volcanoes of the western Aleutians, Semisopochnoi is 20 km wide at sea level and contains an 8-km-wide caldera. Mount Cerberus was constructed within the caldera during the Holocene, along with Sugarloaf Peak outside the caldera to the SSE. Many documented historical eruptions originated from Cerberus.
Photo by Steve Ebbert, 1997 (U.S. Fish and Wildlife Service).
Authority Sources
Related Volcanoes
Basic Information
This page shows basic data from the Smithsonian Global Volcanism Program. For more detailed information, visit the official Smithsonian page.