Alaska Volcano Observatory | Semisopochnoi
Source: https://avo.alaska.edu/activity/Semisopochnoi.php
Archived: 2026-04-23 17:14
Alaska Volcano Observatory |
Semisopochnoi
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Semisopochnoi
Description
Current Activity
Past Activity
Images
Maps
Bibliography
Prepare
Ash Forecasting
Facts
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:
Your browser does not support the audio element.
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
Distance from Anchorage: 1309 mi (2107 km)
Subfeatures:
Sugarloaf Peak
Lakeshore Cone
Mount Young
Anvil Peak
Pochnoi
Ragged Top
Three-quarter cone
Description
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 February 27, 2026, 11:55 am.
Recent Updates
Small explosion on February 22. Volcano remains at GREEN/NORMAL.
Feb 27th, 2026 11:55:44 AM
Small explosion detected seismically and in infrasound.
Feb 23rd, 2026 11:58:06 AM
No eruptive activity has been observed. Aviation Color Code and Volcano Alert Level lowered to GREEN/ NORMAL.
Aug 4th, 2023 02:57:33 PM
Volcanic unrest has declined with no eruptive activity detected in 3 months. Aviation Color Code and Volcano Alert Level lowered to GREEN/NORMAL.
Aug 3rd, 2023 03:42:10 PM
Seismicity is low. No unusual activity observed in mostly cloudy satellite and web camera views.
Aug 3rd, 2023 12:32:32 PM
Webcams
Amchitka [AHT, 381 ft]
Semisopochnoi [CETU, 1041 ft]
Semisopochnoi [CEPE, 1113 ft]
Webicorder
Color Code Timeline
An explanation of color codes and alert levels can be found
here
.
Loading Past Activity...
Loading Images...
Loading Maps...
Loading Bibliography...
Before an eruption
Sign up for volcanic activty alerts from the Volcano Notification System (US Geological Survey)
Ashfall & Preparedness Information
Ashfall impacts & preparedness (US Geological Survey)
Volcanic health hazards & impacts (International Volcanic Health Hazards Network)
Ash Alert! Pamphlet (AK Division of Homeland Security & Emergency Management)
Volcanic Ashfall (AK Department of Environmental Conservation, Division of Air Quality)
During and After an Eruption
National Weather Service Ashfall Warnings are posted here when in effect
Alaska Division of Homeland Security and Emergency Management
Ted Stevens Airport Flight Status (Anchorage)
Fairbanks Airport Flight Status
Other Useful Links
Citizen Science: Ashfall collection instructions
Citizen Science: IS ASH FALLING? Report your observations (yes or no!)
Ash Forecasting
Mathematical models developed by the USGS forecast various aspects of how a volcanic ash plume will interact with wind—where, how high, and how fast ash particles will be transported in the atmosphere, as well as where ash will fall out and accumulate on the ground. AVO runs these models when a volcano is restless by assuming a reasonable hypothetical eruption, to provide a pre-eruptive forecast of areas likely to be affected. During an ongoing eruption, AVO will update the forecast with actual observations (eruption start time and duration, plume height) as they become available.
View the current airborne ash cloud models for Semisopochnoi
Ashfall thickness forecast
The Ash3d model was developed by the USGS to forecast how a volcanic ash plume will interact with wind and where ash will fall out and accumulate on the ground. AVO runs these models twice daily when a volcano is restless by assuming a reasonable
hypothetical
eruption altitude and duration. The map shows the model results of ashfall thickness for areas that are likely to be affected, if one were to occur. During an ongoing eruption, AVO will update the forecast with actual observations (eruption start time and duration, plume height) as they become available, and these plots will be automatically updated. The
National Weather Service
Anchorage Forecast Office will issue the official ashfall warning product and post them at
weather.gov/afc
THESE PRODUCTS MAY NOT BE CURRENT.
During an actual eruption, see National Weather Service forecasts of ashfall:
https://weather.gov/afc
.
Ashfall Forecast
Click on the X on the graphic (upper right) to expand the map to show the map legend.
Ashfall Start Time
This map shows the modeled estimate of the time it would take for ashfall to begin following an eruption. It corresponds to the ashfall thickness forecast map shown above. This map uses the start time of either the twice-daily hypothetical model runs (time shown in the legend) or the actual eruption start time (if one were to occur). In the case of an actual eruption, the National Weather Service Anchorage Forecast Office will issue the official ashfall warning product that includes the ashfall start time and post them at
weather.gov/afc
THESE PRODUCTS MAY NOT BE CURRENT.
During an actual eruption, see National Weather Service forecasts of ashfall:
https://weather.gov/afc
.
Ashfall Start Times Forecast
Click on the X on the graphic (upper right) to expand the map to show the map legend.
USGS Volcano Notification Service
Contact AVO
Privacy
Accessibility
Information Quality
FOIA
This website is supported by the U.S. Geological Survey under Cooperative Agreement Grant G22AC00137 and G23AC00070.
Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey.
×
Semisopochnoi
Alaska Volcano Observatory
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Webcams
Ashcam GUI
News
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Explore Data
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Glossary
Images
Get Involved
About AVO
About AVO
Staff
Study Areas
Alert Levels
FAQ
Semisopochnoi
Description
Current Activity
Past Activity
Images
Maps
Bibliography
Prepare
Ash Forecasting
Facts
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:
Your browser does not support the audio element.
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
Distance from Anchorage: 1309 mi (2107 km)
Subfeatures:
Sugarloaf Peak
Lakeshore Cone
Mount Young
Anvil Peak
Pochnoi
Ragged Top
Three-quarter cone
Description
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 February 27, 2026, 11:55 am.
Recent Updates
Small explosion on February 22. Volcano remains at GREEN/NORMAL.
Feb 27th, 2026 11:55:44 AM
Small explosion detected seismically and in infrasound.
Feb 23rd, 2026 11:58:06 AM
No eruptive activity has been observed. Aviation Color Code and Volcano Alert Level lowered to GREEN/ NORMAL.
Aug 4th, 2023 02:57:33 PM
Volcanic unrest has declined with no eruptive activity detected in 3 months. Aviation Color Code and Volcano Alert Level lowered to GREEN/NORMAL.
Aug 3rd, 2023 03:42:10 PM
Seismicity is low. No unusual activity observed in mostly cloudy satellite and web camera views.
Aug 3rd, 2023 12:32:32 PM
Webcams
Amchitka [AHT, 381 ft]
Semisopochnoi [CETU, 1041 ft]
Semisopochnoi [CEPE, 1113 ft]
Webicorder
Color Code Timeline
An explanation of color codes and alert levels can be found
here
.
Loading Past Activity...
Loading Images...
Loading Maps...
Loading Bibliography...
Before an eruption
Sign up for volcanic activty alerts from the Volcano Notification System (US Geological Survey)
Ashfall & Preparedness Information
Ashfall impacts & preparedness (US Geological Survey)
Volcanic health hazards & impacts (International Volcanic Health Hazards Network)
Ash Alert! Pamphlet (AK Division of Homeland Security & Emergency Management)
Volcanic Ashfall (AK Department of Environmental Conservation, Division of Air Quality)
During and After an Eruption
National Weather Service Ashfall Warnings are posted here when in effect
Alaska Division of Homeland Security and Emergency Management
Ted Stevens Airport Flight Status (Anchorage)
Fairbanks Airport Flight Status
Other Useful Links
Citizen Science: Ashfall collection instructions
Citizen Science: IS ASH FALLING? Report your observations (yes or no!)
Ash Forecasting
Mathematical models developed by the USGS forecast various aspects of how a volcanic ash plume will interact with wind—where, how high, and how fast ash particles will be transported in the atmosphere, as well as where ash will fall out and accumulate on the ground. AVO runs these models when a volcano is restless by assuming a reasonable hypothetical eruption, to provide a pre-eruptive forecast of areas likely to be affected. During an ongoing eruption, AVO will update the forecast with actual observations (eruption start time and duration, plume height) as they become available.
View the current airborne ash cloud models for Semisopochnoi
Ashfall thickness forecast
The Ash3d model was developed by the USGS to forecast how a volcanic ash plume will interact with wind and where ash will fall out and accumulate on the ground. AVO runs these models twice daily when a volcano is restless by assuming a reasonable
hypothetical
eruption altitude and duration. The map shows the model results of ashfall thickness for areas that are likely to be affected, if one were to occur. During an ongoing eruption, AVO will update the forecast with actual observations (eruption start time and duration, plume height) as they become available, and these plots will be automatically updated. The
National Weather Service
Anchorage Forecast Office will issue the official ashfall warning product and post them at
weather.gov/afc
THESE PRODUCTS MAY NOT BE CURRENT.
During an actual eruption, see National Weather Service forecasts of ashfall:
https://weather.gov/afc
.
Ashfall Forecast
Click on the X on the graphic (upper right) to expand the map to show the map legend.
Ashfall Start Time
This map shows the modeled estimate of the time it would take for ashfall to begin following an eruption. It corresponds to the ashfall thickness forecast map shown above. This map uses the start time of either the twice-daily hypothetical model runs (time shown in the legend) or the actual eruption start time (if one were to occur). In the case of an actual eruption, the National Weather Service Anchorage Forecast Office will issue the official ashfall warning product that includes the ashfall start time and post them at
weather.gov/afc
THESE PRODUCTS MAY NOT BE CURRENT.
During an actual eruption, see National Weather Service forecasts of ashfall:
https://weather.gov/afc
.
Ashfall Start Times Forecast
Click on the X on the graphic (upper right) to expand the map to show the map legend.
USGS Volcano Notification Service
Contact AVO
Privacy
Accessibility
Information Quality
FOIA
This website is supported by the U.S. Geological Survey under Cooperative Agreement Grant G22AC00137 and G23AC00070.
Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey.
×