Geologic Background:
Okmok is a 10-km-wide caldera that occupies most
of the E end of Umnak Island, located 120 km SW of Dutch
Harbor in the eastern Aleutian Islands.
Historic Activity:
- Eruptions have occurred in 1805, 1817, 1824, 1830,
1878, 1899, 1931, 1936, 1938, 1943, 1945, and 1958.
- Okmok has had several eruptions in historical time
typically consisting of ash emissions, occasionally to
heights over 10 km but generally to much lower altitudes,
sometimes accompanied by lava flows.
- Lava flows crossed the caldera floor in 1945 and
1958.
Recent Activity:
- In July 1983, a large ash plume extending 100
kilometers from volcano was observed.
- From November 1986 to December 1987 there were been
intermittent reports of steam and ash emissions from a
cinder cone within caldera.
- In February 1988 dark ash emissions began from new
vent at SW base of cone.
- Small earthquake swarm during May 11-15, 2001
Data Sources:
- Smithsonian's SEAN Bulletin (V. 8, No. 8; V. 11, No.
12 to V. 13, No. 2).
- Global Volcanism Network (V. 22, No. 1 to V. 62, No.
8)
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