Okmok, shield volcano on Umnak, SW Alaska, in the Aleutian Islands. Rising to 3,520 ft (1,073 m) and extending over most of NE Umnak, the 22-mi-wide (35-km) volcano contains two 6-mi-wide (10-km) partially overlapping calderas that were formed about 8,250 and 2,400 years ago. An eruption of Okmok that occurred c.43 B.C. may have contributed to climate changes in the Mediterranean region that led to upheaval in the Roman Republic and the rise of the Roman Empire. More recent eruptions occurred in 1945 and from 1986 to 1988.
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