Global Volcanism Program | Image GVP-05386
The broad 35-km-wide Okmok is truncated by two largely overlapping, 10-km-wide calderas. Both calderas formed by voluminous eruptions of tephra and pyroclastic flows during the Holocene, one about 8,250 years ago and the other about 2,400 years ago. Numerous cones and lava domes formed on the caldera floor and flanks. Historical explosive eruptions and lava flows have originated from cones within the caldera.
Photo by John Reeder (Alaska Division of Geological & Geophysical Surveys).