Okmok (United States)

Status Normal Eruption 2008 1073m
Shield (Subduction zone / Intermediate crust (15-25 km))

The broad, basaltic Okmok shield volcano, which forms the NE end of Umnak Island, has a dramatically different profile than most other Aleutian volcanoes. The summit of the low, 35-km-wide volcano is cut by two overlapping 10-km-wide calderas formed during eruptions about 12,000 and 2050 years ago that produced dacitic pyroclastic flows that reached the coast. More than 60 tephra layers from Okmok have been found overlying the 12,000-year-old caldera-forming tephra layer. Numerous satellitic cones and lava domes dot the flanks of the volcano down to the coast, including 1253-m Mount Tulik on the SE flank, which is almost 200 m higher than the caldera rim. Some of the post-caldera cones show evidence of wave-cut lake terraces; the more recent cones, some of which have been active historically, were formed after the caldera lake, once 150 m deep, disappeared. Hot springs and fumaroles are found within the caldera. Historical eruptions have occurred since 1805 from cinder cones within the caldera.

The broad, 35-km-wide Okmok shield volcano is truncated by two spectacular, largely overlapping, 10-km-wide calderas. Both calderas at this dominantly basaltic volcano were formed by voluminous eruptions of dacitic tephra and pyroclastic flows during the Holocene, one about 8250 years ago and the other about 2400 years ago. Numerous satellitic pyroclastic cones and lava domes dot the floor of the caldera and the flanks of the volcano. Explosive eruptions and lava flows have originated from cones within the caldera in historical time.

Photo by John Reeder (Alaska Division of Geological & Geophysical Surveys).

Last updated 2019-08-04 00:28:03