At 1700 on 22 July, H. Wilson observed continuous but minor ash emissions from a cone in the SW part of the caldera. The ash and steam plume rose ~150 m above the cone with a burst of dark gray and light yellow ash. As the plume drifted ESE, most of the ash dropped from the plume and the steam dissipated within 5 km of the cone, although a gray haze could be recognized to 25 km ESE. When Wilson returned at 2030 that day he saw only minor steam emission rising no more than 60 m above the cone. The floor of the cone was visible and no incandescent material was observed.

Geological Summary. 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.

Information Contacts: J. Reeder, Alaska Division of Geological and Geophysical Surveys (ADGGS).