Report on Okmok (United States) — 9 July-15 July 2008
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 9 July-15 July 2008
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2008. Report on Okmok (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 9 July-15 July 2008. Smithsonian Institution and US Geological Survey.
53.43°N, 168.13°W; summit elev. 1073 m
All times are local (unless otherwise noted)
A strong explosive eruption at Okmok began abruptly at 1143 on 12 July after about an hour of rapidly escalating earthquake activity. Ash and gas from the initial explosions reached at least 15 km (50,000 ft) a.s.l on 12 July and drifted as a large cloud S and E above the North Pacific. Satellite tracking of the ash cloud by traditional techniques has been hampered by the high water content due to interaction of rising magma with very shallow groundwater and surficial water inside the caldera. Heavy ashfall occurred on the eastern portion of Umnak Island; a dusting of ash also occurred for several hours in Unalaska/Dutch Harbor.
As of 15 July, based on AVO analysis of satellite data, ash is continuing to erupt from a composite cinder and spatter cone called Cone D in the eastern portion of the 6-mile wide caldera. Seismicity reached a peak at about 1400 on 12 July and has been gradually declining since. The volcano is currently at aviation color code RED and alert level WARNING. All areas immediately around the volcano are considered hazardous.
News media reported that residents of Umnak Island heard thundering noises the morning of 12 July and quickly realized an eruption had begun. After calling the US Coast Guard for assistance, they began to evacuate to Unalaska using a small helicopter. A fishing boat evacuated the remaining residents after heavy ashfall made further flights impossible.
Geologic Background. 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.