HR: 1330h
AN: V42B-0346    [PDF]
TI: The 1817 Eruption of Okmok Caldera, Umnak Island, Alaska: New Insights Into a Complex Historical Eruption in the Eastern Aleutians
AU: * Neal, C A
EM: tneal@usgs.gov
AF: U.S. Geological Survey, Alaska Volcano Observatory 4200 University Drive, Anchorage, AK 99508
AU: Beget, J
EM: ffjeb1@aurora.uaf.edu
AF: Department of Geology and Geophysics, University of Alaska, Fairbanks, AK 99775
AU: Grey, D
EM: ftdmg@uaf.edu
AF: Department of Geology and Geophysics, University of Alaska, Fairbanks, AK 99775
AU: Wolfe, B
EM: benjamin.wolfe@ttemi.com
AF: Tetra Tech EM Inc., 8030 Flint St., Lenexa, KS 66214
AB: Okmok is a 10-km-diameter, late-Holocene caldera on Umnak Island in the eastern Aleutians, 1400 km southwest of Anchorage. The most recent eruption in 1997 was strombolian in character, producing a basaltic-andesite lava flow within the caldera and localized ash fall. Since caldera-formation approximately 2050 14C yrs BP, however, more violent eruptions from vents within the caldera have impacted all flanks of the volcano with tephra fall, ballistics, pyroclastic surges and flows, and lahars. An example of these more violent intracaldera events is the 1817 eruption. Reevaluation of historical accounts of activity at Okmok combined with new geologic mapping and tephra studies suggest that an 1817 eruption included (1) early, largely hydrovolcanic, explosive activity and the production of significant pyroclastic fall and surge deposits extending down the north and east flanks of the volcano; (2) generation of a flood that reached the Bering Sea; (3) late-stage strombolian fountaining and lava flow production. Vents from the 1817 eruption form a 4-km-long arc that parallels the base of the north caldera wall and include a 50-70-m-deep, elongate maar crater erupted through pre-existing tuff cone deposits. Terrace morphology and flood deposits less than 200 14C yrs BP indicate a flooding event down Crater Creek consistent with historical accounts of Aleut village inundation at the coastline in 1817. The later part of the eruption produced a 120-m high cinder and spatter cone and a blocky a'a lava flow field that fills a shallow basin near the outlet of Crater Creek. These preliminary results indicate that Okmok is capable of eruptions far more violent than the largely effusive events of the last century.
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting