Cordilleran Section - 108th Annual Meeting (29–31 March 2012)

Paper No. 1
Presentation Time: 08:30-18:30


ORT, Michael H., Seses, Northern Arizona University, Box 4099, Flagstaff, AZ 86011, UNEMA, Joel A., SESES, Northern Arizona University, Box 4099, Flagstaff, AZ 86011, LARSEN, Jessica F., Geology, University of Alaska Fairbanks, Fairbanks, AK 99775 and SCHAEFER, Janet R., Alaska Division of Geological & Geophysical Surveys, Alaska Volcano Observatory, 3354 College Rd, Fairbanks, AK 99709,

Okmok volcano, in the central Aleutian Islands, Alaska, began erupting explosively on July 12, 2008, with hydrovolcanic explosions continuing through August 23. Initial plume heights reached 16 km asl emanating from a cluster of vents near the center of a 10-km-wide caldera. During the eruption, steep-walled collapse pits, 10 to 200 m in diameter, formed through a pre-existing lava field NW of the vents. These older lava flows were deposited over water-saturated volcaniclastic sediments and pit formation was likely due to collapse as the groundwater and liquefied sediments drained toward the vents during the eruption. Subsurface magma-water interaction at Okmok produced three and possibly four vents and diverse proximal deposits with sectorial distribution. The basal units 1 and 2 are coarse lapilli-ash fallout beds, and unit 3 is a fine-medium ash with abundant accretionary lapilli and ash pellets. Within the caldera, to the N, S and SE of the vent area, a nonsorted explosion breccia of accidental and juvenile blocks occurs between units 2 and 3. To the west of the vent area, deposits from pyroclastic density currents occur at the same stratigraphic position, as do chaotic water-laid deposits across a lake to the north. These likely formed during vent-opening or vent-widening early in the eruption. Two sets of deposits with isolated 2-4 dm duneforms occur in the fine-ash-rich middle to upper parts of most stratigraphic sections in the caldera. These are evidence of two periods of surge production, likely from partial collapse of a column simultaneously producing ash pellets and ash rain. Several black lapilli ash-fall beds within the ash-pellet-rich beds represent periods of increased discharge and wind direction shifts late in the eruption. Fallout, but no density-current, deposits occur outside the caldera and correlate with this sequence. Deposition of the fine-medium ash of the upper sequence is controlled by local weather (rain and wind) and individual layers do not correlate laterally. The tephra record from the 2008 Okmok eruption reveals complex interaction of water and magma: in the conduit through molten fuel-coolant interaction and jetting, as volcanogenic tsunami near lakes, as sheet-wash floods off caldera walls, and through condensation within, and the interaction of rain and mist with, the eruption cloud.