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Water-magma interaction and plume processes in the 2008 Okmok eruption, Alaska

Joel A. Unema, Michael H. Ort, Jessica F. Larsen, Christina A. Neal and Janet R. Schaefer
Water-magma interaction and plume processes in the 2008 Okmok eruption, Alaska
Geological Society of America Bulletin (January 2016) 128 (5-6): 792-806

Abstract

Eruptions of similar explosivity can have divergent effects on the surroundings due to differences in the behavior of the tephra in the eruption column and atmosphere. Okmok volcano, located on Umnak Island in the eastern Aleutian Islands, erupted explosively between 12 July and 19 August 2008. The basaltic andesitic eruption ejected approximately 0.24 km (super 3) dense rock equivalent (DRE) of tephra, primarily directed to the northeast of the vent area. The first 4 h of the eruption produced dominantly coarse-grained tephra, but the following 5 wk of the eruption deposited almost exclusively ash, much of it very fine and deposited as ash pellets and ashy rain and mist. Meteorological storms combined with abundant plume water to efficiently scrub ash from the eruption column, with a rapid decrease in deposit thickness with distance from the vent. Grain-size analysis shows that the modes (although not their relative proportions) are very constant throughout the deposit, implying that the fragmentation mechanisms did not vary much. Grain-shape features consistent with molten fuel-coolant interaction are common. Surface and groundwater drainage into the vents provided the water for phreatomagmatic fragmentation. The available water (water that could reach the vent area during the eruption) was approximately 2.8X10 (super 10) kg, and the erupted magma totaled approximately 7X10 (super 11) kg, which yield an overall water:magma mass ratio of approximately 0.04, but much of the water was not interactive. Although magma flux dropped from 1X10 (super 7) kg/s during the initial 4 h to 1.8X10 (super 5) kg/s for the remainder of the eruption, most of the erupted material was ejected during the lower-mass-flux period due to its much greater length, and this tephra was dominantly deposited within 10 km downwind of the vent. This highlights the importance of ash scrubbing in the evaluation of hazards from explosive eruptions.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 128
Serial Issue: 5-6
Title: Water-magma interaction and plume processes in the 2008 Okmok eruption, Alaska
Affiliation: Northern Arizona University, School of Earth Sciences and Environmental Sustainability, Flagstaff, AZ, United States
Pages: 792-806
Published: 20160108
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 39
Accession Number: 2016-025809
Categories: Quaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2016018
Illustration Description: illus. incl. 2 tables, geol. sketch map
N53°25'60" - N53°25'60", W168°07'60" - W168°07'60"
Secondary Affiliation: University of Alaska-Fairbanks, USA, United StatesU. S. Geological Survey, USA, United StatesState of Alaska Division of Geological and Geophysical Surveys, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201613
Program Name: USGSOPNon-USGS publications with USGS authors
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