HR: 1340h
AN: V53A-0624    [Abstracts]
TI: Variations in Eruptive Dynamics and Magma Withdrawal During the Caldera V Eruption of Ksudach Volcano, Kamchatka
AU: * Andrews, B J
EM: andrewsb@mail.utexas.edu
AF: Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254 United States
AU: Izbekov, P E
EM: pavel@gi.alaska.edu
AF: Alaska Volcano Observatory, University of Alaska Fairbanks, 903 Koyukuk Drive, University of Alaska Fairbanks, Fairbanks, AK 99775-7320 United States
AB: In $\sim$240 A.D., Ksudach Volcano, Kamchatka, erupted 11.6 km$^{3}$ of rhyodacite magma (DRE) to form a 4x6.5 km caldera. Field data indicate that 70$%$ of the deposit is Plinian fall. Mass discharge rate (MDR) of the eruption increased from $<$10$^{8}$ kg/s to as much as 6x10$^{8}$ kg/s prior to caldera collapse; notably, MDR decreased just before or during collapse. Post-collapse MDR was only 2-3x10$^{8}$ kg/s. The timing of caldera collapse corresponds with an abrupt change in pumice color, from white to gray. Despite this color change, both pumice types are rhyodacitic ($\sim$71.5 wt$%$ SiO$_{2}$) and contain greater than 90 vol$%$ rhyolitic glass, with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, magnetite, and ilmenite. Magnetite-ilmenite geothermometry indicate pre-eruptive temperatures of 895 +/-5 $\deg$C for both pumice types at oxygen fugacities of $\sim$-11.5 (log f$_{O2}$). Glass inclusions are approximately 70 wt$%$ SiO2 and up to 7.5 wt$%$ H2O, with no difference observed between pumice types. The only differences between white and gray pumice are variations in vesicle textures and microlite content. Vesicles in white pumice are more voluminous ($>$2x10$^{5}$ $\mu$m$^{3}$ versus 10$^{5}$ $\mu$m$^{3}$) and more elongate (aspect ratios of greater than 5:1 versus 2:1). Gray pumice have plagioclase microlites and ten times more Fe-Ti microlites compared to white pumice. One explanation for the differences in microlite contents and vesicle textures between otherwise indistinguishable pumice is that the white magma decompressed (and hence ascended) faster than the gray magma. A second possibility is that the magma at depsth was decompressed (or cooled) during the eruption, before it ascended to the surface. Experiments in progress will evaluate the different scenarios for producing the textural disparities. Interestingly, whatever caused textural differences between white and gray pumice, it occurred with the end of caldera collapse.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting