HR: 08:15h
AN: V11F-02    [Abstracts]
TI: Vesicle Microtextures and Fragmentation in Basaltic Plinian Eruptions
AU: * Gonnermann, H M
EM: helge@hawaii.edu
AF: University of Hawaii, SOEST, Deptartment of Geology & Geophysics, 1680 East-West Road, Honolulu, HI 96822, United States
AU: Houghton, B F
EM: bhought@soest.hawaii.edu
AF: University of Hawaii, SOEST, Deptartment of Geology & Geophysics, 1680 East-West Road, Honolulu, HI 96822, United States
AU: Sable, J E
EM: sable@hawaii.edu
AF: Montana State University, Deptartment of Earth Sciences, P.O. Box 173480, Bozeman, MT 59717, United States
AB: We set out to establish how basaltic magma erupts at Plinian intensities, by matching vesicle microtextures within the products of basaltic Plinian eruptions to calculated rates for bubble growth and coalescence in basaltic magmas. We find that observed microtextures are consistent with sufficient gas overpressure for magma fragmentation during ascent-driven decompression. Vesicles in pyroclasts from basaltic Plinian eruptions at Mt. Etna, 122 B.C. (Italy) and Mt. Tarawera, 1886 A.D. (New Zealand) have complex, non-spherical shapes indicative of bubble coalescence. Because of surface tension, non-spherical bubbles relax back to spherical shape over time. Preservation of abundant vesicles with "coalescence" shapes in both sample suites implies that bubble coalescence proceeded at a faster rate than shape relaxation, immediately prior to ejection and cooling of the erupting magma. We hypothesize that fast rates of bubble coalescence are the consequence of ascent-driven, decompressive bubble growth. We test this hypothesis by calculating the required decompression rates and matrix viscosities at which bubble growth and coalescence are faster than shape relaxation. Calculated decompression rates are consistent with existing eruption rate estimates, while calculated viscosities are consistent with a melt viscosity affected by high micro-crystallinity (>50%) observed in matrix glasses from both sample suites. When considered together, we find that during eruptive conditions of faster coalescence relative to shape relaxation, gas overpressure within bubbles is predicted to be sufficient to cause brittle fragmentation of the ascending basalt magma.
DE: 8400 VOLCANOLOGY
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting