HR: 10:20h
AN: V22A-01 INVITED    [Abstracts]
TI: From Map Unit to Magma Chamber: Understanding the 2006 Eruption of Augustine Volcano
AU: * Coombs, M
EM: mcoombs@usgs.gov
AF: Alaska Volcano Observatory, U.S. Geological Survey Alaska Science Center 4200 University Drive, Anchorage, AK 99508, United States
AU: Bull, K
EM: kate.bull@alaska.gov
AF: Alaska Volcano Observatory, Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK 99708, United States
AU: Cervelli, P
EM: pcervelli@usgs.gov
AF: Alaska Volcano Observatory, U.S. Geological Survey Alaska Science Center 4200 University Drive, Anchorage, AK 99508, United States
AU: Larsen, J
EM: faust@gi.alaska.edu
AF: Alaska Volcano Observatory, Geophysical Institute University of Alaska Fairbanks, Fairbanks, AK 99708, United States
AU: Mandeville, C
EM: cmandy@amnh.org
AF: American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, United States
AU: Nye, C
EM: cnye@giseis.alaska.edu
AF: Alaska Volcano Observatory, Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK 99708, United States
AU: Tilman, M
EM: fsmrt5@uaf.edu
AF: Alaska Volcano Observatory, Geophysical Institute University of Alaska Fairbanks, Fairbanks, AK 99708, United States
AU: Vallance, J
EM: jvallance@usgs.gov
AF: Cascades Volcano Observatory, U.S. Geological Survey 1300 SE Cardinal Court, Vancouver, WA 98683, United States
AU: Wallace, K
EM: kwallace@usgs.gov
AF: Alaska Volcano Observatory, U.S. Geological Survey Alaska Science Center 4200 University Drive, Anchorage, AK 99508, United States
AU: Webster, J
EM: jdw@amnh.org
AF: American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, United States
AB: In 2006, Augustine Volcano once again sprang to life and erupted ~70 x 10⁶ m³ of magma during three eruptive phases. Variations in magma composition, eruptive style, and deformation of the edifice provide clues to the ascent and interaction of magmas prior to and during the three-month-long eruption. Genetically unrelated end members basaltic andesite (56.5 wt% SiO₂) and dacite (63.3 wt% SiO₂) bracket erupted magma compositions. Products from all three eruptive phases contain both end members though proportions varied with time, and many pyroclasts have mixing textures. Initial Vulcanian explosions in mid-January 2006 (explosive phase) erupted small-volumes (~14 x 10⁶ m³ DRE) of basaltic andesite as ash fall and pyroclastic flows. In late January, explosions transitioned to continuous "boil over" at the vent (continuous phase), producing relatively voluminous (28.5 x 10⁶ m³ DRE) block-and-ash flows that are rich in dacite and banded clasts. In early February, explosive activity gave way to effusion of basaltic andesite lava (effusive phase; 32 x 10⁶ m³ DRE) highlighted by a pulse of increased effusion from March 7–14. Effusive activity ceased by the end of March. Geophysical and petrologic evidence lead us to a hypothetical series of magmatic processes that drove the eruption. The presence of amphibole in all eruptive products indicates that magma storage must have been within the amphibole stability field at depths greater than ~4 km, in agreement with elevated water (2 - 4 wt% by difference) and chlorine (2400 - 4900 ppm) contents in melt inclusions. Precursory unrest took the form of six months of volcano-tectonic earthquakes and island-wide uplift and radial displacement (recorded by GPS), all centered near sea level. Preliminary textural analysis of the explosively erupted basaltic andesite indicates that it did not undergo the degree of decompression-driven crystallization expected if it had accumulated at sea level (~40 MPa) for longer than a few days. This suggests that the precursory geophysical signals were not caused by magma accumulation, but perhaps by gas released from and preceding the ascending magma. We hypothesize that gas-rich basaltic andesite rose from the mid-to-lower crust, through a mid-to-upper crustal dacite magma body, and toward the surface. Mixing textures suggest that the basaltic andesite that erupted during the explosive phase also entrained dacite magma and created a pathway for the mid-to-upper crustal dacite to ascend. A ~10- km-deep deflationary source coincided with the continuous eruption of dacite in late January and early February. Unrimmed amphiboles in continuous-phase dacite are consistent with rapid ascent from within the amphibole stability field. Following continuous dacite eruption, the mostly degassed "tail" of the basaltic andesite continued its ascent. Some basaltic andesite erupted effusively, but shallow inflation suggests that some accumulated in the shallow crust. Subsequent shallow deflation suggests that this basaltic andesite remobilized during the mid- March lava extrusion.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting