HR: 0800h
AN: V31B-0488    [Abstracts]
TI: Bezymianny and Shiveluch Volcanoes, Kamchatka, Russia and Mount St. Helens, WA: Response of Volcanoes That Have Experienced Edifice Collapse
AU: * Shipman, J S
EM: jshipman@giseis.alaska.edu
AF: AVO, University of Alaska, Fairbanks, AK 99775, United States
AU: Pallister, J
EM: jpallist@usgs.gov
AF: USGS, Cascades Volcano Obs., Vancouver, WA 98683, United States
AU: Gorbach, N
EM: n_gorbach@mail.ru
AF: Inst. of Volcanology and Seismology, RAS, P.K., 683006, Russian Federation
AU: Belousova, M
EM: belousov@mail.ru
AF: Inst. of Volcanology and Seismology, RAS, P.K., 683006, Russian Federation
AU: Gavrilenko, M
EM: max_gavrilenko@mail.ru
AF: Inst. of Volcanology and Seismology, RAS, P.K., 683006, Russian Federation
AU: Krivomazova, O
EM: Krivomazova_o.v@mail.ru
AF: Inst. of Volcanology and Seismology, RAS, P.K., 683006, Russian Federation
AU: Izbekov, P
EM: pavel@gi.alaska.edu
AF: AVO, University of Alaska, Fairbanks, AK 99775, United States
AU: Eichelberger, J
EM: jeichelberger@usgs.gov
AF: USGS, Volcano Hazards Program, Reston, VA 20192, United States
AB: Comparison of volcanic sequences characterized by shallow intrusion, edifice collapse, and paroxysmal eruption, subsequent dome-building and intermittent explosive activity enables a broader analysis of the factors that control magmatic systems at arc volcanoes. Three historic eruptions displaying such a sequence of events are Bezymianny Volcano, Kamchatka, 1956 Shiveluch Volcano, Kamchatka, 1964 and Mount St. Helens, WA, 1980. Here we present field observations and results of petrologic analyses to investigate the eruption response by volcanoes which have experienced edifice collapse. On December 24, 2006 Bezymianny experienced a large eruption producing pyroclastic flows, and lahars, followed by another eruption on May 11, 2007. In August, 2007 we observed that a large portion of the dome was removed and a new lava flow was visible at the summit. Bulk chemical trends from 1956 to 2006 show a trend toward more mafic compositions (61 to 57 wt.% SiO₂). Observed increases in matrix crystallinity may be attributed to slower ascent rates. Microprobe analyses of plagioclases show different textural classes and multiple heating events. Oxide thermometry shows an increase in temperature from 800 to 950^oC. These factors indicate a mafic source rejuvenating the system. On February 27, 2005 Shiveluch volcano experienced its largest eruption since 1964. Currently, Shiveluch is in a dome-building phase with associated explosive events, rock falls, and pyroclastic flows. As of July 2007, the dome has almost completely been rebuilt, indicating a rapid extrusion rate. From 1964 to 2005, Shiveluch exhibits a bulk chemical trend toward more silicic compositions (59 to 64 wt.% SiO₂) and a decrease in crystallinity. An increase in the variability of plagioclase composition and zoning in recent deposits suggests reheating. Injection of mafic magma may be reheating the base of the silicic chamber and producing the changes in crystallinity, leading to intensification of Shiveluch activity. Mount St. Helens returned to unrest on September 23, 2004 with small explosions and dome extrusion that reached a volume of 91.7x10⁶ m³ by April 20, 2007. The 2004 dacite is geochemically uniform (65 wt.% SiO₂) and phenocryst-rich, with similar major-element composition to the most evolved 1980-1986 dacite, but with trace-element characteristics that suggest tapping a region of the 5 to 15-km-deep reservoir. Although there is no direct evidence of ongoing mafic recharge in the current eruption, past eruptive episodes indicate such recharge. In addition, larger deformations than expected for the current erupted volume and the 3- year long low-rate eruption suggests ongoing recharge at depth. An intriguing possibility is unloading by sector collapse results in changes in the deep levels of magmatic systems and that these changes require decades to be seen in eruptive products at the surface.
UR: http://fairweather.gps.alaska.edu/PIRE
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting