Experimental and Textural Constraints on the Eruptive Behavior of Basic Magmas in Subduction Zones

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GardnerEAR-0229290Eruptions of basaltic and basaltic andesitic (basic) magma in subduction zones can vary in style between strombolian to intense sub-Plinian (and rarely Plinian). Why can subduction-zone basic magmas erupt in such dramatically different styles, as compared to Hawaiian eruptions? Is it because subduction-related magmas contain greater amounts of gas, or are their ascent paths or ascent rates more conducive to explosive eruptions? This study seeks to answer those questions by investigating the influences that gas content, storage, ascent, and degassing have on the eruptive style of basic magma in subduction zones. The investigators' study focuses on three key eruptions of basic magma from the Aleutian volcanic arc: Shishaldin volcano in 1999, Okmok volcano in 1997, and Westdahl volcano in 1991-92. Those eruptions are ideal for this study because they varied greatly in eruptive style (strombolian, fire fountain, and sub-Plinian), including during each eruption. In addition, all three eruptions were monitored with seismic and geodetic measurements, which place important constraints on magma motion and eruptive character. The investigators propose to examine influences on eruptive style in three related tasks. Task 1 will define textural differences (both crystals and vesicles) between the three eruptions and between different phases of the eruptions. Task 2 will define pre-eruptive gas contents and pre-eruptive storage conditions of the three magmas, with a combination of glass inclusion measurements and experimental petrologic work. Task 3 will define the relationship between decompression (ascent) rate and textural differences in the resulting products, through a series of experimental petrologic work. Results from the three tasks, combined with the existing geophysical constraints on the eruptions, will allow the investigation of the gas contents, storage, ascent, degassing, and eruption of basic magma in the Aleutians. Experiments will be performed in the Experimental Petrology Laboratory at the University of Alaska Fairbanks, and all products will be analyzed in the Advanced Instrumentation Laboratory at the University of Alaska Fairbanks.