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Deconvolving the relationship between subtle shifts in geochemistry and dramatic changes in explosivity at Augustine Volcano, Alaska

Alison Koleszar 1, Jessie Farrell1, Kristina Walowski2, Jessica Zehner3, Matthew Loewen4

  • Affiliations: 1Department of Earth and Environmental Geosciences, Colgate University, Hamilton NY, USA; 2Geology Department, Western Washington University, Bellingham WA, USA; 3Department of Earth Sciences, Montana State University, Bozeman MT, USA; 4USGS-Alaska Volcano Observatory, Anchorage AK, USA

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 36

  • Programme No: 1.7.23

  • Theme 1 > Session 7

Abstract

Augustine Volcano is the most historically active volcano in Alaska's Cook Inlet region, with seven confirmed eruptions since 1883. Underlying these modern ash deposits are thicker, coarser tephras that indicate significantly greater explosivity as recently as 390 ybp. To fill gaps in the eruptive history at Augustine, and by extension, address the factors that control eruptive style at intermediate arc volcanoes, we present a chemostratigraphic study of Tephras B, M, and C, the most explosive eruptions at Augustine in the past 1200 ybp. These eruptions produced lapilli of high-silica andesite, low-silica andesite, and banded pumice comprised of mingled compositions and/or textures. The higher-explosivity eruptions of Tephras B, M, and C contain a higher proportion of high-silica andesite (35-93%) compared to the explosive phase of the 2006 eruption (26-52%; Wallace et al., 2010). We find subtle differences in geochemistry between these eruptions, particularly within the low-silica andesite component, suggesting different mafic parent magmas in the explosive eruptions that produced Tephras B, M, and C. Some, but not all, increases in explosivity within a single eruption (evidenced by shifts to greater median grain size) are associated with a decrease in the relative proportion of high-silica andesite, an increase in the abundance of banded pumice, and subtle shifts in the composition of the high-silica andesite. We generate a chemostratigraphic record for these large explosive late Holocene eruptions to explore concurrent changes in eruption style and geochemistry and compare these results to the well-constrained, lower explosivity eruptions of the past 200 years.