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Date of Award
Spring 2025
Document Type
Masters Thesis
Department or Program Affiliation
Geology
Degree Name
Master of Science (MS)
Department
Geology
First Advisor
Walowski, Kristina
Second Advisor
Koleszar, Alison M.
Third Advisor
Sas, Mai
Abstract
Changes in eruption style at a specific volcano, including variations within a single eruption, are driven by magma properties (e.g., volatile content, SiO2 content, and temperature), architecture of the subsurface magma reservoir, and conditions of magmatic ascent prior to eruption. Augustine Volcano is an active intermediate stratovolcano in the Alaska-Aleutian arc with frequent modern (< 200 ybp) and Holocene eruptions. The six most recent eruptions (including 2006, 1986, and 1976) produced significantly thinner tephra deposits containing smaller clasts than older eruptions, ~400-2,200 ypb, indicating older eruptions likely had much higher explosivity. This study compares the more explosive ~1,100 ybp Tephra C to the less explosive, most recent eruptions to constrain the mechanisms that drive changes in eruptive style. Componentry and grain size analysis of chronological samples from the Tephra C deposit indicate at least three distinct eruptive phases or paroxysms, and a mid-eruption magma mixing event. These three eruptive phases allow for assessment of changes through a single eruptive sequence, as well as between eruptions of differing explosivity. This study characterizes the pre-eruptive conditions of Tephra C by analyzing major element and volatile contents of pyroxene- and plagioclase-hosted melt inclusions and their host phenocrysts from seven samples through the Tephra C stratigraphy. Melt inclusions and host phenocryst compositions from Tephra C record the composition of a single high-silica andesite magmatic component, consistent with the dominant high-silica andesite pumice component. They contain 3–5 wt.% H2O and 200–600 ppm CO2, with minimal stratigraphic variation, suggesting relatively uniform pre-eruptive conditions. Volatile saturation pressures from melt inclusions indicate pre-eruptive magma storage at depths of ~4–9 km, consistent with volatile-saturated, closed-system behavior. Comparison of measured volatile contents to H2O contents determined by plagioclase-melt hygrometry indicates variations in decompression rate across eruptive phases, with more explosive units recording less decoupling between measured and plagioclase-melt hygrometer estimated H2O (∆H2O) and thus faster decompression. The presence of compositionally banded pumice supports the presence of at least one more mafic, albeit minor, magmatic component that is not recorded by the phenocrysts or melt inclusions analyzed in this study. Elevated Cl contents (>4,000 ppm) suggest possible crustal interaction with seawater-derived brines. Results demonstrate that more explosive behavior at Augustine is associated with simpler magmatic architecture, fewer magmatic components, deeper storage, and more rapid decompression, rather than higher initial volatile contents. These findings support a model in which ascent dynamics and storage configuration are key controls on eruptive style and provide an important reference point for interpreting Augustine’s eruptive history and forecasting future activity.
Type
Text
Keywords
volcano, petrology, geochemistry, melt inclusions, volatiles, Alaska, Augustine, eruption style, explosivity, Tephra C
Publisher
Western Washington University
OCLC Number
1523967291
Subject – LCSH
Explosive volcanic eruptions--Alaska--Augustine Volcano; Volcanic eruptions--Alaska--Augustine Volcano; Plagioclase--Inclusions; Pyroxene--Inclusions; Petrology--Alaska--Augustine Volcano; Geochemistry--Alaska--Augustine Volcano; Volcanism--Alaska--Augustine Volcano
Geographic Coverage
Augustine Volcano (Alaska)
Format
application/pdf
Genre/Form
masters theses
Language
English
Rights
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.
Recommended Citation
Brody, Saisha, "Unraveling Changes in Eruption Dynamics at Augustine Volcano, Alaska: Insights from Plagioclase- and Pyroxene-hosted Melt Inclusions" (2025). WWU Graduate School Collection. 1406.
https://cedar.wwu.edu/wwuet/1406