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Date of Award

Summer 2023

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Walowski, Kristina

Second Advisor

DeBari, Susan M., 1962-

Third Advisor

Koleszar, Alison M.

Fourth Advisor

Sas, Mai

Abstract

The eruption behavior of a volcano is intrinsically linked to magmatic evolution. Augustine Volcano is an active stratovolcano in the Aleutian Arc (AK, USA), showing stratigraphic evidence that eruptions before 1,800 C.E. were more explosive, producing thick pumice deposits, compared to the six eruptions observed in the last 200 years. Tephra B, an understudied ~400-year-old pumice fall unit, represents the last more explosive style eruption at Augustine - making it ideal for studying how changes in magmatic evolution can result in changes in eruption style. For this thesis, I used textural zoning patterns and chemical variations (major and trace elements) in plagioclase and pyroxene mineral populations to identify three chemically distinct magmas: two with felsic-intermediate compositions and one with a mafic composition. Population 1 (bright core, dark oscillating rim) and Population 2 (dark core, bright mantle, dark oscillating rim) plagioclase show an increase in An% with no increase in FeO wt.% and comprise >60% of all plagioclase populations, indicating thermal mixing in a single, dominant magma reservoir. Population 3 (dark oscillatory) and Population 4 (sieved) plagioclase increase in both An% and FeO wt.%, indicating the compositional mixing of more than one magma. Further trace element analysis reveals these plagioclase populations likely grew from a subordinate felsic-intermediate magma reservoir. Interaction with a mafic magma is evident by the banded pumice component, presence of mafic minerals, abundance of reversely zoned pyroxenes, and elevated FeO and MgO wt.% in some plagioclase rims. Only a subset of plagioclase has elevated rims suggesting that mixing or mingling of the mafic component was perhaps minor, localized, or incomplete. Compared to the most recent Augustine eruptions, magma mixing is explained by a system of interconnected sills and dikes controlling eruption behavior. Tephra B may be more explosive due to a longer-lived dominant reservoir producing more homogenously mixed felsic-intermediate magmas.

Type

Text

Keywords

Augustine, magma mixing, arc volcanoes, intermediate magma, thermal mixing, volcano, petrology, igneous

Publisher

Western Washington University

OCLC Number

1393192406

Subject – LCSH

Magmatism--Alaska--Augustine Volcano; Volcanology--Alaska--Augustine Volcano; Petrology--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.

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