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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

DeBari, Susan M., 1962-

Second Advisor

Walowski, Kristina

Third Advisor

Shamloo, Hannah I.

Abstract

Mount Baker is a very high-threat Cascade stratovolcano in Northwestern Washington. It last experienced an explosive magmatic eruption ~6.7 ka, producing the trachyandesitic Black Ash (BA) tephra from Sherman Crater (0.2 km3, VEI 4). Despite this threat assessment, no previous petrological studies have focused on the pre-eruptive magma storage conditions and eruption initiation mechanisms for this latest eruption. The BA tephra presents a unique opportunity to investigate the most recently recorded subvolcanic magmatic architecture beneath Mount Baker, as well as the mechanisms responsible for explosiveness and initiation. Here, I present new bulk tephra and mineral geochemical data to constrain the pre-eruptive plumbing system and provide context for eruption. The BA tephra bulk composition is a trachyandesite with 60.1-62.6 wt.% SiO2 and 2.5-3.9 wt.% MgO. The matrix glass contains an average of 68.6 wt.% SiO2. Bulk tephra compositions suggest that the BA tephra was sourced from a combination of magma series, which include high-Mg andesite/high-Mg basaltic andesite (HMA/HMBA) and alkaline series components. HMA/HMBA series magmas are typical of edifice eruptions for the last ~100 ka, whereas the ~9.8 ka eruption from the flank was fed by a low-Mg calc-alkaline series magma, suggesting that edifice and flank eruptions have two different, yet adjacent, sources. Crystals show a limited range of geochemistry (clinopyroxene Mg# 67-81, orthopyroxene Mg# 62-74, and plagioclase An 30-53), with limited zoning patterns and rare disequilibrium features. Based on core Mg# and An, the crystals can be grouped into three co-crystallizing mineral assemblages that crystallized from andesite to evolving dacite liquids (andesite: A, dacite 1 : D1, and dacite 2: D2). These assemblages crystallized at less than 15 km depth at ~6.7 ka, clustering around ~7-9 km depth. The assemblages were crystallized or stored at relatively hot temperatures, including an average of 1059°C for the A assemblage, 1003°C for the D1 assemblage, and 977°C for the D2 assemblage. The limited range of mineral geochemistry and homogeneous matrix glass suggest relatively simple pre-eruptive processes, involving the extraction of a silicic trachyandesitic interstitial melt from a deeper more mafic mush storage region. While the initiation of melt extraction is unclear, the melt migrated upwards due to one of three possible mechanisms (buoyant forces, magma chamber overpressure, or thermal underplating). The interstitial melt progressively crystallized each assemblage and ultimately erupted explosively only because of external magma-water interactions.

Type

Text

Keywords

Geochemistry, Igneous Petrology, Volcanology, Crystal Cargo, Magmatic Storage, Eruption Initiation, Cascade Volcanic Arc, Mount Baker

Publisher

Western Washington University

OCLC Number

1522983945

Subject – LCSH

Geochemistry--Washington (State)--Baker, Mount; Igneous rocks--Washington (State)--Baker, Mount; Volcanology--Washington (State)--Baker, Mount; Magmatism--Washington (State)--Baker, Mount; Magmas--Washington (State)--Baker, Mount

Geographic Coverage

Baker, Mount (Wash.)

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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