DIVERSITY OF PETROLOGY AND ERUPTION INITIATION MECHANISMS BETWEEN MT. SHASTA’S MISERY HILL AND HOTLUM CONE-BUILDING EPISODES

A variety of research suggests that past magmatic and eruptive behavior has predictive power to better understand how volcanic systems may behave in the future. Within the Pacific Northwest, Mt. Shasta is the most voluminous stratocone of the Cascade Volcanic Arc and is ranked the 5^th highest threat volcano in the United States by the USGS. It has experienced four distinct cone-building episodes since 250 ka; these include the Sargents Ridge (250-110 ka), Misery Hill (80-10 ka), Shastina (10-9.4 ka), and Hotlum (6-2 ka) stages. These cone-building events are primarily andesitic in composition, although more evolved dacitic lavas are present. Additionally, small quantities of rhyodacite, primitive magnesian andesite, basaltic andesite, and high-alumina olivine tholeiite have been found along the flanks of the stratocones and other peripheral eruption sites. This compositional variability of Mt. Shasta lavas, as well as the high H₂O content of input magmas, implies a highly complex and dynamic volcanic system.

In this study, we characterize the diversity of mineralogy, geochemistry, parental magma traits, and eruption initiation mechanisms between the Misery Hill and Hotlum eruptive units at Mt. Shasta in order to understand their magmatic origins and the frequency of particular processes preceding their eruptions, which may provide insights into what is likely to instigate future eruptions. Specifically, we utilize petrographic and geochemical analysis of samples from three Misery Hill lava flows and two Hotlum lava flows to determine and compare the relative importance of crystallization, magma mixing, crustal melting, and different eruption mechanisms in their origins. Our preliminary results suggest that magma mixing is ubiquitous in Hotlum and Misery Hill age lavas examined so far and is the likely culprit for initiating these eruptions, based on the presence of glomerocrysts, reversely zoned plagioclase crystals, quenched magmatic inclusions, and zoned phenocrysts of clinopyroxene, hornblende, and biotite. Future work will assemble a comprehensive petrographic analysis of our samples and propose a model for the general sequence of magmatic events which led to the Misery Hill and Hotlum cone-building episodes.