24 Undergraduate Research Posters

This poster session highlights geoscience research conducted and presented by undergraduate students. Abstracts must be written by students (but may include non-student co-authors). Research topics may include any area of the earth sciences.

Thursday, 23 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Timescales of Magma Ascent During Quaternary Effusive Eruptions from Mount Shasta Based on Amphibole Reaction Rim Thicknesses

Paper number: 24-4
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM

Authors

Abstract

Mount Shasta is one of the most voluminous volcanoes in the Cascades arc and represents a major hazard to proximal regional population centers and to the economically vital highways and railroads. This study aims to improve our understanding of Mount Shasta's volcanic hazards by constraining the timescales of magma ascent during several effusive eruptions. We measured thicknesses of amphibole phenocryst reaction rims in several Quaternary lavas and compared them to published results of experimentally-grown amphibole reaction rims produced through decompression experiments of lavas with similar whole-rock composition. Lavas sampled in this study include Haystack Butte, Sargents Ridge, Shastina, and Black Butte. Crystals in all amphibole decompression rims are composed of fine-grained and subhedral to anhedral plagioclase, orthopyroxene, clinopyroxene, magnetite, and ilmenite. We used plane-polarized light microscopy to measure widths of decompression rims, using high-intensity transmitted light to minimize shadows created by rim oxides. Scanning Electron Microscopy was also used to measure a smaller sample of decompression rims in the Black Butte sample due to its extremely thin rims. Using experimentally-produced amphibole reaction rims as a guide, lavas produced by Mount Shasta record multiple ascent durations, whereas one population may be consistent with an ascent timescale of days to weeks, another population may record an ascent duration of months. For example, Haystack Butte decompression rims record an ascent time of >30 days, but 43% of rims have an ascent time of <7 days. Sargents Ridge lavas display decompression rimmed-amphibole recording an ascent time of >30 days, but 18% of decompression rims recorded <13 days' ascent. Shastina lavas display deompression rims recording >30 day ascents, but 36% of Shastina rims record ~12 day ascents. Black Butte lavas are the only samples that show a unimodal population of decompression rims ranging from 7 to 37 um with an average of 29 um, correlating to an ascent time of only ~7 days. The presence of amphibole crystals with diverse decompression-induced reaction rim thicknesses in Mount Shasta lavas likely represent different ascent durations and styles due to complexities of the viscous flow within a conduit, including stop-start ascent, changes in ascent from the margin to the interior of conduits, and changes in mass flux. Future work may include examining whether these distributions are uniform or heterogenous throughout a single flow. 

 

 

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410845

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