75-27 Interpreting Syn-eruptive Conduit Dynamics from Pumice Density of the W Tephra Sequence from Mount St. Helens.

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Determination of eruption dynamics of pre-historic volcanic eruptions requires inferences based on stratigraphic and dispersal characteristics of tephra deposits and on textures of individual pumice clasts. Pumice density is an easily measured property that reflects a portion of the volume of gas bubbles trapped in the melt at fragmentation. Changes in gas exsolution and accumulation directly affect the explosivity of an eruption, thereby providing information about inter-eruptive and syn-eruptive degassing from eruption sequences. Here, we analyze pumice density changes in an eruption sequence from the Mount St. Helens Kalama eruptive period, in the W tephra sequence (1479-1482 CE). We report data of three of the five Kalama age tephra-producing eruptions (Wn, We, Wa), which were separated by intervening dome building events, to understand potential changes in degassing and outgassing that led to eruption style changes. Wn and We are the most explosive eruptions of the sequence; Wn deposits were sampled from both the finer-grained base and a coarser-grained middle stratigraphic layer. The We deposits had the bottom few centimeters sampled to capture the opening phase and the middle layer collected to represent the sustained eruption. Wa represents a less explosive and less voluminous eruption and was sampled from a single layer. Pumice densities of tens of clasts from each sample were measured using Archimedes’ principle. Density distributions are broader in the base layer of both Wn and We (0.64-1.01 and 0.75-1.49 g/cm³, respectively) than from the more homogeneous and unimodal layer above (0.56-0.85 and 0.66-0.97 g/cm³, respectively). The density distribution of pumice from Wa is also wide (0.49-1.28 g/cm³). In general, average pumice densities correlate inversely with grain size, perhaps indicating clearing of the shallow conduit during low eruption intensity eruptions. In both the Wn and We, however, this phase of eruption transitioned to a more intense Plinian phase, perhaps tapping greater depths in the storage system. In contrast, the Wa eruption ended before transitioning to a Plinian phase. Future work will include larger sampling of W tephra, including the other two eruptions, and micro-CT scans of the pumice to identify bubble structures. This study provides foundational research into the smaller eruptions of the Kalama period to further contextualize the controls on eruption style in the Mount St. Helens system.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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