QUANTIFYING THE RELATIONSHIP BETWEEN VOLUME AND POROSITY IN PUMICE AND THE IMPACT OF POST-FRAGMENTATION EXPANSION: A CASE STUDY FROM MEDICINE LAKE VOLCANO, CA

The amount, size, and shape, as well as the state of connectivity of gas bubbles during magma ascent, partly control whether magma will erupt explosively or effusively. If gas can efficiently escape the magma via permeable pathways, the magma will tend to erupt as a flow. If the gas remains trapped in the bubbles as melt viscosity increases, then bubble growth will be impeded, and bubble overpressure may build, causing fragmentation (i.e., an explosive eruption).

Characterizing magma porosity and bubble connectivity before fragmentation is therefore crucial for understanding eruption dynamics and parameterizing accurate models. This proves tricky, however, as volcanic pyroclasts can undergo post-fragmentation expansion, during which bubble growth, permeable outgassing, and quenching speed compete to determine final porosity.

To better inform tephra sampling and measurement procedures, we seek to elucidate the relationship between the porosity of a sample and its size using pyroclasts from the Plinian phase of the 1060 CE Glass Mountain eruption of Medicine Lake Volcano, CA. Currently, most porosity measurements are made on 1.6-to-3.2-cm-diameter clasts. Here we examine a broader range of pumice volumes, between ~0.3 cm³ and 50 cm³. For samples smaller than ~8 cm³, a new method to measure particle size using a particle size analyzer is presented. The volume of larger pumices is measured following a standard immersion method that relies on water displacement. Samples are analyzed using helium pycnometry to obtain the connected and total porosity.

Preliminary analyses on ~300 samples within a limited range of volumes show that total porosity generally increases from ~55% at ~0.9 cm³ to ~77% at ~16 cm³. This trend will be evaluated in the coming months. Results of this study will be used in a numerical model to determine to what extent an increase in porosity can be attributed to post-fragmentation expansion versus the pre-fragmentation characteristics of the magma.