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Mechanisms of degassing: inducing magma vesiculation via impact

Klara Heinrigs , Jackie E. Kendrick, Anthony Lamur, Yan Lavallée

Abstract

Volatiles drive magma ascent and volcanic eruption style. As they exsolve from supersaturated melts during ascent, they form a porous network that may "fossilise" into the cold lava counterpart. Our understanding of gas overpressure as a driving force behind eruption dynamics directly relies on our interpretation of these remnant porous networks. Are those networks only capturing decompression driven vesiculation? Can other mechanisms also force magmas to foam and thus contribute to the final porous network geometry? For example, can external, rapid stress perturbations (such as those observed during earthquakes, or during impact of ballistic ejecta) also trigger vesiculation? We conduct a series of experiments on obsidian samples from Newberry Crater, Oregon and Long Valley Caldera, California, employing a newly designed drop tower apparatus that allows impact testing of material at high temperatures. The sample's thermal properties were analysed to constrain a temperature window above Tg but below their vesiculation temperature at atmospheric pressure. We then bring the samples to temperatures below their vesiculation temperature and impact them at different impact energies. We show that impacts serve as a possible mechanism to induce exsolution of volatiles from the melt by introducing kinetic energy to the system. We further quantify the effect of impact energy on intensity and behaviour of vesiculation. Our findings have implications on interpreting eruption trigger and dynamics as external, rapid stress perturbations are a viable mechanism for volatile exsolution at any level in the plumbing system but also in the plume and ballistics.