Effects of thermal quenching on mechanical properties of pyroclasts

Contact with water can promote magma fragmentation. Obsidian chips and glass spheres typically crack when quenched. Vesicular pyroclasts are made of glass, so thermal quenching by water may damage them. If water enters eruption columns, or if pyroclastic density currents interact with water, hot pumice can be quenched. We performed a set of experiments on air fall pumice from Medicine Lake, California. We made quenched samples by heating natural clasts to 600 °C, quenching them in water at 21 °C, drying them at 105 °C, and then cooling them to room temperature. We compare these samples with untreated air fall pumice from the same deposit, hereafter referred to as regular pumice. We tested whether quenched pumice would 1) shatter more easily in collisions and 2) abrade faster. We also tested whether individual clasts lose mass upon quenching, and whether they increase in effective wet density, two measurements which may help characterize the magnitude of clast damage during quenching. We also compare pre-quenching and post-quenching textures using X-ray microtomography (μXRT) images. Results from collision experiments show no clear difference between quenched pumice and regular pumice. Quenched pumice abraded faster than regular pumice. On average 0.3% of mass may have been lost during quenching. Effective wet density increased 1.5% on average, as measured after 5 minutes of immersion in water. Overall, we find modest differences between quenched pumice and regular pumice in experiments and measurements. The experimental results imply that quenching may damage small parts of a clast but tends not to cause cracks that propagate easily through the clast. Post-quenching μXRT imaging shows no obvious change in clast texture. This is in stark contrast to non-vesicular glass that develops large cracks on quenching. We present four factors that explain why pumice is resistant to damage from thermal quenching: thin glass films experience lower transient thermal stresses, many internal surfaces are initially vapor cooled, vesicles can arrest cracks, and cracks from thermal quenching may not occur in the locations most susceptible to fracture in later collisions.