Drop it like it’s hot - viscoelastic deformation of rhyolitic melt at high strain rates

Deformation of magmas and hot rocks occurs at a range of strain rates in natural systems including rapid deformation as magma is sheared against the conduit wall upon ascent or during collapse of parts of the volcanic edifice. The initiation of cracks and fractures in magma is crucial to the development of permeable pathways through which volatiles may degas and alleviate overpressure in the system. Yet, experimental data on the deformation of hot magmas at high strain rates remains sparse, with the majority of tests conducted at strain rates on the order of 10^-5 s^-1. Using a drop tower equipped with a furnace, we subject high temperature (890 – 950 °C) rhyolitic obsidians to high strain rate impacts (10⁰-10² s^-1) at various impact energies.  Our results indicate a strong effect of both temperature and strain rate on the peak stress recorded in the melts. Despite being far above their glass transition temperature (717 °C), the samples all deform in a brittle manner, owing to the ratio between relaxation to observation timescales which is expressed as the dimensionless Deborah number (De). At colder temperatures (890 – 930 °C), samples behave predominantly elastic-brittle whereas at higher temperature (950 °C) the increasing viscous component of deformation weakens the melt, causing lower peak stresses and more comprehensive fragmentation. Our findings provide insights into how changes in temperature, energy and strain rate affect the rupture behaviour of melts, thereby improving our understanding of dynamic magmatic processes such as magma-conduit interaction upon magma ascent.