Shear Rate-Dependent Disequilibrium Rheology and Dynamics of Basalt Solidification

Magmas and lavas undergo a range of shear rates during transport and emplacement. Further, transport of magma and lava occurs at subliquidus conditions where the melt crystallizes at varying temperature, pressure, and oxygen fugacity. Transport efficiency and eruption style are governed by magma rheology, which evolves during cooling, crystallization and degassing. Quantification of magma rheology rests almost exclusively on experimentation at constant temperature and shear rate. We present the first study on the effect of shear rate on subliquidus basalt rheology at conditions relevant to lava flows and shallow magmatic systems. The results reveal that basalts reach their rheologic death or cutoff temperature (T_cutoff; i.e., the point at which the sample rheologically solidifies and flow stops) at higher temperatures when flowing faster, whereas crystallization is suppressed when the shear rate is low. We explore the implications of shear-enhanced crystallization for modeling and forecasting of lava flow hazards and our understanding of magma and lava transport/storage systems.