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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.

Plain Language Summary Natural magmatic processes span several orders of magnitude in deformation rate. The presented approach is the first to systematically document the resulting effects on the magma/lava transport properties. Our paper presents an experimental study on the shear rate dependence of the crystallization/solidification dynamics and rheology of basalts at experimental conditions that mimic the natural environment. These were made possible through application of a newly developed experimental apparatus and method. The measurements show that at a specific subliquidus temperature, the so called rheological cutoff temperature, the effective viscosity of basalts increases dramatically, leading to solidification of the melt. Systematic investigation of this rheological cutoff shows a drastic, previously unrecognized, dependence on shear rate, cooling rate, and composition. High deformation rates result in more intense crystallization at higher temperatures, whereas crystallization is suppressed to lower temperatures in the absence of deformation. The presented results open an entirely new field of studies in magma rheology and have important implications for the forecasting of natural hazards, lava flow emplacement, and the understanding of magma and lava storage and transport systems in general. We discuss the implications of the experimental results for lava flow emplacement, magma migration, and for the computational modeling of lava flows for hazard assessment.