Rheological behavior of partly crystallized silicate melts under variable shear rate

High‐temperature experiments were performed in order to shed new light on the shear‐rate controls on magma viscosity. We compare viscosity data from three different compositions: an andesite from the Calbuco volcano, a basalt from the Etna 122 BCE Plinian eruption and a synthetic pyroxenite with a composition similar to the Canadian Theo’s Flow. In addition to the determination of melt viscosity (at 1545–1715 K), we performed viscosity determinations at subliquidus conditions in partially crystallized materials, under controlled shear rates of 0.1 and 1 s−1 and at temperatures of 1483, 1493, and 1503 K for the Calbuco andesite, Etna basalt, and synthetic pyroxenite, respectively. The two different shear rates allow us to retrieve information about shear‐rate influences on viscosity of partly crystallized systems. A decrease in viscosity is observed with increase in shear rate. This behavior is known in the literature as the “shear thinning effect.” Our data show that changes of shear rates from 0.1 to 1 s−1 may cause a viscosity difference of 0.5 to one order of magnitude. This effect should be taken into account when considering magmatic processes occurring in volcanic conduits. The rheological properties of partly crystallized systems could drastically change depending on the dynamics of the magmatic system. © 2021 American Geophysical Union.