Rock avalanches are uncommon, large, catastrophic events in which granular masses of rock flow at high speeds, commonly with unusually long runouts. Necessary conditions for rock avalanche occurrence include high relief, steep slopes, and a prefractured (granular) groundmass. An additional parameter, antecedent shear of the granular mass in the direction of failure, may be prerequisite. In many instances the basement mass of the eventual landslide underwent internal strain subparallel to the direction of mass wasting, commonly focused along a basal surface (e.g., fault). New experimental data from laboratory experiments indicate that shear of a granular mass produces subtle anisotropy in grain arrangements that strongly affects the subsequent distribution of deformation and rearrangement, the failure threshold, and the rate at which steady state conditions are reached. This phenomenon may help to explain the spatial clustering of rock avalanche deposits. If memory of prior shear is also relevant on larger scales, taking deformation history into account could greatly enhance prediction of rock avalanche occurrence.
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