Dynamic rock fragmentation causes low rock-on-rock friction

The low frictional resistance to rock-on-rock sliding reported in large blockslides, in coseismic fault rupture and in laboratory-scale rock friction tests has been attributed to a variety of causes. Herein we propose a mechanical explanation for the reduced friction, which seems likely to be universally relevant to complement other mechanisms. Rock-on-rock sliding of intact brittle rocks always generates a layer of comminuted debris. Rock must fail in order to form and further comminute debris; at local strain rates >>100 s(-1), recycling of elastic strain energy stored in accomplishing fragmentation generates instantaneous, local, GPa-range isotropic pressures similar to the rock's Hugoniot elastic limit (Q). Under rapid strain, simultaneously fragmenting grains deliver large normal forces to the boundaries of the comminuting layer, reducing the confining stress on the debris (and hence its resistance to shear), thus lowering the frictional resistance to slip. This behaviour corresponds quantitatively to published laboratory data on granite friction; to the dynamics of low-angle blocksliding and faulting; and to our data on rapid shearing of fragmenting dry coal.