Frictional Melting Processes in Planetary Materials: From Hypervelocity Impact to Earthquakes

Frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip. Within planetary materials, which are mainly natural ceramics, frictional melting occurs at high strain rates (typically >10⁻² s⁻¹) and at slip velocities greater than 0.1 m s⁻¹. The pathway to friction melting is controlled by the mechanical properties of a rock's constituent minerals, especially fracture toughness. Minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt. The product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix. This cools to form the rock type known as pseudotachylyte, and at even higher strain rates, it forms shock veins in meteorites and in impact craters, which may contain high-pressure mineral polymorphs. The generation of melt on sliding surfaces can lubricate earthquake faults, facilitate the post-shock modification of impact craters, and make landslides more hazardous.