A fragmentation-spreading model for long-runout rock avalanches

Based on the observation that deposits of large rock avalanches consist pre dominantly of intensely fragmented rock debris, it is proposed that the pro cesses of rock fragmentation are significant causes of the peculiar distrib ution of mass in these deposits, and of the correspondingly long runout. Ro ck fragmentation produces high-velocity fragments moving in all directions, resulting in an isotropic dispersive stress within the translating rock ma ss. A longitudinal dispersive force consequently acts in the direction of r educing mass depth and tends to cause the rear part of the avalanche to dec elerate and halt and the front part to accelerate. The result is greater lo ngitudinal spreading of the travelling mass compared with nonfragmenting gr anular avalanches. The longer runout results from this additional fragmenta tion-induced spreading.