Rock avalanches are extreme and destructive mass movements in which large volumes of rock (typically >1 million cubic metres) travel at high speeds covering large distances, the occurrence of which is highly unpredictable. These avalanches cover a far larger distance horizontally than the initial fall height thus the distal part of the deposit travels further than would normally be expected for a coherent mass sliding downslope. In a simple conceptual model, large scale rock avalanches can be considered to have two key phases that may contribute to the high mobility of the avalanche; a phase that involves the downslope acceleration of the material from the source and arunout phase. In this model, the transition between the two phases occurs at the change in topography from the steep mountainside to the near-horizontal valley floor. These phases are numerically modelled using PFC3D as a high rate of normal compression during the slope transition and high velocity shear during runout. The roles of normal and shear loading within a rock avalanche are examined using breakable agglomerates within a matrix of fines. Results suggest that, in spite of propagating rock avalanches possessing high shear rates, particularly near the base, high strain rate normal loading is of greater importance to the production of high mobility.