Granular mass flows of rock fragments are studied in the lab by means of a high-speed video camera at 2000 frames per second. These granular flows are generated using beds of pumice fragments positioned on a rough rotating disk, whose angular velocity is controlled by a motor. The experimental apparatus allows an understanding of the arrangement of the particles in granular mass flows with relatively small and relatively large values of the Savage number (the Savage number represents the ratio between grain collision stresses and gravitational grain contact stresses). In particular, these flows develop a basal layer of agitated and colliding particles underneath a relatively rigid upper layer. Our experimental results suggest the validity, on average, of the Coulomb's relationship between shear and normal forces at the base of granular mass flows irrespective of their Savage number value. In Coulomb's equation the shear stresses do not depend on the shear rate. We expect the Coulomb friction law to be valid also in moving pyroclastic flows. Our experiments suggest that the collisions and subsequent comminution of pumice fragments in moving pyroclastic flows could provide ash for the overriding ash clouds. In our experiments the amount of ash generated by particle-particle and particle-boundary interactions increases as the value of the Savage number increases. In nature, part of this ash may also simply move toward the base of the flows because of kinetic sieving.