Abstract
The main parts of pyroclastic flow deposits show various characteristic features, such as density and size segregation of large fragments in the vertical direction. To understand the mechanisms that produce these features, we numerically simulated granular flows on an inclined rough surface with compositional impurities differing in size and density from matrix particles. The simulations show that, in flows with heterogeneous size and density distributions, the impurities rise or sink in the matrix according to their density. With increasing size of impurities and increasing density difference between impurities and matrix particles, the rate of rising or sinking of impurities increases. Observations of density and size segregation in agitated granular systems indicate that impurities are subject to buoyancy and granular viscous forces. The behavior of impurities in the central parts of flows obeys the laws governing the flow of viscous fluids.
