Constraining the effect of clast density within natural granular flows Eliza Calder (firstname.lastname@example.org) (max 200 words) Examples are presented where the presence of low density pumice and high density lithic clasts determine differential behaviour a) within the same pyroclastic flow and b) in sequential pyroclastic flows travelling down the same drainage system. These examples are drawn from the 1993 pyroclastic flows at Lascar volcano, Chile and the 1996-1999 block-and-ash and pumice flows from Soufriere Hills Volcano, Montserrat. A conceptual model for the derivation of pumice-rich and lithic-rich facies variations of the Lascar pyroclastic flows is presented. Density segregation occurred progressively within the flow as it travelled downslope. Lithic rich portions, with a higher internal coefficient of friction, deposited on relatively steep slopes (14-6°). Pumice-rich portions of the flow are mobile in comparison, and are not deposited until the flow reaches slopes of < 5°. On Montserrat, block-and-ash flows derived from dome collapse and pumice flows generated during vulcanian explosions displayed contrasting mobilities as indicated by relationships between runout distances, areas inundated, and flow velocity. The pumice flows, are substantially more mobile and clast density is considered to be one of the factors responsible. Measured modal densities for > 3cm clasts are 2300 kg m-3 for block-and-ash deposits and between 900-1500 kg m-3 for pumice flow deposits.