Constraining the effect of clast density within natural granular flows

Eliza Calder (e.s.calder@open.ac.uk)

(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.