THE AERODYNAMIC BEHAVIOR OF VOLCANIC AGGREGATES

A large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 mum in diameter. The majority of these particles become incorporated into a w ide range of aggregate types, the aerodynamic behaviour of which has n ot been determined by either direct observation or in the laboratory. In the absence of such data, theoretical models of fallout from volcan ic plumes make necessarily crude assumptions about aggregate densities and fall velocities. Larger volcanic ejecta often consists of pumice of lower than bulk density. Experimental data are presented for the fa ll velocities of porous aggregates and single particles, determined in systems analogous to that of ejecta falling from a volcanic plume. It is demonstrated that the fall of aggregates may be modelled in identi cal fashion to single particles by using a reduced aggregate density d ependent on the porosity, and a size corresponding to an enclosing sph ere. Particles incorporated into aggregates attain a substantially hig her fall velocity than single particles. This is due to the larger phy sical dimensions of the aggregate, which overcomes the effect of lower aggregate density. Additionally, the internal porosity of the aggrega te allows some flow of fluid through the aggregate and this results in a small increase in fall velocity. The increase in fall velocity of p articles incorporated into aggregates, rather than falling individuall y, results in the enhanced removal of fine material from volcanic plum es.