Particle aggregation in volcanic eruption columns

We develop a model to describe the formation of aggregates in a volcanic er uption column. The model combines a description of the rate of collision an d sticking of particles with a model of the vertical transport in the erupt ion column. We thereby determine the evolution of the grain size distributi on as a function of height in the eruption column. We consider aggregates i n which liquid water provides the binding agent. For sufficiently large eru ption columns we find that this limits the vertical extent of the zone wher e aggregates may form since near the source, all water is in the vapor form , while in the upper part of the column the mixture becomes very cold and f reezes. However, in many cases, we find that the particles spend sufficient time in the central region, where the water is in the liquid form, that a substantial amount of aggregation occurs. Furthermore, we predict that owin g to the reduction in the binding efficiency of water as particle size incr eases [Gilbert and Lane, 1994], there is a relatively narrow size distribut ion of aggregates at the plume top. We also model the airfall deposits asso ciated with this aggregate-rich distribution of particles which is injected to the top of the eruption column. We show that the relatively narrow size distribution of particles at the top of the column, coupled with the gravi tational settling and transport by ambient winds, may lead to enhanced depo sition close to the source and in some cases secondary thickening of the de posit. The relatively near-source deposition of fine ash in these deposits is associated with the fallout of aggregates. As a simple application, we s how that the present dynamical aggregation model is consistent with the sec ondary thickening of the deposit from the May 18, 1980, eruption of Mount S t. Helens.