Abstract. We develop a model to describe the formation of aggregates in a volcanic eruption column. The model combines a description of the rate of collision and sticking of particles with a model of the vertical transport in the eruption column. We thereby determine the evolution of the grain size distribution as a function of height in the eruption column. We consider aggregates in which liquid water provides the binding agent. For sufficiently large eruption columns we find that this limits the vertical extent of the zone where 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 freezes. 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 owing to the reduction in the binding efficiency of water as particle size increases [Gilbert and Lane, 1994], there is a relatively narrow size distribution of aggregates at the plume to•. We also model the airfall deposits associated 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 gravitational settling and transport by ambient winds, may lead to enhanced deposition close to the source and in some cases secondary thickening of the deposit. The relatively near-source deposition of fine ash in these deposits is associated with the fallout of aggregates. As a simple application, we show that the present dynamical aggregation model is consistent with the secondary thickening of the deposit from the May 18, 1980, eruption of Mount St. Helens.
Abstract. We study the dynamics of sedimentation and reentrainment of particles from the umbrella cloud above an axisymmetric, turbulent, particle-laden buoyant plume. We develop a model to show that the reentrainment of particulate material into the uprising plume will cause the particle flux to increase by a factor of e between the plume source and the umbrella cloud. A buoyant plume rising in an environment of uniform density may thereby become negatively buoyant if its particle loading becomes suf[iciently high. We compare the predictions of the model to a series of laboratory experiments and show that at high particle loadings the plume undergoes an oscillatory collapse. This periodic collapse generates dense gravity flows down the flanks of the plume. In the context of volcanic eruptions, such an instability, associated with particle recycling may lead to the formation of interleaved fall and flow deposits, such as have been observed near the collapse horizon in a number of pyroclastic deposits.
We have developed a new theoretical model of an eruption column that accounts for the re-entrainment of particles as they fall out of the laterally spreading umbrella cloud. The model illustrates how the mass flux of particles in the plume may increase with height in the plume, by a factor as large as 2.5 because of this recycling. Three important consequences are that (1) the critical velocity required to generate a buoyant eruption column for a given mass flux increases, (2) the total height of rise of the column may decrease, and (3) we infer that in relatively wind-free environments, for eruption columns near the conditions for collapse, the recycling of particles may lead to an unsteady oscillating motion of the plume, which, in time, may lead to the formation of interleaved fall and flow deposits.
A radial temperature distribution is applied to the top of a cylinder of rotating stably stratified fluid. Thermal wind shear drives the interior flow. Linearized theory predicts, and laboratory experiments confirm, that when the stratification is large enough it completely suppresses the Ekman pumping into the interior. The interior velocity field, which is primarily azimuthal, responds by satisfying the no-slip boundary conditions without the need of Ekman layers on the horizontal surfaces. Moreover, for large stratification a thermal boundary layer beneath the top surface traps the thermal disturbance applied at the upper surface. The greatest azimuthal velocity occurs at the base of this layer. Below this layer the azimuthal velocity viscously diffuses downward with thermal wind adjusting the temperature. The Rossby radius of deformation based on this layer depth is the cylinder's radius divided by the square root of the Prandtl number. Detailed measurements of the velocity field generated in the cylinder by the heating are compared with the theory in the case where the Ekman layers are eliminated by stratification. The theory and experiments agree qualitatively well over a range of four orders of magnitude of imposed parameters and over a large parameter range the quantitative comparison is also very good.
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