Nature of Convective Instabilities in Explosive Volcanic Clouds Inferred by Analog Experiments

Understanding the mechanisms controlling the dynamics of a volcanic cloud generated by the rise and spread of an explosive eruption is a central issue in volcanology for the assessment of associated hazards. The last decades have seen the development of sophisticated numerical simulations and particle-tracking models with the aim of better understanding and forecasting the transport and sedimentation of the solid fraction in the cloud. In these models, the lateral spreading of an umbrella cloud is strongly influenced by stratospheric winds and its loss of mass with time is assumed be controlled by the opposing effects of particles settling and turbulent diffusion. However, recent observations suggest that additional spatially complex and time-dependent phenomena may govern the dynamics in a volcanic cloud. Here we investigate the mechanisms governing the lateral transport and residence time of ash in the atmosphere using analog experiments. In these experiments, a mixture of small particles and fresh water is injected upwards at a fixed rate into a chamber containing a salt water layer beneath a fresh water layer. Our results show that the formation of a thin particle-rich layer at the base of the cloud (a particle boundary layer) can dramatically modify its dynamics and lead to a variety of behaviors not detected previously. Depending on the conditions imposed at the source and on the magnitude of the density gradient in the environment, the cloud may either break up into discrete layers or release material as dense batches of particle-laden fluid. In natural eruptions the formation of this dense layer is found to be mainly controlled by the grain size distribution and to a lesser extent the altitude reached by the plume. An exhaustive review of field data available in the literature suggests that several past eruptions meet the required conditions to form a particle boundary layer. This study shows that large convective instabilities induced by the presence of a particle-rich layer may lead to the generation of a new type of hazardous flow emerging downward from a volcanic umbrella cloud.