The Role of Convective Instabilities from Ash Clouds in Proximal Deposition of Fine Ash

Abstract:

Gravitational instabilities forming at the bottom of spreading volcanic clouds have been observed in many explosive eruptions. They can generate vertical fingers which have been observed to have higher concentrations than the associated plumes. Characterization of volcanic fingers only relies on qualitative observations and quantitative analysis of dedicated experiments.

For the first time we present a quantitative description of volcanic fingers developed during the Eyjafjallajökull 2010 eruption (Iceland) as a combination of video imaging and deposit observations. We conclude that the observed fingers on May 4^th, 2010 moved downwards at a speed of 1±0.5 m s^-1 and were advected laterally by the local wind at a similar speed as the associated cloud (e.g. about 8.5±0.8 m s^-1). They continuously formed at the base of the cloud from about 1.4 km from the vent, with an average width and spacing of 168±26 m and 180±60 m, respectively. We could also correlate the sedimentation of fingers with observations of various types of particles aggregates with calculated velocities ranging between 0.1 and 7 m/s. We conclude that the aggregates with the largest terminal velocities sedimented independently of the fingers, while smaller aggregates could form either in the cloud or in the fingers.

We have complemented our field data with dedicated experiments in order to investigate the evolution of particle concentration in the mixing region that results from propagation of turbulent convective instabilities. The experimental configuration was similar to that of previous studies with an aqueous suspension of water and spherical glass beads initially overlying a lower density sugar solution. Experiments demonstrate that once fingers are developed, they control sedimentation. In fact, particles are embedded in the fluid and cannot settle individually. This confirms that when settling driven instabilities is the dominating dynamics, particles are likely to settle faster following observed fingers and this enhances the settling of fine particles.

Our study shows that within an ash cloud, no single explanation for fine particle deposition close to source is supported by the observations, and that a range of distinctive origins is possible.