Experimental investigation of volcanic particle aggregation in the absence of a liquid phase

[1] Understanding the dispersal and deposition of fine-grained silicate particles from volcanic plumes is key to interpreting ash fall deposits and predicting hazards for future eruptions. It is known that many of these particles fall incorporated into delicate, dry aggregates whose sedimentation characteristics have not been previously investigated. Here we present the results of laboratory experiments on aggregates of small, dry silicate particles produced by the fragmentation of pumice collected from the 18 May 1980 Mount St. Helens fall deposit. The aggregation process is driven by electrostatic charges naturally imparted to the particles during the fracture process. For particle fall distances of similar to1 m, images of the in-flight aggregates show that they commonly have irregular shapes and are up to similar to800 mum in size. Strobe photography was used to determine aggregate fall velocities and, by representing aggregates as falling spheres, suggested that they had densities of similar to100-200 kg m(-3). Comparable densities were obtained from experiments where equivalent fall velocities were assumed for aggregates and single particles which had been transported similar distances within a horizontal airflow. These dispersal experiments produced bimodal particle size distributions, similar to those observed in the 18 May 1980 Mount St. Helens deposits, and suggest that the aggregates were composed mainly of particles <70 mu m in diameter. Our experimental results are in agreement with aggregate size and density estimates previously used within several theoretical plume sedimentation models in order to explain some features of natural ash deposits.