Chapter 3 - Ash Aggregation in Volcanic Clouds

Abstract

Particle aggregation governs the sedimentation of small volcanic tephra particles from ash clouds generated by explosive volcanic eruptions. Particle aggregates have larger sizes, lower densities, and higher terminal fall velocities than individual ash particles. Critically, this process reduces the atmospheric lifetime of volcanic ash clouds. Aggregation increases sedimentation rates in proximal regions and correspondingly decreases airborne ash mass loadings in distal regions. Aggregation processes and products vary with distance from a volcano. Proximal aggregates are larger (up to centimeter size), may have concentric internal structures, may be strongly bound (cemented and/or frozen), and can be associated with pyroclastic density currents. Distal aggregates are smaller (submillimeter), less well structured, weakly bound, and are deposited exclusively from the base of ash clouds. Bonding forces for individual ash particles include hydrostatic bonds (liquid and ice water) and electrostatic forces, and the availability of liquid determines the aggregate morphology. Gravitational convective instabilities observed in recent eruption clouds influence aggregation rates. Recently, much progress has been made in documenting and understanding aggregation through field studies, laboratory experiments, and numerical modeling. However, there remains uncertainty over the roles of eruption source parameters (eg, particle size distribution, erupted mass, eruption column height, cloud liquid content, and temperature), the eruption plume temperature lapse rate, and environmental parameters in determining the nature and distribution of aggregates. This chapter will provide an overview of aggregates observed falling out of recent volcanic clouds, aggregates found in deposits, and key formation mechanisms.