Most particles less than a few hundred micrometers in diameter, which fall from volcanic plumes, do so as aggregates. Eye witness accounts confirm that aggregation occurs across the full range of climates on Earth. Aggregates are preserved in deposits of Precambrian to Holocene age. Despite the ubiquitous nature of aggregates their formation mechanisms are not well understood. Through interpretation of field observations, SEM images and field and laboratory experiments we review (1) likely mechanisms of particle aggregation and (2) morphological types of aggregates.
We envisage aggregation as a two stage process. Particles in plumes need first to collide and second to bind. The likelihood of particle collisions is controlled by particle concentration, particle fall velocities and electrostatic forces. Once particles are in contact, the likelihood that they will remain together during transport depends on the magnitudes of surface tension and electrostatic forces, and growth of secondary minerals and ice crystals which act to cement particles together. We show that liquids on particle surfaces play a key role in the morphologic type of aggregate formed. For this reason, we classify aggregates along a spectrum ranging from dry aggregate to accretionary lapillus to particle-laden liquid drop, reflecting the increasing role of liquid. Field and SEM observations of ash erupted during the earliest phase of the April 2010 summit eruption of Eyjafjallajökull show that aggregates are dominated by particle-laden liquid drops and accretionary lapilli proximal to the vent (in Iceland). Distal samples collected in the UK are non-spherical, well cemented, low porosity aggregates.