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Linking cloud microphysics and volcanic sedimentation

Linking cloud microphysics and volcanic sedimentation

Explosive volcanic eruptions, such as the 18 May 1980 eruption of Mount St. Helens, USA, can generate ash clouds that reach the stratosphere and deposit tephra 1000s km from the volcano. Predicting how long the particles remain airborne and where ash-fall may occur is an essential component of hazard mitigation performed by volcanic ash transport and dispersion models. The majority of these models calculate sedimentation rates based on gravitational settling of single particles and often have difficulty reproducing the observed sedimentation of the finest size fraction (<100 microns) 100-1000s km downwind from the volcano. This discrepancy results from particle aggregation, a poorly understood process, which is also responsible for forming distal mass deposition maxima observed 100s km from the volcano in many recent ash deposits.

Image as described adjacent

Mammatus clouds on the volcanic cloud generated by the 18 May 1980 eruption of Mount St. Helens, USA (image © Douglass Miller)

To stimulate advances in the understanding of volcanic cloud sedimentation, Research Associate Dr Adam Durant initiated the development of several sedimentological databases in collaboration with Dr. Bill Rose (Michigan Tech) for recent distal tephra fall deposits from the 18 May 1980 Mount St. Helens (USA) eruption, 14 October 1974 Fuego (Guatemala) eruption, and August and September 1992 Crater Peak (USA) eruptions. Particle size (using laser diffraction particle size analysis) and shape (using 2-D image analysis and scanning electron microscope stereology) and the distribution of fallout were compiled to generate the most comprehensive sedimentological characterisation of distal tephra fall deposits to date. Based on our laboratory studies of ice nucleation, combined with a review of previous studies detailing airborne measurements, meteorological observations of mammatus clouds and numerical modelling, we were able to interpret these data in a fresh context and propose a new model of volcanic sedimentation based on the formation of hydrometeors. This work promises to guide the development of a new generation of volcanic ash transport and dispersion models.