Scrutinising Volcanic Grain Size Distributions – The Influence of Particle Shape, Density and Number

Abstract:

The grain size of explosive volcanic products (tephra) provides insight into fragmentation and ash transport processes. The size of volcanic ash also determines the risk to human health and the grain size distribution (GSD) is a crucial input parameter for ash dispersion models. Despite the importance of grain size and shape characterisation in volcanology, however, data compilation and comparison across different studies can be challenging because of the range of methods used to measure size. Here we present an analytical technique for quantifying the size and shape of volcanic tephra that uses the principle of dynamic image analysis (DIA; ISO 13322-2), which avoids many of the confines of other methodologies. Using the Camsizer X2, a particle analyser manufactured by Microtrac MRB (formerly Retsch Technology), we developed a protocol to quantify tephra size and shape from 0.8 µm - 8 mm. Advantages of this method include the computation of multiple measures of particle size, GSDs reported according to the volume percent in each grain size fraction, quantified particle number distributions, and the concurrent quantification of size and shape. We demonstrate the advantages of this approach using a range of synthetic (glass spheres) and natural samples (Mount Saint Helens 1980; Mt Mazama ~7 ka and Hverfjall Fires ~2.5 ka). Importantly, this approach provides an alternative to sieving, which indirectly sorts according to size and shape. We also use DIA (volume %) and sieve analyses (mass %) in parallel to provide an approximate measure of particle density across the size array. A comprehensive understanding of particle size, shape and density is critical for estimating particle settling velocities used for modelling ash dispersion. At the same time, concurrent measurements of particle size, shape and number provide important insights into the fragmentation process.