Measuring the size of non-spherical particles and the implications for grain size analysis in volcanology

Abstract To quantify the size of tephra, two practical challenges must be addressed: wide range particle sizes (10−8 - 101 m) and diversity shape, density optical properties. Here we use dynamic image analysis (DIA) to simultaneously characterise shape tephra samples from Mount Mazama, Krafla, St. Helens Campi Flegrei. The Camsizer X2 instrument used in this study, which has a measurement 0.8 μm – 8 mm, avoids need overlap different methods principles for fine ( 125 μm) sizes. Importantly, DIA does not require an assumption also allows grain distributions (GSDs) using multiple definitions. Quantification by long axis area equivalent sphere diameter, example, make GSDs compatible with outputs other such as laser diffraction sieving. Parallel mass-based (sieving) volume-based highlight effects variations on analysis; concentrations dense crystals within narrow range, particular, can affect their interpretations. We show that important effect apparent distal tephra. Extreme shapes, platy glass shards typical Campanian Ignimbrite deposits, appear coarser than tephras if is quantified according axis. These results have implications ash dispersion models, where input assume reported measurements are volume-equivalent diameters. conclude only suitable characterising, simultaneously, particles but provide new insights into properties useful both modelling studies explosive volcanism.