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

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