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Investigating the evolution of grain shape during transport inside various small-volume pyroclastic density currents over complex topographies

Ashley Paschall, Aurelie Germa, Sylvain J. Charbonnier

Abstract

Small-volume (< 0.5 km3) pyroclastic density currents (PDCs) occur frequently and pose severe threats to populations and infrastructures surrounding active volcanoes. They are responsible for the two deadliest eruptions of the 21st century (Merapi 2010, Fuego 2018). Regardless of the generation mechanism, the complex topography of active volcanoes largely controls the dynamics, mobility and impacts of small-volume PDCs. A first compilation of grain shape parameters obtained from three well-constrained major PDC events (Merapi 2010, Calbuco 2015, Colima 2015) combined with detailed pre-, syn- and post-eruption data allows us to better understand the influence of varying topographic parameters on internal flow dynamics. Using image analysis with a Keyence VHX-7000 digital microscope, we observed ~5000 grains (of 250-300 microns size) per sample to measure circularity, roundness, solidity, convexity, and aspect ratio. Combining the data sets from the three targeted eruptions led to the identification of quantifiable relationships between grain shapes, deposit footprint and surficial/sedimentological features, retaining capacity of the receiving landscape, and scale-dependent tendencies for PDCs to overspill confined channels. For example, grains from valley-confined deposits are less circular and less convex than grains from overbank deposits of the same PDC unit, the latter also showing increases in all parameters with distance. Moreover, each type of PDC deposit yields grain shape parameters that plot within specific ranges of values, which are narrower for unconfined PDC units. These findings aim to expand on previous classifications of volcanic ash morphology to include data from small-volume PDCs to broaden the spectrum of explosive processes considered.