Experimental constraints on volcanic ash generation and clast morphometrics in pyroclastic density currents and granular flows

Abstract

Pyroclastic density currents (PDCs) are a prominent hazard of volcanic activity; however, fully quantitative observations are lacking and little direct evidence exists to constrain the parameters controlling ash production and runout. Here, we use rotary tumbling experiments to investigate ash generation efficiency and clast morphometrics in the dense basal flow of PDCs. We observe greater ash generation with periodic ash removal and with higher starting mass. By scaling to the bed height and clast diameter we obtain a general description for ash production in all experiments as a function of flow distance that we parameterise in dimensionless space. We also show that ash production correlates with clast shape changes and with the Inertial number for our experiments. This work introduces some of the first systematic and generalizable experimental parameterizations of ash production and clast evolution in PDCs and should advance the ability to understand flow mobility and associated hazards.