The in-flight cooling of ballistic pyroclasts during mafic explosive eruptions : a numerical model

Ogbuagu, C.C. and Lin, K.C. and Jones, T.J. and De Angelis, S. (2025) The in-flight cooling of ballistic pyroclasts during mafic explosive eruptions : a numerical model. Journal of Volcanology and Geothermal Research, 468: 108463. ISSN 0377-0273

Abstract

Explosive volcanic eruptions pose a threat to nearby populations and infrastructure. The erupted pyroclasts (i.e., variably molten droplets) can ballistically travel large distances from the vent, cause range of hazards, and produce a range of deposit types depending on the temperature at which they land. To mitigate these hazards and to perform enhanced quantitative textural analyses of the erupted pyroclasts, there is a need to improve our understanding of the transport and cooling dynamics of such ballistic pyroclasts. In this study, we developed and coupled both a transport and a transient cooling model that account for the in-flight cooling of ballistic pyroclasts of different sizes, launch angles, and exit velocities. The transport model developed solves equations for a translating spherical body in two-dimensional (2-D) space, and our cooling model solves the Fourier heat equation for spherical bodies. Our two models were then coupled using a set of dimensionless equations that describe relationships between Nusselt-Reynolds-Prandtl numbers. These relationships provide a way to estimate the heat transfer coefficient, based on ambient flow conditions around the pyroclast, at different times during the pyroclast transport. Together, our coupled models can describe the trajectory, distance reached, viscosity changes, and cooling profiles (i.e., core to rim temperature) of pyroclasts during mafic explosive eruptions. As an example, we show how our model can be used to predict the temperature of pyroclasts within lava fountains with varying ambient temperatures and discuss the possible textural outcomes of ejected pyroclasts in-flight and upon landing. Thus, our model can be used to predict pyroclast types and textures (e.g., rheomorphic, breadcrusted) at set distances from the vent and used forensically to determine eruptive conditions from deposits of past eruptions.