The in-flight deformation of pyroclasts : insights from analogue experiments

Ogbuagu, Chiedozie C. and Jones, Thomas J. (2025) The in-flight deformation of pyroclasts : insights from analogue experiments. Bulletin of Volcanology, 87 (5): 33. ISSN 0258-8900

Abstract

Explosive eruptions of basaltic and other low-viscosity, magmas are widespread and mostly characterised by lava fountains and jets that produce a diverse array of pyroclasts. Considering the hazardous nature and potentially long duration of most low-viscosity eruptions, understanding the eruption dynamics and range in pyroclast characteristics (i.e., their shape and size) remains an important goal. Analogue experiments provide a robust means of studying the same dynamic processes within a controlled laboratory environment. In this study, we used scaled analogue experiments to investigate the in-flight deformation of pyroclasts. A total of 30 experiments, grouped into three different experiment sets, were conducted to cover a range of pyroclast properties (e.g., viscosity) and eruption conditions. Analogue fluid droplets were released at various heights in the presence of a controlled background air flow, replicating pyroclast transport within a fountain or jet after primary magmatic fragmentation. High-speed videography and accompanying image analysis documented any change in the droplet shape during flight. Droplets falling from constant height, with increasing background gas velocities increasingly deformed, progressing from regimes of positive and negative deformation, through oscillatory deformation to bag-stamen deformation, at dimensionless Weber numbers (We) ranging from 0.52 to 8.26. This study, through an experimentally validated in-flight deformation map, illustrates that all explosive eruptions of low viscosity magma (e.g., carbonatites, kimberlites, basanites, basalts) are susceptible to in-flight pyroclast deformation and potential secondary fragmentation. These processes need to be considered when examining pyroclasts within the stratigraphic record and inverting textures for dynamic eruption processes.