Tying Textures of Breadcrust Bombs to the Dynamics of Their Transport Mechanisms

Mary Benage, Georgia Institute of Technology

Photo of Mary Benage

Pyroclastic Density Currents (PDCs) are a dangerous natural hazard during volcanic eruptions. The danger and opacity of PDCs makes it impossible to have in-situ thermal or concentration measurements. Pyroclasts that were altered during their transport within PDCs therefore provide unique insight into the thermal evolution of the currents. Breadcrust bombs are pyroclasts that are molten when erupted and cooled during transport. Convective and radiative cooling creates a dense, glassy skin called a rind that is relatively non-vesicular. The relative size of bubbles in the clast and the rind thickness provides an important indicator of its cooling history. In order to tie their textures to their transportation history, thermal evolution and initial water concentration, we first model the clasts through two end-member transportation mechanisms, a ballistic parabolic path and a path inside in a PDC. The clasts are modeled as Lagrangian particles in a multiphase numerical model (Eulerian-Eulerian-Lagrangian [EEL]). The EEL model tracks the averaged conditions the Lagrangian particles experience in transport. The clast morphology is then modeled using conditions from the EEL model to calculate the heat flux and cooling of the clast that is combined with coupled viscosity and bubble growth models. The coupled models are a novel approach to model the dynamic relationship between cooling, viscosity, bubble growth and loss of water that causes the rind on breadcrust bombs. The results of the model show that dense PDCs with little entrainment have less variation in rind thickness than the ballistic clasts that experience more changes in ambient temperature. Also, the rind thickness is dependent on the initial water content and initial temperature of the magma at eruption. The study of breadcrust bomb textures provides information about some of the initial eruption conditions and their cooling history. We conclude that breadcrust bombs are useful thermal proxies for PDCs.

Abstract Author(s): Mary Benage, Josef Dufek, & Wim Degruyter