Modeling the Morphology of Volcanic Clasts as a Proxy for the Thermal Evolution of Pyroclastic Density Currents

Mary Benage, Georgia Institute of Technology

An important and unanswered question for Pyroclastic Density Currents (PDCs) is the amount of air entrainment or cooling that occurs during transport down the flanks of volcanoes. This thermal history of PDCs is critical in determining current dynamics and deposit characteristics. We model the morphology of volcanic clasts and use them as a proxy for the thermal state of the current. The volcanic clasts are modeled as Lagrangian particles in a coupled multiphase numerical model (Eulerian-Eulerian-Lagrangian [EEL]) that tracks the heat exchange between the current and the clasts. It is through the varying cooling regimes of clasts that we can model different morphologies.

The volcanic clast morphology is modeled by coupling heat transfer, viscosity, and bubble growth models. The cooling of the clast is calculated by the convective and radiative heat transfer of the clast to the surrounding environment. The temperature profile of the clast is then used, along with composition of the clast, to calculate the distribution of viscosity. Viscosity can have a significant impact on bubble growth within the clast. A numerical bubble growth model is used to calculate post-eruption bubble evolution and the clast morphology, such as rind thickness and bubble size distributions.

The first step toward better understanding the thermal history of a PDC was to determine if a volcanic clast’s path (either ballistic or within a PDC) determines its morphology. The results show that ballistic clasts rapidly cool and produce thicker rinds. However, the bubble sizes and rinds are also a function of the clast’s initial water concentration. Therefore, the modeled volcanic clast’s morphology is dependent on not only the cooling rate but also the initial water content. Future work will be to compare the modeled clast morphologies to those in the deposits at Tungurahua Volcano to constrain the thermal history of the PDCs.

Abstract Author(s): Mary C Benage and Josef Dufek