The thermal evolution and dynamics of pyroclastic density currents from boiling-over eruptions

Mary Benage, Georgia Institute of Technology

The thermal history of pyroclastic density currents (PDCs) is critical in determining the current dynamics and deposit characteristics. The thermal history of these flows depends on the particles’ internal rate of heat transfer and heat exchange between a gas phase. A multiphase numerical model coupled with a Lagrangian model (Eulerian-Eulerian-Lagrangian) was used to model and track the internal heat transfer and changing temperature and viscosity of individual pyroclasts/breadcrust bombs. Viscosity can have a significant impact on bubble growth within the clast. A numerical bubble growth model was used to calculate post-eruption bubble evolution and the breadcrust bomb morphology. The first step toward better understanding the thermal history of a PDC was to determine if a pyroclast’s path (either ballistic or within flow) determines its morphology. The results show that if the PDC stays relatively hot and does not entrain a significant amount of air then a pyroclast entrained in the current will likely have a thinner rind than one from a ballistic trajectory. The bubble growth results exhibit the behavior we see in the field, which were small vesicles along the rind and larger vesicles in the interior. Future work will verify the modeled breadcrust bomb morphologies to the deposits, as well as compare the EEL model results to grain size distribution and run-out distance. The verified thermal results will be used to determine the amount of air entrained in the flow during transport. In the end, the verified numerical models of the changing breadcrust bomb morphology due to thermal properties, the thermal history of the PDCs (such as amount of entrainment or particle density), and grain size distribution will provide a better understanding of PDC dynamics from boiling-over eruptions and improve hazard assessments.

Abstract Author(s): Benage, M.C.; Dufek, J.; Degruyter, W.