Influence of eruption parameters on the thermofluid dynamics of collapsing volcanic columns

Abstract

A complex two‐phase flow model was employed to study the influence of eruption parameters on the thermofluid dynamics of collapsing volcanic columns. The physical model accounts for thermal and mechanical nonequilibrium between the gas and solid phases and for mixing of water vapor leaving the vent with the pressure‐ and temperature‐stratified atmosphere. The effects of particle collisions and pressure were modeled by a kinetic theory of granular flow. Depending on the eruption, the simulations were performed from several to tens of minutes after the beginning of the eruptions. Initial conditions for simulations involved different vent diameters, exit velocities, and temperatures of the two phases, particle diameters, and water vapor contents of the flow mixture. After an initial period of fountain building, characterized by column collapse and formation of pyroclastic flows and material recycling from the collapsed column into the fountain, the simulated columns developed different structures, producing stationary or oscillatory behaviors which depend on the geometry and physical parameters of the gas and pyroclasts at the vent. Such behaviors of the collapsed columns may strongly influence the dynamics of pyroclastic flows, causing variations of the flow properties and strong pulsations of their mass flow rates. The application of spectral analysis to timewise distributions of some properties at different points in the fountains and pyroclastic flows permitted identification of the main frequencies associated with the nonstationary column phenomena. The columns at the transition between the collapsing and plinian behaviors developed characteristic suspended flows that spread radially at the fountain height level for several kilometers from the vent and their heads intermittently collapsed due to the changing atmospheric dynamics above and below the suspended flows. The results from computer simulations of collapsing volcanic columns are consistent with previous simple and complex modeling approaches, laboratory experiments, and field observations. The results also show that the thermofluid dynamics of collapsing volcanic columns may produce very complex depositional structures in pyroclastic flows and should therefore provide important clues in the interpretations of field deposits associated with explosive volcanic eruptions.