Effects of vent overpressure on buoyant eruption columns: Implications for plume stability

Abstract

Volcanic plumes resulting from explosive volcanic eruptions present a variety of hazards depending on their behavior. Buoyant plumes heat and entrain enough of the surrounding air to rise high into the atmosphere, disrupting air traffic and causing regional ash fall. Alternatively, collapsed plumes produce dangerous fast-moving lateral flows of hot ash and gas. The transition between these behaviors and the nature of each hazard is dependent on the fluid dynamics of the volcanic plume, which is largely determined by the conditions at the vent. Most treatments of volcanic plumes for hazard assessment assume that the eruptive fluid exits the vent at pressures equal to atmospheric pressure or that pressure equalizes quickly with little effect on the flow. Here we show that vent pressures greater than atmospheric lead to rapid expansion of the plume and the development of standing shock waves that change the behavior of the entire eruption column. We simulate two volcanic plumes with the same heat flow (J s − 1) at the vent; one exits the vent at atmospheric pressure (pressure-balanced) and the other at four times atmospheric pressure (overpressured). The two simulated plumes have the same radius after the initial rapid decompression of the overpressured case. These plumes show drastically different behavior due to the presence of standing shock waves in the overpressured case despite having the same heat flow at the vent and the same area available for entrainment of ambient air. Both simulated plumes exhibit buoyant rise but the overpressured plume collapses with a regular periodicity. These simulations suggest that the dynamics of a steady-state overpressured vent may result in plumes that oscillate between buoyant rise and collapse, providing a mechanism for the deposition of intraplinian pyroclastic flows.