This is a past event.

Amanda B. Clarke
Arizona State University
Tempe, AZ

Vulcanian eruptions are impulsive explosions that occur as a result of rapid decompression of a volcanic conduit containing bubbly magma. Upon decompression, a leading shock wave may form and travel away from the vent into the atmosphere and a decompression-induced fragmentation wave travels down the conduit. At the fragmentation front the bubbly magma is disrupted into a gas-solid mixture, propelled upward, and ejected from the vent at velocities up to 400 m s-1. Typically only a portion of the magma in the conduit is fragmented and evacuated such that Vulcanian eruptions characteristically last only seconds to minutes.

Results of two relevant experimental studies are presented here. The first examines the initial burst phase and leading shock waves via 1-D shock-tube experiments in which mixtures of air and spherical particles are rapidly decompressed into a low-pressure environment via diaphragm rupture. Maximum gas-particle mixture velocities decrease with increasing particle diameter for a given initial pressure ratio across the diaphragm. Experiments with particles produce weaker and more slowly propagating shock waves relative to experiments with air alone. Comparison of experimental data to theoretical and computational solutions produced two key conclusions: 1) the effective interphase drag coefficient during high-acceleration stages of an eruption is less than values previously used in multiphase models of explosive eruptions; therefore a new formulation is prescribed; and 2) leading shock waves are formed by the gas phase alone, not the solid-gas mixture, with shock wave characteristics reflecting losses due to drag between gas and particles; shock wave calculations should consider these losses rather than treat the system as a perfectly-coupled pseudogas.

The second set of experiments examines the subsequent propagation of the pyroclastic jet or plume by injecting discrete pulses of (negatively or positively) buoyant fluids into fresh water. Dimensional analysis, based on two source parameters, total injected momentum and total injected buoyancy, identifies a universal scaling relationship for the initial propagation of short-duration impulsive flows; the non-dimensional, time-varying velocity varies as the square root of the time-varying, non-dimensional ratio of source parameters. The relationship successfully describes experiments and several well-documented Vulcanian eruptions over a wide-range of conditions. Vertical flow front velocity (u) decays with time (t) and the decay trend depends on vent conditions: u in experiments with momentum only (neutrally buoyant fluids) decays as t-3/4, while u in experiments with both momentum and buoyancy decays as t-1. Note that these trends are distinct from the classic case of a buoyant thermal (u~t-1/2). These formulations expand the range of theoretical relationships appropriate for Vulcanian explosions, which have typically been treated as steady plumes or discrete thermals.

The utility of both sets of experiments is demonstrated by estimating pre-eruption pressures, time-varying vent mass flux and total mass erupted for several well-documented eruptions, with results comparing favorably to independent estimates.

Suggested Reading Material

A. B. CLARKE, J. C. PHILLIPS & K. N. CHOJNICKI
An investigation of Vulcanian eruption dynamics using laboratory analogue experiments and scaling analysis
 

K. Chojnicki, A. B. Clarke, and J. C. Phillips
A shock-tube investigation of the dynamics of gas-particle mixtures: Implications for explosive volcanic eruptions

Hosts John Lyons (jlyons@mtu.edu) and Stephanie Tubman (sctubman@mtu.edu)