Dynamics of magma flow in volcanic conduits with variable fragmentation efficiency and nonequilibrium pumice degassing

Abstract

Numerical models of volcanic conduit dynamics assume that the foamy structure of magma is totally destroyed at fragmentation, neglecting the presence of vesicular fragments in the above gas‐particle flow region. The abundance of pumice among the volcanic pyroclasts is, however, the outstanding evidence that such a complete atomization assumption is an oversimplification. The present one‐dimensional, multiphase, nonequilibrium magma ascent modeling rejects such an assumption, taking into account the presence of pumice, ash, and loose crystals in the turbulent flow region above fragmentation. A parametric study is conducted on the role of pumice abundance, related to a fragmentation efficiency parameter, in affecting the magma ascent dynamics. Additionally, different hypotheses on pumice degassing behavior are evaluated. The numerical results show that the presence of even relatively small amounts of pumice translates into a large decrease of continuous gas volume fraction, while other flow quantities are mainly affected by the efficiency of pumice degassing. A decreased degassing efficiency translates into significant velocity and pressure decrease, mixture density increase, and further gas volume fraction decrease, that overall result in a decrease of the mechanical energy content of the erupted magma by an amount comparable to that due to water content decrease by several weight percent. Postprocessing of numerical results based on the dynamics of porous flow of gas in pumice allows one to relate the efficiency of pumice degassing to a measurable pumice texture parameter, namely the degree of gas bubble coalescence, which is therefore identified as a major factor affecting the ascent dynamics of silicic magma.