Two-dimensional gas loss for silicic magma flows: toward more realistic numerical models

Abstract

Among the physical parameters that govern magma flows, gas content plays a major role. In particular, the presence of gas controls the magma viscosity and velocity gradients, which characterize the eruptive style. Depending on whether gas can escape the volcanic conduit or not, the magma pressure will increase (or not), leading to an effusive or an explosive regime. Gas loss is thus a key point for understanding the physics of magma flows. Its role during the magma ascent is studied numerically. In this paper, I introduce, for the first time, both horizontal and vertical magma permeabilities into a 2-D magma flow model. The magma permeability is defined specifically for every point of the conduit as a function of the gas content and the bubble shape. Starting from an (unrealistic) closed system, I calculate iteratively the evolution of a degassing dyke to reach the stationary state for an open system. Gas loss does not occur homogeneously within the conduit, which leads to degassed magma sections presenting both vertical and lateral discrepancies. Its major influence lies within 500 m below the surface, where it leads to the formation of a 30–100 m thick degassed cover at the top of the conduit and the presence of 1–2 m thick degassed material near the conduit walls. Globally, the range of gas content decreases from 60–90 vol. % for a closed system down to 10–60 vol. % for an open system. This distribution in gas content and the associated flow are in good agreement with several field observations.