Volatile Degassing From Magma Chambers as a Control on Volcanic Eruptions

The loss of volatiles from a magma reservoir affects the magmatic overpressure responsible for driving ground deformation and eruptions. Although the high-temperature metamorphic aureole around a magma chamber is typically considered to have low permeability, recent theoretical, experimental, and field studies have highlighted the role of transient permeability in magmatic systems. Also, direct measurements suggest that passive degassing is a significant component of total volatile loss in both basaltic and silicic volcanoes. Consequently, the effective permeability of the crust when magma is present in the system can be many orders of magnitude larger than that of exhumed rock samples. We develop a fully coupled porothermoelastic framework to account for both the flow of volatiles as well as associated effects on the stress state of the crust and calculate an analytical solution for spherical geometry. We then combine a magma chamber box model with these solutions to analyze eruption dynamics in magmatic systems. We find that in addition to viscous relaxation, magma recharge, and cooling timescales, the pore pressure diffusion timescale exerts a first-order control on volcanic eruptions with moderately high crustal permeabilities of order 10⁻¹⁷ to 10⁻¹⁹ m². We describe a parameter space to identify which components dominate in different regimes for volcanic eruptions according to these different timescales.