Bubbledrive-1: A numerical model of volcanic eruption mechanisms driven by disequilibrium magma degassing

Abstract

We have developed a numerical model for magma dynamics in volcanic conduits that involves conduit flow driven by a detailed treatment of bubble growth. The “Bubbledrive-1” model can be characterized by several unique aspects, including treatment of the transient problem, specific eruption triggers, disequilibrium degassing, a full-scale bubble growth module, and steady-state eruption in the case of constant recharge from below. The general conditions for the model are homogeneous two-phase flow, a simple fragmentation threshold, pre-existing bubbles, and parameters corresponding to magma compositions of dacite and rhyolite. The advantage of using a full-scale bubble growth model in our treatment of conduit flow makes it possible to track the level of oversaturation throughout the conduit in space and time. The model accounts for dynamic decompression, in which the evolving interaction between decompression rate (due to magma ascent), and diffusive addition of gas into bubbles (due to decreased solubility with decreasing pressure) serves as the driving force of magma flow in the conduit. The ability of the model to run the transient case enables us to explore specific triggers to initiate an eruption, which proceeds with duration and eruption energy depending on conduit geometry and several other factors. Our numerical study revealed several notable features of volcanic systems that may shed additional light on natural systems. Each of the model runs was directed at elucidating specific aspects of eruptions, and the suite of runs indicated some common features that may play a key role in all silicic eruptions. We find that conduit geometry is critical in determining the nature of eruption. Conduits lacking a magma chamber have smooth and relatively quiet eruption styles. Conduits that include a magma chamber involve eruptions that start out like those without chambers, but then evolve into a long and uniform eruption that ends with a violent explosion as the chamber is emptied. The uniform phase is maintained by vesiculation within the chamber, and the blast occurs when the fragmentation level descends down into the magma chamber. Resistance to flow imposed by the narrow conduit above the chamber acts as a “buffer”, stabilizing the eruption during the steady state phase until the chamber is depleted in material. Discharge and vent velocity are proportional to the conduit/chamber depth and are controlled by the initial concentration of dissolved volatiles. Maximum oversaturation is observed immediately below the fragmentation level of the magma column. Oversaturation at the vent remains remarkably constant at 1 wt.% of H 2O for all cylindrical conduits with no recharge from below. Oversaturation at the vent is much more complex for conduits that contain magma chamber at 2 km depth. Our numerical study indicates that the primary factors that control eruption behavior are magma composition and conduit shape. This suggests that future observation programs could provide valuable information by determining sub-edifice structure so that better constraints on geometry can be derived.