The propagation of a dyke driven by gas-saturated magma

Abstract

SUMMARY A new mathematical and numerical model is presented for the propagation of a pressure- and buoyancy-driven dyke filled with volatile-saturated magma and a gas cap at its upper part. The model accounts for coupling between conduit flow of a bubbly magma, gas filtration through the magma, gas accumulation in a gas cap and elastic deformation and fracturing of the host rock. All these processes allow studying different regimes of dyke propagation. The rate of propagation of dykes is controlled by the rate of the fracturing at the tip and by the flow rate of magma inside the dyke. When high energy is needed to fracture the host rock and magma viscosity is low, the rate of propagation is controlled by the rate of fracturing (fracturecontrolled regime). When the energy to fracture the host rock is low, propagation is controlled by the magma flow rate (magma-controlled regime). We study the transition between these regimes for the case of a constant magma vesicularity and constant mass of gas in the cap. Under these conditions, the propagation of the dyke is self-similar. In the fracture-controlled regime the propagation rate only weakly depends on the amount of the gas in the gas cap, whereas at the magma-controlled regime it is significantly enhanced with increase the mass of gas at the cap. The gas pressure in the cap opens the dyke in front of the magma and allows magma flow rates that are significantly higher than predicted by models that ignore the gas cap. The maximum propagation rate is obtained at the transition between the fracture- and magma-controlled regimes. If the gas mass in the gas cap is high enough, a gas pocket can separate from the magma as a distinct unconnected pocket and propagate as a gas-filled crack at a constant velocity. Pressure decreases during ascent leads to higher vesicularity and faster gas filtration through the magma and into a gas cap. Gradual increase of the mass of gas in the cap is important in accelerating the propagation rate of dykes.