Pressure Changes Associated with Dike Intrusion: Effect of Vesiculation in Magma

Vesiculation in magma, which has been studied to understand observed geologic samples, is one of the most probable candidates for driving force of magma migration beneath active volcanoes. By incorporating elasticity of the surrounding medium into the bubble growth model proposed by Prousevitch et al. (1993), magma pressurization is evaluated for dike intrusion. Dike is assumed to be a crack filled with magma buried in infinite elastic medium, and magma consists of melt and numerous small spherical gas bubbles. Melt is compressible liquid saturated with volatiles such as water, and the gas in bubbles is approximated as a perfect gas. When magma is pressurized, a stress concentration occurs at the elastic body of the crack tips, and the crack extends its length in a short time. This causes a pressure drop in magma when volume of magma is constant during the extension. With the elapse of time, the gas bubble grows to remove the pressure difference between the gas bubbles and melt. Finally, the pressures of the gas and melt reach an equilibrium condition. From the mass conservation of water in magma and pressure balance between gas bubbles, melt and surrounding elastic medium, the magma pressure and bubble radius in the final stage when bubble growth reach an equilibrium are related to the initial condition of magma and the pressure drop induced by dike intrusion. Calculation results show that the pressure drop caused by a dike intrusion is recovered by the excess pressure of gas bubbles when initial bubble radius is sufficiently small. Pressure drop is effectively recovered by vesiculation for thick dikes under a low confined pressure. These pressure recovery processes suggest that the dike can migrate and extend without a new supply when magma is rich in tiny bubbles of volatiles.