FLUID MECHANISM OF PRESSURE RISE IN VOLCANIC (MAGMATIC) SYSTEMS WITH MASS EXCHANGE

Abstract

In a closed constant-volume system filled with incompressible (or difficulty compressible fluid), the gas pressure in an ascending bubble is always greater than the instantaneous (local) lithostatic pressure, i.e., greater than the pressure of the gases dissolved in the magma. The result is that the chemical potential of the gas ascending in the bubble is greater than that of the gas dissolved in the magma. Because of that, mass exchange takes place between the bubble and the magma. The authors demonstrate that in the presence of gas exchange, i.e., in the case of correction for vertically varying partial pressure of the gas dissolved in the magma, the ascent of bubbles in a magma saturated with volatiles is also accompanied by a pressure rise in the system. Outgassing of a magma saturated with volatiles in a closed constant-volume system is a feedback process: ascent of the gas causes a rise in pressure and the generation or propagation of a crack, accompanied by an earthquake, and an increase in the volume of the system, with a decline in the pressure in it. The generation and propagation of cracks, along which the magma travels to the surface, involves the mechanism of hydrofracturing. In the case of the fluid fracturing mechanism, the overpressure is produced by gas, and it decreases during the crack propagation as a result of an increase in the volume of the system and expansion of the gas, so that crack development by this mechanism proceeds intermittently.