Could Non-Condensible Gases Affect Hydrothermal Eruptions

Abstract

This paper extends work done previously by the present author (with others) in attempts to construct mathematical models, based on physical conservation laws, which describe the way a hydrothermal eruption progresses. It is assumed that hot fluid which is initially at rest in a porous structure just below the ground surface begins to vent to the atmosphere, beginning at the surface. A "flashing front" progresses downwards through the rock-fluid system, with fluid above moving upwards, driven by pressures which decrease towards atmospheric pressure at the surface. The ground surface is meanwhile eroded away by the lift forces created by the upward-moving fluid which, by the time it gets to the surface, is far less dense than at the flashing front and is moving at high speed. In past work, the fluid was assumed to be pure water (McKibbin, 1989, 1990; Bercich & McKibbin, 1992, 1993). However, as mentioned in McKibbin (1989), the initial motivation for this sequence of studies was to find out what effect the characteristics of a geothermal fluid containing significant amounts of a non-condensible gas such as C02 or H2S could have on the model of a hydrothermal eruption. In this paper, the presence of non-condensible gases is allowed for in the model; C02 is chosen to represent such components. It is found that large overpressures of non-condensible gases would be needed to significantly affect the erupting fluids potential for lift-driven erosion compared with that of pure water.