Composition-Eruptive Activity Relation of Volcanic Gases : Discussions by Means of Thermodynamics of Irreversible Processes

Abstract

A volcanic gas phase which is formed by various chemical reactions within a magma reservoir is transferred through a channel or fissure and is emitted into the air. Consequently a system (volcanic gas + magma) is open for gases. A continuous emission of gases shows that the system is not in a state of equilibrium. A steady-state emission is assumed and the thermodynamics of irreversible processes is applied. It is concluded that the steady-state emission is possible when a pressure difference between the magma reservoir and the earth’s surface is fixed as a constant, while a temperature difference is changed as free. As the first approximation, a binary gas mixture of H₂O-H₂S is assumed for a simplified model of volcanic gas. Applying the results thus obtained to fumaroles at Showa-Shinzan, it is inferred that the ascending gas increases its H₂S concentration in nearly ten times while the temperature drop is negligible.