Phase equilibria trigger for explosive volcanic eruptions

Abstract

A mechanism for explosive volcanic eruptions based on multicomponent phase equilibria modelling of four explosive volcanic systems is proposed. In each system, either isochoric or isobaric crystallization, where either crystals or crystals and exsolved fluid are chemically fractionated from melt, leads inevitably to near-solidus dynamical instability culminating in violent explosive eruption. Driving this instability is a dramatic increase in the volume fraction of fluid bubbles in the magma exceeding the limit for magma fragmentation. This phenomenon is independent of magma decompression. Country rock may be weak, allowing magma to remain in lithostatic equilibrium with its host rock, or strong, leading to divergence of magma and lithostatic pressure. Bubbles may be retained or expelled during crystallization. Instability is the inevitable outcome of crystallization at shallow levels in the crust for all four systems, regardless of the mechanical state of host country rock. We speculate that this phase equilibria mechanism driving explosive eruptions has general significance. Copyright 2008 by the American Geophysical Union.