Temporal evolution of large esplosive eruptions

We propose a simple physical model for the temporal evolution of large explosive volcanic eruptions, based on the sudden decompression of a magma reservoir at shallow depth. The magma is water-saturated and the sudden decompression results in vesiculation that drives it out of the chamber. The viscoelastic response of the magma chamber walls plays an important role in controlling the style of the eruption. A temporal increase in the magma discharge rate is predicted when relaxation times are longer than 10⁵s. The eruption ends when the chamber becomes sufficiently underpressured and collapses. We investigate the influence of the magma chamber geometry on the eruptive dynamics and the role played by the initial decompression and the gas bubble density. We are able to correctly reproduce the pattern of magma discharge rate and the duration of the May 18, 1980 eruption of Mt. St. Helens (Washington, U.S.A.).