The slow boiling of magma chambers and the dynamics of explosive eruptions

Explosive eruptions are fed by shallow magma chambers, and are mainly characterized by a water-saturated magma, a temporal-increasing eruption-rate and deep seismicity following the later stages of eruption. We develop two simple physical models based on the decompression of a water-saturated magma and the non-equilibrium growth of bubbles that drives the magma out of the chamber. The eruption ends when sufficient decompression is attained to cause magma chamber collapse and restoration of the initial pressure conditions. The first model considers a single step decompression; the second one a multi-step decompression. The models predict that the duration of the eruptive phase is mainly controlled by the bubble number densities, and the initial decompression. Pressure decrease of the order 0.1-1 MPa results in eruption lasting days-hours. The erupted volume (magnitude) and eruption rate (intensity) are mainly controlled by the volume of the magma chamber, by the strength of the rocks surrounding the reservoir, and the physical properties of the magma (water diffusivity and bubble density). We simulated the eruptions at Mt St Helens, 1980, and Vesuvius 79 AD. The models correctly predict the duration and evolution of the eruptions with the initial phases characterized by a slow eruption rate with the formation of a plinian column and the subsequent increase, resulting eventually in pyroclastic flow activity. (C) 2001 Elsevier Science B.V. All rights reserved.