Factors controlling the temporal evolution of explosive eruptions

A simple model, based on observations of recent explosive eruptions, is proposed for the patterns of variable discharge rate during explosive eruptions. Such eruptions are characterized by a slowly increasing waxing phase and a phase of more rapid waning activity. This behaviour is associated with volatile saturation of the magma and a rigid response of the magma chamber walls until brittle fractures occur. The opening of the reservoir and the escape of the vapour phase cause an initial decompression of the magma. The emptying of the reservoir and the rigid behaviour of the wall rocks permit a further decompression of the magma during the course of the eruption. The waxing phase is related to a progressive increase in the rate of vesiculation of saturated melt that re-equilibrates the decompression of the magma. The following increase of discharge rate permits an early Plinian phase which culminates in collapse of the eruption column and emplacement of pyroclastic flows. The emission of abundant lithic fragments, making up the roof of the chamber, signals the beginning of the waning phase. The eruption lasts until the vesiculation, caused by the initial decompression and emptying of the chamber, can counterbalance the lithostatic load. Collapse of the chamber occurs as soon as the pressure becomes lower than the lithostatic load by an amount similar to the strength of rocks. The collapse tends to re-establish the original pressure conditions preventing further vesiculation and thereby resealing the magma chamber.