CONTROL OF MAGMA VOLATILE CONTENT AND CHAMBER DEPTH ON THE MASS ERUPTED DURING EXPLOSIVE VOLCANIC-ERUPTIONS

Explosive volcanic eruptions are triggered when magma is sufficiently overpressured to open up a fracture to the surface and the eruption co ntinues until this overpressure is relieved. Here we show that the mas s erupted from the cl-lamber during the eruption depends critically on whether the magma becomes saturated in its volatiles. Unsaturated Liq uid mag-ma may only deform elastically and is therefore relatively inc ompressible. Thus the eruption of relatively small fractions, 0.001-0. 01, of the magma in the chamber is sufficient to relieve the chamber o verpressure. In contrast, if the magma is saturated and contains exsol ved bubbles of val,ol, then the mixture becomes much more compressible and the chamber overpressure is only relieved when a fraction of orde r 0.01-0.1 of the initial mass in the chamber has erupted. In such cas es the mass of exsolved bubbles of vapor in the magma as well as the d epth and vertical extent of the chamber have a dominant control on the mass erupted. It is also shown that for a given total volatile conten t, since the mass of volatiles exsolved as gas increases with the crys tal content of the magma, the mass erupted also tends to increase with magmatic crystal content. In a layered chamber the volatile content o f each layer and the depth and. vertical extent of the chamber again e xert the main controls on the mass erupted. We use our model to interp ret chamber sizes in a number of well-documented historic eruptions.