Bubble plumes generated during recharge of basaltic magma reservoirs

CO2 is relatively insoluble in basaltic magma at low crustal pressures. It therefore exists as a gas phase in the form of bubbles in shallow crustal r eservoirs. Over time these bubbles may separate gravitationally from the ma gma in the chamber. As a result, any new magma which recharges the chamber from deeper in the crust may be more bubble-rich and hence of lower density than the magma in the chamber. Using scaling arguments, we show that for t ypical recharge fluxes, such a source of low-viscosity, bubble-rich basalt may generate a turbulent bubble plume within the chamber. We also show that the bubbles are typically sufficiently small to have a low Reynolds number and to remain in the Row. We then present a series of analogue laboratory experiments which identify that the motion of such a turbulent bubble-drive n line plume is well described by the classical theory of buoyant plumes. U sing the classical plume theory we then examine the effect of the return fl ow associated with such bubble plumes on the mixing and redistribution of b ubbles within the chamber. Using this model, we show that a relatively deep bubbly layer of magma may form below a thin foam layer at the roof. If, as an eruption proceeds, there is a continuing influx at the base of the cham ber, then our model suggests that the bubble content of the bubbly layer ma y gradually increase. This may lead to a transition from lava flow activity to more explosive fire-fountaining activity. The foam layer at the top of the chamber may provide a flux for the continual outgassing from the flanks of the volcano [Ryan, Am. Geophys. Union Geophys. Monogr. 91 (1990)] and i f it deepens sufficiently it may contribute to the eruptive activity [Vergn iolle and Jaupart, J. Geophys. Res. 95 (1990) 2793-3001]. (C) 2001 Elsevier Science B.V. All rights reserved.