MAGMA MIXING BY CONVECTIVE ENTRAINMENT

RECENT studies of volcanic eruptions have brought to light a puzzling sequence: the ejection of dense and mixed liquids followed by a larger volume of unmixed, less dense liquid(1-4). Although these studies hav e shown that the mixing is associated ,vith the injection of basaltic liquid into a more silicic magma chamber, a paradox remains. Basaltic magma is denser than silicic magma, and should flow along the floor of the magma chamber, This configuration impedes mixing of the two liqui ds, and appears incompatible with the early eruption of basaltic liqui d from the top of the chamber. Previously proposed mixing processes ex plain neither the eruption sequence nor the eruption of dense liquid d uring the time of lowest eruption rate(1-8). We report here fluid-mech anical experiments that suggest a solution: a thermal plume forms over the replenishment inlet, dragging basaltic liquid upward by viscous c oupling, and producing mixing only in a localized area without affecti ng the bulk of the chamber.