Suppression of large-scale magma mixing by melt–volatile separation

Many volcanic eruptions are triggered by the injection of hot basic magma into a subsurface reservoir containing cooler and less dense silicic magma. As the basaltic magma cools and crystallises, it may become volatile saturated and exsolve bubbles. Here we present a new quantitative model and supporting laboratory experiments which identify that if a sufficient number of bubbles remain in suspension, then the bulk density of the basalt may fall below that of the silicic magma. However, if the basalt has sufficiently low viscosity, or the cooling rate is sufficiently small, then the bubbles can rise through the basalt, suppressing the large-scale overturn, and forming an intermediate bubbly layer at the interface with the more viscous silicic magma. As the small bubble plumes then rise from this foam and transport vesicular basalt into the upper layer, the chamber now remains density stratified.