Mafic inclusions are lower in bulk density than the andesitic and dacitic lavas in which they occur, and apparently represent a foam inflated during rapid cooling of wet mafic magma in contact with cooler, more silicic reservoir magma. This process occurs as mafic magma enters the base of the reservoir, so that the mafic/silicic interface becomes unstable in Rayleigh–Taylor fashion. Mixing in these reservoirs thus depends on water content of mafic magma and pressure.