 The partial melting is induced by a lowering of the melting temperature, the solidus, by the input of water and other volatiles supplied by phase transitions in the subducting slab.When the buoyant partial melt rises upwards through the mantle, it will usually stall at the base of the crust and pond there.This is because the crust is usually less dense than the underplating magma, and this is the point at which the ascending magma reaches a level of neutral buoyancy.