Underplating is the accumulation of partial melts at the base the crust where an ocean plate is subducting under continental crust. Underplating is the result of partial melts being produced in the mantle wedge above a subducting plate. The partial melting is induced by a lowering of the solidus by the input of water and other volatiles supplied by phase changes in the subducting slab. When the 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.
The melt will remain here until it fractionates/evolves enough (through MASH processes) so that the remaining melt is less dense than the surrounding rock and the melt will then continue up into the crust, leaving behind the heavier mafic minerals which were crystallized during fractional crystallization. The assemblage of minerals remaining behind are typically mafic or ultramafic, and are responsible for the observed seismic anomaly which indicates underplated material.
See also