The transformation into magma of a denser plate as it dives under another, less dense, plate. A subduction zone occurs where rocks of an oceanic plate are forced to plunge below much thicker continental crust. As the plate descends it melts and is released into the magma below the Earth’s crust. Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth’s subduction zones; see Vannucchi et al. (2008) Nature 451 who provide excellent diagrams. The subduction of features such as ridges and seamounts increases basal erosion and the subsidence of the accretionary wedge (Morgan (2008) Geophys. Res. Abstr. 10, EGU2008–A-00560). See also Chiarabba et al. (2008) Earth and Plan. Sci. Letts 268, 3–4.