Plates pull apart and basaltic magma moves up to fill the voids. Short path to the surface. Produces mid-oceanic ridges. Water-rich plates descend into the hot mantle and melting occurs. Long path to the surface.
Dominant magma is andesitic - much assimilation.
Water and other volatiles contribute to the explosive nature of the volcanism. Hawaiian islands and Yellowstone are two examples. Remelting of the continental crust - process poorly understood.