Melting above the subducted slab produces mafic magma, as described above, yet intermediate rocks (andesite, diorite) between oceanic and continental lithosphere, explain how this intermediate magma forms. (a) A relatively small amount of intermediate magma occurs in volcanic island arcs. Remembering that melting above the subducting slab produces mafic magma, explain the origin of the intermediate magma.
(b) Much more intermediate magma erupts in continental arcs. Why?

Answer and Explanation:

A. Intermediate magma occurs with the partial melting of the overlying ultramafic oceanic crust. This melting is caused by the fluids released by the subducted slab. As a result of this parvial melt there is a magmatic differentiation where the formed magma leaves a material residue, called intermediate residue, which is assimilated by the basaltic magma, thus forming the intermediate magma.

B. This is because as the intermediate magma is produced, the continental crust is either assimilated by it, or melted and assimilated by the magma.