Volcanoes form at different plate boundaries because of the plates divergent and convergent nature. the plates are always in motion, however minimal they may be. When the plates move apart from each other, the magma from below comes up to fill in the vacant space and thus a volcano is formed. It may be the other way round also and that is the magma forces the plates to move away and this results in the formation of a volcano. When one of plates dives under another plate, then the pressure creates melting of the mantle and thereby forms magma which in turn creates volcanoes.