A convergent plate boundary forms a volcanic island because when the plates collide there is going to be a mountainous type form with a crack in the center. Magma will heat up in the mantle and rise to the surface creating an island.

A convergent boundary usually involves one plate over-riding another plate. The lower plates descent involves heat generation, probably mostly by friction and partially by assimilated chemical reaction. As the subducted plate moves deeper and heat increases magmas are generated and this molten rock begins to move upward rather like a baloon rising in air. The magma separates from the subducting plate and thereby removes heat from the system. As the magma plume ascends toward the surface, the lighter elements in the magma begin to dissassociate from the melt giving rise to, sometimes explosive, eruptions at the surface. Pyroclastic volcanos.