Island arcs are formed by the subduction of an oceanic plate beneath another oceanic plate. The oceanic plate that is subducted beneath the other plate melts as it is pushed dâ¦own into the mantle and creates a source of magma. The magma rises and forms volcanoes which create the islands of the island arc. The Hawaiian islands are formed by a mantle plume which is roughly in the center of the Pacific plate. The plate is moving across the plume and the islands are being formed in a line as the plate moves.