Complex sentence: Since Oceanic plates are composed of basalt, a lighter material than what composes the upper mantle, that rises up through the remaining oceanic plate as it is melted to form volcanos in the shape of an arc.
Split into simpler sentences: Since Oceanic plates are composed of basalt they are lighter than the material that composes the upper mantle. As this lighter basaltic material rises up through the remaining plate it forms volcanos in the shape of an arc.