The magma comes from the mantle layer. In the mantle layer there's enormous pressure that tries to find it's way up. On the places where the crust layer is thinner, the pressure from the mantle manages to break it. Once it starts to make cracks through the crust the magma, under the high pressure from bellow, starts to come through the cracks to the surface, thus causing volcanic activities, both on the sea beds and the on the ground.