There's always some oceanic crust attached to a continent. In the East African rift system A small section of East Africa is poised to drift slowly to the northeast, leaving a narrow ocean basin between itself and the rest of Africa. As the rift extends, it develops extension faults parallel to it. This extends all the way to a broad, shallow continental shelf. The rise is at the base of the slope.
Volcanic isles form on overlying crust, as magma blurps up from melting subducted crust.
Formed above subduction zone regions where the subducted plate is at a depth of 80 km or deeper. They result from the rise of magma generated by the partial melting of subducted lithosphere. Ocean to continent Oceanic crust subducts. Coarser-grained basalt rock. Also, subduction zones cease to function, as there's no more oceanic crust to do the subducting.