The divergent plate boundaries are the places where the tectonic plates move away from each other. As they move part, which is actually driven by the convection currents in the mantle, they leave a gap between them of very thin and cracked crust. The convection currents manage to push the magma up with easy through this gap. As the magma pushes upward, it reaches the ocean floor, where the temperatures is significantly lower and it quickly cools off and forms new igneous rocks, or rather new crust. Since the magma is constantly rising and constantly making new crust, the new crust is pilling up, forming a mid ocean ridge with intense volcanic activity. Eventually, the rising magma will manage to come out on the surface and create volcanic islands, thus a chain of volcanic islands, which can even result in the formation of new large land of mass.
