The convection cell is basically a self-contained in which there is a motion, where the hotter fluid is moving upward because of the lesser density, while the cooler fluid is moving downward because it is denser. This leads to constant circular movement of the magma in the mantle layer, and the one that moves upward is constantly creating big pressure on the crust above it, penetrating through the cracks in it or melting it and intruding. As the magma pushes upward, it manages to get through the thinner and cracked crust between the plates that move away from each other at the divergent boundary. The magma reaches the ocean floor and quickly cools off, creating new crust, and that continues constantly, with more and more magma coming out and more and more new crust pilling up, eventually creating an underwater mountain range of volcanic origin known as mid-ocean ridge.
