The convection cell in the mantle is essentially the self-sustaining, constant movement of the magma inside this layer. This works in a very simple way. The hotter magma is less dense, so it is pushed upward toward the crust above it, while the cooler magma is denser, and it moves downward where it gets hotter, and the process goes on and one. The magma that goes upward toward the crust, manages to push through the thinner and cracked crust between the plates that move away from each other, thus on the divergent plate boundaries. As the magma penetrates through the crust it manages to get to the ocean floor, thus causing volcanic activity. This magma cools of very quickly and creates new crust. Because the magma constantly rises up and creates new crust, the crust piles up and forms an underwater mountain of volcanic origin that constantly has volcanic activity because of the rising magma. This volcanic underwater mountain range is called mid-ocean ridge.
This is why S-waves arrive as secondary waves at the Earth's surface. There is another importantdifference between P-waves and S-waves. Although both can pass through solid rock, only P-waves can also pass through gases and liquids.
