Heat supplies the energy that causes motion. The deeper you go into toward the Earths core, the hotter it gets. At the core it is really hot. All that heat tries to rise toward the surface. In the Mantle, the rock is partially melted and is able to move. As it is heated, the material in the mantle moves toward the surface. As the mantle material rises, it cools. When it reaches the Earths crust, it is mostly stopped. A little of the material can break through the surface, but not all. Instead, it begins to move horizontally. The mantle material moves horizontally away from a mid-ocean ridge crest. Toward the surface, the mantle material starts to cool. As it cools it sinks back down into the mantle. These areas are where deep sea trench occur. The material sinks back down to the core. The system operates like a giant conveyor belt. The motion due to heating and cooling is called convection. 
What happens when the mantle material that is near the surface starts to cool?
It sinks back down into the mantle