As molten magma rises at a mid-ocean ridge it heats the rocks around it. The heat expelled by the magma expands the lithosphere and asthenosphere at the ridge, pushing them above the surrounding ocean floor. As time passes, the heated rocks cool and their density increases. When this happens gravity pulls the weighty lithosphere away from the mid-ocean ridge, down the sloping asthenosphere, which, in turn, allows new molten magma to well up. This fresh magma will eventually become new lithosphere.
Scientists believe the cooling rock exerts a force on spreading lithospheric plates, helping to drive their movements. The force is called ridge push.