After about 25 million years of cooling and shrinking, the edge becomes so dense that gravity can pull it down into the asthenosphere. There, the intense heat and increased pressure due to the great depth change the crust of the sunken plate edge into even denser rock. Because of the additional density, gravity pulls the plate edge into the asthenosphere even more strongly. This sinking action is known as slab-pull because the sinking edge pulls the remainder of the slablike plate behind it. Many scientists believe that slab-pull is the main action driving the motion of plates with sinking edges.
Gravity also causes plates to slide downhill away from ocean ridges.
This sliding force is called ridge-push. Another cause of plate movement is the simple pushing of plates against one another. Scientists believe that large plates shove some small plates about. The rise of mantle plumes and other movements of mantle rock may also affect the motion of tectonic plates slightly. The circulation of mantle rock as it rises to the top of the asthenosphere, cools, and then sinks is known as a convection current.