There are mountain ranges in the middle of the ocean floor. Because of the higher elevation gravity pushes down on the edges of the plates. This is called ridge push.
Step-by-step explanation:
Ridge push is a simple explanation of the height of the landforms based on the gravity and the elasticity of the underlying rock. It says when a landform gets too much higher, the weight of the overlying rock and soil pushes the landform back to plains. Thereby an equilibrium is maintained.
Slab pull is a theory proposed which visualizes the earth interior as a pool of hot molten lava that has a convection current going on. It explains why the crust of the earth continuously moves slowly and forms mountains and other rift valleys.