6. Mechanisms for Plate Motion

The energy source for plate tectonics is Earth’s internal heat while the forces moving the plates are the “ridge push” and “slab pull” gravity forces. So heat and gravity work together to keep the plates constantly moving.

The slow flowing mantle convects, much like a pot of water on the stovetop.  Rocks heat up deeper in the mantle and rise (remember, for the same composition, hotter rock is less dense than colder rock) where they cool and then sink, starting the convection cycle over again (see GIF below).  It was once thought that mantle convection alone could drive plate motions, pushing plates along from below through traction. While mantle convection definitely plays a role, it cannot explain why some plates move faster than others, or faster than the convection currents beneath them.

The other piece to this is the role of gravity through ridge-push and slab-pull. Ridge-push refers to gravity causing the lithosphere to slide downhill away from the elevated mid-ocean ridges. Slab-pull refers to the weight of subducting slabs (which are cold and dense) dragging the rest of the plate down into the mantle at subduction zones.

Current dynamic models have plates moving as part of a gravity-driven convection system that requires mantle convection, ridge-push, and slab pull all working together to drive plate motion.

References

GIFs: IRIS. (2017). Plate Tectonics – What are the forces that drive plate tectonics? CC by 4.0. Found here.