system
Given the following post, respond as if you're commenting on Reddit:
human
Why exactly do the tectonic plates move in different directions?

I fully understand that tectonic plates are just pieces of crust that move because of the convection cells of lava and rock beneath the surface moving them towards a certain direction. But my question is *why* do the different convection cells all move in different directions, what determines that? So for example, the Indo-Australian and Pacific plates have a convergent border, so my question is why does the Indo-Australian plate's convection cell move east and the Pacific Plate's convection cell move west rather than both of them just moving in the same direction?
gpt
There are a few different forces driving the motion of tectonic plates.

*Slab pull* is thought to be the main one. The part of the plate that is subducting is old, (relatively) cold, making it dense, making it sink in the mantle. It pulls the surface part of the plate along. Like a tablecloth sliding off a table because of the weight of the part already hanging down.

*Slab suction* is also associated with the subduction. The sinking slab drives flow in the surrounding mantle which then acts on other parts of plates. Slab suction means that even if the slab breaks it can still drive plate motion.

The fastest moving plates are the ones with most subduction, and this is taken as a major piece of evidence for subduction-related forces being dominant.

*Ridge push* is from mid-ocean ridges. The plate near the ridge is new, warm, so it's less dense and sits high. Away from the ridge it's colder and more dense and sits low. So the plate basically slides downhill. Ridge push is estimated as 5-10% as strong as slab pull plus slab suction.

Mantle convection acting on the underside of the plates was once assumed to drive the motion, but it's now thought to be a very minor part. For it to be significant the convection cells would need to be comparable in size to the plates themselves, but evidence based on density measurements of the mantle by seismic waves indicate any convection cells are actually much smaller. Thus to the extent they exert forces on plates, they'll be pushing different bits of the same plate different ways with little overall effect.