system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What determines the direction (and speed) of tectonic plates?

Or why are the continents drifting in the direction they are drifting?
gpt
There are convection currents in the mantle of the earth that rise and sink cyclically. These are what the plates float on and move around on.

 We can tell where they were in the past by deposits of fossils unique to an area in a specific rock layer. For example, if there’s tropical shells in a layer, then above there’s cold water starfish fossils, then that part must have moved from warm to cold part of the earth. Too, the magnetic field aligns magma very predictably so you can tell how old the crust is, and which way it was oriented originally. There’s more methods, but those are some of the basics!