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Mostly by looking at patterns across continents. For example the Appalachian mountains and Scottish highlands were once part of the same mountain range. Geologists were able to figure that out by closely examining those mountain ranges. With enough patterns like that you can start to reconstruct the plate movement.
I recently learned a really weird piece of evidence geologists apparently look at: heavy oxygen trapped in rocks. Turns out water with oxygen18 is slower to evaporate than water with the lighter oxygen16, which apparently means that rocks absorb oxygen18 water more readily than oxygen16 water, and therefore landmasses tend to accumulate oxygen18 leaving less in the seas... which means, when geologists find really old sea floor rocks full of oxygen18, it suggests that there was a lot more oxygen18 in the seas back then, which suggests that there was very little dry land to absorb oxygen18, meaning ancient earth might have been a water world, which is what we see in the start of the video.
How do they figure this out?
Mostly by looking at patterns across continents. For example the Appalachian mountains and Scottish highlands were once part of the same mountain range. Geologists were able to figure that out by closely examining those mountain ranges. With enough patterns like that you can start to reconstruct the plate movement.
Plus there's a handful of things we know are absolutely true, like California moving x inches per year because we have very accurate measuring
Then you can extrapolate
I recently learned a really weird piece of evidence geologists apparently look at: heavy oxygen trapped in rocks. Turns out water with oxygen18 is slower to evaporate than water with the lighter oxygen16, which apparently means that rocks absorb oxygen18 water more readily than oxygen16 water, and therefore landmasses tend to accumulate oxygen18 leaving less in the seas... which means, when geologists find really old sea floor rocks full of oxygen18, it suggests that there was a lot more oxygen18 in the seas back then, which suggests that there was very little dry land to absorb oxygen18, meaning ancient earth might have been a water world, which is what we see in the start of the video.
https://www.astronomy.com/science/ancient-earth-may-have-been-a-water-world-with-no-dry-land/
*which also has obvious implications for the origins of life, as the article mentions—none of Darwin's "warm little ponds"