How did scientists prove the existence of tectonic plates.
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Geology, mostly…you can take samples on each side of plate boundaries and show that samples from one plate match samples from that same plate in other locations while samples from the other side of the boundary match the samples from the other plate.
It was based on several theories each which individually suggested tectonic plates existed but when combined had so much evidence behind it that the theory of plate tectonics was basically irrefutable. For example, visual evidence that the entire landmasses of Earth used to fit together (like a puzzle that was ripped part) was supported by another theory based on fossil record evidence (plants and animals) from across the world. Somebody out there was kicking around the simplest explanation that made all the evidence make sense, and plate tectonics was it.
How plate technics was first theorized and then proven follows a very similar story arc as evolution. Evolution prior to modern science proving it true with DNA research was propped up by a series of theories. Each of these theories alone could only partially point towards Evolution, but when viewed together it was hard to come up with any theory that tied all the evidence together better.
From what i was taught the biggest piece of evidence of plate tectonics was the fact that rocks and fossil records on opposite sides of oceans showed the same rocks and species, species which couldn’t possibly have flown or swum across an entire ocean and it would be impossible that they evolved independently and become identical
There are some good historical answers here, but on top of all this it looks like nobody has mentioned the fact that we can measure precisely enough to actually tell the speed of horizontal movement using things like lasers and satellites.
It’s frankly astonishing what we can do now
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