GOOD QUESTION I’M GLAD YOU ASKED.
it’s true, compared to literally every other rocky planet and planetoid in our solar system, Earth is just extremely… liquidous.
(and I’m not just talking about about our extremely spicy liquid iron core, either)
stuff MOVES here, where it really has no business doing so if you compare Earth to the rest of the solar system on paper.
continents skitter their way across the surface in a billion-year-long dance, smashing together, breaking apart, smashing together again.
on the other rocky planets, tectonic plates are locked tightly in place against their neighbors, or never even existed in the first place.
so what gives? why is Earth letting gigantic sections of its outer crust bonk around unsupervised in a way that would get it shouted at in a busy supermarket?
maybe invest in a couple of these, Earth
well, like just about every other weird goddam thing on this planet, it’s because of water!
I’m sure you know about plate tectonics at this point, where the great continental plates float around on top of Earth’s mantle aimlessly and sometimes make your house fall down when they bonk into each other? well, turns out it’s a little more complicated than that!
see, they aren’t really “floating” on top of Earth’s mantle, because the mantle is a plastic solid, like a stiff putty. if enough pressure is applied to it it’ll deform, but it’s definitely nowhere NEAR liquid.
sorry to bust every single scifi planet-destruction scene ever, I guess?
so what the hell is going on with the tectonic plates? why are they so peppy and fresh? well, it’s because the entire ocean snuck down there when we weren’t looking, that’s why.
see, the places where tectonic plates meet often aren’t exactly geologically stable (as you probably know if you live anywhere near one) and they’re DEFINITELY not waterproof. which is funny, actually, because most of them are… on the seafloor…
so what’s been happening over the past, oh, 4 billion years or so, is that water has just been leaking down out of the oceans and into the mantle itself, creating a thick layer of mantle that’s been completely saturated with superheated water!
there is more water in this layer of rock than in all the surface oceans of the Earth combined.
so the tectonic plates aren’t really skidding around on top of the MANTLE, they’re hydroplaning on the water IN THE MANTLE. which explains why they hit each other so much, actually. hope Planetary Triple-A covers that.
yowch.
anyway, the moral of the story is that Planet Earth contains more than twice as much water as you thought it did, and it makes the continents go brrrrr.
surprise?