Learning Journal, 6-15-04
Jun. 15th, 2004 09:43 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
kilroyOk. Poked briefly on the web, and ask.com produced the following unhelpful answer: other planets don't have plate tectonics because they don't have hot cores. Oh yeah, and the earth's core is somehow like an electromagnet. This produced more confusion than anything else: why is the earth's core actually hot? Why isn't anywhere else's and how do we know they aren't hot? What the hell is the deal with the electromagnet?
Shelved that for later, picked up Rare Earth again a few hours later and flipped to the plate tectonics chapter. This proved immensely helpful, but I had to re-read bits a few times in various sequences to finally get it. Lemme see if I can summarize. The mantle is "liquid" in the same way that glass is; it's effectively solid for all intents and purposes, but it does shift very, very slowly in the usual liquid way. However, since it moves like a liquid there's convection inside the planet; cells where hot mantle rises, drifts for a while until it cools down from contact with, say, seawater or the continents, then sinks again. This is easy enough to get, but this is also unfortunately where it gets weird.
Inside the convection cell there's differentiation: less dense material floats to the top, so the upper mantle, even though it's still rock floats on the rest of the mantle. The floaty bits of the mantle are basically made of basalt, which is what you find on the bottom of the ocean. In other words, the bottom of the ocean is actually floating on the rest of the mantle-- that is, on top of the convection cell. Now, the lower mantle drifts along as it cools, and it takes the basalt with it. That means that the basalt spreads out from the original upwelling. As it does so, more light, liquid basalt material floods up to plug the hole; until it too spreads, and is replaced by more basalt, etc.
So far so good. The thing is, like all convection cells, eventually the mantle convection runs out of juice. The mantle starts to cool down, contract, gain density, attract chunks of heavier rock, and eventually wants to sink. Normally this would be no problem; the material would sink back down towards the core, where it'd get heated up again, get blown into another convection cell, and the process would repeat. Right? Not quite. See, on its long journey across the bottom of the ocean, that basalt has absorbed a lot of water. Chemically the upper-upper mantle is a new animal entirely; it's a hydrated material with a different composition and --very importantly-- a lower melting point. So when the convection juice runs out and the basalt starts to sink, partway down again as it starts to heat up the hydrated basalt decides "Oh, I can melt now!" and bubbles back up to the surface. Which makes volcanoes, among other things. Now, this new lava is lighter again than basalt; it floats above the basalt, which is in turn floating above the lower mantle. And when this new second-degree upwelling cools, it forms granite: our continents as we know them.
So, in short, our continents are corks floating on oil floating on water. Oh yeah, and tectonic plates heading down cause volcanoes to shoot up. To a certain extent the plates ramming into each other causes the upthrusting of mountains-- but it's not the biggest part of the equation. Think about it: almost every real serious mountain range has active volcanoes-- because most real mountains are (or were) honest to god volcanoes.
Other interesting tidbit: because this process has been going on for a few billion years, the amount of landmass has actually been increasing with time. There's a lot more land now than there was when the dinosaurs were hanging around. The Aleutian islands may eventually be the backbone of a new continent.
Questions: Where'd the water come from and why's it on top? Why do plates form in lines and fronts (why do convection cells)-- in other words, why are the upwellings mostly lines and not point sources? How does hydrated-basalt lava get all the way back to the surface-- what's the pressure mechanism for volcanoes?
Shelved that for later, picked up Rare Earth again a few hours later and flipped to the plate tectonics chapter. This proved immensely helpful, but I had to re-read bits a few times in various sequences to finally get it. Lemme see if I can summarize. The mantle is "liquid" in the same way that glass is; it's effectively solid for all intents and purposes, but it does shift very, very slowly in the usual liquid way. However, since it moves like a liquid there's convection inside the planet; cells where hot mantle rises, drifts for a while until it cools down from contact with, say, seawater or the continents, then sinks again. This is easy enough to get, but this is also unfortunately where it gets weird.
Inside the convection cell there's differentiation: less dense material floats to the top, so the upper mantle, even though it's still rock floats on the rest of the mantle. The floaty bits of the mantle are basically made of basalt, which is what you find on the bottom of the ocean. In other words, the bottom of the ocean is actually floating on the rest of the mantle-- that is, on top of the convection cell. Now, the lower mantle drifts along as it cools, and it takes the basalt with it. That means that the basalt spreads out from the original upwelling. As it does so, more light, liquid basalt material floods up to plug the hole; until it too spreads, and is replaced by more basalt, etc.
So far so good. The thing is, like all convection cells, eventually the mantle convection runs out of juice. The mantle starts to cool down, contract, gain density, attract chunks of heavier rock, and eventually wants to sink. Normally this would be no problem; the material would sink back down towards the core, where it'd get heated up again, get blown into another convection cell, and the process would repeat. Right? Not quite. See, on its long journey across the bottom of the ocean, that basalt has absorbed a lot of water. Chemically the upper-upper mantle is a new animal entirely; it's a hydrated material with a different composition and --very importantly-- a lower melting point. So when the convection juice runs out and the basalt starts to sink, partway down again as it starts to heat up the hydrated basalt decides "Oh, I can melt now!" and bubbles back up to the surface. Which makes volcanoes, among other things. Now, this new lava is lighter again than basalt; it floats above the basalt, which is in turn floating above the lower mantle. And when this new second-degree upwelling cools, it forms granite: our continents as we know them.
So, in short, our continents are corks floating on oil floating on water. Oh yeah, and tectonic plates heading down cause volcanoes to shoot up. To a certain extent the plates ramming into each other causes the upthrusting of mountains-- but it's not the biggest part of the equation. Think about it: almost every real serious mountain range has active volcanoes-- because most real mountains are (or were) honest to god volcanoes.
Other interesting tidbit: because this process has been going on for a few billion years, the amount of landmass has actually been increasing with time. There's a lot more land now than there was when the dinosaurs were hanging around. The Aleutian islands may eventually be the backbone of a new continent.
Questions: Where'd the water come from and why's it on top? Why do plates form in lines and fronts (why do convection cells)-- in other words, why are the upwellings mostly lines and not point sources? How does hydrated-basalt lava get all the way back to the surface-- what's the pressure mechanism for volcanoes?