Transcript of Isostatic equilibrium

>> Did you ever wonder why the earth has high places and low places? We've got these places called continents that stick up out of the ocean, sometimes miles above sea levels, and then these other areas that we call ocean basins which lie on average two to three miles below sea level. And there's not a lot in between. High places and low places. Why is that? It's unusual among the planets that we know. Well, it turns out it's because the earth is made of two very distinct kinds of crust, what we call continental crust and oceanic crust. I've got here on the board a generalized sketch of the continental crust and the oceanic crust. And the two kinds of crust lie on this very thick region of the earth beneath that we call the mantle. And that extends down to about halfway down to the center of the earth. You can see on my sketch that there's one very clear difference between the continental crust and the oceanic crust. The continental crust is a lot thicker. It averages in thickness about 35 kilometers, which is about 22 miles. Whereas, the oceanic crust averages only about eight kilometers in thickness, which is about 5 miles. So there's a big difference in thickness. There's another important difference which is in their density. I'll show you in a minute the kind of rock that makes up the continental crustal: rock like granite and its relatives. And that continental crustal rock is a relatively low density rock. Its average density is about 2.7 grams per cubic centimeter. If you're not familiar with the metric system think about water and how heavy a gallon of water is. Water is about 1 gram per cubic centimeter. And so the continental crustal rock is about 2.7 times heavier for a given amount than water. The oceanic crust is made of different minerals and the rock there is a little denser than the granite, about 3 grams per cubic centimeter. In other words, it's about 10% denser than the continental crust. And the mantle rock upon which both of these crusts lie is the densest of all. It's about 3.3 grams per cubic centimeter up in the upper part of the mantle and gets denser as you go farther down. And that turns out to be an important difference. The continental crust is the least dense of the three, and both of these kinds of crusts are less dense than the mantle. And as I'll show you in a minute, both kids of crust actually float in this heavier denser rock beneath them that makes up the mantle. I have here a piece of granite. This is the main rock of the continental crust. You can see that it's light in color, although it's got some darker minerals in it and you may even be able to see from there that the minerals in it are visible, they're about the size of peppercorns, so it's what we call "coarse-grained" (big minerals). And this is a fairly low-density rock and it's the main rock that makes up the continental crust. In contrast, I've got her a piece of volcanic basalt from the ocean floor. This is the main rock of the oceanic crust. You can see it's dark in color and also the crystals are much tinier. As I run my fingers over the surface I can feel more like a sandpaper texture. So, peppercorn-sized crystals versus sand-sized crystals. And this is the most common rock on earth, really, because it forms from volcanic eruptions at mid-ocean ridges as the seafloor spreads and grows. This stuff, volcanic basalt, paves the entire bottom of the ocean floor. You can also see it on islands like Hawaii, it's the same basic kind of rock. But it's the most common rock on earth. Between these two rocks, the basalt that makes up the oceanic crust and the granite that makes up the continental crust, we've got most of the rocks on earth covered right here. In this demonstration, this pan of water represents the earth's mantle. Now, the earth's mantle is not actually liquid molten rock. It's solid, but it's so hot that over long periods of time it does behave somewhat like a liquid in that it moves and deforms. To represent my two kinds of crust, I've got two blocks of wood. This first block of wood is a piece of redwood. And if you know anything about redwood you know it's a fairly light, low density type of wood. And you can see I've cut this block to be quite thick. So this represents my continental crust: thick and low density. And then here I've got a piece of cherry. Cherry's a hardwood and so it's a denser wood. And I've also cut the piece to be thinner, and so this thin, higher density block of wood represents my oceanic crust. And when I put my -- float my continental crust in the mantle, you can see it sits up high. And when I put my oceanic crust into the mantle you can see that it sits down low. So the thick, low-density rock of the continents floats high enough on the mantle to stick up out of the ocean and form dry land, whereas the thin high-density rock of the ocean floor sags down low in the mantle to sit several miles below sea level. If we didn't have any of this stuff on earth, rocks like granite forming thick, low-density rock, there would hardly be any dry land on earth. We would be a planet completely covered by ocean water with maybe just a few islands poking out. And so, every day I hope you do what I do which is to thank the thick light rocks of the continents for floating high in the mantle and forming dry land because otherwise we wouldn't be here talking about it. Now, in fact, I don't actually thank the continental crust every day, but you do get my point. Right?