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Compositional and mechanical layers of the earth

Crust, mantle, core, lithosphere, asthenosphere, mesosphere, outer core, inner core. Created by Sal Khan.

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Video transcript

What I want to do in this video is try to get a better understanding of the structure of the earth. And we're actually going to think about it in two different ways. So let me just draw half of the earth over here. That's my best shot at drawing half of a circle. And what we're going to do is think about it in two ways. And on the left-hand side we're going to think about it as the compositional layers, or the chemical layers. So over here we're going to think about the chemical structure, or the composition of the layer. And on the right-hand side we're going to think about the mechanical properties of the layer. And when I say the mechanical properties I'm really just saying is that layer kind of a solid, rigid layer? Is it kind of a liquid layer? Or is it something in between, a kind of a putty-like non-rigid, solid layer? So let's think about it on the chemical or the compositional side first, because to some degree this is simpler. So the outermost layer is the crust. That's the layer that we're sitting on right here, right now, I'm assuming. I'm assuming you're on the planet. So this right here is the crust. It is the outermost, it's obviously solid. We'll think about that when we talk about the mechanical side of things. And it's also the thinnest layer. And crust is not uniform. There's both oceanic crust and continental crust. And let me draw the crust on this side as well. So let me draw some crust over here. We've got some crust right over there. And there is both oceanic crust and continental crust. So oceanic crust is thinner crust. So let's say this part right here-- Let me draw some thicker crust right over here. We'll call the thicker stuff the continental crust, which is thicker and less dense than the oceanic crust. So what I'm doing in this light green color, this is continental. So this right here is continental. And then in this kind of more fluorescent green, this is oceanic crust. And the oceanic crust is pretty thin. It's on the order of about 5 or 10 kilometers. So let's just call this, this is approximately 5 to 10 kilometers thick. And when I talk about oceanic crust I'm not talking about the oceans. I'm not talking about the liquid part, the water. I'm talking about the rock that kind of holds the water, the rock underneath the oceans. And so this is 5 to 10 kilometers thick. If you were to go to the bottom of the ocean and you were to sit on the rock and then drill you'd have to drill about 5 to 10 kilometers to get through that layer, this compositional layer. So this is 5 to 10 kilometers. And the continental crust is about 10 to 70 kilometers thick. And obviously, they are both rigid. They are both solid rock. Now below, when you think about composition, or what the layers are made up of, the next layer below that, this is actually the biggest layer of the earth by volume, is the mantle. Let me draw it like that. I always have trouble drawing the right-hand side of the circle. So this is the mantle right over here. This is all the mantle. And once again, we differentiate it from the crust because it's composed of different types of rock. Now, you go even deeper, and let me give you the depths here. So the mantle starts right below the crust, right below the oceanic and the continental crust. And it goes about 2,900 kilometers deeper. So it's much, much, much thicker than the crust. The crust is on the order of 5 to maybe 70 kilometers thick. This is much, much thicker. So even though I've drawn the crust fairly thin I didn't draw it thin enough relative to how thick I've drawn the mantle. This isn't drawn to scale. Now, you go even deeper than that you get the densest part of the earth. And that is the core. And there's going to be couple of themes here, especially when we think about the mechanical properties of the earth, is the deeper you get you're going to get denser elements, and you're going to have more heat and more pressure. And the reason why you're going to have denser elements is when Earth was first forming and it was kind of in this molten state, the denser elements just kind of sunk to the bottom, and the lighter elements would kind of rise to the top. They would have this buoyancy because they're less dense than everything around it. And really even the gases would kind of bubble up, would essentially bubble up and form our atmosphere. So that's why, in general, the densest things are in the center and the least dense things are on the outside. They're in our atmosphere. And the core, once again, its composition is fundamentally different than the mantle and the crust. We believe that it's mainly metals, and in particular iron and nickel. So that's the structure of the layers of the earth from a composition point of view, from a chemical point of view. Now, let's kind of think about the same layers, but we're going to think more in terms of what's liquid, what's rigid and solid, and what's in between. So the outermost rigid layer of the earth is made up of the crust, both the continental and the oceanic crust, and kind of the coolest top layer of the mantle. So let me draw that in pink. So this layer right over here. So what I'm drawing in pink is the cool, rigid, solid part of the mantle. So it is solid rock, the part of the mantle that is solid rock. It's composition is different than say, the continental crust, but they are both rigid. So if you combine this top-most layer of the mantle with the crust then you're talking about the lithosphere. So this is the lithosphere. And this essentially gets you about, the lithosphere, depending on where you are on the surface of the earth is 10 to 200 kilometers thick. And most of the time it's closer to the high end of this range. The 10 is kind of where you have hot spots in the mantle and it's essentially been able to kind of dissolve parts of the lithosphere. Well, we'll talk more about that when we talk about the actual plate tectonics of it all. And when we talk about plate tectonics the plates are actually lithospheric plates. It's actually the lithosphere that's moving on top of the lower layers of the mantle. So the lithosphere, it is rigid. It is solid. It's made up of the crust and the uppermost layer of the mantle. Now, you go a little bit deeper, the temperatures and the pressures increase. But now the temperatures have increased enough, you have the same composition as the uppermost, the rigid part of the mantle, but the temperatures have now gone up enough that it now turns into not quite a liquid. We won't call it a liquid. It actually still transmits the type of waves that liquids would not transmit. It's more of like a putty type texture. It has fluid properties. It can flow. It's way more viscous than what we would associate with most fluids. So it's not rigid and solid. It can have convection going on in it, but it's not a liquid. It still will transmit certain types of waves that liquids won't. And this is called the asthenosphere, kind of this jelly, putty layer. And it's like that because it's so hot that the rock has somewhat melted. So this layer right here in magenta is the asthenosphere. I've seen some spellings where there's an "e" after the "a." I think that's maybe the European spelling. And the asthenosphere obviously starts right below the lithosphere. It's what the lithospheric plates-- when we talk about plate tectonics-- are riding on top of. It's kind of the gummy material that allows it to actually move, that allows the rigid layer to actually move on top. So it starts below the lithosphere, and it ends at around 660 kilometers deep. So this right here is 660 kilometers deep. And then you go even deeper than that, and now the pressures are so big that even though the temperatures are even higher the pressures are so big that the same material can't have fluid motion anymore. It's essentially been jammed together. So you can imagine if you have things that are somewhat fluid, that means that the molecules can kind of slide past each other, maybe very slowly. But if you increase the pressure enough they'll be jammed into each other. And that's essentially what happens in the next layer of the mantle. All of these layers of the mantle are made up of the same thing. It's just difference of temperature and pressure. And so that next layer of the mantle is called the mesosphere. This is called the mesosphere, but there's also a layer of our atmosphere, the layer right above the stratosphere that's called the mesosphere, and so don't get confused here. These are two different mesospheres. And this layer, the pressure is so big that now we are rigid again. We are kind of definitely solid. None of this debate about a little bit of fluid motion because the pressures are so big. Now, you go a little bit deeper, we are now in the core, the metallic core, and the temperatures are so high that even though the pressures are high, because we have a compositional change, we're at pressures where this type of mesospheric rock is rigid, but metals at these temperatures actually can be fluid, can actually be liquid. And so we actually have a liquid outer core. The entire core, as far as we know, is made up of the same stuff. Just the outer part of the core, the temperatures are high enough to melt the metal, but the pressures aren't so high enough to make them solid. The pressures are definitely high enough to make kind of more rocky material solid, but not the metals. And then you go even deeper, now even though the temperature keeps going up, the pressure is so strong that even the metals are solid. So this is the solid inner core. So when you think about the mechanical properties the innermost-- And just so you know the total distances we're talking about, the outer core starts at-- Actually I didn't tell you where the mesosphere-- So the mantle ends at about 2,900 kilometers deep. So that's clearly where the mesosphere ends as well, because the mesosphere is kind of the lower mantle. So this is 2,900 kilometers deep. Then you go even deeper, you're in the liquid outer core. And that extends from about 2,900 kilometers deep to about 5,100 kilometers deep. So I frankly should make the liquid core in my drawing even wider. So this depth right over here is about 5,100 kilometers deep. And then, obviously, then you have the center of the earth, and the entire radius of the earth is about 6,400 kilometers. So hopefully that clarifies things when you hear people talking about the lithosphere, or the mantle. They're really talking about mechanical versus composition. When we talk about mechanical solid inner core, liquid outer core, essentially solid mesosphere. It's rigid. Then you have something kind of a spongy, somewhat fluid, not solid, not liquid asthenosphere that the lithospheric plates can ride on top of. And then you have your actual rigid, solid lithosphere made up of the uppermost part of the mantle and the crust.