If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:50

Compositional and mechanical layers of the earth

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 a half of the earth over here that's my best shot at drawing half of a circle and we're going to do it is think about 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 the chemical structure or the composition or the composition 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 a putty like non rigid solid layer so let's think about it on the chemical or the compositional side first because the 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 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 is both oceanic crust and continental crust let me draw the crust on this side as well so let me draw some crust over here I'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 that this part right here let me draw some thicker crust 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 continental and then in this kind of more fluorescent green this is oceanic oceanic crust and the oceanic crust is pretty thin it's on the order of about five or ten kilometers so let's just call this this is approximately five to ten kilometers thick and when I talk about oceanic crust I'm not talk about the oceans I'm going to write the liquid part the water I'm talking about the och that kind of holds the water the rock underneath the oceans and so this is five to ten kilometers thick if you were to go to the bottom of the ocean and you were to kind of sit on the rock and then drill you'd have to drill about five to ten kilometres to get through that layer this compositional layer so this is five to ten kilometres and the continental crust is about ten to 70 kilometers thick ten to the 70 kilometers thick and obviously they are both rigid they are both solid solid rock now below when you think about composition or the what what what the layers are made up of the next layer below then this is actually the biggest layer of the earth by volume is the mantle is the mantles let me draw it like that I always have trouble drawing the right-hand side of this circle so let me draw so this is the mantle right over here let me write down this is all this is all the mantle and once again we differentiate it from the cross because it's composed of different types of rock now you go even deeper and let me give you the depth here so the mantle starts right below the crust right below the oceanic and the continental crust Oceanic and continental and it goes about 2,900 2,900 kilometers deeper so it's much much much thicker than the crust the crust is on the order of a 5 to maybe 70 kilometers thick this is a much much thicker so even even though I've drawn the crust fairly thin it 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 kind of the densest part of the earth and that is the core that is the core and there's going to be a couple of themes here especially when you think about the mechanical properties of the earth is the deeper you get 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 its molten State the denser elements just kind of sunk to the bottom and the lighter elements would sink would kind of rise to the top they would have this buoyancy because there they're less dense than everything around it and really the you know 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 is mainly metals and in particular iron and nickel so that's the structure 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 gonna think more in terms of what's liquid what's rigid in solid and what's in between so the outermost rigid layer of the earth the outermost rigid layer of the earth is made up of the crust both of the continental and the oceanic crust and the kind of the coolest top layer of the of the mantle so let me draw that 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 this is the cool so it is solid rock the part of the mantle that's solid rock it's composition is different than say the continental crust but they are both rigid so if you combine this this topmost this topmost layer of the mantle with the crust then you're talking about the lithosphere so this is the lithosphere the lithosphere and this essentially gets you about the lithosphere depending on where you are I mean we depending on where you are on on the surface of the earth it's 10 to 200 kilometers thick so 10 to 200 kilometers thick and most most of the time it's closer to the high end of this range the 10 is kind of where you have hotspots in the mantle and to essentially been able to kind of dissolve parts of the lithosphere and essentially create new what 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 lit the spheric plates it's actually the lithosphere that's moving on top of the lower layer the lower layers of the mantle so that little it the sphere it is rigid it is solid it's made up of the crust and the upper layer of the mantle 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 or something it has fluid properties it can flow it's way more viscous than what we would associate with most fluids but it's 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 liquid won't and this is called the asthenosphere kind of this jelly putty layer and it's that it's jet it's it's it's like that because it's so hot that the rock is somewhat melted so this is this layer right here in magenta is the asthenosphere asthenosphere I've seen some spellings where there's an e after the a I think that's do 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 its kind of the gummy material that allows it to actually move that allows the rigid layer to actually move on top and it goes so it starts below the starts below the lithosphere and it ends at around six hundred and sixty 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 even the temperatures are heated higher the pressures are so big that what 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 they can they can kind of that means that they can the molecules can kind of slide past each other maybe very slowly but if you increase the pressure enough 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 a difference of temperature and pressure and so that next layer of the mantle is called the mesosphere mezzo it's called the mesosphere and you shouldn't you shouldn't this is called the mesosphere but there's also a layer of our atmosphere that's called the layer right above the stratosphere that's called the mesosphere and so don't get confused here these are two different measures fears and this layer the pressure is so big that now we are rigid again we are kind of you know definitely solid none of this debate about you know a little bit of fluid motion because the pressures are so big now you go a little bit deeper you go a little bit deeper we are now in the core the 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 metallic but metals at these temperatures actually can be fluid can actually be liquid and so we actually have a liquid we actually have a liquid outer core liquid outer 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 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 the pressure even though the temperature keeps going up the pressure is so strong that even the metals are solid so this is the solid solid inner core so when you think about the mechanical properties the innermost and just just so you know that total distances we're talking about the outer core starts at let me actually didn't tell you where the mesosphere so the mesosphere ends I saw the the mantle ends at about 2900 kilometers deep so that's clearly where the mesosphere ends as well because the mesosphere is kind of the lower mantle so this is 2900 kilometers deep then you go even deeper you're in the liquid outer core and that extends from about 2900 kilometers deep to about 50 100 kilometers deep so I really should I frankly should make the liquid core in my drawing even wider so this extends to about this depth right over here is of about 50 100 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 that 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 a 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