Compositional and Mechanical Layers of the Earth Crust, mantle, core, lithosphere, asthenosphere, mesosphere, outer core, inner core
Compositional and Mechanical Layers of the Earth
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- What I want to do in this video is to 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
- a half of a circle. We're going to think about it in
- two different ways. On the left 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 a solid, rigid
- layer, is it kind of a liquid layer, or is it something
- in between, a kind of putty-like, non-rigid, solid layer.
- So, let's think about it on the chemical or
- compositional side first, because to some degree
- this is simpler.
- The outermost layer is the crust.
- That's the layer we're sitting on right here, right now,
- I'm assuming - assuming you're on the planet.
- It is the outermost and it is obviously solid - we'll
- think about that when we talk about the mechanical
- side of things.
- And it's also the thinnest layer.
- Now, crust is not uniform - there's both oceanic crust
- and continental crust.
- Let me draw the crust on this side as well.
- And there's both oceanic crust and continental crust.
- So, oceanic crust is thinner crust.
- So, let's say that this part right here - let's draw
- some thicker crust - we'll call the thicker part
- continental crust, which is thicker and less dense
- than oceanic crust. So, what I'm doing in this light
- green color, this is continental.
- And then in this more fluorescent green is oceanic
- And the oceanic crust is pretty thin, in the order of
- five to ten kilometers thick.
- Now, when I talk about oceanic crust I'm not talking
- about the oceans, the liquid part, the water, I'm
- talking about the rock that kind of holds the
- The rock underneath the ocean.
- So, this is five to ten kilometers thick.
- If you were to go to the bottom of the ocean, sit on
- the rock, then drill, you would have to drill five to ten
- kilometers to get through that layer, this
- compositional layer.
- So, this is five to ten kilometers.
- And the continental crust is about ten to seventy
- kilometers thick.
- And obviously they are both rigid, they are both
- solid rock.
- Now below, when you think of composition, 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.
- So, 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.
- And once again we differentiate it from the crust
- because it's composed of different types of rock
- Now, we go even deeper.
- Let me give you the depths here. So, the mantle starts
- below the crust, right below the continental and
- oceanic crust, and it goes about 2,900 kilometers
- So it is much, much, much thicker than the crust.
- The crust is on the order of maybe 5 to 70 km thick.
- This is much, much thicker. So, even though I drew
- the crust thin, I didn't draw it thin enough relative to
- how thick I've draw 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.
- There's going to be a couple of themes here,
- especially when we think of the mechanical
- properties of the earth, is that the deeper you get,
- you're going to get denser elements and you're going
- to have more heat and more pressure.
- The reason why you're going to have denser elements
- Is when the Earth was first
- forming and it was kind of in its molten state,
- the denser elements sunk to the bottom, and the
- lighter elements would kindc of rise to the top.
- They would kind of rise to the top. They would have
- this bouyancy because they are less dense than
- everything around it. Even the gases would bubble
- up and form our atmosphere.
- So, that's why in general, the densest things are
- at the center and the least dense things are on
- the outside - they're in our atmosphere.
- And the core, once again, it's composition is
- fundamentally different than the mantle and crust.
- We believe that it is mainly metals, in particular
- iron and nickel.
- So that's the structure of the layers of the earth from
- a composition point of view, a chemical point of
- Now, let's kind of think of 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 oceanic crust,
- and the coolest top layer of the mantle. Let me draw
- that in pink.
- So what I'm drawing in pink is the cool, rigid, solid
- part of the mantle.
- So it is solid rock, part of the mantle is solid rock,
- it's composition is different from the 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
- So this is the lithosphere.
- And this essentially gets you about, depening on
- where you are on the surface of the earth, is 10-200
- kilometers thick. And most of the time, it's closer to
- the high end of the range. The 10 is kind of where you
- have hot spots in the mantle and it's essentially been
- able to dissolve part of the lithosphere, we'll talk
- about that more when we talk about plate tectonics.
- These are actually lithospheric plates, and the
- lithosphere is actually moving on top of the lower
- layers of the mantle.
- The lithosphere it is rigid, 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 pressures increased, 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 it now into not quite a
- liquid, we won't call it a liquid, it still transmits
- a certain type of waves that liquids wouldn't transmit.
- it's more of like a putty-like texture, it has fluid-like
- properties, it can float, it's way more viscous than what
- we'd associate with most liquids, 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 some types of waves
- that liquids won't.
- This is called the asthenosphere, this kind of jelly,
- putty layer. It's like that, because it's so hot that the
- rock has somewhat melted.
- So, this is, this layer right here, is the asthenosphere.
- I've seen some spellings that have an 'e' after the a,
- I think that's the european spelling.
- The aesthenosphere obviously starts right below the
- It's what the lithospheric plates, when we are talking
- about tectonic plates, are riding on top of.
- It's kind of the gummy layer that allows the rigid
- layer to kind of move, 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 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 they 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 a difference of pressure.
- So, the next layer of the mantle is called the
- This is called the mesosphere, but there is also a
- layer of our atmosphere right above the stratosphere
- that's called the mesosphere.
- And so, don't get confused here, these are two
- different mesosphere-s.
- And this layer, the pressure is so big, that now we
- are rigid again. We are kind of definetely solid now.
- None of this debate about a little bit of fluid motion
- because the pressure is so big.
- Now you go a little bit deeper, you are in the core,
- the metallic core, the temperatures are so high, that
- even though the pressures are high because we
- have a compositional change, we are at pressures
- where this type of mesospheric rock is rigid, but
- metals at this temperature can be fluid, can actually
- be liquid.
- 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 metals,
- but the pressures aren't so high enough to make
- them solid.
- They're definitely high enough to kind of make more
- rocky materials solid, but not the metals.
- And then you go even deeper, now the temperature
- keep going up, the pressure is so strong that even
- the metals are solid.
- So, this is the solid, 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 - 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.
- This is 2900 kilometers deep.
- Then you go even deeper.
- Now, you're in the liquid outer core, and that extends
- from about 2900 kilometers deep to about 5100
- kilometers deep.
- So I really should, I frankly should make the
- liquid core in my drawing a little wider.
- So, this depth is about 5100 kilometers deep.
- And then obviously, you have the center of the earth,
- and the entire radius of the earth is about 6400
- So, hopefully that clarifies things when you hear
- people talking about the lithosphere or the mantle,
- they're really talking about mechanical versus
- And when we talk about mechanical, solid inner core,
- liquid outer core, essentially solid mesosphere,
- and then you have something, kind of a spongy,
- somewhat fluid, not-solid, not-liquid aesthenosphere
- that the lithosphere 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
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