Compositional and Mechanical Layers of the Earth

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Compositional and Mechanical Layers of the Earth : Crust, mantle, core, lithosphere, asthenosphere, mesosphere, outer core, inner core

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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
crust.
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
water
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
deeper.
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
view.
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
lithosphere.
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
lithosphere.
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
mesosphere.
Mesosphere.
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
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.
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
crust.