Compositional and mechanical layers of the earth Video Lecture - Class 1

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.