What I want to do in this
video is talk a little bit about plate tectonics. And you've probably
heard the word before, and are probably,
or you might be somewhat familiar with
what it discusses. And it's really just the idea
that the surface of the Earth is made up of a bunch
of these rigid plates. So it's broken up into
a bunch of rigid plates, and these rigid plates move
relative to each other. They move relative to
each other and take everything that's
on them for a ride. And the things that are on
them include the continents. So it literally is talking about
the movement of these plates. And over here I have
a picture I got off of Wikipedia of
the actual plates. And over here you have
the Pacific Plate. Let me do that in
a darker color. You have the Pacific Plate. You have a Nazca Plate. You have a South American Plate. I could keep going on. You have an Antarctic Plate. It's actually,
obviously whenever you do a projection
onto two dimensions of a surface of a sphere,
the stuff at the bottom and the top look much bigger
than they actually are. Antarctica isn't
this big relative to say North America
or South America. It's just that we've
had to stretch it out to fill up the rectangle. But that's the Antarctic
Plate, North American Plate. And you can see that
they're actually moving relative to each other. And that's what these
arrows are depicting. You see right over
here the Nazca Plate and the Pacific Plate are
moving away from each other. New land is forming here. We'll talk more about
that in other videos. You see right over here in the
middle of the Atlantic Ocean the African Plate and the South
American Plate meet each other, and they're moving away
from each other, which means that new land, more plate
material I guess you could say, is somehow being
created right here-- we'll talk about that
in future videos-- and pushing these
two plates apart. Now, before we go into the
evidence for plate tectonics or even some of the more details
about how plates are created and some theories as to
why the plates might move, what I want to do
is get a little bit of the terminology of plate
tectonics out of the way. Because sometimes people
call them crustal plates, and that's not exactly right. And to show you the
difference, what I want to do is show you two different
ways of classifying the different
layers of the Earth and then think about how they
might relate to each other. So what you traditionally
see, and actually I've made a video that goes into
a lot more detail of this, is a breakdown of the
chemical layers of the Earth. And when I talk about
chemical layers, I'm talking about what
are the constituents of the different layers? So when you talk
of it in this term, the top most layer, which is the
thinnest layer, is the crust. Then below that is the mantle. Actually, let me show
you the whole Earth, although I'm not going
to draw it to scale. So if I were to draw
the crust, the crust is the thinnest outer
layer of the Earth. You can imagine the blue
line itself is the crust. Then below that,
you have the mantle. So everything between the
blue and the orange line, this over here is the mantle. So let me label the crust. The crust you can literally
view as the actual blue pixels over here. And then inside of the
mantle, you have the core. And when you do this
very high level division, these are chemical divisions. This is saying that
the crust is made up of different types of elements. Its makeup is different
than the stuff that's in the mantle, which is
made up of different things than what's inside the core. It's not describing the
mechanical properties of it. And when I talk about
mechanical properties I'm talking about whether
something is solid and rigid. Or maybe it's so hot and
melted it's kind of a magma, or kind of a plastic solid. So this would be the
most brittle stuff. If it gets warmed up, if rock
starts to melt a little bit, then you have
something like a magma, or you can view it as like a
deformable or a plastic solid. When we talk about plastic,
I'm not talking about the stuff that the case of your
cellphone is made of. I'm talking about
it's deformable. This rock is deformable
because it's so hot and it's somewhat melted. It kind of behaves like a fluid. It actually does
behave like a fluid, but it's much more viscous. It's much thicker
and slower moving than what we would
normally associate with a fluid like water. So this a viscous fluid. And then the most fluid would,
of course, be the liquid state. This is what we
mean when we talk about the mechanical properties. And when you look at
this division over here, the crust is solid. The mantle actually has some
parts of it that are solid. So the uppermost part
of the mantle is solid. Then below that, the
rest of the mantle is kind of in this magma,
this deformable, somewhat fluid state, and
depending on what depth you go into the
mantle there are kind of different levels of fluidity. And then the core,
the outer level layer of the core, the
outer core is liquid, because the
temperature is so high. The inner core is made
up of the same things, and the temperature
is even higher, but since the pressure is
so high it's actually solid. So that's why the mantle,
crust, and core differentiations don't tell you whether it's
solid, whether it's magma, or whether it's really a liquid. It just really tells
you what the makeup is. Now, to think about
the makeup, and this is important for
plate tectonics, because when we talk
about these plates we're not talking
about just the crust. We're talking about
the outer, rigid layer. Let me just zoom
in a little bit. Let's say we zoomed
in right over there. So now we have the
crust zoomed in. This right here is the crust. And then everything
below here we're actually talking about the upper mantle. We haven't gotten too deep
in the mantle right here. So that's why we call
it the upper mantle. Now, right below the
crust, the mantle is cool enough that it is
also in real solid form. So this right here
is solid mantle. And when we talk
about the plates were actually talking about
the outer solid layer. So that includes both the
crust and the solid part of the mantle. And we call that
the lithosphere. When people talk
about plate tectonics, they shouldn't say
crustal plates. They should call these
lithospheric plates. And then below the
lithosphere you have the least viscous
part of the mantle, because the temperature is high
enough for the rock to melt, but the pressure
isn't so large as what will happen when you go into
the lower part of the mantle that the fluid can actually
kind of move past each other, although still pretty viscous. This still a magma. So this is still kind
of in its magma state. And this fluid
part of the mantle, we can't quite call
it a liquid yet, but over large periods of time
it does have fluid properties. This, that essentially
the lithosphere is kind of riding on top of,
we call this the asthenosphere. So when we talk about the
lithosphere and asthenosphere we're really talking
about mechanical layers. The outer layer, the solid
layers, the lithosphere sphere. The more fluid layer right
below that is the asthenosphere. When we talk about the
crust, mantle, and core, we are talking about
chemical properties, what are the things
actually made up of.