Before we go into
possible theories as to why plates actually move,
what I want to do in this video is think a little bit about why
we see the geological features we do see at plate boundaries. And in particular, I want
to focus on the features we see at divergent
plate boundaries, where the plates are moving
away from each other, or where new land
is being created like we saw in the mid-oceanic
ridges, where we see new land being created
right in the center and moving outwards from them. So to do that, let's think
about the different layers. And actually I want to
make one quick correction on the last video. Over here I had drawn these
arrows going in that direction. And based on how I
defined them they should have been
going into the page. And so they should have
had these X's there. Now, with that out
of the way, let's draw a little diagram
of what happens in the early stages of these
divergent plate boundaries. So you might have
your just your crust, and maybe it's
continental crust. So this right here
is the Earth's crust. And then you have the
solid part of the mantle, and the combination of
them is the lithosphere. And then you have
the liquid part, or the super hot
part of the mantle. So this down here is magma. It hasn't solidified. It's hot enough to be
in the liquid state. And all of this combined,
so this right here we consider the mantle. Now, there's some
debate, and we'll talk about this
in the next video, of how hot spots actually form. It could be these
mantle plumes that start at the border between
the mantle and the core. It could be some type
of convection currents in the actual mantle. We'll talk more about
that in the next video, or maybe a few videos from now. But let's take it for
granted that hot spots form in the mantle. So let's say we have an
area of magma right here that is particularly hot. Let me do this in another color. I'll do it in pink. So this is particularly
hot magma here. And we know, or
maybe we don't know, well, you'll learn right now,
if you take the same material and you make it
hotter it's going to become less dense, because
the particles essentially are going to bump into each
other with more kinetic energy and have more space
in between them. And so this really
hot part of the magma, or this really hot
part of the mantle, it is going to move upwards,
because it is less dense. It will have buoyancy. And as it moves upwards, it will
heat up the things around it. And it will eventually make
its way into the lithosphere. And it'll kind of be able to
break through the lithosphere because it's so hot it can
kind of melt its way through. So let's fast forward this. Let's fast forward
this a little bit. So this is step one up here. Now step two. This hot magma is rising
now through the lithosphere, and so it's going to
create a hot spot. It's going to create a
dome in the lithosphere and actually on the crust. And so it might look like this. So the crust is now going
to have a dome in it. And this was the
original lithosphere. And it's now kind of been
broken in two by this hot spot. So the lithosphere
is now broken in two, or it's about to be broken
in two by this hot spot. So all of this is
still the lithosphere. I'll just write litho for short. This up here is the crust. And if you take any rigid
material, and the crust and the lithosphere for
that matter, they're rigid, and you push outward on
it, it won't stretch nicely like a nice elastic balloon. It'll start to crack and have
to be pulled apart in order to kind of take the
pushing from below. So this crust is going
to start to crack. And actually, the best
example where you see this is actually in like
sourdough bread that has really hard
shells around it. You see sourdough bread. Let me see if I can draw
a roll of sourdough bread. It has all of these
cracks in the surface. And that's because the
outer layer, the outer shell of the bread, is really rigid. And so the inside heats up and
the surface area has to expand. These kind of rifts
form in the bread to allow that kind of rigid
shell to actually expand. And that exact same
thing would happen to the crust, or actually
the entire lithosphere. So let me draw this
hot spot again. Let me do it in that pink color. Now the hot spot
has gotten this far. This is the hot magma
right over here. And if we fast forward
even a little bit more, then you could
actually have the crust starting to be
fully pulled apart. So you fast forward
a little bit more, the bottom boundary
of the lithosphere maybe now starts to look
something like this. The magma has kind of broken
through the hardened part, the rigid part of the mantle. So maybe it looks
like this right now. You have your hot
spot right over here. It's gotten that far now. And the crust on top, the actual
what we would kind of normally see has now been pulled
apart to kind of have to cover this new surface area. So now it kind of looks
something like this. So it's been pushed apart. Let me see how well
I can draw this. So now it's been pushed
apart, and as it gets pushed apart it kind of
thins out a little bit as you can imagine it doing. It's almost exactly
as the bread analogy. When you look at bread like
this the rift, the depressions where it was expanding
most vigorously, those parts of the bread
are actually thinner, like these parts of the
bread are actually thinner, and they're not as hard as
the parts that moved away. And you see that exact same
thing happening with the land. And all of this
stuff is continuously getting pushed
outward, essentially to kind of make space
for this hot spot. Now, this step
right over here, you might have a volcano or
two, but more important, you're going to have what's
called a rift valley. Right now we're assuming that
we're not below sea level yet, or we're assuming that
this kind of depression in the land that
you see here hasn't come in contact with
another body of water. And so it'll just kind
of become a little valley in between higher land. And you actually
see that on Earth. And the most famous is
the African Rift Valley. That's right about
this region here. Actually, I have
a better diagram that depicts the African
Rift Valley right over here. It's this whole region
of Africa is actually kind of a big valley created
by a hot spot right over there. Now, as the hot spot kind
of keeps maturing eventually some of the rift will
become so depressed that it will actually
be below sea level. Remember, all this land up
here is being stretched apart. So let me go to the next step. The next step will
be right over here. The land on top is now
maybe below sea level in this next step, and
it comes in contact with maybe an ocean or a sea. And so now it might
look like this. So now the land is
super thin on top. I'll do my best to draw it. So it's super thin
on top, and remember it kind of keeps
getting pulled apart from this bubble of hot magma
that's essentially coming up from below. Let me draw it like this. This is all solid rock here. What I drew in
orange is the crust. This is kind of the
rocky part of the mantle. So the combination
is the lithosphere. And now you have the hot
magma coming up like this. And it might peek through
every now and then and create a volcano there. Maybe it'll peak through
and create a volcano there. But in general, it's going
to keep pushing the land up and outwards. And so this land, even
though you're saying hey, it's being pushed up, because
of the outward motion, this land over here
is going to be lower than the land around it,
like the loaf of bread. If it gets low enough
and comes in below sea level actually and comes in
contact with a body of water, or even if it doesn't
actually, water will start to gather over there. And once again, we
actually see that in the rift forming between
the African and Arabian plates. The Red Sea is actually an
example of exactly that. The Arabian Plate is moving
away from the African Plate because of this hot spot. This is pushing all of the land
up and out right over here. And so this is going out. That is going out. It's moving outward
in every direction. And so it creates
these depressions where water can flow inwards. The Rift Valley hasn't had
water flow into it the way the Red Sea has just yet, but
if it kept happening eventually it's going to get low enough
so that the water will flow into it. So the Red Sea is exactly that. You essentially have
the Indian Ocean flowing into this rift that
formed from this hot spot. And then if you fast forward
a bunch so that finally the magma can kind of surface. So let's fast forward from
even this point even more. So let's fast forward
even more, and let's say now the land has been
pushed a good bit apart. Now the hot spot has
actually surfaced. Now the crust might look
something like this. So it's been pushed apart
a good bit at this point. Now we're talking about on the
order of hundreds of thousands of years or tens of
thousands of years. So the land, for example,
the land that was here, this part of the land
might now be out here. And this part of the land
might now be out here. What's going to happen
is that this hot spot is going to continue
to fuel, and we're assuming everything's
underwater at this point. Since this depression that
was created is now so low the crust was stretched thin. We're going to assume that
all of this is underwater. The hot spot is
essentially going to come out of
underwater volcanoes and start creating what's now--
this body of water's gotten large enough that we can
call it a mid-oceanic ridge. And so it'll actually start
creating an actual ridge with volcanoes in the center. So that's why one, we see things
like the Rift Valley in Africa, we see things like the Red Sea. And maybe even more
importantly, that's why we see something like the
mid-Atlantic rift in the middle of the Atlantic
Ocean, where you have all of this depressed
land that was essentially analogous to that Rift Valley
but it's at a much later stage. And that's why it's
able to collect water, because when the
land was pushed out and stretched thin water was
able to flow into it, going back to the bread
analogy, essentially when this bread was baking
and this part of the crust pushed outwards, you
had this rift form, and then if there was
some water on the bread, or if it was raining,
or if it was connected to a body of water,
water would've eventually flowed in here. And if that bread
kept growing this rift would have kept
growing, eventually to the size of the Atlantic
Ocean in our theoretical bread. And so that's why you have
this huge depressed area where the ocean can form,
but in the middle of it you kind of have this you
have this submersed you have this actual
submersed mountain chain, this submersed
chain of volcanoes, this submersed ridge where
the land actually does go up a little bit because of all
that magma flowing directly out of it. So hopefully that
clears up a little bit. That was always confusing to
me why you see uplifted land but then everything around the
uplifted land is much lower, and why the whole thing is
submersed as it's moving away. So hopefully that clears
things up a little bit.