- The earth's surface is divided
into a sort of mosaic of plates.
Here they are,
and these plates move around,
and the point about this mosaic of plates
as we're looking at it here is
you can't really recognize where the continents are
from the shapes of these plates.
The continents are really just passengers.
If I take this one off, for example,
there's Africa hiding underneath it, right,
but the borders of the African plate are not the edges
of the African continent.
So let's say we were just regarding these things
as passengers sitting on the place.
As we'll see in this lecture,
they're actually more interesting than that,
and let me remind you
of how these plates actually move around.
This is a map of the Pacific sea floor
with the water taken away to remind you
that the plates are created at mid-ocean ridges.
Here's the ridge in the Southeast Pacific here
where sea floor
and the plate is created by sea floor spreading,
and it moves away from the ridge and is eventually destroyed
in the deep trenches around the edge of the Pacific,
like around here, near Japan in the Northwest,
and the plates move from the ridges to the trenches,
and all you need to know about plate tectonics,
the beauty, the simplicity of the whole thing is
that to find the borders of the plates,
the edges of the plates,
you just need to know where the earthquakes are.
You just join the dots on a map of earthquakes.
That's your plate.
To find out how fast they're moving,
all you need to know is how fast the ridges are spreading,
and we know how to do that, too.
Now you can see the problem in Asia.
If we come back to Asia and look
at a map of the earthquakes in Asia,
here are the earthquakes.
Tibet is this big high white region in the middle.
You can see there's no problem joining the dots
in the oceans.
Here's a mid-ocean ridge in the Indian ocean.
Here are trenches in Indonesia
and in Japan up here, no problem at all,
nice narrow bands of earthquakes,
but what do we do in central Asia, in Tibet,
in this area here?
How can you possibly join the dots?
They're spread out all over the place, these earthquakes,
and remember, the earthquakes happen when rocks break
or slide about.
So the earthquakes are telling us that things are going on.
You can't possibly join the dots here.
It's a crazy thing to think of doing,
except that it's so easy to do in the oceans,
and when plate tectonics was discovered in 1967,
it was immediately obvious
that the continents were doing something odd.
Couldn't quite understand what was going on.
You can't find where the plate boundaries are
on the continents,
and it took much, much longer to sort out
what the continents were up to,
and it's really only in the last five years or so
that we have a kind of language
for talking about how the continents work.
One of the first clues along the way here is
that the continents are very, very old,
and I have here a rock from the continents.
This rock I'm holding in my hand is
about 3000 million years old.
That's incredibly old.
The earth itself is 4 1/2 thousand million years old,
and this rock is getting on then for that sort of age,
for that the age of the earth,
and then when I tell you that the oldest rocks in the ocean,
the oldest rocks you see on the sea floor are
only 200 million or even less,
we can see there's a huge difference.
The difference between 3000 million years old
for the continents and 200 or less for the oceans is huge.
The oceans are really young,
and the continents are incredibly old,
and that's an important clue to what's going on,
which we have to deal with.
Now, let's return to our problem in Asia.
One of the things we learned how to do
in an earlier lecture was how to rewind the continents
to take them backwards to where they were in the past,
and that's an obvious place to start,
and we'll do that now.
Here is what the continents looked like
150 million years ago,
India tucked up against Africa and Antarctica.
This started to break up.
India breaks away from Gondwanaland,
that supercontinent, about a hundred million years ago
and moves North towards Asia,
and as it gets closer to Asia,
it's slowing down a little now,
and here it is in its present position.
Now, as India was an island in the Indian ocean here,
the whole of that ocean
between India and Asia was vanishing underneath Asia,
and we need to remind ourselves of what actually happens.
(paper rustling)
This is a model to show what happens in a trench
where, in this case,
the sea floor is going to be sliding underneath the land,
and this could be India as a passenger
on an island in the Indian ocean
with the Indian ocean sea floor here
sliding down underneath Asia.
This would be Asia, Southern Tibet.
Let's remind ourselves what actually happens.
As we move, this let's move it,
(low rumbling)
we get shallow earthquakes here
on the interface between those two plates as they go down.
As this plate is being pushed down into the mantle,
we get deep earthquakes within this plate here.
These red lights are showing deep earthquakes
in the plate going down as it breaks up.
Now, remember that these rocks which are entering the trench
on the sea floor are wet rocks.
They have water in them,
and that water is what triggers melting down here.
So somewhere down here, we have melting,
and that magma rises to the surface.
A lot of it will freeze here to make granite,
but some will come to the surface as volcanoes,
along the top here and on the Southern edge of Asia,
(sizzling)
and all this is fine.
We actually do see in Southern Tibet big areas
of granite that would have been formed as India
but the Indian Ocean was sliding underneath Asia.
All that is fine.
We see that,
and on top of that granite will have been volcanoes
of this sort, okay,
but now, let's see what happens
when India actually gets closer to the trench.
So let's just move India right up
into the trench then within, please.
Right there, and now, what's gonna happen now?
What we see today is no volcanoes at all
in Southern Tibet.
There are no active volcanoes,
nor are there any deep earthquakes anymore.
They've all switched off.
India is not going down the hatch.
It's not being pushed back into the earth anymore.
Something else is happening instead.
One thing we know that going on is
that India is still moving.
This is not like a car crash (hand pounding)
where everything just stops.
We know from the spreading in the Indian Ocean ridge
that India is still moving north towards Asia.
It's about 50 millimeters per year.
Now, it was moving much faster.
It was moving at about 150 millimeters per year
before the collision,
and it slowed down to 50 millimeters per year now,
but since the collision about 40 or 50 million years ago,
India has shoved about 2000 kilometers into Asia.
That's a huge distance.
That's from here to Rome
India has plowed into Asia,
and obviously, that has something to do
with why the Himalayas and Tibet are there.