MANNING: Mountains are amongst the most spectacular
and beautiful features of our planet.
Yet, when you think about it, they are quite unusual.
Mountains are found only in a few parts of the world.
For many years, geologists have been exploring the mountains,
trying to explain their origins.
But only recently have they finally begun to understand
how and why the world's great mountain ranges are formed.
I'm here at Delphi in Greece.
It's the place where the ancient Greeks came to get the answers to some difficult questions.
One of the things that puzzled them, though I doubt if they consulted the oracle,
was why seashells are found throughout these hills.
How can a sea creature, like this scallop, be found at the top of a mountain?
To answer that, geologists today have had to ask a much more fundamental question,
"Why are there mountains in the first place?"
One geologist who's been wrestling with this problem is Philip England.
Over the last 20 years, he's developed a radical new theory of how mountains are made.
It's a theory that sees mountains not as ancient and fixed,
but as active and dynamic features of our planet.
And Greece is one of the most geologically active places in the world.
These are the earthquakes in Greece over the past 30 years or so.
And what you can see, of course, is that this huge region over here,
600 kilometres or so on the side which is simply covered with earthquakes.
And that's what we're interested in, that's why we're here.
MANNING: This pattern of earthquakes is a vital clue
in the problem of how mountains are made.
The significance of these earthquakes started to become clear 35 years ago.
ANNOUNCER: The keynote of cordiality...
MANNING: In the early '60s, the Soviets and the Western nations
agreed to a partial ban on nuclear testing.
Tests in the atmosphere and underwater were banned.
ANNOUNCER: But the Russian refusal to allow inspection teams on their own territory
has prevented any agreement on underground tests.
Just take a look at these pictures showing the upheaval of 12 million tons of earth
in an American underground test.
MANNING: Huge underground explosions like this are effectively man-made earthquakes.
So the US Air Force decided to keep an eye on the Soviets
by setting up a global network of seismometers, the standard way of monitoring earthquakes.
Of course, the global seismic network can detect earthquakes as well as nuclear explosions.
That's right. We've been able to monitor earthquakes
by using instruments since the turn of the century.
But we never had a coordinated network all the way around the world.
And that gave us the first global picture of where earthquakes occur.
That's the crucial point and that's the exciting point, actually, at that time.
Yeah.
I mean, just look at this thin line of earthquakes, the thin red line
down the middle of the Atlantic here, sweeping round into the Indian Ocean.
And off into the Pacific as well.
MANNING: The discovery of this extraordinary pattern of earthquakes was very exciting
because it confirmed the new theory of plate tectonics.
The earthquakes clearly outline the boundaries of the large plates
which make up the planet's surface and which are constantly moving around the globe.
But significantly, not all the earthquake zones were so neatly defined.
Some of these earthquake zones are extremely narrow, but here is a much broader one.
Yes. You can still see the plate boundary coming all the way through Indonesia,
around the top of India and along here.
You see there's a lot of earthquakes marking the edge of the plate.
But there is a very broad zone here. Something else is going on here.
MANNING: And what they noticed was that these broad zones of earthquakes
seemed to occur wherever there were high mountain ranges.
But what was the link between earthquakes and mountains?
The most mountainous region in the world is the small Himalayan kingdom of Nepal,
jammed between the flat planes of India to the south
and the very high plateau of Tibet to the north.
Since the 1960s, geologists have been searching for clues
linking the Himalayan earthquakes to the mountains.
Jean-Philippe Avouac has recently made a critical discovery.
But he doesn't work in the high mountains.
Instead, he spends his time in the foothills
where the rivers flow down from the high Himalayas
onto the plains of India.
He is walking over the rounded pebbles of an old riverbed which is now high and dry.
As you can see, there is no more water flowing here.
And the reason for that is that the present river is far down below to the right.
So the problem here was to understand what's going on.
MANNING: And when he looked closely at the riverbed, he found charcoal.
Old pieces of wood.
Here you have a small piece of charcoal.
And this kind of matter, it can be dated.
For example, this terrace was dated to be 4,000 years old.
And as you can see, it overhangs the present riverbed by about 20 metres.
MANNING: So these pieces of charcoal show that water last flowed here on this ancient riverbed
several thousand years ago.
Now, most people would imagine that this is the result
of the river cutting down through the rocks.
But Jean-Philippe thinks something else has happened.
What we found is that the whole land around is rising,
bringing up the old riverbed,
while the valley stays at the same position.
MANNING: Further upstream, he has dated three older riverbeds
even higher above the present river.
The top one is hidden by the trees.
So the rocks have been uplifted at about one centimetre per year on average.
But actually, that was not a continuous process.
The rocks were uplifted by very large earthquakes,
such as the one that occurred in 1934.
And during those earthquakes, the rocks here are uplifted suddenly
by about two metres.
MANNING: Jean-Philippe's measurements show that the land here
has been rising by a centimetre a year.
This is the fastest rate that's ever been measured.
(SPEAKING IN NEPALl)
But there are at least 15 small earthquakes a day recorded here in Nepal.
Every single one represents land levels changing.
It's now clear that seismologists are actually watching mountains being built.
(BEEPING)
An earthquake, basically, is some kind of rock failure in the rocks at depth.
When the rocks fail,
they break along a fault that slips suddenly.
And each time a fault slips, it's an earthquake.
MANNING: Seismometers show not only where the rocks have slipped,
but also by how much the rocks have been displaced.
And these displacements, when they accumulate over million of years,
are creating the mountains.
MANNING: Jean-Philippe's work here shows that the Himalayas are still very active,
still growing.
The amazing conclusion is that vast mountain ranges
can form in a relatively short period of geological time.
But if the earthquakes are creating the mountains,
then what are the forces creating the earthquakes?
Bob Spicer and Leonore Hoke are exploring the mountains in western Nepal.
(BOTH CHATTERING)
MANNING: Leonore has worked in the high ranges of the Alps and the Andes.
Bob is an unusual combination.
He's a botanist and a geologist.
They're trekking up to the high Himalayas
searching for evidence of the fundamental forces creating these mountains.
- This looks promising. - Yeah. It's quite dark.
There's low oxygen conditions and preservation's likely to be quite good.
(GROANS)
SPICER: Here's a nodule.
So, has it got the characteristic ammonite?
No, it's just the shale.
SPICER: There's quite a lot of them here.
Well, we are here at 3,600 metres above sea level.
And where I'm standing is black shale.
And, in fact, when we look closely, there are nodules in here. And...
In fact, here is quite a nice one.
And if you're lucky, and we crack these nodules open...
Some of these nodules contain fossils.
Here we have a beautiful coiled shell, in fact, it's an ammonite,
and this tells us that the sediment is marine.
And from the detailed structure of that ammonite, we can tell the age of this sediment,
which is about 150 million years old.
SPICER: We want that one. How much for this one?
MAN: 200. SPICER: 200?
(MAN CHATTERING)
MANNING: Similar ammonites can be found along the entire length of the high Himalayas.
They are a sign of an ancient ocean which has long since vanished.
And it seems that most of the mountains here are made of rocks originally laid down
on the floor of this ocean.
- We're in the purple stuff here, aren't we? - Yeah, in there.
And that's been mapped as lower Jurassic.
And it looks like limestone beds, quite uniformly dipping away from us.
SPICER: Yeah, and the beds seem to get thicker as we go up.
And that's pretty much... pretty much what we see over there.
So I think what we're dealing with is a very large fold.
I'll measure the inclination.
They seem to have an apparent dip of about 30 degrees.
SPICER: These beds are incredibly contorted.
MANNING: Immense forces must have squeezed and folded these ancient limestone beds,
the remains of dead marine organisms pushing them up from the ocean floor
to make this part of the Himalayas.
Incredible to think that that's basically a beach that's up on end now.
HOKE: It's vertical, is it? SPICER: Yeah.
MANNING: So how did these rocks get here?
In the 1960s, geologists could not have answered that question.
An S-shaped fold.
MANNING: But then came the realisation that the continents were moving around the globe.
It really is remarkable to realise that the highest mountain range in the world
is made up of rocks that were once an ocean floor. How did this come about?
Originally, India wasn't here at all.
There was a great ocean through this part of the world.
And India was part of a massive continent to the south of that ocean.
It broke off eventually and started moving northwards until it eventually met Asia,
and in that process, squashed the ocean up in front of it.
MANNING: India was once part of a continent called Gondwana,
which was sitting near the South Pole.
Around 85 million years ago, India broke away
and moved rapidly northwards before colliding with Asia.
Here was the force that geologists were looking for.
It's this collision of continents which is causing all the earthquakes
and creating the highest mountain range in the world.
So the first part of the answer to the mystery of mountains
lies in the theory of plate tectonics.
As the continents collide, they push up the mountains.
But it's the events after the collision which continue to puzzle scientists.
Mike Searle and Roberto Weinberg
are trying to work out exactly when the Himalayas became so high.
To do this, they need to understand the events which occur
deep within the Earth's crust as mountains are built.
The evidence they are looking for lies in the rocks
close to the high and spectacular peaks around Mount Everest.
I'm standing here amongst the highest, some of the highest mountains of the world.
And in front of me I have the Nuptse-Lhotse ridge.
Nuptse is the mountain on the left and Lhotse is the highest peak on the right.
This ridge is continuously above 8,000 metres for about six or seven kilometres.
In the distance, underneath that mushroom cloud is the highest mountain of the world,
Everest, at 8,800 metres.
And as we swing around, in the distance there we have Makalu, another giant 8,000-metre peak.
And as we swing around further towards the south,
here we have the magnificent face of Ama Dablam.
MANNING: It's only when they reach the glaciers below the towering peaks that they can start work.
This is where they find the rocks which show what happened after India collided with Asia.
SEARLE: Whoa, lots of biotite.
- Any granite? - One minute.
Yeah, this is the main granite pluton, which is biotite, muscovite, garnet...
MANNING: But these are not rocks from the ocean floor.
They're finding granites, rocks which were once molten.
And these granites reveal a crucial aspect of the Himalayas that lies hidden deep underground.
Following the collision, India continued to penetrate northwards into Asia,
deforming the rocks by squeezing them and thickening them.
Now, this thickening process not only pushed rocks up, it also pushed rocks down.
And at the deepest levels that these rocks went,
the increased temperatures and pressures ultimately resulted in melting.
The temperatures were high enough to melt this rock to form granites
like we see in the high mountains around us here.
MANNING: When mountains form, they develop huge roots.
They're rather like icebergs, with far more rock being pushed down than up.
As the rocks are forced down into the Earth's interior to form this mountain root, they melt.
Then, as the mountains continue to be built, this molten rock rises,
cooling and solidifying to form granite.
But when did all this happen?
SEARLE: What we are particularly interested in doing
is dating the timing that this rock solidified to form this granite.
We have minute quantities of radioactive minerals,
which we can use for dating this rock.
And when we do that, we find that most of the granites in this area
solidified around 20 million years ago.
And the amazing thing is that this 20-million-year age
is actually the same along the whole of the Himalayan chain.
So I think this probably marks the climax of mountain building along the Himalayas.
MANNING: Mike Searle's work is confirming that since the first moments of the collision with Asia
some 55 million years ago, India has continued to plough its way northwards,
moving at 5 centimetres per year.
And it wasn't until some 20 million years ago that the mountain building here reached its peak.
The picture of a mountain range formed by colliding continents seemed complete.
But impressive though the Himalayas are, the staggering fact is
that they're just a small part of an even bigger mountain range.
By the mid-1970s, most geologists accepted that the theory of plate tectonics
could explain the creation of the Himalayas.
But at the same time that the Himalayas were forming,
there was forming to the north of them this vast plateau, Tibet.
And some geologists began to question whether plate tectonics could be the whole answer.
MANNING: The theory explained when and why mountains were formed,
but it had very little to say about how large and high these mountain ranges should be.
And it was this question that Philip England was determined to answer.
It's important to know the size of the object you're dealing with.
Most people concentrate on the Himalaya which run along the south of the Tibetan plateau,
but you must realise that these are a very small range of mountains.
They're barely 200 kilometres wide.
And stretching to the north of them is this vast plateau of Tibet.
MANNING: The Tibetan plateau extends for nearly 2,000 kilometres to the north of the Himalayas.
And it's very high,
on average, just as high as the Himalayas, well over five kilometres.
And it's just as active. There are earthquakes everywhere.
The problem was simply this.
Plate tectonics is a theory about the movement of large, rigid plates over the surface of the Earth.
And Philip realised that the collision of two rigid plates
could not produce something the size and shape of Tibet.
ENGLAND: Now, we knew that plate tectonics was not the explanation for what we see.
So we thought, why not abandon completely the idea of rigid plates
and treat the continents as though they were some kind of fluid?
That's pretty hard to understand. I mean, Mount Everest appears rather solid.
How can you deal with massive mountains as if they were a fluid?
I agree, it doesn't make sense when you think about it first of all,
because when you stand on the surface of the Earth, you see these great big mountains
made out of rigid rocks that you could build skyscrapers out of, or whatever.
But what you have to realise is that the surface of the Earth
that we think of as rigid is really only a very thin skin
on what's going on down below.
Below the Earth's surface and not very far below the Earth's surface,
we find rocks that are much more like fluids.
ENGLAND: So what we thought is that we could treat the continents
as though the surface were being carried along
by something that is much more fluid-like underneath.
MANNING: But if the rocks are behaving like a fluid,
how does India pushing into Asia make the huge plateau of Tibet?
If Asia is a fluid, there are going to be two forces acting on it.
There's going to be a force produced by India moving northwards into Asia,
squashing up the crust, making the crust thicker and higher.
But at the same time, there's the force of gravity acting on this fluid,
which is going to tend to make it spread, fall away under its own weight.
We can see how that might be if we imagine a blob of syrup
standing in for Asia.
If we drop a pile of that syrup onto the bench here, you can see
- flowing away under its own weight. - Gravity's taking it.
Yeah. That's right.
Now, let's imagine this spoon is India.
And if I move this slowly into the fluid,
you can see that we're building up a plateau in front of India.
MANNING: Yes, and of course if you stop, gravity begins to assert itself...
ENGLAND: Then if you stop, exactly. Gravity asserts itself and the fluid flows away.
Now, of course, syrup is not a...
Diddling around with syrup is not really the way to investigate the mechanics of mountains.
And one has to solve this problem properly.
MANNING: In the early '80s, Philip and his colleagues
wrestled with the problem of creating Tibet,
using the fundamental laws of fluid motion.
Could they get a fluid to look something like the vast ranges in Asia?
It took them over five years working on the equations
before they got a shape that resembled Tibet.
ENGLAND: But in the end, we were fairly pleased
with this simple view of the deformation.
And here's the result of one of our experiments.
We've grown a region of high ground
something like 2,000 kilometres across.
- It's roughly the shape of the Tibetan plateau. - Exactly. Exactly.
So we had a nice, simple theory.
We had a balance between the compression of India,
crunching the mountains up,
and their tendency to flow apart under their own weight,
which gave us a plateau roughly the same size and shape
as the present Tibetan plateau
and roughly the right height as well.
MANNING: So it looked like their fluid theory of mountains
could explain the shape of Tibet.
But the theory also made an extraordinary prediction.
When the force pushing up the mountain stops,
then the mountains will start to flow away.
And Philip England has found somewhere where mountains do appear to be flowing down,
Greece.
This is where St Paul preached to the Corinthians, is it?
So they say.
And as you can see, it's underneath the water now.
MANNING: So how can we be sure which has happened,
I mean, sea level rising or land sinking?
Well, we know pretty well where sea level has been for the past 2,000 years
and this is nothing to do with sea level change.
- This is the ground sinking. - Right.
- And this is what's happening all over Greece. - Yes.
MANNING: And the way this is happening is clearly visible in the rocks.
Look at that.
ENGLAND: It's incredible. It's so smooth. MANNING: It's like glass.
Well, what sort of process can produce rock like that?
It looks as if it's been polished by... in a factory.
Well, this is part of a fault
that stretches 10 or 15 kilometres beneath our feet.
- Like right down into the crust? - That's right.
And what you've got to imagine is that maybe a million years ago
there was another piece of rock, just like this, up against this surface.
But every time there's an earthquake,
this piece of rock, that's gone now, would have slid down
maybe a metre or so past this rock, polishing it smooth as it goes past.
And what was here is now 10, maybe 15 kilometres below our feet. It's gone.
MANNING: And this is what geologists call a normal fault?
ENGLAND: That's right.
This is the opposite of mountain building.
Here the land's falling away.
MANNING: Philip and his team have recently started to work out how fast Greece is moving.
And they have to climb to the top of almost every mountain to do their measurements.
MAN: 18.9.
- F. - MAN: F.
MANNING: Using GPS,
Global Positioning Satellites,
they're measuring the hundreds of movements on the faults throughout Greece.
(RECEIVER BEEPING)
Right now, the display on the receiver tells me
that we're recording the distance to nine satellites.
What we can do to make the measurement more accurate
is to leave the antenna above the mark on the ground for, say, 24 hours
and measure the distance repeatedly to the satellites.
So at the end of the day, we might end up
with, say, several thousand measurements of distance.
To give you an idea of how accurately we can do this,
we're about 100 kilometres now from Athens
and we can measure that distance to five millimetres.
That's a distance like that.
1-11...
MANNING: Some of the survey points they're using were set up in the 1890s
when the whole of Greece was surveyed extremely accurately.
ENGLAND: The measurements they were making a hundred years ago were state-of-the-art.
They were measuring angles to something like two parts per million.
I think that's astonishing with just visual sighting
- using theodolites at that time. - That's it. That's it.
ENGLAND: So we were very lucky to have these data.
We can come back now
and see how much each of these survey points has moved.
Up in the north here, you can see there hasn't been much movement.
Down here in the centre of Greece, where we are,
the motion has been as much as two metres.
And down at the very bottom, four to five metres in the past hundred years.
I mean, that seems astonishingly fast in geological terms, isn't it?
Yes, this is the same rate at which India is moving into Asia.
And it looks as if Greece is consistently moving down to the southwest.
It's all in one direction.
That's right. But,
notice here in the centre, the movement has been two metres.
Here, it's been four or five metres. So the ground is stretching like this.
And, yes, it's moving in one direction and there's a reason for that.
Here is the deepest part of the ocean floor.
You remember I told you about syrup on a plate? Well, here it is.
It's flowing downhill
to the deepest part of the ground
and stretching as it goes.
- Finding its own level. - Yes.
This is the Gulf of Corinth, one of the places where Greece is stretching apart
and all those steep slopes that you can see going off into the distance,
those are the normal faults that are doing the stretching.
And the ground is dropping down in the middle there.
Right. Because the edges stretch and the centre is sinking.
That's it.
MANNING: It is an incredible thought
that in a few million years' time,
this lovely country will have disappeared under the sea.
But despite all the successes of the fluid theory,
Philip England began to realise that there was a problem with it.
This first cropped up when geologists started to examine
photographs of the Earth from space.
Here's a photograph taken from the shuttle.
You can see the Himalaya coming along the south here, a thin band of mountains,
and stretching away to the north the Tibetan plateau, flat as you like.
And that's the Indian plains there
and you can even see the curvature of the Earth. It's extraordinary.
ENGLAND: That's right. It's a huge area.
And what you can see here are a few problematic features.
You can see this gap in the ground.
This is almost certainly a normal fault.
And there's another one up here.
Perhaps another one over here.
Why do you say that's problematic?
Well, they seem to suggest that Tibet is stretching
but to be honest, we thought they weren't that important,
- and we tended to neglect them for a while. - You lived with them for a bit.
For a bit, yes.
MANNING: It's didn't fit with the theory because normal faults are found
where mountains are sinking, like Greece.
They had not predicted this for Tibet,
which they assumed was still growing.
Something strange was happening.
When scientists were at last given permission to visit Tibet,
the problem became even more apparent.
(DOG BARKING)
Peter Molnar had also been puzzled by the normal faults in Tibet.
But even he was surprised by what he found.
MOLNAR: We're standing here in a valley,
a valley extending over to a range of mountains over here.
And you'll notice
that although the mountains are dissected with deep valleys,
the crests of ridges that reach back are all at about the same height.
You can see a whole string of these going along.
Sometime in the past, those ridges and this valley here were at the same elevation.
The valley has dropped relative to the ridges.
This is not unique. There are hundreds of normal faults like this in Tibet today.
And this fault is active.
There's likely to be an earthquake on this fault any time.
MANNING: Faced with this overwhelming evidence,
geologists could no longer ignore these faults.
There was a paradox here.
Although India is still moving into Asia, pushing up the mountains,
it seemed that Tibet was sinking like Greece.
And then another clue emerged
that something strange had happened to Tibet.
(THUNDER RUMBLING)
Every summer, the Indian subcontinent is drenched by monsoon rains.
Climatologists now know that the monsoon is a direct result of the height of Tibet.
During the summer months, the plateau heats up,
warming the upper atmosphere,
drawing in moist air from the Indian Ocean.
As it passes over India and the Himalayas,
the moisture condenses as rain, giving rise to the annual downpour.
But climatologists have discovered that the rains were much less intense in the past.
The present monsoon only started about 10 million years ago.
The implications of this were startling.
Climatologists were suggesting that about this time,
Tibet must have suddenly increased in height.
It had appeared that Tibet and the Himalaya
reached their peak height about 20 million years ago,
but the fact that the monsoon intensified 10 million years ago
seems to imply that the plateau had greatly increased in height.
And when people started looking at the normal faults,
then it appeared as though they too had begun about 10 million years ago.
So there are these two lines of evidence that suggest
that something crucial happened about that time.
That's right.
The normal faults are telling us that Tibet is too high
to be supported by the push from India.
And there are two possibilities.
One, India had stopped pushing.
There's no evidence of that, though, is there?
No, India has been moving at roughly the same speed
for the whole of the last 50 million years.
So the alternative, which must be the right alternative,
is that Tibet greatly increased in height
at about this 10-million-year interval.
MANNING: If somehow Tibet had got suddenly higher 10 million years ago,
then maybe it became too high to be supported,
even by the relentless push from India.
And so it started to sink.
But why should this have happened?
Philip has recently had an ingenious idea which depends on the fact that Tibet,
just like any mountain, has an underlying root.
We've known for about a hundred years that mountains are a bit like icebergs
and we might draw a picture a bit like this.
Here's India moving into Asia,
crunching up the crust, making the Himalaya and Tibet like this.
And underneath, making a root, if you like, of the iceberg,
70 kilometres or so of crust.
And we understood this problem.
The push from India was stopping these high mountains collapsing...
- Holding them up. - Holding them up. That's it.
And what we'd overlooked
was that at the same time that India is scrunching up the crust,
it was also thickening the top of the upper mantle here.
Now, this stuff is denser than the crust
and was holding it down.
And what we think is that about 10 million years ago,
this layer sank away into the mantle.
And as this weight was removed, the crust bobbed up.
So, rather like the drop of honey falls off the bottom of the spoon,
- it flowed down. - That's it. That's it.
MANNING: His surprising conclusion is that the huge root under Tibet
was acting like an anchor and holding Tibet down.
And 10 million years ago when Tibet lost the bottom of its root,
it suddenly bobbed up.
When that happened, it rose so far
that it became too high to be supported by the push of India
and so soon afterwards, started to sink down again and flow away.
It was an extraordinary idea.
Many geologists were sceptical.
After all, there was no way of telling exactly how the height of Tibet had changed in the past.
But then Bob Spicer realised that as a botanist, he might be able to do this.
Rocks don't change with altitude but plants do.
SPICER: This is a monsoon forest
and the reason why I'm here is that I'm interested in
the leaves which are formed in this kind of situation.
Here, where it's hot and very wet, leaf size is quite large,
and the tip of the leaf has got a little projection on,
which sort of sheds the water off when it rains.
The other feature, which is apparently suggestive of warm conditions,
is this margin.
Now, there are no teeth on there. It's very, very smooth.
These are features which are adaptations
to the very warm, very humid environment.
MANNING: But that environment changes with altitude.
Driving up into the Himalayas,
the differences in vegetation are easy to spot.
2,000 metres in altitude
is like moving from the climate of North Africa to Britain.
I'm looking at the size and the shape and other features of the leaves,
such as these microscopic teeth
and the pointed tips.
Now, these features are fairly typical of leaves which are growing in quite cool environments.
We tend to find that vegetation reflects very, very strongly the climate in which it's growing.
MANNING: And the same rules apply to fossil leaves.
Bob has been collecting fossils down at sea level
and at altitude in the Himalayas and Tibet.
These fossil leaves come from the Tibetan plateau.
SPICER: They're dated at 11 million years old.
And we can look at a similar collection of leaves
from a site that we know was at sea level
and we can look at the difference in temperature
between the fossils at sea level and those at this site.
Now, that difference actually equates to an altitude difference
of about two-and-a-half kilometres.
So, we know that this fossil site 11 million years ago
was sitting at two-and-a-half kilometres above sea level.
MANNING: Right.
And what height did you find these fossils at today?
Well, we go back and collect here now, it's hard work.
It's four-and-a-half kilometres above sea level now.
So in the last 11 million years, this area has risen by nearly two kilometres.
So what's been happening?
Well, it looks as if the ideas relating to the fluid theory are probably right.
What seems to have happened is that Tibet rose
to about two-and-a-half to three kilometres 11 million years ago,
and then the bottom dropped off, allowing Tibet to spring up rather suddenly.
MANNING: The thought that Tibet sprang up quickly by such a huge amount, two kilometres,
is quite staggering.
And just as intriguing is that this sudden growth is the reason Tibet is now collapsing.
But Tibet seems not to be unique.
This whole process appears to be part of the pattern of mountain building around the world.
In the western United States lies the Basin and Range Province.
It's a vast area of mountains and deep valleys.
One of the valleys is particularly well-known.
Below me is the infamous Death Valley of California.
It was given that name by some of the pioneers,
the 49ers who tried to use it as a shortcut to the gold fields.
It's a long way from Tibet, but from this valley and the surrounding mountains,
one may catch a glimpse of Tibet's future.
MANNING: Most astonishing, this whole flat valley basin. Lowest point in the Western Hemisphere.
SPICER: That's right. 86 metres below sea level here. Of course, what's happening...
MANNING: But Death Valley hasn't always been below sea level.
Sometime in the past when it was high, like Tibet, this area lost part of its mountain root.
SPICER: What's seen here in Death Valley, that is the Valley floor sinking,
is seen all over the Basin and Range Province.
MANNING: You mean that huge area is slowly sinking down?
That's right. It used to be much higher, and all that is consistent with the idea
that this big root, which used to be underneath it, has dropped off.
- It's dropped down into the mantle, then? - That's right.
And the crust here is thinning, it's stretching.
And that is opening up all the faults and the valleys are dropping down.
So the crust is slowly collapsing under its own weight?
Well, the big mountain edifice is spreading out sideways.
MANNING: The whole of the Basin and Range Province is now sinking.
And because, like Tibet, it has lost its root, this is now happening quite rapidly.
How rapidly has recently been calculated by Jack Wolfe.
There are some things that look like... elm, is it?
Yeah, there's an elm and... Yeah, that's one of the oaks.
That's the big tree, sequoia, of the Sierra Nevada.
Have you got dates on this?
Yeah, there are several dates in the section and this would appear to be about 16 million.
So, at 16 million years, how high do you think this was above sea level?
We generally have come down about 1,500 metres.
In other words, it was about 1,500 metres higher at 16 million years ago than it is today.
MANNING: For a whole mountain range
to fall down one-and-a-half kilometres is quite something.
It seems probable that in a few tens of millions of years,
parts of Tibet, one of the highest places in the world,
will be like Death Valley, one of the lowest.
In the last 30 years, geologists have changed our view of mountains completely.
No longer should we see them as permanent and fixed,
but as young and active features of our world.
We now know that mountains are made when continents collide.
But what's been more surprising to learn recently is that mountains can ebb and flow.
In geological terms, they are formed very rapidly and collapse just as rapidly.
The Himalayas and Tibet are just the latest of the high mountains to form
during the long and turbulent history of our planet.
But as mountains are formed, it has also become clear
that they have had an unexpected and far-reaching impact on the rest of the world.
As mountains rise and fall, they've had dramatic effects on the Earth's climate.
And it's just that relationship between changes to the Earth and changes to our climate
that we're going to be looking at in our next programme.