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MANNING: Mountains are amongst
the most spectacular
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and beautiful features of our planet.
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Yet, when you think about it,
they are quite unusual.
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Mountains are found
only in a few parts of the world.
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For many years, geologists
have been exploring the mountains,
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trying to explain their origins.
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But only recently
have they finally begun to understand
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how and why the world's
great mountain ranges are formed.
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I'm here at Delphi in Greece.
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It's the place where the ancient Greeks came
to get the answers to some difficult questions.
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One of the things that puzzled them,
though I doubt if they consulted the oracle,
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was why seashells are found
throughout these hills.
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How can a sea creature, like this scallop,
be found at the top of a mountain?
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To answer that, geologists today have had to ask
a much more fundamental question,
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"Why are there mountains in the first place?"
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One geologist who's been wrestling
with this problem is Philip England.
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Over the last 20 years, he's developed
a radical new theory of how mountains are made.
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It's a theory that sees mountains
not as ancient and fixed,
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but as active and dynamic features of our planet.
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And Greece is one of the most
geologically active places in the world.
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These are the earthquakes in Greece
over the past 30 years or so.
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And what you can see, of course,
is that this huge region over here,
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600 kilometres or so on the side
which is simply covered with earthquakes.
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And that's what we're interested in,
that's why we're here.
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MANNING: This pattern of earthquakes
is a vital clue
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in the problem of how mountains are made.
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The significance of these earthquakes
started to become clear 35 years ago.
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ANNOUNCER: The keynote of cordiality...
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MANNING: In the early '60s,
the Soviets and the Western nations
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agreed to a partial ban on nuclear testing.
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Tests in the atmosphere
and underwater were banned.
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ANNOUNCER: But the Russian refusal
to allow inspection teams on their own territory
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has prevented any agreement
on underground tests.
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Just take a look at these pictures showing
the upheaval of 12 million tons of earth
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in an American underground test.
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MANNING: Huge underground explosions like this
are effectively man-made earthquakes.
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So the US Air Force
decided to keep an eye on the Soviets
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by setting up a global network of seismometers,
the standard way of monitoring earthquakes.
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Of course, the global seismic network can detect
earthquakes as well as nuclear explosions.
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That's right.
We've been able to monitor earthquakes
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by using instruments
since the turn of the century.
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But we never had a coordinated network
all the way around the world.
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And that gave us the first global picture
of where earthquakes occur.
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That's the crucial point and that's
the exciting point, actually, at that time.
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Yeah.
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I mean, just look at this thin line of earthquakes,
the thin red line
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down the middle of the Atlantic here,
sweeping round into the Indian Ocean.
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And off into the Pacific as well.
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MANNING: The discovery of this extraordinary
pattern of earthquakes was very exciting
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because it confirmed the new theory
of plate tectonics.
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The earthquakes clearly outline the boundaries
of the large plates
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which make up the planet's surface and which
are constantly moving around the globe.
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But significantly, not all the earthquake zones
were so neatly defined.
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Some of these earthquake zones are extremely
narrow, but here is a much broader one.
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Yes. You can still see the plate boundary
coming all the way through Indonesia,
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around the top of India and along here.
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You see there's a lot of earthquakes
marking the edge of the plate.
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But there is a very broad zone here.
Something else is going on here.
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MANNING: And what they noticed was that
these broad zones of earthquakes
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seemed to occur wherever
there were high mountain ranges.
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But what was the link
between earthquakes and mountains?
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The most mountainous region in the world
is the small Himalayan kingdom of Nepal,
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jammed between the flat planes
of India to the south
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and the very high plateau of Tibet to the north.
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Since the 1960s,
geologists have been searching for clues
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linking the Himalayan earthquakes
to the mountains.
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Jean-Philippe Avouac
has recently made a critical discovery.
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But he doesn't work in the high mountains.
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Instead, he spends his time in the foothills
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where the rivers flow down
from the high Himalayas
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onto the plains of India.
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He is walking over the rounded pebbles
of an old riverbed which is now high and dry.
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As you can see,
there is no more water flowing here.
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And the reason for that is that the present river
is far down below to the right.
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So the problem here was to understand
what's going on.
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MANNING: And when he looked closely
at the riverbed, he found charcoal.
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Old pieces of wood.
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Here you have a small piece of charcoal.
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And this kind of matter, it can be dated.
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For example, this terrace
was dated to be 4,000 years old.
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And as you can see, it overhangs
the present riverbed by about 20 metres.
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MANNING: So these pieces of charcoal show that
water last flowed here on this ancient riverbed
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several thousand years ago.
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Now, most people would imagine
that this is the result
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of the river cutting down through the rocks.
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But Jean-Philippe thinks
something else has happened.
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What we found is that
the whole land around is rising,
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bringing up the old riverbed,
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while the valley stays at the same position.
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MANNING: Further upstream,
he has dated three older riverbeds
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even higher above the present river.
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The top one is hidden by the trees.
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So the rocks have been uplifted at about
one centimetre per year on average.
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But actually, that was not a continuous process.
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The rocks were uplifted by very large earthquakes,
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such as the one that occurred in 1934.
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And during those earthquakes,
the rocks here are uplifted suddenly
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by about two metres.
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MANNING: Jean-Philippe's measurements
show that the land here
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has been rising by a centimetre a year.
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This is the fastest rate that's ever been measured.
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(SPEAKING IN NEPALl)
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But there are at least 15 small earthquakes
a day recorded here in Nepal.
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Every single one represents land levels changing.
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It's now clear that seismologists
are actually watching mountains being built.
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(BEEPING)
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An earthquake, basically,
is some kind of rock failure in the rocks at depth.
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When the rocks fail,
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they break along a fault that slips suddenly.
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And each time a fault slips, it's an earthquake.
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MANNING: Seismometers show not only where
the rocks have slipped,
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but also by how much
the rocks have been displaced.
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And these displacements,
when they accumulate over million of years,
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are creating the mountains.
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MANNING: Jean-Philippe's work here
shows that the Himalayas are still very active,
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still growing.
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The amazing conclusion is that
vast mountain ranges
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can form in a relatively short period
of geological time.
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But if the earthquakes are creating the mountains,
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then what are the forces
creating the earthquakes?
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Bob Spicer and Leonore Hoke are exploring
the mountains in western Nepal.
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(BOTH CHATTERING)
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MANNING: Leonore has worked in the high ranges
of the Alps and the Andes.
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Bob is an unusual combination.
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He's a botanist and a geologist.
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They're trekking up to the high Himalayas
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searching for evidence of the fundamental forces
creating these mountains.
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- This looks promising.
- Yeah. It's quite dark.
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There's low oxygen conditions
and preservation's likely to be quite good.
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(GROANS)
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SPICER: Here's a nodule.
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So, has it got the characteristic ammonite?
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No, it's just the shale.
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SPICER: There's quite a lot of them here.
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Well, we are here at 3,600 metres above sea level.
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And where I'm standing is black shale.
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And, in fact, when we look closely,
there are nodules in here. And...
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In fact, here is quite a nice one.
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And if you're lucky,
and we crack these nodules open...
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Some of these nodules contain fossils.
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Here we have a beautiful coiled shell,
in fact, it's an ammonite,
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and this tells us that the sediment is marine.
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And from the detailed structure of that ammonite,
we can tell the age of this sediment,
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which is about 150 million years old.
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SPICER: We want that one.
How much for this one?
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MAN: 200.
SPICER: 200?
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(MAN CHATTERING)
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MANNING: Similar ammonites can be found
along the entire length of the high Himalayas.
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They are a sign of an ancient ocean
which has long since vanished.
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And it seems that most of the mountains here
are made of rocks originally laid down
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on the floor of this ocean.
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- We're in the purple stuff here, aren't we?
- Yeah, in there.
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And that's been mapped as lower Jurassic.
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And it looks like limestone beds,
quite uniformly dipping away from us.
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SPICER: Yeah, and the beds seem
to get thicker as we go up.
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And that's pretty much...
pretty much what we see over there.
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So I think what we're dealing with
is a very large fold.
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I'll measure the inclination.
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They seem to have an apparent dip
of about 30 degrees.
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SPICER: These beds are incredibly contorted.
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MANNING: Immense forces must have squeezed
and folded these ancient limestone beds,
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the remains of dead marine organisms
pushing them up from the ocean floor
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to make this part of the Himalayas.
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Incredible to think that that's basically
a beach that's up on end now.
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HOKE: It's vertical, is it?
SPICER: Yeah.
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MANNING: So how did these rocks get here?
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In the 1960s, geologists could not
have answered that question.
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An S-shaped fold.
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MANNING: But then came the realisation
that the continents were moving around the globe.
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It really is remarkable to realise
that the highest mountain range in the world
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is made up of rocks that were once an ocean floor.
How did this come about?
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Originally, India wasn't here at all.
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There was a great ocean
through this part of the world.
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And India was part of a massive continent
to the south of that ocean.
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It broke off eventually and started moving
northwards until it eventually met Asia,
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and in that process,
squashed the ocean up in front of it.
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MANNING: India was once part
of a continent called Gondwana,
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which was sitting near the South Pole.
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Around 85 million years ago, India broke away
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and moved rapidly northwards
before colliding with Asia.
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Here was the force
that geologists were looking for.
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It's this collision of continents
which is causing all the earthquakes
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and creating the highest
mountain range in the world.
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So the first part of the answer
to the mystery of mountains
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lies in the theory of plate tectonics.
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As the continents collide,
they push up the mountains.
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But it's the events after the collision
which continue to puzzle scientists.
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Mike Searle and Roberto Weinberg
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are trying to work out
exactly when the Himalayas became so high.
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To do this, they need to understand
the events which occur
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deep within the Earth's crust
as mountains are built.
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The evidence they are looking for lies in the rocks
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close to the high and spectacular peaks
around Mount Everest.
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I'm standing here amongst the highest,
some of the highest mountains of the world.
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And in front of me
I have the Nuptse-Lhotse ridge.
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Nuptse is the mountain on the left
and Lhotse is the highest peak on the right.
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This ridge is continuously above 8,000 metres
for about six or seven kilometres.
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In the distance, underneath that mushroom cloud
is the highest mountain of the world,
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Everest, at 8,800 metres.
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And as we swing around, in the distance there
we have Makalu, another giant 8,000-metre peak.
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And as we swing around further
towards the south,
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here we have the magnificent face
of Ama Dablam.
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MANNING: It's only when they reach the glaciers
below the towering peaks that they can start work.
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This is where they find the rocks which show
what happened after India collided with Asia.
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SEARLE: Whoa, lots of biotite.
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- Any granite?
- One minute.
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Yeah, this is the main granite pluton,
which is biotite, muscovite, garnet...
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MANNING: But these are not rocks
from the ocean floor.
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They're finding granites,
rocks which were once molten.
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And these granites reveal a crucial aspect of
the Himalayas that lies hidden deep underground.
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Following the collision, India continued
to penetrate northwards into Asia,
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deforming the rocks by squeezing them
and thickening them.
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Now, this thickening process not only pushed
rocks up, it also pushed rocks down.
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And at the deepest levels that these rocks went,
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the increased temperatures and pressures
ultimately resulted in melting.
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The temperatures were high enough to melt
this rock to form granites
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like we see in the high mountains around us here.
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MANNING: When mountains form,
they develop huge roots.
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They're rather like icebergs, with far more rock
being pushed down than up.
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As the rocks are forced down into the Earth's
interior to form this mountain root, they melt.
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Then, as the mountains continue to be built,
this molten rock rises,
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cooling and solidifying to form granite.
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But when did all this happen?
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SEARLE: What we are particularly
interested in doing
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is dating the timing that this rock solidified
to form this granite.
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We have minute quantities of radioactive minerals,
227
00:20:40,887 --> 00:20:43,560
which we can use for dating this rock.
228
00:20:45,127 --> 00:20:49,359
And when we do that, we find
that most of the granites in this area
229
00:20:49,607 --> 00:20:52,326
solidified around 20 million years ago.
230
00:20:54,727 --> 00:20:57,764
And the amazing thing is that
this 20-million-year age
231
00:20:57,847 --> 00:21:01,283
is actually the same
along the whole of the Himalayan chain.
232
00:21:01,447 --> 00:21:06,441
So I think this probably marks the climax
of mountain building along the Himalayas.
233
00:21:10,487 --> 00:21:15,436
MANNING: Mike Searle's work is confirming that
since the first moments of the collision with Asia
234
00:21:15,527 --> 00:21:20,647
some 55 million years ago,
India has continued to plough its way northwards,
235
00:21:20,727 --> 00:21:23,639
moving at 5 centimetres per year.
236
00:21:24,887 --> 00:21:30,519
And it wasn't until some 20 million years ago
that the mountain building here reached its peak.
237
00:21:32,207 --> 00:21:36,962
The picture of a mountain range formed
by colliding continents seemed complete.
238
00:21:38,087 --> 00:21:41,796
But impressive though the Himalayas are,
the staggering fact is
239
00:21:41,887 --> 00:21:45,596
that they're just a small part
of an even bigger mountain range.
240
00:21:47,847 --> 00:21:52,682
By the mid-1970s, most geologists accepted
that the theory of plate tectonics
241
00:21:52,767 --> 00:21:55,486
could explain the creation of the Himalayas.
242
00:21:55,567 --> 00:21:58,240
But at the same time
that the Himalayas were forming,
243
00:21:58,327 --> 00:22:02,764
there was forming to the north of them
this vast plateau, Tibet.
244
00:22:03,847 --> 00:22:08,477
And some geologists began to question whether
plate tectonics could be the whole answer.
245
00:22:11,007 --> 00:22:14,522
MANNING: The theory explained when
and why mountains were formed,
246
00:22:14,607 --> 00:22:19,806
but it had very little to say about how large
and high these mountain ranges should be.
247
00:22:20,607 --> 00:22:24,885
And it was this question that Philip England
was determined to answer.
248
00:22:27,447 --> 00:22:31,235
It's important to know the size
of the object you're dealing with.
249
00:22:31,327 --> 00:22:37,277
Most people concentrate on the Himalaya
which run along the south of the Tibetan plateau,
250
00:22:37,607 --> 00:22:40,679
but you must realise that these are
a very small range of mountains.
251
00:22:40,767 --> 00:22:43,076
They're barely 200 kilometres wide.
252
00:22:43,167 --> 00:22:47,365
And stretching to the north of them
is this vast plateau of Tibet.
253
00:22:50,887 --> 00:22:56,484
MANNING: The Tibetan plateau extends for nearly
2,000 kilometres to the north of the Himalayas.
254
00:22:57,007 --> 00:22:58,281
And it's very high,
255
00:22:58,367 --> 00:23:02,679
on average, just as high as the Himalayas,
well over five kilometres.
256
00:23:03,287 --> 00:23:07,075
And it's just as active.
There are earthquakes everywhere.
257
00:23:13,047 --> 00:23:15,242
The problem was simply this.
258
00:23:15,327 --> 00:23:20,845
Plate tectonics is a theory about the movement
of large, rigid plates over the surface of the Earth.
259
00:23:21,487 --> 00:23:24,638
And Philip realised that
the collision of two rigid plates
260
00:23:24,727 --> 00:23:27,878
could not produce something
the size and shape of Tibet.
261
00:23:29,287 --> 00:23:34,202
ENGLAND: Now, we knew that plate tectonics
was not the explanation for what we see.
262
00:23:34,687 --> 00:23:39,044
So we thought, why not abandon
completely the idea of rigid plates
263
00:23:39,127 --> 00:23:42,244
and treat the continents as though
they were some kind of fluid?
264
00:23:42,327 --> 00:23:47,162
That's pretty hard to understand.
I mean, Mount Everest appears rather solid.
265
00:23:47,327 --> 00:23:51,036
How can you deal with massive mountains
as if they were a fluid?
266
00:23:51,127 --> 00:23:54,437
I agree, it doesn't make sense
when you think about it first of all,
267
00:23:54,527 --> 00:23:58,202
because when you stand on the surface
of the Earth, you see these great big mountains
268
00:23:58,287 --> 00:24:02,599
made out of rigid rocks that you could
build skyscrapers out of, or whatever.
269
00:24:02,727 --> 00:24:07,801
But what you have to realise
is that the surface of the Earth
270
00:24:08,047 --> 00:24:11,357
that we think of as rigid
is really only a very thin skin
271
00:24:11,447 --> 00:24:13,563
on what's going on down below.
272
00:24:13,647 --> 00:24:17,162
Below the Earth's surface
and not very far below the Earth's surface,
273
00:24:17,247 --> 00:24:19,841
we find rocks that are much more like fluids.
274
00:24:21,407 --> 00:24:24,160
ENGLAND: So what we thought is that
we could treat the continents
275
00:24:24,247 --> 00:24:27,876
as though the surface were being carried along
276
00:24:27,967 --> 00:24:31,084
by something that is much more
fluid-like underneath.
277
00:24:33,007 --> 00:24:35,919
MANNING: But if the rocks
are behaving like a fluid,
278
00:24:36,087 --> 00:24:40,285
how does India pushing into Asia
make the huge plateau of Tibet?
279
00:24:40,687 --> 00:24:44,646
If Asia is a fluid,
there are going to be two forces acting on it.
280
00:24:44,727 --> 00:24:49,801
There's going to be a force produced
by India moving northwards into Asia,
281
00:24:50,207 --> 00:24:53,563
squashing up the crust,
making the crust thicker and higher.
282
00:24:54,247 --> 00:24:58,763
But at the same time,
there's the force of gravity acting on this fluid,
283
00:24:58,847 --> 00:25:03,443
which is going to tend to make it spread,
fall away under its own weight.
284
00:25:04,087 --> 00:25:08,046
We can see how that might be
if we imagine a blob of syrup
285
00:25:08,127 --> 00:25:10,004
standing in for Asia.
286
00:25:10,447 --> 00:25:14,122
If we drop a pile of that syrup onto
the bench here, you can see
287
00:25:14,207 --> 00:25:16,038
- flowing away under its own weight.
- Gravity's taking it.
288
00:25:16,127 --> 00:25:17,242
Yeah. That's right.
289
00:25:17,327 --> 00:25:20,000
Now, let's imagine this spoon is India.
290
00:25:20,087 --> 00:25:22,999
And if I move this slowly into the fluid,
291
00:25:23,247 --> 00:25:27,081
you can see that we're building up
a plateau in front of India.
292
00:25:28,007 --> 00:25:30,680
MANNING: Yes, and of course if you stop,
gravity begins to assert itself...
293
00:25:30,767 --> 00:25:35,158
ENGLAND: Then if you stop, exactly.
Gravity asserts itself and the fluid flows away.
294
00:25:35,447 --> 00:25:37,915
Now, of course, syrup is not a...
295
00:25:38,527 --> 00:25:42,679
Diddling around with syrup is not really the way
to investigate the mechanics of mountains.
296
00:25:42,767 --> 00:25:45,281
And one has to solve this problem properly.
297
00:25:47,847 --> 00:25:50,202
MANNING: In the early '80s,
Philip and his colleagues
298
00:25:50,287 --> 00:25:52,721
wrestled with the problem of creating Tibet,
299
00:25:52,807 --> 00:25:55,685
using the fundamental laws of fluid motion.
300
00:26:02,567 --> 00:26:06,845
Could they get a fluid to look something like
the vast ranges in Asia?
301
00:26:08,167 --> 00:26:11,284
It took them over five years
working on the equations
302
00:26:11,367 --> 00:26:14,404
before they got a shape that resembled Tibet.
303
00:26:16,087 --> 00:26:18,157
ENGLAND: But in the end, we were fairly pleased
304
00:26:18,247 --> 00:26:20,681
with this simple view of the deformation.
305
00:26:21,607 --> 00:26:23,916
And here's the result of one of our experiments.
306
00:26:24,007 --> 00:26:26,475
We've grown a region of high ground
307
00:26:27,207 --> 00:26:29,437
something like 2,000 kilometres across.
308
00:26:29,527 --> 00:26:32,963
- It's roughly the shape of the Tibetan plateau.
- Exactly. Exactly.
309
00:26:33,047 --> 00:26:34,924
So we had a nice, simple theory.
310
00:26:35,007 --> 00:26:38,283
We had a balance between
the compression of India,
311
00:26:38,367 --> 00:26:40,005
crunching the mountains up,
312
00:26:40,087 --> 00:26:43,397
and their tendency to flow apart
under their own weight,
313
00:26:43,527 --> 00:26:46,246
which gave us a plateau
roughly the same size and shape
314
00:26:46,327 --> 00:26:48,158
as the present Tibetan plateau
315
00:26:48,247 --> 00:26:50,477
and roughly the right height as well.
316
00:26:51,487 --> 00:26:54,160
MANNING: So it looked like
their fluid theory of mountains
317
00:26:54,247 --> 00:26:56,317
could explain the shape of Tibet.
318
00:26:56,967 --> 00:26:59,879
But the theory also made
an extraordinary prediction.
319
00:27:00,847 --> 00:27:03,645
When the force pushing up the mountain stops,
320
00:27:03,967 --> 00:27:06,959
then the mountains will start to flow away.
321
00:27:07,727 --> 00:27:12,243
And Philip England has found somewhere
where mountains do appear to be flowing down,
322
00:27:12,967 --> 00:27:14,195
Greece.
323
00:27:16,487 --> 00:27:19,285
This is where St Paul
preached to the Corinthians, is it?
324
00:27:19,367 --> 00:27:20,800
So they say.
325
00:27:20,887 --> 00:27:24,084
And as you can see,
it's underneath the water now.
326
00:27:24,367 --> 00:27:26,437
MANNING: So how can we be sure
which has happened,
327
00:27:26,527 --> 00:27:28,916
I mean, sea level rising or land sinking?
328
00:27:29,007 --> 00:27:32,283
Well, we know pretty well where sea level
has been for the past 2,000 years
329
00:27:32,367 --> 00:27:34,358
and this is nothing to do with sea level change.
330
00:27:34,447 --> 00:27:36,199
- This is the ground sinking.
- Right.
331
00:27:36,287 --> 00:27:39,199
- And this is what's happening all over Greece.
- Yes.
332
00:27:41,487 --> 00:27:45,321
MANNING: And the way this is happening
is clearly visible in the rocks.
333
00:27:47,647 --> 00:27:49,205
Look at that.
334
00:27:52,167 --> 00:27:55,159
ENGLAND: It's incredible. It's so smooth.
MANNING: It's like glass.
335
00:27:55,247 --> 00:27:57,841
Well, what sort of process
can produce rock like that?
336
00:27:57,927 --> 00:28:01,636
It looks as if it's been polished by...
in a factory.
337
00:28:02,207 --> 00:28:04,846
Well, this is part of a fault
338
00:28:04,967 --> 00:28:08,755
that stretches 10 or 15 kilometres
beneath our feet.
339
00:28:08,847 --> 00:28:10,644
- Like right down into the crust?
- That's right.
340
00:28:10,727 --> 00:28:13,321
And what you've got to imagine
is that maybe a million years ago
341
00:28:13,407 --> 00:28:17,764
there was another piece of rock,
just like this, up against this surface.
342
00:28:17,847 --> 00:28:19,599
But every time there's an earthquake,
343
00:28:19,687 --> 00:28:22,599
this piece of rock, that's gone now,
would have slid down
344
00:28:22,687 --> 00:28:27,761
maybe a metre or so past this rock,
polishing it smooth as it goes past.
345
00:28:28,047 --> 00:28:33,758
And what was here is now 10,
maybe 15 kilometres below our feet. It's gone.
346
00:28:35,047 --> 00:28:38,483
MANNING: And this is what
geologists call a normal fault?
347
00:28:38,567 --> 00:28:39,556
ENGLAND: That's right.
348
00:28:39,647 --> 00:28:41,444
This is the opposite of mountain building.
349
00:28:41,527 --> 00:28:43,836
Here the land's falling away.
350
00:28:56,847 --> 00:29:02,160
MANNING: Philip and his team have recently
started to work out how fast Greece is moving.
351
00:29:02,647 --> 00:29:07,323
And they have to climb to the top of almost
every mountain to do their measurements.
352
00:29:14,367 --> 00:29:16,039
MAN: 18.9.
353
00:29:17,527 --> 00:29:19,518
- F.
- MAN: F.
354
00:29:21,487 --> 00:29:22,966
MANNING: Using GPS,
355
00:29:23,167 --> 00:29:25,078
Global Positioning Satellites,
356
00:29:25,207 --> 00:29:29,041
they're measuring the hundreds of movements
on the faults throughout Greece.
357
00:29:29,127 --> 00:29:30,958
(RECEIVER BEEPING)
358
00:29:32,407 --> 00:29:34,921
Right now, the display on the receiver tells me
359
00:29:35,007 --> 00:29:37,646
that we're recording the distance
to nine satellites.
360
00:29:37,727 --> 00:29:40,719
What we can do
to make the measurement more accurate
361
00:29:41,007 --> 00:29:45,080
is to leave the antenna above the mark
on the ground for, say, 24 hours
362
00:29:45,327 --> 00:29:48,319
and measure the distance
repeatedly to the satellites.
363
00:29:48,407 --> 00:29:50,443
So at the end of the day, we might end up
364
00:29:50,527 --> 00:29:53,837
with, say, several thousand
measurements of distance.
365
00:29:54,727 --> 00:29:57,844
To give you an idea of
how accurately we can do this,
366
00:29:57,927 --> 00:30:00,521
we're about 100 kilometres now from Athens
367
00:30:00,687 --> 00:30:02,837
and we can measure that distance
to five millimetres.
368
00:30:02,927 --> 00:30:04,758
That's a distance like that.
369
00:30:05,647 --> 00:30:07,717
1-11...
370
00:30:07,807 --> 00:30:11,686
MANNING: Some of the survey points
they're using were set up in the 1890s
371
00:30:11,767 --> 00:30:14,998
when the whole of Greece
was surveyed extremely accurately.
372
00:30:17,167 --> 00:30:21,160
ENGLAND: The measurements they were making
a hundred years ago were state-of-the-art.
373
00:30:21,247 --> 00:30:24,796
They were measuring angles
to something like two parts per million.
374
00:30:24,887 --> 00:30:28,641
I think that's astonishing with just visual sighting
375
00:30:28,727 --> 00:30:32,003
- using theodolites at that time.
- That's it. That's it.
376
00:30:36,007 --> 00:30:38,077
ENGLAND: So we were very lucky
to have these data.
377
00:30:38,167 --> 00:30:39,600
We can come back now
378
00:30:39,687 --> 00:30:42,884
and see how much each of these
survey points has moved.
379
00:30:42,967 --> 00:30:46,243
Up in the north here, you can see
there hasn't been much movement.
380
00:30:46,327 --> 00:30:48,363
Down here in the centre of Greece, where we are,
381
00:30:48,447 --> 00:30:50,915
the motion has been as much as two metres.
382
00:30:51,087 --> 00:30:55,000
And down at the very bottom,
four to five metres in the past hundred years.
383
00:30:55,087 --> 00:30:58,796
I mean, that seems astonishingly fast
in geological terms, isn't it?
384
00:30:58,887 --> 00:31:02,436
Yes, this is the same rate
at which India is moving into Asia.
385
00:31:02,927 --> 00:31:07,955
And it looks as if Greece is consistently moving
down to the southwest.
386
00:31:08,047 --> 00:31:09,685
It's all in one direction.
387
00:31:09,767 --> 00:31:11,041
That's right. But,
388
00:31:11,127 --> 00:31:14,802
notice here in the centre,
the movement has been two metres.
389
00:31:14,927 --> 00:31:18,806
Here, it's been four or five metres.
So the ground is stretching like this.
390
00:31:18,887 --> 00:31:22,482
And, yes, it's moving in one direction
and there's a reason for that.
391
00:31:22,567 --> 00:31:25,445
Here is the deepest part of the ocean floor.
392
00:31:25,527 --> 00:31:28,599
You remember I told you about syrup on a plate?
Well, here it is.
393
00:31:28,687 --> 00:31:30,643
It's flowing downhill
394
00:31:30,727 --> 00:31:32,843
to the deepest part of the ground
395
00:31:32,927 --> 00:31:34,280
and stretching as it goes.
396
00:31:34,367 --> 00:31:36,198
- Finding its own level.
- Yes.
397
00:31:38,847 --> 00:31:42,760
This is the Gulf of Corinth, one of the places
where Greece is stretching apart
398
00:31:42,847 --> 00:31:46,317
and all those steep slopes that you can see
going off into the distance,
399
00:31:46,407 --> 00:31:49,285
those are the normal faults
that are doing the stretching.
400
00:31:49,367 --> 00:31:51,881
And the ground is dropping down
in the middle there.
401
00:31:51,967 --> 00:31:56,006
Right. Because the edges stretch
and the centre is sinking.
402
00:31:56,087 --> 00:31:57,202
That's it.
403
00:31:59,087 --> 00:32:01,203
MANNING: It is an incredible thought
404
00:32:01,287 --> 00:32:03,039
that in a few million years' time,
405
00:32:03,127 --> 00:32:07,120
this lovely country
will have disappeared under the sea.
406
00:32:10,287 --> 00:32:13,359
But despite all the successes of the fluid theory,
407
00:32:13,447 --> 00:32:16,962
Philip England began to realise
that there was a problem with it.
408
00:32:17,367 --> 00:32:20,484
This first cropped up
when geologists started to examine
409
00:32:20,567 --> 00:32:22,762
photographs of the Earth from space.
410
00:32:22,967 --> 00:32:24,878
Here's a photograph taken from the shuttle.
411
00:32:24,967 --> 00:32:28,926
You can see the Himalaya coming along
the south here, a thin band of mountains,
412
00:32:29,007 --> 00:32:32,158
and stretching away to the north
the Tibetan plateau, flat as you like.
413
00:32:32,247 --> 00:32:34,124
And that's the Indian plains there
414
00:32:34,207 --> 00:32:36,801
and you can even see the curvature of the Earth.
It's extraordinary.
415
00:32:36,887 --> 00:32:39,276
ENGLAND: That's right. It's a huge area.
416
00:32:39,367 --> 00:32:42,325
And what you can see here
are a few problematic features.
417
00:32:42,407 --> 00:32:44,238
You can see this gap in the ground.
418
00:32:44,327 --> 00:32:46,397
This is almost certainly a normal fault.
419
00:32:46,487 --> 00:32:48,205
And there's another one up here.
420
00:32:48,287 --> 00:32:50,323
Perhaps another one over here.
421
00:32:50,407 --> 00:32:52,602
Why do you say that's problematic?
422
00:32:52,687 --> 00:32:54,962
Well, they seem to suggest that Tibet is stretching
423
00:32:55,047 --> 00:32:57,481
but to be honest,
we thought they weren't that important,
424
00:32:57,567 --> 00:33:00,286
- and we tended to neglect them for a while.
- You lived with them for a bit.
425
00:33:00,367 --> 00:33:01,959
For a bit, yes.
426
00:33:02,807 --> 00:33:05,958
MANNING: It's didn't fit with the theory
because normal faults are found
427
00:33:06,047 --> 00:33:08,561
where mountains are sinking, like Greece.
428
00:33:08,647 --> 00:33:10,638
They had not predicted this for Tibet,
429
00:33:10,727 --> 00:33:13,321
which they assumed was still growing.
430
00:33:13,447 --> 00:33:15,722
Something strange was happening.
431
00:33:25,967 --> 00:33:29,676
When scientists were at last
given permission to visit Tibet,
432
00:33:29,767 --> 00:33:32,042
the problem became even more apparent.
433
00:33:56,447 --> 00:33:57,766
(DOG BARKING)
434
00:34:00,207 --> 00:34:04,485
Peter Molnar had also been puzzled
by the normal faults in Tibet.
435
00:34:04,927 --> 00:34:07,805
But even he was surprised by what he found.
436
00:34:07,887 --> 00:34:10,003
MOLNAR: We're standing here in a valley,
437
00:34:10,087 --> 00:34:12,965
a valley extending over
to a range of mountains over here.
438
00:34:13,047 --> 00:34:14,685
And you'll notice
439
00:34:14,847 --> 00:34:17,839
that although the mountains
are dissected with deep valleys,
440
00:34:17,927 --> 00:34:21,363
the crests of ridges that reach back
are all at about the same height.
441
00:34:21,447 --> 00:34:24,200
You can see a whole string of these going along.
442
00:34:24,887 --> 00:34:29,836
Sometime in the past, those ridges
and this valley here were at the same elevation.
443
00:34:31,127 --> 00:34:33,766
The valley has dropped relative to the ridges.
444
00:34:34,647 --> 00:34:39,038
This is not unique. There are hundreds
of normal faults like this in Tibet today.
445
00:34:39,207 --> 00:34:40,720
And this fault is active.
446
00:34:40,807 --> 00:34:44,516
There's likely to be an earthquake
on this fault any time.
447
00:34:47,607 --> 00:34:49,837
MANNING: Faced with this overwhelming evidence,
448
00:34:49,927 --> 00:34:53,124
geologists could no longer ignore these faults.
449
00:34:54,087 --> 00:34:55,918
There was a paradox here.
450
00:34:56,007 --> 00:34:59,636
Although India is still moving into Asia,
pushing up the mountains,
451
00:34:59,727 --> 00:35:02,799
it seemed that Tibet was sinking like Greece.
452
00:35:04,047 --> 00:35:05,799
And then another clue emerged
453
00:35:05,887 --> 00:35:08,879
that something strange had happened to Tibet.
454
00:35:11,927 --> 00:35:13,599
(THUNDER RUMBLING)
455
00:35:17,527 --> 00:35:22,157
Every summer, the Indian subcontinent
is drenched by monsoon rains.
456
00:35:23,407 --> 00:35:28,322
Climatologists now know that the monsoon
is a direct result of the height of Tibet.
457
00:35:29,607 --> 00:35:32,167
During the summer months, the plateau heats up,
458
00:35:32,247 --> 00:35:33,919
warming the upper atmosphere,
459
00:35:34,007 --> 00:35:36,760
drawing in moist air from the Indian Ocean.
460
00:35:37,967 --> 00:35:40,401
As it passes over India and the Himalayas,
461
00:35:40,487 --> 00:35:44,526
the moisture condenses as rain,
giving rise to the annual downpour.
462
00:35:46,167 --> 00:35:51,287
But climatologists have discovered that the rains
were much less intense in the past.
463
00:35:51,367 --> 00:35:55,201
The present monsoon
only started about 10 million years ago.
464
00:35:55,927 --> 00:35:58,725
The implications of this were startling.
465
00:35:58,927 --> 00:36:02,602
Climatologists were suggesting
that about this time,
466
00:36:02,727 --> 00:36:05,764
Tibet must have suddenly increased in height.
467
00:36:07,327 --> 00:36:09,602
It had appeared that Tibet and the Himalaya
468
00:36:09,687 --> 00:36:12,360
reached their peak height
about 20 million years ago,
469
00:36:12,447 --> 00:36:16,725
but the fact that the monsoon
intensified 10 million years ago
470
00:36:16,807 --> 00:36:20,277
seems to imply that the plateau
had greatly increased in height.
471
00:36:20,367 --> 00:36:23,200
And when people started looking
at the normal faults,
472
00:36:23,287 --> 00:36:26,597
then it appeared as though they too
had begun about 10 million years ago.
473
00:36:26,687 --> 00:36:28,917
So there are these two lines
of evidence that suggest
474
00:36:29,007 --> 00:36:31,123
that something crucial happened about that time.
475
00:36:31,207 --> 00:36:32,640
That's right.
476
00:36:32,727 --> 00:36:36,037
The normal faults are telling us
that Tibet is too high
477
00:36:36,127 --> 00:36:38,436
to be supported by the push from India.
478
00:36:38,607 --> 00:36:39,881
And there are two possibilities.
479
00:36:39,967 --> 00:36:42,083
One, India had stopped pushing.
480
00:36:42,167 --> 00:36:43,885
There's no evidence of that, though, is there?
481
00:36:43,967 --> 00:36:46,322
No, India has been moving
at roughly the same speed
482
00:36:46,407 --> 00:36:48,602
for the whole of the last 50 million years.
483
00:36:48,687 --> 00:36:51,406
So the alternative,
which must be the right alternative,
484
00:36:51,487 --> 00:36:53,876
is that Tibet greatly increased in height
485
00:36:54,207 --> 00:36:56,641
at about this 10-million-year interval.
486
00:36:58,527 --> 00:37:02,964
MANNING: If somehow Tibet had got
suddenly higher 10 million years ago,
487
00:37:03,167 --> 00:37:05,920
then maybe it became too high to be supported,
488
00:37:06,007 --> 00:37:08,680
even by the relentless push from India.
489
00:37:09,327 --> 00:37:11,443
And so it started to sink.
490
00:37:12,167 --> 00:37:14,601
But why should this have happened?
491
00:37:15,047 --> 00:37:19,802
Philip has recently had an ingenious idea
which depends on the fact that Tibet,
492
00:37:19,927 --> 00:37:23,203
just like any mountain, has an underlying root.
493
00:37:24,527 --> 00:37:28,725
We've known for about a hundred years
that mountains are a bit like icebergs
494
00:37:28,807 --> 00:37:31,765
and we might draw a picture a bit like this.
495
00:37:32,607 --> 00:37:35,644
Here's India moving into Asia,
496
00:37:36,487 --> 00:37:40,366
crunching up the crust,
making the Himalaya and Tibet like this.
497
00:37:41,047 --> 00:37:45,245
And underneath, making a root,
if you like, of the iceberg,
498
00:37:45,407 --> 00:37:48,046
70 kilometres or so of crust.
499
00:37:48,847 --> 00:37:50,280
And we understood this problem.
500
00:37:50,367 --> 00:37:53,837
The push from India
was stopping these high mountains collapsing...
501
00:37:53,927 --> 00:37:56,122
- Holding them up.
- Holding them up. That's it.
502
00:37:56,207 --> 00:37:57,720
And what we'd overlooked
503
00:37:57,807 --> 00:38:01,595
was that at the same time
that India is scrunching up the crust,
504
00:38:01,687 --> 00:38:05,202
it was also thickening
the top of the upper mantle here.
505
00:38:05,287 --> 00:38:08,199
Now, this stuff is denser than the crust
506
00:38:08,527 --> 00:38:10,643
and was holding it down.
507
00:38:11,167 --> 00:38:14,364
And what we think is
that about 10 million years ago,
508
00:38:14,567 --> 00:38:18,845
this layer sank away into the mantle.
509
00:38:19,847 --> 00:38:24,204
And as this weight was removed,
the crust bobbed up.
510
00:38:25,047 --> 00:38:29,484
So, rather like the drop of honey
falls off the bottom of the spoon,
511
00:38:29,567 --> 00:38:31,842
- it flowed down.
- That's it. That's it.
512
00:38:35,447 --> 00:38:39,406
MANNING: His surprising conclusion
is that the huge root under Tibet
513
00:38:39,527 --> 00:38:42,724
was acting like an anchor and holding Tibet down.
514
00:38:44,607 --> 00:38:48,361
And 10 million years ago
when Tibet lost the bottom of its root,
515
00:38:48,447 --> 00:38:50,358
it suddenly bobbed up.
516
00:38:52,647 --> 00:38:55,002
When that happened, it rose so far
517
00:38:55,087 --> 00:38:59,000
that it became too high
to be supported by the push of India
518
00:38:59,087 --> 00:39:03,239
and so soon afterwards,
started to sink down again and flow away.
519
00:39:05,047 --> 00:39:06,844
It was an extraordinary idea.
520
00:39:06,927 --> 00:39:08,883
Many geologists were sceptical.
521
00:39:09,127 --> 00:39:13,996
After all, there was no way of telling exactly
how the height of Tibet had changed in the past.
522
00:39:21,007 --> 00:39:25,762
But then Bob Spicer realised that as a botanist,
he might be able to do this.
523
00:39:28,447 --> 00:39:32,201
Rocks don't change with altitude but plants do.
524
00:39:37,767 --> 00:39:40,156
SPICER: This is a monsoon forest
525
00:39:40,287 --> 00:39:43,643
and the reason why I'm here
is that I'm interested in
526
00:39:43,847 --> 00:39:47,283
the leaves which are formed
in this kind of situation.
527
00:39:47,727 --> 00:39:51,606
Here, where it's hot and very wet,
leaf size is quite large,
528
00:39:52,127 --> 00:39:55,403
and the tip of the leaf
has got a little projection on,
529
00:39:55,487 --> 00:39:58,081
which sort of sheds the water off when it rains.
530
00:39:58,527 --> 00:40:03,043
The other feature, which is
apparently suggestive of warm conditions,
531
00:40:03,127 --> 00:40:04,799
is this margin.
532
00:40:04,887 --> 00:40:08,323
Now, there are no teeth on there.
It's very, very smooth.
533
00:40:09,167 --> 00:40:11,806
These are features which are adaptations
534
00:40:11,887 --> 00:40:15,004
to the very warm, very humid environment.
535
00:40:18,687 --> 00:40:21,565
MANNING: But that environment
changes with altitude.
536
00:40:22,367 --> 00:40:24,039
Driving up into the Himalayas,
537
00:40:24,127 --> 00:40:27,244
the differences in vegetation are easy to spot.
538
00:40:31,247 --> 00:40:33,317
2,000 metres in altitude
539
00:40:33,407 --> 00:40:37,116
is like moving from the climate
of North Africa to Britain.
540
00:40:42,287 --> 00:40:46,121
I'm looking at the size and the shape
and other features of the leaves,
541
00:40:46,207 --> 00:40:48,926
such as these microscopic teeth
542
00:40:49,487 --> 00:40:51,717
and the pointed tips.
543
00:40:51,807 --> 00:40:57,404
Now, these features are fairly typical of leaves
which are growing in quite cool environments.
544
00:41:00,087 --> 00:41:05,684
We tend to find that vegetation reflects very,
very strongly the climate in which it's growing.
545
00:41:08,007 --> 00:41:10,885
MANNING: And the same rules
apply to fossil leaves.
546
00:41:11,207 --> 00:41:13,960
Bob has been collecting fossils down at sea level
547
00:41:14,167 --> 00:41:16,806
and at altitude in the Himalayas and Tibet.
548
00:41:21,207 --> 00:41:24,199
These fossil leaves come from the Tibetan plateau.
549
00:41:25,327 --> 00:41:28,125
SPICER: They're dated at 11 million years old.
550
00:41:28,687 --> 00:41:31,645
And we can look at a similar collection of leaves
551
00:41:31,767 --> 00:41:34,645
from a site that we know was at sea level
552
00:41:35,807 --> 00:41:40,278
and we can look at the difference in temperature
553
00:41:40,367 --> 00:41:43,962
between the fossils at sea level
and those at this site.
554
00:41:44,047 --> 00:41:48,199
Now, that difference actually equates
to an altitude difference
555
00:41:48,407 --> 00:41:51,524
of about two-and-a-half kilometres.
556
00:41:51,967 --> 00:41:55,676
So, we know that this fossil site
11 million years ago
557
00:41:56,327 --> 00:41:59,080
was sitting at two-and-a-half kilometres
above sea level.
558
00:41:59,167 --> 00:42:00,282
MANNING: Right.
559
00:42:00,367 --> 00:42:03,643
And what height did you find
these fossils at today?
560
00:42:04,087 --> 00:42:07,204
Well, we go back and collect
here now, it's hard work.
561
00:42:07,287 --> 00:42:10,723
It's four-and-a-half kilometres
above sea level now.
562
00:42:10,847 --> 00:42:16,877
So in the last 11 million years,
this area has risen by nearly two kilometres.
563
00:42:17,287 --> 00:42:19,323
So what's been happening?
564
00:42:19,407 --> 00:42:24,879
Well, it looks as if the ideas relating
to the fluid theory are probably right.
565
00:42:25,167 --> 00:42:28,000
What seems to have happened is that Tibet rose
566
00:42:28,087 --> 00:42:32,603
to about two-and-a-half to three kilometres
11 million years ago,
567
00:42:32,687 --> 00:42:38,557
and then the bottom dropped off,
allowing Tibet to spring up rather suddenly.
568
00:42:41,407 --> 00:42:46,561
MANNING: The thought that Tibet sprang up
quickly by such a huge amount, two kilometres,
569
00:42:46,647 --> 00:42:48,399
is quite staggering.
570
00:42:48,567 --> 00:42:53,846
And just as intriguing is that this sudden growth
is the reason Tibet is now collapsing.
571
00:42:54,847 --> 00:42:57,566
But Tibet seems not to be unique.
572
00:42:57,767 --> 00:43:03,205
This whole process appears to be part of the
pattern of mountain building around the world.
573
00:43:22,207 --> 00:43:26,837
In the western United States
lies the Basin and Range Province.
574
00:43:36,207 --> 00:43:39,404
It's a vast area of mountains and deep valleys.
575
00:43:40,087 --> 00:43:43,124
One of the valleys is particularly well-known.
576
00:43:49,007 --> 00:43:53,000
Below me is the infamous
Death Valley of California.
577
00:43:54,047 --> 00:43:56,356
It was given that name by some of the pioneers,
578
00:43:56,447 --> 00:44:00,759
the 49ers who tried to use it
as a shortcut to the gold fields.
579
00:44:03,087 --> 00:44:08,320
It's a long way from Tibet, but from this valley
and the surrounding mountains,
580
00:44:08,607 --> 00:44:11,758
one may catch a glimpse of Tibet's future.
581
00:44:22,567 --> 00:44:28,483
MANNING: Most astonishing, this whole flat valley
basin. Lowest point in the Western Hemisphere.
582
00:44:28,567 --> 00:44:34,085
SPICER: That's right. 86 metres below
sea level here. Of course, what's happening...
583
00:44:35,207 --> 00:44:38,597
MANNING: But Death Valley hasn't
always been below sea level.
584
00:44:38,687 --> 00:44:44,717
Sometime in the past when it was high,
like Tibet, this area lost part of its mountain root.
585
00:44:48,567 --> 00:44:52,765
SPICER: What's seen here in Death Valley,
that is the Valley floor sinking,
586
00:44:52,847 --> 00:44:55,156
is seen all over the Basin and Range Province.
587
00:44:55,247 --> 00:44:59,365
MANNING: You mean that huge area
is slowly sinking down?
588
00:44:59,687 --> 00:45:04,681
That's right. It used to be much higher,
and all that is consistent with the idea
589
00:45:04,847 --> 00:45:08,920
that this big root, which used to be
underneath it, has dropped off.
590
00:45:09,047 --> 00:45:11,766
- It's dropped down into the mantle, then?
- That's right.
591
00:45:11,847 --> 00:45:15,556
And the crust here is thinning, it's stretching.
592
00:45:15,727 --> 00:45:20,517
And that is opening up all the faults
and the valleys are dropping down.
593
00:45:20,607 --> 00:45:24,395
So the crust is slowly collapsing
under its own weight?
594
00:45:24,967 --> 00:45:29,279
Well, the big mountain edifice
is spreading out sideways.
595
00:45:30,687 --> 00:45:34,600
MANNING: The whole of
the Basin and Range Province is now sinking.
596
00:45:34,687 --> 00:45:39,522
And because, like Tibet, it has lost its root,
this is now happening quite rapidly.
597
00:45:41,807 --> 00:45:45,402
How rapidly has recently
been calculated by Jack Wolfe.
598
00:45:48,007 --> 00:45:51,886
There are some things that look like... elm, is it?
599
00:45:51,967 --> 00:45:56,597
Yeah, there's an elm and...
Yeah, that's one of the oaks.
600
00:45:58,687 --> 00:46:02,316
That's the big tree, sequoia, of the Sierra Nevada.
601
00:46:02,407 --> 00:46:04,125
Have you got dates on this?
602
00:46:04,207 --> 00:46:09,645
Yeah, there are several dates in the section
and this would appear to be about 16 million.
603
00:46:10,567 --> 00:46:14,958
So, at 16 million years, how high
do you think this was above sea level?
604
00:46:15,167 --> 00:46:20,480
We generally have come down
about 1,500 metres.
605
00:46:20,567 --> 00:46:26,881
In other words, it was about 1,500 metres higher
at 16 million years ago than it is today.
606
00:46:31,207 --> 00:46:32,606
MANNING: For a whole mountain range
607
00:46:32,687 --> 00:46:36,521
to fall down one-and-a-half kilometres
is quite something.
608
00:46:48,727 --> 00:46:52,242
It seems probable that
in a few tens of millions of years,
609
00:46:52,327 --> 00:46:55,603
parts of Tibet,
one of the highest places in the world,
610
00:46:55,687 --> 00:46:58,759
will be like Death Valley, one of the lowest.
611
00:47:04,887 --> 00:47:09,677
In the last 30 years, geologists have changed
our view of mountains completely.
612
00:47:10,847 --> 00:47:14,203
No longer should we see them
as permanent and fixed,
613
00:47:14,287 --> 00:47:17,324
but as young and active features of our world.
614
00:47:21,007 --> 00:47:24,761
We now know that mountains are made
when continents collide.
615
00:47:24,847 --> 00:47:30,319
But what's been more surprising to learn recently
is that mountains can ebb and flow.
616
00:47:30,967 --> 00:47:35,995
In geological terms, they are formed
very rapidly and collapse just as rapidly.
617
00:47:40,447 --> 00:47:44,679
The Himalayas and Tibet are just
the latest of the high mountains to form
618
00:47:44,767 --> 00:47:48,043
during the long and turbulent history
of our planet.
619
00:47:51,447 --> 00:47:54,439
But as mountains are formed,
it has also become clear
620
00:47:54,527 --> 00:47:59,442
that they have had an unexpected
and far-reaching impact on the rest of the world.
621
00:48:08,247 --> 00:48:13,082
As mountains rise and fall, they've had
dramatic effects on the Earth's climate.
622
00:48:14,207 --> 00:48:19,884
And it's just that relationship between
changes to the Earth and changes to our climate
623
00:48:20,287 --> 00:48:23,279
that we're going to be looking at
in our next programme.