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MANNING: "Then the volcano
gave vent to the lavas.
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"I could see their long streams
extending over the slopes,
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"like meshes of flowing hair.
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"But descend into the crater of the volcano
called Snaefells, O audacious traveller,
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"and you will reach the centre of the Earth."
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Here in this desolate volcanic landscape,
those words might seem easy to believe,
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that somewhere here might be a direct route
to the source of the Earth's energy.
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In his adventure classic
Journey to the Centre of the Earth,
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Jules Verne has his hero, professor Lidenbrock,
investigate the interior of our planet
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by the simple expedient of climbing down
an extinct volcano here in Iceland.
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That's just fantasy, of course.
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But what isn't fantasy
is the intense scientific curiosity
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about what's going on beneath our feet.
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Most of the time, the surface of our world
gives no hint of what lies below.
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But there are a few places on Earth
where we can get a glimpse
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of what's happening inside the planet.
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(HELICOPTER HOVERING)
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MAN ON RADIO: That is just one fantastic sight.
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You can really seehow much of this cone has fallen in.
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These talus slopes are all very recent.
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Like, all the loose boulders we see down here
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have fallen off the crater walls since March.
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MANNING: This is Mount Kilauea
on the island of Hawaii.
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Scientists from the US Geological Survey
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regularly fly up here
to keep an eye on the lava pond.
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We're standing on the crater rim of Pu'u'O'o cone.
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This is the active vent on the east rift zone
of Kilauea Volcano.
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A potential hazard here is,
when the pond is so low,
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there's not much supporting
the inside of the crater.
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So we'll have
large chunks of the crater rim that fall
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and if that happens, pond level will then
rise and fall, rise and fall
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and you'll also get spatter and...
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Of course, you don't wanna be standing
on a chunk of rim that might actually go down in.
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MANNING: Mount Kilauea is the world's
most active volcano.
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Geologists suspect that all this volcanism
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reflects some remarkable phenomenon
deep within the planet.
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But the problem scientists face is that
the Earth's interior is totally inaccessible.
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Yet in a sense, scientists have managed
to descend to the very centre of the Earth.
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In a remarkable series of experiments,
they've probed our planet to its core.
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They've shown how the same process
that created this volcano
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also keeps the entire surface of our planet
in constant motion.
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There's no better place to see that motion
than Iceland.
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The island sits astride an extraordinary break
in the crust,
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a rift that marks the boundary
between two of the vast plates
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that make up the Earth's surface.
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Here, geologists like Bob White
can study at first hand the way the plates move.
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So, we're on the edge of the two plates here
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with the American plate over here
and the Eurasian plate behind us.
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Yes, that's right.
There's a zone of about 50 kilometres wide
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where the movement is taken up
and this is just one of those fissures
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that the movement is taken up on
as America separates from Eurasia.
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But over this zone,
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there's about 20 millimetres a year
of movement on average,
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about that much movement.
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MANNING: The movement
that's so visible in Iceland
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is in fact happening all over the world.
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By the late '60s, scientists had worked out
that beneath the crust,
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the entire surface of the planet is divided up
into a small number of pieces, plates.
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These plates are all slowly moving,
carrying the continents with them.
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The theory that described this motion
was called plate tectonics.
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It was the biggest step forward
scientists had ever made
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in understanding the Earth.
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SCIENTIST: Everything then will fit together.
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MANNING: But there was something missing.
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They knew the plates were moving,
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but scientists had no idea why they were moving.
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SCIENTIST: This is a question as to particularly
pressure and temperature.
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MANNING: Nobody was more worried
about this problem
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than one of the architects of the theory,
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Dan McKenzie.
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By the beginning of the '70s,
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we had some understanding
of what was going on on the surface.
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'Cause we'd been able to map the motions
of the plates, really, in quite some detail,
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all over the world.
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But what we really didn't have at that stage
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was any decent understanding
of how these motions were maintained.
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And the obvious place to look
is underneath the plates in the mantle.
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'Cause they clearly weren't being driven
by the winds from the outside. Right?
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It had to be an internal process.
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MANNING: The first hint
of what lies inside our planet
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came from scientists like Bryndis Brandsdottir,
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who spends much of her time monitoring
the many earthquakes which shake Iceland.
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Basically we use different kinds of sensors
to monitor different things within the Earth.
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For instance, you can see earthquakes
which occurred in the Hengill region late last night
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and early this morning.
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But with the right sort of instruments
you can do more than just record local activity,
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where earthquakes are and when they occur?
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Yes. The different waveforms
can give us information
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about the different properties of the Earth
as a whole.
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So you're picking up waves which have come
right through the Earth from a very long distance?
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- Yes. From the other side of the globe.
- Right.
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Actually the biggest earthquakes,
they make the Earth ring like a bell.
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And there are huge waves
travelling through the Earth
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and those waves give us informations
about various places within the Earth.
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MANNING: By putting together information
from thousands of earthquakes,
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scientists built up their first complete picture
of the Earth's interior.
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At its centre lies the core,
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an immense ball of liquid iron
the size of the planet Mars.
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Beyond the core is the vast bulk of the planet,
the mantle,
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which is composed of solid rock.
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The plates are simply the upper layer
of the mantle,
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with the crust just a thin veneer on top.
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But this picture gave no hint of any movement
in the mantle
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that could be driving the motion of the plates.
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To make progress,
scientists needed a different approach.
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Here in Lovgrund, a Swedish island in the Baltic,
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scientists have long been intrigued
by a mysterious phenomenon.
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PELTIER: Hello, Martin.
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MANNING: Canadian geophysicist Dick Peltier
has made the journey from Toronto
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to meet Martin Ekman and see things for himself.
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The sea has always been important here.
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People notice when it changes.
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In 1940, Lisa Nygren was a young woman.
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Lisa tells that she's about 35 years old here.
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EKMAN: And you can see that in front of
the boathouse here there is a lot of water.
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Nowadays there is no water here any longer.
It is completely dry.
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Just gravel and grass.
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(NYGREN SPEAKING IN SWEDISH)
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MANNING: It seemed to the people here
that the sea was slowly ebbing away.
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(MARTIN SPEAKING IN SWEDISH)
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(SPEAKING IN SWEDISH)
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She tells me that when she was young
there was water here.
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It was filled with water,
the small inlet from the bay.
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PELTIER: You mean you could bring boats
right up into what are now fields?
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EKMAN: Yes, rowing boats.
PELTIER: Mm-hm.
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MANNING: Peltier and Ekman are not
the first scientists to study the phenomenon.
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Two and a half centuries ago,
the famous Swedish physicist Anders Celsius
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noticed the same thing.
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EKMAN: This is the famous rock of Lovgrund.
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And here is the sea level mark
made by Celsius in 1731.
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So, 250 years ago, sea level was here.
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Today, it's down there.
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MANNING: Thanks to Celsius's foresight,
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Peltier and Ekman can calculate
the rate of apparent sea level drop here.
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Over one centimetre each year.
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The problem is, there's absolutely
no evidence whatsoever
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of any significant change
to global sea levels for 10,000 years,
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since levels rose at the end of the Ice Age.
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Perhaps it's not the sea that is falling,
but Scandinavia which is rising.
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PELTIER: It's astonishing.
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EKMAN: Believe it or not, this is a beach.
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We are now 200 metres up a mountain
and 30 kilometres from the sea.
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9,000 years ago, this was at sea level
and this was a seashore.
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You can see the stones here have been
reworked by the sea waves.
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Over the years, this has been lifted up
nearly 200 metres and this is the result.
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MANNING: All the evidence suggests
that since the ice melted 10,000 years ago,
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Scandinavia has slowly but steadily
been rising out of the sea.
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Scientists have come up
with a very ingenious explanation for this.
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If you think back to the last great glacial period,
about 20,000 years ago,
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we know the whole of this area
was covered with a colossal ice sheet.
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It was about three kilometres thick
over Scandinavia
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and the huge weight of this sheet
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pressed the crust of Scandinavia
down into the Earth's mantle.
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And then about 10,000 years ago,
relatively quickly, the ice disappeared
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and the crust of Scandinavia bobbed back
and it's still rising today.
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Now, that's a very ingenious explanation
but it does require one rather remarkable thing,
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which is that the rocks of the Earth's mantle
can behave like a fluid, and flow.
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00:14:04,367 --> 00:14:07,803
MANNING: What sort of stuff
is rock-solid yet flows?
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Understanding that would turn out to be the key
to the mystery of why the plates move.
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But to get that key, scientists would have
to perform an ingenious series of experiments,
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which would take them
deep into the planet's interior.
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The first step was to get hold
of some rock from the mantle.
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It's rare stuff, but just occasionally
volcanic eruptions bring to the surface
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small, unmelted fragments of the mantle.
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- Do you often find such objects?
- Well, while you walk around...
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00:14:45,767 --> 00:14:48,440
MANNING: Eckhard Salje
spends his time studying them.
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00:14:49,447 --> 00:14:51,438
In any sort of volcanic event,
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you have a certain chance
to have some basalt coming up.
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And sometimes, when you're very lucky,
you find these green patches on here
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and this green material is actually olivine.
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00:15:03,527 --> 00:15:08,362
MANNING: So this is the lava material
and that's really a piece of the Earth's mantle.
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- Which has been pushed up.
- It is basically stuck into it
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and transported with it up to the surface.
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00:15:13,767 --> 00:15:15,359
And what is olivine?
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00:15:15,447 --> 00:15:20,726
Well, olivine is a silicate.
It has magnesium, iron and so forth in there.
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00:15:21,687 --> 00:15:26,442
Normally it's very fine-grained
as you can see there, but if you're really lucky,
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- you can find crystals.
- Those are crystals.
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00:15:29,327 --> 00:15:32,797
Yes, yes. They are rather big and nice.
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00:15:33,527 --> 00:15:35,916
SALJE: Again, this typically greenish colour.
MANNING: Yes.
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MANNING: That's pure olivine?
SALJE: That's pure olivine. Yes. Yes.
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00:15:39,727 --> 00:15:44,323
Now, they are nice
but of course they don't tell us very much
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as they are, about the conditions
as they are down in the Earth.
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Because normally this stuff is existing
at very high temperatures and pressures.
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That's right. That's right. And what we want to do
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is to simulate the same conditions, but in the lab.
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MANNING: So a tiny sample of olivine is squeezed
between the jaws of a specialised vise
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00:16:05,367 --> 00:16:07,403
called a diamond anvil.
189
00:16:09,727 --> 00:16:12,366
To understand how pressure affects the sample,
190
00:16:12,447 --> 00:16:16,156
Ekhard uses X-rays to look deep
into the structure of the crystal.
191
00:16:19,247 --> 00:16:23,604
The scientists employ these techniques
to descend ever deeper into the Earth.
192
00:16:23,687 --> 00:16:26,155
They're revealing how the behaviour of our planet
193
00:16:26,247 --> 00:16:30,843
is governed by what happens at the atomic level,
inside the mantle.
194
00:16:36,207 --> 00:16:38,562
Although the sample always remains solid,
195
00:16:38,647 --> 00:16:42,765
the intense pressures can disrupt
the structure of the crystals.
196
00:16:44,007 --> 00:16:48,205
And that means the solid material can slowly flow.
197
00:16:51,007 --> 00:16:53,316
Suppose this is a piece of the Earth's mantle.
198
00:16:53,407 --> 00:16:55,921
Seismologists tell us,
based on earthquake observations,
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00:16:56,007 --> 00:16:57,679
that this material is solid.
200
00:16:57,767 --> 00:17:00,122
It's elastic, it's resilient.
201
00:17:01,287 --> 00:17:04,597
What could be more elastic or more resilient?
202
00:17:04,767 --> 00:17:09,045
However, on a longer timescale,
its behaviour is completely different.
203
00:17:27,287 --> 00:17:30,438
So, the mantle of the Earth is
both a solid and a fluid.
204
00:17:30,527 --> 00:17:32,563
It's all a question of timescale.
205
00:17:34,167 --> 00:17:38,126
MANNING: So over long periods of time,
the solid mantle is flowing.
206
00:17:38,807 --> 00:17:42,277
But what does this flow have
to do with the motion of the plates?
207
00:17:42,767 --> 00:17:46,760
Scientists would find the answer to that
when they voyaged even deeper,
208
00:17:47,007 --> 00:17:49,123
down to the iron core.
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00:17:51,447 --> 00:17:52,846
We have here a piece of iron
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00:17:52,927 --> 00:17:56,715
in which we're going to recreate the conditions
at the centre of the Earth.
211
00:17:57,567 --> 00:18:00,445
And we have to do that
by generating very high pressures.
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00:18:00,527 --> 00:18:03,997
And we have to melt it
under those very high pressures.
213
00:18:04,727 --> 00:18:08,117
And we're going to stick it in this,
the hydrogen gas gun.
214
00:18:12,647 --> 00:18:16,560
Now, Keith is loading the gun now
with about seven pounds of gunpowder.
215
00:18:16,887 --> 00:18:20,357
When the gun is fired, it pushes this heavy piston
216
00:18:20,487 --> 00:18:22,682
down this hydrogen-filled tube,
217
00:18:22,927 --> 00:18:25,919
which starts out
at ten times atmospheric pressure,
218
00:18:26,007 --> 00:18:28,282
but the time we arrive here,
219
00:18:28,607 --> 00:18:31,326
it's about 2,000 times atmospheric pressure.
220
00:18:31,407 --> 00:18:33,762
Takes this disc, bursts it.
221
00:18:33,847 --> 00:18:37,157
The hydrogen comes out of here
and pushes on this projectile,
222
00:18:37,247 --> 00:18:39,158
accelerating it down this tube,
223
00:18:39,847 --> 00:18:42,202
at about 10,000 Gs
224
00:18:42,287 --> 00:18:44,755
up to a velocity
of about eight kilometres a second.
225
00:18:44,847 --> 00:18:47,077
Which is why we call it
"the fastest gun in the west".
226
00:18:47,167 --> 00:18:51,001
Once it gets here, it impacts on the target,
generating very high pressures,
227
00:18:51,087 --> 00:18:52,998
melting the iron target.
228
00:18:53,087 --> 00:18:57,080
And Zhang Yigou has about 50 billionths
of a second to measure that temperature.
229
00:18:59,527 --> 00:19:01,245
I'll zero the counters.
230
00:19:01,487 --> 00:19:03,557
MANNING: Neil Holmes is going to all this trouble
231
00:19:03,647 --> 00:19:08,163
because he wants to answer
a very basic question about the Earth's core.
232
00:19:08,247 --> 00:19:09,919
How hot is it?
233
00:19:10,367 --> 00:19:12,005
Okay, everything looks good over here.
234
00:19:12,087 --> 00:19:15,557
MANNING: Since scientists are pretty sure
that the core contains molten iron,
235
00:19:15,647 --> 00:19:19,799
the thinking is that its temperature must be close
to the melting point of iron.
236
00:19:19,887 --> 00:19:22,196
Calibrating target chamber.
237
00:19:23,487 --> 00:19:26,206
MANNING: But not the melting point
at the surface.
238
00:19:27,607 --> 00:19:29,086
KEITH: Bringing up the X-rays.
239
00:19:29,167 --> 00:19:30,236
MANNING: The melting point,
240
00:19:30,327 --> 00:19:33,046
under the stupendous pressures
at the centre of the planet.
241
00:19:33,127 --> 00:19:35,243
- HOLMES: Okay, whenever you're ready, Keith.
- Ready to fire.
242
00:19:35,327 --> 00:19:36,919
HOLMES: Just go ahead. Yeah.
243
00:19:37,007 --> 00:19:38,725
(BEEPING)
244
00:19:53,567 --> 00:19:56,843
Well, there's not much left in here.
245
00:19:58,207 --> 00:20:00,243
That used to be a part of some of it.
246
00:20:00,327 --> 00:20:01,680
I don't know what.
247
00:20:02,167 --> 00:20:05,637
So, the whole target,
everything we started with, is destroyed.
248
00:20:06,047 --> 00:20:08,402
But when the projectile hit the iron,
249
00:20:08,967 --> 00:20:11,197
the iron made a brief flash of light.
250
00:20:14,047 --> 00:20:17,278
The colour of that light
tells us what the temperature was.
251
00:20:17,367 --> 00:20:21,565
What it tells us is that the melting temperature
is 6,200 centigrade.
252
00:20:22,447 --> 00:20:25,803
This is over a thousand degrees hotter
than the surface of the sun.
253
00:20:25,927 --> 00:20:29,761
What this means is the core of the Earth
is a glowing ball of hot iron,
254
00:20:29,847 --> 00:20:32,407
3,000 miles in diameter.
255
00:20:33,167 --> 00:20:35,601
And slowly, over the eons,
256
00:20:35,687 --> 00:20:39,441
that heat is gradually moving out
toward the surface of the Earth.
257
00:20:41,767 --> 00:20:44,679
MANNING: So, at the Earth's centre,
beneath the mantle,
258
00:20:44,767 --> 00:20:48,362
there's a heat source
hotter than the surface of the sun.
259
00:20:54,527 --> 00:20:57,485
Anybody who's ever put a pan of water on a stove
260
00:20:57,567 --> 00:21:01,446
knows that when you heat a fluid from below
it starts moving.
261
00:21:01,967 --> 00:21:04,435
A little bit of dye shows what happens.
262
00:21:05,247 --> 00:21:07,761
Hot material comes up from the bottom,
263
00:21:07,847 --> 00:21:11,601
rises to the surface, where it cools
and sinks back again.
264
00:21:12,287 --> 00:21:15,006
This is the process known as convection.
265
00:21:15,487 --> 00:21:20,959
To geologists, the obvious question was,
"Is this what's happening inside the Earth?"
266
00:21:21,567 --> 00:21:23,797
Now, it was Lord Rayleigh in the 19th century
267
00:21:23,887 --> 00:21:27,880
who discovered
that convection depends on several factors.
268
00:21:27,967 --> 00:21:30,800
One, the thermal properties of the fluid.
269
00:21:31,007 --> 00:21:35,717
Secondly, the viscosity of the fluid.
And thirdly, how deep the layer is.
270
00:21:36,407 --> 00:21:39,365
Now, thinking about the mantle,
we know how deep the mantle is
271
00:21:39,447 --> 00:21:41,961
from the earthquake evidence.
272
00:21:42,047 --> 00:21:47,201
We know about the thermal properties from
the work that's been done with diamond anvils.
273
00:21:47,287 --> 00:21:50,484
And we can get a good estimate of the viscosity
274
00:21:50,567 --> 00:21:54,401
from the speed with which
Scandinavia is rebounding
275
00:21:54,487 --> 00:21:57,047
after the pressure of the ice has been removed.
276
00:21:57,487 --> 00:22:01,685
When geophysicists plugged these values
into Rayleigh's formula,
277
00:22:01,767 --> 00:22:05,806
they discovered that not only can
the Earth's mantle convect,
278
00:22:05,887 --> 00:22:09,766
it must convect,
and must be convecting very vigorously.
279
00:22:22,007 --> 00:22:24,965
So the situation now
is really very much more interesting
280
00:22:25,047 --> 00:22:27,800
because we have this mobile Earth,
281
00:22:27,887 --> 00:22:31,516
which is mobile because it's transporting heat
282
00:22:31,687 --> 00:22:35,396
from the interior towards the surface.
283
00:22:35,527 --> 00:22:37,882
Both upwards by hot material rising
284
00:22:37,967 --> 00:22:40,527
and downwards by cold material sinking.
285
00:22:40,607 --> 00:22:43,599
And the thing acts really
just like an engine, right?
286
00:22:43,687 --> 00:22:46,121
It's converting heat, right,
287
00:22:46,207 --> 00:22:49,597
the heat which it's transporting,
into some sort of work.
288
00:22:49,687 --> 00:22:53,475
And it's this work
which is driving the plates around on the surface.
289
00:22:55,287 --> 00:22:58,040
MANNING: It's not difficult to see
how the motion of the plates
290
00:22:58,127 --> 00:23:00,880
relates to convection in the mantle.
291
00:23:01,567 --> 00:23:04,798
At the mid-ocean ridges, hot mantle is flowing up.
292
00:23:04,887 --> 00:23:09,005
And as it cools, it forms new plate
which moves away from the ridge.
293
00:23:10,167 --> 00:23:12,203
Eventually, it's cold and dense enough
294
00:23:12,287 --> 00:23:15,484
to sink back down again into the planet's interior.
295
00:23:21,407 --> 00:23:25,923
So, the complex motion of the plates
is actually part of a simple process
296
00:23:26,087 --> 00:23:29,875
by which the Earth is slowly losing
its massive store of heat.
297
00:23:42,007 --> 00:23:44,999
But what about the volcanism
in places like Hawaii?
298
00:23:45,647 --> 00:23:48,081
How did that fit into mantle convection?
299
00:23:55,287 --> 00:23:58,563
When Dan McKenzie started experimenting
with convection,
300
00:23:58,767 --> 00:24:01,440
he made a totally unexpected discovery.
301
00:24:03,567 --> 00:24:06,525
What we wanted to do was actually see
302
00:24:06,607 --> 00:24:10,361
what the general sort of things
303
00:24:10,447 --> 00:24:13,484
that fluids like the Earth's mantle could do.
304
00:24:13,567 --> 00:24:15,956
So what we wanted was a fluid
305
00:24:16,047 --> 00:24:19,756
which had properties
which were like those of the mantle.
306
00:24:19,847 --> 00:24:22,919
And the one we chose was Lyle's Golden Syrup.
307
00:24:24,007 --> 00:24:28,842
The arrangement is that you have
hot water underneath, right, here
308
00:24:28,927 --> 00:24:32,920
which actually causes
this layer of syrup to convect.
309
00:24:33,007 --> 00:24:37,956
Because it's heated from the bottom by the water
and cooled from the top by the atmosphere.
310
00:24:38,447 --> 00:24:42,156
And we can see what's going on by shining light
311
00:24:42,247 --> 00:24:45,125
through this layer onto the mirror at the bottom.
312
00:24:45,207 --> 00:24:47,437
And then makes an image on the screen.
313
00:24:47,527 --> 00:24:50,519
And you can see
314
00:24:50,967 --> 00:24:54,880
the dark parts on the screen
are where hot fluid is rising
315
00:24:55,087 --> 00:24:58,636
and the bright ones
are where cold fluid is sinking.
316
00:24:58,767 --> 00:25:01,327
So it comes up in a plume in the middle
317
00:25:01,407 --> 00:25:03,796
and then goes down round the outside.
318
00:25:03,927 --> 00:25:06,441
And you can see that the whole of this screen
319
00:25:06,527 --> 00:25:09,758
is covered with all these little cells.
320
00:25:11,687 --> 00:25:15,282
We could heat it in different ways,
we could do all kinds of different things.
321
00:25:15,367 --> 00:25:18,200
We could alter the top temperature
and alter the viscosity contrast.
322
00:25:18,287 --> 00:25:21,996
Really doesn't make much difference.
You always get this pattern.
323
00:25:22,087 --> 00:25:23,566
Lots of little cells.
324
00:25:23,647 --> 00:25:26,764
Not really at all like what we wanted for plates.
325
00:25:26,847 --> 00:25:28,803
That's a bit perplexing at first sight.
326
00:25:28,887 --> 00:25:30,923
Yes. Yes, absolutely.
327
00:25:31,007 --> 00:25:33,567
It was also rather a disappointment.
328
00:25:33,647 --> 00:25:36,002
Because what we really wanted to understand
329
00:25:36,087 --> 00:25:38,885
was how we could drive the plates.
330
00:25:38,967 --> 00:25:41,959
But this then gave us the idea
331
00:25:42,047 --> 00:25:45,323
that maybe on Earth,
the same thing was going on.
332
00:25:45,487 --> 00:25:47,921
And that underneath these big plates
333
00:25:48,007 --> 00:25:51,795
there were perhaps lots of little cells
like this which were convecting.
334
00:26:09,847 --> 00:26:13,886
MANNING: Could one of Dan McKenzie's
convecting cells lie beneath Hawaii?
335
00:26:23,767 --> 00:26:27,999
If so, then all this lava
is coming from a plume of mantle rock
336
00:26:28,087 --> 00:26:30,555
that has risen from deep within the planet.
337
00:26:37,207 --> 00:26:39,767
MAN: Why don't you stand about here
and give me enough coil...
338
00:26:39,847 --> 00:26:43,601
MANNING: Lava flows in underground channels
down from the main vent.
339
00:26:43,687 --> 00:26:47,043
That gives Carl Thornber and Dave Berkovici
340
00:26:47,127 --> 00:26:49,800
a chance to go fishing for a sample.
341
00:26:49,887 --> 00:26:53,163
THORNBER: Perfect! Absolutely perfect shot.
342
00:26:53,887 --> 00:26:56,640
Okay? Are we gonna...
Ready to go ahead with this sample?
343
00:26:56,887 --> 00:26:57,876
Okay.
344
00:26:58,087 --> 00:27:00,521
Hey, hang on to the end of that.
345
00:27:04,167 --> 00:27:05,282
Okay.
346
00:27:05,567 --> 00:27:06,841
Looks good.
347
00:27:10,727 --> 00:27:12,877
We're at the top of that lava fall.
348
00:27:13,847 --> 00:27:15,326
And it's in.
349
00:27:16,527 --> 00:27:18,597
Oh, Jesus. That's moving.
350
00:27:22,247 --> 00:27:24,477
Okay, I'm trying to get a good sample.
351
00:27:24,607 --> 00:27:26,086
Man, it's hot.
352
00:27:30,487 --> 00:27:31,636
Come on, baby.
353
00:27:33,527 --> 00:27:35,324
Okay, we're coming up.
354
00:27:35,887 --> 00:27:37,366
We got a big one.
355
00:27:39,567 --> 00:27:41,762
Okay, we're ready to pull and quench.
356
00:27:42,327 --> 00:27:45,399
Okay, here we go. One, two, three!
357
00:27:46,047 --> 00:27:47,446
Let's go.
358
00:27:51,567 --> 00:27:53,444
- THORNBER: That was a good sample.
- Yeah.
359
00:27:55,687 --> 00:27:57,040
THORNBER: Anyone for tea?
360
00:28:02,567 --> 00:28:05,320
THORNBER: Routine analysis of fresh lava samples
361
00:28:05,407 --> 00:28:08,365
allows us to predict
what the volcano might do next.
362
00:28:08,447 --> 00:28:10,881
But perhaps more significantly...
363
00:28:10,967 --> 00:28:12,002
Some of these are still hot.
364
00:28:12,087 --> 00:28:16,205
More significantly, in terms of
where all these basalts come from,
365
00:28:16,287 --> 00:28:19,882
this stuff contains an extraordinary proportion
of helium-3,
366
00:28:20,007 --> 00:28:22,999
which is relatively rare in volcanic rocks.
367
00:28:23,287 --> 00:28:26,916
Now, one might wonder where this helium-3
comes from.
368
00:28:27,607 --> 00:28:30,838
One source where helium-3 is still present
within the Earth today
369
00:28:30,927 --> 00:28:32,406
is at the core-mantle boundary.
370
00:28:32,487 --> 00:28:37,083
And if that's the case, then one might expect that
this stuff came from way down there.
371
00:28:41,087 --> 00:28:44,477
MANNING: The composition of the lava indicates
that it's come from mantle rock
372
00:28:44,567 --> 00:28:47,604
that has risen rapidly
from deep down in the Earth.
373
00:28:49,047 --> 00:28:52,676
And there's another line of evidence
that Hawaii sits above a plume.
374
00:28:56,767 --> 00:28:59,406
One of the major clues
that we're standing above a mantle plume
375
00:28:59,487 --> 00:29:01,443
is that the Hawaiian islands themselves
376
00:29:01,527 --> 00:29:05,281
form an almost perfectly linear chain
that moves off to the northwest.
377
00:29:06,087 --> 00:29:08,476
MANNING: The plume is sitting
under the Pacific plate
378
00:29:08,567 --> 00:29:11,923
which has been moving across it
for 100 million years.
379
00:29:12,687 --> 00:29:15,884
So the volcanic islands
the plume created in the past
380
00:29:15,967 --> 00:29:18,640
have been carried away by the moving plate.
381
00:29:21,487 --> 00:29:24,763
BERKOVICl: In time,
an entire line of volcanic islands
382
00:29:24,847 --> 00:29:27,600
was effectively burnt into the Pacific plate.
383
00:29:27,687 --> 00:29:31,805
In fact, this island and Kilauea Volcano
are moving off the centre of the plume
384
00:29:31,887 --> 00:29:35,846
and giving way to a new volcano, Loihi,
which is forming underwater
385
00:29:35,967 --> 00:29:38,686
about 15 miles to the southeast of this island.
386
00:29:39,007 --> 00:29:42,204
And in about 200,000 years,
Loihi will breach sea level
387
00:29:42,287 --> 00:29:44,721
and become the next Hawaiian island.
388
00:30:02,207 --> 00:30:05,882
MANNING: Once scientists realised
that mantle plumes really do exist,
389
00:30:06,167 --> 00:30:08,362
they began to see them everywhere.
390
00:30:09,007 --> 00:30:12,682
McKENZIE: Where the plumes are coming up,
they push up the surface a little bit.
391
00:30:12,767 --> 00:30:16,885
And so, what we've been able to do,
quite recently, actually,
392
00:30:16,967 --> 00:30:22,325
is to map the whole of the convective circulation
underneath the plates.
393
00:30:22,407 --> 00:30:25,365
And you can see that very clearly here
in the Pacific.
394
00:30:25,447 --> 00:30:27,677
Here's South America and North America
395
00:30:27,767 --> 00:30:29,325
and Australia and Japan.
396
00:30:29,407 --> 00:30:30,681
And Hawaii in the middle.
397
00:30:30,767 --> 00:30:34,521
And you can see these red patches
are where plumes are coming up.
398
00:30:34,607 --> 00:30:36,404
And the biggest of these is Hawaii.
399
00:30:36,487 --> 00:30:38,842
And what you're looking at here
is essentially the fact
400
00:30:38,927 --> 00:30:40,724
that the sea floor's all been pushed up
401
00:30:40,807 --> 00:30:42,877
by the plume coming up.
402
00:30:43,247 --> 00:30:47,001
We can see this pattern of rising plumes
403
00:30:47,127 --> 00:30:48,765
all over the Earth's surface,
404
00:30:48,847 --> 00:30:50,439
everywhere we look.
405
00:30:56,007 --> 00:30:57,599
MANNING: So beneath the crust,
406
00:30:57,687 --> 00:30:59,405
beneath the plates,
407
00:30:59,487 --> 00:31:02,524
the rocks of the mantle move to their own rhythm.
408
00:31:06,247 --> 00:31:09,364
Rising from deep within the planet's interior,
409
00:31:09,447 --> 00:31:12,883
plumes of hot rock push up towards the surface.
410
00:31:16,447 --> 00:31:20,884
Less than 20 years ago, plumes were no more
than a theoretical possibility.
411
00:31:21,567 --> 00:31:25,003
Today, they are recognised as major features
of the planet
412
00:31:25,127 --> 00:31:29,757
which can remain stable for tens,
perhaps hundreds of millions of years.
413
00:31:30,327 --> 00:31:34,764
And it's becoming clear that over Earth history
they've played a major role
414
00:31:34,847 --> 00:31:37,077
in shaping the surface of the planet.
415
00:32:03,647 --> 00:32:06,605
These are the Kanheri caves in western India,
416
00:32:06,807 --> 00:32:12,165
a large temple complex carved by Buddhist monks
out of a single vast lava flow.
417
00:32:13,327 --> 00:32:16,046
This flow is the result of a volcanic eruption
418
00:32:16,127 --> 00:32:19,324
far bigger than anything
humanity has ever witnessed.
419
00:32:20,847 --> 00:32:25,875
Yet this is just one of dozens of such flows
which blanket much of western India.
420
00:32:27,447 --> 00:32:29,324
Piled one on top of the other,
421
00:32:29,407 --> 00:32:33,764
they form this extraordinary mountain range,
the so-called Deccan Traps.
422
00:32:37,207 --> 00:32:41,041
Here is evidence of volcanism
on a truly gigantic scale.
423
00:32:43,607 --> 00:32:47,202
Most geologists assume
that the eruptions which produced this landscape
424
00:32:47,287 --> 00:32:50,324
must have continued for tens of millions of years.
425
00:32:51,447 --> 00:32:57,124
But in 1985, the French geophysicist
Vincent Courtillot came to the Deccan.
426
00:32:57,967 --> 00:33:02,757
He was interested in the way
the Earth's magnetic field has changed over time.
427
00:33:03,927 --> 00:33:05,246
When we first came here,
428
00:33:05,327 --> 00:33:08,956
our purpose was to measure
the magnetisation of lava in the Deccan.
429
00:33:09,527 --> 00:33:12,803
When a lava flow cools
below a certain critical temperature,
430
00:33:12,887 --> 00:33:16,402
it will lock in the direction
of the Earth's magnetic field.
431
00:33:16,487 --> 00:33:20,321
It has a memory of what
the Earth's field was like at the time of cooling.
432
00:33:20,407 --> 00:33:23,922
And we know the Earth's field has been reversing,
flipping many times,
433
00:33:24,007 --> 00:33:26,680
approximately once every million years.
434
00:33:29,287 --> 00:33:34,441
MANNING: So, frozen into the lava layers
should be a history of these magnetic reversals.
435
00:33:34,967 --> 00:33:39,961
Courtillot was expecting to find dozens of them
recorded in the Deccan lava flows.
436
00:33:45,367 --> 00:33:48,245
What he actually found astonished him.
437
00:33:57,927 --> 00:34:01,397
COURTILLOT: When we came here,
the estimates for ages of the Deccan lava
438
00:34:01,487 --> 00:34:04,399
ranged over as wide as 50 million years.
439
00:34:04,767 --> 00:34:09,397
So we expected to have recorded
maybe 50, 60 reversals of the field.
440
00:34:10,647 --> 00:34:14,003
When we had completed the sampling
and the work in our laboratory,
441
00:34:14,087 --> 00:34:19,957
much to our surprise, we found that only two
reversals had been frozen in the lava.
442
00:34:20,327 --> 00:34:22,921
That only two reversals had been recorded
443
00:34:23,007 --> 00:34:27,125
was proof that the whole volcanism
could not have lasted much more
444
00:34:27,207 --> 00:34:28,640
than, say, a million years.
445
00:34:28,727 --> 00:34:30,957
This was an enormous out rate indeed.
446
00:34:31,047 --> 00:34:32,526
It could only have been caused
447
00:34:32,607 --> 00:34:36,316
by the massive eruption of the Earth's surface
of a mantle plume head.
448
00:34:39,407 --> 00:34:43,286
MANNING: Enough lava was erupted here
to cover the United States
449
00:34:43,367 --> 00:34:45,801
to a depth of nearly one kilometre.
450
00:34:49,367 --> 00:34:52,598
Vincent Courtillot believes
such a stupendous event
451
00:34:52,687 --> 00:34:56,600
can only have been the result
of a powerful new plume splitting the crust
452
00:34:56,687 --> 00:34:58,962
and breaking through to the surface.
453
00:34:59,247 --> 00:35:02,045
COURTILLOT: When a mantle plume
comes to the surface of the Earth,
454
00:35:02,127 --> 00:35:05,039
the plume head will bulge, deform the crust
455
00:35:05,127 --> 00:35:08,961
and then burst out as a lava flow
with enormous volumes.
456
00:35:10,887 --> 00:35:15,438
These plumes, which are born inside the mantle
and erupt to the surface of the Earth,
457
00:35:15,527 --> 00:35:19,998
will modify the whole landscape of a continent
in a very durable fashion.
458
00:35:24,727 --> 00:35:27,560
MANNING: And it hasn't happened only in India.
459
00:35:33,127 --> 00:35:37,359
Iceland, too, shows the unmistakable fingerprints
of a plume.
460
00:35:37,847 --> 00:35:41,044
But here, the plume has done more
than alter the landscape.
461
00:35:41,287 --> 00:35:44,757
There's evidence that it's changed the motion
of the plates,
462
00:35:44,847 --> 00:35:47,839
reshaping an entire region of the globe.
463
00:36:01,247 --> 00:36:04,876
Iceland presents us with a mystery in some ways.
464
00:36:05,447 --> 00:36:09,076
If we go back to the time
when the continents were joined together
465
00:36:09,167 --> 00:36:12,159
to form one supercontinent, Pangaea
466
00:36:12,407 --> 00:36:15,956
then the east coast of Greenland
and North America
467
00:36:16,047 --> 00:36:20,518
and the west coast of northern Europe
fit together remarkably well.
468
00:36:21,047 --> 00:36:23,197
That's a very satisfying result.
469
00:36:23,287 --> 00:36:26,279
The only trouble is where's Iceland?
470
00:36:26,687 --> 00:36:30,965
It's as if we've done a jigsaw puzzle
and completed the picture
471
00:36:31,087 --> 00:36:34,602
and then discovered
that we've got one piece left over.
472
00:36:35,967 --> 00:36:39,960
The answer is connected to the rift
that runs across the island.
473
00:36:43,167 --> 00:36:46,637
This is, in fact, part of the vast mid-ocean ridge,
474
00:36:46,727 --> 00:36:49,685
the great rift that runs up
the centre of the Atlantic,
475
00:36:49,767 --> 00:36:52,520
where the ocean basin is slowly opening up.
476
00:36:54,287 --> 00:36:59,077
Iceland is above the water
only because it's buoyed up by a mantle plume.
477
00:37:03,167 --> 00:37:08,241
It's the one place one can get down inside the rift
without getting wet.
478
00:37:13,927 --> 00:37:15,280
BRANDSDOTTIR: Down here.
479
00:37:19,047 --> 00:37:22,881
MANNING: So, we're really inside
the Mid-Atlantic rift?
480
00:37:22,967 --> 00:37:28,200
Yes. This is one of the major fissures
of south of Krafla volcano.
481
00:37:28,767 --> 00:37:31,759
And it's obviously hot.
482
00:37:31,847 --> 00:37:34,964
Yes, it's 47 degrees at the moment.
483
00:37:35,087 --> 00:37:38,079
MANNING: All right for the hand but not,
I think, for total immersion.
484
00:37:38,167 --> 00:37:41,204
But it used to be
a popular bathing place before it heated.
485
00:37:44,087 --> 00:37:48,444
MANNING: As the rift pulls apart,
new lava wells up from the plume below,
486
00:37:48,527 --> 00:37:52,645
constantly adding to the island
as the Atlantic slowly widens.
487
00:37:54,007 --> 00:37:58,478
These lavas were extruded
between 10,000 and 8,000 years ago.
488
00:37:58,607 --> 00:38:00,518
And then following that period,
489
00:38:00,607 --> 00:38:05,123
there was a hiatus of eruptive activity
for about 5,000 years.
490
00:38:05,287 --> 00:38:09,041
Since then we've had five rifting events here.
491
00:38:09,167 --> 00:38:11,681
- Three in historical times.
- Right.
492
00:38:11,767 --> 00:38:16,795
So the people who settled this region,
they settled on a new lava flow.
493
00:38:19,487 --> 00:38:23,400
MANNING: So Iceland was never part
of the jigsaw puzzle of Pangaea.
494
00:38:23,487 --> 00:38:27,480
It's grown up above the plume
since the Atlantic started to open up.
495
00:38:27,847 --> 00:38:30,680
Which raises a really intriguing question.
496
00:38:31,487 --> 00:38:35,639
Could the plume under Iceland
have caused the rift in the first place
497
00:38:35,727 --> 00:38:39,515
and triggered the opening of the Atlantic
58 million years ago?
498
00:38:41,127 --> 00:38:45,803
Clues to what happened then can still be found
on the west coast of Scotland.
499
00:38:56,647 --> 00:39:00,037
On the island of Mull, I met Chris Nicholas,
500
00:39:00,127 --> 00:39:03,642
who showed me some
extraordinary natural rock formations.
501
00:39:04,367 --> 00:39:06,801
So we're walking on seashore here
502
00:39:06,887 --> 00:39:10,562
but this looks almost like a man-made wall here.
503
00:39:10,647 --> 00:39:12,319
It does at first glance,
504
00:39:12,407 --> 00:39:16,320
but you can see the ridge
snakes off across the beach there.
505
00:39:16,407 --> 00:39:20,082
And it's actually made of quite a hard rock.
506
00:39:20,447 --> 00:39:23,757
- Made of tiny little crystals.
- Yeah.
507
00:39:23,847 --> 00:39:27,635
So it's actually cooled and solidified magma
508
00:39:27,727 --> 00:39:30,161
that's come from deep down in the crust.
509
00:39:30,367 --> 00:39:33,803
So it's come up
through a split in the bedrock here?
510
00:39:33,887 --> 00:39:36,003
Yes, well to give us this sort of ridge shape,
511
00:39:36,087 --> 00:39:41,366
it must have come up through a crack or fissure
through these surrounding muds.
512
00:39:41,687 --> 00:39:44,997
So there was a lot of volcanic activity in this area.
513
00:39:45,087 --> 00:39:46,486
Yeah, there's not just one ridge here.
514
00:39:46,567 --> 00:39:49,604
There are several of them, all in parallel
or cutting across each other.
515
00:39:49,687 --> 00:39:54,203
And they all date from almost exactly
the same time, about 58 million years ago.
516
00:39:54,287 --> 00:39:58,917
So we can be fairly sure at that time
there was a great pulse of igneous activity.
517
00:39:59,007 --> 00:40:01,396
A great pulse of melt below the crust
518
00:40:01,927 --> 00:40:05,681
produced a series of fissures
and a series of eruptions
519
00:40:05,767 --> 00:40:09,521
which built most of the islands
you see in Scotland.
520
00:40:11,767 --> 00:40:13,962
MANNING: Volcanic activity on this scale
521
00:40:14,047 --> 00:40:17,881
is evidence for an event
similar to that which caused the Deccan Traps,
522
00:40:18,567 --> 00:40:22,037
a new mantle plume head which split the crust.
523
00:40:22,487 --> 00:40:25,160
NICHOLAS: About 60 million years ago,
524
00:40:25,247 --> 00:40:28,637
Greenland was sitting next to Scotland.
525
00:40:28,727 --> 00:40:30,683
So you'd be able to see it just out there.
526
00:40:30,767 --> 00:40:33,645
The Atlantic as we know it just didn't exist.
527
00:40:33,807 --> 00:40:40,042
And what we think happened is
that a big hot plume of mantle material came up
528
00:40:40,407 --> 00:40:43,365
between Greenland and Scotland
529
00:40:43,807 --> 00:40:45,525
and domed the crust.
530
00:40:45,607 --> 00:40:49,395
And eventually it split,
and the two continents rifted apart.
531
00:40:52,847 --> 00:40:56,044
As the plume rifts
these two continental blocks apart,
532
00:40:56,127 --> 00:40:58,595
it was stretching the crust either side.
533
00:40:58,687 --> 00:41:03,078
And it was fracturing.
And up through all the fractures and fissures
534
00:41:03,167 --> 00:41:06,603
was coming this hot mantle material
on either side of the rift.
535
00:41:10,927 --> 00:41:12,599
It was pouring out flood basalts
536
00:41:12,687 --> 00:41:15,201
on the Scottish side
to give us everything we have here.
537
00:41:15,287 --> 00:41:16,959
But it was also, on the other side of the rift,
538
00:41:17,047 --> 00:41:20,084
it was pouring out
the basalts that you see in eastern Greenland.
539
00:41:20,167 --> 00:41:23,045
So the rocks here and the rocks there
are of the same age?
540
00:41:23,127 --> 00:41:25,118
Yeah, same age. They went straight across, yes.
541
00:41:25,207 --> 00:41:28,085
And what happened to this plume, subsequently?
542
00:41:28,167 --> 00:41:31,477
Well, Greenland is now
about 2,000 kilometres away.
543
00:41:32,047 --> 00:41:36,996
But we think that the plume itself is still sitting
under the Mid-Atlantic rift out there,
544
00:41:37,087 --> 00:41:39,317
under Iceland.
545
00:41:40,887 --> 00:41:43,526
MANNING: So the rocks
on both sides of the Atlantic
546
00:41:43,607 --> 00:41:47,202
bear witness to the impact of a plume
from deep within the mantle.
547
00:41:54,287 --> 00:41:57,757
As the plume pushed up against
the crust, it split,
548
00:41:57,847 --> 00:42:00,202
rifting Greenland off from Europe.
549
00:42:01,207 --> 00:42:05,086
Once split, Europe and Greenland
continued to move apart
550
00:42:05,167 --> 00:42:07,920
creating a new ocean basin in the north Atlantic.
551
00:42:09,207 --> 00:42:12,563
Ever since then,
the plume has been sitting under the rift
552
00:42:12,767 --> 00:42:14,439
producing Iceland,
553
00:42:14,527 --> 00:42:16,961
the largest volcanic island on the planet.
554
00:42:40,887 --> 00:42:45,881
The plume that's now sitting under Iceland
has produced some enormous changes
555
00:42:45,967 --> 00:42:50,643
in the 60 million years or so since it started
to rise through the Earth's mantle.
556
00:42:51,527 --> 00:42:54,405
Without that event in the Earth's interior,
557
00:42:54,487 --> 00:42:58,480
the whole of this area of the globe
would be totally different.
558
00:43:00,567 --> 00:43:05,561
But plumes may have even more profound effects
than the shaping of the planet's surface.
559
00:43:06,127 --> 00:43:09,278
As a biologist, what I find particularly intriguing
560
00:43:09,367 --> 00:43:12,882
is the suggestion that they may change
the course of evolution.
561
00:43:31,447 --> 00:43:37,044
The vast Deccan lava flows were all erupted in,
geologically speaking, a blink of an eye.
562
00:43:37,127 --> 00:43:38,355
But when?
563
00:43:38,447 --> 00:43:43,362
SAHNl: Usually, they may extend up to
100 metres and if you look very carefully...
564
00:43:43,447 --> 00:43:47,679
MANNING: Palaeontologist Ashok Sahni
has helped Vincent Courtillot
565
00:43:47,767 --> 00:43:51,077
pin down exactly
when the plume head hit the surface.
566
00:43:52,447 --> 00:43:56,565
Could this event have had global consequences
for living things?
567
00:43:59,967 --> 00:44:03,004
SAHNl: These here are the teeth of mammals.
568
00:44:03,087 --> 00:44:07,126
These mammalian teeth were recovered
in sediments
569
00:44:07,927 --> 00:44:10,077
lying in-between the Deccan flows.
570
00:44:10,567 --> 00:44:12,956
They belong to very primitive mammals
571
00:44:13,047 --> 00:44:16,517
which are related to the present-day shrews.
572
00:44:17,207 --> 00:44:20,404
These mammals evolved at a fairly fast rate.
573
00:44:20,887 --> 00:44:25,597
And it's possible to tell from these teeth
exactly where we are in time.
574
00:44:25,687 --> 00:44:30,681
They give us a date
for the initiation of the Deccan volcanics.
575
00:44:31,287 --> 00:44:36,122
And this date happens to be 65 million years.
576
00:44:36,647 --> 00:44:41,641
They belong to an era that is what we call
the Cretaceous period.
577
00:44:42,087 --> 00:44:46,797
And surprisingly, it is this Cretaceous period,
the end of this Cretaceous period,
578
00:44:46,887 --> 00:44:49,799
which also saw the extinction of the dinosaurs.
579
00:45:14,927 --> 00:45:17,646
If you ask someone, "What killed the dinosaurs?"
580
00:45:17,727 --> 00:45:21,242
He or she is likely to tell you,
"Well, it was the impact of an asteroid."
581
00:45:21,327 --> 00:45:23,477
And they might well be right.
582
00:45:23,887 --> 00:45:27,038
But that cannot explain all of the extinctions.
583
00:45:27,127 --> 00:45:30,597
Palaeontologists tell us
that well before the impact hit the Earth,
584
00:45:30,687 --> 00:45:33,076
species had begun slowly disappearing.
585
00:45:33,167 --> 00:45:37,922
When I mean slowly, I mean in the course of tens
of thousands or hundreds of thousands of years.
586
00:45:38,007 --> 00:45:41,443
This is precisely the timescale
of the Deccan eruptions.
587
00:45:41,527 --> 00:45:44,405
This is what makes me prefer that explanation.
588
00:45:46,407 --> 00:45:49,285
COURTILLOT: As the large plume head
came from inside the Earth,
589
00:45:49,367 --> 00:45:52,325
it started erupting lava flow after lava flow.
590
00:45:55,767 --> 00:45:59,282
You should imagine these lava flows
pumping into the atmosphere
591
00:45:59,367 --> 00:46:01,039
vast amounts of gases.
592
00:46:03,767 --> 00:46:08,238
Sulphur dioxide, carbon dioxide, chlorine
and also large amounts of dust
593
00:46:08,327 --> 00:46:09,760
obscuring the skies.
594
00:46:09,847 --> 00:46:12,645
Obscurity would cause cold, possibly freezing.
595
00:46:12,727 --> 00:46:16,276
And then acid rains would follow.
Real hell everywhere.
596
00:46:23,927 --> 00:46:25,804
And when the Deccan had finished erupting,
597
00:46:25,887 --> 00:46:30,278
60% of the species
had disappeared from the surface of the Earth.
598
00:46:36,167 --> 00:46:39,318
MANNING: Among the survivors
were those tiny shrew-like mammals,
599
00:46:39,407 --> 00:46:40,760
our own ancestors,
600
00:46:40,847 --> 00:46:44,840
who were to flourish in a world freed
from the grip of the large reptiles.
601
00:46:47,127 --> 00:46:49,038
If Vincent Courtillot is right,
602
00:46:49,167 --> 00:46:54,082
then in a sense, we human beings
owe our existence to events deep within the Earth.
603
00:46:56,887 --> 00:47:00,004
The link between the Earth's activity
and the course of evolution
604
00:47:00,087 --> 00:47:03,284
is something I'll be exploring later in the series.
605
00:47:15,687 --> 00:47:20,556
Jules Verne's hero, Professor Lidenbrock,
finally made it to the centre of the Earth
606
00:47:20,647 --> 00:47:25,198
to find it populated,
ironically enough, by dinosaurs.
607
00:47:29,847 --> 00:47:32,839
The real scientists
who've explored the Earth's interior
608
00:47:33,207 --> 00:47:36,597
have found something in its way
just as surprising.
609
00:47:37,247 --> 00:47:40,683
That beneath all the complex changes
we see at the surface,
610
00:47:40,807 --> 00:47:43,275
there is an underlying simplicity,
611
00:47:43,367 --> 00:47:46,996
the vast churning heat engine
of the Earth's mantle,
612
00:47:47,127 --> 00:47:50,437
an engine which drives the dance
of plate tectonics,
613
00:47:50,527 --> 00:47:54,361
and reforms our world in a myriad different ways.
614
00:47:58,727 --> 00:48:03,039
In Iceland, you can really sense
the power of the interior
615
00:48:03,127 --> 00:48:05,163
that shaped the Earth's surface.
616
00:48:06,607 --> 00:48:08,916
As the Earth cools by convection,
617
00:48:09,007 --> 00:48:13,956
so the continents shift over its surface
and rift apart.
618
00:48:14,567 --> 00:48:18,640
But plate tectonics means
that continents don't just separate,
619
00:48:18,727 --> 00:48:20,479
they must also collide.
620
00:48:21,047 --> 00:48:23,925
And it's what happens
when two continents collide
621
00:48:24,007 --> 00:48:26,805
that's the subject for our next programme.
622
00:48:27,305 --> 00:49:27,564
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