Earth Story (1998) s01e04 Episode Script

MANNING: "Then the volcano gave vent to the lavas.
"I could see their long streams extending over the slopes, "like meshes of flowing hair.
"But descend into the crater of the volcano called Snaefells, O audacious traveller, "and you will reach the centre of the Earth.
" Here in this desolate volcanic landscape, those words might seem easy to believe, that somewhere here might be a direct route to the source of the Earth's energy.
In his adventure classic Journey to the Centre of the Earth, Jules Verne has his hero, professor Lidenbrock, investigate the interior of our planet by the simple expedient of climbing down an extinct volcano here in Iceland.
That's just fantasy, of course.
But what isn't fantasy is the intense scientific curiosity about what's going on beneath our feet.
Most of the time, the surface of our world gives no hint of what lies below.
But there are a few places on Earth where we can get a glimpse of what's happening inside the planet.
(HELICOPTER HOVERING) MAN ON RADIO: That is just one fantastic sight.
You can really see how much of this cone has fallen in.
These talus slopes are all very recent.
Like, all the loose boulders we see down here have fallen off the crater walls since March.
MANNING: This is Mount Kilauea on the island of Hawaii.
Scientists from the US Geological Survey regularly fly up here to keep an eye on the lava pond.
We're standing on the crater rim of Pu'u'O'o cone.
This is the active vent on the east rift zone of Kilauea Volcano.
A potential hazard here is, when the pond is so low, there's not much supporting the inside of the crater.
So we'll have large chunks of the crater rim that fall and if that happens, pond level will then rise and fall, rise and fall and you'll also get spatter and Of course, you don't wanna be standing on a chunk of rim that might actually go down in.
MANNING: Mount Kilauea is the world's most active volcano.
Geologists suspect that all this volcanism reflects some remarkable phenomenon deep within the planet.
But the problem scientists face is that the Earth's interior is totally inaccessible.
Yet in a sense, scientists have managed to descend to the very centre of the Earth.
In a remarkable series of experiments, they've probed our planet to its core.
They've shown how the same process that created this volcano also keeps the entire surface of our planet in constant motion.
There's no better place to see that motion than Iceland.
The island sits astride an extraordinary break in the crust, a rift that marks the boundary between two of the vast plates that make up the Earth's surface.
Here, geologists like Bob White can study at first hand the way the plates move.
So, we're on the edge of the two plates here with the American plate over here and the Eurasian plate behind us.
Yes, that's right.
There's a zone of about 50 kilometres wide where the movement is taken up and this is just one of those fissures that the movement is taken up on as America separates from Eurasia.
But over this zone, there's about 20 millimetres a year of movement on average, about that much movement.
MANNING: The movement that's so visible in Iceland is in fact happening all over the world.
By the late '60s, scientists had worked out that beneath the crust, the entire surface of the planet is divided up into a small number of pieces, plates.
These plates are all slowly moving, carrying the continents with them.
The theory that described this motion was called plate tectonics.
It was the biggest step forward scientists had ever made in understanding the Earth.
SCIENTIST: Everything then will fit together.
MANNING: But there was something missing.
They knew the plates were moving, but scientists had no idea why they were moving.
SCIENTIST: This is a question as to particularly pressure and temperature.
MANNING: Nobody was more worried about this problem than one of the architects of the theory, Dan McKenzie.
By the beginning of the '70s, we had some understanding of what was going on on the surface.
'Cause we'd been able to map the motions of the plates, really, in quite some detail, all over the world.
But what we really didn't have at that stage was any decent understanding of how these motions were maintained.
And the obvious place to look is underneath the plates in the mantle.
'Cause they clearly weren't being driven by the winds from the outside.
Right? It had to be an internal process.
MANNING: The first hint of what lies inside our planet came from scientists like Bryndis Brandsdottir, who spends much of her time monitoring the many earthquakes which shake Iceland.
Basically we use different kinds of sensors to monitor different things within the Earth.
For instance, you can see earthquakes which occurred in the Hengill region late last night and early this morning.
But with the right sort of instruments you can do more than just record local activity, where earthquakes are and when they occur? Yes.
The different waveforms can give us information about the different properties of the Earth as a whole.
So you're picking up waves which have come right through the Earth from a very long distance? - Yes.
From the other side of the globe.
- Right.
Actually the biggest earthquakes, they make the Earth ring like a bell.
And there are huge waves travelling through the Earth and those waves give us informations about various places within the Earth.
MANNING: By putting together information from thousands of earthquakes, scientists built up their first complete picture of the Earth's interior.
At its centre lies the core, an immense ball of liquid iron the size of the planet Mars.
Beyond the core is the vast bulk of the planet, the mantle, which is composed of solid rock.
The plates are simply the upper layer of the mantle, with the crust just a thin veneer on top.
But this picture gave no hint of any movement in the mantle that could be driving the motion of the plates.
To make progress, scientists needed a different approach.
Here in Lovgrund, a Swedish island in the Baltic, scientists have long been intrigued by a mysterious phenomenon.
PELTIER: Hello, Martin.
MANNING: Canadian geophysicist Dick Peltier has made the journey from Toronto to meet Martin Ekman and see things for himself.
The sea has always been important here.
People notice when it changes.
In 1940, Lisa Nygren was a young woman.
Lisa tells that she's about 35 years old here.
EKMAN: And you can see that in front of the boathouse here there is a lot of water.
Nowadays there is no water here any longer.
It is completely dry.
Just gravel and grass.
(NYGREN SPEAKING IN SWEDISH) MANNING: It seemed to the people here that the sea was slowly ebbing away.
(MARTIN SPEAKING IN SWEDISH) (SPEAKING IN SWEDISH) She tells me that when she was young there was water here.
It was filled with water, the small inlet from the bay.
PELTIER: You mean you could bring boats right up into what are now fields? EKMAN: Yes, rowing boats.
PELTIER: Mm-hm.
MANNING: Peltier and Ekman are not the first scientists to study the phenomenon.
Two and a half centuries ago, the famous Swedish physicist Anders Celsius noticed the same thing.
EKMAN: This is the famous rock of Lovgrund.
And here is the sea level mark made by Celsius in 1731.
So, 250 years ago, sea level was here.
Today, it's down there.
MANNING: Thanks to Celsius's foresight, Peltier and Ekman can calculate the rate of apparent sea level drop here.
Over one centimetre each year.
The problem is, there's absolutely no evidence whatsoever of any significant change to global sea levels for 10,000 years, since levels rose at the end of the Ice Age.
Perhaps it's not the sea that is falling, but Scandinavia which is rising.
PELTIER: It's astonishing.
EKMAN: Believe it or not, this is a beach.
We are now 200 metres up a mountain and 30 kilometres from the sea.
9,000 years ago, this was at sea level and this was a seashore.
You can see the stones here have been reworked by the sea waves.
Over the years, this has been lifted up nearly 200 metres and this is the result.
MANNING: All the evidence suggests that since the ice melted 10,000 years ago, Scandinavia has slowly but steadily been rising out of the sea.
Scientists have come up with a very ingenious explanation for this.
If you think back to the last great glacial period, about 20,000 years ago, we know the whole of this area was covered with a colossal ice sheet.
It was about three kilometres thick over Scandinavia and the huge weight of this sheet pressed the crust of Scandinavia down into the Earth's mantle.
And then about 10,000 years ago, relatively quickly, the ice disappeared and the crust of Scandinavia bobbed back and it's still rising today.
Now, that's a very ingenious explanation but it does require one rather remarkable thing, which is that the rocks of the Earth's mantle can behave like a fluid, and flow.
MANNING: What sort of stuff is rock-solid yet flows? Understanding that would turn out to be the key to the mystery of why the plates move.
But to get that key, scientists would have to perform an ingenious series of experiments, which would take them deep into the planet's interior.
The first step was to get hold of some rock from the mantle.
It's rare stuff, but just occasionally volcanic eruptions bring to the surface small, unmelted fragments of the mantle.
- Do you often find such objects? - Well, while you walk around MANNING: Eckhard Salje spends his time studying them.
In any sort of volcanic event, you have a certain chance to have some basalt coming up.
And sometimes, when you're very lucky, you find these green patches on here and this green material is actually olivine.
MANNING: So this is the lava material and that's really a piece of the Earth's mantle.
- Which has been pushed up.
- It is basically stuck into it and transported with it up to the surface.
And what is olivine? Well, olivine is a silicate.
It has magnesium, iron and so forth in there.
Normally it's very fine-grained as you can see there, but if you're really lucky, - you can find crystals.
- Those are crystals.
Yes, yes.
They are rather big and nice.
SALJE: Again, this typically greenish colour.
MANNING: Yes.
MANNING: That's pure olivine? SALJE: That's pure olivine.
Yes.
Yes.
Now, they are nice but of course they don't tell us very much as they are, about the conditions as they are down in the Earth.
Because normally this stuff is existing at very high temperatures and pressures.
That's right.
That's right.
And what we want to do is to simulate the same conditions, but in the lab.
MANNING: So a tiny sample of olivine is squeezed between the jaws of a specialised vise called a diamond anvil.
To understand how pressure affects the sample, Ekhard uses X-rays to look deep into the structure of the crystal.
The scientists employ these techniques to descend ever deeper into the Earth.
They're revealing how the behaviour of our planet is governed by what happens at the atomic level, inside the mantle.
Although the sample always remains solid, the intense pressures can disrupt the structure of the crystals.
And that means the solid material can slowly flow.
Suppose this is a piece of the Earth's mantle.
Seismologists tell us, based on earthquake observations, that this material is solid.
It's elastic, it's resilient.
What could be more elastic or more resilient? However, on a longer timescale, its behaviour is completely different.
So, the mantle of the Earth is both a solid and a fluid.
It's all a question of timescale.
MANNING: So over long periods of time, the solid mantle is flowing.
But what does this flow have to do with the motion of the plates? Scientists would find the answer to that when they voyaged even deeper, down to the iron core.
We have here a piece of iron in which we're going to recreate the conditions at the centre of the Earth.
And we have to do that by generating very high pressures.
And we have to melt it under those very high pressures.
And we're going to stick it in this, the hydrogen gas gun.
Now, Keith is loading the gun now with about seven pounds of gunpowder.
When the gun is fired, it pushes this heavy piston down this hydrogen-filled tube, which starts out at ten times atmospheric pressure, but the time we arrive here, it's about 2,000 times atmospheric pressure.
Takes this disc, bursts it.
The hydrogen comes out of here and pushes on this projectile, accelerating it down this tube, at about 10,000 Gs up to a velocity of about eight kilometres a second.
Which is why we call it "the fastest gun in the west".
Once it gets here, it impacts on the target, generating very high pressures, melting the iron target.
And Zhang Yigou has about 50 billionths of a second to measure that temperature.
I'll zero the counters.
MANNING: Neil Holmes is going to all this trouble because he wants to answer a very basic question about the Earth's core.
How hot is it? Okay, everything looks good over here.
MANNING: Since scientists are pretty sure that the core contains molten iron, the thinking is that its temperature must be close to the melting point of iron.
Calibrating target chamber.
MANNING: But not the melting point at the surface.
KEITH: Bringing up the X-rays.
MANNING: The melting point, under the stupendous pressures at the centre of the planet.
- HOLMES: Okay, whenever you're ready, Keith.
- Ready to fire.
HOLMES: Just go ahead.
Yeah.
(BEEPING) Well, there's not much left in here.
That used to be a part of some of it.
I don't know what.
So, the whole target, everything we started with, is destroyed.
But when the projectile hit the iron, the iron made a brief flash of light.
The colour of that light tells us what the temperature was.
What it tells us is that the melting temperature is 6,200 centigrade.
This is over a thousand degrees hotter than the surface of the sun.
What this means is the core of the Earth is a glowing ball of hot iron, 3,000 miles in diameter.
And slowly, over the eons, that heat is gradually moving out toward the surface of the Earth.
MANNING: So, at the Earth's centre, beneath the mantle, there's a heat source hotter than the surface of the sun.
Anybody who's ever put a pan of water on a stove knows that when you heat a fluid from below it starts moving.
A little bit of dye shows what happens.
Hot material comes up from the bottom, rises to the surface, where it cools and sinks back again.
This is the process known as convection.
To geologists, the obvious question was, "Is this what's happening inside the Earth?" Now, it was Lord Rayleigh in the 19th century who discovered that convection depends on several factors.
One, the thermal properties of the fluid.
Secondly, the viscosity of the fluid.
And thirdly, how deep the layer is.
Now, thinking about the mantle, we know how deep the mantle is from the earthquake evidence.
We know about the thermal properties from the work that's been done with diamond anvils.
And we can get a good estimate of the viscosity from the speed with which Scandinavia is rebounding after the pressure of the ice has been removed.
When geophysicists plugged these values into Rayleigh's formula, they discovered that not only can the Earth's mantle convect, it must convect, and must be convecting very vigorously.
So the situation now is really very much more interesting because we have this mobile Earth, which is mobile because it's transporting heat from the interior towards the surface.
Both upwards by hot material rising and downwards by cold material sinking.
And the thing acts really just like an engine, right? It's converting heat, right, the heat which it's transporting, into some sort of work.
And it's this work which is driving the plates around on the surface.
MANNING: It's not difficult to see how the motion of the plates relates to convection in the mantle.
At the mid-ocean ridges, hot mantle is flowing up.
And as it cools, it forms new plate which moves away from the ridge.
Eventually, it's cold and dense enough to sink back down again into the planet's interior.
So, the complex motion of the plates is actually part of a simple process by which the Earth is slowly losing its massive store of heat.
But what about the volcanism in places like Hawaii? How did that fit into mantle convection? When Dan McKenzie started experimenting with convection, he made a totally unexpected discovery.
What we wanted to do was actually see what the general sort of things that fluids like the Earth's mantle could do.
So what we wanted was a fluid which had properties which were like those of the mantle.
And the one we chose was Lyle's Golden Syrup.
The arrangement is that you have hot water underneath, right, here which actually causes this layer of syrup to convect.
Because it's heated from the bottom by the water and cooled from the top by the atmosphere.
And we can see what's going on by shining light through this layer onto the mirror at the bottom.
And then makes an image on the screen.
And you can see the dark parts on the screen are where hot fluid is rising and the bright ones are where cold fluid is sinking.
So it comes up in a plume in the middle and then goes down round the outside.
And you can see that the whole of this screen is covered with all these little cells.
We could heat it in different ways, we could do all kinds of different things.
We could alter the top temperature and alter the viscosity contrast.
Really doesn't make much difference.
You always get this pattern.
Lots of little cells.
Not really at all like what we wanted for plates.
That's a bit perplexing at first sight.
Yes.
Yes, absolutely.
It was also rather a disappointment.
Because what we really wanted to understand was how we could drive the plates.
But this then gave us the idea that maybe on Earth, the same thing was going on.
And that underneath these big plates there were perhaps lots of little cells like this which were convecting.
MANNING: Could one of Dan McKenzie's convecting cells lie beneath Hawaii? If so, then all this lava is coming from a plume of mantle rock that has risen from deep within the planet.
MAN: Why don't you stand about here and give me enough coil MANNING: Lava flows in underground channels down from the main vent.
That gives Carl Thornber and Dave Berkovici a chance to go fishing for a sample.
THORNBER: Perfect! Absolutely perfect shot.
Okay? Are we gonna Ready to go ahead with this sample? Okay.
Hey, hang on to the end of that.
Okay.
Looks good.
We're at the top of that lava fall.
And it's in.
Oh, Jesus.
That's moving.
Okay, I'm trying to get a good sample.
Man, it's hot.
Come on, baby.
Okay, we're coming up.
We got a big one.
Okay, we're ready to pull and quench.
Okay, here we go.
One, two, three! Let's go.
- THORNBER: That was a good sample.
- Yeah.
THORNBER: Anyone for tea? THORNBER: Routine analysis of fresh lava samples allows us to predict what the volcano might do next.
But perhaps more significantly Some of these are still hot.
More significantly, in terms of where all these basalts come from, this stuff contains an extraordinary proportion of helium-3, which is relatively rare in volcanic rocks.
Now, one might wonder where this helium-3 comes from.
One source where helium-3 is still present within the Earth today is at the core-mantle boundary.
And if that's the case, then one might expect that this stuff came from way down there.
MANNING: The composition of the lava indicates that it's come from mantle rock that has risen rapidly from deep down in the Earth.
And there's another line of evidence that Hawaii sits above a plume.
One of the major clues that we're standing above a mantle plume is that the Hawaiian islands themselves form an almost perfectly linear chain that moves off to the northwest.
MANNING: The plume is sitting under the Pacific plate which has been moving across it for 100 million years.
So the volcanic islands the plume created in the past have been carried away by the moving plate.
BERKOVICl: In time, an entire line of volcanic islands was effectively burnt into the Pacific plate.
In fact, this island and Kilauea Volcano are moving off the centre of the plume and giving way to a new volcano, Loihi, which is forming underwater about 15 miles to the southeast of this island.
And in about 200,000 years, Loihi will breach sea level and become the next Hawaiian island.
MANNING: Once scientists realised that mantle plumes really do exist, they began to see them everywhere.
McKENZIE: Where the plumes are coming up, they push up the surface a little bit.
And so, what we've been able to do, quite recently, actually, is to map the whole of the convective circulation underneath the plates.
And you can see that very clearly here in the Pacific.
Here's South America and North America and Australia and Japan.
And Hawaii in the middle.
And you can see these red patches are where plumes are coming up.
And the biggest of these is Hawaii.
And what you're looking at here is essentially the fact that the sea floor's all been pushed up by the plume coming up.
We can see this pattern of rising plumes all over the Earth's surface, everywhere we look.
MANNING: So beneath the crust, beneath the plates, the rocks of the mantle move to their own rhythm.
Rising from deep within the planet's interior, plumes of hot rock push up towards the surface.
Less than 20 years ago, plumes were no more than a theoretical possibility.
Today, they are recognised as major features of the planet which can remain stable for tens, perhaps hundreds of millions of years.
And it's becoming clear that over Earth history they've played a major role in shaping the surface of the planet.
These are the Kanheri caves in western India, a large temple complex carved by Buddhist monks out of a single vast lava flow.
This flow is the result of a volcanic eruption far bigger than anything humanity has ever witnessed.
Yet this is just one of dozens of such flows which blanket much of western India.
Piled one on top of the other, they form this extraordinary mountain range, the so-called Deccan Traps.
Here is evidence of volcanism on a truly gigantic scale.
Most geologists assume that the eruptions which produced this landscape must have continued for tens of millions of years.
But in 1985, the French geophysicist Vincent Courtillot came to the Deccan.
He was interested in the way the Earth's magnetic field has changed over time.
When we first came here, our purpose was to measure the magnetisation of lava in the Deccan.
When a lava flow cools below a certain critical temperature, it will lock in the direction of the Earth's magnetic field.
It has a memory of what the Earth's field was like at the time of cooling.
And we know the Earth's field has been reversing, flipping many times, approximately once every million years.
MANNING: So, frozen into the lava layers should be a history of these magnetic reversals.
Courtillot was expecting to find dozens of them recorded in the Deccan lava flows.
What he actually found astonished him.
COURTILLOT: When we came here, the estimates for ages of the Deccan lava ranged over as wide as 50 million years.
So we expected to have recorded maybe 50, 60 reversals of the field.
When we had completed the sampling and the work in our laboratory, much to our surprise, we found that only two reversals had been frozen in the lava.
That only two reversals had been recorded was proof that the whole volcanism could not have lasted much more than, say, a million years.
This was an enormous out rate indeed.
It could only have been caused by the massive eruption of the Earth's surface of a mantle plume head.
MANNING: Enough lava was erupted here to cover the United States to a depth of nearly one kilometre.
Vincent Courtillot believes such a stupendous event can only have been the result of a powerful new plume splitting the crust and breaking through to the surface.
COURTILLOT: When a mantle plume comes to the surface of the Earth, the plume head will bulge, deform the crust and then burst out as a lava flow with enormous volumes.
These plumes, which are born inside the mantle and erupt to the surface of the Earth, will modify the whole landscape of a continent in a very durable fashion.
MANNING: And it hasn't happened only in India.
Iceland, too, shows the unmistakable fingerprints of a plume.
But here, the plume has done more than alter the landscape.
There's evidence that it's changed the motion of the plates, reshaping an entire region of the globe.
Iceland presents us with a mystery in some ways.
If we go back to the time when the continents were joined together to form one supercontinent, Pangaea then the east coast of Greenland and North America and the west coast of northern Europe fit together remarkably well.
That's a very satisfying result.
The only trouble is where's Iceland? It's as if we've done a jigsaw puzzle and completed the picture and then discovered that we've got one piece left over.
The answer is connected to the rift that runs across the island.
This is, in fact, part of the vast mid-ocean ridge, the great rift that runs up the centre of the Atlantic, where the ocean basin is slowly opening up.
Iceland is above the water only because it's buoyed up by a mantle plume.
It's the one place one can get down inside the rift without getting wet.
BRANDSDOTTIR: Down here.
MANNING: So, we're really inside the Mid-Atlantic rift? Yes.
This is one of the major fissures of south of Krafla volcano.
And it's obviously hot.
Yes, it's 47 degrees at the moment.
MANNING: All right for the hand but not, I think, for total immersion.
But it used to be a popular bathing place before it heated.
MANNING: As the rift pulls apart, new lava wells up from the plume below, constantly adding to the island as the Atlantic slowly widens.
These lavas were extruded between 10,000 and 8,000 years ago.
And then following that period, there was a hiatus of eruptive activity for about 5,000 years.
Since then we've had five rifting events here.
- Three in historical times.
- Right.
So the people who settled this region, they settled on a new lava flow.
MANNING: So Iceland was never part of the jigsaw puzzle of Pangaea.
It's grown up above the plume since the Atlantic started to open up.
Which raises a really intriguing question.
Could the plume under Iceland have caused the rift in the first place and triggered the opening of the Atlantic 58 million years ago? Clues to what happened then can still be found on the west coast of Scotland.
On the island of Mull, I met Chris Nicholas, who showed me some extraordinary natural rock formations.
So we're walking on seashore here but this looks almost like a man-made wall here.
It does at first glance, but you can see the ridge snakes off across the beach there.
And it's actually made of quite a hard rock.
- Made of tiny little crystals.
- Yeah.
So it's actually cooled and solidified magma that's come from deep down in the crust.
So it's come up through a split in the bedrock here? Yes, well to give us this sort of ridge shape, it must have come up through a crack or fissure through these surrounding muds.
So there was a lot of volcanic activity in this area.
Yeah, there's not just one ridge here.
There are several of them, all in parallel or cutting across each other.
And they all date from almost exactly the same time, about 58 million years ago.
So we can be fairly sure at that time there was a great pulse of igneous activity.
A great pulse of melt below the crust produced a series of fissures and a series of eruptions which built most of the islands you see in Scotland.
MANNING: Volcanic activity on this scale is evidence for an event similar to that which caused the Deccan Traps, a new mantle plume head which split the crust.
NICHOLAS: About 60 million years ago, Greenland was sitting next to Scotland.
So you'd be able to see it just out there.
The Atlantic as we know it just didn't exist.
And what we think happened is that a big hot plume of mantle material came up between Greenland and Scotland and domed the crust.
And eventually it split, and the two continents rifted apart.
As the plume rifts these two continental blocks apart, it was stretching the crust either side.
And it was fracturing.
And up through all the fractures and fissures was coming this hot mantle material on either side of the rift.
It was pouring out flood basalts on the Scottish side to give us everything we have here.
But it was also, on the other side of the rift, it was pouring out the basalts that you see in eastern Greenland.
So the rocks here and the rocks there are of the same age? Yeah, same age.
They went straight across, yes.
And what happened to this plume, subsequently? Well, Greenland is now about 2,000 kilometres away.
But we think that the plume itself is still sitting under the Mid-Atlantic rift out there, under Iceland.
MANNING: So the rocks on both sides of the Atlantic bear witness to the impact of a plume from deep within the mantle.
As the plume pushed up against the crust, it split, rifting Greenland off from Europe.
Once split, Europe and Greenland continued to move apart creating a new ocean basin in the north Atlantic.
Ever since then, the plume has been sitting under the rift producing Iceland, the largest volcanic island on the planet.
The plume that's now sitting under Iceland has produced some enormous changes in the 60 million years or so since it started to rise through the Earth's mantle.
Without that event in the Earth's interior, the whole of this area of the globe would be totally different.
But plumes may have even more profound effects than the shaping of the planet's surface.
As a biologist, what I find particularly intriguing is the suggestion that they may change the course of evolution.
The vast Deccan lava flows were all erupted in, geologically speaking, a blink of an eye.
But when? SAHNl: Usually, they may extend up to 100 metres and if you look very carefully MANNING: Palaeontologist Ashok Sahni has helped Vincent Courtillot pin down exactly when the plume head hit the surface.
Could this event have had global consequences for living things? SAHNl: These here are the teeth of mammals.
These mammalian teeth were recovered in sediments lying in-between the Deccan flows.
They belong to very primitive mammals which are related to the present-day shrews.
These mammals evolved at a fairly fast rate.
And it's possible to tell from these teeth exactly where we are in time.
They give us a date for the initiation of the Deccan volcanics.
And this date happens to be 65 million years.
They belong to an era that is what we call the Cretaceous period.
And surprisingly, it is this Cretaceous period, the end of this Cretaceous period, which also saw the extinction of the dinosaurs.
If you ask someone, "What killed the dinosaurs?" He or she is likely to tell you, "Well, it was the impact of an asteroid.
" And they might well be right.
But that cannot explain all of the extinctions.
Palaeontologists tell us that well before the impact hit the Earth, species had begun slowly disappearing.
When I mean slowly, I mean in the course of tens of thousands or hundreds of thousands of years.
This is precisely the timescale of the Deccan eruptions.
This is what makes me prefer that explanation.
COURTILLOT: As the large plume head came from inside the Earth, it started erupting lava flow after lava flow.
You should imagine these lava flows pumping into the atmosphere vast amounts of gases.
Sulphur dioxide, carbon dioxide, chlorine and also large amounts of dust obscuring the skies.
Obscurity would cause cold, possibly freezing.
And then acid rains would follow.
Real hell everywhere.
And when the Deccan had finished erupting, 60% of the species had disappeared from the surface of the Earth.
MANNING: Among the survivors were those tiny shrew-like mammals, our own ancestors, who were to flourish in a world freed from the grip of the large reptiles.
If Vincent Courtillot is right, then in a sense, we human beings owe our existence to events deep within the Earth.
The link between the Earth's activity and the course of evolution is something I'll be exploring later in the series.
Jules Verne's hero, Professor Lidenbrock, finally made it to the centre of the Earth to find it populated, ironically enough, by dinosaurs.
The real scientists who've explored the Earth's interior have found something in its way just as surprising.
That beneath all the complex changes we see at the surface, there is an underlying simplicity, the vast churning heat engine of the Earth's mantle, an engine which drives the dance of plate tectonics, and reforms our world in a myriad different ways.
In Iceland, you can really sense the power of the interior that shaped the Earth's surface.
As the Earth cools by convection, so the continents shift over its surface and rift apart.
But plate tectonics means that continents don't just separate, they must also collide.
And it's what happens when two continents collide that's the subject for our next programme.