Earth Story (1998) s01e03 Episode Script
Ring of Fire
MANNING: The High Andes of Bolivia.
In the 16th century, rumours of fabulous wealth drew Spanish adventurers to this lonely place.
What they found here exceeded their wildest dreams: A mountain of silver they called the Cerro Rico.
To the Spanish, the Cerro Rico was a gift from God.
It yielded tens of thousands of tons of silver, enough to finance the entire Spanish Empire.
But why should so much mineral wealth be concentrated in one spot? And it's not just here.
All around the Pacific Ocean, there are rich deposits of copper, silver and gold.
In fact, you might call the rim of the Pacific the Ring of Gold, but it's more usually called the Ring of Fire because all around the margin of the Pacific basin, associated with the mineral deposits, there are huge chains of volcanoes.
(RUMBLING) At first sight there might not seem to be any connection between volcanoes and mineral wealth, but in fact there is.
And that link gives us a clue to one of the most puzzling mysteries about the Earth.
How did the dry land, the continents on which we live, how did they form? Three-and-a-half billion years ago, our planet was covered by a single vast ocean.
Scientists believe there were no large land masses, just hundreds of volcanic islands.
Yet today, nearly a third of the Earth's surface is covered by dry land.
So where did it all come from? How did these huge continents with their mineral riches form? The answer lies in the connection between earthquakes and volcanoes and in the scientific revolution which has linked them together.
Scientists have discovered that the entire surface of the globe, which seems so fixed and immobile, is in fact constantly moving.
And that it's the way the surface moves that holds the key to how the continents grow.
But the mystery of the continents has taken over 30 years to solve.
Throughout that time, John Dewey has been one of the leading scientists trying to understand the origin of the dry land around us.
The west coast of Scotland holds a special interest for him.
The rocks here are the oldest in the British Isles.
It's always marvellous being in the West Highlands.
This is a dramatic part, isn't it? - The northwest.
- It's gorgeous, it's gorgeous.
Even more gorgeous will be the whiskey at 6:00.
Drinking at 6:00.
Never before, I know.
MANNING: When John Dewey first came here over 30 years ago, geologists knew very little about how the continents were formed.
Paradoxically, they understood far more about the rocks at the bottom of the ocean.
When geologists haul rocks off the sea floor, they find that most of them are very similar.
Often they are surprisingly glassy and sharp, evidence that they were recently erupted from an undersea volcano.
And it turns out that most of the ocean floor is made of relatively young volcanic rock, all of it erupted within the last 200 million years.
MAN: Look at that glassy one there MANNING: Once geologists were able to go down to the ocean floor itself, they soon found fields of solidified lava, and the volcanoes themselves.
It became clear that these volcanoes were part of a single vast chain of volcanic mountains that runs down the middle of the world's ocean basins, all the way around the planet.
These volcanoes are continually producing new rock.
The ocean floor is constantly growing.
So by the late '60s, the origin of the ocean floor was absolutely clear.
But on land the picture was completely different.
DEWEY: Utterly different.
We see this incredibly complicated pattern of deformations, melting, re-melting.
So this lovely swirling pattern with bands, that's telling you something about the history of the rock.
What it's telling us, Aubrey, is that these rocks have a very long and very complicated history a very long time ago, and had to be explained in completely different ways to the rocks we see out in the oceans.
MANNING: But if ocean rocks are produced by undersea volcanoes, could land volcanoes have a role in the growth of the continents, volcanoes like those in Bolivia? Each year the people who live at the foot of the silver mountain called Cerro Rico celebrate the source of their prosperity.
But for many years, the region has also attracted scientists like Rodney Grapes and Leonore Hoke.
HOKE: 20 litres.
That should be enough.
(CHILDREN SINGING) MANNING: This year they're being joined by two British geologists, Rick Thomas and Catrin Jones.
The team are heading for one of the most volcanically active regions on Earth.
They plan to test a theory which links volcanoes to the formation of the continents.
Okay, so where are we heading? I think, really, we've got to go straight towards that big volcano 'cause there's nothing else that's going to give us a good bearing.
HOKE: So we have to go, what is it, 310 GRAPES: We'll have to rely on your bearing then.
HOKE: Okay, well, then let's go.
MANNING: Their journey takes them across the Salar, a dry salt lake, part of the vast Altiplano, a 4,000-metre-high plateau in the middle of the Andes.
HOKE: There are the odd potholes, but I think it's the best road in Bolivia.
You can go flat-out 120 kilometres if the car makes it.
MANNING: Between them and the Pacific coast lies their goal, a chain of active volcanoes, the highest in the world.
(HISSING) As they near the volcanoes, the team start to see signs at the surface of tremendous activity deep below them.
The volcanoes here erupt a characteristic type of rock, called andesite, after the Andes.
Not just the volcanic cones, but the entire landscape here is made of this andesite and other volcanic rocks.
Since the '60s, scientists studying the growth of the continents, like Rick Thomas, have been working out how the eruptions here can distribute material over such a wide area.
Volcanoes in the Andes are notoriously explosive.
They can eject tons of ash and fragments of volcanic rock into the atmosphere.
The reason for this behaviour is a surprising one.
THOMAS: The liquid rock at depth contains a lot of water, and that water's happily dissolved when it's down deep.
As it comes close to the surface, it froths up, the magma, the liquid rock, and it causes it to explode, just like when you uncork a champagne bottle.
Same sort of thing's happening here.
It froths out and what we get is fragments of rock which are full of bubbles within the rock and you get lots of these pumices which we can see here.
If you'd been here during one of these eruptions, you'd have seen vast billowing clouds in the distance coming towards you, kilometres away.
But in a minute or so, because they travel so fast, hundreds of miles an hour, quite easily, they would have engulfed you.
And this ash cloud would have just blasted on down the valley and just filled most of what we can see of that plain.
MANNING: As the clouds of ash and pumice settle, they compress together to form new rock layers.
Over time, these layers gradually build up, adding material to the surface.
But could a similar process be happening deep below the volcanoes? And might this be the key to how continents grow? Clues to what's happening beneath the volcanoes can be found in the gases that continuously stream out of them.
Straight ahead of us is Ollague with its fumarole on top.
Yes, a nice conical volcano, that, actually.
And you can see some MANNING: Leonore plans to collect a gas sample from the active fumaroles at the summit, which are continuously producing steam and other vapours.
THOMAS: Actually quite clearly with the fumarole there.
And you come down the right shoulder and you just might be able to make out Irruputuncu.
HOKE: And that's where we're heading.
MANNING: If Leonore can work out what's happening here, the results may have global implications, because scientists now realise that the explosive volcanoes of the Andes and the rocks that they produce are far from unique.
Matthew Thirlwall has collected rock samples from volcanoes all over the world.
This is a sample of pumice from the Andes, but we can find almost identical pumice in several other places on Earth.
Here we have pumice almost exactly the same with these gas bubbles in it from Mount Saint Helens in Washington State.
And this is a typical example of andesite, the characteristic rock in these areas.
And you can see, you can find almost identical andesites in several other recent eruptions.
For example, here we have a sample from Mount Pinatubo in the Philippines, very, very similar samples.
Some 4,000 miles of Pacific between them.
MANNING: So the volcanoes of the Pacific Rim are all basically similar.
Hundreds of them are continually erupting in an extraordinary arc that runs up the western coast of the Americas from Patagonia, through Central America and right up to Alaska.
It continues across the Bering Straits through Japan and on into Indonesia and the rest of the Pacific basin.
This is the Ring of Fire.
It dawned on scientists that these volcanoes were gradually building up a ring of andesitic rock all around the Pacific.
And then they discovered a link between these rocks and those which make up the continents as a whole.
As they analysed thousands of samples from all over the world, a surprisingly simple picture emerged.
Though the rocks looked different, they were all made from the same basic ingredients.
What's more, the recipe hardly varied at all from continent to continent.
Even more significant, this basic recipe was essentially the same as that of the volcanic rocks in the Ring of Fire.
DEWEY: If one were to take all the continental crust of the whole world, grind it up, put it in a giant mixing bowl, stir it up, then measure its average composition, it would be very, very similar to that of these rocks.
And it would consist of three basic minerals: Quartz, feldspar and hornblende.
So this is the stuff of which the continents are made? This is the very stuff of which the core of the continents is made.
Let's take a piece off.
Like this.
Here you can see those three minerals, I'll give you the hand lens.
Here we see those three minerals.
- You see the pale translucent quartz.
- Yeah.
- You see the rather milky feldspar.
- Yeah.
And you can see the dark green hornblende.
If you go to the Andes and look at the volcanic rocks of the Andes and indeed the plutonic deep rocks injected under the volcanoes in the deep crust, these are the three minerals that you will find.
So you're saying that's the same stuff of which the Andes are made? That's right, and of course that immediately begins to give one an inkling that possibly these rocks were formed in the same way.
And not only that, that much of the continental crust must have been formed in the same way as we see in the Andes at the present day.
MANNING: So it looks as if somehow the continents have all been created by volcanoes like these, gradually building up the crust over geological time.
But there was a problem with this idea.
How could something happening around the Pacific explain rocks in Scotland? This was a complete mystery.
But then the Ring of Fire provided another vital clue.
The region was periodically rocked by earthquakes.
On Good Friday, 1964, one of the largest ever recorded struck Alaska.
(CRASHING) (GULLS CAWING) I was leaning in my cabin getting a cigarette and all of a sudden it started shaking.
We was evidently right about over the epicentre.
These big sand spouts were shooting up in the air.
You can't believe how that boat shook and rattled.
It was scary.
MANNING: The very next day a US government geologist, George Plafker, flew in to investigate what had happened.
WOMAN ON PA: May I have your attention please From the initial reports we realised it was a major disaster and a great earthquake and it was in my field area.
Our mission was to assess the damage.
And of course, as a scientist, I was interested in trying to figure out the cause of this earthquake.
MANNING: Although he didn't know it then, George's work would usher in a revolution in our understanding of the Earth, a revolution that would finally solve the mystery of the continents.
But his immediate problem was just getting around.
PLAFKER: When we first came up, all we'd heard was that the roads and railroads were cut, and that there was a great deal of landsliding and wave damage in the coastal areas.
All I seen was a big black wall of water out there between this island and that mainland over there.
I said, "Oh, my gosh, tidal wave," and I just kept running.
I didn't even look back.
MANNING: Amongst all the stories of eyewitnesses, confusing reports of sea level changes were also emerging.
Some witnesses claimed the sea level had dropped, others that it had risen.
George's hunch was that something fundamental had happened to the land itself and that working this out would lead him to the cause of the earthquake.
He decided to examine the shoreline in detail.
This is what we have to do to get around this shoreline.
MANNING: And it was here that he made his first remarkable discovery.
PLAFKER: This water level is a little bit below mean high tide.
Mean high tide normally reaches to about the top of this rock.
And you can see these barnacles on the rock and seaweed, which grow up to that level.
Let me show you something fascinating now.
Here is a line of dead barnacles, and this line is about six feet above the living barnacles.
When I came here in 1964, some of these barnacles were still alive.
MANNING: George realised that during the few moments of the earthquake, the land here had been jacked up six feet.
What was more, George had found a way to measure accurately land level changes over the whole enormous area affected.
He visited hundreds of small coves and inlets, following the line of barnacles.
Now they're 13 feet above the level at which barnacles live at the present time.
PLAFKER: I spent all of the summer of 1964 and most of the summer of 1965 studying a stretch of coast more than 500 miles from one end to the other.
MANNING: Where the shoreline was uplifted, plants soon colonised the new land.
PILOT: Portage is about 270 degrees MANNING: But when he flew northwest, further inland, it was a completely different story.
PILOT: We'll go across Naked Island.
MANNING: Before the earthquake, Portage had been a thriving rail stop.
But George learned that it was now being flooded at high tides and icebergs were floating up the river, into the town.
We stood over there where the railroad station used to be and watched these icebergs slam into the bar building and bounce off.
- Jesus! - They'd slam into the house and it would go crunch.
That night the water came in to the top of the bar, where people sat to drink, it was clear up to that.
And the old guy that worked for us stayed in the bar that night.
The rest of us all fled across the street.
Then we all go back over to see if he's all right.
And of course, he was.
He was drunk.
MANNING: George realised that in this region, land had gone down, nearly two metres at Portage.
The subsequent inundation of seawater killed large tracts of pine forest.
When George plotted his measurements on a map, he found that a striking picture emerged.
Along the coastal regions, an area about the size of Great Britain had risen during the earthquake, up to 12 metres, while further inland, another vast area had gone down about two metres.
That was something that was absolutely new and different.
Nobody had ever seen anything like that before, probably on the face of the Earth.
MANNING: George Plafker had discovered movements of the Earth's crust on a totally unprecedented scale, and that accounted for the huge landslides and the tidal waves.
What he still had to explain was why parts of the Alaskan crust had risen whilst others had sunk.
And the explanation he came up with changed our view of the Earth forever.
DEWEY: When George was studying the great 1964 Alaskan earthquake, we knew at that stage already that movements of the Earth's crust and shaking of the Earth's crust through earthquakes were linked with the big structures in the crust we call faults.
For example, the great San Andreas Fault of California, a break in the crust, that cuts the surface of the Earth along a very great distance.
Here we have a couple of rather nice slides.
- And this one - Good heavens! MANNING: You really can see a break on the Earth's surface there.
DEWEY: And of course that is perhaps what George was looking for in Alaska.
It was such a huge earthquake, they must have expected something - very, very conspicuous on the surface.
- A very, very substantial large structure cutting the surface over a very long distance, that's right, yes.
MANNING: But in Alaska there was no sign of a large fault cutting the surface.
PLAFKER: Despite all of our efforts, we didn't find such a fault.
And this was a major puzzle, and it's something that bothered me for a long time.
MANNING: George knew there had to be a fault somewhere.
Perhaps he was looking in the wrong place? Finally, after much thinking about it, I realised that the fault we were looking for actually underlays the entire region of uplift and subsidence, and that we couldn't see it because it came to the surface out to sea.
MANNING: George's insight was that the fault was underground, sloping gently beneath the entire region.
He knew that about 100 kilometres off the Alaskan coast lay a mysterious deep trench in the ocean floor.
Oceanographers had been aware of it for some time, but they couldn't explain what it was.
George was convinced that this was where his gigantic fault reached the surface, underwater.
DEWEY: So if I draw a cross-section of Alaska, I come from the north, through the land, across the volcano and down gradually steeper and deeper, then down into a deep ocean trench like this.
What George found, if you remember, of course, was that the great Alaskan earthquake of 1964 was produced on a fault plane, dipping gently under Alaska like this, with this sense of motion.
That is, the under part was slipping down under Alaska, in that way.
And of course this is very important because if one projects that fault out along its length, lo and behold, it projects to exactly the position of the oceanic trench.
So the reason that nothing was visible on the surface, Earth's surface up here, was that the break actually hits the edge of the crust beneath the sea? That's right.
MANNING: A major fault like this would have caused repeated earthquakes.
George realised that it was what happened between earthquakes, as the crust was squeezed, that could explain why some places had gone down while others had risen.
I have here a model, a very simplified representation of the continental margin of Alaska.
This represents the fault.
For several centuries, between earthquakes, there is gradual compression and uplift of the area above that fault.
And at the time of an earthquake it slips suddenly, causing uplift on the seaward side and subsidence in the area where there was a bulge.
MANNING: And this land movement was not a one-off event.
When he looked, George had found signs of past earthquakes engraved in the landscape.
This series of steps was created as the land was repeatedly jacked out of the sea every few hundred years by sudden movement on the fault.
But it was what was happening beneath the fault that was really significant.
George suggested that with this sense of motion, the whole of the Pacific ocean floor is slipping, flowing down along this inclined plane underneath Alaska.
And of course the expression of this are intermittent earthquakes.
About every 700 or 800 years a great earthquake occurs like this on this great inclined plane.
And we have a slip of maybe 15, 20 metres or something like that.
You wait another 800 years, there's another slip.
So over geological time, you keep on adding that up and you have this tremendous flow of the Pacific Ocean floor down back into the mantle.
This is an astonishing picture.
These colossal areas of the sea floor - are sliding beneath the crust of Alaska.
- That's right.
MANNING: George Plafker and his colleagues suddenly realised the full significance of the ocean trench off the coast of Alaska.
Because this trench circled virtually the whole Pacific margin, it suggested that the entire floor of the Pacific Ocean was sliding beneath the surrounding continents, sinking back into the Earth's interior.
And this was just part of an even bigger picture.
Geologists had been puzzled about the ultimate fate of the sea floor after it had been created at the mid-ocean ridge.
The whole cycle was now obvious.
As ocean floor was being created in the middle of the oceans, so it was being destroyed at the trenches at ocean margins in what scientists came to call subduction zones.
And this was happening all over the planet.
Together, the ocean trenches and the mid-ocean ridges formed a network of lines, boundaries that divided the globe into vast slowly moving regions, the plates.
It was an extraordinary revelation.
Geologists suddenly realised that the entire surface of our world is in constant motion.
And so the theory of plate tectonics was born.
And scientists started thinking about the Earth in a completely new way.
LECTURER: And it's really quite an extraordinary idea, but remarkably simple.
Plate tectonics.
We've all heard about it, and rightly so.
It's done for our understanding of the Earth what the theory of evolution did for our understanding of life.
The continents move with the plates.
Throughout geological time they have completely rearranged themselves, sometimes coming together and sometimes splitting apart, opening and closing oceans, reshaping the face of the Earth.
What had begun with a line of dead barnacles ended with a new view of our planet.
As the new theory sank in, scientists realised that the movement of the plates, which was the cause of all the earthquakes around the Pacific, might also be responsible for the volcanoes.
Leonore Hoke believes that this link can finally explain the mystery of the continents.
Deep beneath her feet, ocean plate is sinking into the Earth's interior.
Now, it's very interesting, because we have volcanoes round about at in the position where the subducting oceanic plate is 100 to 120 kilometres below the surface, where we are here now.
And that's where we have volcanoes and we know from looking at the volcanoes that they produce a lot of molten rock and they produce a lot of fluids, like water.
MANNING: Geologists suspect that some of this water comes directly from the sinking ocean plate and may explain why the volcanoes are here.
Leonore plans to test this idea by collecting a sample from high on the volcano.
Well, this is my blue barrel, which contains all the sample equipment which I need.
And there are all sorts of things in it, like this gas mask which protects me from obnoxious gases, funnels to catch gas bubbles with.
It contains this tubing which I need and it contains these containers and I packed them at sea level in New Zealand.
They pop when you open them because we're really at high altitude here.
And here are these special glass containers into which I will suck the gas.
HOKE: So we're ready then? (SPEAKING SPANISH) - Benito.
- Benito.
Leonore.
(MAKING INTRODUCTIONS IN SPANISH) MANNING: It's 6 a.
m.
On the Bolivian border with Chile.
Leonore, Catrin and their guide are on their way to their chosen sampling area.
They're heading for the crater, the most dangerous place on the active volcano Irruputuncu.
The climbers are following a rarely used footpath made by locals to collect sulphur from the summit.
They're climbing to nearly 6,000 metres.
At this height oxygen levels have almost halved, making it difficult to breathe.
It's hard work, isn't it? - Yeah.
Biting wind.
- Yeah.
Sulphurous smell is getting stronger as well, isn't it? Well, we're getting closer to the fumarole.
That's good.
JONES: Yeah.
Another hour or so.
- Brilliant view.
- Yeah.
- That's one of the rewards of it.
- Yeah.
It's amazing.
Feel like a bird up here.
MANNING: The higher they go, the steeper the climb, as the sticky volcanic rock has built up the slopes of the crater.
After four hours, they are within sight of their goal.
This is the solid material that the volcano is made from.
It's dark, almost black.
But covering all these rocks is sulphur which is coming from the gases that are coming up from the crater behind me.
It's beautiful, yellow, crystalline material.
But the gases are also containing water.
MANNING: It's this water, together with other gases rising through the crater, that Leonore has come to collect.
I'm taking a condensate sample.
That means I'm collecting the steam and I guide it through this glass container which is immersed in snow and that cools the steam.
And I'm collecting the water at the bottom of this and you will see, later on, there's quite a lot of water in this steam.
I'm having a look now how much water there is.
I take it out of the snow.
(GASPING) It's great to get out of this.
Hot steam, it's 120 degrees.
But what I've collected here is some water.
You can see it in here.
It's quite a lot coming out of this fumarole.
Now, the chemistry of this water will tell us where this water is coming from and it will be analysed back at the lab.
I haven't measured the temperature of this.
I'm gonna do this after.
- You've got yellow all over it.
- We got covered in sulphur.
- GRAPES: Stings your eyes and things.
- Yeah.
- It was kind of like hell.
- It's itchy.
The devil was about to pop his head out any minute.
JONES: Okay, time to go.
- Guys, you'll be freezing.
Be quick.
- Ooh, it's not warm.
MANNING: Back in Wellington, New Zealand, Leonore heads straight for her laboratory.
She begins by examining the gases she collected from the volcano.
HOKE: You can measure the different gas components in the sample and it contains helium, it contains argon, it contains CO2, it contains nitrogen and many other gas components.
MANNING: But it's the concentration of nitrogen which catches her attention.
It goes almost off the scale.
HOKE: That's good.
We have here quite a high signal coming from the nitrogen.
MANNING: Nitrogen is the most common gas in air, but the proportion in the sample is far too high.
It suggests a completely different source.
Leonore turns her attention to the water in the sample.
We can also measure the isotopes of these gas components by using a mass spectrometer.
MANNING: By analysing the atoms in the water, Leonore is hoping to test a startling idea.
She suspects that some of the water in the sample might come from the sea.
If so, it would support her growing conviction that the nitrogen could be coming from dead marine organisms, known to be nitrogen-rich, buried in the sea floor.
Could the flask from Bolivia contain a sample of the Pacific Ocean? The hydrogen is very close to the hydrogen isotopes of seawater.
So that suggests that some component within that water is probably seawater.
HOKE: So we can say from that that a lot of that nitrogen is biogenic nitrogen produced in ocean sediments and released during the subduction process.
MANNING: So the journey up the volcano has yielded an amazing result.
High up at the summit are traces of marine life, and the Pacific Ocean itself, which have completed a long and extraordinary journey through the Earth.
JONES: Excellent.
The whole story is contained in one glass flask.
You suffered with me.
I sense the completion of one of life's key cycles here because the sea floor is pulling down with it a lot of dead animal and plant material.
This is being stewed up here and a lot of it is coming out as nitrogen and carbon dioxide from the volcano, which goes into the atmosphere, re-dissolves in the oceans, goes back through living material again.
It's a cycle.
That's right.
And of course, a large part of the cyclic process explains the origin of the continents because this flux of wet rock coming down here, getting hotter and hotter, and eventually they get so hot that the water is expelled MANNING: What happens next is the key to how the continents grow.
The expelled water has a strange effect on the rocks of the Earth's interior.
Like scattering salt on an icy road, the water reduces the melting point of the surrounding rocks and so they start to melt.
This molten rock rises to the surface to erupt as a line of volcanoes.
But these volcanoes are like the tip of an iceberg.
Only a small proportion of the molten rock ever reaches the surface.
The surface volcanoes are merely the surface manifestation of a huge process of the addition of new hot material to the crust.
And if this has been going on for some considerable period of time, which we know it has, you can actually make very large amounts of continental crust in this way.
JONES: There's a huge volume of volcanic material being added to the crust.
Below us you can imagine more molten rock ponding underneath you at some level.
As you drive along, you're witnessing the growth of the continental crust.
It's quite an exciting thought.
Here is a place where, for millions of years, the continent has been growing and it's going to continue probably for many more millions of years.
MANNING: But as the plates have moved throughout geological time, the places where the continents grow have shifted over the Earth.
In the Lake District, Matthew Thirlwall showed me how volcanoes once built the crust in Britain, 450 million years ago.
THIRLWALL: Formed by ash and pumice just as in Bolivia.
- This, then, is all volcanic rock? - Yes, everything we see over here.
It was produced by big eruptions of the style that you saw in the Pacific.
Below us, there is something like 8,000 metres of volcanic rocks.
Was this then a great subduction zone like the Ring of Fire? That's right.
From the andesitic composition of these volcanic rocks, we can tell that there was almost certainly a subduction zone beneath the Lake District.
And not just beneath the Lakes but beneath Wales, southern Ireland and even into the eastern United States as well.
Worldwide, the composition of continental crust is andesitic, right the way back through time.
Subduction zones were the sites of continental crust formation, not just at the present day at the Ring of Fire, but right the way back until the earliest parts of Earth history.
MANNING: So once these gentle hills were as violent and dangerous as the Ring of Fire.
As the plates moved, they cooled and died.
The Cerro Rico too, was once an active volcano some 16 million years ago.
These tunnels have yielded 60,000 tons of precious metal, a significant fraction of all the silver in human hands.
And the much older volcanoes in the Lake District have also brought up their own rich dividend.
Not gold and silver, but copper and lead, crucial elements in Britain's Industrial Revolution.
So what is this association between volcanism and mineral richness? What is the connection? The connection is that the hot rocks at depth heat up water and the water dissolves metals from the volcanic rocks, transports the metals up to the surface and then deposits them as the pressure drops.
And it's those concentrations that provide the workable copper and lead, - over in these hills up here.
- Yeah.
So really, the volcanoes have done all the work? They have, yeah.
They've brought all this stuff up and we just have to pick it off the surface.
More or less, yes.
MANNING: The people of the Cerro Rico have long believed the silver is a gift from Pachamama, their god of the Earth.
In some ways, they've been proved right.
Scientists now see how as the plates of the Earth move, creating the land on which we live, our planet also yields up its wealth.
But the movement of these plates presents us with a really challenging question.
What is the driving force? What engine moves the plates across the planet? That's what I'll be finding out in the next programme when we journey to the centre of the Earth.
In the 16th century, rumours of fabulous wealth drew Spanish adventurers to this lonely place.
What they found here exceeded their wildest dreams: A mountain of silver they called the Cerro Rico.
To the Spanish, the Cerro Rico was a gift from God.
It yielded tens of thousands of tons of silver, enough to finance the entire Spanish Empire.
But why should so much mineral wealth be concentrated in one spot? And it's not just here.
All around the Pacific Ocean, there are rich deposits of copper, silver and gold.
In fact, you might call the rim of the Pacific the Ring of Gold, but it's more usually called the Ring of Fire because all around the margin of the Pacific basin, associated with the mineral deposits, there are huge chains of volcanoes.
(RUMBLING) At first sight there might not seem to be any connection between volcanoes and mineral wealth, but in fact there is.
And that link gives us a clue to one of the most puzzling mysteries about the Earth.
How did the dry land, the continents on which we live, how did they form? Three-and-a-half billion years ago, our planet was covered by a single vast ocean.
Scientists believe there were no large land masses, just hundreds of volcanic islands.
Yet today, nearly a third of the Earth's surface is covered by dry land.
So where did it all come from? How did these huge continents with their mineral riches form? The answer lies in the connection between earthquakes and volcanoes and in the scientific revolution which has linked them together.
Scientists have discovered that the entire surface of the globe, which seems so fixed and immobile, is in fact constantly moving.
And that it's the way the surface moves that holds the key to how the continents grow.
But the mystery of the continents has taken over 30 years to solve.
Throughout that time, John Dewey has been one of the leading scientists trying to understand the origin of the dry land around us.
The west coast of Scotland holds a special interest for him.
The rocks here are the oldest in the British Isles.
It's always marvellous being in the West Highlands.
This is a dramatic part, isn't it? - The northwest.
- It's gorgeous, it's gorgeous.
Even more gorgeous will be the whiskey at 6:00.
Drinking at 6:00.
Never before, I know.
MANNING: When John Dewey first came here over 30 years ago, geologists knew very little about how the continents were formed.
Paradoxically, they understood far more about the rocks at the bottom of the ocean.
When geologists haul rocks off the sea floor, they find that most of them are very similar.
Often they are surprisingly glassy and sharp, evidence that they were recently erupted from an undersea volcano.
And it turns out that most of the ocean floor is made of relatively young volcanic rock, all of it erupted within the last 200 million years.
MAN: Look at that glassy one there MANNING: Once geologists were able to go down to the ocean floor itself, they soon found fields of solidified lava, and the volcanoes themselves.
It became clear that these volcanoes were part of a single vast chain of volcanic mountains that runs down the middle of the world's ocean basins, all the way around the planet.
These volcanoes are continually producing new rock.
The ocean floor is constantly growing.
So by the late '60s, the origin of the ocean floor was absolutely clear.
But on land the picture was completely different.
DEWEY: Utterly different.
We see this incredibly complicated pattern of deformations, melting, re-melting.
So this lovely swirling pattern with bands, that's telling you something about the history of the rock.
What it's telling us, Aubrey, is that these rocks have a very long and very complicated history a very long time ago, and had to be explained in completely different ways to the rocks we see out in the oceans.
MANNING: But if ocean rocks are produced by undersea volcanoes, could land volcanoes have a role in the growth of the continents, volcanoes like those in Bolivia? Each year the people who live at the foot of the silver mountain called Cerro Rico celebrate the source of their prosperity.
But for many years, the region has also attracted scientists like Rodney Grapes and Leonore Hoke.
HOKE: 20 litres.
That should be enough.
(CHILDREN SINGING) MANNING: This year they're being joined by two British geologists, Rick Thomas and Catrin Jones.
The team are heading for one of the most volcanically active regions on Earth.
They plan to test a theory which links volcanoes to the formation of the continents.
Okay, so where are we heading? I think, really, we've got to go straight towards that big volcano 'cause there's nothing else that's going to give us a good bearing.
HOKE: So we have to go, what is it, 310 GRAPES: We'll have to rely on your bearing then.
HOKE: Okay, well, then let's go.
MANNING: Their journey takes them across the Salar, a dry salt lake, part of the vast Altiplano, a 4,000-metre-high plateau in the middle of the Andes.
HOKE: There are the odd potholes, but I think it's the best road in Bolivia.
You can go flat-out 120 kilometres if the car makes it.
MANNING: Between them and the Pacific coast lies their goal, a chain of active volcanoes, the highest in the world.
(HISSING) As they near the volcanoes, the team start to see signs at the surface of tremendous activity deep below them.
The volcanoes here erupt a characteristic type of rock, called andesite, after the Andes.
Not just the volcanic cones, but the entire landscape here is made of this andesite and other volcanic rocks.
Since the '60s, scientists studying the growth of the continents, like Rick Thomas, have been working out how the eruptions here can distribute material over such a wide area.
Volcanoes in the Andes are notoriously explosive.
They can eject tons of ash and fragments of volcanic rock into the atmosphere.
The reason for this behaviour is a surprising one.
THOMAS: The liquid rock at depth contains a lot of water, and that water's happily dissolved when it's down deep.
As it comes close to the surface, it froths up, the magma, the liquid rock, and it causes it to explode, just like when you uncork a champagne bottle.
Same sort of thing's happening here.
It froths out and what we get is fragments of rock which are full of bubbles within the rock and you get lots of these pumices which we can see here.
If you'd been here during one of these eruptions, you'd have seen vast billowing clouds in the distance coming towards you, kilometres away.
But in a minute or so, because they travel so fast, hundreds of miles an hour, quite easily, they would have engulfed you.
And this ash cloud would have just blasted on down the valley and just filled most of what we can see of that plain.
MANNING: As the clouds of ash and pumice settle, they compress together to form new rock layers.
Over time, these layers gradually build up, adding material to the surface.
But could a similar process be happening deep below the volcanoes? And might this be the key to how continents grow? Clues to what's happening beneath the volcanoes can be found in the gases that continuously stream out of them.
Straight ahead of us is Ollague with its fumarole on top.
Yes, a nice conical volcano, that, actually.
And you can see some MANNING: Leonore plans to collect a gas sample from the active fumaroles at the summit, which are continuously producing steam and other vapours.
THOMAS: Actually quite clearly with the fumarole there.
And you come down the right shoulder and you just might be able to make out Irruputuncu.
HOKE: And that's where we're heading.
MANNING: If Leonore can work out what's happening here, the results may have global implications, because scientists now realise that the explosive volcanoes of the Andes and the rocks that they produce are far from unique.
Matthew Thirlwall has collected rock samples from volcanoes all over the world.
This is a sample of pumice from the Andes, but we can find almost identical pumice in several other places on Earth.
Here we have pumice almost exactly the same with these gas bubbles in it from Mount Saint Helens in Washington State.
And this is a typical example of andesite, the characteristic rock in these areas.
And you can see, you can find almost identical andesites in several other recent eruptions.
For example, here we have a sample from Mount Pinatubo in the Philippines, very, very similar samples.
Some 4,000 miles of Pacific between them.
MANNING: So the volcanoes of the Pacific Rim are all basically similar.
Hundreds of them are continually erupting in an extraordinary arc that runs up the western coast of the Americas from Patagonia, through Central America and right up to Alaska.
It continues across the Bering Straits through Japan and on into Indonesia and the rest of the Pacific basin.
This is the Ring of Fire.
It dawned on scientists that these volcanoes were gradually building up a ring of andesitic rock all around the Pacific.
And then they discovered a link between these rocks and those which make up the continents as a whole.
As they analysed thousands of samples from all over the world, a surprisingly simple picture emerged.
Though the rocks looked different, they were all made from the same basic ingredients.
What's more, the recipe hardly varied at all from continent to continent.
Even more significant, this basic recipe was essentially the same as that of the volcanic rocks in the Ring of Fire.
DEWEY: If one were to take all the continental crust of the whole world, grind it up, put it in a giant mixing bowl, stir it up, then measure its average composition, it would be very, very similar to that of these rocks.
And it would consist of three basic minerals: Quartz, feldspar and hornblende.
So this is the stuff of which the continents are made? This is the very stuff of which the core of the continents is made.
Let's take a piece off.
Like this.
Here you can see those three minerals, I'll give you the hand lens.
Here we see those three minerals.
- You see the pale translucent quartz.
- Yeah.
- You see the rather milky feldspar.
- Yeah.
And you can see the dark green hornblende.
If you go to the Andes and look at the volcanic rocks of the Andes and indeed the plutonic deep rocks injected under the volcanoes in the deep crust, these are the three minerals that you will find.
So you're saying that's the same stuff of which the Andes are made? That's right, and of course that immediately begins to give one an inkling that possibly these rocks were formed in the same way.
And not only that, that much of the continental crust must have been formed in the same way as we see in the Andes at the present day.
MANNING: So it looks as if somehow the continents have all been created by volcanoes like these, gradually building up the crust over geological time.
But there was a problem with this idea.
How could something happening around the Pacific explain rocks in Scotland? This was a complete mystery.
But then the Ring of Fire provided another vital clue.
The region was periodically rocked by earthquakes.
On Good Friday, 1964, one of the largest ever recorded struck Alaska.
(CRASHING) (GULLS CAWING) I was leaning in my cabin getting a cigarette and all of a sudden it started shaking.
We was evidently right about over the epicentre.
These big sand spouts were shooting up in the air.
You can't believe how that boat shook and rattled.
It was scary.
MANNING: The very next day a US government geologist, George Plafker, flew in to investigate what had happened.
WOMAN ON PA: May I have your attention please From the initial reports we realised it was a major disaster and a great earthquake and it was in my field area.
Our mission was to assess the damage.
And of course, as a scientist, I was interested in trying to figure out the cause of this earthquake.
MANNING: Although he didn't know it then, George's work would usher in a revolution in our understanding of the Earth, a revolution that would finally solve the mystery of the continents.
But his immediate problem was just getting around.
PLAFKER: When we first came up, all we'd heard was that the roads and railroads were cut, and that there was a great deal of landsliding and wave damage in the coastal areas.
All I seen was a big black wall of water out there between this island and that mainland over there.
I said, "Oh, my gosh, tidal wave," and I just kept running.
I didn't even look back.
MANNING: Amongst all the stories of eyewitnesses, confusing reports of sea level changes were also emerging.
Some witnesses claimed the sea level had dropped, others that it had risen.
George's hunch was that something fundamental had happened to the land itself and that working this out would lead him to the cause of the earthquake.
He decided to examine the shoreline in detail.
This is what we have to do to get around this shoreline.
MANNING: And it was here that he made his first remarkable discovery.
PLAFKER: This water level is a little bit below mean high tide.
Mean high tide normally reaches to about the top of this rock.
And you can see these barnacles on the rock and seaweed, which grow up to that level.
Let me show you something fascinating now.
Here is a line of dead barnacles, and this line is about six feet above the living barnacles.
When I came here in 1964, some of these barnacles were still alive.
MANNING: George realised that during the few moments of the earthquake, the land here had been jacked up six feet.
What was more, George had found a way to measure accurately land level changes over the whole enormous area affected.
He visited hundreds of small coves and inlets, following the line of barnacles.
Now they're 13 feet above the level at which barnacles live at the present time.
PLAFKER: I spent all of the summer of 1964 and most of the summer of 1965 studying a stretch of coast more than 500 miles from one end to the other.
MANNING: Where the shoreline was uplifted, plants soon colonised the new land.
PILOT: Portage is about 270 degrees MANNING: But when he flew northwest, further inland, it was a completely different story.
PILOT: We'll go across Naked Island.
MANNING: Before the earthquake, Portage had been a thriving rail stop.
But George learned that it was now being flooded at high tides and icebergs were floating up the river, into the town.
We stood over there where the railroad station used to be and watched these icebergs slam into the bar building and bounce off.
- Jesus! - They'd slam into the house and it would go crunch.
That night the water came in to the top of the bar, where people sat to drink, it was clear up to that.
And the old guy that worked for us stayed in the bar that night.
The rest of us all fled across the street.
Then we all go back over to see if he's all right.
And of course, he was.
He was drunk.
MANNING: George realised that in this region, land had gone down, nearly two metres at Portage.
The subsequent inundation of seawater killed large tracts of pine forest.
When George plotted his measurements on a map, he found that a striking picture emerged.
Along the coastal regions, an area about the size of Great Britain had risen during the earthquake, up to 12 metres, while further inland, another vast area had gone down about two metres.
That was something that was absolutely new and different.
Nobody had ever seen anything like that before, probably on the face of the Earth.
MANNING: George Plafker had discovered movements of the Earth's crust on a totally unprecedented scale, and that accounted for the huge landslides and the tidal waves.
What he still had to explain was why parts of the Alaskan crust had risen whilst others had sunk.
And the explanation he came up with changed our view of the Earth forever.
DEWEY: When George was studying the great 1964 Alaskan earthquake, we knew at that stage already that movements of the Earth's crust and shaking of the Earth's crust through earthquakes were linked with the big structures in the crust we call faults.
For example, the great San Andreas Fault of California, a break in the crust, that cuts the surface of the Earth along a very great distance.
Here we have a couple of rather nice slides.
- And this one - Good heavens! MANNING: You really can see a break on the Earth's surface there.
DEWEY: And of course that is perhaps what George was looking for in Alaska.
It was such a huge earthquake, they must have expected something - very, very conspicuous on the surface.
- A very, very substantial large structure cutting the surface over a very long distance, that's right, yes.
MANNING: But in Alaska there was no sign of a large fault cutting the surface.
PLAFKER: Despite all of our efforts, we didn't find such a fault.
And this was a major puzzle, and it's something that bothered me for a long time.
MANNING: George knew there had to be a fault somewhere.
Perhaps he was looking in the wrong place? Finally, after much thinking about it, I realised that the fault we were looking for actually underlays the entire region of uplift and subsidence, and that we couldn't see it because it came to the surface out to sea.
MANNING: George's insight was that the fault was underground, sloping gently beneath the entire region.
He knew that about 100 kilometres off the Alaskan coast lay a mysterious deep trench in the ocean floor.
Oceanographers had been aware of it for some time, but they couldn't explain what it was.
George was convinced that this was where his gigantic fault reached the surface, underwater.
DEWEY: So if I draw a cross-section of Alaska, I come from the north, through the land, across the volcano and down gradually steeper and deeper, then down into a deep ocean trench like this.
What George found, if you remember, of course, was that the great Alaskan earthquake of 1964 was produced on a fault plane, dipping gently under Alaska like this, with this sense of motion.
That is, the under part was slipping down under Alaska, in that way.
And of course this is very important because if one projects that fault out along its length, lo and behold, it projects to exactly the position of the oceanic trench.
So the reason that nothing was visible on the surface, Earth's surface up here, was that the break actually hits the edge of the crust beneath the sea? That's right.
MANNING: A major fault like this would have caused repeated earthquakes.
George realised that it was what happened between earthquakes, as the crust was squeezed, that could explain why some places had gone down while others had risen.
I have here a model, a very simplified representation of the continental margin of Alaska.
This represents the fault.
For several centuries, between earthquakes, there is gradual compression and uplift of the area above that fault.
And at the time of an earthquake it slips suddenly, causing uplift on the seaward side and subsidence in the area where there was a bulge.
MANNING: And this land movement was not a one-off event.
When he looked, George had found signs of past earthquakes engraved in the landscape.
This series of steps was created as the land was repeatedly jacked out of the sea every few hundred years by sudden movement on the fault.
But it was what was happening beneath the fault that was really significant.
George suggested that with this sense of motion, the whole of the Pacific ocean floor is slipping, flowing down along this inclined plane underneath Alaska.
And of course the expression of this are intermittent earthquakes.
About every 700 or 800 years a great earthquake occurs like this on this great inclined plane.
And we have a slip of maybe 15, 20 metres or something like that.
You wait another 800 years, there's another slip.
So over geological time, you keep on adding that up and you have this tremendous flow of the Pacific Ocean floor down back into the mantle.
This is an astonishing picture.
These colossal areas of the sea floor - are sliding beneath the crust of Alaska.
- That's right.
MANNING: George Plafker and his colleagues suddenly realised the full significance of the ocean trench off the coast of Alaska.
Because this trench circled virtually the whole Pacific margin, it suggested that the entire floor of the Pacific Ocean was sliding beneath the surrounding continents, sinking back into the Earth's interior.
And this was just part of an even bigger picture.
Geologists had been puzzled about the ultimate fate of the sea floor after it had been created at the mid-ocean ridge.
The whole cycle was now obvious.
As ocean floor was being created in the middle of the oceans, so it was being destroyed at the trenches at ocean margins in what scientists came to call subduction zones.
And this was happening all over the planet.
Together, the ocean trenches and the mid-ocean ridges formed a network of lines, boundaries that divided the globe into vast slowly moving regions, the plates.
It was an extraordinary revelation.
Geologists suddenly realised that the entire surface of our world is in constant motion.
And so the theory of plate tectonics was born.
And scientists started thinking about the Earth in a completely new way.
LECTURER: And it's really quite an extraordinary idea, but remarkably simple.
Plate tectonics.
We've all heard about it, and rightly so.
It's done for our understanding of the Earth what the theory of evolution did for our understanding of life.
The continents move with the plates.
Throughout geological time they have completely rearranged themselves, sometimes coming together and sometimes splitting apart, opening and closing oceans, reshaping the face of the Earth.
What had begun with a line of dead barnacles ended with a new view of our planet.
As the new theory sank in, scientists realised that the movement of the plates, which was the cause of all the earthquakes around the Pacific, might also be responsible for the volcanoes.
Leonore Hoke believes that this link can finally explain the mystery of the continents.
Deep beneath her feet, ocean plate is sinking into the Earth's interior.
Now, it's very interesting, because we have volcanoes round about at in the position where the subducting oceanic plate is 100 to 120 kilometres below the surface, where we are here now.
And that's where we have volcanoes and we know from looking at the volcanoes that they produce a lot of molten rock and they produce a lot of fluids, like water.
MANNING: Geologists suspect that some of this water comes directly from the sinking ocean plate and may explain why the volcanoes are here.
Leonore plans to test this idea by collecting a sample from high on the volcano.
Well, this is my blue barrel, which contains all the sample equipment which I need.
And there are all sorts of things in it, like this gas mask which protects me from obnoxious gases, funnels to catch gas bubbles with.
It contains this tubing which I need and it contains these containers and I packed them at sea level in New Zealand.
They pop when you open them because we're really at high altitude here.
And here are these special glass containers into which I will suck the gas.
HOKE: So we're ready then? (SPEAKING SPANISH) - Benito.
- Benito.
Leonore.
(MAKING INTRODUCTIONS IN SPANISH) MANNING: It's 6 a.
m.
On the Bolivian border with Chile.
Leonore, Catrin and their guide are on their way to their chosen sampling area.
They're heading for the crater, the most dangerous place on the active volcano Irruputuncu.
The climbers are following a rarely used footpath made by locals to collect sulphur from the summit.
They're climbing to nearly 6,000 metres.
At this height oxygen levels have almost halved, making it difficult to breathe.
It's hard work, isn't it? - Yeah.
Biting wind.
- Yeah.
Sulphurous smell is getting stronger as well, isn't it? Well, we're getting closer to the fumarole.
That's good.
JONES: Yeah.
Another hour or so.
- Brilliant view.
- Yeah.
- That's one of the rewards of it.
- Yeah.
It's amazing.
Feel like a bird up here.
MANNING: The higher they go, the steeper the climb, as the sticky volcanic rock has built up the slopes of the crater.
After four hours, they are within sight of their goal.
This is the solid material that the volcano is made from.
It's dark, almost black.
But covering all these rocks is sulphur which is coming from the gases that are coming up from the crater behind me.
It's beautiful, yellow, crystalline material.
But the gases are also containing water.
MANNING: It's this water, together with other gases rising through the crater, that Leonore has come to collect.
I'm taking a condensate sample.
That means I'm collecting the steam and I guide it through this glass container which is immersed in snow and that cools the steam.
And I'm collecting the water at the bottom of this and you will see, later on, there's quite a lot of water in this steam.
I'm having a look now how much water there is.
I take it out of the snow.
(GASPING) It's great to get out of this.
Hot steam, it's 120 degrees.
But what I've collected here is some water.
You can see it in here.
It's quite a lot coming out of this fumarole.
Now, the chemistry of this water will tell us where this water is coming from and it will be analysed back at the lab.
I haven't measured the temperature of this.
I'm gonna do this after.
- You've got yellow all over it.
- We got covered in sulphur.
- GRAPES: Stings your eyes and things.
- Yeah.
- It was kind of like hell.
- It's itchy.
The devil was about to pop his head out any minute.
JONES: Okay, time to go.
- Guys, you'll be freezing.
Be quick.
- Ooh, it's not warm.
MANNING: Back in Wellington, New Zealand, Leonore heads straight for her laboratory.
She begins by examining the gases she collected from the volcano.
HOKE: You can measure the different gas components in the sample and it contains helium, it contains argon, it contains CO2, it contains nitrogen and many other gas components.
MANNING: But it's the concentration of nitrogen which catches her attention.
It goes almost off the scale.
HOKE: That's good.
We have here quite a high signal coming from the nitrogen.
MANNING: Nitrogen is the most common gas in air, but the proportion in the sample is far too high.
It suggests a completely different source.
Leonore turns her attention to the water in the sample.
We can also measure the isotopes of these gas components by using a mass spectrometer.
MANNING: By analysing the atoms in the water, Leonore is hoping to test a startling idea.
She suspects that some of the water in the sample might come from the sea.
If so, it would support her growing conviction that the nitrogen could be coming from dead marine organisms, known to be nitrogen-rich, buried in the sea floor.
Could the flask from Bolivia contain a sample of the Pacific Ocean? The hydrogen is very close to the hydrogen isotopes of seawater.
So that suggests that some component within that water is probably seawater.
HOKE: So we can say from that that a lot of that nitrogen is biogenic nitrogen produced in ocean sediments and released during the subduction process.
MANNING: So the journey up the volcano has yielded an amazing result.
High up at the summit are traces of marine life, and the Pacific Ocean itself, which have completed a long and extraordinary journey through the Earth.
JONES: Excellent.
The whole story is contained in one glass flask.
You suffered with me.
I sense the completion of one of life's key cycles here because the sea floor is pulling down with it a lot of dead animal and plant material.
This is being stewed up here and a lot of it is coming out as nitrogen and carbon dioxide from the volcano, which goes into the atmosphere, re-dissolves in the oceans, goes back through living material again.
It's a cycle.
That's right.
And of course, a large part of the cyclic process explains the origin of the continents because this flux of wet rock coming down here, getting hotter and hotter, and eventually they get so hot that the water is expelled MANNING: What happens next is the key to how the continents grow.
The expelled water has a strange effect on the rocks of the Earth's interior.
Like scattering salt on an icy road, the water reduces the melting point of the surrounding rocks and so they start to melt.
This molten rock rises to the surface to erupt as a line of volcanoes.
But these volcanoes are like the tip of an iceberg.
Only a small proportion of the molten rock ever reaches the surface.
The surface volcanoes are merely the surface manifestation of a huge process of the addition of new hot material to the crust.
And if this has been going on for some considerable period of time, which we know it has, you can actually make very large amounts of continental crust in this way.
JONES: There's a huge volume of volcanic material being added to the crust.
Below us you can imagine more molten rock ponding underneath you at some level.
As you drive along, you're witnessing the growth of the continental crust.
It's quite an exciting thought.
Here is a place where, for millions of years, the continent has been growing and it's going to continue probably for many more millions of years.
MANNING: But as the plates have moved throughout geological time, the places where the continents grow have shifted over the Earth.
In the Lake District, Matthew Thirlwall showed me how volcanoes once built the crust in Britain, 450 million years ago.
THIRLWALL: Formed by ash and pumice just as in Bolivia.
- This, then, is all volcanic rock? - Yes, everything we see over here.
It was produced by big eruptions of the style that you saw in the Pacific.
Below us, there is something like 8,000 metres of volcanic rocks.
Was this then a great subduction zone like the Ring of Fire? That's right.
From the andesitic composition of these volcanic rocks, we can tell that there was almost certainly a subduction zone beneath the Lake District.
And not just beneath the Lakes but beneath Wales, southern Ireland and even into the eastern United States as well.
Worldwide, the composition of continental crust is andesitic, right the way back through time.
Subduction zones were the sites of continental crust formation, not just at the present day at the Ring of Fire, but right the way back until the earliest parts of Earth history.
MANNING: So once these gentle hills were as violent and dangerous as the Ring of Fire.
As the plates moved, they cooled and died.
The Cerro Rico too, was once an active volcano some 16 million years ago.
These tunnels have yielded 60,000 tons of precious metal, a significant fraction of all the silver in human hands.
And the much older volcanoes in the Lake District have also brought up their own rich dividend.
Not gold and silver, but copper and lead, crucial elements in Britain's Industrial Revolution.
So what is this association between volcanism and mineral richness? What is the connection? The connection is that the hot rocks at depth heat up water and the water dissolves metals from the volcanic rocks, transports the metals up to the surface and then deposits them as the pressure drops.
And it's those concentrations that provide the workable copper and lead, - over in these hills up here.
- Yeah.
So really, the volcanoes have done all the work? They have, yeah.
They've brought all this stuff up and we just have to pick it off the surface.
More or less, yes.
MANNING: The people of the Cerro Rico have long believed the silver is a gift from Pachamama, their god of the Earth.
In some ways, they've been proved right.
Scientists now see how as the plates of the Earth move, creating the land on which we live, our planet also yields up its wealth.
But the movement of these plates presents us with a really challenging question.
What is the driving force? What engine moves the plates across the planet? That's what I'll be finding out in the next programme when we journey to the centre of the Earth.