The Building of the Earth The Living Planet

Our planet, the Earth, is, as far as we know, unique in the universe.

It contains life. Even in its most barren stretches, there are animals.

Around the equator, where those two essentials for life,

sunshine and moisture, are most abundant, great forests grow,

and here plants and animals proliferate in such numbers

that we still have not even named all the different species.

Here, animals and plants, insects and birds, mammals and man

live together in intimate and complex communities,

each dependent on one another.

Two thirds of the surface of this unique planet are covered by water,

and it was here indeed that life began.

From the oceans, it has spread even to the summits of the highest mountains,

as animals and plants have responded to the changing face of the Earth.

This river, the Kali Gandaki, has cut its way, in the most remarkable fashion,

right through the highest range of mountains in the world, the Himalaya.

To the east of me rises Annapurna, over 26,000 feet high.

To the west, Dhaulagiri, even higher.

Their two summits are a mere 22 miles apart,

and I am four vertical miles below them.

And that makes this the deepest valley in the world.

At this altitude, about 7,000 feet, it's quite warm,

and animal and plant life on the flanks of the valley is both rich and abundant.

The blossoms on these trees may well look familiar.

Flowers like them grow in gardens all over the world.

But these are wild plants and this is their original home. They're rhododendrons.

And here they are food for monkeys, grey langurs,

reminders that the hot plains of Southern Nepal and the tropics

are not far away to the south.

But they aren't just monkey food. They are the rhododendrons' advertisements,

attracting birds and insects which will sip their nectar, gather their pollen,

and so bring about their fertilisation.

The ring-necked parakeet also comes from the tropics.

Here, it's at the top of its range.

Any higher and the weather will be too cold for it.

Beneath the rhododendrons live several species

of those most splendid of Asia's birds, the pheasants.

The blood pheasant, for all its delicate beauty, is a plainer member of the family.

The cock Tragopan is surely the most magnificent.

Until, that is, you see a cock Impeyan pheasant,

with the coronet of a peacock

and the burnished, metallic iridescence of a tropical butterfly.

The Impeyan's hen, like those of all pheasants, is comparatively dull.

This deepest of all valleys in the world

enables you to walk within a few days

from the tropics, in its lower reaches,

to the equivalent of the poles on the slopes high above,

and to see as you make the journey

how closely animals and plants

are matched to the changing circumstances.

As you walk higher, the rhododendron forest gets thinner and hung with moss.

The air is moist and it can be quite warm during the day.

And now, in summer, there are orchids here.

On the ground beneath, flowers appear in close-packed bunches,

protecting one another from the night frosts.

The little Himalayan panda is certainly very well protected against the cold.

Not only does it have warm, dense fur,

but, like many animals that spend time in the snow,

it has hair on the soles of its feet.

That keeps its feet warm on the snow and stops it from sliding around on ice.

Now, in the summer, it also helps in getting a grip on wet, slippery branches.

It's primarily a vegetarian, collecting buds and leaves and fruit,

but it also takes eggs from a bird's nest, if it can find one.

On the ground, and scarcely bigger than the panda,

one of the shyest animals of the Himalayan forests, a musk deer.

In these tangled trees, antlers would be a considerable handicap,

and the musk deer doesn't develop them.

A male fights instead with the short, sharp tusks in his upper jaw.

They feed on moss, lichen and leaves,

and are so agile and well-adapted to a mountain life

that they can climb steep cliffs in search of food.

When a musk deer or any other animal of any size dies, the vultures come.

These are griffons,

very similar to those that circle the skies above Indian villages

down in the hot foothills.

They are common in this forest up to 7,000 or 8,000 feet.

So the lives of all these creatures are connected,

one with the other, either directly or indirectly,

and all are ultimately dependent upon the vegetation.

But of course both animals and plants are also greatly affected by the physical environment.

I've climbed several thousand feet now and things are beginning to change.

It's getting colder, and the rhododendrons are giving way to fir trees,

and that will mean a change in the animals that live here.

The yellow-throated marten has a broad taste in food.

It takes fruit on occasion, catches a few insects now and then,

but it relishes about all, small rodents, like mice and squirrels,

and there are quite a lot to be found here.

Even when winter comes, and the forests are deep in snow, it will remain active.

But it's a great traveller, and if the weather becomes very cold indeed,

it will descend to lower altitudes for a spell.

The Himalayan bear is capable of living very high indeed.

Its thick fur protects it against severe cold,

but its range is not limited by temperature so much as by food supply.

In spite of its size, it seldom tackles any animal bigger than a mouse,

and it lives for most of the time on just ants, grubs, nuts and leaves,

so it seldom goes any higher than the forest can grow.

And now, getting on for 10,000 feet up, the forest is beginning to thin.

In summer, there's not much rain here, for most has fallen at lower altitudes.

In winter, it gets extremely cold.

Those conditions don't suit rhododendrons. Here only conifers flourish in large numbers.

High though we are, the Kali Gandaki is still a very broad river.

Remarkably, and indeed mysteriously,

it doesn't rise from the flanks of these giant mountains but cuts right through them.

The people of the foothills

have long since recognised the value of this extraordinary corridor

that leads right through the Himalayas,

and all summer trains of mules trudge up the valley,

taking barley and buckwheat to trade with Tibetans for wool and salt.

All the way up the valley are villages where the muleteers can stay and rest,

but during the summer few do so.

Most trudge tirelessly upwards for as long as there's daylight.

A lammergeier, the bearded vulture,

a mountain bird that soars around the high valleys of Asia

and still in a few remote parts of Europe,

but nowhere higher than this.

And a sign that now we are getting really high - snow cock.

Its dappled white plumage gives it camouflage against the broken snow

that even now, in summer, can fall at these altitudes.

They forage for seeds and rootlets in the thin turf.

There are no trees now, just a few small shrubs

and dry, withered grass.

But that's enough for the tahr.

It is neither a true sheep or a true goat, but related equally to them both.

It will eat almost anything that's green, and is grateful to find it in this bleak land.

Another typically mountain creature -

the red-billed chough, a kind of crow.

They search the rocks for insects, grubs, odd seeds.

They will take most things.

Their cousins, yellow-billed choughs, go as high as any bird in the world,

riding the rising wind currents to the height of the snow peaks themselves.

Flowers at this altitude can only come from small cushion plants,

huddled together against the cold.

Higher still, little can grow except lichens.

Now it's so cold that growth may only be possible for a few days in the year.

And yet, in these bleak regions, people live.

To help plough the fields, they use the yak,

a domesticated creature that once roamed wild on the plains of Tibet,

the only large mammal that lives permanently as high as man.

The people, Bhotias and Sherpas, grow not only barley but potatoes,

a crop that was first cultivated by the Incas in the Andes

and was introduced here a century or so ago.

These highland people are well-adapted to life at these altitudes.

Their blood contains a particularly high number of red corpuscles

and so can carry more oxygen in it than a lowlander's can.

Certainly, when it comes to walking at these high altitudes,

they're very much better adapted than I am.

So, all the living creatures in these high valleys are adapted to their environment,

both their biological environment and their physical environment.

And yet, in terms of biological history, those adaptations are very recent indeed.

These immense mountains, the eternal hills, are in fact far from eternal.

They are younger than the plains of India to the south,

or the plateau of Tibet to the north.

They were raised to their present height about 65 million years ago

from the bottom of the sea.

And what is the evidence for that extraordinary statement?

It can be found all over the place, just up here.

These slopes are littered...

with fragments like these.

This is obviously a shell that's been turned to stone, a fossil.

Although there are no molluscs alive today exactly like this one,

there are some which are sufficiently similar for us to be sure that it lived

in water. And if we analyse the rock in which it's embedded,

it's clear that that was mud laid down at the bottom of a sea.

But I am as far as possible as it is to be from the sea.

Not only I am in the middle of Asia, hundreds of miles from the sea,

but I am over two vertical miles above its level.

What forces could possibly have raised the seabed to these heights?

Well, we now know that those forces are still in action,

that these mountains are still rising

and that land is still being created.

I'm in Iceland.

This fantastic fountain of fire

rising 200 feet or so into the air behind me

is molten rock.

Fine ash is falling all around,

there are gusts of choking, poisonous gas,

and it's so hot that this is just about as close as I can get to it.

The sheer weight of these molten ingots of rock

prevents them being swept away from the vent by the gale,

so there's little danger of them suddenly coming our way.

Less dramatic than the fire fountain, but perhaps more sinister

is this tide of black slag that is slowly creeping over the surface of the land.

In parts, it's red-hot and molten and flows like treacle,

but on the edges it's cooled enough for me to be able to handle it.

It's black, it's heavy, and it's called basalt.

Basalt like this has been welling up from deep in the Earth's crust

over since the beginning of the history of our planet.

A flow may travel for as much as 25 miles.

Sometimes it moves no faster than a man can walk,

but sometimes it races along at an extraordinary speed,

40 miles an hour, and nothing - nothing - can stop it.

Sometimes so much lava is produced

that it accumulates in flows 100 feet or so thick.

Then the centre layers of it cool exceptionally slowly and very evenly,

and this is the result.

Here, at the Giant's Causeway, the top of the lava flow has been eroded away,

for the eruptions took place 50 million years ago.

The cooling contractions have produced the effect you see in drying mud,

though here the cracks extend to a much greater depth

to produce six-sided columns about a foot and a half across.

In the Hebrides, there's another lava flow that erupted at about the same time

and formed Fingal's Cave.

The blanketing layer of lava that slowed down the cooling of the interior is still uneroded,

and beneath it the near-perfect basalt columns rise almost 20 feet high.

Basalt that doesn't contain very much gas

wells out from below almost quietly.

But if the lava has been extruded under great pressure,

it may be full of gas, and then it behaves very differently.

Sometimes a flow sweeps down over a forest,

incinerating the trees in its path.

Most dramatic of all, the lava sometimes wells up inside a crater and can't escape.

Then it forms that most fearsome of nature's spectacles, a lava lake,

like this one in Nyiragongo in Africa.

This lava is over 1,000 degrees centigrade, 2,000 degrees Fahrenheit.

The bubbles of gas that burst from its surface may be 50 feet across.

Sometimes, having got rid of much of its gas, like beer losing its fizz,

it sinks back down the vast pipe up which it rose

and returns to the lava chamber a mile or so below.

But lava lakes fed by pipes are not common.

Basalt more usually comes to the surface of the Earth in a rather different way.

These Icelandic volcanoes erupt from huge cracks or fissures

which regularly open up in a line which runs right across the width of the island.

But that line itself is only the northern end of a huge line of weakness

that runs for thousands of miles southwards from Iceland

right round the side of the globe.

Iceland lies between Norway and Greenland, south of the Arctic Circle.

The crack, ridged over by lava, is for the most part underwater,

which is why its existence wasn't known until the beginning of this century.

From Iceland, it runs midway between Europe and Africa

to the east and the Americas to the west.

In places, it rises above the sea to form volcanic islands -

The Azores, the Cape Verdes, Ascension, St Helena, Tristan da Cunha.

But below the surface the lava is also continually erupting,

unseen by human eyes until only a few years ago.

The clouds of gas come from the lava itself.

They're not steam. The pressure of the water in these depths

prevents that from being produced.

The heat is rapidly absorbed by the vastness of the ocean itself

so that the lava cools and congeals

much more quickly than it would do in the air.

Eruptions like these, at great depths, built the Atlantic ridge.

But the basalt forms not only the ridge itself

but the sea floor on either side.

By dating it chemically,

we know that the farther it is from the centre of the ridge,

the older it is.

This means that basalt is welling up in a molten state at the ridge

and then, as it solidifies, is moving away on either side.

We still don't fully understand the forces that power the process,

but it seems that 50 to 60 miles below the Earth's surface

it's so hot that the rocks are molten and currents in them

are welling up beneath the ridge, causing eruptions,

and then flowing away on either side,

pulling the great plates of the ocean floor with them.

It was this movement that dragged apart Africa and South America

and created the Atlantic Ocean.

Similar things have happened in the Pacific.

The great plate that forms the eastern part of the ocean floor

is moving towards the west coast of America.

But where it meets the continent, it dives downwards,

perhaps pulled by the descending current in the crust below,

producing a deep trench in the ocean floor.

As it goes down, it takes with it some of the sediments

that have accumulated at the bottom of the ocean and also some water.

These new ingredients melt

and interact with the rocks of the interior

to produce a mixture that is crucially different from the lava

that erupted at the ocean's ridge.

For one thing, it contains much more dissolved gas and steam.

As it rises up in the cracks on the edge of the continent,

it cools and solidifies, choking the vents.

The effect is like screwing down the safety valve of a boiler.

Mount St Helens on the Pacific coast of North America.

On May 18th 1980,

with an explosion 500 times as powerful

as the atomic blast at Hiroshima,

it blew away three-quarters of a cubic mile of rock.

The forests around the mountain were totally destroyed.

Trees 200 feet tall lay scattered like matchsticks.

Geologists, weeks beforehand,

watching a huge bulge develop on the side of the mountain,

had warned of the coming catastrophe.

Even so, over 60 people stayed and were killed.

On the northern side of the volcano,

there were not even trees to be seen.

A huge avalanche of rock, blown out by the blast,

had slid for 15 miles down the side of the mountain, burying everything.

Behind it, Mount St Helens lay wrecked.

Its summit was over 1,000 feet lower,

and at the back of a huge amphitheatre,

a mile wide from which the rock had come,

another ominous bulge was developing,

swathed in jets of steam.

Almost a century earlier, on the opposite side of the Pacific,

another catastrophic eruption had taken place on the tiny island of Krakatau,

in the straits between Java to the east and Sumatra to the west.

In 1883 it was an island five miles long and three miles wide,

with three volcanic peaks on it, the highest rising to almost 3,000 feet.

But those peaks were dormant.

There had been no sign of any volcanic activity within living memory.

But in August of that year,

people on the coast of Java began to hear a series of explosions.

A great column of smoke rose above Krakatau.

Pieces of lava the size of a house were being thrown high into the air.

The explosions continued day after day.

The column of smoke rose up until it was five miles or so up into the sky.

Ships that were sailing nearby

had their decks covered in ash and pumice,

and at night electric flames played over the rigging.

Day after day this continued.

And as it was doing so, it was emptying the lava chamber

deep in the crust beneath the sea,

and that was the cause of the greatest catastrophe of all.

Because on the morning of August 27th, Monday, at 10 o'clock,

the roof of that lava chamber collapsed.

Millions of tonnes of sea water poured onto the red-hot lava.

So did millions of tonnes of rocks.

And this produced a titanic explosion.

The noise was almost certainly

the loudest noise that has ever echoed round the Earth

in recorded history.

It was heard 2,000 miles away in Australia.

3,000 miles away on the small island of Rodriguez in the South Atlantic,

the commander of the garrison heard it

and thought it was distant gunfire at sea.

The explosion also produced a tempest of wind,

which swept out entirely round the globe seven and a half times

before it finally died away.

But most catastrophic of all, the explosion produced a tidal wave.

It swept towards the coasts and as it approached became a wall of water

over a 100 feet high.

It crashed into the harbours, it picked up a naval gunboat with a crew of 28

and lifted it bodily for over a mile inland

and dumped it on a top of hill.

And it overwhelmed village after village.

Over 36,000 people were killed.

The pall of ash brought darkness

over an area of 100 miles or so for several days.

But when it cleared away, the island of Krakatau was unrecognisable.

Three-quarters of the main island had disappeared.

The two nearby islets were buried beneath massive deposits of ash.

And where the tallest peak had stood, the sea was 900 feet deep.

But not for long. 44 years later another island rose from the boiling sea.

They called it Anak Krakatau - the child of Krakatau.

Compared with the explosions of its parent,

its eruptions are still trivial bubblings.

Now, after more than 50 years of fitful activity,

Krakatau's child has built itself a new cone.

It's still not very big, less than 1,000 feet high.

Sporadically, it explodes.

But often it's easy enough to walk round its rim.

The fumes that boil up from its crater

are partly steam and partly sulphurous gas,

and the sulphur condenses on the rocks, coating them yellow.

All volcanic eruptions spew out sulphur in one form or another,

including those underwater.

Here it doesn't form yellow crystals,

but reacts with the sea water to produce clouds of black sulphides.

These smokers, nearly two miles deep on the floor of the Pacific,

are one of the most extraordinary scientific discoveries of recent years.

The sulphides they produce are food for microscopic bacteria.

They, in turn, are consumed by a group of creatures unlike any seen before.

These are giant tube-worms 11 feet long.

They have neither mouth nor gut but absorb bacteria through their thin skin.

And these are clams, two feet across.

They too consume the bacteria.

The heated water rising above the smokers

causes currents along the sea bottom

that sweep small particles to the vents

so there's a whole community of creatures feeding on them.

Small, white, blind crabs. Strange fish, hitherto unknown.

Until this bizarre colony was discovered,

we had believed that all creatures on Earth

derived their energy through plants from the sun.

Even the deep sea creatures fed on fragments falling from the sunlit surface.

But here were animals that owed nothing to the sun

and were sustained through bacteria by the chemical energy of volcanoes.

But volcanoes don't remain active for ever.

Eventually, there is some shift deep in the Earth's crust

and the focus of the intense heat moves away slightly

and the eruptions come to an end.

But if water should percolate down from the surface through the rocks

and approach the magma chamber,

it's still so hot that the water is superheated and forced up again,

like water in the spout of a boiling kettle.

On the way, it may dissolve minerals from the rocks through which it passes,

and then, as it emerges as hot springs, the minerals will be deposited in terraces,

like these in Rotorua, in New Zealand.

In some parts, the superheated steam on its way to the surface

has dissolved the softer rocks through which it passes and brings them up as boiling mud.

Elsewhere, the boiling water shoots spasmodically into huge fountains,

and the whole area is wreathed in steam.

Such a place is typical of land where volcanic fires are on the wane.

Famous hot springs of Yellowstone in the Rocky Mountains of North America

are also heated by a vast chamber of molten rock

some distance down beneath the surface.

The water welling up from these crystal-clear, chemically rich pools

is so hot that no creature can live in them.

When they trickle over the brim, they begin to cool,

and there rich colonies of bacteria and mats of algae begin to grow.

In places, they flourish so thickly

that they break the surface and divert the flow of water

so that in parts they're cool enough to allow brine flies to settle.

The flies come to feed on the algae.

And here, too, they mate.

They lay their eggs directly in the warm mat of the algae.

Each has a long white thread to its case, like a seed.

The eggs, however, are far from safe.

They're seized by mites that clamber about over the algae.

Spiders, too, prowl around the grazing herds.

A slightly larger fly moves among the brine flies.

It too is a killer, devouring the grubs.

So the algal mats support a closely-knit interdependent community,

all nourished by chemicals in the water and energised by the volcanic heat.

But in the end it's destroyed by its own success.

Increasing numbers of grubs eat the algae begin to weaken the mat.

Eventually it gives way, the channel clears and scalding water gushes down,

killing a generation of grubs

and many hunters and parasites that live on them.

Now the process has to start all over again.

The hot volcanic springs of the Rift Valley in Africa

also support their own crops of bacteria and the small algae that feed on them.

But here the creatures that come to harvest them are much bigger.

Flamingoes, sometimes as many as a million of them on this one lake.

These lesser flamingoes feed entirely on single-celled algae

that proliferate in vast quantities in these steaming soda-rich waters.

Flocks like these remove 150 tonnes

of these microscopic plants from this lake every day.

Their bills have sieves inside them

which strain off the algae as the water passes through them.

It's easy to see how creatures can benefit

from the chemical riches of volcanoes dissolved in the waters of hot springs.

It's more difficult to imagine how any living thing

could derive nourishment from a basalt lava flow.

Its surface in many places is as smooth and as hard as glass,

and neither frost nor roots of plants can initially make any impression on it.

Centuries may pass after an eruption

before there's any sign of the surface of such a flow beginning even to weather.

This flow for example on the flanks of Mount Kilauea in Hawaii is some 3,000 years old,

and yet still it shows the rippled, ropy surface that formed when it was liquid.

But in the end the surface does erode and plants do get root in the cracks.

They in turn can support all kinds of other life,

and so the lava flow is eventually colonised,

but not only on its surface but in its depths.

For these basaltic lava flows are often not as solid as they seem.

When the lava first flows out of the vent like a river,

that on the outside of the flow will cool quicker and solidify,

forming walls on either side of the flow.

The top too cools quicker, and that causes a crust to form over the flow,

so that eventually the lava is flowing down a long tunnel.

When that happens, the walls and ceiling of the tunnel act as insulation,

keeping the heat in, so that the lava flow remains liquid

and so continues for mile after mile.

When eventually the supply of lava stops,

that tunnel may drain, leaving a long cavern like this one.

Out of the reach of rain and frost and even dust,

the surface of the lava looks exactly as it must have done,

when the last trickle was draining away,

and the floor was so hot that anything touching it would be turned to a cinder.

Molten lava had dripped from the ceiling, it had swilled round the sides

and spurted out in little dribbles from cracks in the newly congealed walls.

But living organisms have already moved in.

These roots belong to trees that are growing on the surface of the lava flow.

They've found their way down through the cracks, and here they dangle,

catching water as it percolates through the lava and trickles down them.

Among the rootlets, there are animals that live nowhere else in the world.

Normally, these creatures are in total darkness.

Nearly all of them, like this cricket, have lost their pigment.

Many of them have also lost their wings and their eyes.

In the blackness, they find their way about by touch,

and, like many cave insects elsewhere, have developed long legs and antennae.

Some, like this bug, are scavengers.

Others, like the centipede, hunt.

And the millipedes feed on the roots.

So, in these extraordinary lava caverns,

there is yet another community of interdependent creatures

that have come into existence since the volcanoes erupted.

The colonisation of volcanic ash presents different problems.

The difficulty here is not the glassy hardness of the rock

but quite the reverse, its insubstantial dustiness.

Mount St Helens is still a wasteland.

It's now, as I speak, some two and a quarter years

since the volcano erupted.

I'm some three miles from the crater,

and still the scene is one of devastation and sterility.

It's not just that this unweathered ash is not very fertile,

but it's also that it's so loose that it's difficult for plants to get root.

But that possibility is always here.

Here, for example, in this crevice,

there are the seeds of the willow herb,

or, as they call it in these parts, fireweed,

that have been blown up from the valleys below.

I don't suppose these particular ones will manage to get root here,

but in the end some plant will,

and in the end the process of colonisation will begin.

Krakatau's child is just 57 years old.

Its flanks too are covered for the most part with ash,

and they're still buried regularly with new layers from fresh eruptions,

yet the process of colonisation is already under way.

Not only are there giant grasses, like this wild sugar cane,

but trees - a casuarina.

If you want to see what a century of colonisation by plants can bring about,

have a look at that fragment of old Krakatau over there.

We know from first-hand reports that 100 years ago

there was nothing here but sterile ash many feet deep.

Within three years, 34 different species of plants had reappeared.

Ten years later there were twice that number,

and over 100 species of birds and insects as well.

Some seeds must have floated here from Java, some 20 miles away,

and they still continue to do so.

Other smaller ones were probably carried here by birds,

either on their feet or in their stomachs.

But the ash is still here beneath the lattice of roots of the jungle trees.

Somehow or other, rats and lizards and pythons have all reached here.

There are now many hundreds of different species of plants,

and the winds have assisted the passage of a great number of flying insects,

whose descendants now form large and permanent populations,

pollinating the flowers, feeding on their fruits,

collecting their rotting leaves and indeed feeding on one another.

As yet there are no larger mammals, no monkeys or squirrels,

no hunting cats or mongoose, as there are in Java or Sumatra.

But as far as smaller creatures are concerned,

the number of species is increasing all the time.

And on the flanks of volcanoes all round the world,

men clear fields and plant crops,

even though they know they may be sitting on a time bomb.

These rice fields lie on the flanks of one of Krakatau's near neighbours,

Gunung Agung in Bali.

Only 20 years ago it erupted,

killing 2,000 people and leaving 150,000 homeless.

But still the Balinese will not leave fields that are so fertile

they can produce two or three rich harvests of rice every year.

Gunung Agung, Krakatau and the rest of the violently explosive volcanoes

that run in a chain along Sumatra and Java and the Indonesian islands

stand on the line of the crack in the Earth's crust

where the basalt plate forming the floor of the Indian Ocean

meets the partly submerged edge of the continent of Asia.

This junction already existed 65 million years ago,

when India was an isolated island in the middle of that ocean.

Since then, as the ocean floor has continued to spread,

the continents have shifted and India has moved towards Asia.

As the two continents approached,

the sediments between them crumpled and eventually piled up over the junction,

so instead of the line between them being marked by a chain of volcanoes,

it's buried deep beneath an immense range of mountains, the Himalaya.

So these great peaks of sandstone and limestone rising five miles into the sky

are not only the highest mountains in the world, but among the youngest.

And the process has not yet come to an end.

India is still moving north at the rate of two inches a year,

compacting itself ever more tightly against the continental mass of Asia,

and the Himalaya are, infinitesimally, getting higher and higher.

And that is how this ammonite, this sea-living creature,

came to rest over two miles high in the Himalaya.

And that too is the explanation of how the Kali Gandaki river

managed to cut its way clean through the highest range of mountains in the world.

It was flowing south from the ancient plateau of Tibet

even before the great mass of India collided with Asia.

As the sediments between the two land masses buckled and rose over millions of years,

the river maintained its course,

cutting down through the rocks as swiftly as they rose.

And so now it still flows south to the plains of India,

and does so through the deepest gorge in the world.

Mountain ranges have been created in this way several times,

during the history of the Earth

The Himalaya are just the most recent.

As they are worn down, so they create different environments

in which animals and plants can live.

So we have begun our portrait of the planet up on the roof of the world,

and we will go from high altitudes to low, from the poles to the equator.

And in the next programme we'll go even higher,

to the most inhospitable environment of all, the world of snow and ice.

Subtitles by Red Bee Media Ltd