Horizon

The landscapes of Earth have been shaped by volcanoes.

We've long been in awe of their destructive beauty.

But only recently have we discovered that volcanism exists beyond Earth.

The planets and moons of the solar system have volcanoes that are even

more extraordinary than those on our home planet.

Rivers of lava once raced across our moon.

It's an amazing thought that you could have been standing on Earth

and looked up at the moon, and seen these massive eruptions happening.

The largest volcano of the solar system,

three times the height of Everest, is on Mars.

The most violent volcano is on a moon of Jupiter.

Huge, icy geysers fountain out into space from a moon orbiting Saturn.

We have not closed the book on volcanism across the solar system by any means.

But what's most remarkable is what volcanic activity elsewhere in

the solar system has told scientists about our own planet, Earth.

What the Earth was like at its birth,

why we have the geology and the atmosphere we do.

And even how life on Earth, and possibly elsewhere, originated.

Way back in the ninth century AD,

a band of Vikings discovered Iceland.

They experienced volcanic eruptions for the first time.

To explain their devastation,

they evoked the terrible wrath of gods such as Surtr, the fire giant.

A Viking poet wrote, "In the beginning, all was cold and grim."

"Then came Surtr with a crashing noise."

"Bright and burning, he bore a flaming sword."

A millennium later, and a team of international scientists has also

travelled to the land of fire and ice.

This small country has more types of

volcanoes and geological wonders packed into it

than anywhere else in the world.

For the team, it allows them to compare the volcanism of Earth with

volcanoes found elsewhere in the solar system.

You've got these continual cycles of glaciers and volcanoes.

Absolutely brilliant.

Yeah, you have a really diverse range of volcanic features here,

and I think it's a good place to see the importance of volcanism.

For geologist Jim Head,

Iceland is a familiar landscape.

In the 1960s,

he was teaching the Apollo astronauts all about rocks

before they headed off to the moon.

We took them everywhere we could

that would give them geological information.

Iceland was clearly one of those.

And I think it's completely perfect, actually,

that we are here today in Iceland, studying the volcanoes that

actually propelled the astronauts to go to the moon.

Five, four, three, two, one.

The Apollo missions weren't just about the space race.

They were also the most ambitious geological field trips of all time.

A key aim was to discover if volcanoes

had helped create the moon.

And, if so, were any still active?

Before the Apollo programme,

we didn't even know whether the moon had volcanism.

For example, some people thought it was a cold moon,

some people thought it was a warm moon,

which had heating inside and volcanism.

So this is a big question - was it even volcanic rock?

2,000 feet, 2,000 feet.

47 degrees. Roger.

These dark-looking plains of the moon

were particularly tantalising to scientists.

They're called the seas, or the maria.

Beautiful view! Isn't that something?

Magnificent desolation.

To find out exactly what they were,

the first Apollo landing was to Mare Tranquillitatis,

the Sea of Tranquillity.

OK, ready for me to come out?

All set.

As the astronauts explored the dusty and rocky surface,

they recognised basalt - the most common volcanic rock found on Earth.

And lots of it.

When you erupt molten rock on a moon, liquid rock on the moon,

it actually is one sixth gravity,

so it's much less gravity than we see on the Earth.

It looks like a collection of just about

every variety of rock you could find.

If the lava is coming up from great depths, given the gravity, etc,

you'll get a lot of lava coming up,

commonly much more than you see on the Earth,

and so it flows great distances,

and so we have lava flows that go over 1000 kilometres,

like, incredible, it would go

halfway across the United States, no problem.

Another mysterious feature found on the moon

was these winding canyons, or sinuous rilles.

These channels were up to 400 metres deep and over 100km long.

Clues as to what created them can be found back on Earth.

Under the south-west of Iceland are curious tunnels through solid rock.

They appear almost man-made.

Gro Pedersen is exploring one.

In the depths of the tunnel,

she hopes to find evidence of what used to flow through it.

You can actually see how the lava has been running along the wall here,

and you can see also that it was very hot in here,

because some of this lava re-melted,

and basically was dribbling down the wall. You see that here.

It's a lava tube and, long ago, lava was surging through these tunnels.

One of the very exciting things people found on the moon

was these sinuous rilles and,

of course, before people actually had been on the moon,

they were thought to potentially be water eroded.

But then people have gone to the moon,

and it has been studied much more and we've found out that these

sinuous rilles were always connected with the maria,

the moon lava that we have up there.

Perhaps these sinuous rilles were once enclosed lava tubes.

So one of the things that you see here, obviously,

is that we have what we call skylights,

so the roof has collapsed.

If all of the roof collapses, you will end up with a valley,

like something you see on the moon.

But you can also see the tubes on the moon by a string of skylights,

just as we see here, one hole after the other, and you just follow them,

you trace them down and you can see that these are within lava flows.

But when did these eruptions take place?

And why did they eventually stop?

The answer would come in small bags of volcanic rocks

brought home by the astronauts.

On Earth, they could be accurately dated.

So when the moon rocks were brought back, it's, like, unbelievable.

OK, this we can tell, four-billion-year-old rocks.

These are the keys to the understanding of the solar system.

Like other planetary bodies made of rock,

the moon was a mass of hot molten magma as it was forming.

It's an amazing thought that you could have been standing on Earth

and looked up at the moon and seen these massive eruptions happening.

But all the time, it was cooling -

being relatively small, a quarter the diameter of the Earth,

the moon cooled down quickly.

By three billion years ago,

almost all the lava and interior magma had solidified

into one big lump of cold rock.

No more volcanoes.

But you see the remnants of it.

I mean, when you look at the sky and you look at the moon,

you see the evidence of the volcanism,

because you see the dark areas, the basalt,

which has filled in the craters.

Understanding how the moon lost its volcanoes

helps explain why Earth remains so active.

Being larger allowed the Earth to retain much of its original heat.

And so today, our planet is a dynamic and ever-changing world,

rather than a dead one.

So, the discovery on the moon of lava flows

gave us pause to think about how this worked

on other planetary bodies.

How does volcanism work on Mars?

So, the lunar exploration really opened up

a field of, really, planetary volcanology.

Exploring our neighbour, Mars,

also reveals secrets about Earth's geology.

When probes first reached the red planet,

one feature stood out above swirling sandstorms.

The volcano Olympus Mons.

Olympus Mons is enormous, it's about 25km high.

On Earth, you would be looking at something ridiculously high.

Most commercial aircraft fly 10-15 kilometres.

So you're looking at something that is towering way above

what commercial aircraft might fly.

Its base covers an area the size of France.

It's three times the height of Mount Everest.

Making it the largest volcano ever discovered in the solar system.

Finding out how it grew to be so colossal

tells scientists more about the volcanoes of Earth.

That's why three of the team have come together to study this volcano.

Icelanders call it Skjaldbreidur, which means "broad shield",

as side on, it's reminiscent of a Viking shield.

Although small in stature, it's of great significance.

This shield volcano is the one over...

about which all the other volcanoes of this type are called,

in the solar system and on the Earth.

So this is the first one, in many senses,

the first one to be named the shield.

It's only 1,000 metres high,

a 25th the height of Olympus Mons,

but crucially, it's the same type of shield volcano.

At the summit is the crater.

Wow, now you can see the crater.

Yeah. Fantastic. Wow!

That's very nice.

I mean, you could even have come skiing up here. Oh, wow. Yeah.

Then we can imagine, like, a lava lake.

Yeah, just round the top.

Yeah. Dribbling over where we are now. Yeah.

Around the rim are mysteriously-shaped rocks.

They look almost like fossilised snakes.

Yet they give a hint how this type of volcano forms,

and what gives it the distinctive shield shape.

This is a type of lava we call entrail,

and it's a bit like the entrails from the inside of a human body

or any animal body.

They're characteristically quite thin.

I mean, you can see from the shape of my hand,

it's a couple of hand widths.

Shield volcanoes comprise lavas that are very runny,

because the shapes of them,