All language subtitles for Super Volcanoes BBC Horizon 2000-02-03 EN

In 1971 heavy rain fell across much of east Nebraska.

In the summer palaeontologist Mike Voorhies travelled
to the farmland around the mid-west town of Orchard.

What he was to discover exceeded his wildest dreams.

It was a sight of sudden, prehistoric disaster.

Voorhies's digging revealed the bones of 200 fossilised rhinos,

together with the prehistoric skeletons of
camels and lizards, horses and turtles.

Dating showed they had all died abruptly 10 million years ago.

It suddenly dawned on me that this
was a scene of a mass catastrophe

of a type that I'd never, never encountered before.

The cause of death, however, remained a mystery.
It was not from old age.

I could tell by looking at the teeth that these
animals had died in their prime.

What was astounding was that here were
young mothers and their, and their babies,

big bull rhinos in the prime of life and here
they were dead for no, no apparent reason.

For the animals at Orchard death had come suddenly.

There was another strange feature to the skeletons,

an oddity which offered a crucial clue
about the cause of the catastrophe.

We saw that all of these skeletons were covered with very peculiar growth,

soft material that I first thought was a mineral deposit.

Then we noticed that it was cellular.
It's biological in origin so there was something actually

growing on those bones.

I had no idea what that stuff was, never seen anything like it.

A palaeo-pathologist, Karl Reinhard,
was sent a sample of the bones.

This specimen is typical of the rhino bones.

You see this material, in this case it's a whitish
material that's deposited on the surface

of the original bone.

This is peculiar to me, but as I thought back in
my experience I realised that this was similar

to something that turns up in the veterinary world,
a disease called Marie's disease.

Marie's is a symptom of deadly lung disease.
Every animal at Orchard seemed to be infected.

One of the clues was that all of the animals had it.
Now that is a very important observation

for all the diseases, all the animals to exhibit this disease
there had to be some universal problem.

Scientists discovered the universal problem was ash.

10 million years ago ash had choked them to death.

It may have been a bit like pneumonia
with the lungs filling with fluid,

except in this case the fluid would have
been blood for the ash is very sharp.

There'd be microscopic shards of ash

lacerating the lung tissue and, and causing the bleeding.

I would imagine these animals as stumbling around the thick ash,

spitting up blood through their mouths and
gradually dying in a most miserable way.

Only a volcano could have produced so much ash,

yet the wide flat plains of Nebraska have no volcanoes.

I remember some of my students and I sitting
around after a day's digging and just

speculating where did this stuff come from?
There, there are no volcanoes in Nebraska now.

As far as we know there never have been.

We, we obviously had to have volcano somewhere

that, that produced enough ash to
completely drown the landscape here,

but where that was really was anybody's guess.

One geologist in Idaho realised there had been a volcanic eruption

which coincided with the disaster at Orchard 10 million years ago,

but the site was halfway across North America.

It seemed like a really fascinating story which made me think,

because I had been working on--

volcanic rocks in south-western Idaho that
potentially could make lots of ash

and,

and there was some age dates on that that were
around 10 million years and I began to wonder wow,

could this situation in Nebraska have really been
caused by some of these large eruptions that

evidently had happened in south-western Idaho.

The extinct volcanic area, Bruneau Jarbridge,

was 1600 kilometres away, a vast distance.

How could this eruption have blasted so much ash so far?

Bonnichsen was sceptical.

Volcanoes will spew ash for a few tens or maybe a few hundreds of miles.

This ash, and it's like two metres thick,

in Nebraska is 1600 kilometres or more
away from its potential source,

so that's an amazing thing. There really had been no previous

documentation, to my knowledge, of phenomenon like that.

Despite his doubts Bonnichsen decided to compare
the chemical content of ash from the two sites.

He analysed samples from both Bruneau Jarbridge and Orchard

and plotted their mineral composition
on a graph looking for similarities.

If you have a group of rocks
that are very similar to one another

they should be a closely spaced cluster of pods.

We had these analyses come out from the Orchard site

and I thought I'd try the clock again and see
how close they were to one another.

By golly, they fall right in the same little
trend as the Bruneau Jarbridge samples.

Bonnichsen's hunch had proved correct.

Bruneau Jarbridge was responsible for the catastrophe at Orchard.

An eruption covering half of North America with two metres of ash

was hundreds of times more powerful than any normal volcano.

It seemed almost unbelievable,

but then Bruneau Jarbridge was that rarest
of phenomena which scientists

barely understand and the public knows nothing about:

a supervolcano.

Supervolcanoes are--

eruptions and explosions of catastrophic proportions.

When you actually sit down and think about these
things they are absolutely apocalyptic in scale.

It's difficult to conceive of a, of an eruption this big.

Scientists have never witnessed a supervolcanic eruption,

but they can calculate how vast they are.

Super eruptions are often called VEI8 and
this means that they sit at point 8

on what's known as a volcano explosivity index.

Now this runs from zero up to 8. It's actually a measure
of the violence of a volcanic eruption

and each point on it represents an eruption
10 times more powerful than the previous one,

so if we take Mount St. Helens,
for example, which is a VEI5,

we can represent that eruption by a cube

of this sort of size, this represents here the amount
of material ejected during that eruption.

If you go up step higher and look at a VI6,
something of the Santorini size for example,

then we can represent the amount of material
ejected in Santorini by a cube of this sort of size,

but if we go up to VEI8 eruptions then we're dealing
with something on an altogether different scale,

a colossal eruption and you can represent a VI8,
some of the biggest VI8 eruptions

by a cube of this, this sort of size.
It's absolutely enormous.

Normal volcanoes are formed by a column of magma,

molten rock, rising from deep within the Earth,

erupting on the surface and hardening in layers down the sides.

This forms the familiar dome or cone-shaped mountains.

Most people's idea of a volcano is a lovely symmetrical cone

and this involves magma coming up, reaching the surface,

being extruded either as lava or as explosive eruptions as, as ash

and these layers of ash and lava gradually accumulate
until you're left with a, a classic cone shape.

Vulcanologists know this smooth flowing magma
contains huge quantities of volcanic gases,

like carbon dioxide and sulphur dioxide.

Because this magma is so liquid these
gases bubble to the surface, easily escaping.

There are thousands of these normal volcanoes throughout the world.

Around 50 erupt every year,

but supervolcanoes are very different in almost every way.

First, they look different.

Rather than being volcanic mountains, supervolcanoes
form depressions in the ground.

Despite never having seen a supervolcano erupt,

by studying the surrounding rock scientists have
pieced together how supervolcanoes are formed.

Like normal volcanoes they begin when a column
of magma rises from deep within the Earth.

Under certain conditions, rather than breaking
through the surface, the magma pools

and melts the Earth's crust turning the rock itself into more thick magma.

Scientists don't know why, but in the case of supervolcanoes

a vast reservoir of molten rock eventually forms.

The magma here is so thick and viscous
that it traps the volcanic gases

building up colossal pressures over thousands of years.

When the magma chamber eventually does erupt

its blast is hundreds of times more powerful than
normal draining the underground reservoir.

This causes the roof of this chamber to
collapse forming an enormous crater.

All supervolcano eruptions form these subsided craters.

They are called calderas.

The main factor governing the size of eruptions
is really the amount of available magma.

If you've accumulated an enormous volume of magma in the crust

then you have at least a potential for a very, very large eruption.

The exact geological conditions needed to create
a vast magma chamber exist in very few places,

so there are only a handful of supervolcanoes in the world.

The last one to erupt was Toba 74,000 years ago.

No modern human has ever witnessed an eruption.

We're not even sure where all the supervolcanoes are.

Yellowstone National Park, North America.

Ever since people began to explore Yellowstone
the area was known to be hydrothermal.

It was assumed these hot springs and
geysers were perfectly harmless,

but all that was to change.

I first came to Yellowstone in the mid-1960s to
be a part of a major restudy of the geology

of Yellowstone National Park,

but at that point I had no idea of what we were to find.

When geologist Bob Christiansen first began
examining Yellowstone rocks

he noticed many were made of compacted ash.

But he could see no extinct volcano or caldera crater,

there was no give-away depression.

We realised that Yellowstone had been
an ancient volcanic system.

We suspected that it had been a caldera volcano,

but we didn't know where the caldera was
or specifically how large it was.

As he searched throughout the Park looking for the volcanic caldera

Christiansen began to wonder if he was mistaken.

Then he had a stroke of luck.

NASA decided to survey Yellowstone from the air.

The Space Agency had designed infrared
photography equipment for the moon shot

and wanted to test it over the Earth.

NASA's test flight took the most revealing
photographs of Yellowstone ever seen.

What was so exciting about looking at the remote sensing imagery

was the sense that showed it in one,
one sweeping view of what this truly was.

Christiansen hadn't been able to see
the ancient caldera from the ground

because it was so huge.
It encompassed almost the entire Park.

An enormous feature.

70 kilometres across, 30 kilometres wide.

This had been a colossal supervolcano.

Certainly one of the largest known anywhere on earth.

Bob Christiansen was determined to find out
when Yellowstone had last erupted.

He began examining the sheets of hardened ash,
dozens of metres thick

blasted from the ground during the eruption.

What he found was 3 separate layers.

This meant there had been 3 different eruptions.

When Christiansen and his team dated the
Yellowstone ash he found something unexpected.

The oldest caldera was formed by a vast eruption 2 million years ago.

The second eruption was 1.2 million years old

and when he dated the third and most recent eruption

he found it occurred just 600,000 years ago.

The eruptions were regularly spaced.

Quite amazingly we realised that there was
a cycle of caldera-forming eruptions,

these huge volcanic eruptions about every 600,000 years.

Yellowstone was on a 600,000 year cycle

and the last eruption was just 600,000 years ago.

Yet there was no evidence of volcanic activity now.

The volcano seemed extinct.

That reassuring thought was about to change.

There was another geologist who was fascinated by Yellowstone's volcanic history.

Like Bob Christiansen, Professor Bob Smith has been
studying the Park for much of his career.

In 1973 he was doing field work,
camping at one end of Yellowstone Lake.

I was working at the south end of this lake at a place called Peal Island.

I was standing on the island one day and
I noticed a couple of unusual things.

The, the boat dock that we normally would
use at this place seemed to be underwater.

That evening as I was looking over
the expanse of the south end of the lake

I could see trees that were being inundated by water.

I took a look at these trees and they were be, being inundated with

water a few inches, maybe a foot deep

and it was very unusual for me to see
that because nowhere else in the lake

would the lake level have really changed.
What did it mean? We did not know.

Smith commissioned a survey to try to find out
what was happening at Yellowstone.

The Park had last been surveyed in the 1920s when the elevation,

the height above sea-level, was measured
at various points across Yellowstone.

The idea was to survey their elevations and
to compare the elevations in the mid-70s

to what they were in 1923

The two sets of figures should have been similar,
but as the survey team moved across the Park,

they noticed something unexpected:
the ground seemed to be heaving upwards.

The conclusion kind of hit me in the face
and said this caldera has uplifted

at that time 740 millimetres in the middle of the caldera.

As the measuring continued, an explanation
for the submerged trees began to emerge.

The ground beneath the north of Yellowstone was bulging up,
tilting the rest of the Park downwards.

This was tipping out the sound end of the lake
inundating the shoreside trees with water.

The vulcanologist realised only one thing could make the Earth heave in this way:

a vast living magma chamber.
The Yellowstone supervolcano was alive

and if the calculations of the cycle were correct,
the next eruption was already overdue.

Well this gave us a real shiver of nervousness if you will about

the fact that we have been through this 600,000 year cycle

and that the last eruption was about 600,000 years ago.

The scientists had found the largest single active volcanic system yet discovered.

There were many things they needed to find out.

How big was the magma chamber deep underground,

how widespread would the effects of an eruption
be and crucially, when would it happen?

To answer any of these questions vulcanologists knew they first had to understand

Yellowstone's mysterious magma chamber.

It's incredibly important to understand what's
happening inside of the magma chamber

because that pressure and that heat, the fluid is what's triggering the final eruption.

It's like understanding the primer in a bullet.

Understanding the magma chamber would be very difficult.

Smith and his team needed to discover the size
of something 8 kilometres below the ground.

They began harnessing information from an ingenious source:

earthquakes.

Well, what we have here is a seismometer.
This is the working end of a seismograph,

the device that's used to record earthquakes.
It is able to pick up the smallest of earthquakes in,

in Yellowstone plus it picks up moderate to large
earthquakes around the world, it is so sensitive.

Like many thermal areas, Yellowstone has
hundreds of tiny earth tremors each year.

They are harmless, but in his seismographic lab

Smith has been using them to trace
the size of the magma chamber.

Earthquakes are essentially telling you the pulse.

They tell you the real time pulse of how
the caldera is deforming, of how faults are fracturing.

Bob Smith's 22 permanent seismographs
are spread across the Park.

They detect the sound-waves which come
from earthquakes deep underground.

These waves travel at different speeds depending
on the texture of what they pass through.

Soundwaves passing through solid rock

go faster than those travelling through molten rock or magma.

By measuring the time they take to reach the seismographs

Smith can tell what they've passed through.

Eventually this builds up a picture of what lies beneath the Park.

The magma chamber we found extends
basically beneath the entire caldera.

It's maybe 40-50 kilometres long,

maybe 20 kilometres wide and it has a thickness of about 10 kilometres.

So it's a giant in volume and essentially encompasses

a half or a third of the area beneath Yellowstone National Park.

The eruption here 3,500 years ago,

although not VEI8 in scale, did have a small magma chamber.

Professor Steve Sparks has spent much of his career studying Santorini.

When I first came to Santorini and started to look at the pumice

deposits from these caldera forming
eruptions I found evidence of a dramatic change

in the power and violence of the eruption.

By examining the layers of Santorini pumice

Sparks discovered magma chambers could
erupt with almost unimaginable force

and spread their devastation widely.

There's dramatic evidence of a sudden increase in the power.

Huge blocks about 2 metres in diameter

were hurled out of the volcano reaching 7 kilometres
and smashing into the ground

and to do that the blocks must have been thrown from the volcano

at hundreds of metres per second, about the speed
of Concorde and you can imagine this enormous

red rock crashing in and breaking up on impact.

To understand why caldera volcanoes could erupt with such power

Sparks replicated their violence at one trillionth of the scale.

In the lab he modelled a reaction which occurs in
the magma chamber of an erupting caldera.

The problem is we can't go into a
magma chamber so the next best thing to do

is to go to the laboratory and try and simulate
what happens in the magma chamber

and in the pathway to the surface.

Sparks believed escaping volcanic gas trapped in the magma

might be responsible for the violence of the eruptions.

Into a glass flask - the magma chamber -
he poured a mixture of pine resin and acetone.

the pine resin mimicked the magma,

the acetone modelled trapped volcanic gases
like carbon dioxide and sulphur dioxide.

Pine resin is a very sticky, stiff material so
it has some properties which are rather like

magma and we thought that if we could get a, a gas which dissolved in

pine resin, like acetone, then we could get a,
a laboratory system which would represent

the, the natural case.

Sparks then created a vacuum above the flask to mimic
the depressurisation that occurs in the magma chamber

when a supervolcano begins its eruption

and the dissolved volcanic gas can expand.

When the vacuum reached the liquid it caused a dramatic change.

The dissolved acetone suddenly became a gas.

This made the resin expand causing violent frothing
and blasting the contents out of the chamber.

These experiments give us tremendous insight

into the tremendous power of gases coming out of solution

and enabled to drive these very dramatic explosive flows.

But experiments in the laboratory cannot answer
the biggest question of all surrounding Yellowstone:

when will it next erupt?

Scientists face a problem.
They have never seen a supervolcano erupt.

Until a VEI8 eruption is observed and analysed

no-one knows what the telltale precursors
would be to a Yellowstone eruption.

Nobody wants to see a global disaster of course

and yet we'll never really fully understand the
processes involved in these supervolcanic eruptions

until one of them happens.

74,000 years ago a supervolcano erupted here in Sumatra.

The resultant caldera formed Lake Toba,

100 kilometres long, 60 kilometres wide.

It was, in short, colossal.

Scientists are only now beginning to understand
the effects of so much ash on the planet's climate.

This is the ocean core repository at Columbia University in America.

It contains thousands of drill samples
from seabeds round the world,

a historical keyhole through which scientists,

like Michael Rampino can view volcanic history.

The size of the Toba eruption was enormous.

We're talking about, about 3,000 cubic kilometres
of material coming out of that volcano.

That's about 10,000 times the size of the 1980 Mount St. Helens eruption

which people think of as a large eruption, a truly super eruption.

This is an ocean drilling core from the central Indian Ocean.

It's about 2,500 kilometres from the Toba volcano

and here are 35 centimetres of ash deposited after the Toba eruption.

It shows that this Toba eruption was a supervolcanic event,

it was much, much bigger than any other volcanic
eruption we see in the geological record.

Chemical analysis of the ash tells us that
this eruption was rich in sulphur,

would have released a tremendous amount of
sulphur dioxide and other gases into the stratosphere

which would have turned into sulphuric acid aerosols

and affected the climate of the Earth for years.

For a long time scientists have known that
volcanic ash can affect the global climate.

The fine ash and sulphur dioxide blasted into the stratosphere

reflects solar radiation back into space
and stops sunlight reaching the planet.

This has a cooling effect on the Earth.

In the year following the 1991 eruption of Mount Pinatubo for instance

the average global temperature fell by half a degree Celsius.

By comparing the amount of ash ejected by past
volcanoes with their effect on the Earth's temperature,

Rampino has estimated the impact of the Toba
eruption on the global climate 74,000 years ago.

I'm plotting a simple graph where one side
there's sulphur released in millions of tons

by volcanic eruptions and on the other side
there's a cooling in degree Celsius that we saw

after these volcanic eruptions.

I'm plotting as points the historical eruptions

like Mount St. Helens, Krakatoa,
Pinatubo, Tambora.

There's a nice correlation between the sulphur
released into the atmosphere and the cooling.

Because of this relationship between the
sulphur released by large volcanoes

and global cooling, Rampino can calculate the drop
in temperature caused by the Toba eruption.

We can see this kind of plot predicts

that the Toba eruption was so large that the temperature change after Toba

in degrees Celsius would have been about
a 5 degree global temperature drop,

very significant, very severe global cooling.

Five degrees Celsius average drop in global
temperature would have been devastating

causing Europe's summers to freeze
and triggering a volcanic winter.

Five degrees globally would translate into 15 degrees or so

of summer cooling in the temperate to high latitudes.

The effects on agriculture, on the growth of plants,

on life in the oceans would be catastrophic.

This global catastrophe would have continued
for years, dramatically affecting life on Earth,

but what impact did it have on humans?

The answer may be buried not inside
the ancient rocks, but deep within us all.

Lynn Jorde and Henry Harpending are
scientists specialising in human genetics.

Since the early 1990s they have been studying mitochondrial DNA

using the information to investigate mankind's past.

Most of our genetic information is stored in the nuclei of our cells,

but a small, separate quantity exists in another component,

the part which produces the cells' energy, the mitochondria.

Mitochondria have their own genes.
It's a small number of genes, a small amount of DNA,

but it's distinct from the rest of the DNA in the cell

and because of the way mitochondria are
transmitted from one generation to the next,

they're, they're inherited only from the mother so they give us a record

of the maternal lineage of a population.

Mitochondrial DNA is inherited only by the mother.

All mutations are passed on from mother to child,
generation after generation at a regular rate.

Over time, the number of these mutations accumulate in a population.

Every event that takes place in our past,
every major event, a population increase,

a population decrease, or the exchange
of people from one population to another

changes the composition of the mitochondrial DNA in that population,

so what happens is that we have a record
of our past written in our mitochondrial genes.

By knowing the rate of mutation of mitochondrial DNA

and by a complex analysis of the distribution of these mutations,

the geneticists can estimate the size of populations in the past.

Several years ago they began seeing a
strange pattern in their results.

We expected that we would see a pattern consistent
with a relatively constant population size.

Instead, we saw something that departed
dramatically from that expectation.

We saw a pattern much more consistent

with a dramatic reduction in population size at some point in our past.

This confirmed what other geneticists have noticed.

Given the length of time humans have existed,
there should be a wide range of genetic variation,

yet DNA from people throughout the world is surprisingly similar.

What could have caused this?

The answer is a dramatic reduction of
the population some time in the past:

a bottleneck.

We imagine the population diagrammed like this.

In the distant past back here we have a large population,

then a bottleneck looking like this and then a
subsequent enlargement of population size again,

so we would have families of people in the distant past with--

a significant amount of genetic diversity,

but when the bottleneck occurs,
when there's a reduction in population size

perhaps only a few of those families would survive the bottleneck.

We have a dramatic reduction in genetic diversity
during this time when the population is very small

and then after the bottleneck the people
who would we, who we would see today

would be descendants only of those who survived,

so they're going to be genetically much more similar
to one another reducing the amount of genetic variation.

It seemed so incredible, you know the idea that all of us,

now there's 6 billion people on Earth,

and what the data were telling us was that we,

you know our species was reduced to,

you know, a few thousand. Suddenly it hit us,
we had something to say about human history.

Our population may have been in such a precarious position

that only a few thousand of us may have been
alive on the whole face of the Earth

at one point in time, that we almost went extinct,

that some event was so catastrophic

as to nearly cause our species to cease to exist completely.

It is an astonishing revelation, but the key was
to find out when and why it happened.

Because mitochondrial DNA mutates at an average rate

these scientists believe, controversially, that they
can narrow down the date of the bottleneck.

Mutations in the mitochondria take place with clocklike regularly,

so the number of mutations give us a clock
essentially that we can use to approximately date

the major event. In the case of a population bottleneck

we think that this would have occurred roughly 70-80,000 years ago,

give or take some number of thousands of years.

As for what caused this dramatic reduction in population

the geneticists had no idea.

Henry Harpending began touring
universities to talk about the bottleneck.

He was invited by anthropologist Stanley Ambrose
to give a lecture to his students.

Well Stanley is full of ideas,

he's the kind of scientist that

plucks things from all over and puts them together.

I sat in on the lecture and he started talking
about this human population bottleneck

and I thought what could have caused it

and at that point I broke out into a sweat.
I went up to Henry and said

I've just read a paper, and it's on the top of my desk now, that

may have an explanation for why this population bottleneck occurred.

I didn't read it till a week later and when I read it

you know it was like somebody kicking you in the face.
There it was.

The paper was about the super eruption of
a volcano called Toba in Sumatra.

This team of scientists believe the bottleneck
occurred between 70 and 80,000 years ago,

although this date is hotly debated.

Toba erupted in the middle of this period, 74,000 years ago.

If there really is a connection

this research has terrifying implications
for a future Yellowstone eruption.

It could well be of a similar size and ferocity to Toba.

Like Toba, it would have a devastating impact,
not just on the surrounding region, North America,

but on the whole world.

If Yellowstone goes off again, and it will,

it'll be disastrous for the United States
and eventually for the whole world.

Vulcanologists believe it would all start with
the magma chamber becoming unstable.

You'd start seeing bigger earthquakes, you may see--

parts of Yellowstone uplifting as magma intrudes
and gets nearer and nearer the surface.

And maybe an earthquake sends a rupture through the brittle layer,

you've broken the lid of the pressure cooker.

This would generate sheets of magma which
will be probably rising up to 30, 40, 50 kilometres

sending gigantic amounts of debris into the atmosphere.

Where we are right now would be gone.
We would be instantly incinerated.

Pyroclastic flows will cover that whole region,

maybe kill tens of thousands of people in the surrounding area.

You're getting a, an eruption which we can barely imagine.

We've never seen this sort of thing.

You wouldn't be able to get within 1,000 kilometres
of it when it was going like this.

The ash carried in the atmosphere and
deposited over large areas of the United States,

particularly over the great plains, would have devastating effects.

The area that would be affected is,
is the bread basket of North America

in effect and it produces an enormous
amount of grain on a global scale really.

That's, that's, that's the problem and you would see nothing.

The harvest would vanish virtually overnight.

All basic economic activity would
certainly be impacted by this

and let alone changes in the climate that could possibly be induced.

The climatic effects globally from that eruption

will be produced by the plume of material
that goes up into the atmosphere.

That'll spread worldwide and will have
a cooling effect that will probably

knock out the growing season on a global basis.

We can't really overstate the effect of these huge eruptions.

Civilisation will start to creak at the seams in a sense.

The fact that we haven't seen one in historic time or documented

means the human race really is not attuned
to these things because they're such a rare event.

It's really not a question of if it'll go off,
it's a question of when

because sooner or later one of these
large super eruptions will happen.