Nova: Killer Volcanoes

Our planet is capable of unleashing extreme chaos.

Volcanoes, earthquakes, hurricanes, and floods

can cause untold devastation.

We may think we've seen

the worst Mother Nature can throw at us,

but scientists struggling to understand these disasters

are discovering evidence that even more extreme events

have struck in the past.

JEAN CHRISTOPHE KOMOROWSKI: So this is about 13 times

more powerful than the Pompeii eruption.

They're uncovering clues

that the worst catastrophes in history

could strike again.

Nearly 1,000 years ago,

a disaster shook the world.

This is one of the largest eruptions

in the last 10,000 years.

A volcano so powerful

it chilled the entire planet.

But where was it?

No one knew the source of the eruption.

The clues are here, buried and hidden all around the world.

Now, scientists come together

to scour our volatile Earth, to solve the mystery

of killer volcanoes.

Right now, on NOVA.

Of all the forces of nature,

volcanoes are among the most dangerous.

They have the power

to kill millions and disrupt the fabric of modern life.

Volcanoes can have a global impact.

Today, there are more than 1,500

active volcanoes on Earth.

About 50 erupt every year.

Many are well known, like Vesuvius in Italy,

and Mount St. Helens in Washington State.

But could there be other slumbering giants

that we have never heard of?

Volcanoes that were once

even more powerful and destructive

than today's monsters?

That's what a series of clues is suggesting.

Scientists working across the world

have begun to find evidence of a cataclysmic event,

a mysterious eruption that could have been

one of the largest in human history.

And the trail starts in a very unexpected place.

Here, in the heart of London,

archaeologists uncover a surprise.

While excavating a medieval cemetery,

they happened upon a series of mass graves

on the edges of the burial ground.

Over 4,000 men, women and children,

packed into large pits.

The cause of death was not obvious.

So what killed so many people,

and why were they all buried together?

When we find mass burial pits,

we know that there's been a lot of people

dying very quickly, and something has gone very wrong.

As a first step to identifying the killer,

archaeologist Don Walker and his team conducted

radiocarbon tests, to find out how long ago they died.

The hope was that they could tie their deaths

to some historical event.

But what they found merely deepened the mystery.

The victims all died around 1250 in the Common Era.

That ruled out

one of the most notorious mass killers of the past,

the Black Death, which ravaged Europe

about a century later, in 1348.

So what could have caused this mass killing?

Don Walker heads to the British Library

to consult ancient historical records from the period.

This manuscript is a history of England.

Over 750 years old, it was written in Latin

by a monk named Matthew Paris.

Among these chronicles,

one account stands out--

a description of bitterly cold weather around London

in the spring and early summer of 1258

that kills crops and livestock

and leads to a deadly famine.

It says owing to the scarcity of wheat,

a large number of poor people died,

and dead bodies were found in all directions,

swollen and livid.

Then Walker discovers

a description that seems to match the discovery

of the mass graves.

When several corpses were found,

large and spacious holes were dug in the cemeteries,

and a great many bodies were laid in them together.

And of course, as soon as I saw that, I got very excited,

because it's exactly the kind of thing that we found

at St. Mary Spital, where they were digging these huge pits.

According to this text,

the famine killed over 15,000 people in London.

That's 30% of the city's population at the time.

You're talking about something

that was perhaps nearly as deadly as the Black Death.

And possibly as widespread too.

Other sources reveal the far-reaching impact

of the extreme weather and its disastrous effect on crops.

There are various records from this period--

1258, 1259, 1260,

in various parts of Europe and as far as Japan,

that do attest to extreme impacts

in terms of famine, in particular.

Something devastating

was plunging much of the Northern Hemisphere

into a pattern of bitter winters

and summers blighted by torrential rain.

We thought perhaps this was something to do

with some catastrophic event.

Only one type of natural disaster

could have such a widespread impact on the climate--

a massive volcanic eruption.

But which volcano was the culprit?

The closest volcanoes are a thousand miles away in Iceland.

In 2010, when a volcano known as Eyja erupted,

it sent an ash cloud over Europe

that disrupted air travel for weeks,

stranding people all over the world.

And historical records reveal that in 1783,

eruptions from the Laki volcano

caused mass deaths across Europe.

It seems like an Icelandic volcano

could be to blame for the mysterious 13th century event,

but Walker came across another possibility.

He became intrigued by one of the most massive eruptions

in recorded history,

even though it occurred just 200 years ago

and was located much farther away--

Mount Tambora in Indonesia.

In April 1815, eyewitness accounts record

that Mount Tambora erupted explosively.

And this is one of the largest eruptions

of the last 10,000 years.

It's estimated over 60,000 people died

in the shadow of this volcano.

But the eruption had an even more far-reaching impact.

In northern Europe and North America,

the year after the eruption, 1816, is known as

the year without a summer.

In New York State, it snowed in June.

In Europe, cold weather led to the worst famine for a century.

The climate change

caused agricultural failures, poor harvests,

it pushed up grain prices,

with many people perishing from malnourishment.

So perhaps another powerful eruption

could have caused the year without a summer in 1258.

We began to think that perhaps this might be something to do

with what had happened back in the 13th century.

Perhaps this was why the people starved.

But to pinpoint the location

of the mysterious volcano, they needed to know more

about its size and type of eruption.

They look at one of the largest volcanic events

in recent memory...

the 1991 eruption of Mount Pinatubo in the Philippines.

Unlike volcanoes that erupt by pouring out rivers

of molten lava over a long period of time,

Pinatubo erupted suddenly and violently...

...when water and gases trapped inside the magma

exploded with tremendous force,

shattering the rock into millions of tiny particles

and sending them high into the atmosphere.

Explosive eruptions like this are the most dangerous of all.

Pinatubo killed 847 people locally,

and left over 200,000 homeless.

But as with the other big Indonesian volcano,

Mount Tambora, this eruption also had far-reaching effects.

NASA's satellites were able to monitor the eruption.

Pinatubo blasted out one cubic mile of superheated ash,

but it also ejected hundreds of millions of tons

of volcanic gases

in a plume 22 miles high.

The ash and the gases

are dispersed through the atmosphere by the wind,

and they can travel thousands of kilometers,

tens of thousands of kilometers.

But unlike heavy ash

that soon falls out of the atmosphere,

the lightweight gases persist for much longer.

The most impactful gases are the sulphur-rich gases.

And those gases will form little droplets of sulphuric acid

in a large cloud.

The tiny drops of sulphuric acid are called aerosols.

And in a large cloud, high in the upper atmosphere,

these aerosols caused enough sunlight to reflect

out into space to cool the planet.

The satellite data revealed that this sulphuric acid mist

had a much more dramatic impact than the ash.

It blocked enough sunlight to cool the entire planet

by one degree Fahrenheit for two years.

That doesn't sound very much, but it actually masks

much stronger regional variations.

And that translates into real impact on the ground