Climate collapse. Toxic turmoil. Mass extinction.

Worse than a killer asteroid, or nuclear war, they are Earth's most destructive Supervolcanoes.

North America, the time was six hundred and forty thousand years ago, long before humans

arrived on the continent.

Amid one of nature's great mountain building projects, the Rockies, vast columns of smoke

began to rise high into the atmosphere.

And soon a smokey haze wrapped the globe.

A thick blanket of ashe spread over the western United States.

Geologists have traced this event to a depression in the land known as a caldera, in the heart

of Yellowstone National Park in Wyoming.

Today, we venture to Yellowstone to admire its spectacles of steam and boiling mud.

All around, thermal energy is underfoot, like a pressure cooker gathering steam.

The heat escapes in an array of smoldering caldrons, steam vents, and hot springs, like

the famous Grand Prismatic.

Here, superheated water rises up from deep below ground, then cools and sinks back down

in a constant cycle.

Geysers are powered by boiling water that moves up through constricted channels in the

rocks. When enough pressure builds, steam and water escape in jets that can blast high

into the air.

Along the Yellowstone River, heat welling up from below softened the underlying rocks

to such an extent that water released by rapid melting at the end of last ice age was able

to carve out one of the most spectacular river canyons in the world.

Visitors to Yellowstone may never suspect they are atop one of the world's largest active

volcanoes.

The last time it blew, it sent an estimated 1000 cubic kilometers of dirt, rocks, ashe,

dust, and soot into the atmosphere. It's difficult to grasp the sheer scale of that eruption.

Compare it to recent experience.

In 1980, Mt. St Helens blasted out just 2.79 cubic kilometers of ashe, less than three

tenths of a percent of what Yellowstone ejected.

In 1991, Mt. Pinatubo in the Philippines released 10 cubic kilometers of material, along with

20 million tons of sulfur dioxide.

A volcano twice that size blew up on the Indonesian island of Krakatoa in 1883.

It unleashed the equivalent force of 10,000 Hiroshima bombs, and tsunamis that killed

36,000 people.

The same year, the Norwegian artist Edvard Munch painted The Scream, with a sunset likely

inspired by the effects of smoke from Krakatoa. "Clouds like blood and tongues of fire," he

wrote, "hung above the blue-black fjord and the city.''

This is Anak Krakatoa, or child of Krakatoa, an island that has risen in the years since.

It is a modest but unruly cone that may be building again toward another big one some

time in the future.

The modern standard was set by the eruption of Tambora, eight times larger than Krakatoa,

in 1815.

In its aftermath, 1816 became known as the "Year Without a Summer." Crops failed and

livestock died in much of the Northern Hemisphere, resulting in the worst famine of the 19th

century.

The public has long been aware of the hazards of an erupting volcano: explosions, lava,

and debris flows, like the ones dramatized in the movie Dante's Peak.

The record of recent volcanoes, from Tambora to Pinatubo, has added an additional global

hazard.

In these images captured by astronauts aboard the space shuttle, you can see the sulfurous

haze that covered the Earth in Pinatubo's wake. It blocked enough sunlight to send global

temperatures down by about a half a degree Celsius.

Mt. Pinatubo erupted at a time when scientists had begun to use high-powered computers to

model the response of Earth's climate to large-scale disruptions.

Based on their success modeling the circulation of particles from Pinatubo, one group sought

to explore the consequences of another type of eruption: limited nuclear war.

They focused on India and Pakistan, two countries that were engaged in a nuclear arms race.

The experiment assumed 100 Hiroshima-sized bombs. The intense heat of cities burning

sent over five million tons of smoke rising into the stratosphere.

With no rain at that altitude to bring them down, soot particles lingered for years. They

absorbed far more solar radiation than the brighter sulfuric acid particles emitted by

volcanoes.

As a result, according to the study, global temperatures dropped by 1.2 degrees Celsius,

equivalent to the "Year without a summer" after Tambora.

That shortened the next growing season by 10-30 days, resulting in widespread crop failures.

Finally, in two to three years, the smoke began to clear and the climate steadily recovered.

But those impacts are dwarfed by what would happen in the wake of a supervolcano. Take

the last major Yellowstone eruption, 640 thousand years ago.

Scientists at the Max Planck Institute in Germany used an ensemble of computer models

to study its impact on Earth's climate.

Their virtual eruption sends up a giant cloud of ashe and dust that's taken by high altitude

prevailing winds.

In a month's time, the cloud has spread over much of the northern hemisphere.

The simulation tracks the bitter consequences.

Solar radiation at Earth's surface falls off in an uneven pattern.

The darkest point occurs around 18 months after the eruption, with the mid latitudes

of North America and Europe experiencing a steep drop in sunlight.

Air temperatures fall too, hitting their lowest point at 18 months. In some places, they fall

by an average of 10 degrees celsius.

That leads to the rapid growth of sea ice in the Artic. More sea ice means that the

Earth reflects even more solar energy back into space.

With cooler surface temperatures, rainfall levels decline and oceans and land areas retain

more carbon dioxide.

These factors all lead to a drop in biological productivity.

With food supplies lasting just weeks or days in some regions, human populations would be

subject to serious losses.

From their steep initial drop, average global temperatures recover gradually, approaching

pre-eruption levels only after two decades.

The study found that changes to the rate carbon is taken up and released back into the air

lasts even longer, two centuries.

As powerful as the Yellowstone eruption was, it still does not approach the greatest supervolcanoes

in history.

This is Toba, a large mountain lake on the Indonesian island of Sumatra.

It's the crater left behind by a super eruption 74,000 years ago. What made Toba super is

the sheer scale of the eruption.

Over fourteen terrible days, Toba blasted 2,800 cubic kilometers of material into the

air. That's almost three times Yellowstone.

Its grim aftermath was a global cataclysm, including a global cold spell and persistent

drought.

One theory holds that Toba brought the human species perilously close to extinction.

What causes a catastrophe on this scale? The answer takes us back to the birth of our sun,

around five billion years ago. It began in a swirl of debris, likely drawn together by

shock waves from a stellar explosion.

Amid the chaos of the early solar system, gravity drew clumps of rock and dust and gas

together.

A fiery young planet formed, molten at first.

Millions of years passed. Earth's surface gradually cooled, but its interior remained

hot. Add to that, heat generated deep within the planet by the radioactive decay of uranium,

thorium, and potasium.

To this day, heat energy is rising steadily from Earth's interior, a vivid reminder of

the fires that burn inside it.

The heat makes its way to the surface through a vast middle region called the mantle.

It punches through where Earth's crust is thinnest, in the middle of oceans. That causes

massive plates that line the surface of the planet to push apart, often flooding the ocean

floor with lava.

These oceanic plates collide with thicker continental plates.

That drives them down into the Earth, along with volcanism's secret ingredient: water.

Some rocks, when mixed with water, melt more readily. They form a reservoir of magma deep

below ground. As more magma enters the reservoir, the pressure increases.

That forces magma up to the surface, and a volcano erupts.

A supervolcano like Toba or Yellowstone begins with a much larger reservoir of magma.

A recent study showed that the larger it gets, the more bouyant the magma becomes amid the

solid crustal rocks that surround it.

Over time this bouyant reservoir pushes up on the terrain above it.

The rock above the magma begins to break. Channels, or dykes, form, to release the rising

magma.

The volcano does not so much erupt, as it explodes.

The land collapses onto the magma, helping to propel it up and out.

The thermal features that dot Yellowstone are small in scale, yet they too are fueled by

an immense dome of magma deep underground.

By tracking the pattern of seismic waves from small earthquakes, scientists have been able

to follow it down to its source.

A kilometer below, surface water enters layers of heated rocks, then shoots back to the surface

in hot springs and geysers.

Deeper still, ten kilometers down, they've located the upper reaches of an immense reservoir

of magma.

It measures over a hundred kilometers wide and four hundred kilometers deep.

Scientists now believe it's partly the result of the the Pacific Ocean Plate diving below

North America,mand partly the product of a hot spot. It's similar to a deep plume of

magma that fuels Hawaii's volcanoes.

Over the last 16 million years, as the continent of North America has moved, inch by inch,

over the Yellowstone hot spot, the volcano has reawakened time and again.

The next to the last time, it ejected 5,000 cubic kilometers of material into the atmosphere,

almost twice as much as Toba.

Toba is known to have erupted at least three times in the last one million years. There

are signs that it's gearing up for another.

Some parts of the caldera, including a large island in the middle called Samosir, have

risen due to a refilling of the magma chamber.

Indonesian scientists recently reported they have detected its presence, starting at a

depth of 20 to 100 kilometers down.

The volcano is not active now. But it's in one of the most geologically active regions

in the world, including the nearby Sumatran fault and the Sumatran Subduction Zone.

This region is known for major earthquakes and a string of volcanoes, including Krakatoa

and Tambora.

If or when Toba does erupt again, it still won't hold a candle to Earth's greatest eruptions.

Roll back the geological clock, to a time around 65 million years ago. A giant asteroid

slammed into the Earth. It wreaked havoc on the climate and may have led to the extinction

of the dinosaurs.

The impact coincided with another type of catastrophe.

That was a time when today's continents were taking shape. The Himalayan mountains were

beginning to rise up. And India had not yet crashed into Asia.

In central India, one of the largest volcanic structures on Earth made its presence felt.

From the so-called Deccan Traps, the Earth literally spilled its guts.

Over a period lasting perhaps 30,000 years, 500,000 cubic kilometers of lava flooded onto

the Indian landscape.

The Deccan traps eruption was about 500 times larger than the last Yellowstone eruption.

An even larger, more destructive event occurred around 235 million years ago in what's now

Russia.

Back then, all of Earth's land masses were joined together in a supercontinent known

as Pangea.

In a period known as the Permian, amphibians and early reptiles flourished. Insects were

everywhere.

Fern plants, dominant for eons, were beginning to give way to conifers and palms.

In the oceans, amid vibrant coral reefs, invertebrates and fish inhabited the open water.

Then, in the geological blink of an eye, it was over.

For a period lasting up to ten million years, countless small eruptions generated several

thousand times more lava than Yellowstone, spreading over an area the size of Western

Europe.

What it brought was not volcanic winter, but rather hell on Earth.

The atmosphere became choked with carbon dioxide and methane gas. The Earth belched deadly

toxins, including high levels of mercury. Temperatures around the world soared.

The oceans turned acidic, their waters depleted of oxygen.

By the time it was over, up to 70 percent of all species on land, and 90% of marine

species, had disappeared. Ecosystems around the planet collapsed in what scientists call

the "great dying."

And that was not even the largest volcanic eruption in Earth's history. For now, that

honor goes to an undersea eruption that began 125,000 years ago, creating a plateau thirty

kilometers thick called Ontong Java. It released about 100 million cubic kilometers of magma.

Scientists believe large eruptions like Ontong Java or Siberian Traps occur where broad regions

of magma, called large igneous provinces, push toward the surface.

The Earth is pocked with dozens of them. They are spread out across all the continents and

oceans.

Thankfully, these regions have erupted only very rarely in Earth's history.

Super eruptions on the scale of Yellowstone and Toba, linked to the movement of crustal

plates, are more frequent, perhaps every 100,000 years or so.

Because so little is known about what triggers them, there's no telling when or where the

next one will occur. Are we due?

Scientists at Yellowstone are watching for early signs: earthquakes, rising heat, the

restless stirrings of a planet under pressure.

Someday, somewhere, Earth will once again vent its rage.

When it does, we can be sure that it will change our world forever.