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  • They are eruptions so vast, so Earth-shattering, they have changed the history of our planet.

  • 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.