Cosmic Journeys – Supervolcanoes

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. 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.

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