What If Every Volcano on Earth Erupted at Once?
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Whether it's glowing lava snaking into the sea or lightning blooming in billowing ash clouds, the sight of an erupting volcano inspires awe and wonder.

Now imagine 1,500 of these suckers all shooting off at once. That's how many active volcanoes dot the Earth, plus an unknown number hidden under the ocean. Every day, between 10 and 20 volcanoes are erupting somewhere on Earth, but scientists say the chance of every volcano on the planet erupting at once is so small that it's impossible. But what if it did happen? Would Earth as it we know it survive?

Not likely, said Parv Sethi, a geologist at Radford University in Virginia. Even if only the volcanoes on land blasted in sync, the effects would trigger an environmental domino chain many, many times more powerful than a nuclear winter, Sethi said. "Things will become so bad that I wouldn't want to survive on an Earth like this," he told Live Science. [Top 10 Ways to Destroy Earth]

The two big hazards from a worldwide volcanic cataclysm are ash and volcanic gases. (While the explosions and outpourings of lava would be deadly to people living close by, the number of deaths would pale compared to those caused by the ensuing climate change.)

Plunged into darkness

Sethi predicts that a thick layer of ash would blanket the Earth, completely blocking incoming sunlight.

"The planet would be pitched into complete dark, and that is going to devastate photosynthesis, destroy crop yields and cause temperatures to plunge," Sethi said. The ash would linger in the atmosphere for up to 10 years, he added.

Yet, not every volcano on Earth is primed to pump out large amounts of ash; some, like Hawaii's volcanoes, usually put out gentle lava flows. But the list of 1,500 potential active volcanoes, compiled by the U.S. Geological Survey, includes whoppers like the Yellowstone supervolcano, which could cover the contiguous United States in a thin layer of ash.

Biting cold

Acid rain would wipe out any crops that survived burial by ash, Sethi said. Volcanic gases include nasties such as hydrochloric acid, hydrogen fluoride, hydrogen sulfide and sulfur dioxide, which can become acid rain when they condense high in the atmosphere. That acid rain would contaminate groundwater and the ocean's surface. Ocean acidification would kill off corals and marine creatures with hard shells. The extinctions would travel up the ocean food chain, wiping out fish and other marine life.

Researchers have documented a similar connection between ocean acidification, mass extinctions in Earth's past and volcanic mega-eruptions called flood basalts. For instance, these huge lava outpourings have been correlated to extinctions at the end of the Permian Period 252 million years ago, the Triassic Period 201 million years ago, and the end of Cretaceous Period 65 million years ago.

"Flood basalts and mass extinction events are linked," said Paul Renne, a geologist at the Berkeley Geochronology Center in California who specializes in figuring out the age of rocks.

Explosive volcanic blasts also shoot ash, dust and gas into the stratosphere. These particles reflect sunlight away from Earth and can significantly cool the planet, albeit briefly. For instance, the eruption of Mount Pinatubo in 1991 — one of the two largest eruptions in the 20th century — cooled parts of the world by up to 0.7 degrees Fahrenheit (0.4 degrees Celsius) for two years. [Countdown: History's Most Destructive Volcanoes]

Turn up the heat

Volcanic eruptions do release the greenhouse gas carbon dioxide, which could help offset global cooling from ash and stratospheric particles. But Sethi wondered whether 1,500 simultaneous volcanic eruptions would overwhelm Earth's systems.

"It's going to be like turning the knob on a gas stove to the broil setting," he said. "The only question is whether it is actually going to change the [atmosphere's] composition so much that we have carbon dioxide poisoning in the atmosphere. Either way, we're going to be cooked, so to speak," Sethi said.

Ancient black shales, a type of marine rock, indicate comparable calamities happened in Earth's history, said Sethi, who studies these Cretaceous Period rocks. The rock record suggests carbon dioxide levels soared in the Cretaceous, killing off marine life in some parts of the ocean and shutting down ocean circulation. During the Late Cretaceous Period, some 90 million years ago, carbon dioxide levels in the atmosphere were around 2.5 times today's levels, scientists think.

The tiniest survive

So what life could survive this deadly volcanic outburst?

"It will be the time of the extremophiles," Sethi predicts. These organisms already live in extremely acidic environments, such as Yellowstone's hot springs, or in deep undersea vents, protected from the surface devastation. "It would be like a clean slate for these organisms to do their evolutionary thing."

The sci-fi strategy is also possible: A few humans live on in orbit or in deep underground bunkers built by well-financed governments or rich moguls, waiting for the atmosphere to clear.

"The lucky ones would be the dead ones in this scenario," Sethi said.

Follow Becky Oskin @beckyoskin. Follow LiveScience @livescience, Facebook & Google+. Originally published on Live Science.