By Natalie Wolchover, Life's Little Mysteries Staff Writer:

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A rough estimate based on geologic records indicates there's a 1-in-10,000 chance of a "supereruption" at Yellowstone during our lifetimes. However, given the erratic nature of volcanoes, that number doesn't mean much. The bulging pocket of magma swishing around beneath Old Faithful might never blow its lid again. Or, it might put on a surprise fireworks show next Independence Day. Scientists just don't know.

But if or when it blows, what will actually happen? Will it be the end of us all, or just a big knock to the tourism industry in Wyoming?

Each of the three past supereruptions of the Yellowstone hotspot spewed more than 1,000 cubic kilometers of magma into the environment — the benchmark of a "supervolcano." According to Jacob Lowenstern, scientist-in-charge at the Yellowstone Volcano Observatory, that's a large enough eruption to cover much of North America in an ash blanket of varying thickness.

"The ash is thick (more than about 30 centimeters of ash) near the eruption source and a small fraction of a millimeter once you move 2,000 miles away. It's fair to say that a trace of ash would be found over most of the United States, though it would only be thick enough to collapse roofs in the states closest to Yellowstone," Lowenstern told Life's Little Mysteries.

With enough warning, the states near Yellowstone could be evacuated, which would largely avoid a tremendous loss of life caused by the downpour of ash, the scientists said. But that's just in the short term; the aftermath would be the rub. For several days, ash would hang in the air, making it difficult to breathe. And that blanket of ash covering the country would smother vegetation and pollute the water supply, quickly leading to a nationwide food crisis. "A lot of people would perish," said Stephen Self, director of the Volcano Dynamics Group at the Open University in the U.K. He envisions American refugees lining up at the Mexican border. [5 Ways the World will Radically Change This Century]

Perhaps foreign governments would come to our aid and embark on a major ash cleanup operation, but without such an effort, inhospitable conditions would persist in the midwestern U.S. for about a decade. "The records show that [new] vegetation starts to take hold about 10 years after supereruptions. It depends on how much rainfall the area receives, as rainfall is the main way you clear ash off the land," Self said.

As for the rest of the world, it would face a few years of mild climate change caused by the supereruption's ash cloud, which would wrap around the globe, casting Earth in shadow for several days and altering the chemical composition of the atmosphere for a decade or so. However, recent research shows the global impacts of supervolcanoes are less severe than scientists once thought, and a Yellowstone supereruption might be especially unimposing because its magma contains minimal sulfur. Sulfur gas produces particles called aerosols, which can cool the climate by blocking sunlight.

"The huge volume of magma means there would still be some sulfur injected into the atmosphere, but work has shown that you reach a sort of limit in the amount of aerosols you can produce with sulfur gas. It means that our earlier suggestions that there would be a severe temperature change is not right," Self said. [What If Earth's Magnetic Poles Flip?]

Based on the new models, the scientists now think the vast majority of Earth's species would weather a Yellowstone supereruption just fine (except, of course, for those knocked out due to proximity of the initial blast). They don't see any evidence in the geologic record of mass extinctions coinciding with supereruptions, and they don't predict extinctions to result from such geologic events in the future.

"The last time Yellowstone erupted, no extinctions took place," said Michael Rampino, a biologist and geologist at New York University. "Supereruptions are not extinction-level events," he said, but added that they can obviously cause problems for civilization.

These are scientists' best guesses, but they probably won't be around to check their answers. Yellowstone's last full-scale outburst occurred 640,000 years ago, and the ones before that occurred 1.3 million and 2.1 million years ago — but each of these events was a tad smaller than the one before it. This geologic hotspot could be growing cold. Or it might have one last hurrah.

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