In Yellowstone, the rim of a supervolcano caldera is visible in the distance. Photo courtesy of the National Park Service.

In 2012, director Roland Emmerich’s love letter to the Mayan apocalypse (oh hey, wasn't that supposed to happen today?), our heroes barely manage to escape Yellowstone National Park before it explodes beneath them. This not-so-subtle sci-fi sequence is actually based on something real: Underneath Yellowstone is a supervolcano. What distinguishes this kind of volcano from regular volcanoes, and what will happen if—or when—it erupts?

Regular versus Super Volcano

There are several types of volcanoes: Cinder, composite or stratovolcanoes, shield volcanoes, and lava domes, which all rise above the earth. But supervolcanoes like the one under Yellowstone are calderas, vast sunken areas formed when the volcano expels all of its magma, and the land comes back to rest in the empty chamber. These calderas can be as big as 60 miles across (the current Yellowstone caldera, which sits on several older calderas, measures about 28 miles by 47 miles).

Click to enlarge.

When Mount St. Helens, a stratovolcano located in Washington, erupted in 1980, the event rated a 5 on the Volcanic Explosivity Index (VEI) and expelled one cubic kilometer of ash. But supervolcanoes register 8 on the VEI and typically expel ten thousand times the quantity of magma and ash expelled during the Mount St. Helens eruption. The last Yellowstone eruption, which occurred 640,000 years ago, "spewed out nearly 240 cubic miles of debris," according to the USGS.

The Yellowstone is just one of a few supervolcanoes scattered around the globe. An incomplete list includes Taupo in New Zealand, which was the most recent to erupt, about 26,000 years ago. Before that was a supereruption of Lake Toba on Sumatra, Indonesia, which occurred 69,000 to 77,000 years ago. There's a supervolcano under Pompeii, and one in Chile, too. There may be more we haven't yet discovered.

What Happens If A Supervolcano Erupts?

Below Yellowstone's surface—in some places as little as 5 or 6 miles—is a reservoir of solid rock and magma. Below that is a 45-mile-wide plume of molten rock that comes from at least 410 miles beneath the Earth surface (this is what fuels Yellowstone's incredible geysers and geothermal pools). Bob Smith, who first described Yellowstone as "a living, breathing caldera" in 1979, says in his book, Windows into the Earth, that if the caldera were to erupt, "Devastation would be complete and incomprehensible."

First there would be swarms of earthquakes, then a huge blast that would wipe Yellowstone off the map. Clouds of ash and gas would burn everything in their paths. Ash would cover most of North America, destroying food sources. Some speculate that a supereruption from the Yellowstone caldera would instantly kill 87,000 people. Others speculate that such an eruption would lower the temperature of the Earth by at least 21 degrees, and might even block out the sun.

Recently, a study determined that if or when Yellowstone next erupts, it will probably be centered in one of three parallel fault zones running north-northwest across the park.

Still, there's probably not a reason to worry. Chances are, a supereruption won't occur in our lifetimes. Number crunchers have determined that only 1.4 supereruptions occur every million years, and, according to the USGS, the chances of Yellowstone erupting are slim: just 1 in 730,000, or 0.00014 percent, a year. Sorry, conspiracy theorists. Maybe at the end of the next b'ak'tun.