National Geographic explains:

Yellowstone is a volcano, and not just any volcano. The oldest, most famous national park in the United States sits squarely atop one of the biggest volcanoes on Earth . . . The last three super-eruptions have been in Yellowstone itself. The most recent, 640,000 years ago, was a thousand times the size of the Mount St. Helens eruption in 1980, which killed 57 people in Washington. But numbers do not capture the full scope of the mayhem. Scientists calculate that the pillar of ash from the Yellowstone explosion rose some 100,000 feet, leaving a layer of debris across the West all the way to the Gulf of Mexico. Pyroclastic flows-dense, lethal fogs of ash, rocks, and gas, superheated to 1,470 degrees Fahrenheit-rolled across the landscape in towering gray clouds. The clouds filled entire valleys with hundreds of feet of material so hot and heavy that it welded itself like asphalt across the once verdant landscape. And this wasn't even Yellowstone's most violent moment. An eruption 2.1 million years ago was more than twice as strong, leaving a hole in the ground the size of Rhode Island.

It's worth reading the rest of this article - it beautifully captures the pyrotechnic scientific mystery that is Yellowstone Park. Photographer Mark Thiessen captured the blowholes where Yellowstone lets off scalding, mineral-rich water.

via National Geographic (Thanks Marilyn Terrell!)

Located in Montana and Wyoming, Yellowstone is famous for its geysers, including "Old Faithful," which blasts steam into the air like clockwork every day. Now geologists studying the recent mini-quakes in the park say we might be in for a big blast. Such blasts tend to come about once every 600 thousand years, and we haven't seen one for roughly that amount of time.

The last big explosion in Yellowstone, according to Scientific American, was roughly 640 thousand years ago, and it covered about 240 cubic miles in hot ash, scalding rocks, and magma. But don't worry yet, says SciAm's David Biello:

Although the earthquake swarm continues, according to the Yellowstone Volcano Observatory, the volcano alert level remains normal. And a slew of larger earthquakes have occurred throughout the western U.S., Alaska, Puerto Rico and even Pennsylvania in the past week without incident, according to the U.S. Geological Survey.

In recent years, Yellowstone's caldera has been rising thanks to uplifting magma beneath it—leading to more cracks, hot springs and even more frequent eruptions of Steamboat Geysers. Paired with the earthquakes, such magma movement might presage an eruption—either big or small. Unfortunately, scientists can't really predict when the next such eruption will happen, and the range of possibilities is large: from later today to a million years from now.

How will we know if we should start worrying? The real warning signs will be rapid changes in the shape of the ground as well as volcanic gases leaking from the ground, neither of which have been sighted—yet.

Right now, in some dark Hollywood pitch meeting, Jerry Bruckheimer is mud-wrestling with Michael Bay over the rights to a movie about this potential explodey Yellowstone disaster.

SOURCE: Scientific American

Thanks, Robert Atlas!

Photo by Nina Raingold/Getty Images.