If the supervolcano underneath Yellowstone National Park ever had another massive eruption, it could send ash for thousands of miles across the United States, smothering crops and shutting down flights and power plants all over. It’d likely be a disaster.

But that doesn’t mean we should all start freaking out. The odds of that happening are thankfully pretty low. The Yellowstone supervolcano — thousands of times more powerful than a regular volcano — has only had three truly enormous eruptions in history. One occurred 2.1 million years ago, one 1.3 million years ago, and one 664,000 years ago.

And despite what you sometimes hear in the press, there’s no indication that we’re due for another “super-eruption” anytime soon. In fact, it’s even possible that Yellowstone might never have an eruption that large again.

Even so, the Yellowstone supervolcano remains an endless source of apocalyptic fascination — and it’s not hard to see why. Last week, a team of scientists published a paper in Geochemistry, Geophysics, Geosystems exploring what a Yellowstone super-eruption might actually look like. Among other things, they found the volcano was capable of blanketing the northern Rockies in three feet of ash and could spread material as far as Miami and Washington, DC:

Ash, ash, everywhere

yellowstone super-eruption ash

An example of the possible distribution of ash from a month-long Yellowstone supereruption. (US Geological Survey)

When I called up one of the study’s co-authors, Jacob Lowenstern of the US Geological Survey, he stressed that the paper was not any sort of prediction of the future. “Even if Yellowstone did erupt again, you probably wouldn’t get that worst-case scenario,” he says. “What’s much, much more common are small eruptions — that’s a point that often gets ignored in the press.” (And even those small eruptions are very rare.)

Lowenstern is the Scientist-In-Charge of the Yellowstone Volcano Observatory in Menlo Park, California. So I talked to him further about what we actually know about the Yellowstone supervolcano, what its eruptions might look like, and why the odds of disaster are low.

What is the Yellowstone supervolcano?

yellowstone side view

(National Park Service)

Lurking beneath Yellowstone National Park is a massive reservoir of molten magma some six miles deep. That heat is responsible for many of the park’s famous geysers and hot springs. And the shifting magma can produce earthquakes in the region.

And yes, on rare occasions throughout history, that volcano has erupted. The vast, vast majority of those eruptions have been smaller lava flows — with the last occurring at the Pitchstone Plateau some 70,000 years ago.

But the reason why Yellowstone gets so much attention is the remote possibility of catastrophic “super-eruptions.” A super-eruption is anything that measures magnitude 8 or more on the Volcano Explosivity Index, in which at least 1,000 cubic kilometers (or 240 cubic miles) of material gets ejected. That’s enough material to bury Texas five feet deep.

These super-eruptions are thousands of times more powerful than even the biggest eruptions we’re used to. Here’s a chart from USGS comparing the Yellowstone super-eruptions with the Mt. St. Helens eruption of 1980. The difference is staggering:

Super-eruptions vs ordinary eruptions

super-eruptions vs ordinary eruptions

(US Geological Survey)

Again, Yellowstone has had three of these really massive eruptions in its history — 2.1 million years ago, 1.3 million years ago, and 664,000 years ago.

That last super-eruption, at Yellowstone Lava Creek, ejected so much material from below that it left a 34-mile-by-50-mile depression in the ground — what we see today as the Yellowstone Caldera:

Location of past Yellowstone super-eruptions

yellowstone map

(National Park Service)

It’s worth noting that Yellowstone isn’t the only supervolcano out there — there are quite a few around the world. The last known super-eruption occurred in Lake Taupoin New Zealand 26,000 years ago.

More dramatically, there was the gargantuan Toba eruption 74,000 years ago that triggered a dramatic multiyear global winter and (according to some) may have nearly wiped out the nascent human race.

On average, the Earth has seen roughly one super-eruption every 100,000 years, although that’s not an ironclad law.

So what would a Yellowstone eruption look like?

Let’s just reiterate that the odds of any sort of Yellowstone eruption, big or small, are very low. But if we’re speaking hypothetically…

The most likely eruption scenario in Yellowstone is a smaller event that produced lava flows (similar to what’s happening at Iceland’s Bárðarbunga right now) and possible a smaller explosion. This would likely be precipitated by a swarm of earthquakes in a small region of the park as the magma made its way to the surface.

Now, in the unlikely event of a much bigger super-eruption, we’d see something different. “We’d likely first see intense seismic activity across the entire park,” Lowenstern says. It could take weeks or months for those earthquakes to break up the rocks above the magma before an eruption. (Though, caveat: No one’s ever seen one of these before, so it’s tough to know exactly how it’d unfold.)

And let’s say we did have a super-eruption — an event that was 1,000 times more powerful than a regular volcanic eruption, ejected at least 240 cubic miles of material, and lasted weeks or months. Most of the hot lava would actually stay within a relatively small radius within the park. In fact, only about one-third of the material would actually make it up into the atmosphere.

The ash that did make it into the atmosphere, however, could spread far and wide — doing plenty of damage. In their recent paper, Lowenstern and his colleagues looked at both historical ash deposits and used advanced modeling to conclude that an eruption would create an umbrella cloud, expanding evenly in all directions. (This was actually a surprising and interesting finding, as the researchers explain here.)

Where the ash fell would depend on weather conditions at the time. But in a super-eruption, you could conceivably get three or more feet of ash in the northern Rockies region, a few inches of ash in the Midwest, and a fraction of an inch on both coasts:

Modeling the spread of ash from a Yellowstone super-eruption

yellowstone ash

(Mastin et al 2014)

That would be very bad news. That much ash could do serious damage to crops and livestock throughout the Midwest and Great Plains. And even a few inches of ash can clog roadways, cause serious respiratory problems, block sewer lines, and even short out transformers. Air travel would have to shut down over much of the country.

An eruption that big would also cool the planet temporarily

A volcanic eruption that big would also have major effects on the climate. Volcanoes can emit sulfur aerosols that reflect sunlight back into the atmosphere cool the climate. These particles are short-lived in the atmosphere, so the effect is only temporary, but it can still be dramatic.

When Pinatubo erupted in 1991, it cooled the planet by about 1°C (1.8°F) for a few years. The Tambora eruption in 1815 cooled the planet enough to damage crops around the world — possibly leading to famines in some areas. And those were relatively tiny eruptions compared to what a supervolcano is, in theory, capable of.

Yikes! So what are the odds of a Yellowstone super-eruption?

Very, very low. In fact, it’s even possible Yellowstone might never erupt again.

Right now, there’s no sign of a pending eruption. Yellowstone park does continue to get earthquakes and ground deformation, but that’s nothing out of the ordinary. “Yellowstone is behaving as it has for the past 140 years,” the USGS points out. “Odds are very high that Yellowstone will be eruption-free for the coming centuries.”

The USGS also notes that, if you simply took the past three eruptions, the odds of Yellowstone erupting in any given year are 0.00014 percent — lower than the odds of getting hit by a civilization-destroying asteroid. But even that’s not a good estimate, since it’s not certain that Yellowstone erupts on a regular cycle or that it’s “overdue” for another eruption. In fact, there might never be a big eruption in Yellowstone again.

“The Earth will see super-eruptions in the future, but will they come in Yellowstone? That’s not a sure thing,” says Lowenstern. “Yellowstone’s already lived a good long life. It may not even see a fourth eruption.”

Volcanoes, after all, do die out. The magma chamber below Yellowstone is being affected by two opposing forces — the heat from below and the relative cold from the surface. If less heat comes in from below, then the chamber could conceivably freeze, eventually turning into a solid granite body.

It’s also worth noting that the volcanic hotspot underneath Yellowstone is slowly migrating to the northeast (or, more accurately, the North American tectonic plate above the hotspot is migrating southwest). You can see the migration below:

The volcanic hotspot is sloooooowly moving northeast

path of yellowstone hotspot

(USGS)

On a long enough time scale, the hotspot will move out from under Yellowstone — and the Yellowstone supervolcano would, presumably, die out. That means if we’re going to see another volcano emerge further to the northeast, the hotspot would have to heat up and melt the cold crust first. And that process could take a million years.

“It’s hard to get our minds around something like a million years,” Lowenstern says. “Humans are a relatively brand-new species. But Earth’s been around a very long time, and these systems take a long time to do what they do.”

Further reading

— The US Geological survey has an excellent FAQ on the Yellowstone supervolcano. They also have a great rundown of the most recent paper modeling a super-eruption.

— Here’s a fascinating (and very accessible) paper Lowenstern wrote in 2006 explaining how scientists actually monitor the Yellowstone volcanic system. A key line: “One obstacle to accurate forecasting of large volcanic events is humanity’s lack of familiarity with the singals leading up to the largest class of volcanic eruptions.”