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You've probably heard of supervolcanoes.

They're like normal volcanoes, except … you know … more volcano.

You've probably also heard that some of them — like Yellowstone — are ticking

time bombs ready to go off at any moment and wreak havoc on civilization as we know it.

This idea pops up in lots of scary news headlines and thrilling action movies, and let's be

honest, with the word “super” in the name, it's just a matter of time before the Avengers

have to fight one somehow.

And yet, despite all the hype, the one group of people who aren't particularly worried

about these volcanoes are the scientists who study them.

The truth is that supervolcanoes are real and they can cause unbelievable devastation,

but they're not really something we have to worry about.

One of the tools geologists use for measuring volcanic eruptions is the awesomely-named Volcanic Explosivity Index.

This is a scale that categorizes volcanoes by how much tephra they spew out — tephra

being the solid rock in the eruption, from big volcanic bombs to very fine ash.

Like the earthquake Richter scale, the magnitudes are logarithmic, so every step up in number

represents a ten-fold increase in volume of erupted rock.

The 1980 eruption of Mount St. Helens — which was the most destructive volcanic event in

the history of the United States — was a 5 on the VEI scale because it released around

1.3 cubic kilometers of tephra.

On the other hand, the eruptions of Kilauea in Hawai'i tend to have a VEI magnitude of 0.

This is because while the volcano produces a lot of semifluid rock, or lava, it doesn't

spit up a lot of solid stuff.

That's not to say that Kilauea isn't dangerous, it's just isn't very explosive.

The highest an eruption can score on the explosivity index is magnitude 8.

It's reserved for volcanoes that release at least 1000 cubic kilometers of tephra in a single eruption.

That's 1 trillion cubic meters of rock and ash — enough to fill 4 million olympic-sized swimming pools.

These have sometimes been called super-eruptions, so the volcanoes that can produce them have

come to be called supervolcanoes, though the term initially came from pop culture, not the scientific literature.

Supervolcanoes aren't common, but there are several around today.

If you go visit one, though, don't expect to see a tall ominous mountain.

Instead, look for an enormous pit.

You see, after a volcano erupts, the ground will sometimes collapse into the now-empty magma chamber.

This creates a bowl-shaped depression called a caldera.

And supervolcano calderas are so big they're almost hard to spot.

A visitor to beautiful Lake Toba on the island of Sumatra might not even realize that the

100-kilometer-long lake is actually a flooded caldera.

It was created in the aftermath of a super-eruption that occurred around 74,000 years ago.

Of course, perhaps the most famous supervolcano in the world sits beneath Yellowstone National

Park in northwest Wyoming.

A large section of the park is taken up by the Yellowstone caldera, which is roughly

72 kilometers long and 48 kilometers wide.

Deep below the park, the magma chambers of the supervolcano are still very much active,

and it shows on the surface.

Yellowstone is famous for its hot springs, geysers, and other hydrothermal features which

are created when water is superheated underground by the volcano's magma.

But while the volcano may still be hot, the last time it actually super-erupted was around

640,000 years ago.

Humans as a species didn't even exist then, and we wouldn't for another 300,000 years or so.

Geologists can track ancient volcanic eruptions by studying the petrified layers of ash or

lava they leave behind.

The rock record of the United States holds evidence of 3 major Yellowstone eruptions,

2 of which were super-eruptions.

The first one occurred 2.1 million years ago, and was one of the largest known volcanic

eruptions in history, covering more than half of the United States in an estimated 2,450

cubic kilometers of tephra.

The more recent Yellowstone super-eruption of 640,000 years ago was quite a bit smaller,

producing about 1,000 cubic kilometers of material.

It's the one that created the caldera you can go visit today.

And worldwide, the most recent known super-eruption was that of Taupō in New Zealand which exploded

a mere 27,000 years ago.

Super-eruptions may be devastating, but they're not frequent.

Based on past eruptions, geologists have estimated they occur every 17,000 to 45,000 years, or so.

But those numbers are just an average, not a prediction.

Volcanoes don't operate on precise cycles.

That's just not how they work.

The forces that lead to an eruption don't build at a constant rate.

So there isn't a volcano on Earth that's “overdue” for an eruption of any kind,

let alone a super-eruption.

And while that lack of clock-like activity means we can't predict exactly when a supervolcano

will erupt, if Yellowstone was about to, volcanologists would know because it's the best-studied

supervolcano around.

It's what's called a hotspot volcano.

These happen when a particularly hot region of the Earth's mantle melts a lot of nearby

rock into magma, which then rises toward the surface.

Over the last several million years, the North American continental plate has been sliding

slowly to the west, while the mantle hotspot has stayed relatively still.

This has created a line of volcanoes stretching from the west coast of North America to Yellowstone's

current home in Wyoming.

In the future, as the crust continues to move, Yellowstone will be pulled away from the hotspot,

and perhaps a new volcanic center will form farther to the east.

But for the time being, Yellowstone is still very much an active volcano.

It's fueled by a pair of enormous underground magma chambers, the size of which we only

came to appreciate in 2015 when a study explored Yellowstone's nether regions using a technique

called seismic tomography.

It's like a CT-scan, except using earthquake tremors instead of X-rays.

Earthquakes create seismic waves, and because these waves travel differently through different

materials — such as molten rock versus cooler solid rock — scientists can examine earthquake

data to image what's going on underground.

The study was the first to visualize both partially-molten magma chambers beneath Yellowstone,

together comprising more than 50,000 cubic kilometers in volume.

Now, a magma chamber isn't just a big tub of glowing goo.

It's a region of hot crust where some of the pore space between the rock is taken up by molten magma.

In these particular magma chambers, the researchers estimated that between 5 and 15% of the rock is liquidy.

And these magma chambers are the main sources of Yellowstone's volcanic activity — activity

which, by the way, includes a lot more than just the occasional super-eruption.

Over the last several hundred thousand years, Yellowstone has produced around 80 non-explosive

eruptions of lava, the last one occurring around 70,000 years ago.

And even more common than that are steam explosions.

At least 26 have happened during the 126 years that scientists have been monitoring the region.

Super-eruptions, by comparison, are extremely rare, in part because it takes exceptional

circumstances to set them off.

Standard volcanoes tend to blow because magma flows into the volcano's interior, increasing

the pressure like air flowing into a balloon until it pops.

But this mechanism isn't likely to cause a super-eruption, since the magma chamb ers

of supervolcanoes tend to be larger and have hotter, more flexible walls.

As new magma flows in, the chambers can simply expand.

So two 2014 studies concluded that super-eruptions are instead caused by a high level of magma buoyancy.

See, magma is hotter and less dense than solid rock, so buried magma naturally tends to rise if it can.

Imagine you've forced an inflated beach ball underwater in a pool.

The air is much less dense than the water, so if you let the ball go, it launches quickly

up and through the surface.

Using computer models and laboratory simulations, the two studies concluded that super-eruptions

happen much the same way.

Once enough magma builds up, the crust can't hold it in any more, and it erupts.

And geologists don't think this is likely to happen anytime soon.

For one thing, magma chambers typically need to be at least 50% molten before they can

erupt, and as I mentioned, Yellowstone's are 15% molten at best.

Besides, if a super-eruption were about to happen, the changing underground conditions

would create noticable changes on the surface as well.

Sure, it's tempting to read about a big earthquake or the changing geyser patterns

at Yellowstone and wonder if the end is nigh, but it's important to remember that that

kind of thing is part of the volcano's normal activity.

Yellowstone experiences between 1,000 and 3,000 earthquakes each year, most of which

are too small for people to notice.

Also, those geysers have always been dynamic features.

And the ground continually rises and falls as things underneath shift about.

So all of that seemingly dramatic activity isn't a sign of impending doom … it's a Tuesday.

If a super-eruption were building, earthquakes would be far more frequent, more intense,

and more concentrated in particular areas.

And we would see similar striking changes to patterns of geyser activity and ground movement.

There may even be chemical clues of an impending major eruption in Yellowstone or other supervolcanoes.

A 2018 study examined the history of major volcanic eruptions in the Phlegraean Fields

of Italy, and characterized how the composition of magma changed as eruptions approached.

Based on that, they concluded that the region is beginning to build to its next major eruption...

but that it will probably be tens of thousands of years before it actually blows.

And across the board, volcanologists say we'll get years or decades if not centuries of warning

before a supervolcano erupts.

Still, they're keeping an eye on them just in case.

For example, the Yellowstone Volcano Observatory monitors geologic activity all across the park.

So you can rest assured that if anything changes, we'll know about it.

Right now, the United States Geological Survey says that the chances of a Yellowstone super-eruption

happening in the next few thousand years are quote “exceedingly small.”

In fact, it's not even a sure thing that Yellowstone will ever have another super-eruption.

Thanks to the movement of the crust, the Yellowstone caldera is already slightly to the northeast

of the major magma chamber beneath it.

Yellowstone might simply be dragged away from its heat source before it gets the chance

to blow its top again.

So there's really no reason to lose sleep worrying about a super-eruption.

But, just for imagination's sake … what would happen if one of them did go off, say, tomorrow?

Well, the eruption could have impacts all over the globe.

The region nearest to the supervolcano would experience the absolute worst versions of

the standard volcano arsenal, including lava flows, pyroclastic clouds, earthquakes, toxic

gases, and more.

If Yellowstone went off, for instance, it's unlikely anyone in the park would survive.

Meanwhile, most of the rest of the United States would have to worry about ash.

Here in Missoula, almost a third of a meter of volcanic ash could rain down, and even

places like Orlando and DC could receive a few millimeters of the stuff.

Those tiny particles of rock and volcanic glass can clog up engines, damage lungs, and

even collapse the roofs of buildings if enough falls.

So a Yellowstone super-eruption would cause medical, economic, and structural chaos across the country.

And, wherever in the world it happened, a super-eruption would almost certainly mess with global climate.

The eruption of Mount Tambora in 1815 pumped so much volcanic ash and gas into the atmosphere

that global temperatures dropped about 3 °C the following year.

And it only ranked a 7 on the VEI scale.

A super-eruption might do the same thing on a larger scale, impacting just about every

corner of the globe.

But, though there'd be some havoc, it wouldn't be the end of the world.

None of Yellowstone's previous eruptions are linked to major extinction events.

Neither are the eruptions of any other supervolcanoes, for that matter.

We might fear for our own safety because for a long time, scientists have thought the Toba

super-eruption 74,000 years ago caused a decline in human populations.

But even that has been called into question by more recent archeological data.

Regardless, all of this really isn't something we need to worry about today.

I can't stress this enough: volcanologists say there's no reason to think any supervolcano

is going off anytime soon.

Still, researchers will continue to monitor and collect data on supervolcanoes

Because, someday in the future — in several thousand years, maybe — Yellowstone or another

supervolcano will start stirring, and future humans will be really glad we spent all this

time studying supervolcanoes when it does.

Until then, we can leave the doomsday stories to Hollywood.

In order to understand and predict volcanic eruptions, volcanologists need serious math

chops to understand their data.

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