Say the word “supervolcano” and you’ll immediately think of Yellowstone and its violent past – but what about its future? When will the next super-eruption be?

The thing is, this is far from the only supervolcano on Earth, and many of them could easily rival the power of Wyoming’s own. So where are these supervolcanoes and which one of them really is the most dangerous in the world?

Let’s take a look, but here’s a spoiler for you – it's probably not Yellowstone.

What the Heck is a Supervolcano Anyway?

Spoiler alert – there’s no such thing as a supervolcano.

Supervolcanoes, as we colloquially know them, normally have a few common characteristics, including a massive cauldron-like crater (a “caldera”) and a vast magma source. Generally speaking, this moniker is casually attached to volcanoes that produce highly infrequent and intensely explosive blasts registering at the upper end of the Volcanic Explosivity Index (VEI).

And what, pray tell, is the VEI? Devised by a pair of inventive volcanologists back in 1982, it’s the only numerically standard way to define how “explosive” an eruption was by looking at a few criteria, including the ash plume height, the amount of volcanic material ejected, and how often this type of eruption occurs.

It’s not a perfect scale, but basically, VEI 0-1 events happen continuously all the time (see: Hawaii’s Kilauea) and produce lava slowly over time. They are almost never explosive. On the other end of the scale, VEI 7-8 events produce city-to-country-sized amounts of volcanic debris, very rapidly, once every 1,000 to  50,000 years.

In the last 36 million years, there have been 42 VEI 8 eruptions. Of these, some are considered to be super-eruptions made by supervolcanoes, while prolonged, continental outbursts of lava (see: Deccan Traps) do not seem to make the cut with most volcanologists.

So, yeah – pretty vague. It’s a term that’s essentially been popularized by both the media and organizations like the United States Geological Survey (USGS).

^{The Great Prismatic Spring in Yellowstone National Park. Lorcel/Shutterstock}

Hide and Seek

All things considered, here’s a selection of the world’s potentially dormant (not extinct) supervolcanoes, those capable of producing incredibly violent, voluminous eruptions:

1 – Yellowstone Caldera, Wyoming

2 – Lake Toba, Indonesia

3 – Taupo, New Zealand

4 – Campi Flegrei, Italy

5 – Long Valley Caldera, California

6 – Valles Caldera, New Mexico

7 – Aira caldera, Japan

So if you’re based in the US, your descendants might be in a spot of bother.

The Classic Caldera

Now comes the tricky part of working out which one is more likely to destroy the world – or get near enough. Place your bets now!

In order to do this, we need to look at the eruption history of these monsters. So let’s start with the classic, Yellowstone. This caldera is 72 kilometers (45 miles) across – so large that you can only really see it from space.

The three eruptions at Yellowstone – 2.1 million, 1.3 million, and 640,000 years ago – formed distinct, interwoven calderas, with the latest being named the Yellowstone Caldera. The most powerful of the three was the first, registering at a VEI 8, which produced 2,500 times the volume of volcanic debris as the 1980 cataclysm at Mount St. Helens.

The least powerful blast, still a VEI 6-7, was the second, with the most recent being the second-most powerful, another VEI 8. Smaller, crater-forming, and lava-effusing blasts have happened, but one day, at some point, another super-eruption will likely take place.

Speaking of which, if you’ll peek at those dates, you might have noticed that it erupts once every 660,000-800,000 years, which suggests that the next blast will take place around 50,000 years from now. Some scientists, however, think Yellowstone’s already overdue for another blast by around 20,000 years, but there’s not enough data to say either way.

Today, the magma source under Yellowstone is unbelievably massive. For years, geophysicists thought there was just a shallow, fairly sizable magma cache under the National Park – but there’s actually another, deeper one that was discovered in 2015. In total, there’s enough molten rock down there to fill up to 14 Grand Canyons right up to the brim.

When it erupts, pretty much the entire chamber will empty out onto the surface in an explosive decompression event. Pyroclastic flows will essentially wipe out anything in the National Park, but the real danger to the nation – and the world – is the ash fallout.

^{Yellowstone's former ash fallouts, mapped. USGS}

Based on the ancient eruptions, thousands upon thousands of cubic kilometers of ash will blanket about 60-70 percent of the US within a day or two, with much of this area ending up smothered in a layer of ash 1 meter (3.3 feet) thick. This will bring agriculture to a grinding halt, cause millions of buildings to collapse under the weight, and cause millions of people to suffer from respiratory problems, perhaps fatally.

It will be a disaster for the US, a near-apocalyptic catastrophe. There’s also a good chance that we’d lose a few years or decades of summer, as the sulfur particles emitted during the eruption will block out the Sun to some degree for as long as they linger in the once-blue skies.

However, as the USGS note, the last 20 eruptions have been lava effusions, which will only reach the boundaries of the National Park. In fact, the yearly probability of another caldera-forming eruption can be approximated as one in 730,000 or 0.00014 percent. That’s really quite low.

A super-eruption is likely to be tens of thousands of years away yet, and even a lava flow will not take place for several centuries. There’s just no sign that the magma chamber is restless right now – it’s still slowly just filling up, and biding its time.

Home of the Brave

^{The sky above supervolcanoes often turn red for days or weeks after they unleash their fury. R.T.Wohlstadter/Shutterstock}

So what of the others?

Let’s stick to the US for now. The Long Valley Caldera “super-erupted” and formed its caldera 760,000 years ago. It was less powerful than the Yellowstone event, and it’s since been involved in plenty of major eruptions, but not anything like its original explosion.

Although another super-eruption is possible – as the magma system beneath it is still definitely active – the chances are that there will be smaller eruptions taking place here in the next few hundred years or so, but nothing major.

Valles is similar, having super-erupted 1.25 million years ago, formed a caldera, and experienced some concerning activity, but it has not super-erupted since. Despite the fact that there’s clearly an active magma source beneath it, it’s likely to be a shadow of its former self, and there’s no sign of an impending super-eruption in the future.

So in terms of the US, Yellowstone’s the Big Bad – it’s the most likely to super-erupt next, while the other two may never reach VEI 7-8 ever again. But what about the rest of the world?

We can take the beautifully viridian Aira caldera out of the equation straight away. Although this vast ancient bowl did have a rather violent past - although arguably not reaching the VEI 8 level of paroxysm - all evidence points towards Mount Sakurajima, a stratovolcano nested within it, as the sole source of its output these days – and although this will one day cause trouble for the nearby, moderately-sized Kagoshima City, it will never change the world.

So what about Taupo? Now a gorgeous crater lake, it is also responsible for two of the world’s most violent eruptions. Around 1.25 to 1 million years ago, it erupted so powerfully that much of New Zealand’s North Island was completely covered in hot ash.

Unusually for supervolcanoes, though, its most powerful eruptions took place after its initial formation. Around 26,500 years ago, the so-called VEI 8 Oruanui eruption produced pyroclastic flows so extensive that they buried the North Island beneath 200 meters of it (about 660 feet). Significant ash fallout spread around much of the regional Pacific Ocean.

If the same happened again today, it would kill most of the 3.6 million people that live there.

^{Lake Taupo, as seen from space. NASA}

Then, another blast during the year 180, although “just” a VEI 7 blast, produced similarly violent pyroclastic flows that covered an area equivalent to 25 New York Cities. The eruption produced so much ash, the skies over Italy and China turned red.

There have been plenty of blasts in between these dates, and the next eruption at the site is expected to be mildly explosive, but it’ll likely only harm those in the region of the lake. New Zealand authorities note that only three eruptions since the Oruanui event have produced pyroclastic flows.

There is no simple pattern to these eruptions that can give volcanologists any idea as to when it will erupt or even super-erupt again, if at all. Taupo is just too unpredictable, but if it did induce another caldera-forming eruption, it would essentially be the Yellowstone of the Southern Hemisphere in terms of its destructive ability.

The Final Two

Indonesia’s Toba made its debut 1.5 million years ago, but that’s not the eruption we’re interested in here.

Around 73,000 years ago, a colossal blast produced a caldera 100 kilometers (62 miles) long. This eruption manufactured so much volcanic material that the world was thought to have been plunged into a six-year-long volcanic winter.

Within days, South Asia was smothered by an ash layer 15 centimeters (6 inches) deep, with closer areas being buried in ash and pyroclastic flow deposits hundreds of meters deep.

^{Resurgence at Toba. OregonStateCEOAS via YouTube}

This was not just a VEI 8 event. This was the largest volcanic eruption in the last 2.5 million years, and for some time it was thought that it almost brought humanity to the point of extinction – although this has since been questioned.

Worryingly, there is a magma source beneath Toba today that’s the same size as the one beneath Yellowstone, and it’s clearly dynamic – the center of Lake Toba is rising skywards, indicating that the magma beneath is expanding outwards.

Around 100,000 people live on the island at the heart of Lake Toba, and a super-eruption would doom them and be a long-term disaster for the wider region. However, as there has only been one super-eruption at the site, scientists can’t predict when or if it will super-erupt again.

Still, there’s been no activity in modern times to suggest the magma is about to burst forth, so it probably isn’t worth worrying about at the moment.

Then, there’s Campi Flegrei, under the Bay of Naples. It’s a bit of a baby, having carved out its caldera 40,000 years ago during a VEI 6-7 blast. By the USGS definition, this barely makes it on the list as a supervolcano. Nowadays, the magma beneath this beast seems to mostly come out of Vesuvius, which hasn’t killed many people since 1631.

If you think this one is an underdog, though, you might want to think again.

Firstly, a million people live inside its crater, so any super-eruption would instantly kill them. The surrounding region is also densely packed with people, so any massive eruption would quickly bury a good chunk of Italy’s population, while the ash cloud would likely suffocate a good portion of Europe.

Secondly, the entire caldera keeps swelling up and deflating, and scientists can’t be sure why. Currently, the ground is rising upwards at a noticeable tick, but between 1982 and 1984, the ascension rate was 24 times that of the present. Back then, volcanologists thought it was magma pushing at the roof of the chamber, but it’s likely this was expanding gas instead.

Either way, something is going on down there that indicates an active magmatic system exists. A recent study noted that it’s entering a “critical state” where it could be ready for an eruption. With only one caldera-forming event to base this on, no one can say for sure when it will super-erupt again – but it’s likely that it will.

The Final Countdown

^{Campi Flegrei is hidden beneath the Bay of Naples, pictured here. Right at the center of this image, you can see the much more famous Vesuvius. NASA/JPL-Caltech}

Predicting when the world will experience another super-eruption is near-impossible thanks to their rarity. Even those with patterns of eruption may not obey them in the future.

However, if we had to pick the most dangerous supervolcano on the planet, we’d have to say Campi Flegrei. Despite it’s less violent résumé, any eruption there would be a true calamity of the highest order. Ultimately, it’s not about size after all.

As for the rest, consider them wildcards.

Remember, boys and girls, that there is also a chance that the next super-eruption will be the one to form a brand new caldera somewhere else in the world. Maybe, just maybe, the most dangerous supervolcano in the world doesn’t even exist yet.