The supervolcano

Yellowstone National Park is a geological masterpiece of hot springs and active geysers, but beneath lies a sleeping giant. 


When transfixed by the geothermal features of Yellowstone National Park, it is easy to forget what powers it all. The sights can be so beautiful they are spellbinding: the spectacular hot springs, the geysers, the lava formations, the fumaroles; not to mention the wildlife of grizzly bears, wolves and bison; the scenic landscape of wild forests, majestic waterfalls and large canyons.

Indeed, Yellowstone is easily among earth’s greatest geological treasures, and has been recognised as such. The 9,000-square-kilometre area (3,478 square miles) was the very first national park of the United States, created in 1872, with UNESCO listing it in 1978. It contains half of the world’s geothermal features, including the largest concentration of geysers. There are fumaroles: spots so hot that water fizzes off into steam once hitting them. There are colourful hot springs. And there are mammoth terraces: a variation of a hot spring, where hot water ascends through ancient limestone deposits, creating terraces of similar appearance to a frozen waterfall.

Such features are there for all to see. What is not is the giant volcano beneath. Down in the deep, a giant chamber of magma powers the park’s geothermal activity, which mainly arises through the combination of hot water and rocks clashing with cold water streams running down from the mountains.


Some 640,000 years ago, a giant eruption took place here. An unfathomable amount of ash was spewed into the air. In the aftermath, fragments of lava heightened the landscape. In the centre, above where the underlying magma chamber had just been emptied, the ground sank by a few hundred metres. This is today called the ‘Yellowstone caldera’, and is where the national park is now based. There have been no further eruptions. But, as the activity on the surface shows, the volcano is still alive.

In this way, Yellowstone sits atop of a ticking geological time bomb. And it is one of unimaginable devastation. Through the deposits from two earlier eruptions, the oldest some 2.1 million years ago, geologists can make educated guesses of how large a new eruption could be. To demonstrate, Michael Branney, senior lecturer in volcanology at the University of Leicester, compares Yellowstone’s oldest eruption to that of Mount Pinatubo in 1991, which plunged the Philippines into chaos. “There were more than half a million people evacuated from that eruption, and that had a volume of about five to eight cubic kilometres,” he says. “Then you compare that to Yellowstone, which had a volume of more than 2,000 cubic kilometres.”


One can see why scientists call it a supervolcano. According to Ivan Savov, lecturer in subduction zone geochemistry at the University of Leeds, another eruption would cause scenes akin to those in a Hollywood blockbuster. It would affect not only the United States, but Europe too. “We’re talking tens of centimetres of ash all the way to the American coast,” he says. “That means no traffic, no crops for a few years, and consequently no animals surviving. It would have a global economic impact. This is what we can imagine.”

Luckily, volcanoes do not erupt to a clockwork-like schedule, and there is no reason to think that Yellowstone will erupt in the immediate future. That said, the underlying magma chamber has been active since measurements started, and scientists agree that another outburst will happen at some point. The timing, however, is difficult to predict. “People sometimes don’t fully appreciate the timescales involved,” says Branney. “When geologists talk about something that is likely to happen ‘in the future’, the human race might well have died out before that. So you have to get things into perspective.”

Photos: Julie Lubick, Lorcel, Robynrg [all via].