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What Would Happen if a Supervolcano Erupted?

Krakatau Volcano erupting at night
The volcano Krakatau erupting at night

On 27 August 1883, humans heard the loudest sound in recorded history. It was so loud that, despite being made in Indonesia, the sound was heard by people in Australia. It was the sound of our planet unleashing its fury on the island of Krakatoa, during a series of volcanic eruptions which destroyed most of the island, triggered vast tidal waves and claimed well over 36,000 lives, with some estimates putting the figure far higher.

The warning signs had been there, with volcanic activity steadily intensifying in the days leading up to the deadly denouement. Black clouds of ash had filled the sky, like a Biblical vision of the end times. Then, on the 27 August, came a series of titanic, inconceivably loud explosions, rupturing the eardrums of sailors on ships more than 40 miles away.

An aerial view of Krakatoa
An aerial view of Krakatoa

The volcanic ash scattered throughout the atmosphere actually changed the colour of the sky. Over in London, the poet Gerard Manley Hopkins wrote of how the post-Krakatoa sunsets resembled “inflamed flesh”. It’s even thought the intense background of Munch’s The Scream painting is a realistic depiction of how the skies looked in the wake of the eruption.

Krakatoa was, in other words, a Pretty Major Event. Like the eruption of Vesuvius in AD 79, which ravaged the Ancient Roman city of Pompeii, it was a brutal demonstration of what volcanoes are capable of doing.

But what if it was far, far worse? What if there was a force that had the potential to be many times greater, and more catastrophic, than even these eruptions? Such a thing does exist: the supervolcano.


Krakatoa and Vesuvius are examples of “stratovolcanoes”. Think the classic, cone-shaped type that a child might come up with if you asked them to draw a volcano

Supervolcanoes, on the other hand, are a completely different beast. Instead of jutting into the sky like a pointy mountain, a supervolcano lies hidden, within a sprawling sunken crater known as a caldera, formed by a previous massive eruption, under which a vast simmering sea of magma builds and builds, until something gives.

You can hike across the caldera of a supervolcano and have no idea of the apocalyptic forces massing beneath your feet. Indeed, the most famous supervolcano happens to also be a major tourist attraction in the United States: Yellowstone National Park.


Yellowstone is a grand landscape of mountains, rivers, wildlife-filled forests and whooshing geysers like the iconic Old Faithful. These geysers are the only real clue as to what’s cooking under the Earth’s crust in this beautiful region. In recent years, it’s been discovered there’s even more molten rock beneath Yellowstone than we once thought: enough to fill the Grand Canyon about 11 times

"The supervolcano threat is substantially greater than the asteroid or comet threat"

The Yellowstone region last suffered a major eruption around 630,000 years ago. According to Dr Robert B. Smith, an expert on supervolcanoes, if Yellowstone was to blow today, “devastation would be complete and incomprehensible”. In case that’s not alarming enough, Brian Wilcox – a NASA researcher who helped study ways to protect us from extra-planetary hazards – says that “the supervolcano threat is substantially greater than the asteroid or comet threat”.

So what would actually happen? There would be the initial incineration of the surrounding region, thanks to the out-pouring of lava and super-heated gas. But the really world-changing effects would come from the thick, black ash that would be thrown across the United States like an infernal blanket. In the case of a Yellowstone super-eruption, vast stretches of Idaho, Wyoming, Montana and other states would be smothered.

Not only would countless numbers of people die, their lungs ripped open by the sharp ash particles, but gases released from the Earth would also combine with the atmosphere to lower temperatures, in the manner of a nuclear winter. Crops would fail, technology would fail and there would likely be an economic crash on an unprecedented scale.

Would humanity survive? Probably, yes. But whether civilisation would is an entirely different matter. Suddenly, even Krakatoa and Vesuvius seem like small-fry by comparison.