Super Volcanoes
How It Works|Issue 113

The world’s largest and deadliest volcanoes could envelop the whole planet in ash

Laura Mears

Supervolcanoes are some of the most destructive natural structures on the planet. Classified only after they have erupted, they eject more than 1,000 cubic kilometres of lava in one go. They’re a thousand times more powerful than a standard volcano and so large that they could blanket the whole Earth in ash.

The ground collapses above them when they explode, and the scars that mark their positions consume so much of the landscape that they become virtually invisible. Yet bubbling pools of magma still seethe below the surface, venting hot steam and gas through the ever-weakening crust above.

Scientists grade volcanic eruptions on a scale of 0 to 8, known as the Volcanic Explosivity Index (VEI). The tiniest volcanoes at the bottom of the scale gently dribble magma, while the behemoths at the top spit out hundreds of tons at a time. There are only around 40 of these supervolcanoes worldwide, but just ten remain potentially active. They sit atop hotspots where magma leaks up from the Earth’s mantle. Bubbles of molten rock accumulate under the ground, building pressure that stretches the Earth at its seams. Between major eruptions, pockets of heat leak out as spurts of lava, water and gas, but eventually the pressure becomes too much. The crust melts and cracks, heaving above the liquid rock below.

When supervolcanoes erupt in earnest the impact is catastrophic. Lava explodes upwards or bursts out in sheets, forming vast splatters and fast-moving lava plains. The temperature of the liquid rock can be as low as 300 degrees Celsius or as high as 1,160 degrees Celsius. It might advance slower than walking speed or vent at more than 60 kilometres per hour, and it tears through everything in its path.

Alongside the lava, supervolcanoes spew gas and ash. The heaviest particles settle within days, forming a blanket around the eruption that can be tens of centimetres thick. They smother crops and damage the eyes and lungs of animals.

Up in the sky, sulphur compounds react with the air, creating clouds that then unleash acid rain. Pollution races through the watercourse, impacting wildlife far from the source of the eruption. Tiny fragments of ash can remain airborne for months, scattering the sunlight and changing the climate across the globe.

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