There are about 1,500 potentially active volcanoes around the world. To a scientist though, a volcano counts as active if it has erupted in the last 10,000 years and only around 550 have erupted in recorded history. But the exact number is much less important than their size.


Eruptions are classified using the Volcanic Explosivity Index (VEI). A VEI of 1 results in at least 10,000m3 of material flung into the air, and each successive number corresponds to 10 times more ejected material. The largest eruption ever recorded was Mount Tambora in 1815, with a VEI of 7. The explosion could be heard 2,600km away and more than 160km3 of material was thrown into the atmosphere.

This single volcano caused temperatures in the northern hemisphere to drop by 0.5°C for six months, as dust blocked out the Sun. The year 1816 was dubbed ‘the year without a summer’ and, combined with disrupted rainfall patterns, caused harvests around the globe to fail.

That was one volcano. There are 32 known VEI 7 or VEI 8 volcanoes and if they all erupted at once it would easily be an extinction-level event for the majority of living things, including humanity.

The initial eruptions would cause most aeroplanes in the air to crash, collapse buildings in the western USA, South America, southern Europe and the Far East, and cause power grids to fail across most of the developed world. Drinking water supplies would be rendered toxic by ash, global temperatures could fall by 15°C for six months, and acid rain would make agriculture impossible for the next decade.

Luckily, this is almost guaranteed never to happen. Volcanoes are driven by localised upwellings of magma beneath the Earth’s crust. Every volcano erupting simultaneously would be like every bubble in a fizzy drink breaking the surface at once.

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Asked by: Callum Reid, Sunderland


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luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.