What Happens When a Supervolcano Erupts?
Supervolcanoes are capable of causing a global catastrophe. The last "super-eruption" was 26,500 years ago. So when will the next one be? And what is it expected to be like?
There are more than 1,000 active volcanoes on our planet. Most of them are located at the fault lines where two tectonic plates meet. When these plates are moving apart, a fissure can form and liquid rock - magma - is able to bubble up to the surface. Volcanoes can also form when the plates are moving towards each other. In that situation, one of the plates dives down beneath the other one and sinks deep into the ground. Rising temperatures and pressures then cause part of the plate itself to melt and form magma. When the volcano erupts, that magma is ejected as lava.
The volcanic scale
It is best not to be too close to a "regular" volcanic eruption, but you need to stay even further away from a supervolcano. They explode with a lot more force than Iceland's famous Eyjafjallajökull, or even Italy's infamous Vesuvius.
Volcanoes are classified using the volcanic explosivity index (VEI), which measures the size of an eruption. This is a scale from 0 to 8, taking into account the height of the smoke plume and the volume of lava, ash and debris ejected. Eyjafjallajökull achieved a VEI‑4 during its eruption in 2010. Vesuvius - when it destroyed Pompeii in 79 - was a VEI‑5. A volcano needs to have an eruption scoring of 8 to be classified as a supervolcano.
Super-eruption
No-one has ever seen a supervolcano erupt. The most recent "super-eruption" occurred at Taupō volcano in New Zealand about 26,500 years ago. The 1815 eruption of Mount Tambora in Indonesia does give us a good idea of what to expect, though. That was a VEI‑7, just short of qualifying as a supervolcano, but its effects were still felt all over the world. A giant cloud of ash blotted out the sunlight, heralding a year in which there was no summer, followed by a series of crop failures and famine throughout the world.
Yellowstone
One of the most famous of all supervolcanoes is located underneath Yellowstone National Park in the United States. The last super-eruption was 640,000 years ago. At that time a thousand cubic kilometres of volcanic material was suddenly thrown into the air. It is barely imaginable what that would mean for today's inhabited America, not to mention its effects on the whole world. So much smoke and ash is released into the atmosphere when a supervolcano erupts that temperatures can easily fall by ten degrees and then remain low for years. That is why supervolcanoes are ranked among the top three most catastrophic global natural disasters.
Low probability
Fortunately, the likelihood of a "super-eruption" like this is very low. The general trend of the volcanic explosivity index is: the larger the eruption, the less frequently it occurs. Dozens of small volcanoes erupt in the world each year, but scientists have "only" identified 42 super-eruptions in the past 36 million years. The United States Geology Survey, which closely monitors the supervolcano beneath Yellowstone, says the annual probability of an eruption is 0.00014%. That's about the same probability as an asteroid one kilometre in diameter hitting the Earth.