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Aside from giant asteroid strikes, supervolcanoes are the most devastating of all natural disasters; they have been known in the past to cause mass extinctions and long-term climate changes. The last known supervolcano eruption, believed to have occurred around 70,000 years ago on the site of today’s Lake Toba in Sumatra, Indonesia, caused a “volcanic winter” that blocked out the sun for six to eight years. According to one theory, it may have killed more than half the world’s population at the time. The supervolcano that erupted in Wyoming 600,000 years ago, in what is now Yellowstone National Park, ejected more than 1,000 cubic meters of lava and ash into the atmosphere–enough to bury a large city several kilometers deep. By comparison, the 1991 eruption of Mount Pinatubo in the Phillipines, which caused a 0.4 degree drop in average global temperature for the following year, was 100 times less forceful than the Yellowstone eruption.

Such mammoth volcanoes don’t function the same way as normal volcanoes, which erupt when magma fills their chambers and spews through an opening when the pressure reaches a critical point. As the chambers of supervolcanoes are so much larger, they don’t get overpressurized to such a degree. For this reason, scientists have long believed that an earthquake or other tectonic event is necessary to break open the Earth’s crust and allow the magma to erupt. Now, however, an international team of researchers has concluded that a supervolcano can erupt at any time simply because of its massive size, without any outside trigger, making such cataclysmic eruptions both less predictable and more likely to occur.

According to the new study’s conclusions, published this week in Nature Geoscience magazine, supervolcanic eruptions are caused by a phenomenon called the “buoyancy effect.” This occurs when the molten magma inside the mostly underground supervolcano reaches critically high pressures due to the much denser rock around it, forcing it to the surface. In tests conducted at the European Synchrotron Radiation Facility in Grenoble, France, researchers aimed to simulate the intense pressure and heat inside the caldera of a supervolcano in order to understand how it erupted. After loading synthetic magma inside a diamond chamber, they fired high-energy X-rays inside to measure the changes that occurred as the pressure inside the chamber rose. Their experiment showed that the transition from solid to liquid magma alone creates a pressure strong enough to crack more than 10 kilometers of Earth’s crust above the caldera. When magma penetrates these cracks and rises to the surface, it will expand violently, causing an explosion.

The scientists’ conclusions mean that while such globally devastating supervolcanic eruptions occur only every 100,000 years or so, they could occur at any time. According to the Grenoble study’s lead author, Wim Malfait, “This is something that, as a species, we will eventually have to deal with. It will happen in future.” By understanding how such supervolcanoes work, the researchers hope they can predict future eruptions. There are 20 known supervolcanoes on Earth, including Lake Toba, Lake Taupo in New Zealand and the Phlegraean Fields near Naples, Italy. None are in danger of erupting in the near future, said Jean-Philippe Perrillat, another of the study’s authors, but scientists are currently working on methods to monitor the pressure of underground magma in order to predict whether an eruption becomes imminent. According to Malfait, the possibility of another supervolcano erupting is comparable to “an asteroid impact – the risk at any given time is small, but when it happens the consequences will be catastrophic.”

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