Everything in the natural world can be explained mathematically, from why Mt. Everest is the height and shape it is, to why galaxies and planets move the way they do, to the likelihood of an earthquake having a given strength. For volcanoes, there's a mathematical relationship between the frequency and magnitude of an eruption that goes a little like this: the more magma is involved in your eruption, the less likely it is to occur. Just like with earthquakes and many other natural disasters, small events are incredibly common, while bigger and bigger events become increasingly less likely.

For almost all of the volcanic eruptions we've ever seen, this relationship holds. But when researchers consider supervolcanoes—behemoths like the one beneath Yellowstone that can spew out up to 120 cubic miles of magma—the mathematical rule breaks down. Supervolcanic eruptions are so rare that the relationship that explains all the other eruptions can't explain how rare they are.

In two new studies, both published in Nature Geoscience, researchers explain that regular volcanic eruptions formulas don't cover supervolcanoes because supervolcanoes are not just really, really big volcanoes—they're fundamentally different things, driven by different forces.

Small volcanic eruptions (the ones we're used to, even dramatics examples, like Mount St. Helens' 1980 eruption) are typically triggered when molten magma burbles up from deep within the Earth into the volcano's magma chamber, explains Alexandra Witze for Nature. This causes an increase in pressure, and when the pressure climbs high enough, the volcano erupts.

Supervolcanoes, however, can't be triggered that way. Big pools of magma in a supervolcano heat up the surrounding rock, making it less likely to get overpressured and pop. Instead, supervolcanoes' magma chambers steadily fill, the pool of magma growing larger and larger. Magma, however, is lighter than the surrounding rock, and this buoyancy puts pressure on the top of the magma chamber. When there's enough magma in the chamber, the top of the chamber cracks open and the devastating flood of magma comes bursting out.

The downside of this realization, says the BBC, is that it means supervolcanoes are able to erupt because of their sheer size alone– if the magma chamber is full enough, then an eruption is right around the corner. On the other hand, knowing that supervolcanoes operate under a different set of rules than regular volcanoes means that we might be better able to predict when one is getting close, maybe even giving us a chance to do something about it.

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