Supervolcanoes can create devastating eruptions and repeat them again and again—one way is through vast, multi-level reservoirs of magma that generate impossibly hot water.

A team from the Russian Academy of Sciences studying Toba—a supervolcano in Indonesia which erupted 75,000 years ago—have come up with a new model that is also consistent with Yellowstone in the United States. They used seismic mapping to look deep into the earth beneath the volcano, and detected that magma had formed 150 kilometres below the surface, where the tectonic plate melts while sliding under its neighbour. The magma then rose to a depth of 75 kilometres, forming a huge reservoir. Water was fed into this through the surrounding fracture zone.

With a temperature of about 1300ºC, the water shoots out of the mantle and into the upper crust where it melts the rocks, creating another reservoir of partially molten rock saturated with water.

“This is a very dangerous mixture, kind of a huge bomb that is charged for hundreds of thousands of years,” says lead scientist Ivan Koulakov.

In the lead-up to an eruption, the water heats further and turns to steam, the intense pressure blowing new cracks in the crust, into which runs more boiling water, causing more melting. It’s this avalanche-like process that creates an eruption on a supervolcano scale.