Density variations between magma and the rock surrounding it means that the lava within the supervolcano’s caldera can produce big enough forces to break through the earth’s crust, allowing the molten rock and ash to erupt from the surface, the scientists said.

“The difference in density between the molten magma in the caldera and the surrounding rock is big enough to drive the magma from the chamber to the surface,” said Jean-Philippe Perrillat of the National Centre for Scientific Research in Grenoble.

“The effect is like the extra buoyancy of a football when it is filled with air underwater, which forces it to the surface because of the denser water around it,” Dr Perrillat said.