Supereruptions are among the most devastating natural disasters—spewing up more than 1000 cubic kilometers of ash and lava, sufficient to drop global temperatures by 10°C for a decade. But what causes these terrifying events? Two research teams have recreated the conditions inside a supervolcano’s magma chamber—digitally and experimentally—to answer this question. Their results, published online today in Nature Geoscience, reveal two distinct trigger mechanisms, related to the evolution of magma reservoirs. Young magma chambers tend to be smaller and in cooler settings. The injection of new magma from deeper in the crust causes increasing pressure in the chamber, forcing it to expand. These stresses can lead to fracturing, allowing magma to escape to the surface in small eruptions. As magma chambers grow in volume, however, the impact of newly injected material diminishes. At the same time, the heating and accumulated damage to the surrounding rock causes the chamber walls to develop a more viscous (rather than elastic) response to stress. Together, these lower the risk of overpressurization and eruptions become less frequent as chambers head into a storage phase, wherein the buoyancy of the magma (compared with denser rock) becomes the dominant source of pressure on the surrounding crust. This alone, the researchers discovered, can actually be enough to trigger an eruption—forcing the chamber roof to crack all the way to the earth’s surface (as seen, developing, in the picture above). Still unclear, however, is whether this must always result in a catastrophic chamber collapse (releasing all the magma in one supereruption), or whether smaller, pressure-relieving releases could occur. It is hoped that greater understanding of these volcanic triggers may lead to an improved ability to predict dangerous eruptions in the future.