Eruptions from supervolcanoes - the most hazardous type of volcano on Earth - may be controlled by large, dense magma reservoirs, which keep magma stored away beneath a thick overlying crust until an eruption takes place. These findings, published in Nature Communications this week, stem from a new seismic model of the magma paths beneath the Toba supervolcano in Indonesia.

The Toba caldera supervolcano last erupted 74,000 years ago and had a devastating effect on the global climate and biosphere at the time. Despite the fact that supervolcanoes such as Toba and Yellowstone have the potential to cause dramatic eruptions with global consequences, much is still unclear about why these volcanoes are associated with such large eruptions, how such large volumes of magma are generated and why there is such a long time gap between eruptions.

Ivan Koulakov and colleagues present a new model based on seismic data which reveals a complex, multilevel magmatic plumbing system beneath Toba down to a depth of over 150 km. They find that, at a depth of around 150 km, gases and basic (low in SiO2) melts are generated from the subducting plate. These melts then rise to a depth of around 75 km creating a large basic magma reservoir. The authors note that a similar type of reservoir has also been identified at Yellowstone. Their findings indicate that the stalling of dense magma in these reservoirs beneath the overlying crust may be a key control as to why very long periods of time pass between eruptions at supervolcanoes around the world.

They further show that, only when the magma reservoir reaches a critical level of overpressure due to the trapped volatiles, the reservoir empties and the large amount of magma begins to rise through the crust, eventually leading to a large scale eruption. Although subduction continues to occur at Toba and another extremely large eruption may be likely in the future, there is no evidence to suggest that an eruption is imminent, caution the authors.