The term supervolcano was first used in a TV documentary to describe eruptions of more than 1,000 cubic kilometres of magma. Such volcanoes are devastating, but very rare. The last one happened at Toba in Sumatra, Indonesia, 74,000 years ago.
The active volcanic system at Yellowstone National Park has resulted in super-eruptions - the most recent happened 640,000 years ago. Yellowstone's volcanism is powered by magma rising from a hotspot. The relatively thick continental crust causes large amounts of magma to build up underground.
Image: Grand Prismatic Spring, one of the many thermal springs in the volcanically active Yellowstone National Park (credit: Douglas Faulkner/SPL)
Yellowstone is one of a small number of active supervolcanoes.
The volcanic system beneath Yellowstone is huge and may one day cause a super-sized eruption of ash and lava, as it has in the past. However, such an eruption is extremely unlikely to happen anytime soon.
How big is the magma chamber at Yellowstone?
Seismologists study how earthquake waves travel through the Earth to map the huge magma chamber below Yellowstone National Park.
Scientists find evidence of a supervolcano's devastating power.
Archaeologists working in Orchard, Nebraska, found hundreds of fossilised prehistoric animals that were killed by ash from a volcanic eruption. When they traced the source of the ash, they discovered that it was a supervolcano 1,600km away in Bruneau Jarbridge, Idaho. The hotspot that powered the Bruneau Jarbridge eruption 12-10.5 million years ago caused a trail of prehistoric super-eruptions as the North American plate moved over it.
A supervolcano is any volcano capable of producing a volcanic eruption with an ejecta volume greater than 1,000 km3 (240 cu mi). This is thousands of times larger than normal volcanic eruptions. Supervolcanoes can occur when magma in the mantle rises into the crust from a hotspot but is unable to break through the crust, and pressure builds in a large and growing magma pool until the crust is unable to contain the pressure (this is the case for the Yellowstone Caldera). They can also form at convergent plate boundaries (for example, Toba).
Although there are only a handful of Quaternary supervolcanoes, supervolcanic eruptions typically cover huge areas with lava and volcanic ash and cause a long-lasting change to weather (such as the triggering of a small ice age) sufficient to threaten species with extinction.