A supervolcano is a volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for such an eruption is greater than 1,000 cubic kilometers (240 cubic miles).
Location of Yellowstone hotspot over time. Numbers indicate millions of years before the present.
Satellite image of Lake Toba, the site of a VEI 8 eruption c. 75,000 years ago
Supervolcanoes occur when magma in the mantle rises into the crust but is unable to break through it and pressure builds in a large and growing magma pool until the crust is unable to contain the pressure. This can occur at hotspots (for example, Yellowstone Caldera) or at subduction zones (for example, Toba).
The term "supervolcano" was first used in a volcanic context in 1949.[note 1]
Its origins lie in an early 20th-century scientific debate about the geological history and features of the Three Sisters volcanic region of Oregon in the United States. In 1925, Edwin T. Hodge suggested that a very large volcano, which he named Mount Multnomah, had existed in that region.[note 2] He believed that several peaks in the Three Sisters area were the remnants of Mount Multnomah after it had been largely destroyed by violent volcanic explosions, similar to Mount Mazama. In 1948, the possible existence of Mount Multnomah was ignored by volcanologist Howel Williams in his book The Ancient Volcanoes of Oregon. This book was reviewed in 1949 by another volcanologist, F. M. Byers Jr. In the review, Byers refers to Mount Multnomah as a supervolcano.
More than fifty years after Byers' review was published, the term supervolcano was popularised by the BBCpopular science television program Horizon in 2000, referring to eruptions that produce extremely large amounts of ejecta.
Eruptions that rate VEI 8 are termed "super eruptions". Though there is no well-defined minimum explosive size for a "supervolcano", there are at least two types of volcanic eruptions that have been identified as supervolcanoes: large igneous provinces and massive eruptions.
Large igneous provinces
Map of large Flood Basalt igneous provinces worldwide
Large igneous provinces, such as Iceland, the Siberian Traps, Deccan Traps, and the Ontong Java Plateau, are extensive regions of basalts on a continental scale resulting from flood basalt eruptions. When created, these regions often occupy several thousand square kilometres and have volumes on the order of millions of cubic kilometers. In most cases, the lavas are normally laid down over several million years. They release large amounts of gases.
The Réunion hotspot produced the Deccan Traps about 66 million years ago, coincident with the Cretaceous–Paleogene extinction event. The scientific consensus is that a meteor impact was the cause of the extinction event, but the volcanic activity may have caused environmental stresses on extant species up to the Cretaceous–Paleogene boundary. Additionally, the largest flood basalt event (the Siberian Traps) occurred around 250 million years ago and was coincident with the largest mass extinction in history, the Permian–Triassic extinction event, although it is unknown whether it was solely responsible for the extinction event.
Such outpourings are not explosive, though lava fountains may occur. Many volcanologists consider Iceland to be a large igneous province that is currently being formed. The last major outpouring occurred in 1783–84 from the Laki fissure, which is approximately 40 km (25 mi) long. An estimated 14 km3 (3.4 cu mi) of basaltic lava was poured out during the eruption (VEI 4).
Volcanic eruptions are classified using the Volcanic Explosivity Index, or VEI. It is a logarithmic scale, which means that an increase of one in VEI number is equivalent to a tenfold increase in volume of erupted material. VEI 7 or VEI 8 eruptions are so powerful that they often form circular calderas rather than cones because the downward withdrawal of magma causes the overlying rock mass to collapse into the empty magma chamber beneath it.
Known super eruptions
Based on incomplete statistics, at least 60 VEI 8 eruptions have been identified. Below is a list of well-known super-eruptions.
VEI 8 eruptions have happened in the following locations.
^The term entered the English language already in a 1925 book, Conquering the World by Helen Bridgeman, about Indonesia to refer to an Indian Ocean sunset. 
^Subsequent research proved that each peak of the Three Sisters was formed independently, and that Mount Multnomah never existed.
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Overview and Transcript of the original BBC program
Yellowstone Supervolcano and Map of Supervolcanoes Around The World
USGS Fact Sheet – Steam Explosions, Earthquakes, and Volcanic Eruptions – What's in Yellowstone's Future?
Scientific American's The Secrets of Supervolcanoes
Supervolcano eruption mystery solved, BBC Science, 6 January 2014