Supervolcano is the popular term for a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They are the most dangerous type of volcano. A supervolcano is capable of producing a volcanic eruption with ejecta greater than 1,000 cubic kilometers (240 cubic miles). This is thousands of times larger than most historic volcanic eruptions.
Volcanologists and geologists do not refer to "supervolcanoes" in their scientific work, since this is a blanket term that can be applied to a number of different geological conditions. Since 2000, however, the term has been used by professionals when presenting to the public. The term megacaldera is sometimes used for calderas with supervolcano characteristics, such as the Blake River Megacaldera Complex in the Abitibi greenstone belt of Ontario and Quebec, Canada. Eruptions that rate VEI 8 are termed "super eruptions".
Supervolcanoes occur when magma in the Earth rises into the crust from a hotspot but is unable to break through the crust. Pressure builds in a large and growing magma pool until the crust is unable to contain the pressure. This kind of eruption is typically sufficient to cause a long-lasting change to weather (such as the triggering of an ice age) sufficient to threaten the extinction of species, and cover huge areas with lava and ash.
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. 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.
Eruptions resembling supervolcanoes have been identified by the Voyager spacecraft on the moons of Jupiter and Saturn. The outer solar system volcanoes are mostly cryovolcanoes, not magma volcanoes. Theoretically a supervolcano could be a cryovolcano, but none has been found within the solar system so far.
The Discovery Channel highlighted six known supervolcanoes:
Yellowstone - Wyoming
Long Valley - Eastern California
Valles Caldera - northern New Mexico
Lake Toba - North Sumatra, Indonesia
Taupo Volcano - North Island, New Zealand
Aira Caldera - Kagoshima Prefecture, Kyushu, Japan.
Yellowstone Has Bulged as Magma Pocket SwellsNational Geographic - January 19, 2011
Yellowstone National Park's supervolcano just took a deep "breath," causing miles of ground to rise dramatically, scientists report. The simmering volcano has produced major eruptions three times in the past 2.1 million years. Yellowstone's caldera, which covers a 25- by 37-mile (40- by 60-kilometer) swath of Wyoming, is an ancient crater formed after the last big blast, some 640,000 years ago. Since then, about 30 smaller eruptions including one as recent as 70,000 years ago have filled the caldera with lava and ash, producing the relatively flat landscape we see today.
In the News ...
New Supervolcano Found in Hong Kong National Geographic - September 6, 2012
Much of Hong Kong and its Victoria Harbour, including this rocky islet, lies atop the remnants of an ancient supervolcano with an 11-mile (18 kilometer) wide caldera, Chinese authorities revealed last week. But even though the volcano has produced some gigantic eruptions, it poses no risk to today's millions of Hong Kong residents. It's been extinct since it last erupted and largely collapsed into the ocean 140 million years ago, Hong Kong's Geotechnical Engineering and Development Department said.