<P> Volcanism at Yellowstone is relatively recent, with calderas that were created during large eruptions that took place 2.1 million, 1.3 million, and 630,000 years ago . The calderas lie over a hotspot where light, hot, magma (molten rock) from the mantle rises toward the surface . While the Yellowstone hotspot is now under the Yellowstone Plateau, it did help to create the eastern Snake River Plain (to the west of Yellowstone) through a series of huge volcanic eruptions . The hotspot appears to move across terrain in the east - northeast direction, but in fact the hotspot is much deeper than terrain and remains stationary while the North American Plate moves west - southwest over it . </P> <P> Over the past 18 million years or so, this hotspot has generated a succession of violent eruptions and less violent floods of basaltic lava . Together these eruptions have helped create the eastern part of the Snake River Plain from a once - mountainous region . At least a dozen of these eruptions were so massive that they are classified as supereruptions . Volcanic eruptions sometimes empty their stores of magma so swiftly that the overlying land collapses into the emptied magma chamber, forming a geographic depression called a caldera . </P> <P> The oldest identified caldera remnant straddles the border near McDermitt, Nevada--Oregon, although there are volcaniclastic piles and arcuate faults that define caldera complexes more than 60 km (37 mi) in diameter in the Carmacks Group of southwest - central Yukon, Canada, which are interpreted to have formed 70 million years ago by the Yellowstone hotspot . Progressively younger caldera remnants, most grouped in several overlapping volcanic fields, extend from the Nevada--Oregon border through the eastern Snake River Plain and terminate in the Yellowstone Plateau . One such caldera, the Bruneau - Jarbidge caldera in southern Idaho, was formed between 10 and 12 million years ago, and the event dropped ash to a depth of one foot (30 cm) 1,000 miles (1,600 km) away in northeastern Nebraska and killed large herds of rhinoceros, camel, and other animals at Ashfall Fossil Beds State Historical Park . The United States Geological Survey ("USGS") estimates there are one or two major caldera - forming eruptions and 100 or so lava extruding eruptions per million years, and "several to many" steam eruptions per century . </P> <P> The loosely defined term "supervolcano" has been used to describe volcanic fields that produce exceptionally large volcanic eruptions . Thus defined, the Yellowstone Supervolcano is the volcanic field which produced the latest three supereruptions from the Yellowstone hotspot; it also produced one additional smaller eruption, thereby creating West Thumb Lake 174,000 years ago . The three supereruptions occurred 2.1 million, 1.3 million, and approximately 630,000 years ago, forming the Island Park Caldera, the Henry's Fork Caldera, and Yellowstone calderas, respectively . The Island Park Caldera supereruption (2.1 million years ago), which produced the Huckleberry Ridge Tuff, was the largest, and produced 2,500 times as much ash as the 1980 Mount St. Helens eruption . The next biggest supereruption formed the Yellowstone Caldera (~ 630,000 years ago) and produced the Lava Creek Tuff . The Henry's Fork Caldera (1.2 million years ago) produced the smaller Mesa Falls Tuff, but is the only caldera from the Snake River Plain - Yellowstone hotspot that is plainly visible today . </P>

When was it discovered that yellowstone was a volcano