Volcanism
Yellowstone lies over a hotspot where light, hot, molten mantle rock rises towards the surface. While the Yellowstone hotspot is now under the Yellowstone Plateau, it previously helped 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.
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 they cause the overlying land to collapse into the emptied magma chamber, forming a geographic depression called a caldera. Calderas formed from explosive supereruptions can be as wide and deep as mid- to large-sized lakes and can be responsible for destroying broad swaths of mountain ranges.
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 is 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 (30.48 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. Within the past 17 million years, 142 or more caldera-forming eruptions have occurred from the Yellowstone hotspot.
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. The three super eruptions occurred 2.1 million, 1.3 million, and 640,000 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 (640,000 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 (SRP-Y) hotspot that is plainly visible today.
Non-explosive eruptions of lava and less-violent explosive eruptions have occurred in and near the Yellowstone caldera since the last supereruption. The most recent lava flow occurred about 70,000 years ago, while a violent eruption excavated the West Thumb of Lake Yellowstone around 150,000 years ago. Smaller steam explosions occur as well; an explosion 13,800 years ago left a 5 kilometer diameter crater at Mary Bay on the edge of Yellowstone Lake (located in the center of the caldera). Currently, volcanic activity is exhibited via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser, plus recorded ground swelling indicating ongoing inflation of the underlying magma chamber.
The volcanic eruptions, as well as the continuing geothermal activity, are a result of a great cove of magma located below the caldera's surface. The magma in this cove contains gases that are kept dissolved only by the immense pressure that the magma is under. If the pressure is released to a sufficient degree by some geological shift, then some of the gases bubble out and cause the magma to expand. This can cause a runaway reaction. If the expansion results in further relief of pressure, for example, by blowing crust material off the top of the chamber, the result is a very large gas explosion.
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