Yellowstone Caldera

The Yellowstone Caldera is a volcanic caldera and supervolcano in Yellowstone National Park in the Western United States. It is sometimes referred to as the Yellowstone Supervolcano. The caldera and most of the park are in the northwest corner of Wyoming. 
The caldera is about 55 by 72 km. It was formed during the last of the three supereruptions that occurred over the last 2.1 million years.
Volcanism at Yellowstone is relatively recent. The calderas lie over a hotspot under the Yellowstone Plateau. Light and hot magma (molten rock) from the mantle rises toward the surface. The hotspot is much deeper than the terrain: it is stationary, while the North American Plate moves west-southwest over it.
Calderas appear to move, but this is an illusion. The plates of the Earth move, and hot spots stay in the same place (see Hawaiian Islands). As the Americas moved west, they ran over one or two hot spots. This is the one now in Wyoming.