The volcano at Yellowstone National Park is called Yellowstone Caldera, and is a super volcano. The caldera measures 35 by 45 miles. It formed as a result of the last of three super eruptions that occurred in the last 2.1 million years.
The calderas lie over a hotspot where light, hot molten rock from the mantle rises toward the surface. This particular hotspot has erupted many times over the last 18 million years, shaping and creating much of the Snake River and other landmarks. The volcanic nature of the region is also known to cause many earthquakes, with at least a thousand being readable every year, most of which cannot be felt.
As of 2015, scientists see no evidence that another cataclysmic eruption like the ones that occurred millions of years ago will occur at Yellowstone in the foreseeable future, but the volcano is under constant surveillance. Nothing can be done to prevent an eruption, but scientists say another eruption isn't likely for the next 10,000 years. In the case of an eruption, it is predicted that the volcano could produce more than 240 cubic miles of lava, which is the basic requirement to be classified as a super volcano. The pure heat from the last eruption still powers the park's geysers today.