Yellowstone is the site of a huge ancient volcano, whose remains are called the Yellowstone Caldera. The caldera is the largest volcanic system in North America.

The cause of the volcano was a hotspot in the Earth's mantle, over which the American continental plate moved. 640,000 years ago a huge volcanic explosion blew 240 cubic miles (1000 km2) of rock and lava into the air. It was 1000 times larger than the Mount St. Helens eruption.[3]

Yellowstone is still geologically active, and is monitored by the U.S. Geological Survey. There is ground movement, geysers and swarms of small earthquakes. The floor of the caldera has been rising, but experts say there is no immediate danger of eruption.[4]