Yellowstone is has the largest and most diverse collection of natural thermal features in the world. The Yellowstone Caldera is the largest volcano in North America. It is classified as a super volcano. The last eruption in Yellowstone 640,000 years ago was 1,000 times larger than the eruption of Mount St. Helens.
During the month of October 2009, 218 earthquakes were located in Yellowstone National Park, with the largest one measuring a magnitude of 2.5.
Because of the geological activity, Yellowstone has some of the most famous geysers in the world including Old Faithful, Castle Geyser, Lion Geyser, Beehive Geyser, and Steamboat Geyser which is the tallest geyser in the world. Unfortunately, Yellowstone Park's volcano is expected to erupt again sometime in the far future. It is expected that this eruption will be devastating - 1,000 times worse than Mount St. Helens.
Below: Path of the Yellowstone hot spot over the past 15 million years. Numbers indicate millions of years.