The volcanic eruptions that occurred in this area are characterized by sudden outpourings of hot gas, ash, pumice, and rock. These explosions left enormous depressions, which are called calderas. The last caldera explosion occurred about 600,000 years ago and obliterated most of the physical evidence of the preceding two caldera explosions. This event is known as the Yellowstone Caldera, and it destroyed about 1,000 square miles of the central portion of present-day Yellowstone. By comparison, the 1980 eruption of Mount St. Helens blew up less than one square mile, while the largest known historical
volcanic eruption occurred in 1883 on the island of Krakatoa in Indonesia, destroying about 12 square miles.
Today it is difficult to see much of the original Yellowstone Caldera. Not only does it cover an immense area, but following the caldera explosion and continuing for the next 500,000 years, there was a series of lava flows that filled in most of the caldera. This period of lava flow ended about 75,000 years ago. Since that time the forces of water and ice have reshaped Yellowstone once again.
Yellowstone was glaciated at least three distinct times. During the last glaciation (called the Pinedale Glaciation), an enormous icefield built up in the Absaroka Mountains southeast of the present-day parkand in the Gallatin Mountains north of the park. Glaciers flowed from these icefields into the Yellowstone area. The mass of ice centered in what is today the Yellowstone Lake basin and grew to a depth of more than 3,000 feet. Eventually this ice covered about 90% of Yellowstone. By about 8,500 years ago the ice had all melted. Even though a few snowfields may persist in the highest areas of the park today, there are no glaciers in Yellowstone.
The landscape that you see in Yellowstone today is the result of both violent episodes and slow-moving processes that have occurred over thousands of years. It is often hard for us to comprehend the scope of geologic time since written human history goes back only 7,000 years and because most of us only know our family history for three or four generations. Until the fires of 1988, many repeat visitors to Yellowstone noticed very little change in the landscape. But, this is a dynamic place, and the one constant that remains true for Yellowstone is change.