The Yellowstone Caldera
The vast volcanic system beneath Yellowstone National Park, source of three of the largest eruptions known
Most visitors to Yellowstone National Park come for Old Faithful, the bison and the rainbow-colored hot springs, and many do not realize they are standing inside a volcano. The geysers and steaming pools are fed by heat from a large body of partly molten rock a few kilometers below ground. Over the past 2.1 million years the Yellowstone volcanic system has produced three enormous caldera-forming eruptions, two of them among the largest known on Earth, which is why it is often called a supervolcano. The U.S. Geological Survey stresses that another eruption of that size is very unlikely in the foreseeable future and that the most likely hazards are hydrothermal explosions and earthquakes.
The youngest caldera, formed 631,000 years ago, measures roughly 72 by 48 kilometers and covers much of the center of the park in northwestern Wyoming. Its floor is a high plateau around 2,400 meters above sea level, rimmed by mountains and partly filled by Yellowstone Lake. Beneath it, seismic studies have imaged a magma reservoir between about 5 and 17 kilometers deep that is mostly solid, with estimates of the melt fraction ranging from about 5 to 20 percent. More than 10,000 thermal features, including about half of the world's active geysers, dot the landscape. The ground rises and falls by centimeters over years as magma and hot fluids move, and some 1,500 to 2,500 earthquakes are located in the region in a typical year, nearly all too small to feel.
The three great eruptions came about 2.08 million, 1.3 million and 631,000 years ago. The first, the Huckleberry Ridge eruption, produced around 2,500 cubic kilometers of material, and the last, the Lava Creek eruption, about 1,000. Smaller lava flows followed, the most recent about 70,000 years ago. The Yellowstone Volcano Observatory, a USGS-led partnership with Yellowstone National Park, the University of Utah and other agencies, tracks earthquakes, ground deformation, gas and heat, and keeps the volcano at alert level Normal. In July 2024 a hydrothermal explosion at Biscuit Basin hurled rocks across a boardwalk, and a smaller one there on June 13, 2026 opened new hot-water features. The observatory reported background activity through mid-2026, with no significant caldera uplift or subsidence since January.