Think of Yellowstone as a gigantic pressure cooker, fueled by a massive supervolcano. Water from rain and snowmelt, much of it centuries-old, percolates through cracks in the Earth’s crust until heated by molten rock reservoirs deep below. The water then filters upward, eventually finding release in the thousands of geysers, hot springs, and other hydrothermal wonders.
Eruptions of this supervolcano expel so much material that the crust caves in, creating a craterlike depression called a caldera. Yellowstone is known as a supervolcano because of the violence and size of its explosions.
The plume of hot rock has been calculated at more than 600 miles deep. But scientists suspect it actually descends as far as 1,800 miles, all the way to what’s known as the Earth’s outer core-mantle boundary.
The reservoirs and plume are superheated, spongelike rock holding pockets of molten material called magma. The reservoirs’ heat, which originates in the plume, is what keeps the area’s geysers boiling.
Ancient rain and snowmelt seep down to just above the volcano’s magma reservoirs, until they are superheated and rise again through the fractures. Volcanic heat and gases help propel steam and water toward the surface, where they escape through hot springs or geysers.
Hot water rises from a deep reservoir into a teapot-shaped chamber. As water and gases fill the sealed space, pressure builds, preventing boiling. Some water spills into the spout, releasing pressure and allowing the water in the chamber to boil. Steam and water then blast up the spout.
Pressure builds behind a narrow constriction until steam shoots through. Some water splashes out, then jets of steam and water explode, rising on average 130 feet. As the chamber drains, pressure drops, and the process begins again.
The park’s hydrothermal features cluster in basins at the margins of lava flows or near faults. Rivers and streams are heated as they pass through these basins. Heat and escaping gases are also evidence of the subterranean forces that lie below Yellowstone.
Design and Development: Manuel Canales, Daisy Chung, Daniela Santamarina, Swordsweeper Industries. Art and Animation: Ronald Paniagua, Charles Preppernau, Hernan Cañellas. Maps: Charles Preppernau, Andrew Umentum. Text: Eve Conant. GIS: Theodore A. Sickley. Consultants: Duncan Foley, Pacific Lutheran University; Robert Fournier, USGS; Henry Heasler, Cheryl Jaworowski, National Park Service; Jacob Lowenstern, USGS; Robert B. Smith, Jamie Farrell, University of Utah. Sources: K. A. Barrick, Environmental Reviews, 2010 (Vanishing Geysers); S. Hurwitz, et al., Journal of Geophysical Research, 2014 (Shaken Faith); NPS (hydrothermal features); USGS; USDA National Agriculture Imagery Program (map imagery).