On March 1, 1872, Yellowstone became the first National Park reserve declared anywhere in the world, by President Ulysses S. Grant. In 1978 it was designated a World Heritage Site. Although it is commonly assumed that the park was named for the yellow rocks seen in the Grand Canyon of the Yellowstone, the park’s name comes from the Yellowstone River that flows through it, which is in turn named after sandstone bluffs found farther down its course in eastern Montana.
Long before any recorded human history in Yellowstone, a massive volcanic eruption spewed an immense volume of ash that covered all of the western U.S., much of the Midwest, parts of the US east coast, northern Mexico, and some areas in Canada. The eruption left a caldera approximately 34 by 45 miles (55 by 72 km). See volcanoes for background; Yellowstone is classed as a supervolcano and its last eruption is thought to have been a VEI-8 event with over 1000 km3 of ejecta, a thousand times more powerful than the 1980 eruption of Mt. St. Helens. The Yellowstone super volcano is believed to erupt every 600,000 to 900,000 years with the last event occurring 640,000 years ago. Its eruptions are among the largest known to have ever occurred on Earth, producing drastic climate change in the aftermath.
With half of the earth’s geothermal features, Yellowstone holds the planet’s most diverse and intact collection of geysers, hot springs, mudpots, and fumaroles. Its more than 300 geysers make up two thirds of all those found on earth. Combine this with more than 10,000 thermal features comprised of brilliantly colored hot springs, bubbling mudpots, and steaming fumaroles, and you have a place like no other.
Yellowstone’s hydrothermal features would not exist without the underlying magma body that releases tremendous heat. They also depend on sources of water, such as from the mountains surrounding the Yellowstone Plateau. There, snow and rain slowly percolate through layers of permeable rock riddled with cracks. Some of this cold water meets hot brine directly heated by the shallow magma body. The water’s temperature rises well above the boiling point but the water remains in a liquid state due to the great pressure and weight of the overlying water. The result is superheated water with temperatures exceeding 400 °F (200 °C).
The superheated water is less dense than the colder, heavier water sinking around it. This creates convection currents that allow the lighter, more buoyant, superheated water to begin its journey back to the surface following the cracks and weak areas through rhyolitic lava flows. This upward path is the natural “plumbing” system of the park’s hydrothermal features. Once it reaches the surface, the various colors of the pools are due to different types of bacteria growing in different temperatures.