The Yellowstone supervolcano is the volcanic field beneath Yellowstone National Park, centred on a caldera about 72 by 45 km (45 by 28 mi) across in north-west Wyoming, with older parts reaching into Idaho and Montana. Driven by the Yellowstone hotspot, it has produced three caldera-forming eruptions: the Huckleberry Ridge Tuff about 2.08 million years ago (more than 2,450 km³ of ash-flow), the Mesa Falls Tuff about 1.3 million years ago (about 280 km³) and the Lava Creek Tuff about 631,000 years ago (about 1,000 km³), which made the present caldera. The last lava flow came about 70,000 years ago, and the Yellowstone Volcano Observatory does not consider an eruption imminent. Today the volcano mostly heats water, and that heat, together with the lava plateaus and the caldera lake, shapes where Yellowstone's animals and microbes live.
The park covers 8,983 km² (3,468 sq mi), 96 percent of it in Wyoming, and sits at the centre of the roughly 80,940 km² Greater Yellowstone Ecosystem. Well over half of the world's geysers and hydrothermal features are inside it. The sections below pair each of the park's defining species with the piece of volcanic ground it depends on.
Thermus aquaticus and the Microbial Mats of the Lower Geyser Basin
The smallest residents of the supervolcano have had the largest effect beyond it. In 1969 Thomas D. Brock and Hudson Freeze of Indiana University described a new heat-loving bacterium, Thermus aquaticus, which they had isolated from Mushroom Spring in the Lower Geyser Basin, near Great Fountain Geyser and White Dome Geyser. At the time, scientists thought bacteria could not survive much above about 55 °C. The organism's heat-stable enzyme, Taq polymerase, later made the polymerase chain reaction practical; Kary Mullis, who developed PCR at Cetus Corporation, received the 1993 Nobel Prize in Chemistry.
The same kind of life paints the park's hot springs. Grand Prismatic Spring in the Midway Geyser Basin, the largest hot spring in Yellowstone and the third largest in the world, is about 110 m (370 ft) across and 50 m deep and discharges around 2,100 litres of water at about 70 °C every minute. Its sterile, deep-blue centre is ringed by mats of thermophilic bacteria and archaea whose colours depend on temperature and on the balance of chlorophyll and carotenoids, so they tend to be orange and red in summer and dark green in winter. In 2016 researchers from Uppsala University reported a new class of archaea, Hadesarchaea, from the park's Culex Basin, able to turn carbon monoxide and water into carbon dioxide and hydrogen.
Bison in Hayden Valley and the Lamar Valley: From 23 Survivors to About 6,000
Bison were never wiped out in Yellowstone, and no other place in the United States can say the same. When the 19th-century slaughter had all but finished the species, a remnant of 23 animals survived in Pelican Valley, east of Yellowstone Lake. In 1902 the park held fewer than 50 bison, and a captive herd of 21 plains bison was brought in and ranched in the Lamar Valley until the 1960s, when the park switched to natural regulation. By 2023 the herd numbered about 6,000, the largest public herd in the country, a level that led to a cull of about 1,150 animals.
The herd splits into two groups that follow volcanic geography. The central interior herd, around 1,400 animals, ranges from the Madison River valley into Hayden Valley and the Upper and Lower Geyser Basins, all inside the caldera. Hayden Valley is itself a relic of the volcano: Yellowstone Lake once stood 61 m (200 ft) higher and stretched north across the valley to the foot of Mount Washburn. The northern range herd, around 2,300 animals, lives outside the caldera, from Gardiner, Montana, across the Blacktail Plateau into the Lamar Valley.
Bison leaving the park in winter remain a political problem. About half have been exposed to brucellosis, and since the Interagency Bison Management Plan was agreed between Montana and the federal government, the population has been held between about 2,400 and 5,500, with up to 900 animals a year removed at the boundary. No case of wild bison passing brucellosis directly to cattle has been recorded.
Gray Wolves and Elk on the Northern Range Since January 1995
The Lamar Valley lies on the park's northern range, outside the young caldera. The older mountains on this side of the park, such as Mount Washburn, are eroded remnants of Absaroka stratovolcanoes, active between about 53 and 43.7 million years ago, long before the hotspot arrived; their rocks cover about 23,000 km² of Montana and Wyoming, including roughly a third of the park. It was here that wolves came back. Park Service hunters had killed 136 wolves by 1926, the year the last official wolf killing in the park took place. In January 1995 officials captured 14 wolves near Hinton, Alberta, and held them in acclimation pens at Crystal Creek, Rose Creek and Soda Butte Creek in the Lamar Valley; between 21 and 31 March 1995 all 14 were released. Seventeen more wolves captured in Canada arrived in January 1996 and were released that April.
A 2024 survey counted 9 packs and 108 wolves inside the park. Their return reshaped the food web. Within two years coyote numbers had halved, and coyotes moved from open meadows to steeper ground. Elk, which number more than 30,000 across the park and are its most numerous large mammal, declined sharply in the northern herd after the mid-1990s and began using more forested cover. Researchers at Oregon State University link the recovery of aspen to these changed grazing habits. The northern Rocky Mountain wolf population was removed from the endangered species list on 27 February 2008.
Cutthroat Trout, Lake Trout and Sand Verbena in Yellowstone Lake's Caldera Basin
Yellowstone Lake fills part of the caldera floor. It covers 352 km² (136 sq mi) at 2,357 m (7,733 ft), making it the largest high-elevation lake in North America, and reaches 120 m deep. Its western arm, West Thumb, is a smaller caldera about 6 by 8 km nested inside the big one, formed around 160,000 years ago. On the north shore, Mary Bay is a 5 km crater left by a hydrothermal explosion about 13,800 years ago, one of more than 20 large blast craters made in the last 14,000 years. The lake is also moving: uplift of the Sour Creek dome north of Fishing Bridge has been tilting it southward, widening northern beaches and flooding the southern arms.
This is the core range of the Yellowstone cutthroat trout, one of 18 fish species in the park. In 1994 non-native lake trout were discovered in the lake, probably introduced illegally from Lewis Lake as early as 1989. They prey on the smaller cutthroat trout, and the decline that followed spread up the food chain to predators such as the North American river otter, which relies on cutthroat trout in the lake's spawning streams. The park now runs a lake trout removal programme, anglers must kill any lake trout they catch, and since 2001 all native sport fish have been catch-and-release.
The lake's sandy shores hold the park's only endemic plant, the Yellowstone sand verbena. It belongs to a genus of about 30 species that otherwise live in the warm deserts of the western United States and Mexico, yet it grows at about 2,360 m, and some botanists think geothermal warmth let it survive the plateau's winters. Surveys found four populations, and trampling cut its numbers by 56 percent between 1998 and 2010. Its white flowers open in July and August.
The lake and its rivers also anchor the park's birds. Some 311 species have been reported in Yellowstone, almost half of them nesting there; 26 pairs of bald eagles were documented nesting in 1999, and the common loon, harlequin duck, osprey and trumpeter swan are listed as species of special concern because they are rare in the park.
Grizzly Bears and Lodgepole Pine on the Central and Madison Plateaus
Since the Lava Creek eruption, some 80 eruptions of rhyolitic lava have nearly filled the caldera, forming the broad Madison, Pitchstone, Central and Solfatara plateaus. Lodgepole pine grows across them; it makes up 80 percent of the park's forests, and its cones open in the heat of fire. In 1988 fires burned more than a third of the park. Surveys afterwards found the toll on wildlife was small: about 345 elk, 36 deer, 12 moose, 6 black bears and 9 bison.
Grizzly bears range across these forests and meadows. In 2017 an estimated 700 lived in the Greater Yellowstone Ecosystem, about 150 of them wholly or partly within the park, and their listing as a threatened species has been removed and restored several times in court since 1975. Black bears share the park with them, seen most often on the northern range and in the Bechler area in the south-west corner. Among the rarer animals, cougars number only about 25 park-wide.
All of this sits on ground that keeps moving. Between 2004 and 2008 the caldera floor rose almost 75 mm a year, more than three times any rate measured since 1923, and a swarm of 1,620 small earthquakes struck between 17 January and 1 February 2010. A magnitude 4.8 quake on 30 March 2014 was the strongest since 1980. To see how Yellowstone compares with other giant collapse craters, read our ranking of the largest calderas in the world and the guide to Sumatra's Toba supervolcano, and for the park's animals beyond the hot ground, see our Yellowstone wildlife ecosystem guide.