2,219,791 acres (899,015 hectares), the world's first national park (est. 1872), NW Wyo., extending into Montana and Idaho. It lies mainly on a broad plateau in the Rocky Mts., on the Continental Divide, c.8,000 ft (2,440 m) above sea level, surrounded by mountains from 10,000 to 14,000 ft (3,048–4,267 m) high. The area, a huge craterlike volcanic basin (caldera), is a geological "hot spot" and the site of several massive eruptions, the most recent occurring some 600,000 years ago. The plateau is mostly formed from once-molten lava.

Volcanic activity is evidenced by nearly 10,000 hot springs, 200 geysers, and many vents and mud pots. The more prominent geysers are unequaled in size, power, and variety. Old Faithful, the best known although not the largest, erupts every 40 to 70 min and shoots c.11,000 gal (41,640 liters) of water some 150 ft (46 m) high. Mammoth Hot Springs, a series of five terraces with reflecting pools, continues to grow as residue from the mineral-rich water is deposited.

The park also has petrified forests, lava formations, and the "black glass" Obsidian Cliff. Eagle Peak, 11,370 ft (3,466 m), is the highest point. Yellowstone Lake, the Grand Canyon of the Yellowstone, and waterfalls are notable features on the Yellowstone River, which crosses the park. The park has a wide variety of flowers and other plant life. Bears, mountain sheep, elk, bison, moose, many smaller animals, and more than 200 kinds of birds inhabit Yellowstone, which is one of the world's largest wildlife sanctuaries. Fires in 1988 burned about 36% of the park, but animal and plant life rebounded quickly, as the nutrient influx in the ash nourished the soil.

See also National Parks and Monuments, table.

See J. Muir, Yellowstone National Park (1979); B. T. Scott, The Geysers of Yellowstone (rev. ed. 1986); G. Wuerthner, Yellowstone & the Fires of Change (1988).

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