Yellowstone National Park, established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872, is a national park located primarily in the U.S. state of Wyoming, though it also extends into Montana and Idaho. Yellowstone was the first national park in the world, and is known for its wildlife and its many geothermal features, especially Old Faithful Geyser, one of the most popular features in the park. It has many types of ecosystems, but the subalpine forest is dominant.

Native Americans have lived in the Yellowstone region for at least 11,000 years. The region was bypassed during the Lewis and Clark Expedition in the early 1800s. Aside from visits by mountain men during the early to mid-1800s, organized exploration did not begin until the late 1860s. The U.S. Army was commissioned to oversee the park just after its establishment. In 1917, administration of the park was transferred to the National Park Service, which had been created the previous year. Hundreds of structures have been built and are protected for their architectural and historical significance, and researchers have examined more than 1,000 archaeological sites.

Yellowstone National Park spans an area of 3,468 square miles (8,980 km2), comprising lakes, canyons, rivers and mountain ranges. Yellowstone Lake is one of the largest high-altitude lakes in North America and is centered over the Yellowstone Caldera, the largest supervolcano on the continent. The caldera is considered an active volcano; it has erupted with tremendous force several times in the last two million years. Half of the world's geothermal features are in Yellowstone, fueled by this ongoing volcanism. Lava flows and rocks from volcanic eruptions cover most of the land area of Yellowstone. The park is the centerpiece of the Greater Yellowstone Ecosystem, the largest remaining, nearly intact ecosystem in the Earth's northern temperate zone.

Hundreds of species of mammals, birds, fish and reptiles have been documented, including several that are either endangered or threatened. The vast forests and grasslands also include unique species of plants. Grizzly Bears, wolves, and free-ranging herds of bison and elk live in the park. Forest fires occur in the park each year; in the large forest fires of 1988, nearly one third of the park burned. Yellowstone has numerous recreational opportunities, including hiking, camping, boating, fishing and sightseeing. Paved roads provide close access to the major geothermal areas as well as some of the lakes and waterfalls. During the winter, visitors often access the park by way of guided tours that use either snow coaches or snowmobile.

Geography

Approximately 96 percent of the land area of Yellowstone National Park is located within the state of Wyoming. Another 3 percent is within Montana, with the remaining 1 percent in Idaho. The park is 63 miles (101 km) north to south, and 54 miles (87 km) west to east by air. Yellowstone is 2,219,789 acres (898,317 ha; 3,468.420 sq mi) in area, larger than the states of Rhode Island or Delaware. Rivers and lakes cover 5 percent of the land area, with the largest water body being Yellowstone Lake at 87,040 acres (35,220 ha; 136.00 sq mi). Yellowstone Lake is up to 400 feet (120 m) deep and has 110 miles (180 km) of shoreline. At an elevation of 7,733 feet (2,357 m) above sea level, Yellowstone Lake is the largest high altitude lake in North America. Forests comprise 80 percent of the land area of the park; most of the rest is grassland.

The Continental Divide of North America runs diagonally through the southwestern part of the park. The divide is a topographic feature that separates Pacific Ocean and Atlantic Ocean water drainages. About one third of the park lies on the west side of the divide. The origins of the Yellowstone and Snake Rivers are near each other but on opposite sides of the divide. As a result, the waters of the Snake River flow to the Pacific Ocean, while those of the Yellowstone find their way to the Atlantic Ocean via the Gulf of Mexico.

The park sits on the Yellowstone Plateau, at an average elevation of 8,000 feet (2,400 m) above sea level. The plateau is bounded on nearly all sides by mountain ranges of the Middle Rocky Mountains, which range from 9,000 to 11,000 feet (2,700 to 3,400 m) in elevation. The highest point in the park is atop Eagle Peak (11,358 ft/3,462 m) and the lowest is along Reese Creek (5,282 ft/1,610 m). Nearby mountain ranges include the Gallatin Range to the northwest, the Beartooth Mountains in the north, the Absaroka Range to the east, and the Teton Range and the Madison Range to the southwest and west. The most prominent summit on the Yellowstone Plateau is Mount Washburn at 10,243 feet (3,122 m).

Yellowstone National Park has one of the world's largest petrified forests, trees which were long ago buried by ash and soil and transformed from wood to mineral materials. This ash and other volcanic debris, are believed to have come from the park area itself. This is largely due to the fact that Yellowstone is actually a massive caldera of a supervolcano. There are 290 waterfalls of at least 15 feet (4.6 m) in the park, the highest being the Lower Falls of the Yellowstone River at 308 feet (94 m).

Three deep canyons are located in the park, cut through the volcanic tuff of the Yellowstone Plateau by rivers over the last 640,000 years. The Lewis River flows through Lewis Canyon in the south, and the Yellowstone River has carved two colorful canyons, the Grand Canyon of the Yellowstone and the Black Canyon of the Yellowstone in its journey north.

Geology

Yellowstone is at the northeastern end of the Snake River Plain, a great U-shaped arc through the mountains that extends from Boise, Idaho some 400 miles (640 km) to the west. This feature traces the route of the North American Plate over the last 17 million years as it was transported by plate tectonics across a stationary mantle hotspot. The landscape of present-day Yellowstone National Park is the most recent manifestation of this hotspot below the crust of the Earth.

The Yellowstone Caldera is the largest volcanic system in North America. It has been termed a "supervolcano" because the caldera was formed by exceptionally large explosive eruptions. The current caldera was created by a cataclysmic eruption that occurred 640,000 years ago, which released 240 cubic miles (1,000 km³) of ash, rock and pyroclastic materials. This eruption was 1,000 times larger than the 1980 eruption of Mount St. Helens. It produced a crater nearly a two thirds of a mile (1 km) deep and 52 by 28 miles (84 by 45 km) in area and deposited the Lava Creek Tuff, a welded tuff geologic formation. The most violent known eruption, which occurred 2.1 million years ago, ejected 588 cubic miles (2,450 km³) of volcanic material and created the rock formation known as the Huckleberry Ridge Tuff and created the Island Park Caldera.

A smaller eruption ejected 67 cubic miles (280 km³) of material 1.2 million years ago, forming the Henry's Fork Caldera and depositing the Mesa Falls Tuff.

Each of the three climax eruptions released vast amounts of ash that blanketed much of central North America falling many hundreds of miles away. The amount of ash and gases released into the atmosphere probably caused significant impacts to world weather patterns and led to the extinction of many species, primarily in North America.

A subsequent minor climax eruption occurred 160,000 years ago. It formed the relatively small caldera that contains the West Thumb of Yellowstone Lake. Later, two smaller eruptive cycles, the last one ending about 70,000 years ago, buried much of the caldera under thick lava flows.

Each eruption is in fact a part of an eruptive cycle that climaxes with the collapse of the roof of a partially emptied magma chamber. This creates a crater, called a caldera, and releases vast amounts of volcanic material, usually through fissures that ring the caldera. The time between the last three cataclysmic eruptions in the Yellowstone area has ranged from 600,000 to 900,000 years, but the small number of such climax eruptions cannot be used to make an accurate prediction for future volcanic events.

Between 630,000 and 700,000 years ago, Yellowstone Caldera was nearly filled in with periodic eruptions of rhyolitic lavas such as those that can be seen at Obsidian Cliffs and basaltic lavas which can be viewed at Sheepeater Cliff. Lava strata are most easily seen at the Grand Canyon of the Yellowstone, where the Yellowstone River continues to carve into the ancient lava flows. The canyon is a classic V-shaped valley, indicative of river-type erosion rather than erosion caused by glaciation.

The most famous geyser in the park, and perhaps the world, is Old Faithful Geyser, located in Upper Geyser Basin. Castle Geyser, Lion Geyser and Beehive Geyser are in the same basin. The park contains the largest active geyser in the world—Steamboat Geyser in the Norris Geyser Basin. There are 300 geysers in Yellowstone and a total of at least 10,000 geothermal features altogether. Half the geothermal features and two-thirds of the world's geysers are concentrated in Yellowstone.

In May 2001, the U.S. Geological Survey, Yellowstone National Park, and the University of Utah created the Yellowstone Volcano Observatory (YVO), a partnership for long-term monitoring of the geological processes of the Yellowstone Plateau volcanic field, for disseminating information concerning the potential hazards of this geologically active region.

In 2003, changes at the Norris Geyser Basin resulted in the temporary closure of some trails in the basin. New fumaroles were observed, and several geysers showed enhanced activity and increasing water temperatures. Several geysers became so hot that they were transformed into purely steaming features; the water had become superheated and they could no longer erupt normally. This coincided with the release of reports of a multiple year United States Geological Survey research project which mapped the bottom of Yellowstone Lake and identified a structural dome that had uplifted at some time in the past. Research indicated that these uplifts posed no immediate threat of a volcanic eruption, since they may have developed long ago, and there had been no temperature increase found near the uplifts. On March 10, 2004, a biologist discovered 5 dead bison which apparently had inhaled toxic geothermal gases trapped in the Norris Geyser Basin by a seasonal atmospheric inversion. This was closely followed by an upsurge of earthquake activity in April 2004. In 2006, it was reported that the Mallard Lake Dome and the Sour Creek Dome— areas that have long been known to show significant changes in their ground movement— had risen at a rate of 1.5 to 2.4 inches (3.8 to 6.1 cm) per year from mid–2004 through 2006. As of late 2007, the uplift has continued at a reduced rate. These events inspired a great deal of media attention and speculation about the geologic future of the region. Experts responded to the conjecture by informing the public that there was no increased risk of a volcanic eruption in the near future.

Yellowstone experiences thousands of small earthquakes every year, virtually all of which are undetectable to people. There have been six earthquakes with at least magnitude 6 or greater in historical times, including a 7.5 magnitude quake that struck just outside the northwest boundary of the park in 1959. This quake triggered a huge landslide, which caused a partial dam collapse on Hebgen Lake; immediately downstream, the sediment from the landslide dammed the river and created a new lake, known as Earthquake Lake. Twenty-eight people were killed, and property damage was extensive in the immediate region. The earthquake caused some geysers in the northwestern section of the park to erupt, large cracks in the ground formed and emitted steam, and some hot springs' normally clear water turned muddy. A 6.1 magnitude earthquake struck inside the park on June 30, 1975, but damage was minimal. For three months in 1985, 3,000 minor earthquakes were detected in the northwestern section of the park, during what has been referred to as an earthquake swarm, and has been attributed to minor subsidence of the Yellowstone caldera. Beginning on April 30, 2007, sixteen small earthquakes with magnitudes up to 2.7 occurred in the Yellowstone Caldera for several days. These swarms of earthquakes are common, and there have been 70 such swarms between 1983 and 2008. In December 2008, over 250 earthquakes were measured over a four day span under Yellowstone Lake, the largest measuring a magnitude of 3.9. In January 2010, more than 250 earthquakes were detected over a two day period. Seismic activity in Yellowstone National Park continues and is reported hourly by the Earthquake Hazards Program of the U.S. Geological Survey.

Biology and ecology

Yellowstone National Park is the centerpiece of the 20 million acre/31,250 square-mile (8,093,712 ha/80,937 km2) Greater Yellowstone Ecosystem, a region that includes Grand Teton National Park, adjacent National Forests and expansive wilderness areas in those forests. The ecosystem is the largest remaining continuous stretch of mostly undeveloped pristine land in the continental United States,considered to be the world's largest intact ecosystem in the northern temperate zone (although the area is mostly not temperate but subalpine, and all the national forest lands surrounding the National Park are not intact). With the successful wolf reintroduction program, which began in the 1990s, virtually all the original faunal species known to inhabit the region when white explorers first entered the area can still be found there.

Climate

Yellowstone climate is greatly influenced by altitude, with lower elevations generally found to be warmer year round. The record high temperature was 99 °F (37 °C) in 2002, while the coldest temperature recorded is −66 °F (−54.4 °C) in 1933. During the summer months of June through early September, daytime highs are normally in the 70 to 80 °F (21 to 27 °C) range, while nighttime lows can go to below freezing (0 °C)—especially at higher altitudes. Summer afternoons are frequently accompanied by thunderstorms. Spring and fall temperatures range between 30 and 60 °F (-1 and 16 °C) with cold nights in the teens to single digits (-5 to -20 °C). Winter in Yellowstone is very cold with high temperatures usually between zero to 20 °F (-20 to -5 °C) and nighttime temperatures below zero °F (-20 °C) for most of the winter.

Precipitation in Yellowstone is highly variable and ranges from 15 inches (380 mm) annually near Mammoth Hot Springs, to 80 inches (2,000 mm) in the southwestern sections of the park. The precipitation of Yellowstone is greatly influenced by the moisture channel formed by the Snake River Plain to the west that was, in turn, formed by Yellowstone itself. Snow is possible in any month of the year, with averages of 150 inches (3,800 mm) annually around Yellowstone Lake, to twice that amount at higher elevations.

Tornadoes in Yellowstone are rare; however, on July 21, 1987, the most powerful tornado recorded in Wyoming touched down in the Teton Wilderness of Bridger-Teton National Forest and hit Yellowstone National Park. Called the Teton–Yellowstone tornado, it was classified as an F4, with wind speeds estimated at between 207 and 260 miles per hour (333 and 420 km/h). The tornado left a path of destruction 1 to 2 miles (1.6 to 3.2 km) wide, and 24 miles (39 km) long, and leveled 15,000 acres (6,100 ha; 23 sq mi) of mature pine forest.

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