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Yellowstone Caldera
The northeastern part of Yellowstone Caldera, with the Yellowstone River flowing through Hayden Valley and the caldera rim in the distance
Elevation	10,308 feet (3,142 m) at Mount Sheridan[1]
Location	Wyoming, U.S.
Range	Rocky Mountains
Coordinates	44°24′N 110°42′W / 44.4°N 110.7°W / 44.4; -110.7 (Yellowstone Caldera)Coordinates: 44°24′N 110°42′W / 44.4°N 110.7°W / 44.4; -110.7 (Yellowstone Caldera)
Topo map	USGS Yellowstone National Park
Type	Caldera[2]
Age of rock	2.1 million years[2]
Last eruption	640,000 years ago (magmatic) / 1350 BC ± 200 years ago (hydrothermal)[2]
Easiest route	hike/auto/bus

The Yellowstone Caldera is the volcanic caldera in Yellowstone National Park in the United States. The caldera is located in the northwest corner of Wyoming, in which the vast majority of the park is contained. The major features of the caldera measure about 55 kilometers (34 mi) by 72 kilometers (45 mi) as determined by geological field work conducted by Bob Christiansen of the United States Geological Survey in the 1960s and 1970s. After a BBC television science program coined the term supervolcano in 2005, it has often been referred to as the Yellowstone Supervolcano.

Contents 1 Volcanism 1.1 Earthquakes 1.2 Volcanic hazards 1.3 Hydrothermal explosion hazard 2 Origin 3 In popular culture 4 See also 5 Further reading 6 References 7 External links

[edit] Volcanism

Yellowstone, like Hawaii, is believed to lie on top of an area called a hotspot where light, hot, molten mantle rock rises towards the surface. While the Yellowstone hotspot is now under the Yellowstone Plateau, it previously helped create the eastern Snake River Plain (to the west of Yellowstone) through a series of huge volcanic eruptions. Although the hotspot's apparent motion is to the east-northeast, the North American Plate is really moving west-southwest over the stationary hotspot deep underneath.^[3]

Over the past 17 million years or so, this hotspot has generated a succession of violent eruptions and less violent floods of basaltic lava. Together these eruptions have helped create the eastern part of the Snake River Plain from a once-mountainous region. At least a dozen or so of these eruptions were so massive that they are classified as supereruptions. Volcanic eruptions sometimes empty their stores of magma so fast that they cause the overlying land to collapse into the emptied magma chamber, forming a geographic depression called a caldera. Calderas formed from explosive supereruptions can be as wide and deep as mid- to large-sized lakes and can be responsible for destroying large parts of mountain ranges.

The oldest identified caldera remnant straddles the border near McDermitt, Nevada-Oregon. Progressively younger caldera remnants, most grouped in several overlapping volcanic fields, extend from the Nevada-Oregon border through the eastern Snake River Plain and terminate in the Yellowstone Plateau. 142 or more caldera-forming eruptions have occurred from the Yellowstone hotspot within the past 17 million years.^[4]

The loosely-defined term 'supervolcano' has been used to describe volcanic fields that produce exceptionally-large volcanic eruptions. Thus defined, the Yellowstone Supervolcano is the volcanic field which produced the latest three supereruptions from the Yellowstone hotspot. The three super eruptions occurred 2.1 million, 1.3 million and 640,000 years ago; forming the Henry's Fork Caldera, the Island Park Caldera, and Yellowstone calderas, respectively^[5]. The Island Park Caldera supereruption (2.1 million years ago), which produced the Huckleberry Ridge Tuff, was the largest and produced 2,500 times as much ash as the 1980 Mount St. Helens eruption. The second largest supereruption formed the Yellowstone Caldera (630,000 years ago) and produced the Lava Creek Tuff. The Henry's Fork Caldera (1.2 million years ago) produced the smaller Mesa Falls Tuff but is the only caldera from the SRP-Y hotspot that is plainly visible today.^[6]

Non-explosive eruptions of lava and less violent explosive eruptions have occurred in and near the Yellowstone caldera since the last supereruption. The most recent lava flow occurred about 70,000 years ago while the largest violent eruption excavated the West Thumb of Lake Yellowstone around 150,000 years ago. Smaller steam explosions occur as well; an explosion 13,800 years ago left a 5 kilometer diameter crater at Mary Bay on the edge of Yellowstone Lake (located in the center of the caldera).^[7][8] Currently, volcanic activity is exhibited only via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser.

The volcanic eruptions, as well as the continuing geothermal activity, are a result of a large chamber of magma located below the caldera's surface. The magma in this chamber contains gases that are kept dissolved only by the immense pressure that the magma is under. If the pressure is released to a sufficient degree by some geological shift, then some of the gases bubble out and cause the magma to expand. This can cause a runaway reaction. If the expansion results in further relief of pressure, for example, by blowing crust material off the top of the chamber, the result is a very large gas explosion.

[edit] Earthquakes

Due to the volcanic and tectonic nature of the region, the Yellowstone Caldera experiences between 1000 and 2000 measurable earthquakes a year, though most are relatively minor, measuring a magnitude of 3 or less. Occasionally, numerous earthquakes are detected over a short period of time, an event known as an earthquake swarm. In 1985, more than 3000 earthquakes were measured over several months. More than 70 smaller swarms have been detected between 1983 and 2008. The USGS states that these swarms are caused by movements of magma & hydrothermal fluids.^[9] The most recent swarm occurred in December 2008 and continued into January 2009, with more than 500 quakes detected under the northwest end of Yellowstone Lake over a seven day span, with the largest registering a magnitude of 3.9.^{[10][11][12][13]}

[edit] Volcanic hazards

The last full-scale eruption of the Yellowstone Supervolcano, the Lava Creek eruption which happened approximately 640,000 years ago^[14], ejected approximately 240 cubic miles (1000 cubic kilometres) of rock and dust into the sky.^[8]

Geologists are closely monitoring the rise and fall of the Yellowstone Plateau, which averages ±0.6 inches (about ±1.5 cm) yearly, as an indication of changes in magma chamber pressure.^[15][16]

The upward movement of the Yellowstone caldera floor – almost 3 inches (7 centimeters) per year for the past three years – is more than three times greater than ever observed since such measurements began in 1923.^[17] From mid-Summer 2004 through mid-Summer 2008, the land surface within the caldera has moved upwards, as much as 8 inches at the White Lake GPS station.^[18] The U.S. Geological Survey, University of Utah and National Park Service scientists with the Yellowstone Volcano Observatory maintain that they "see no evidence that another such cataclysmic eruption will occur at Yellowstone in the foreseeable future. Recurrence intervals of these events are neither regular nor predictable.".^[8]

[edit] Hydrothermal explosion hazard

Studies and analysis may indicate that the greater hazard comes from hydrothermal activity which occurs independently of volcanic activity. Over 20 large craters have been produced in the past 14,000 years since the glaciers retreated from Yellowstone, resulting in such features as Mary Bay, Turbid Lake and Indian Pond.

Lisa Morgan, a USGS researcher, explored this threat in a 2003 report, and in a recent talk postulated that an earthquake may have displaced more than 77 million cubic feet (2,200,000 m³) (576,000,000 US gallons) of water in Yellowstone Lake, creating huge waves that essentially unsealed a capped geothermal system leading into the hydrothermal explosion that formed Mary Bay.^[19][20]

Further research shows that earthquakes from great distances do reach and have effects upon the activities at Yellowstone, such as the 1992 7.3 magnitude Landers earthquake in California’s Mojave Desert that triggered a swarm of quakes from more than 800 miles (1,300 km) away and the Denali fault earthquake 2,000 miles (3,200 km) away in Alaska that altered the activity of many geysers and hot springs for several months afterwards.^[21]

The head of the Yellowstone Volcano Observatory, Jake Lowenstern, has proposed major upgrades and extended monitoring since the U.S. Geological Survey classified Yellowstone as a “high-threat” system.^[22]

[edit] Origin

The source of the Yellowstone hotspot is controversial. Some geoscientists hypothesize that the Yellowstone hotspot is the effect of an interaction between local conditions in the lithosphere and upper mantle convection.^[23][24] Others prefer a deep mantle origin (mantle plume).^[25] Part of the controversy is due to the relatively sudden appearance of the hotspot in the geologic record. Additionally, the Columbia Basalt flows appeared at the same approximate time, causing speculation about their origin.^[26]

[edit] In popular culture

• Supervolcano, a two-part docudrama about a hypothetical eruption of the Yellowstone caldera
• When Yellowstone Erupts, a documentary about the hypothetical after-effects of the Yellowstone caldera eruption, and warning signs that scientists are looking for.
• End Day, an apocalyptic docu-drama with five scenarios, the fourth being the eruption of the Yellowstone supervolcano.

[edit] See also

• Long Valley Caldera, Valles Caldera, La Garita Caldera - examples of other calderas close to but not related to Yellowstone.
• Iceland hotspot and Iceland plume describes aspects of volcanic processes

[edit] Further reading

• Breining, Greg, Super Volcano: The Ticking Time Bomb beneath Yellowstone National Park (St. Paul, MN: Voyageur Press, 2007). A popularized scientific look at the Yellowstone area's geological past and potential future. ISBN 978-0-7603-2925-2
• Vazquez, J.A., and Reid, M.R., 2002, Time scales of magma storage and differentiation of voluminous rhyolites at Yellowstone caldera, Wyoming: Contributions to Mineralogy & Petrology, v. 144, p. 274-285
• Sutherland, Wayne, and Sutherland, Judy, Yellowstone Farewell (SPUR RIDGE, 2003). A novel looking at an eruption in the Yellowstone Caldera written by a practicing Wyoming geologist. Contains a wealth of technical details on the geology of western Wyoming.

[edit] References

[edit] External links

• The Snake River Plain and the Yellowstone Hot Spot
• Yellowstone Volcano Observatory
  • YVO FAQ relating to Supervolcano
• Supervolcano documentary from BBC