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| Yellowstone Caldera | |
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 The northeastern part of Yellowstone Caldera, with the Yellowstone River flowing through Hayden Valley and the caldera rim in the distance |
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| Elevation | 10,308 feet (3,142 m) at Mount Sheridan |
| Location | Wyoming, U.S. |
| Range | Rocky Mountains |
| Coordinates | |
| Topo map | USGS Yellowstone National Park |
| Type | Caldera |
| Age of rock | 70,000 – 2.1 million years |
| Last eruption | 640,000 years ago |
| Easiest route | hike/auto/bus |
The Yellowstone Caldera is the volcanic caldera in Yellowstone National Park in the United States. It is located in the northwest corner of Wyoming, measuring about 55 kilometers (34 mi) by 72 kilometers (45 mi). The caldera was discovered based on 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 2000, it has often been referred to as the "Yellowstone supervolcano."
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Yellowstone, like the Hawaiʻi hotspot, is believed to lie on top of one of the planet's few dozen hotspots where light, hot, molten mantle rock rises towards the surface. The Yellowstone hotspot has a long history. Over the past 17 million years or so, successive eruptions have flooded lava over wide stretches of Washington, Oregon, California, Nevada, and Idaho, forming a string of comparatively flat calderas linked like beads, as the North American plate moves across the stationary hotspot. The oldest identified caldera remnant is straddling the border near McDermitt, Nevada-Oregon. The caldera's apparent motion to the east-northeast forms the Snake River Plain. However, what is actually happening is the result of the North American plate moving west-southwest over the stationary hotspot deep underneath.
Currently, volcanic activity is exhibited only via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser, but within the past two million years, it has undergone three extremely large explosive eruptions, up to 2,500 times the size of the 1980 Mount St. Helens eruption. The three eruptions happened 2.1 million years ago, 1.3 million years ago, and the most recent such eruption produced the Lava Creek Tuff 640,000 years ago and spread a layer of volcanic ash over most of the North American continent. Additionally, non-explosive eruptions of lava flows have occurred in and near the caldera since the last major eruption; the most recent of these was about 70,000 years ago. Craters of the Moon National Monument in Idaho is the result of volcanic activity between 11,000 and 2,000 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).[1] 142 or more caldera-forming eruptions have occurred from the Yellowstone hotspot within the past 17 million years. [2]
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.
A full-scale eruption of the Yellowstone caldera could result in millions of deaths. The last full-scale eruption, the so-called Lava Creek eruption, ejected approximately 240 cubic miles of rock and dust into the sky.
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.[3][4] 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 [1]. The U.S. Geological Survey, University of Utah and National Park Service scientists with the Yellowstone Volcano Observatory "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."[5].
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) of water in Yellowstone Lake, creating huge waves that essentially unsealed a capped geothermal system leading into the hydrothermal explosion that formed Mary Bay.[6][7]
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.[8]
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.[9]
The source of the Yellowstone hotspot is controversial. Some geoscientists theorize that the Yellowstone hotspot is the effect of an interaction between local conditions in the lithosphere and upper mantle convection.[10][11] Others prefer a deep mantle origin (mantle plume).[12] 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.[13]