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The Yellowstone Caldera is the volcanic caldera and supervolcano located in Yellowstone National Park in the United States, sometimes referred to as the Yellowstone Supervolcano. The caldera and most of the park are located in the northwest corner of Wyoming. The major features of the caldera measure about .〔as determined by geological field work conducted by Bob Christiansen of the United States Geological Survey in the 1960s and 1970s.〕 The caldera formed during the last of three supereruptions over the past 2.1 million years: the Huckleberry Ridge eruption 2.1 million years ago (which created the Island Park Caldera and the Huckleberry Ridge Tuff), the Mesa Falls eruption 1.3 million years ago (which created the Henry's Fork Caldera and the Mesa Falls Tuff) and the Lava Creek eruption 640,000 years ago (which created the Yellowstone Caldera and the Lava Creek Tuff). ==Volcanism== Volcanism at Yellowstone is relatively recent with calderas that were created during large eruptions that took place 2.1 million, 1.3 million, and 640,000 years ago. The calderas lie over a hotspot where light, hot, molten rock from the mantle rises toward 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. The hotspot appears to move across terrain in the east-northeast direction, but in fact the hotspot is much deeper than terrain and remains stationary while the North American Plate moves west-southwest over it. Over the past 18 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 of these eruptions were so massive that they are classified as supereruptions. Volcanic eruptions sometimes empty their stores of magma so swiftly that they cause the overlying land to collapse into the emptied magma chamber, forming a geographic depression called a caldera. The oldest identified caldera remnant straddles the border near McDermitt, Nevada-Oregon, although there are volcaniclastic piles and arcuate faults that define caldera complexes more than in diameter in the Carmacks Group of southwest-central Yukon, Canada, which are interpreted to have formed 70 million years ago by the Yellowstone hotspot.〔(【引用サイトリンク】url=http://www.ldeo.columbia.edu/~manders/SRP_erupt.html )〕 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. One such caldera, the Bruneau-Jarbidge caldera in southern Idaho, was formed between 10 and 12 million years ago, and the event dropped ash to a depth of one foot (30 cm) away in northeastern Nebraska and killed large herds of rhinoceros, camel, and other animals at Ashfall Fossil Beds State Historical Park. The United States Geological Survey ("USGS") estimates there are one or two major caldera-forming eruptions and 100 or so lava extruding eruptions per million years, and "several to many" steam eruptions per century.〔(Yellowstone Volcanic Hazards, USGS ). Volcanoes.usgs.gov (March 1, 2012). Retrieved on December 31, 2013.〕 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; it also produced one additional smaller eruption, thereby creating West Thumb Lake〔West Thumb Lake is not to be confused with West Thumb Geyser Basin. The caldera created West Thumb Lake, and the underlying Yellowstone hotspot keeps West Thumb Geyser Basin active. See (Fig. 22 ). See also :File:Yellowstone Caldera map2.JPG.〕 174,000 years ago. The three super eruptions occurred 2.1 million, 1.3 million, and 640,000 years ago, forming the Island Park Caldera, the Henry's Fork Caldera, and Yellowstone calderas, respectively.〔Newhall, Christopher G.; Dzurisin, Daniel (1988) (Historical Unrest at Large Calderas of the World ): U.S. Geological Survey Bulletin 1855〕 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 next biggest supereruption formed the Yellowstone Caldera (640,000 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 Snake River Plain-Yellowstone (SRP-Y) hotspot that is plainly visible today.〔This qualitative statement is easily verified by reviewing the Yellowstone area in Google Earth〕 Non-explosive eruptions of lava and less-violent explosive eruptions have occurred in and near the Yellowstone caldera since the last supereruption.〔(【引用サイトリンク】url=http://pages.uoregon.edu/bindeman/Bindemanetal2008.pdf )〕〔(【引用サイトリンク】url=http://pages.uoregon.edu/bindeman/Supervolcanoes.pdf )〕 The most recent lava flow occurred about 70,000 years ago, while a 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 diameter crater at Mary Bay on the edge of Yellowstone Lake (located in the center of the caldera).〔 〕 Currently, volcanic activity is exhibited via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser, plus recorded ground swelling indicating ongoing inflation of the underlying magma chamber. The volcanic eruptions, as well as the continuing geothermal activity, are a result of a great cove of magma located below the caldera's surface. The magma in this cove 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. According to the analysis of earthquake data in 2013, the magma chamber is long and wide. It also has underground mass, of which 6–8% is filled with molten rock. This is about 2.5 times bigger than scientists had previously imagined it to be; however, scientists believe that the proportion of melt in the chamber is much too low to allow another supereruption.〔(【引用サイトリンク】title=USGS: Volcano Hazards Program – Yellowstone Volcano Observatory Featured Articles Archive )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yellowstone Caldera」の詳細全文を読む スポンサード リンク
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