43°00′N113°30′W / 43.000°N 113.500°W /43.000; -113.500
TheSnake River Plain is ageologic feature located primarily within the U.S. state ofIdaho. It stretches about 400 miles (640 km) westward from northwest of the state ofWyoming to the Idaho-Oregon border. The plain is a wide, flat bow-shaped depression and covers about a quarter of Idaho. Three major volcanicbuttes dot the plain east ofArco, the largest beingBig Southern Butte.
Most of Idaho's major cities are in the Snake River Plain, as is much of its agricultural land.
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TheSnake River Plain can be divided into three sections: western, central, and eastern. The western Snake River Plain is a large tectonicgraben or rift valley filled with several kilometers of lacustrine (lake) sediments; the sediments are underlain byrhyolite andbasalt, and overlain by basalt. The western plain began to form around 11–12Ma (million years ago) with the eruption of rhyolite lavas andignimbrites. The western plain is not parallel toNorth American Plate motion and lies at a high angle to the central and eastern Snake River Plains. Itsmorphology is similar to othervolcanic plateaus such as theChilcotin Group in south-centralBritish Columbia, Canada.
The eastern Snake River Plain traces the path of the North American Plate over theYellowstone hotspot, now centered inYellowstone National Park. The eastern plain is a topographic depression that cuts acrossBasin and Range mountain structures, more or less parallel to North American Plate motion. It is underlain almost entirely by basalt erupted from largeshield volcanoes. Beneath the basalts are rhyolite lavas and ignimbrites that erupted as thelithosphere passed over the hotspot.
The central Snake River plain is similar to the eastern plain but differs by having thick sections ofinterbedded lacustrine (lake) and fluvial (stream) sediments, including theHagerman fossil beds.
Island Park andYellowstone Calderas formed as the result of enormous rhyolite ignimbrite eruptions, with single eruptions producing up to 600 cubic miles (2,500 km3) of ash.Henry's Fork Caldera, measuring 18 miles (29 km) by 23 miles (37 km), may be the largest symmetricalcaldera in the world. The caldera formed when a dome of magma built up and then drained away. The center of the dome collapsed, leaving a caldera. Henry's Fork Caldera lies within the older and largerIsland Park Caldera, which is 50 miles (80 km) by 65 miles (105 km). Younger volcanoes that erupted after passing over the hotspot covered the plain with young basalt lava flows in places, includingCraters of the Moon National Monument.
The Snake River Plain has a significant effect on the climate ofYellowstone National Park and the adjacent areas to the south and west of Yellowstone. Over time, the Yellowstone hotspot left a 70-mile (110 km) wide channel through theRocky Mountains. This channel is in line with the gap between theCascade Range and theSierra Nevada. The result is amoisture channel extending from the Pacific Ocean to Yellowstone. Moisture from the Pacific Ocean streams onshore in the form of clouds and humid air. It passes through the gap between the Sierra and Cascades and into the Snake River Plain where it is channeled through most of the Rocky Mountains with no high plateaus or mountain ranges to impede its progress.[1] It finally encounters upslope conditions at the head of the Snake River Valley atAshton, Idaho; atIsland Park, Idaho; at theTeton Range east ofDriggs, Idaho; and at theYellowstone Plateau of Yellowstone National Park where the channeled moisture precipitates out as rain and snow.[2] The result is a localized climate on the eastern side of the Rockies that is akin to a climate on the west slope of the Cascades or the northern Sierra. The head of the Snake River Valley, the Tetons, and theYellowstone Plateau receive much more precipitation than other areas of the region, and the area is known for being wet, green, having many streams, and having abundant snow in winter.
Although the topography of the Plain has largely gone unchanged for several million years, this region's climate has not been so constant. Current climatic conditions began to characterize the region in the early Pleistocene (approximately 2.5 million years ago). However, the arid climate of today was born from the gradual dissipation of a climate defined by greater moisture and narrower ranges of annual temperatures.[3][4]
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