Snake River Plain

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Big Southern Butte

The Snake River Plain is a geological feature of (primarily) the American state of Idaho. It stretches about 400 miles (640 km) westward from northwest of the state of Wyoming to the Idaho-Oregon border. The plain is a wide flat bow-shaped depression, and covers about a quarter of Idaho. Three major buttes dot the plain east of Arco, the largest being Big Southern Butte.

Many of Idaho's major cities are in the Snake River Plain as is much of its agricultural land. Also located within Snake River Plain is the Idaho National Laboratory.

[edit] Geology

The Snake River Plain traces the path of the North American plate over a geologic hot spot, now centered in Yellowstone National Park. The plain can be divided into three sections: western, central, and eastern. The western Snake River Plain is a large tectonic graben or rift valley filled with several km of lacustrine (lake) sediments; the sediments are underlain by rhyolite and basalt, and overlain by basalt. The western plain began to form around 11-12 Ma with the eruption of rhyolite lavas and ignimbrites. The western plain is not parallel to North American Plate motion, and lies at a high angle to the central and eastern Snake River Plains.

The eastern Snake River plain is a topographic depression that cuts across Basin and Range Mountain structures, more or less parallel to North American plate motion. It is underlain almost entirely by basalt erupted from large shield volcanoes. Beneath the basalts are rhyolite lavas and ignimbrites that erupted as the lithosphere passed over the hotspot. The central Snake River plain is similar to the eastern plain, but differs by having thick sections of interbedded lacustrine (lake) and fluvial (stream) sediments, including the famous Hagerman fossil beds.

Island Park and Yellowstone Calderas formed as the result of enormous rhyolite ignimbrite eruptions, with single eruptions producing up to 2500 cubic km of ash. Younger volcanoes that erupted after passing over the hotspot covered the plain with young basalt lava flows in places, including Craters of the Moon National Monument.