The Snake River Plain hosts most of Idaho's large cities, including Boise, Twin Falls and Pocatello, as well as much of the state's agricultural territory. Bridging the Columbia Plateau and the Northern Rocky Mountains, it is a curved structural depression trending from near the head of Hells Canyon along the Idaho-Oregon border southeastward and then northeastward to the Yellowstone Plateau. Swathed where not developed in sagebrush steppe and punctuated by petrified lava flows and scattered volcanic cones, it is a tremendous geological classroom.
Geological Profile
While the western end of the Snake River Plain is a graben, or rift valley, the central and eastern portions were likely blazed out by the North American tectonic plate's passage over a stationary hot spot. Such a feature is an upwelling of magma at the surface from the Earth's mantle. Geologists believe the present location of the hot spot that created part of the Snake River Plain now underlies the Yellowstone Plateau and explains that Rocky Mountain site's dense concentration of geothermal features. This part of the plain proceeds from the Owyhee Plateau northeastward to the Yellowstone highlands, marking the course of the continental plate over the volatile hotspot. A 62-mile network of fractures, called the Great Rift system, angles northwestward to southward across the Eastern Snake River Plain.
Volcanoes
True volcanic domes stud the Snake River Plain. Among them are the well-known cluster of buttes in the eastern Plain near Arco, Idaho. Largest is Big Southern Butte, which dominates the rolling sagebrush-steppe horizon for miles. With its substantial relief, this massive rhyolitic volcano provides habitat for trees and shrubs like quaking aspen, Douglas fir and manzanita that find sparse habitat in the surrounding semiarid basin. Big Southern Butte has great significance among the Shoshone-Bannock people, and once supplied highly desired obsidian, a kind of volcanic glass, that Indian cultures across the West used to make weapons.
Springs
The permeable basalt and unconsolidated sediments of the Snake River Plain harbor huge aquifers. Some of this groundwater issues forth in large springs, the most notable on the Eastern Snake River Plain being American Falls and Thousand Springs. The former discharges some 2,600 cubic feet of water per second, while approximately 5,200 cubic feet per second spill forth from the many outpourings of Thousand Springs. The Snake River Plain aquifers are immensely important to the valley's agricultural and municipal operations, and are, like other tapped-into aquifers the world over, vulnerable to drawdown and reduced recharge.
Craters of the Moon
One of the defining landforms of the Snake River Plain are the great exposed lava beds of Craters of the Moon National Monument and Preserve near Arco. A system of three lava fields shoulders the Great Rift zone, the largest being the Craters of the Moon Lava Field with its 60 or so distinct lava flows and 25 cinder cones. Some of the formations in Craters of the Moon are but 2,000 years old (others are 13,000 years older), which means large areas are still barren of much soil or vegetation.
Tags: River Plain, Snake River, Snake River Plain, Craters Moon, cubic feet, Eastern Snake
