The Snake River Plain is a depression that runs along the North American Plate boundary. It covers over a fourth of Idaho. It stretches about 400 miles in an east-west direction with the concave side of the arc facing the north. The subsurface is known because various water wells and many deep exploration wells for geothermal resources and oil and gas. Geophysical surveys have also yielded much info on the surface of the plain. Many different theories have been brought up on the origin of the plain including a depression, downwarp, graben, and a rift. Although the plain is continuous, the surface features and geophysical features vary significantly among western, central, and eastern parts of the plain.
The Snake River Plain can be divided into three sections: western, central, and eastern. The western is a large tectonic rift valley filled with several km of lake sediments. The sediments underneath are rhyolite and basalt, and they are covered by basalt. The western plain began started forming around 11 Ma with the eruption of rhyolite lavas. The western plain doesn’t run along with the North American Plate motion and lies at a high angle to the central and eastern Snake River Plains. The eastern section traces the path of the North American Plate over the Yellowstone hotspot. The eastern plain is a topographic depression that cuts across different mountain structures that makes it parallel to the North American Plate motion. Underneath it’s almost entirely all basalt that came from the eruption of large shield volcanoes. The central Snake River plain is similar to the eastern plain, but differs by having thick sections of interbedded lake and stream sediments, including the Hagerman fossil beds.
Believe it or not, the Snake River Plain has a dramatic effect on the climate of Yellowstone National Park and the adjacent areas to the south and west of Yellowstone. As the Yellowstone hotspot burned its way through southern Idaho, it left a 70 mile wide channel through the Rocky Mountains. This channel is in line with the gap between the Cascade Range and the Sierra Nevada. The result of this is a moisture channel that extends from the Pacific to Yellowstone. Moisture from the Pacific streams onshore in the form of clouds. It passes through gap between the Sierra and Cascades and on into the Snake River Plain where it goes through most of the Rocky Mountains. It finally bumps into higher conditions at the head of the Snake River Valley at Ashton, Idaho and at Island Park, Idaho at the Teton Range and at the Yellowstone Plateau where the channeled moisture comes down as rain or snow. This results in a localized climate on the eastern side that is totally different than the western side. The affects Yellowstone in many ways, but manages to keep its beauty.