The Snake River flowing below the Teton Range. (Ansel Adams/public domain, https://catalog.archives.gov/id/519904)
The Snake River Plain is a prominent river drainage that cuts a broad “smile” across southern Idaho, easily recognizable from satellite imagery. The geologic history of the Eastern Snake River Plain and the Yellowstone Hotspot track are closely intertwined, but the Western Snake River Plain has a different story to tell.
The Snake River flows more than 1,000 miles from its headwaters in Yellowstone through Jackson Hole, across southern Idaho’s Snake River Plain, and through Hells Canyon before joining the Columbia River in south-central Washington. The river’s course covers an incredibly diverse range of geology and topography. The Snake River also carried flood waters from the massive Bonneville and Missoula Ice Age floods.
Today, the Snake River Plain is home to eight of the 10 most populous cities in Idaho and supports much of the state’s agriculture industry. While the Snake River now flows seamlessly across southern Idaho, the Eastern Snake River Plain and Western Snake River Plain were formed by very different geologic processes.
The Eastern Snake River Plain is a northeast-southwest-trending topographic depression in southeastern Idaho that extends from the Wyoming border to approximately the city of Twin Falls. As the North American tectonic plate moved southwest, the (mostly) stationary Yellowstone hotspot plume heated the crust and generated a significant amount of melt. This resulted in a line of volcanic calderas that get progressively older along the hotspot track from northeast to southwest.
As dense magma generated by the hotspot accumulated in the middle crust, the extra weight caused the crust to sink. The sinking crust accumulated additional sediments and volcanic rocks at the surface, which caused further sinking.
Ultimately, the rocks along the hotspot track subsided by approximately 2.8 miles compared to the surrounding rocks. Faulting is a common way for crustal rocks to move, but in the case of the Eastern Snake River Plain, the subsidence occurred by flexing and warping of the crust. The warping of rock surrounding the Eastern Snake River Plain can be clearly seen in dipping rock layers on the margins of the plain where the Lost River, Lemhi and Beaverhead ranges terminate on the northwest side and the Albion, Sublett, Deep Creek, Bannock, Pocatello and Portneuf ranges terminate on the southeast side.
The Western Snake River Plain is a southeast-northwest-trending topographic depression that extends from approximately the city of Twin Falls to the Oregon border. It is roughly perpendicular to the Eastern Snake River Plain and the Yellowstone Hotspot track.
While Eastern Snake River Plain was formed by warping directly along the Yellowstone hotspot track, the Western Snake River Plain was created by faulting. Passage of the hotspot approximately 12 million years ago triggered extension of the crust north of the hotspot track. Normal faults formed on either side of the Western Snake River Plain: a southwest-dipping fault on the northeast side (now known as the Boise Front fault) and a northeast-dipping fault on the southwest side (now known as the Owyhee Mountains fault).
Motion on these faults dropped the intervening block of crust, called a graben, down relative to the surrounding rock. Western Snake River Plain faulting was most active from about 11 million years ago to about 9 million years ago. Since then, the faults bounding the Western Snake River Plain graben have been moving very slowly. There is no evidence of movement on the Boise Front fault in the last ~2.6 million years; however, the Owyhee Mountains fault does have evidence for fault motion in the past ~500,000 years. The 2020 Stanley earthquake and its aftershocks (which are still ongoing!) are not directly related to the Western Snake River Plain or the Yellowstone Hotspot, but the same extensional forces did play a role in the sequence.
Fault-related subsidence of the Western Snake River Plain created space to accumulate water, sediments and lava flows. In fact, the Western Snake River Plain was occupied by the massive Lake Idaho from about 10 million years ago until about 2.5 million years ago. Lake Idaho fluctuated greatly in size throughout this time.
It left behind deposits of fine-grained sediments approximately 5,500 feet thick covering much of southwest Idaho. Those sediments include spectacular fossil assemblages found in the Hagerman Fossil Beds National Monument, which include horses, peccaries and otters. Around 2.5 million years ago the lake overtopped a drainage divide near present-day Huntington, Oregon, and was captured by the lower Snake River. The lake drained to the north, carving Hells Canyon, the deepest river gorge in North America.
The origin and development of the Snake River Plain illustrates the diverse and far-reaching geologic impacts of the Yellowstone Hotspot. That “smile” that marks the topography across southern Idaho is more than just a pretty face — er, geomorphic feature. It has an amazing geologic story to tell as well!
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.