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November 21, 2022

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.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Zach Lifton, geologist with the Idaho Geological Survey.

Ansel Adams photo of the Snake River and Teton Range
The Snake River flowing below the Teton Range. Photo by Ansel Adams, 1942, public domain,

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 (SRP) 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 (ESRP) and Western Snake River Plain (WSRP) were formed by very different geologic processes.

The ESRP 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 4.5 kilometers (2.8 miles) compared to the surrounding rocks! Faulting is a common way for crustal rocks to move, but in the case of the ESRP the subsidence occurred by flexing and warping of the crust. The warping of rock surrounding the ESRP 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.

Map of southern Idaho and the Snake River Plain
Map of southern Idaho and the Snake River Plain, showing the eastern (ESRP) and western (WSRP) parts of the geologic province. Map by Zach Lifton, Idaho Geological Survey.

The WSRP is a southeast-northwest-trending topographic depression that extends from approximately the city of Twin Falls to the Oregon border and that is roughly perpendicular to the ESRP and the Yellowstone Hotspot track. While the ESRP was formed by warping directly along the Yellowstone hotspot track, the WSRP 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 WSRP: 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. WSRP faulting was most active from about 11 million years ago to about 9 million years ago. Since then, the faults bounding the WSRP 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 WSRP or the Yellowstone Hotspot, but the same extensional forces did play a role in the sequence.

Cross section of the Western Snake River Plain, Idaho
Cross section of the Western Snake River Plain, Idaho. From USGS Groundwater Atlas of the United States, 1994,

Fault-related subsidence of the WSRP created space to accumulate water, sediments, and lava flows. In fact, the WSRP 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 1,700 meters (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!

Reconstructed fossil horse skeleton found at Hagerman Fossil Beds National Monument, Idaho
Reconstructed fossil horse skeleton found at Hagerman Fossil Beds National Monument, Idaho. National Park Service photo,

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