Idaho’s Snake River Plain: A tale of two basins – Idaho Capital Sun

The Snake River Plain is a outstanding river drainage that cuts a broad “smile” throughout southern Idaho, simply recognizable from satellite tv for pc imagery. The geologic historical past of the Japanese Snake River Plain and the Yellowstone Hotspot observe are intently intertwined, however the Western Snake River Plain has a special story to inform.

The Snake River flows greater than 1,000 miles from its headwaters in Yellowstone via Jackson Gap, throughout southern Idaho’s Snake River Plain, and thru Hells Canyon earlier than becoming a member of the Columbia River in south-central Washington. The river’s course covers an extremely various vary of geology and topography. The Snake River additionally carried flood waters from the huge Bonneville and Missoula Ice Age floods.

At the moment, the Snake River Plain is house to eight of the ten most populous cities in Idaho and helps a lot of the state’s agriculture business. Whereas the Snake River now flows seamlessly throughout southern Idaho, the Japanese Snake River Plain and Western Snake River Plain have been fashioned by very totally different geologic processes.

The Japanese Snake River Plain is a northeast-southwest-trending topographic melancholy in southeastern Idaho that extends from the Wyoming border to roughly the town of Twin Falls. Because the North American tectonic plate moved southwest, the (principally) stationary Yellowstone hotspot plume heated the crust and generated a big quantity of soften. This resulted in a line of volcanic calderas that get progressively older alongside the hotspot observe from northeast to southwest.

As dense magma generated by the hotspot gathered within the center crust, the additional weight triggered the crust to sink. The sinking crust gathered further sediments and volcanic rocks on the floor, which triggered additional sinking.

Finally, the rocks alongside the hotspot observe subsided by roughly 2.8 miles in comparison with the encircling rocks. Faulting is a typical approach for crustal rocks to maneuver, however within the case of the Japanese Snake River Plain, the subsidence occurred by flexing and warping of the crust. The warping of rock surrounding the Japanese Snake River Plain might be clearly seen in dipping rock layers on the margins of the plain the place the Misplaced River, Lemhi and Beaverhead ranges terminate on the northwest facet and the Albion, Sublett, Deep Creek, Bannock, Pocatello and Portneuf ranges terminate on the southeast facet.

The Western Snake River Plain is a southeast-northwest-trending topographic melancholy that extends from roughly the town of Twin Falls to the Oregon border. It’s roughly perpendicular to the Japanese Snake River Plain and the Yellowstone Hotspot observe.

Whereas Japanese Snake River Plain was fashioned by warping immediately alongside the Yellowstone hotspot observe, the Western Snake River Plain was created by faulting. Passage of the hotspot roughly 12 million years in the past triggered extension of the crust north of the hotspot observe. Regular faults fashioned on both facet of the Western Snake River Plain: a southwest-dipping fault on the northeast facet (now referred to as the Boise Entrance fault) and a northeast-dipping fault on the southwest facet (now referred to as the Owyhee Mountains fault).

Movement on these faults dropped the intervening block of crust, referred to as a graben, down relative to the encircling rock. Western Snake River Plain faulting was most energetic from about 11 million years in the past to about 9 million years in the past. Since then, the faults bounding the Western Snake River Plain graben have been transferring very slowly. There isn’t any proof of motion on the Boise Entrance fault within the final ~2.6 million years; nevertheless, the Owyhee Mountains fault does have proof for fault movement prior to now ~500,000 years. The 2020 Stanley earthquake and its aftershocks (that are nonetheless ongoing!) should not immediately associated to the Western Snake River Plain or the Yellowstone Hotspot, however the identical extensional forces did play a task within the sequence.

Fault-related subsidence of the Western Snake River Plain created house to build up water, sediments and lava flows. The truth is, the Western Snake River Plain was occupied by the huge Lake Idaho from about 10 million years in the past till about 2.5 million years in the past. Lake Idaho fluctuated vastly in dimension all through this time.

It left behind deposits of fine-grained sediments roughly 5,500 toes thick protecting a lot of southwest Idaho. These sediments embody spectacular fossil assemblages discovered within the Hagerman Fossil Beds Nationwide Monument, which embody horses, peccaries and otters. Round 2.5 million years in the past the lake overtopped a drainage divide close to present-day Huntington, Oregon, and was captured by the decrease Snake River. The lake drained to the north, carving Hells Canyon, the deepest river gorge in North America.

The origin and improvement of the Snake River Plain illustrates the various and far-reaching geologic impacts of the Yellowstone Hotspot. That “smile” that marks the topography throughout southern Idaho is greater than only a fairly face — er, geomorphic function. It has an incredible geologic story to inform as nicely!

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.