Photo by D. Sillman
We left Utah and the remnants of ancient Lake Bonneville behind and drove through the northern edge of the Basin and Range topography in southern Idaho. The short, north-south running mountain ranges and their flat, surrounding sagebrush steppes, though, stop after about 50 miles and suddenly smooth out into a broad, flat plain that is more midwestern than western in appearance. It is like we crossed into Iowa or Kansas (except for the surrounding, hazy rim of distant mountains). We were on the Snake River Plain!
The Snake River Plain is a 400 mile long, 50 to 125 mile wide, arc-shaped depression that runs from a gap between the Sierra Nevada and the Cascade mountains in the west all the way to the Yellowstone Plateau in northwestern Wyoming. The plain is a flat, smile-shaped funnel through which moisture from the Pacific Ocean flows directly into the high elevations around Yellowstone. In the winter, this moisture conduit generates large amounts of snow in northwestern Wyoming!
The plain is bisected by the Snake River which cuts a great canyon through the thick, underlying mass of rock. You can’t see the Snake River from any distance away! You need to drive or walk up to the canyon rim and look down into the broad, deep gash in order to see its shimmering sliver of water.
Photo by D. Sillman
Elevations on the Snake River Plain are a thousand to almost two thousand feet lower than the elevations of the cities back in Utah. Snowville, Utah, near the Idaho border, is 4547 feet above sea level while Twin Falls, Idaho (our destination for the night), in the central region of the Plain, is only 3743.’ The slightly more distant Boise Idaho, in the western section of the Plain, through which we will be driving tomorrow, is just 2730’ above sea level!
The Snake River Plain has extensive mountains to its north and extensive mountains to its south. On a map it looks as though the mountains, that logically should be on the Plain, have either been melted away or mowed down by some force. On the ground, though, it just looks like a flat expanse with little hint as to how it came into being.
The Plain is a site of intense agricultural activity. Irrigation rigs span broad fields of alfalfa and hay, and there are abundant fruit and vegetable stands along the roadside which must be stocked in season with locally grown produce. Some of these fields must have potatoes, too! After all, this is Idaho! Most of the agricultural production of Idaho occurs on the Snake River Plain, and most of the large cities of the state and most of its human population are found here, too.
Invasive Russian olive trees, I. Beal, Flickr
There is an abundance of invasive plants growing in uncultivated areas on the Plain. Extensive roadside patches of Alyssum (“basket-of-gold”) are in bright, yellow flower, and thick, gray-green leafed stands of Russian olive line most of the streambanks and sides of irrigation ditches. I think I see (again, in passing empty fields at 80 mph) clusters of Russian thistle (tumbleweeds!!) (see Signs of Spring 6, April 8, 2021). This is a place with a long history of human use!
So, why is this odd plain here? The causative agent was once directly, deep underground and, possibly, now is “living” (as much as geological entities can be said to be alive!) in northwest Wyoming!
Diagram by Kevinsong, Wikimedia Commons
Picture the physical structure of the Earth: the surface layer (on average, the upper 9 to 12 miles or so) is called the “Crust.” This is the layer we live on. The crust makes up the surface of the terrestrial continents and also the sea beds of the oceans. The continental parts of the crust are primarily made up of granite, a slowly cooled, large crystal, igneous rock, while the floors of the oceans are made up of basalt, a rapidly cooled, small crystal, igneous rock. Basalt, though, can also be found on continents in places where extruded lava has reached the crust surface and then rapidly cooled. Both the continental crust and the oceanic crust are broken up into great plates of rock (the “tectonic plates”) that float about on the hot (but still solid) rocks of the next layer of the Earth that is called the “Mantle.”
The mantle is 1800 miles thick and makes up 84% of the volume of the Earth. The heat in the mantel (which comes from the continuous radioactive decay of elements like uranium, thorium and potassium) causes the mantle rocks to move in great, slow, convection currents, and this movement pushes the tectonic plates in all sorts of directions. Plates crash into each other and slide under each other (like we described in the Laramide orogeny and the formation of the Rocky Mountains (see Signs of Summer 2, June 10, 2021)). Heat from the mantel can also melt crustal rocks and generate magma which can then escape from its subterranean places of origin and spill out across the surface of the crust as volcanic lava!
Yellowstone hot spot diagram. By Kbh3rd, Wikimedia Commons
Interesting, there are spots in the mantel that are stationary, and many of these stationary spots are also extremely hot. The moving tectonic plates of the crust that pass over these non-moving, mantel “hot spots,” then, are heated to a point where the crustal rocks melt, the crust thins out and volcanoes can form. The islands of Hawaii and the Galapagos Archipelago, for example, arose from chains of volcanoes that formed on underlying tectonic plates as they passed over fixed hot spots in the Pacific Ocean.
Path of Yellowstone hot spot,. Diagram by K. Case. Wikimedia Commons
There are dozens to hundreds of these hot spots around the Earth, and one of them, called “The Yellowstone Hot Spot,” sat under the westward moving North American tectonic plate right here in what is now southern Idaho. Over a 12 million year, well-defined sequence of volcanic activity on the moving North American plate, the Yellowstone Hot Spot melted the mountains in the crust over the Snake River Plain and formed the shrunken, flattened topography of the Plain! That hot spot is, of course, currently under Yellowstone Park in northwestern Wyoming and is the energy source for the hot springs, boiling mud pits and geysers of the park. It is also the energy source for the past (and future!) Yellowstone “super-volcanoes” (see Signs of Fall 5, October 5, 2017).
Driving up and out of the Snake River Plain, entering Oregon and then Washington State, we see evidence of even more volcanic activity. The Columbia Plateau and then the volcanoes of the High Cascades are the topics for next week’s blog!