Columbia Plateau

The Columbia Plateau is a large geologic province located throughout large swaths of Washington, Oregon, and Idaho, chiefly east of the Cascade Range and west of the Rocky Mountains. It is composed of hundreds of flood basalt lava flows, dating back 17 to 6 million years ago, and as thick as 11,000’ in some places. Due to the rich volcanic soils in the Columbia Plateau, it has become a center of agriculture. Several cities, including Yakima, Kennewick, Pasco, Richland, Walla Walla, Lewiston, Othello, Hermiston, Quincy, and Moses Lake have sprung up in the region, with agriculture being their main sources of economy. Though the area is home to a rich agricultural scene with extensive farms and many cities, the Columbia Plateau still has its fair share of nature. The land is characterized not only by flat farmland, but also by rolling hills, deep canyons carved by great rivers, gorgeous waterfalls, and a large geologic feature known as the Channeled Scablands. All that is to say that the Columbia Plateau has its fair share of natural places and interesting geology.

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The geological story of the Columbia Plateau begins roughly 17 million years ago during the Miocene Epoch. At this time, massive amounts of lava began erupting from fissures mainly centered in southeastern Washington and northeastern Oregon. This lava was chiefly mafic in composition, erupting as a basalt known as the Columbia River Flood Basalts. The lion’s share of these eruptions occurred between 17 and 14 million years ago, though they continued up until about 6 million years ago in the area. The Columbia River Flood Basalts cover more than 77,200 square miles (200,000 square kilometers), have a volume exceeding 52,800 cubic miles (220,000 cubic kilometers), and have a maximum thickness of 11,000’, with an average thickness of roughly 6,000’. 
 

The rationale behind why the Columbia River Flood Basalts erupted is still a mystery in the geological community, but the most widely accepted hypothesis is that it was due to hot spot magmatism. In geology, a hot spot is an anomalously hot plume of magma from deep in the mantle. One of the fundamental principles of Physics is that heat rises, and as such, this exceedingly hot mantle plume rises towards the surface of the earth- melting the crust and generating volcanoes to form and erupt. Some prime examples of volcanic hot spots include the Hawaii, Iceland, and Yellowstone Hot Spots. The location of volcanic hot spots are independent of tectonic plate boundaries and remain stationary- and can create long chains of volcanoes as the tectonic plate above them moves. This is easily seen in the Hawaiian Hot Spot- it’s in the middle of the Pacific Plate right now, but you can follow the Hawaiian and Emperor Seamounts (extinct volcanoes from the Hawaiian hot spot) all the way to the Kamchatka Peninsula of Russia, showing evidence of the Pacific Plate moving in northwestern fashion. Similarly, the extinct volcanoes of the Yellowstone hot spot can be traced all the way to northwestern Nevada. 

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Scientists are still gathering evidence, but it has been proposed that the Columbia River Flood Basalts are derived from the same hot spot as Yellowstone- and that this hot spot is also responsible for the accreted Siletzia Terrane (a large marine igneous province off the coast of North America from 55 MA that was accreted onto the western margin of North America around 50 MA) in the Coast Range of Oregon and Washington. The Columbia River Flood Basalts created a large igneous province- a very large accumulation of igneous rocks. As the flood basalts erupted, the Earth’s crust gradually sank in the space left by the lava, forming a large yet depressed lava plateau. The flood basalts altered the course of the ancestral Columbia River, forcing it into its northwesterly course today. Lava permeated through valleys and streambeds first, damming water courses to form lakes. As such, several fossils, including leaf impressions, animal bones, insects, and petrified wood can be found in these dried lake beds.

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The geological history of the Columbia Plateau really takes an even more eventful turn from 18,000-13,000 years ago, during the last Ice Age. During this time, a famous geological area known as the Channeled Scablands was created. The Channeled Scablands are a series of interconnected flood channels, coulees, and waterfalls throughout southeastern Washington, and were created by over 40 cataclysmic floods as the massive Cordilleran Ice Sheet just north of this area melted as the climate warmed at the end of the ice age, in events known as the Missoula Floods. Palouse Falls, the official state waterfall of Washington, and a very beautiful waterfall, is a great place to see a Channeled Scabland.

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Deep canyons have been cut in the Columbia Plateau, by the likes of the Snake River, among other rivers and creeks. The Snake River carved out Hells Canyon, the deepest gorge in the United States at a maximum depth of 8,000’ (2,000’ deeper than the Grand Canyon), on the margin of Oregon and Idaho. Furthermore, several deep canyons exist in the Columbia Plateau, such as Joseph Canyon in northeastern Oregon.

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Most recently, much of the Columbia Plateau has been crumpled into a series of synclines and anticlines known as the Yakima Fold and Thrust Belt, extending from approximately Ellensburg, WA to Pendelton, OR. These Anticlines have formed large ridges, such as Manastash Ridge, Umtanum Ridge, Saddle Mountain and the Horse Heaven Hills, among other ridges. These anticlines are bounded by active thrust faults, suggesting active tectonic compression in the area, most likely due to rotation of the North American Plate. Geodesy studies have shown that Oregon is rotating about a point near Lewiston, Idaho, at a rate of roughly 3 millimeters per year, and the Yakima Fold and Thrust Belt is where this rotation is compressing Washington. The most likely source of this rotation is the crustal extension of the Basin and Range province of southern Oregon, Nevada, western Utah, and southern Idaho. The Sierra Nevada and Klamath Blocks are stable areas of crust just west of the Basin and Range, so the east-west motion of the Basin and Range is most likely deflected into a north-south orientation by the Sierra Nevada and Klamath Blocks, causing Oregon to rotate and causing the crustal compression in the Yakima Fold and Thrust Belt.

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Some marquee areas to explore in the Columbia Plateau include Palouse Falls, Grand Coulee, Saddle Mountain, Lewiston, Hells Canyon, Joseph Canyon, Walla Walla, and any area in the Channeled Scablands.