| Columbia River Basalt Group |
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The Columbia River Basalt Group is a Large Igneous Province that lies across parts of the states of Washington , Oregon , and Idaho in the United States . During late Miocene and early Pliocene times, one of the largest Flood Basalt s ever to appear on the Earth 's surface engulfed about 163,700 km&2 (63,000 mile&2) of the Pacific Northwest , forming a Large Igneous Province with an estimated volume of 174,300 km³. Eruptions were most vigorous from 17—14 million years ago, when over 99% of the basalt was released. Less extensive eruptions continued from 14—6 million years ago.1 These lava flows have been extensively exposed by the erosion resulting from the Missoula Floods , which laid bare many layers of the basalt flows at Wallula Gap , the lower Palouse River , the Columbia River Gorge and throughout the Channeled Scablands . FORMATION OF THE COLUMBIA RIVER BASALT GROUP Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 1.8 km (6,000 feet). As the molten rock came to the surface, the earth's crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Columbia River Plateau . The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found Fossil leaf impressions, Petrified Wood , fossil insects, and bones of vertebrate animals. 2 3 ''Portions of this article, including a figure, are adapted from Works Of The United States Government , which are in the Public Domain ''4 Catastrophic beginnings Canyon just downstream of Palouse Falls, the Sentinel Bluffs flows of the Grand Ronde Formation can be seen on the bottom, covered by the Ginkgo Flow of the Wanapum Basalt.]] Prior to 17 million years ago, the Western Cascade Stratovolcano es erupted with periodic regularity for over 20 million years, even as they do today. An abrupt transition to Shield Volcanic flooding occurred in the mid- Miocene . The ultimate cause of this volcanism is still up for debate, however, the most widely accepted idea is that a mantle plume or upwelling (similar to that associated with present day Hawaii) initiated widespread and voluminous basaltic volcanism, ~17 million years ago. While it has been postulated that the flood basalt volcanism was triggered by a meteorite impact in southeastern Oregon, no geologic evidence has been found (as of 11/25/2006) to support this hypothesis. The flows can be divided into major categories: The Grand Ronde Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt. The ages of the various lava flows have been dated by radiometric dating—particularly through measurement of the ratios of isotopes of potassium to argon. .]] Imnaha Basalt The oldest of the flows, from 17.4 million to 17.0 million years ago, is the Imnaha Basalt. The Imnaha flows welled up across northeastern Oregon. Although estimates are that this amounts to about 10% of the total flows, they have been buried under more recent flows, and are visible in few locations. Grand Ronde Basalt The next oldest of the flows, from 17 million to 15.6 million years ago, make up the Grand Ronde Basalt. Units (flow zones) within the Grand Ronde Basalt include the Meyer Ridge and the Sentinel Bluffs units. Geologists estimate that the Grand Ronde Basalt comprises about 85% of the total flow volume. It is characterized by a number of dikes called the Chief Joseph Dike Swarm through which the lava upwelling occurred (estimates range to up to 20,000 such dikes). Many of the dikes were fissures 5-10 meters wide, allowing for huge quantities of magma upwelling. Much of the lava flowed north into Washington as well as down the Columbia River channel to the Pacific Ocean ; the tremendous flows created the Columbia River Plateau . The weight of this flow caused central Washington to sink, creating the broad Columbia Basin in Washington. Wanapum Basalt , Washington. The upper basalt is Roza Member, while the lower canyon exposes Frenchmen Springs Member basalt.]] The Wanapum Basalt is made up of the Eckler Mountain Member (15.6 million years ago), the Frenchmen Springs Member (15.5 million years ago), the Roza Member (14.9 million years ago) and the Priest Rapids Member (14.5 million years ago).5 Saddle Mountains Basalt The Saddle Mountains Basalt, seen prominently at the Saddle Mountains , is made up of the Umatilla Member flows, the Wilbur Creek Member flows, the Asotin Member flows (13 million years ago), the Weissenfels Ridge Member flows, the Esquatzel Member flows, the Elephant Mountain Member flows (10.5 million years ago), the Bujford Member flows, the Ice Harbor Member flows (8.5 million years ago) and the Lower Monumental Member flows (6 million years ago). SEE ALSO EXTERNAL LINKS
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