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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 basalts ever to appear on the earth's surface engulfed about 163,700 km² (63,000 mile²) 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. 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. 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). The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Columbia River Plateau. Flowing from modern Eastern Oregon, the flows would make it as far as the modern Oregon Coast and form portions of the Oregon Coast Range. 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.
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