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Columbia Plateau
The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbia River. In one of various usages, the term "Columbia Basin" refers to more or less the same area as the Columbia Plateau.〔Floyd, Ben, et al. "(Glossary )". (1998) (Hanford Reach Protection and Management Program Interim Action Plan ). Prosser, Washington: Benton County Planning Department.〕
== Geology ==
(詳細はMiocene and early Pliocene times, one of the largest flood basalts ever to appear on the earth's surface engulfed about of the Pacific Northwest, forming a large igneous province.〔(Description: Columbia Plateau Columbia River Basalt. ) United States Geological Survey, accessed October 9, 2007.〕 Over a period of perhaps 10 to 15 million years, lava flow after lava flow poured out, ultimately accumulating to a thickness of more than 6,000 feet (1.8 km).〔 As the molten rock came to the surface, the Earth's crust gradually sank into the space left by the rising lava. The Columbia River Basalt Group consists of seven formations: The Steens Basalt, Imnaha Basalt, Grande Ronde Basalt, Picture Gorge Basalt, Prineville Basalt, Wanapum Basalt, and Saddle Mountains Basalt. Many of these formations are subdivided into formal and informal members and flows.〔http://volcanoes.usgs.gov/observatories/cvo/cvo_columbia_river_basalt.html〕〔
The subsidence of the crust produced the large plateau—a large, slightly depressed lava plain sometimes also known as the Columbia Basin.〔 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.〔 Entities found in these lake beds include fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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