Covering most of central and eastern Washington, northern and northeastern Oregon and portions of western Idaho, the Columbia River Plateau formed from basalt lava flows, 17 to 6 million years ago; in fact, most of the lava, which spread across 65,000 square miles, poured out within a span of 2 million years (17-15 million years ago). Some 300 major vents have been identified and many more small ones are thought to have existed. One of the basalt flows, which ran from western Idaho to the Pacific, is the longest known lava flow in Earth's history. The thickness of the basalt varies across the Plateau, with some areas exceeding 8000 feet.
The cause for these numerous and massive lava flows is the subject of ongoing scientific debate. Most geologists feel that the rise of the Cascades, which began 36 million years ago and continues today, caused the basin between the Cascades and Rockies to stretch and thin out; the development of mantle plumes beneath the basin may have also played a role. Vents likely occured along fracture lines between the many terrains and island arcs that had merged to form the Pacific Northwest.
Today, the massive Plateau has been molded by the Columbia River and its numerous tributaries; the Snake River, which rises east of the Tetons and flows across southern Idaho, cuts through the southeastern portion of the Plateau before merging with the Columbia. As impressive as the Columbia River Plateau is, its total lava flow is but 10% of the flow at the Deccan Traps (India) and Siberian Traps.
