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Since the broad acceptance of the plate-tectonic theory in the late 1960s, mountains have been thought to form through three distinct tectonic processes, all the result of essentially horizontal forces. The authors propose to add a fourth mountain-building mechanism—the vertical motion of hot plumes of material rising from the earth's core-mantle boundary—and site as an example the complicated structures of the southwestern U.S. The westward progress of the North American plate over the relatively stationary Yellowstone plume during the past 75 million years may explain geological features as diverse as the Laramide Orogeny, the distended Basin and Range Province, and the accretion of exotic terranes along the continent's west coast.
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