Traveling west on Sixth Avenue from downtown Denver, the Front Range of the Colorado Rockies looms 2,500 meters above the mile-high city. Yet as magnificent as the panorama may be, it fails to do justice to the Laramide Orogeny, the process that started the Rocky Mountains' growth some 75 million years ago.
Figure 1. Theories of mountain building were revolutionized by the plate-tectonic theory in the 1960s. The horizontal motions of rigid plates of material above a pliable mantle helped explain the creation and destruction of oceans, the generation of mountain belts and sedimentary basins, the distribution of volcanic and earthquake activity, and the locations of ore, oil and gas deposits. Nonetheless, plate-tectonic theorists have had a difficult time accounting for many of the geological details of southwestern North America shown in this shaded relief map. For example, what caused the Rocky Mountains to be uplifted 2,000 kilometers away from a plate margin? Why did the Basin and Range region of Nevada, Utah and Arizona extend by over 100 percent shortly later? What produced the extensive volcanic deposits of the Columbia Plateau and Snake River Plain? The authors propose a fourth mechanism of mountain building, involving the interaction between mantle plumes and subduction zones, which adds a vertical element that helps explain these features.
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