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BRIEF GEOLOGICAL HISTORY OF THE SOUTHWEST

This section briefly describes the formation of the Sierras, the Rockies, and Basin and Range, and the Colorado Plateau.

The Sierras and Cascades formed as the Farallon Plate subducted beneath the North American Plate, creating the volcanic mountain chain (see Plate Tectonics for how this works).

The Rocky Mountains formed when the Farallon Plate, subducting beneath the North American Plate, "stuck,", causing the interior of the North American Plate to compress and fold up into a mountain range.

The Basin and Range formed as the last of the Farallon Plate subducted beneath the North American Plate, causing the North American Plate to form a transform plate boundary with the Pacific Plate. This caused the crust to stretch throughout most of the southwest. As the crust stretched, the tension caused normal faulting throughout the region. The normal faults caused down-dropped valleys (basins) and uplifted mountains (ranges). This is why we have so many mountain ranges separated by valleys in the southwest. The Tucson Basin is one of these down-dropped valleys and the Santa Catalina Mountains and Tucson Mountains are the uplifted ranges. See picture to right of mini-version of Basin and Range.

As for the Colorado Plateau, it withstood the upfolding during the uplift of the Rockies and it withstood the normal faulting of the Basin and Range disturbance. This is why most of the layers of the Grand Canyon are relatively horizontal. However, the Colorodo Plateau was uplifted and rotated somewhat during this time.