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Tucson Mountains moving southwest

Hold mouse cursor over image and
watch what happened millions of years ago

Tucson sits within a valley, between the Santa Catalina Mountains to the north and the Tucson Mountains to the west. How this came to be is an interesting story, and not without debate. The Tucson Mountains long have been labeled "The Tucson Mountain Chaos." But a story emerged in the 1980s that is making sense of the "chaos", but is hard to imagine. It goes like this... The Tucson Mountains were once the top of a large volcano, the bottom of which is the Santa Catalina Mountains. The top of the volcano slid 20 miles to the west, and the area between the Tucson and Catalina Mountains was dropped down to form the valley.

Below, I have presented the story in more detail. But first, you may want to familiarize yourself with some of the background by learning about Plate Tectonics, Four Geological Concepts, Faults and Folds, Rock Types, and A Brief Geological History of the Southwest.

Detachment fault
Place mouse cursor in image to see detachment fault occur
Normal faulting dropping down Tucson valley
Place mouse cursor in image to see what happens after Basin and Range Disturbance drops Tucson Valley and then erosion occurs

Ready? Let me give you the story in more detail as so well-summarized in the Sonorensis [12(1)7-10] by Dave Thayer. About 70 million years ago a huge volcano formed about where the Santa Catalina Mountains are today. The volcano collapsed in on itself, forming a circular basin called a caldera. Then about 30 million years ago crustal stretching began to occur as a result of Plate Tectonics. This stretching caused upper pieces of crust to slide across lower pieces of crust, resulting in detachment faults (see Geological Structures). One of these detachment faults caused the upper part of the caldera to slide 20 miles off the lower part of the caldera. Additional stretching during this period (Basin and Range Disturbance) caused many normal faults to occur across the southwest, including the Tucson area. The normal faulting caused chunks of land to drop, forming valleys (basins), and surrounding chunks of land to stay even or raise up, forming mountains (ranges). Where Tucson sits dropped 10,000' or more to form the valley (which has since filled in with at least 5000' of sediment). The upper caldera that was left higher than the valley became the Tucson Mountains. The lower caldera and underlying granite that was left higher on the north side of the valley became the Santa Catalina and Rincon Mountains. Much of the Tucson Mountains is composed of rhyolite, an extrusive igneous rock that was created during the eruption of the volcano. Much of the Santa Catalina Mountains is composed of granite, an intrusive igneous rock from the solidified magma below the volcano. However, the lower portions of the Santa Catalina Mountains have a layer of gneiss, a metamorphic rock that was created along the detachment fault when the granite was heated and put under pressure when "the Tucson Mountains" slid off the "Santa Catalina Mountains."