The astoningly scenic canyons and cliffs that surround Moab result from erosion powered by flowing water and the force of gravity. Several factors have converged to form the unique red rock country of southeastern Utah: an abundance of colorful sedimentary rocks, a dry climate, and the areaʻs high elevation. The areaʻs overall high elevation is sometimes underappreciated as the central element in the formation of canyon country; but without it, there would be no canyons and no Moab.
Water flowing in rivers and streams is the primary erosional agent shaping land across the globe from the most subtle river valleys to the deepest gorges. A canyon is, by definition, a deep narrow valley. The canyons here have been carved by the Colorado and Green rivers and their tributaries as they incise into the land on their way to the Gulf of California.
Because rivers cut down to their base level (e.g., the lowest they can erode into the land), canyons cannot exist in areas of low elevation. (There is something known as the “Grand Canyon of the Everglades,” which is a rather nonsensical moniker as it is all of 3–4 feet deep.)
The Colorado Plateau (the Four Corners region where Utah, Colorado, New Mexico, and Arizona join) has an average elevation of more than 6,000 feet above sea level. Moab itself sits a bit lower at about 4,000 feet. The plateau obtained its high elevation after the time periods when its rock layers (formations to geologists) were deposited in low regions that were at or near sea level at the time.
The change from a low area where sediments accumulated (e.g., a depositional basin) to the high-elevation Colorado Plateau results from the complexities of the plate tectonics of the entire American West during the past 100 million years or so as three different tectonic plates (North American, Pacific, and Farallon) were interacting with one another. The Pacific plate is currently moving north relative to North America, and a small remnant of the Farallon (the Juan De Fuca) plate is currently subducting underneath Washington and Oregon.
The Laramide orogeny (mountain-building event) the first of the three uplifts that raised the Colorado Plateau. The Laramide orogeny was caused by the subduction of the Farallon plate. The second and third uplifts (e.g., the Mid-Cenozoic and Recent uplifts) both are consequent to this subduction and result from complicated dynamics in the mantle.
Subduction occurs when a thin oceanic plate slides underneath a thick buoyant continental plate. Normally, a subducting oceanic plate dives underneath the continental one at a fairly steep angle. This type of normal subduction occurred in the Sevier orogeny that impacted western Utah about 100 million years ago. Normal subduction creates narrow mountain ranges as the crust is compressed by the tectonic collision and from volcanic activity, such as in today’s Cascade Mountains in the Pacific Northwest.
The Laramide orogeny was different. The Laramide impacted a wide swath of western North America (as far east as Laramie, Wyoming from which it gets its name), and caused the uplift of the Colorado Plateau and the Rocky Mountains. The Colorado Plateau uplift occurred with minimal deformation of the rock layers although there are broad folds in areas such as the Monument Uplift that extends into Canyonlands National Park and the Kaibab Uplift at Grand Canyon.
Flat subduction is what enabled the Laramide orogeny to impact areas far from the tectonic boundary. The Laramide orogeny began when the subducting Farallon plate began descending at progressively shallower angles. This flattening was caused by a particularly thick section of oceanic crust that was part of the Farallon plate. It was not possible for this thick segment to steeply plunge underneath the continent like what normally occurs.
The thickened section of the Farallon plate was most likely the location of a former undersea mountain range (an oceanic plateau or rise). The Shatsky Rise in the northern Pacific today is considered to be a good analog for the thick section of the Farallon plate that caused the Laramide orogeny.
When the Farallon’s Shatsky Rise conjugate was subducted, the subduction angle to became shallower so that much of western North America was impacted. Plate tectonic models show this rise traveling underneath the Colorado Plateau in a northeasterly direction between about 84 and 68 million years ago, leading to this area’s first uplift. The Laramide orogeny also fundamentally changed the geology of southeastern Utah. Instead of being an area where sedimentary rocks were being deposited, it became one where they were being eroded away.
But the story of the Farallon plate doesn’t end with its subduction. Remnants of the plate have lingered in the mantle below the Colorado Plateau since Laramide time. Interactions between the Farallon slab and the surrounding mantle have led to the two additional periods of uplift that will be explored in the next installment of Geology Happenings.
The Sevier and Laramide orogeny figures are modified from Karl Karlstrom et al. 2022. Tectonics of the Colorado Plateau and Its Margins. Annual Review of Earth and Planetary Sciences. Used with permission of Karl Karlstrom.
Diagram courtesy of Karl Karlstrom
Diagram courtesy of Karl Karlstrom
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A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.