Not Just One and Done: Part 2
The First of the Three Uplifts that Raised the Colorado Plateau by Allyson Mathis
Many people, particularly folks who are planning a vacation to Arches and Canyonlands National Parks and Moab’s public lands, do not realize that this area is at high elevation. Unless someone is already inclined to think geologically, it may not occur to them that high elevation is the necessary prerequisite for canyon formation. But after people arrive here, the high elevation becomes evident, at least when they find themselves breathing hard on an uphill hike or when they view the crystal-clear night sky. (High elevation, combined with low humidity, relatively clean air, and low light pollution is the perfect combination for dark skies.)
Moab, even though it is at one of the lowest elevations in southeastern Utah, itself sits nearly twice as high as the well-known mountain town of Asheville, North Carolina. And 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; contrast this to the highest point east of the Mississippi which is 6,684 feet.
So the question becomes: How did the Southwest attain its high elevation? The tectonics (the large-scale processes that impact Earth’s crust) of the American Southwest is a bit more complicated than the textbook example of collisional plate tectonics like where two plates ram into one another (such as the Indian plate colliding with the Asian plate, creating the Himalayas). Along western North America, three tectonic plates (North American, Pacific, and Farallon) have interacted and impacted regions including the Colorado Plateau far from the plate boundaries.
Three separate periods of uplift are responsible for this region’s high elevation. This great elevation has enabled the Colorado River and its tributaries to incise into the land to carve canyons, as well as help sculpt the accompanying cliffs, rock domes, spires, and mesas. Last month’s Geology Happenings column (see the article archive on moabhappenings.com) discussed the first of these uplifts, the Laramide orogeny (mountain-building event), in some detail. This month, we will explore the latter two uplifts that have brought us to today.
The flat subduction of the Laramide orogeny was caused by an extra thick section of the Farallon plate that was too thick to normally descend underneath the continental plate. Diagram courtesy of
Karl Karlstrom.
The Laramide orogeny not only caused uplift, but it also set the stage for additional uplift events. The Laramide was an unusual tectonic event in that the subduction (the process by which one tectonic plate slides under another) that took place during it was at a shallow angle, causing tectonic consequences far from the plate boundary. The Laramide impacted an exceptionally wide swath of the continent, raising the Colorado Plateau and the Rocky Mountains that are even farther east. Diagram courtesy of Karl Karlstrom.
Remnants of the subducted Farallon plate have remained in the mantle underneath the Colorado Plateau since the end of the Laramide. Interactions of this slab with the surrounding mantle has led to the two uplifts that followed it.
The mantle is at very high temperatures, and although it is not molten, it has the consistency of a soft solid. (Butter at room temperature is a good example of a soft solid.) Hence, it has the ability to flow in its solid state. Colder regions sink because they are denser, and hotter areas rise. The tectonic plates essentially ride on top of the mantle, and can rise up or warp down as areas within the mantle do the same.
The La Sal Mountains formed during the time interval of the second period of Colorado River uplift. Photo by Ken Lund, CC BY-SA 2.0.
The second uplift of the Colorado Plateau occurred because the remnants of the Farallon plate underneath the Colorado Plateau broke off and sank through the mantle. A segment of a subducted plate like the Farallon is colder than the mantle surrounding it because it came from the cool surface of the Earth. Hotter areas of mantle flowed up and around the sinking slab and caused uplift of the surface. This event occurred between about 35 and 25 million years ago (the Mid Cenozoic). This upwelling also caused volcanic eruptions in western Utah and western Colorado as well the shallow igneous intrusions that formed the La Sal Mountains east of Moab.
The most recent stage of uplift occurred mostly within the last 10 million years or so and is also ultimately an after-effect of the subduction of the Farallon plate. The subducted Farallon plate fundamentally altered the mantle underneath the Colorado Plateau and caused another cycle of downwelling followed by hot mantle flowing around and above it, again causing uplift.
This complicated tectonic story of the three-part uplift of the Colorado Plateau may not be in most people’s immediate thoughts as they enjoy the canyon scenery from Dead Horse Point, Grand View Point in Canyonlands, or the Needles Overlook. But perhaps some will find the story compelling. Events that happened more than 70 million years ago and processes deep within the Earth played essential roles in setting the stage so that the Colorado River would be able to carve this beautiful landscape that we so love.
The Laramide orogeny, Mid Cenozoic, and mantle tomography 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.
A tomographic (cat-scan-like) cross-section of the mantle. Warm areas are in red, and cool areas (including remnants of subducted Farallon plate) are in blue. Areas of convection where hot mantle rises and cool mantle sinks are shown in white arrows. Diagram courtesy of Karl Karlstrom.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
Many people, particularly folks who are planning a vacation to Arches and Canyonlands National Parks and Moab’s public lands, do not realize that this area is at high elevation. Unless someone is already inclined to think geologically, it may not occur to them that high elevation is the necessary prerequisite for canyon formation. But after people arrive here, the high elevation becomes evident, at least when they find themselves breathing hard on an uphill hike or when they view the crystal-clear night sky. (High elevation, combined with low humidity, relatively clean air, and low light pollution is the perfect combination for dark skies.)
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.