The Evolution of the Colorado Plateau and Colorado River
By Mary Gavan and Dave Wegner
The mighty Colorado River begins its journey from a trickle of water high in the Rocky Mountains of Colorado. Before reaching its final destination, the Sea of Cortez in northwestern Mexico, it travels a distance of more than 1,400 miles. During its course, the river cuts through nearly 2 billion years of earth’s geologic history, representing three distinct geologic provinces (the Rocky Mountain, Colorado Plateau, and Basin and Range Provinces).
To understand the physical Colorado River of today we need to take a step back in geologic time and explore the events that set the stage for the formation of today’s Colorado River, and two of its main tributaries (the Green and San Juan Rivers). First a word or two about geologic time.
The key point to remember is that the geologic evolution of the Colorado River Basin occurred over millions of years and included many geologic processes.
70 to 40 million years ago, as tectonic plates were subducting (diving under) the western margin of North America, a major mountain-building event occurred inland as the Earth’s crust was pushed up by the forces beneath it. This is known as the Laramide Orogeny. The event generated deformation (crustal buckling) of the Earth’s crust, uplifting mountain ranges, forming basins, and creating widespread volcanism in the western United States. The deformation caused by the Laramide Orogeny uplifted the Rocky Mountains nearly 1,000 miles inland from the subduction zone boundary. This event also ultimately led to the uplift of the Colorado Plateau, although timing of the uplifting is generally believed to have occurred later.
An interesting phenomenon related to the Colorado Plateau is that the area only experienced moderate deformation during the same event that uplifted the Rocky Mountains, leaving it mostly undeformed. For this reason, sedimentary rock on the plateau is generally flat-lying.
As the early days of the Colorado River began, the source areas for the two primary tributaries, the Green and San Juan Rivers were also being formed on the edges of the Rocky Mountain Province. For the Green River, the Wind River Range was emerging in today’s central Wyoming, while the San Juan Mountains were rising in the southwest corner of the present state of Colorado. Both of these mountain ranges are composed of igneous (volcanic origin) rock. All three major river components of the Colorado River find their sources high in the mountains and in igneous and metamorphic rocks.
Once the source areas for the rivers were uplifted, erosion in the form of glaciers and precipitation (rain) began to cut away at the land. How does a river form? As water runs downhill, the force of the water and abrasion from sediment that it carries with it, cut into the soil, forming channels. As channels flow downslope they merge in low-lying areas, where they become bigger streams and rivers. From a trickle of water, a river is born.
During the past two million years of earth’s history, glaciation advanced on this region. Remnants of these former glaciers remain today in the form of U-shaped valleys and cirques, forming the scenic and dramatic landscape in Rocky Mountain National Park.
The Laramide mountain-building event left behind a trail of hard, dense, and durable rock, consisting of both igneous and metamorphic rock. The nature of this type of rock and the effect erosion has on it is worth noting. Hard, dense, and durable rock is more resistant to erosion, breaking down slowly in comparison to sedimentary rock. In fact, it’s so slow, that as a river flows through it, little sediment is created or picked up and carried downstream, allowing the river to be clear most of the time.
As the Colorado River emerges from the Rocky Mountain Province it flows southwest from its headwaters and enters a different geologic area, the Colorado Plateau Province. From this point on, the river flows through primarily sedimentary rock with occasional pockets of igneous and metamorphic materials.
Point of context, the Rocky Mountain Province developed 70 to 40 million years ago. Prior to two billion years ago however, the Colorado Plateau did not even exist. An enormous geologic event occurred about 1,750 million years ago, when a group of volcanic islands approached the southern boundary of North America, causing a collision. During this collision event, North America’s exposed southern boundary was near present-day Utah and southern Wyoming. New continental crust was added to the continent in the process that ultimately would become the crystalline basement rock and the foundation for the present day Colorado Plateau. Some of the deepest canyons on the plateau expose this ancient rock, which is made up of both igneous and metamorphic rock.
Erosion went to work again and over millions of years wore the basement rock down to a flat surface. Six hundred million years ago, the surface was near sea level, providing space for accumulation of sediment.
The Colorado Plateau showcases layer upon layer of brilliantly colored sedimentary rock. These layers of sediment were deposited during a time when tectonic activity had ceased between the ocean and the continent 525 to 381 million years ago. Seas periodically submerged the region, contributing layers of limestone, and nearshore sand deposits that ultimately resulted in sandstone. Periodically, the seas would withdraw, allowing erosion of older deposits, and advancement of windblown sand from exposed areas. Approximately 250 million years ago, tectonic activity resumed as the supercontinent of Pangea formed. When this occurred, deposition of marine sediment subsided, and vast areas of sand dunes dominated the region.
As the Colorado River nears the present day northwestern boundary of Arizona and Nevada, it leaves the Colorado Plateau, and enters the final geological province of the river, the Basin and Range Province.
The Basin and Range Province began to develop about 20 million years ago. The cause of this disturbance is highly controversial amongst researchers. One possibility is that tension in the earth’s crust, potentially developed as a result in a change in tectonic activity to the west, stretched and thinned, and in some places resulted in uplift as much as twice the crusts original width. This geologic stretching and extension split the region, resulting in down-dropped basins and narrow, elongated mountain ranges. The topography was characterized by Clarence Dutton (a geologist in the late 1800s) as an “army of caterpillars marching toward Mexico”.
Over time, the mountain ranges were subjected to extensive weathering and erosion that filled these basins with sediment. These sediment filled basins are today the flat, dry deserts that alternate with mountain ranges over and over again in the western United States. The Colorado River flows through these deserts of the Basin and Range Province on its journey to its natural terminus, the Sea of Cortez.
We have examined in these few paragraphs as explanation of how the Colorado River Basin came to be and what forces helped to shape this region into the landscape and rivers visible today. Now, one might ask, “when did the Colorado River become a thru-going river?”
Uplift of the Rocky Mountains created by the Laramide Orogeny, 70 to 40 million years ago, established an elevated platform for canyons to form (remember a river needs an elevation difference so water can flow from high to low). Once the mountains rose, water flowing down them resulted in extended periods of erosion resulting in formation of canyons and the transport of sediment downstream. In the case of the Colorado River, where and when it captured another stream(s) is still a subject of debate. There is, however, general agreement by most researchers that the Colorado River, by at least 6 million years ago, was a river that flowed from the Rocky Mountains to the present Pacific Ocean.
An important aspect of the Colorado River story is that the marine and desert origin rock in the Colorado River Basin produces an immense amount of sediment. Much of that sediment today is trapped by dams and in other places it is captured along the way in the river channel and in the floodplain. Under natural circumstances, the produced sediment will flow downstream, all the way to the Sea of Cortez. That will be the focus of a future discussion.
Sources:
Blakey R, Ranney W. 2008. Ancient Landscapes of the Colorado Plateau. First Edition. Grand Canyon (AZ): Grand Canyon Association.
Easterbrook DJ. 1999. Surface Processes and Landforms. Second Edition. Upper Saddle River (NJ): Prentice-Hall, Inc.
Prothero DR, Dott R Jr. 2004. Evolution of the Earth. Seventh Edition. New York (NY): McGraw-Hill.
