The Colorado Rocky Mountains are the highest portion of the 1,900-mile Rocky Mountain chain that stretches from northern British Columbia, Canada, to southern New Mexico. Colorado contains 78 of the 100 highest peaks in the chain, including the 30 tallest. The mountains are the state’s iconic feature and the primary attraction for the 82.4 million people who visited in 2017. Those visitors spent a record $19.7 billion and placed Colorado ninth on the list of tourist-attracting states.
The Colorado Rockies are spread across several distinct ranges, the main ones being the Front, Sawatch, Park-Gore, Mosquito-Tenmile, Sangre de Cristo, Wet, Elk, White River, and San Juan Ranges. At 14,440 feet, Mount Elbert, in the Sawatch Range, is the highest peak in both the state and the Rockies. Most mountain ranges rise along plate tectonic boundaries and are supported by an unusually thick crust called a crustal root; however, Colorado’s Rockies are unique because they formed far from plate boundaries and lack a crustal root.
Colorado Before the Rockies
Geologists use the term orogeny to refer to mountain-building episodes. The terrain that includes northern Colorado was added to the North American continent about 1.7 billion years ago during a continental collision called the Yavapai Orogeny. The state’s southern part was added in a similar collision, the Mazatzal Orogeny, about 100 million years later. Between the welding of Colorado to North America and the rise of today’s Rockies, two important geologic events occurred: the building of the Ancestral Rocky Mountains about 300 million years ago and the submergence of Colorado beneath the Western Interior Seaway between about 100 million and 70 million years ago.
The Ancestral Rockies consisted of two main mountain ranges. One, known to geologists as Uncompahgria, stood approximately where today’s Uncompahgre Plateau rises in western Colorado. The other, Frontrangia, stood in the same place as today’s Front Range. Whereas today’s Rocky Mountains rise above the Great Plains and Colorado Plateau, Uncompahgria and Frontrangia were mountainous islands that rose from a tropical sea, as Colorado then stood near the equator. Rock eroded from the ranges was deposited along the islands’ coasts and filled the shallow sea in between, where today’s Elk and Sawatch Ranges stand. These layers of conglomerate, sandstone, and mudstone rock form much of the state’s most iconic scenery, including Boulder’s Flatirons, Denver’s Red Rocks Amphitheater, Roxborough State Park, Balanced Rock at Colorado Springs’ Garden of the Gods, and Aspen’s Maroon Bells.
By 150 million years ago, the Ancestral Rockies were eroded down to sea level, and the state was a vast, low-elevation plain reminiscent of Mississippi and Louisiana today. Lazy, meandering rivers that crossed the plain deposited shale and sandstone that make up today’s Morrison Formation, which is famous for its rich trove of dinosaur fossils. Many famous Jurassic dinosaur species, including Stegosaurus and Apatosaurus, were first discovered in Colorado’s Morrison Formation.
About 100 million years ago, the sea level rose, submerging the North American mid-continent. Rivers and erosion from the surrounding land deposited beach sand in Colorado along the flanks of the Western Interior Seaway. Later burial and cementation of that beach sand formed the erosion-resistant Dakota Sandstone, which forms an important petroleum reservoir in the Denver Basin, one of the nation’s most productive petroleum provinces.
Colorado continued to sink for roughly the next 30 million years, eventually falling below sea level. The marine mud that accumulated in that shallow sea composes several important rock formations, the thickest being the Pierre Shale, which exceeds 8,500 feet thick north of Boulder, and its western Colorado equivalent, the Mancos Shale. These shale units contain swelling clay, which presents a major engineering challenge because its movement cracks foundations and heaves pavement.
As the seaway drained from the state between 70 million and 68 million years ago, it left behind beach sand. The Fox Hills Sandstone records the last time Colorado stood at sea level. During the rise of the modern Rocky Mountains, the Fox Hills Sandstone and all older rock layers were tilted down eastward east of the Rockies and down westward west of the mountains. The soft Pierre and Mancos Shale eroded away quickly, as did the equally soft Morrison Formation. That left the erosion-resistant Dakota Sandstone, which was sandwiched in between, to stand as a prominent hogback that marks the foot of the Rocky Mountains. East of the Rockies it forms the famous Dakota Hogback. A roadcut through the hogback marks where Interstate 70 enters the Rockies near Golden and impressive dinosaur footprints cover the Dakota Sandstone at Dinosaur Ridge, just south of the cut. West of the Rockies, the Dakota Sandstone and adjacent rock layers form the Grand Hogback, a dramatic ridge that runs south-southeast from Meeker to New Castle.
Raising the Rockies
The rock layers that accumulated in the Western Interior Seaway allow geologists to confidently reconstruct the state’s pre–Rocky Mountain history, but the modern Rockies offer little such evidence. Experts continue to disagree about how and when today’s mountains were built. Three attributes make the Colorado Rockies one of the world’s most puzzling mountain ranges: first, they stand far from a tectonic plate boundary; second, they lack a crustal root; and third, the adjacent Great Plains and Colorado Plateau stand high above sea level despite experiencing minimal folding and faulting. The presence of these high provinces next to the Rockies is unique among world mountain ranges.
Despite these difficulties, geologists agree that a mountain-building event known as the Laramide Orogeny, which occurred between about 70 million and 45 million years ago, raised Colorado’s mountain ranges. Most also agree that a second, later uplift must have occurred. When and why that second uplift happened are still debated.
Almost all modern Colorado mountain ranges have thrust faults at their bases. Thrust faults occur when the crust is compressed, which happens when tectonic plates converge. Movement on a thrust fault stacks one slab of rock atop another. That stacking forms mountains. During the Laramide Orogeny, a plate consisting of oceanic material was converging with continental North America off the coast of California. Geologists call such oceanic-continental convergences subduction zones; the plate that possesses oceanic crust is denser than the continental plate, so it dives, or subducts, deep into Earth’s mantle (the layer below the Earth’s crust).
Normally mountain ranges rise next to subduction zones, but the oceanic plate’s angle of descent dictates exactly how far from the plate boundary compression will be felt. Before 80 million years ago, the oceanic plate converging with continental North America descended at a “normal” angle of about 40–50 degrees. That angle caused compression near the plate boundary, which formed California’s Sierra Nevada Mountains. But after 80 million years ago, the plate’s descent angle became nearly flat, explaining why volcanoes in the Sierra Nevada went extinct just before the start of thrust-fault activity in Colorado.
Explaining the Second Uplift
Flat-slab subduction can explain why Colorado’s mountains rose far from a plate boundary, but it doesn’t explain the range’s lack of a crustal root or the high elevation of the adjacent Great Plains and Colorado Plateau. The best explanation for those attributes is that the mantle beneath the Rockies is unusually warm. When Colorado’s deep mantle warmed, it expanded and pushed up the overlying crust, lifting the Colorado Rockies as well as the adjacent Great Plains and Colorado Plateau. That warm mantle is also the reason Colorado has so many hot springs.
Geophysicist Gordon Eaton has called this heat-induced swelling the Alvarado Ridge. The Great Plains and the Colorado Plateau form the eastern and western parts of the Alvarado Ridge, respectively. The older Laramide Rocky Mountains sit atop the ridge, which explains why Colorado’s mountains are so much higher than the rest of the Rocky Mountain chain.
When Did the Mantle Warm Up?
While geophysicists have documented the warmth of the mantle beneath the Colorado Rockies, they have been unable to deduce when the mantle warmed up. The when and the why of that mantle heating and the associated second uplift event are the subject of current disagreement and research.
About 38 million years ago, soon after the Laramide Orogeny ended, Colorado erupted in a volcanic episode of giant proportions that lasted until about 24 million years ago; geologists call this episode the Ignimbrite Flare-up. The biggest single volcano ever identified on Earth, the La Garita Caldera, is found in Colorado’s San Juan Mountains. It is one of fifteen giant caldera volcanoes in the San Juans, and more big Ignimbrite Flare-up volcanoes or their eroded remnants form peaks in the Elk, West Elk, Sawatch, and Front Ranges. Geologists are not sure what caused this massive volcanic event, but one idea is that the subducting plate that triggered the Laramide Orogeny’s thrust faults also delivered water to Colorado’s subsurface mantle. The addition of water lowers the rock’s melting temperature, which could explain the volcanic activity Regardless, it is clear that a lot of heat would be necessary to produce such large magma volumes; therefore, many geologists believe the mantle warm-up and raising of the Alvarado Ridge occurred about 38–24 million years ago, in conjunction with the Ignimbrite Flare-up.
Later activity also helps explain the raising of the Alvarado Ridge. About 28 million years ago, just as the Ignimbrite Flare-up was winding down, the Colorado Rockies were stretched and split along the north-south trending Rio Grande Rift. The upper Arkansas River valley, from Leadville to Salida, lies along this rift, as does the Rio Grande River’s southward path along the axis of the San Luis Valley. The Rio Grande Rift’s crustal stretching is similar to the stretching that formed today’s famous East African Rift Valley. Such stretching thins the crust, bringing hot mantle closer to the surface, which in turn causes thermal expansion and associated surface uplift. For that reason, many geologists think the Alvarado Ridge rose about 28 million years ago, simultaneous with formation of the Rio Grande Rift.
Other geologists hypothesize that today’s Colorado Rockies rose to their current height within the last 5 million years. Their primary evidence is that before 5 million years ago, sand and gravel were accumulating across the western Great Plains, producing the Ogallala Formation, the rock unit that forms the important Ogallala Aquifer. Sometime after 5 million years ago, the Ogallala Formation was tilted up to the west and the Arkansas and South Platte Rivers began to erode it. Both the tilting and the erosion might indicate that the Alvarado Ridge rose in the more recent geological past.
The Rockies During the Pleistocene Ice Age
The modern Rockies might have risen 40, 28, or 5 million years ago. Whenever it was, the newly risen mountains were almost certainly gently rolling uplands; they lacked the steep cliffs and spectacular, deep valleys that make today’s mountains so impressive. The mountains did not achieve their current grandeur until big glaciers sculpted them during the Pleistocene Ice Ages, which began about 2.5 million years ago.
Periodic changes in Earth’s orbit, called the Milankovitch Cycles, govern the amount of radiation we receive from the sun. About every 100,000 years, the planet cools by about 5 degrees Celsius, which is enough to cause large glaciers to form. Those glacial intervals alternate with interglacial intervals, when the Earth receives more sunlight and the glaciers melt away. The Earth has been in an interglacial interval for about the last 10,000 years, and the peak of the most recent glacial interval was about 20,000 years agowere. Colorado’s mountains were covered by ice caps, and glaciers stretched as long as thirty-five miles down mountain valleys.
The scouring action of those glaciers deepened the valleys and steepened the ridges and mountain faces, turning the formerly rolling upland into today’s rugged landscape. Calling cards of Colorado’s past glaciers include U-shaped mountain valleys, such as the box canyon where Telluride sits, as well as chains of alpine lakes and the craggy nature of many alpine ridges and peaks. Without the combination of the Laramide Orogeny, the post-Laramide uplift of the Alvarado Ridge, and the sculpting action of the Pleistocene glaciers, Colorado would not boast the mountain landscape that brings pleasure to so many locals and visitors today.