Colorado sits above a geologically active foundation that continues to shape its surface. Deep beneath the state, the lithosphere was weakened by ancient subduction, and the ground is still rising and shifting today. A supervolcano in the San Juans erupted millions of years ago with far greater force than recent Cascade eruptions, a mountainside near Lake City creeps steadily downhill, and in the 1960s, the U.S. military inadvertently triggered a series of earthquakes near Denver. The state’s scenery is impressive, but its underlying geology remains in motion.
The modern Rocky Mountains are something of a puzzle. Most major ranges form at plate boundaries, but these peaks rose hundreds of miles inland. During the Laramide Orogeny, roughly 70 to 40 million years ago, an oceanic plate slid nearly horizontally beneath North America, crumpling the crust far from the coast and lifting Colorado’s mountains. They were not the first range to occupy this ground. Around 300 million years ago, an earlier formation known as the Ancestral Rocky Mountains stood here before eroding almost completely away. That eroded sediment later hardened into the red rock formations visible along the Front Range.
Western Colorado contains the trace of a far larger event. About 28 million years ago, the La Garita Caldera in the San Juan Mountains produced one of the largest volcanic eruptions known on Earth. The resulting deposit, called the Fish Canyon Tuff, represents an estimated 5,000 cubic kilometers of erupted rock. An event of that scale would have buried entire states in ash and altered climate patterns. The state also has younger volcanic features. Near Dotsero, a volcano erupted only about 4,000 years ago, creating a broad crater when rising magma met groundwater.
The state is also part of a continent slowly pulling apart. The San Luis Valley lies at the northern end of the Rio Grande Rift, where the crust is being stretched and thinned. As the land splits, faults slip and magma moves closer to the surface. Colorado is one of the few places where continental rifting can be observed directly.
Some of the oldest exposed rock in the country sits at the bottom of the Black Canyon of the Gunnison, dating back about 1.7 billion years. The canyon is so deep and narrow that parts of its floor receive only minutes of direct sunlight each day. The Painted Wall, a cliff within the canyon, drops about 2,250 feet from rim to river, making it the tallest sheer cliff in Colorado. Farther west, the Arkansas River carved the Royal Gorge, a slot canyon roughly 1,250 feet deep with walls of Precambrian granite. Glenwood Canyon, carved by the Colorado River, is one of the state’s most rugged corridors. Its steep walls shed boulders, and fire-scarred slopes can produce mud flows during storms.
At the foot of the Sangre de Cristo Mountains, the Great Sand Dunes rise up to 750 feet, the tallest dunes in North America. Wind carried sand from an ancient lake bed and piled it against the mountain wall, where opposing winds trap it.
Colorado is the highest state in the nation. Mount Elbert reaches 14,440 feet, the loftiest peak in the Rocky Mountain chain, and the state has more than 50 summits above 14,000 feet. The Continental Divide runs along the crest of the Rockies, directing rainfall either toward the Pacific or the Atlantic Ocean.
The granite of Pikes Peak crystallized from molten rock more than a billion years ago before being uplifted and stripped of its overlying layers. At Garden of the Gods near Colorado Springs, fins of red sandstone stand nearly vertical. The same tilting created Boulder’s Flatirons and the walls of Red Rocks Amphitheater. Along the eastern edge of the Rockies, the Dakota Hogback is a long ridge of tilted sandstone left standing as softer rock eroded away.
Near Morrison, Dinosaur Ridge exposes the Morrison Formation, a Jurassic layer rich with dinosaur bones and footprints. It was here that scientists first described Stegosaurus, now Colorado’s state fossil. The area also yielded Apatosaurus and Allosaurus. In the northwestern corner of the state, Dinosaur National Monument preserves a tilted rock face containing roughly 1,500 dinosaur bones. Along the Purgatoire River in the southeast, the Picket Wire Canyonlands contain more than a thousand dinosaur tracks, the largest documented track site in North America.
At Florissant, west of Pikes Peak, the petrified stumps of giant redwoods stand more than 10 feet across. About 34 million years ago, volcanic eruptions buried a valley in ash and mud, preserving trees and countless insects in fine detail. Colorado’s eastern plains hold evidence of a different past. During the age of dinosaurs, a shallow sea called the Western Interior Seaway split North America in two, leaving behind shale rich with marine fossils. In 2010, construction near Snowmass Village uncovered an ancient lake bed containing the bones of mammoths, mastodons, and a giant ground sloth in good condition.
The Colorado mineral belt, a diagonal stripe running from southwest to northeast, was forged where molten rock injected metals into the crust. Near the town of Creede, an ancient volcanic throat left gold deposits that sparked a major rush. Near Leadville, Bartlett Mountain held the Climax mine, once a major source of molybdenum. Southwestern Colorado’s Uravan mineral belt contained uranium and vanadium in ancient sandstone, fueling a Cold War mining boom that left a legacy of contamination still being addressed.
Human activity has also affected the region’s geology. In the 1960s, the Army pumped toxic wastewater deep underground at the Rocky Mountain Arsenal near Denver. A swarm of earthquakes followed in an area that rarely experienced them. Scientists traced the quakes to the injections, providing early evidence that human activity can trigger seismic events.
Some parts of Colorado move more visibly. The Slumgullion Earthflow near Lake City is a slow-moving landslide that has been creeping downhill for centuries, shifting several feet per year and dragging trees along with it. The same slide dammed a river long ago, creating Lake San Cristobal, one of the state’s largest natural lakes.
Colorado’s steep slopes and heavy snow make it the deadliest avalanche state in the nation. Each winter, snow slabs can break loose and cover roads and backcountry areas. The Maroon Bells, among the most photographed peaks in Colorado, are built of weak sedimentary rock that sheds loose stone. They have claimed enough climbers to earn the nickname “the deadly bells.” The Spanish Peaks, rising from the southern plains, are the eroded cores of ancient volcanic activity, surrounded by walls of rock called dikes that radiate outward from the peaks.
The forces that built Colorado are still operating. The Rockies continue to rise by small amounts, the Rio Grande rift continues to pull the crust apart, and slopes keep creeping and collapsing. The magma that fed ancient volcanoes has not fully cooled. The state’s landscape remains under active construction.
