Precambrian igneous and metamorphic rocks in Unaweep Canyon, Mesa County, Colorado. Photo credit: Vince Matthews for the CGS.

Colorado Geology

From the low-lying eastern plains, to the central peaks soaring more than 14,000 feet above sea level, to the western red-rock canyons: the colorful landscape of Colorado embodies some of the most varied, spectacular, and well-displayed geology in the nation. The evolution of the rocks, climate, life, and structures that formed during the Colorado’s 2.7-billion-year geologic history offers marvelous insights into the global science of geology. This diverse region provides not only rich mineral and energy resources, but also presents geological hazards that deserve great respect. Explore and enjoy this section about Colorado’s magnificent geology.

If you would like to have a wonderful primer on Colorado’s unique geology, order a print copy of the award-winning Messages in Stone: Colorado’s Colorful Geology, a perennial favorite of locals and visitors alike.

On all the broad extent of these United States, certainly no region can be found which presents more facts of interest, more opportunities for investigation, and greater possibilities, than the State of Colorado. — Samuel F. Emmons, geologist on the King Survey of the 40th Parallel from California to Colorado from 1867 to 1872; Director of the Rocky Mountain Division of the United States Geological Survey; and the first president of the Colorado Scientific Society, from his inaugural address.

I do not know of any portion of the West where there is so much variety displayed in the geology as within a space of ten miles square around Colorado City (today’s Colorado Springs). Nearly all the elements of geological study revealed in the Rocky Mountains are shown on a unique scale in this locality. — F. V. Hayden, geological expedition leader to the Colorado region between 1869 to 1876, from the first expedition report.

The geology of Colorado is written in the rocks. From this great book are here presented a few translations of a few paragraphs. The scenery of Colorado is a gallery incomparable. Words lack form and light – the essence and soul of scenery. At best they can but call attention to the elements associated in the picture. They cannot convey the beauty and harmony of the assemblage. — The first director (1908-26) of the Colorado Geological Survey, Russell D. George, in the preface to his 1927 book, Geology and Natural Resources of Colorado.

Geologists divide rocks into three main groups, depending on their modes of origin. Sedimentary rocks are formed from broken or dissolved bits of other rocks or remains of organisms, washed by wind and water and deposited as layers of fragments or as chemical precipitates. They may contain remnants or impressions of fossilized plants or animals. Metamorphic rocks are pre-existing rocks (igneous, sedimentary, or prior metamorphic) changed by heat, pressure, or chemical action. Igneous rocks originate from molten material (magma) cooling deep below the surface of the earth (intrusive igneous rocks) or flowing out and hardening at the surface (extrusive igneous rocks). Many varieties of all three rock types occur in Colorado.

Blown across the land by wind or carried along by water and ice as the land continued to remake itself, loose sediments eventually compressed and cemented into rock and left messages in stone for us to decipher. Sediments include the mud at the bottom of streams, the sand dunes at the foot of the mountains, the chemical precipitates of salt in shallow seas, the beaches at the edge of inland seas, and the graveyards of tiny fossils at the bottom of tropical oceans. In these sedimentary layers, such as the Book Cliffs in western Colorado, the imprints of changing life forms in an ancient world are faithfully recorded.

As the name indicates, metamorphic (meta = change, morph = form) rocks are pre-existing igneous, sedimentary, or metamorphic rocks that have been altered, or metamorphosed, deep within Earth’s crust. The rocks changed form in response to intense fluctuations in temperature, pressure, shearing, stress, or chemical environment. During Colorado’s mountain-building events, the intrusion of igneous bodies increased the crustal temperature to result in contact and regional metamorphism. The dominant metamorphic rock types in Colorado are gneiss, schist, amphibolite, and quartzite.

Igneous rock is formed from magma that has cooled and become solid. Molten rock is extraordinarily hot, sometimes exceeding 2,000 degrees Fahrenheit. If this molten and partially crystallized material (magma) solidifies underground before it reaches the surface, the rock is intrusive or plutonic. If magma travels up through Earth’s crust and reaches the surface, the resulting rocks are extrusive or volcanic.

Magma that reaches the surface forms a variety of volcanic landforms and deposits. Today, resistant volcanic flows cap mesas such as Grand Mesa, White River Plateau, Raton Mesa, and those near Basalt, Colorado. In the southwestern part of the state, ash-flow tuffs cover thousands of square miles. The tuff is created from the ash that is blown from the volcano to blanket the landscape.

An ash-flow eruption creates a roughly circular depression called a caldera. There are at least nineteen calderas in Colorado, making the state one of the world’s best outdoor laboratories in which to study their formation.