A drone shot of the layered Miocene basalts of the Grand Mesa Volcanic Field. The tan Green River Formation (Paleogene)—forming the hoodoos—is seen beneath talus accumulations at the base of the basalt flows, Lands End, Grand Mesa, Colorado, July 2017. Photo credit: Julian Chesnutt for the CGS.

Igneous Rocks

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) crystallizes 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 moves upward and cuts across pre-existing layers of rock forms what are generally known as plutons. The largest plutons are batholiths, such as the granitic rocks of Pikes Peak, which are part of a 1,300-square-mile batholith. Smaller plutons take a variety of shapes, each with its own name, such as stocks, plugs, dikes and sills.

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.

The map below shows the distribution of Proterozoic granitic intrusions in Colorado. Those in green are a billion years old. Those in yellow were originally interpreted as being 1.4 billion years old. Those in fuchsia were originally interpreted as being 1.7 million years old. Recent dating of some of these rocks has revealed more precise and different ages. However, this generalized map still gives a good feel for the distribution and ages of the intrusions.

Plutonic Rocks

Colorado’s Rocky Mountains boast spectacular views of numerous plutonic (intrusive) rocks. These rocks were formed long ago as magma rose from deep sources and solidified before making it all of the way to the surface. As this magma rose, in many places, it brought with it precious minerals such as gold, silver, lead, and molybdenum (used in hardening steel). After millions of years, erosional processes stripped off the overlying rocks, exposing them as we see today.

Please explore Colorado’s Plutonic Rocks by using the menu on the left side of the page to learn about alkalic complexes, batholiths, dikes, kimberlite diatremes, plugs, sills, and stocks.

Volcanic Rocks

Volcanic rocks have cooled from molten material that either flowed out onto the surface of the earth or were blasted up into the air and settled back onto the surface of the earth. Volcanic igneous rocks are widespread throughout Colorado. At one time, about sixty percent of Colorado was covered by volcanic rocks but much of that coverage has since eroded.

The map above shows in red the remnants of the volcanic strata that were erupted during an intense time of violent volcanic activity ranging from about 37 million years ago to about 25 million years ago.Although most of these volcanic rocks in red are from the mid-Tertiary caldera/ashflow event, a smaller number of basaltic volcanic rocks from the late-Tertiary extensional event are also included in the red.

Volcanic rocks come in a variety of forms, depending on the type of eruption from which it originated. For example, in a violent eruption ash fall, ash flow, and lava flow may occur, all of which produce different types of volcanic igneous rock.

Many of these rocks originated in the San Juan volcanic field, which is in the southwestern region of the state. There, many large caldera eruptions generated tremendous amounts of pyroclastic debris (hundreds of cubic miles). These calderas are shown in the map above as tan areas.