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 2/3 of Colorado was covered by volcanic rocks but much has 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.
The Castles, consisting of volcanic rock, in the West Elk Wilderness.