Dikes

Dikes are formed when magma (a mixture of molten material and crystals) rises from below and cuts across pre-existing strata. The magma may follow pre-existing cracks or faults, or may create its own path upward. The magma crystallizes underground and becomes a dike, which is a plutonic or intrusive rock.

Erosion cuts into the earth and allows us to observe the dikes. The magma in a dike may or may not have reached the surface. If the magma pours out onto the surface then it becomes a volcanic or extrusive rock. All extrusive rocks must of necessity have intrusive feeders, usually dikes or plugs.

Colorado is home to examples of every type of known dike structure. Here are a few of Colorado’s amazing array of dikes:


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The Big Wall
The Spanish Peaks region is world famous for its dikes. More than 500 have been mapped. There are several different compositions, ages, and orientations of the dikes. The dikes sometimes stand out in bold relief such as this one named the Big Wall, shown hereat the base ofWest Spanish Peak.
The Devil’s Stairsteps
Click image to seeinformation and imagesof the beautiful dike formation in southern Colorado know as the Devil’s Stairsteps.
The Hogback Dike
Click image to see information and images of the 6-mile-long Hogback Dike of Lathrop State Park.

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Horseshoe Gulch
This dike in Horseshoe Gulch has unusual horizontal columnar jointing. Normally, columnar jointing in lava flows is vertical.The igneous dike outlined by the red lines, intruded across relatively cold, sedimentary strata (wall rock). The columnar joints are interpreted to form normal (at right angles) to the isotherms (lines of equal temperature) in the molten material during cooling and solidification. In lava flows, the isotherms are generally horizontal, getting cooler toward the atmosphere on top of the flow. So, the columnar joints that form at right angles to the isotherms are vertical.

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Horseshoe Gulch
In the Horseshoe Gulchdike, the magma lost heat fastest tothe walls into which it intruded. The isotherms within the molten material were parallel to the wall rock. Therefore, the isothermswere vertical (parallel to the walls of the dike and the columnar jointing (parallel to yellow line) formed at right angles (normal) to the wall rock.
Valmont Dike
The Valmont dike just east of Boulder cuts the Pierre shale.Click image to see more information and images of the Valmont Dike.
Sandstone Dikes
Not all dikes are composed of igneous rock!Click on the image at the left to learn about this special geology and how it forms.

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San Juan Mountain Dikes
At least seven dikes can be seen cutting up through these volcanic deposits in the San Juan Mountains.

Image: V. Matthews
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Trinidad State Park Dike
This basaltic dike cuts Tertiary sedimentary rock in a railroad cut in Trinidad State Park.
Close up of Dike in Trinidad State Park
This basaltic dike cuts Tertiary sedimentary rock in a railroad cut in Trinidad State Park.
Arkansas River Canyon
Proterozoic pegmatite dike in the Arkansas River Canyon.
The Morley Dikes
Located near Raton Pass south of Trinidad, Colorado, these classic dikes intersect coal formations in the area.Click image to see more information and images of theMorley Dikes – and some information about how geology affected the original town of Morley, Colorado, now long abandoned.
Dike Mountain Renamed
This mountain was known for many years as Dike Mountain due to its large swarm of radiating dikes.Explore its new name with this link.