Spanish Peaks Volcanic Dikes, Colorado

Provided and copyright by: Paul Ellner
Summary authors & editors: Paul Ellner

The area surrounding the Spanish Peaks, in Southern Colorado, is known for its spectacular array of volcanic dikes. The peaks themselves are batholic domes, which formed 65 million years ago as magma lakes under the surface. Forty million years later (between 14 and 27 million years ago), the cooled magma was pushed towards the surface by tectonic forces. This upwards push cracked the surrounding sedimentary rocks vertically, with the cracks radiating out from the domes, like spokes from a wheel's hub. These cracks subsequently filled with magma, forming volcanic dikes. The magma in the dikes is different than that which made the Spanish Peaks; it contains mostly potassium and sodium feldspars, and very little quartz. After the overlying rocks wore away, the domes and surrounding radial dikes remained.

Profile rock, depicted above, is one of the over 500 dikes radiating from West Spanish Peak. Note the "windows" visible in the inset, where the rock has worn away from the thin dike. Can you see the “profile”?

Related Links:

• The Spanish Peaks Region
• Dikes of the Spanish Peaks
• Geologic Time
• Geologic Glossary