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University of Missouri Department of Geological Sciences

Rock type:      Quartz Porphyry
Location:        La Veta Pass, CO

gold line

This igneous rock contains relatively large, rounded quartz crystals, smaller angular pink feldspars,

and a fine grey groundmass. The fine groundmass indicates rapid cooling, so it was probably a

shallow intrusion of magma that only narrowly failed to reach the surface and erupt. This rock

formed in the mid-Tertiary (around 30 to 20 million years ago) at the eastern edge of the

Rocky Mountains.

Quartz Porphyry Quartz Porphyry

Photo by Alan Whittington, MU Geology

MU Geology Department
http://geology.missouri.edu/