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Castle Rock Rhyolite

Purple rhyolite embedded in Castle Rock Conglomerate

Castle Rock formation in Rock Park. Blocks of purple rhyolite (Wall Mountain Tuff) embedded in Castle Rock Conglomerate.

Wall Mountain Tuff

A moving cloud of gas and ash...

Mount Princeton, in the Sawatch Range, is about 85 miles west of Colorado Springs. A body of magma which formed in the region of Mount Princeton 36.7 million years ago suddenly found a weak area in the surface rocks and explosively broke through.  A mixture of molten magma, pumice, ash, glass, and rock exploded into the atmosphere and formed a cloud which began moving or "flowing" downslope.  Pyroclastic flows, as such moving clouds are known, can be very hot, with temperatures reaching between 500 and 1,000 degrees C (932 - 1,800 degrees F).  The cloud, perhaps moving at 50 to 100 miles an hour, probably reached what is now the city of Castle Rock, some 90 miles to the northeast, within two hours or less.

The ash which settles out of a pyroclastic cloud is known as tuff. Sometimes the pyroclastic globules, which are very sticky, pick up ash and the combination precipitates out of the cloud as welded tuff.  In other instances they cool quickly and fall in the form of powder or volcanic ash, a form known as soft tuff.  Welded tuff can also take the form of rhyolite, a fine-grained, compact, form considered the volcanic equivalent of granite.  It is usually light brown to gray in color.

The pyroclastic cloud which reached the Castle Rock area was still very hot. As a result, much of what came out of the cloud was a form of welded tuff.  It has been called the Wall Mountain Tuff or, alternatively, Castle Rock Rhyolite.  At the time, the region was probably a flat plain.  The amount of material emerging from the Mount Princeton explosion was substantial, since the deposited layer was, in places, at least twenty feet thick.

The "Castle Rock" formation, for which the town is named, has the appearance of an island standing high above the surrounding plain, bearing the brunt of the volcanic eruption.  The reverse is true. The present formation was one of the lowest points around.  In addition, it is not even part of the original deposit of volcanic material, which somehow avoided erosion, but instead, a form of conglomerate, known as Castle Rock Conglomerate. Over millions of years, water and streams began to cut paths through the rock, creating canyons.  The area seems to have been particularly hard hit by floods around 34 million years ago. The rhyolite broke apart and the chunks fell into a streambed as they eroded from the canyon walls, to be reformed and cemented together as conglomerate.  Below the Castle Rock Conglomerate is Dawson Arkose, a feldspar-rich sandstone.  The ash and tuff from the Mount Princeton eruption was falling on what had been an accumulation of sand, an alluvial fan created by the continued erosion of the then existing Rocky Mountains.

Suggestions for further reading.

Stanley Chernicoff and Donna Whitney, "Geology: An Introduction to Physical Geology, 3rd ed." Houghton Mifflin Company, (New York, NY 2002)
Halka Chronic and Felicie Williams, "Roadside Geology of Colorado, 2nd ed.," Mountain Press Publishing Company, (Missoula, MT 2005).
Thomas W. Henry, Emmett Evanoff, Daniel A. Grenard, Herbert W. Meyer, and David M. Vardiman, "Geological Guidebook to the Gold Belt Byway Colorado, Gold Belt Tour Scenic and Historic Byway Association, (Gunnison, CO 2004). Ralph Lee Hopkins and Lindy Birkel Hopkins, "Hiking Colorado's Geology," The Mountaineers, (Seattle, WA 2000).
Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003).
Frank Press and Raymond Siever, "Earth, 4th ed.," W. H. Freeman and Company, (New York 1986).
Andrew M. Taylor, Ph.D., "Guide to the Geology of Colorado," Cataract Lode Mining Company, (Golden, CO 1999).