|Rocks of the Texas Canyon formation|
|Rocks show spalling caused by freezing and thawing|
Recently, we pulled off Interstate 10 into Arizona’s Texas Canyon, in search of the private, non-profit Amerind Foundation museum. We found ourselves near an upthrust range of mountains intriguingly named the Dragoons. (A dragoon was a calvary soldier outfitted with the heavier armaments of a foot soldier.) It was to the vertical labyrinths of the Dragoon Mountains that the Apache warrior Cochise fled from the Confederate calvary that eventually defeated him.
But it was the nearer rocks that caught our attention. Stacked, balanced, and weirdly-sculptured, they looked like the abandoned playthings of some colossal child. What caused them to look like that?
In the museum, we found a brochure that gave the answer.
The formations in these photographs are part of a body of igneous rock called Texas Canyon quartz monzonite. It’s a granite, and it looks as it does because it was once magma (molten rock) that rose slowly without reaching the surface. It cooled beneath the earth’s crust, forming large crystals of the potassium-rich mineral, feldspar. Between the large crystals, other minerals crystalized: quartz, biotite, and plagioclase. Once the rocks that covered the blocks of quartz monzonite eroded away, they were exposed to natural weathering processes that rounded them at exposed surfaces and along the joints.
Tiny cracks formed between the larger crystals, allowing water and oxygen to invade and weaken the granite, prying the mineral grains apart. Sheets of rock spall off the outer surfaces of boulders along curved cracks or joints when water invades the spaces, then freezes. This helps round the blocks. As the rock around the boulders of quartz monzonite disintegrate, the pedestals, spires, and balanced boulders that form the Texas Canyon outcrop appear.
What a reminder of our creative God at work!
|Chiricahua National Monument, Photo courtesy of National Park Service|
|Texas Canyon quartz monzonite formations|