Thursday, February 2, 2012

God at Work in the Rocks

Rocks of the Texas Canyon formation

Rocks show spalling caused by freezing and thawing

Rocks. Mountain size, or miniaturized to a grain of sand, they’re everywhere. Especially here in the American Southwest, where there’s not much soil or plant life to hide them from view.

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!

Only 40 miles to the east of Texas Canyon, a similar magma intrusion reached all the way to the earth’s surface. It caused a gigantic eruption that spread a thick deposit of lava and ash and left a tremendous crater. Today, in Chiricahua National Monument, one sees oddly-shaped columns and spires, the remnants of that almost 27 million year old ash blanket. The difference between one landscape and the other is all in how the rock reached the surface. 

Chiricahua National Monument, Photo courtesy of National Park Service

Texas Canyon quartz monzonite formations

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