WC went to the Chiricahuas almost two decades ago, chasing birds. The Chiricahua Mountains, located in the southeast corner of Arizona, are a famous North American birding destination, offering a fine selection of habitats – it’s more than 6,000 feet from the valley floor to the top of Chiricahua Mountain – and all of those nearby Mexican bird species that drift over the border. The birding was good, with over 100 species. But the geology was fabulous.1
The tortured recent geologic history of the area started with immense amounts of volcanism. From about 70 million years ago to about 25 million years ago, the Farallon and Kula plates were subducting under the North American plate, much as the remnants of the Farallon plate are subducting under the North American plate further north now. The tectonic effect was similar; the current subduction has created the modern Cascade volcanoes, peaks like the Three Sisters, Mt. Hood, and Mt. St. Helens.
The earlier subduction was under what’s now eastern Arizona, and the subduction created volcanoes like the Turkey Creek Caldera. Those eruptions created a layer of ash some 2,000 feet deep. The ash compressed and welded to create a kind of rock called rhyolite. As the rhyolite cooled, it contracted, creating vertical cracks. Those cracks were the entry point for erosion by water, ice and wind.
Later, as the Basin and Range activity got started up a few millions years ago, the Chiricahua Mountains were created, lifting the rhyolite up as much as 9,000 feet. The new mountains captured more moisture, which ran down through the rhyolite and increased the rate of erosion.
Some 27 million years of erosion have created an immense field of vertical rhyolite columns, called hoodoos, on the westerly side of the Chiricahua Mountains. It’s weird, spectacular and impressive.
Geology is messy, of course. The faulting and stretching of the earth’s crust that has created the Basin and Range province across the west has weakened the crust enough to allow much more recent volcanism, albeit on a much smaller scale. The Basin and Range faulting is something like lifting a trap door; the faults cause one side to rise, like handle side of a trapdoor; the other side is a much less steep slope. The result is a mountain range that is steep on one side and sloping on the other. The steep side exposes what had been the basement rock; generally, the deeper the rock, the older it is. The tilting in Chiricahuas has exposed a range of rock across an immense range of geologic history, going back a half a billion years. The sculpted rhyolite is just the frosting on the geologic cake, as it were.
But it is very impressive frosting, even for someone not interested in geology. WC recommends a visit to Chiricahua National Monument if you are in the area. And the birding is very good, too.
- The photos, alas, are not. These are film images, scanned and digitized. The original photos weren’t all that good. The light at the time was bad (it snowed later that day). The scanning process was pretty poor. The result is pretty dreadful. Someday, maybe WC can get back there with decent equipment. ↩