Anyone who has followed Wickersham’s Conscience for more than a few weeks knows that two of WC’s interests are geology and birds. Those two areas aren’t as unrelated as they might seem; often, geology and geomorphology dictate what birds are where.
WC has already written about High Island, where the geologic accident of a salt dome has created a magnet for birds migrating across the Gulf of Mexico. Without the salt dome, without that bit of geology there wouldn’t be a High Island and the bird refuge that geology has created for migrating songbirds.
WC recently spent a few days in the “Sky Islands” of the American Southwest; specifically, in the Chiricahua (“chir-ih-CAW-hwah”) Mountains in the southeastern corner of Arizona. The surrounding Chihuahuan Desert is at about 4,000 feet; the Chiricauhua Mountains tower 5,000 feet above the desert floor. The mountains pull enough moisture out of the atmosphere and are high enough to be a lot cooler – it snows there in the winter – so that there are Ponderosa Pines and Engelman Spruce growing at the higher elevations. The moisture and the greenery attract birds that would otherwise avoid the desert.
So why are there near-10,000 foot mountains in the middle of the Chihuahuan Desert? The easy answer is Basin and Range faulting, the same extensional stretching of the American West that has created the northwest-southeast trending ranges from southern Idaho south to Mexico. But as is often the case, the easy answer is a bit facile. What came before is at least as important.
For tens of millions of years, the land where the Chiricauhua Mountains are now was at the bottom of a shallow sea on a passive margin of a tectonic plate. It was the westerly margin of North America at the time. Think of the Atlantic seaboard in the United States today: very low vertical relief, layers and layers of mud, sand and eroding off the continent and piling up with limestone and reefs, building up thick enough to lithify.
Things changed starting about 160 million years ago (mya). The Farallon Plate began subducting under the North American Plate. It was a big deal, creating the Rocky Mountains in the Laramide Orogency, among other things, but for our purposes one of the important consequences was a series of volcanic events as the subducted oceanic sea bottom melted and erupted as volcanoes across the Southwest, much as the Oregon Cascades have been created in more recent times. The movement of the subducted Farallon Plate was complex and still only poorly understood, but one of the effects was that a very large bulb of molten rock stayed around long enough to melt much of the existing, native rock, creating the high-silicon, viscous form of erupted rock called rhyolite. And that mix of melted ocean floor, sicilian-rich native rock and water erupted in a cataclysmic caldera eruption – maybe more than one – where the southern half of the Chiricahua Mountains are today.
The eruption(s) blew more than 100 cubic miles of magma out of the volcano and buried a region of at least 1,200 square miles in a thick blanket of hot ash and pumice that fused into today’s rhyolite. For comparison, the 1980 eruption of Mount St. Helens produced only one tenth of a cubic mile of magma and the 1991 eruption of Mount Pinatubo in the Philippines, which was one of the largest eruptions of the century, produced only about one cubic mile of magma. The resulting caldera was about 12 miles across and 5,000 feet deep. The spectacular valleys and ridges of the Chirichuas today are eroded rhyolite. On the northwesterly side of the Chiricahuas, at Chiricahua National Moument, the erosion has created pretty amazing hoodoos.
Only after the volcanism has mostly ended and the San Andres Fault had developed, stretching the western third of North America north and west, did Basin and Range faulting begin and all that the eroding volcanic rock get tilted up into the sky. Even today you can find pillow lavas – lava that erupted seriously under water – at Onion Saddle near the crest of the Chiricahuas – 7,800 feet above sea level, along the roadside of the sketchy Forest Service Road 42, that crosses the mountains.
Without all that geology, and its creation of high elevation climate zones, the Chiricahuas wouldn’t be the birding paradise that they are today. Without the easily eroded rhyolite, there wouldn’t be the deep, shady, water-carved valleys, filled with live oaks, cottonwood and sycamores. Without those valleys, you wouldn’t find nesting Elegant Trogons and Red-faced Warblers. Over the next few weeks, WC will be posting photos of some of that amazing avifauna.
But remember: they are there because of the geology.