a desert orogeny

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“Sometimes, on the right kind of morning, Nevada looks like waves frozen in rock. Long ridges roll away to the horizon. Broad stretches of sage and playa form paler troughs between them. From a distance, it all looks barren, lifeless, baking. But within it lies riches for the senses and the spirit.”

-Carolyn Duffurrena, Fifty Miles From Home (2002)

It can be said that what is present-day Nevada was born when the Farallon section of the Pacific Plate died, subducted under the North American continent anywhere between 20 and 40 million years ago. Beginning as an arc of volcanic islands thrust up from a shallow, equatorial sea, the nascent Nevada eventually cemented itself to the continent, forever thence a terran but still retaining, here and there, traces of its benthic past—the fossil-rich limestone of Black Rock, for instance, or the ichthyosaur graves in the Shoshone Mountains. But in the Miocene, Nevada’s geology was not set in stone, as it were—there was much rearranging to be done. Nevada’s accretion to the North American landmass didn’t preclude further volcanism. On the contrary, the state’s most distinctive topographic features were upcoming, waiting in the desert’s wings, ready to take center stage.

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There is a certain monotony to traveling longitudinally across Nevada. (Admittedly, one might easily argue that to travel any which way in this state is a bore, but humor me for a bit.) Boundless panoramas of sagebrush and greasewood aside, it doesn’t take much tooling around to notice a recurrent theme to the desert’s topography. Often you’ll start in a flat area, say, a playa, and continue west or east on its plane for some 10 to 15 miles. Then you’ll approach a sloping range of dun-colored mountains, gain anywhere from 5,000 to 8,000 feet of elevation, and drop back down into a level basin. This process repeats itself dozens of times across hundreds of miles, an undulating motif of peaks and troughs that define the landscape and frequently stupefy the navigator. This is basin and range country, and it’s not boring, I swear.

The geologic region known as the Basin and Range Province spans two countries and eleven states, stretching from northeast California to central Utah and spreading south to Mexico’s Baja Norte, Sonora and Chihuahua. All of Nevada is encompassed within its bounds, as well as the entirety of the Great Basin. Characteristic to the province are its serried ranks of north-south-trailing mountain ranges, chunks of lithospheric crust tilted up by innumerable faults striping the hyperextended North American continent. These ranges are separated by broad, flat basins that fill with the erosional dross of the adjacent peaks.

This shitty diagram shows the crust, pre-stretch

This shitty diagram shows the crust, pre-stretch

Basin and range topography owes its almost orderly appearance to the tectonic peregrinations of the Pacific Plate. Extending from approximately San Francisco to Tokyo and from Anchorage almost to New Zealand, the Pacific Plate is on a leisurely course headed due northwest, three inches a year, and the North American continent seems reluctant to let it go. As the Pacific Plate shears up and away from the continent, enormous cracks called normal faults split western North America’s crust into roughly parallel blocks, which tilt by their own stupendous weight as one crustal edge slips beneath the other. An upwelling of magma beneath the stretched-thin crust helped to spread these blocks apart—think of the Great Basin’s balloon-like expansion, in which the highest elevations lie at the basin’s center.

Imagine the two plates—North America and Pacific—as abutting pizza slices, pre-cut but not quite separable because the knife didn’t sever the crust all the way, and the cheese coalesced over the boundary. Imagine that, in order to get the pieces apart, you have to prop them up from beneath with your hand (simulating the magmatic bulge underlying the plates). Pulling the Pacific slice away would drag North America’s toppings along with it, stretching out its surface across your hand and possibly causing its cheese to tear, upending the positions of various pepperonis, olives and peppers. It’s a shitty analogy, but now I’ve got you thinking about pizza. Which is good, I guess.

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And this is post-stretch

The ascending edge of these crustal blocks, now thrust up from the plain, becomes subject to an eternity of weathering processes: wind, rain, snow and ice. All the eroded sediment flows down into the gaps between faults, creating wide basins adjoining ranges. In days of yore these basins were filled with water, but few such lakes remain. Most are now dry, barren, leveled by millennia of alluvial deposits. These basins are some of the flattest areas on Earth. Death Valley is an obvious example, but perhaps just as impressive is the Black Rock playa, 3,900 feet above sea level, spreading 186 square miles with scarcely more than a three-foot difference in elevation throughout.

Volcanism begat Nevada, much like any other Earthly place, and its continued influence on the land is everywhere apparent. Geothermal springs abound, a startling incongruity in the desert but evidence all the same of a crust stretched tight over the Earth’s glowing mantle, heating groundwater to the surface. Temblors are a regular occurrence along the ubiquitous faults: Nevada ranks third in the nation (behind California and Alaska) in seismic activity. Every thirty years a magnitude 7 or greater earthquake strikes somewhere in the state—a streak unchallenged for more than 150 years.

The desert looks inert to the human eye, unchanging and implacable. Because we’re not equipped to see events unfolding on a geologic timescale, we infer its past from what evidence is afforded among the bones and rocks of ages, and we piece together the desert’s narrative. We divine its future with the law of uniformitarianism, which states that geologic processes on Earth have been the same since its inception and will likewise operate indefinitely. We know much about the desert’s origins, but we only know so much, being human.

Note the photobombing antelope, lower right

Note the photobombing antelope, lower right

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