How did the Sonoran Desert form? And why should we care about the geologic origins of our region? Geology creates topography, which influences climate, watersheds, temperatures, and migratory pathways. Understanding the geology of a place is like unfolding the story of the land itself, which sets the stage for understanding our local ecology. But it also reminds us that stability is an illusion, impermanence is the nature of things, and geology didn’t just happen in the past, but is active and riotous, transforming our earth every single day! Take a walk with us through time as we discover the geologic origins of the Sonoran Desert.
Act I – The Creation of the North American Cordillera, the Spine of the Continent
We’ll begin in the Mesozoic era, around 250 mya (million years ago). This era follows the late Permian extinction event, the largest mass extinction event that decimated the majority of Paleozoic life! The “supercontinent” Pangea forms in the early Mesozoic Era and the region that is now North America drifts north of the equator. The climate is warm, moist, and tropical. Dinosaurs and mammals roam ancient forests, lakeshores, and floodplains. Myriad lifeforms are deposited into sediment later to be discovered by humans as fossils.
Toward the end of the Mesozoic, Pangea begins to split, creating the Atlantic Ocean. This expanding sea dramatically shifts the global climate, and as the North American plate drifts westward, it eventually meets the Farallon oceanic plate. The subduction zone created during this collision causes crustal compression and volcanic activity, forming the continent’s western mountain ranges including the Rocky Mountains and the Sierra Madre Occidental. Together these ranges, along with the much younger Cascades, form the North American Cordillera, also known as the spine of the continent.
Act II – The Creation of the Basin and Range Province and the Gulf of California
Following millions of years of folding, thrusting and general thickening of the western part of our continent, Act Two begins, peaking around 15-5 mya. Our crust begins stretching and thinning in a process called extension. The lower part, being hotter, behaves like taffy. The upper part, being colder, behaves in a brittle fashion and cracks. Large blocks are separated by these cracks, some of them remaining elevated, others foundering and creating long depressed basins. This episode of extensional faulting produces the topography that a modern-day road tripper traveling in the southwest will observe: mountain range – flat basin – mountain range – flat basin.
This stretched-out region is called the Basin and Range Province, and it includes all four North American deserts! The Sonoran Desert sits within a section of this Province called the Cordilleran Gap, the gap between the southern part of the Rocky Mountains and the northern part of the Sierra Madre Occidental Mountains in Mexico.
Just when things seem to be calming down, around 5 mya, our North American Plate pushes itself right over a major break in the earth’s crust. We override the East Pacific Rise, a spreading center where hot magma exits the depths and creates new sea floor. As a result, a significant slice of Mexico tears away from the mainland, forming the Baja Peninsula and the Gulf of California, the youngest sea in the world!
ACT III – The Creation of the Sky Island and Sonoran Desert Ecosystems
For the next 5-6 million years into present day, the mountains steadily erode away, depositing gravel, sand, and clay into the basins. Tucson actually sits directly on a sediment-filled basin between the Santa Catalina and Tucson Mountain ranges!
Some of the mountains within this province are high enough to support woodlands. These mountains get a special name: Sky Islands. Basin and Range faulting separated and isolated previously formed highlands like islands in a sea. The Cordilleran Gap, where the Sonoran Desert is located, acts as both a biogeographical corridor and barrier, allowing plants and animals from very different regions to mingle while at the same time inhibiting migration along the mountainous spine of the North American Cordillera. The result is a fantastically biodiverse region with many of the major biomes represented, from tundra to desert to tropical dry forest!
As we’ve shown, massive landforms that we see as permanent fixtures of the environment are in fact undergoing constant transformation, a cycle of creation and destruction that serves to remind us of the planetary forces constantly at work on our environment.
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