Tearing the Nation Apart
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Sorry to say it, but New Mexico is tearing the country apart.
Or, to be more precise, it’s stretching it apart. We’re talking plate tectonics, in which slabs of the Earth’s rigid lithosphere drift atop the stiff-but-fluid mantle, gradually rearranging the oceans and continents. To glance at a map, you might think land-locked New Mexico is a safe distance from tectonic plates’ rough-and-tumble borders. But it’s actually a geologically active area—and one that’s threatening the stability of the land around it.
Our continent is just part of the North American plate, which extends underwater (and underground) from Northern Greenland to Southern Mexico, the Pacific coast to the Atlantic’s midpoint. But the plate isn’t a homogeneous slab of crust; its edges are in a state of constant slow-motion flex around a stable heart.
That heart is the craton Laurentia, a thick mass of igneous and metamorphic rock that formed 1-1.5 billion years ago. Back then Laurentia was a continent unto itself, but over the eons it played bumper cars with other cratons around the globe. Collectively these cratons crumpled and stretched the lithosphere between them, continuously remaking the landscape (and oceanscape).
Most recently—about 250 million years ago—Laurentia was part of the last great supercontinent, Pangaea. A supercontinent might seem invincible in its massiveness, but it was actually shot through with weakness. Veins of thin, broken-up crust traced the old borders where smaller continents had crashed and merged.
Eventually the hot mantle churning beneath Pangaea chewed into these weak spots and broke through to the surface. In the case of (future) North America, a steady supply of magma bubbled into the seam between Laurentia and the West African Craton, forming a volcanic ridge that wedged them apart. As they pulled away from each other, the young crust between them stretched thin and sank into a basin that collected water over the millennia. That in-between basin is now the Atlantic Ocean, and the ancient seam between Laurentia and West Africa is the Mid-Atlantic Ridge, which continues to widen the Atlantic inch by inch each year as magma seeps up and pushes outward.
As for Laurentia, the process of tearing away from Pangaea produced a series of rifts along its edge—the sediment-filled basins that define modern-day New England. Think of the region’s inland basins and ranges as Earth’s stretch marks. North America and Africa may be 4,000 miles apart, but they still bear scars from when they broke their supercontinental embrace and let an ocean in.
the San Juan, Española, and albuquerque Basins, Three of the large basins along the center of the rift
So what does all this ancient history have to do with the Land of Enchantment?
Nowadays the same process is underway in New Mexico. Some 30 million years ago, the North American plate’s weak western regions fell victim to the mantle’s constant churning and pushing. Once again, the middle of a stable-seeming continent developed a rift that continues to grow by about 2 millimeters per year.
The Basin and Range Province bears geological “stretch marks” west of the Rio Grande Rift in New Mexico
That rift follows the Rio Grande. Or rather, the Rio Grande follows the rift: unlike most valleys, which are carved into existence by the flow of water, the Rio Grande Valley predated the river that ultimately trickled along its tectonic contours. The river is only 300 feet across at its widest (91 m), whereas the series of low-lying basins it traces are up to 53 miles (86 km) across.
The Rio Grande Rift has created dozens of basins of various sizes, but the largest trace the center of the rift (and the river) from north to south. The San Luis Basin straddles the Colorado border, the Española Basin surrounds Santa Fe, and the Albuquerque Basin includes, well, Albuquerque.
These are all part of a larger region called the Basin and Range Province, a 170,000 square mile expanse of rugged “stretch marks” in the weaker North American crust west of Laurentia’s solidity. So far these basins have mostly filled in with dirt and rocks, with no hint of an ocean in sight. But if the Rio Grande Rift continues to split and spread the land around it, the region could someday fill with water... just like the Gulf of California 5 million years ago, the Red Sea 50 million years ago, and the Atlantic Ocean 180 million years ago. After all, these major bodies of water were once mere dips in dry land. It took several epochs for them to develop into their current deep, water-logged selves, and they’re only continuing to grow.
Fellow Burqueños, for now we’re safe walking along the dry riverbanks in the Bosque. But if we plan to stick around for another eon or two, we better brush up on our swimming skills! And as for the rest of the country? Sorry, guys, but there’s no stopping it: our rift just might tear you in two one millimeter at a time.
A view of the Rio Grande as it flows into northern Albuquerque
This post was brought to you by Halka Chronic’s Roadside Geology of New Mexico, EarthScope’s Rio Grande Rift project, and a walk along the river itself.
