Over the last few years, more than a few people have been surprised when I’ve told them I live and work on an organic vegetable farm in New Mexico. To lots of people out east, my home state is naught but an arid wasteland “out there somewhere” if it’s even a state at all. Honestly, this stereotype is understandable given the preponderance of media that bolsters this caricature — the old westerns and Breaking Bads everyone so adores.
But while New Mexico’s definitely got a lot of dry land — the southern part of the state is largely dominated by the Chihuahuan Desert after all — we’ve also got plenty of forests and grasslands. Plus, along the Rio Grande River, there’s a verdant and fertile avenue that runs north-south right down the middle of the state.
This is where my farm is located: 20 miles south of Albuquerque in the rather absurdly-named village of Bosque Farms. People have been using Rio Grande water to grow crops in this area for centuries. I like knowing that I’m digging my hands in the same dirt the Ancestral Puebloans dug their hands into when they weren’t etching petroglyphs into nearby basalt layers.
Why, you may wonder, does the river flow here? There are many other possible paths leading to the sea; why not take one of these alternatives? Why not go through Arizona or something? The answer, in short, is the Rio Grande Rift Valley.
Like Africa’s Great Rift Valley, the Rio Grande Rift is a long and thin low place where the Earth’s crust is being torn asunder. The resulting chain of en echelon (or, “step-like”) grabens reaches from southern Colorado down into Texas and Mexico. In the north, geologists distinguish three sections: the San Luis Basin, the Española Basin and the Albuquerque Basin. In the south, the valley becomes less defined, dovetailing subtly into other geological provinces.
Awhile back, I did a short write-up on the geologic forces responsible for the ribbed topology of the Basin and Range Province. In it, I said both the Manzano and Sandia Mountains are being lifted in the same way the Sierras and Wasatch are being lifted — by movement along the San Andreas Fault and by post-Farallon crustal rebound that are both causing the southwestern United States to stretch and crack. Actually, this is at best only partly correct.
Big Hot Bubble
As it turns out, the Rio Grande Rift appears to be stretching a little bit faster than the Basin and Range country west of it. It’s also been stretching for a little longer — 29 million years versus 17 million. This means there must be another variable lurking under foot, another engine of crustal elongation and thinning.
While geologists are still hard at work figuring out exactly what’s going on below the Rio Grande Rift, what seems to be a principal driver of the east-west extension is a big bubble of hot mantle slowly oozing its way upward by the power of convection. As this bubble pushes upwards towards the Earth’s surface, the lithosphere parts ways and breaks into huge blocks.
These blocks, in turn, float or sink depending on their mass. Having had huge amounts of material eroded from its top, the Sandia-Manzano block floats several thousand feet higher than the basin below. The Albuquerque Basin, meanwhile, has been shoved downwards by deposits that are about three miles thick.
Basin And Range, Or Something Else?
Unfortunately, a question I’ve not been able to find a clear answer to is whether or not the Rio Grande Rift should be considered a part of the Basin and Range Province or not. (As mentioned above, it’s not easily distinguished from neighboring zones.) Sure, there are unique dynamics here, but this is probably the case with lots of other constituent parts of the Basin and Range. At what point do we say this is part of the Basin and Range but this is not?
For me, confusion over the boundary of the Basin and Range is yet another exemplar of the difficulty of drawing clear lines in the world. As with the art/vandalism dichotomy, it would be hard to come up with necessary and sufficient conditions for being incorporated into any geological province. There are always grey areas, always exceptions.
One thing is clear, though: the Rio Grande Rift is still widening.
- Geologic Tour Of The Rio Grande Rift (New Mexico Tech)
- Stretching, Cracking, Floating, Sinking: A Basin And Range Primer (Farther Still)
- Drawing Lines Between Graffiti And Cultural Treasures (Farther Still)
(Today’s photo of cottonwood forest — which flanks much of the length of the Rio Grande — is courtesy of Mark Pouley’s Flickr. Thanks for posting under a Creative Commons license!)