Stretching, Cracking, Floating, Sinking: A Basin And Range Primer

Mountains form in lots of different ways. Sometimes — as is the case with New Mexico’s Mt. Taylor — sustained volcanic activity piles up huge quantities of lava, ash and other material spewed or otherwise oozed from underground. Other times — like with the Himalayas — stupendous peaks arise when enormous rafts of land slam into each other. These examples are pretty easy to understand.

Basin And Range

But mountain building is a little less straightforward in what geologists call the Basin and Range, a physiographic splotch covering a huge chunk of North America, including New Mexico, Arizona, California, Old Mexico, Utah, Nevada, and a bit of West Texas.

The defining characteristic of the region is a rhythmic topology of alternating mountains and plains that are oriented roughly north-south. The Sandias, Manzanos, Wasatch and Sierra Nevadas are all considered part of the Basin and Range.

Step One: Stretching

It’s thanks to my stint at Timpanogos Cave National Monument last year that I learned the ins and outs of what causes the earth to undulate in this manner. Since the monument is located in the Wasatch, having this knowledge was all but mandatory.

To make the science more digestible for visitors, I did what lots of folks do and divided the 17 million-year creation (it is ongoing) of the Basin and Range Province into three steps.

First there’s stretching. In part due to movement along the San Andreas Fault, in part due to crustal rebound stemming from the total subduction of something called the Farallon Plate, land that comprises the province has been stretching out. In fact, Reno and Salt Lake get over half an inch farther from each other every year.

Cracking, Floating And Sinking

Now, as you might expect given your past experience with pizza dough, all this crustal stretching also leads to crustal thinning. And all this crustal thinning, in it’s turn, leads to cracking that’s perpendicular to the direction of stretching. This is step two: the development of fissures in the planet’s outer shell.

What happens next — step three — blows my dang mind. Having been freed by the cracking, huge blocks of crust begin, depending on their relative density, to float higher or lower on the plastic and white-hot mantel below. The mountains and plains (a.k.a. “horsts” and “grabens” respectively), then, aren’t altogether unlike boats.

When erosion removes rock from the mountaintops, it’s like a ship emptying its ballast and sitting higher in the water as a result. When this debris is offloaded onto adjacent fault blocks, they sink a little lower.

Learn More:

  • An Evening Walk Under A Sky Of Cranes (Farther Still)
  • Trigo Canyon, Part 1: A First Voyage Into The Manzanos (Farther Still)
  • Basin And Range Province (USGS)

(Today’s photo of the Sandia Mountains rising out of the Albuquerque Basin comes from Mike Pendroncelli’s Flickr. Thanks for posting under a Creative Commons license!)

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