The Rio Grande Rift

The Rio Grande Rift runs right through the middle of New Mexico...right under, or along, the Rio Grande River.

Rio Grande River, Fort Selden NM

Many wrongly assume that the Rio Grande Rift was created by the Rio Grande River via erosional processes. It's actually the opposite. It began forming around 35 million years ago via normal faulting and extension in the Earth's crust. The result of these processes was a series of four large, semi-connected basins which would eventually fill up with water flowing downhill into the rift, then forming the river.

Generalized Cross Section of the Rio Grande Rift

Volcanoes, as well as landscapes and landforms shaped and formed by them, are plenty in New Mexico. Because New Mexico receives so little rainfall each year (a mere 15 inches compared to the national average of 37), volcanic landforms have eroded very little in the time since their formation. For this reason, combined with the fact that most wild areas in New Mexico are covered in just low-lying grasses, volcanic landforms have been very easy for geologists to spot and catalogue.

Although there hasn't been much volcanic activity at all in recent history, the Rio Grande Rift was once the site of intense volcanism. Yet, only 1.2 million years ago (not a lot in geologic time), The Valles Caldera, one of the world's youngest and largest (13.7 miles wide!) calderas, was created by the collapse of a magma chamber, triggered by an eruption on the scale of what geologists would classify as a "supervolcano."

Satellite Image of Valles Caldera

About 15 million years into its formation, the rift's basins began to open, exposing tons of mafic, (obviously extrusive) igneous rock, such as basalt.

Something I found very interesting is that geologists believe the rift region to be "dormant" and not "extinct," meaning that at some point in geologic time, there are likely to be major volcanic events in New Mexico.

Sources:
http://academic.emporia.edu/aberjame/student/reardon3/RIFT%20NEW2.html
http://en.wikipedia.org/wiki/Valles_Caldera
http://cires.colorado.edu/science/groups/sheehan/projects/riogrande/faq/
http://aconcagua.geol.usu.edu/~arlowry/RGR/faq.html

Photo Credits: 
http://newswatch.nationalgeographic.com/2014/07/03/along-the-rio-grande-an-innovative-water-market-restores-riverside-habitat/
http://en.wikipedia.org/wiki/Rio_Grande_rift#mediaviewer/File:Riogranderift_deepXsection.png
http://upload.wikimedia.org/wikipedia/commons/0/04/Valle_Caldera,_New_Mexico.jpg