Sonoran Volcanism and Faulting

The Sonora Desert has many exciting geographic forms. One of the more interesting is the Pinacate extrusive volcanic field. In this 600 square mile area, a large shield volcano is surrounded by around 400 cinder cones including a large grouping of maar craters.

The Pinacate Volcanic Field, Sonora, Mexico from Google Earth

The shield volcano, known as Volcan Santa Clara, rises 3,900 feet and was created by flood basalts. Multiple layers of low viscosity lava flowed over long distances. These igneous extrusions of dark colored mafic basalt layers formed a gently sloped mountain. As the mountain grew, layer-by-layer, there would have been little danger to human life. The gentle flowing lava and low angle of the volcano would have given any people in the area time to escape.

Adjacent to the shield volcano are the cinder cone and maar craters. The maar craters are the most interesting because of the way in which they formed. Rising magma interacted with ground water creating explosive steam and ash eruptions. These eruptions would have been dangerous to any humans in the area as tephra and steam ejections would have been quite violent. Also, ash in the air would have made it difficult to breathe.

"Elegante" maar crater with the "Volcan Santa Clara" shield volcano in the background. (NASA Photo)

The Pinacate area is covered in grey basalt from tephra deposits from the cinder cones. As a result, you can easily locate this area on Google Earth as a large grey circle just south of Arizona and north of the Gulf of California.

Geological History of the Sonoran Desert region near Tucson from the Arizona-Sonora Desert Museum

Another characteristic of Sonora is the basin and range topography. This was created by an extension faulting process as the Sonora Desert was stretched west. Grabens formed between fault planes (see illustration C above) and the downthrown blocks became today's basins. This can be recognized from above as the mountains and valleys line up in parallel as shown in the below picture.  

 Sonora Range and Basin Topography from Google Earth

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References:
http://www.biology.ed.ac.uk/research/groups/jdeacon/desertecology/pinacate.htm
http://www.desertmuseum.org/books/nhsd_geologic_origin.php
http://googleearth.com/
http://hvo.wr.usgs.gov/kilauea/update/images.html
http://pdc.org/iweb/volcano.jsp
http://pinacate.org/
http://rst.gsfc.nasa.gov/Sect18/originals/FIG18_54.JPG