Saturday, April 13, 2013

VASQUEZ ROCKS: The World’s Poster Child for Tectonics

Located about 40 km north of Hollywood, California and 15 km southwest of the San Andreas Fault are some the world’s most recognizable rock formations; these are the rugged San Vasquez Rocks. 

The stones themselves are rather unremarkable on first sight: basically, they’re sandstones of about 25 million years of age with a fairly strong tilt, dipping away from a larger anticline. Okay, there are many Oligocene rock formations scattered around the earth with similar tilts and folds. The Vasquez Rocks contain clues to their depositional environment with ample mudcracks, ripple marks, crossbeds and graded beds – always geological fun to find in the field, but by no means rare phenomena. Their rugged topography is due to differential erosion; okay, so what else is so outlandish that these rocks deserve such special tectonic recognition?

The North American Plate was once separated from the Pacific Plate by another plate named the Farallon. About 25 million years ago (yes, the same age as the Vasquez sediments), the Farallon plate was over-ridden by the North American Plate and subducted to great depths (you can see a series of cross-sections of these tectonic motions on our recent Earth Story post: ). Below and within the sedimentary rock formations of Vasquez are basalts; these basalts intruded into the broken cracks and fractures forming above this subducting plate. The relentless movement of the North American Plate continued, and continues still, with the San Andreas fault zone now taking the place of a plate contact between the North American Plate and Pacific plate. The early days of the Vasquez formation records this phase of geologic history, and its sedimentary record includes several mega-cycles of uplift, erosion, and deposition as the new plate margin evolved. A Google Earth view of the Aqua Dulce area shows the anticlinal form of the outcropping Vasquez Rocks as a product of motion between the North American Plate and Pacific Plate along the San Andreas fault.

Perhaps you’re scratching your head at this point, and wondering where on earth you've seen these rocks before? In addition to seeing them “on earth,” they’re also highly visible amongst alien planets. Being conveniently accessible to movie and TV studios, as well as being fairly easy for actors and camera crews to scramble over, the Vasquez Rocks are filmed so frequently as to have become an icon for several Star Trek episodes, Bonanza, and have been used as stunt doubles for Tibet, Egypt and other desert countries.

So, the next time you’re watching the TV and the Vasquez Rocks are featured, remember that they’re not famous only for the battle between Captain Kirk and the Gorn, but also as a geologic landmark known the world over.

Photo: from Wiki Commons by a photographer who identifies him/herself as hear2heal:
Thanks to the Tectonophysics Page for reminding me how special these rocks are:

More information on the World’s Most Highly Recognizable Tectonic Locality:

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