Smoky Mountain Graben, Utah
January 09, 2007
The graben shown above was photographed in south central Utah this past October. A graben is a sunken block of Earth’s crust bounded by parallel faults. Large-scale grabens include Death Valley in California and much of the Great Rift Valley in East Africa. The Basin and Range region of Arizona and Nevada is characterized by parallel grabens and horsts (up-faulted blocks). Most of them trend northwest-southeast. Seen from high above, the region looks like an army of caterpillars heading from Mexico toward Oregon. This landscape was created by extensional forces resulting from relative plate motions along the San Andreas Fault, which runs the length of California.
Small-scale grabens are not as common. Smoky Mountain was named for emissions from underground fires in the coal seams that are common in this area north of Lake Powell. These coal layers also lubricate horizontal motion of the Navajo Sandstone that lies on top. Subsequent to the erosion of the mountain, large blocks of sandstone have shifted laterally, opening up remarkably deep fissures along the flat mountain ridge. Compression of the underlying softer rocks has also allowed this 20-meter wide section to drop about 2-3 meters, creating a classic mini-graben structure.
Note the car "parked" near the far side of the graben. This exciting but remote road is not recommended for passenger cars or wet weather travel.