Feb 21, 2009
“Rock pedestal with exfoliating slabs in the background. Enchanted Rock, Texas. This granite is dated at 1.1 billion years (1.1Ga), likely the product of tectonic convergence between Laurentia (North America)and Gonawaland (South America+Africa), resulting in the super continent, Rondinia. The rock pedestal is the result of differential erosion: the base experiences greater chemical weathering than the upper part due to standing water at the base. Case-hardening on the surface of the top of the pedestal helps resist weathering. Exfoliation is explained as the reaction to the removal of overlying pressure (that is, rocks removed by erosion). Rocks exposed to such removal of overburden experience expansion: the result is exfoliation sheets. At Enchanted Rocks, one can see the exfoliated sheets sliding downslope.