Exfoliation
(c) 2000 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Sometimes rocks weather by peeling off in sheets rather than eroding grain by grain. Exfoliation is scientific Latin for that process. It can happen in paper-thin layers on individual boulders, or it can take place in thick slabs as it does here, in Yosemite Valley, California.
The great white granite domes and cliffs of the High Sierra, like Half Dome, owe their appearance to this type of exfoliation. These rocks were emplaced as molten bodies, or plutons, deep underground, raising the Sierra Nevada range. Erosion then unroofed the plutonic rocks and took away the pressure of the overlying rock. As a result, the solid rock acquired fine cracks through pressure-release jointing. The combined work of gravity, weathering, plant roots, and the expansion of freezing water opened up the joints further and loosened these slabs.
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