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Mechanical or Physical Weathering Gallery


Exfoliation is the process in which rocks weather by peeling off in sheets rather than eroding grain by grain. (more below)
Rock domes peel off in shells
Photo courtesy Josh Hill of Flickr under Creative Commons license
Exfoliation can happen in thin layers on individual boulders, or it can take place in thick slabs as it does here, at Enchanted Rock in Texas.

The great white granite domes and cliffs of the High Sierra, like Half Dome, owe their appearance to exfoliation. These rocks were emplaced as molten bodies, or plutons, deep underground, raising the Sierra Nevada range. The usual explanation is that erosion then unroofed the plutons and took away the pressure of the overlying rock. As a result, the solid rock acquired fine cracks through pressure-release jointing. Mechanical weathering opened up the joints further and loosened these slabs.

However, a recent paper argues that large-scale exfoliation of this type arises from the basic mechanics of gravity on a curved surface, which creates tension beneath a dome (and compression beneath a bowl). Therefore "pressure-release jointing" may be a misnomer, but textbooks have not yet adopted this view.

Small-scale exfoliation is more clearly linked to physical effects of chemical weathering, and is discussed in the Chemical Weathering Gallery.

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