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Exfoliation
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In geology, exfoliation is a erosive process, caused by freeze-thaw cycles. Exfoliation causes rock, such as granite, to erode in large, thin sheets. Exfoliation causes characteristic dome-shaped rock formations, as in granite domes.
Half Dome is a granite dome formation at the eastern end of Yosemite Valley, possibly the Valley's most familiar sight. The granite crest rises more than 1,440 meters (4,737 feet) above the Valley floor.
As late as the 1870s, Half Dome was considered completely unclimbable, but it may now be ascended in several different ways. Thousands of hikers reach the top each year by following a trail from the valley floor. The trailhead is only two miles from Half Dome itself, but the circuitous route is 8.5 miles (13.5 kilometers) long. The final ascent is accomplished by following a pair of metal cables raised on posts up the peak's steep but somewhat rounded east face. The cables were constructed in 1919.
Alternatively, over a dozen rock climbing routes lead directly from the valley up Half Dome's vertical north face.
Half Dome is an excellent example of a exfoliation. Half Dome probably never formed a completely round dome. When Half Dome formed, there were fractures in the granite. Water inside the fractures froze and thawed repeatedly. Glaciers eroded the base of the dome. Finally, about 20% of the dome fell and was carried away by the Tenaya glacier, leaving an almost-vertical cliff.
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