ABSTRACT
Unloading When erosion removes surface material, the confining pressure on the underlying rocks is eased. The lower pressure enables mineral grains to move further apart, creating voids, and the rock expands or dilates. In mineshafts cut in granite or other dense rocks, the pressure release can cause treacherous explosive rockbursts. Under natural conditions, rock dilates at right-angles to an erosional surface (valley side, rock face, or what - ever). The dilation produces large or small cracks (fractures and joints) that run parallel to the surface. The dilation joints encourage rock falls and other kinds of mass movement. The small fractures and incipient joints provide lines of weakness along which individual crystals or particles may disintegrate and exfoliation may occur. Exfoliation is the spalling of rock sheets from the main rock body. In some rocks, such as granite, it may produce convex hills known as exfoliation domes. Half-Dome in Yosemite Valley, California, USA, is a classic exfoliation dome (Plates 7.1 and 7.2). In the original grano - diorite intrusion, exposure to erosion leads to pressure changes that cause the dome to crack, forming shells that fall away from the mountain. Although its name suggests that half the mountain has collapsed in that manner, in fact about 80 per cent still stands. Stone Mountain, Georgia, USA, is an exfoliated inselberg.
