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FIGURE 20.1 This famous natural bridge is an example of a landform created by the sculpturing power of weathering and erosion. It is
Rainbow Bridge in the Rainbow Bridge National Monument, Utah.

Mechanical weathering is the physical breaking up of rocks pieces of rock to break off. This process is called frost wedging
without any changes in their chemical composition. Mechani- (Figure 20.3A). It is an important cause of mechanical weath-
cal weathering results in the breaking up of rocks into smaller ering in mountains and other locations where repeated cycles
and smaller pieces, so it is also called disintegration. If you of water freezing and thawing occur. The roots of trees and
smash a sample of granite into smaller and smaller pieces, you shrubs can also mechanically wedge rocks apart as they grow
are mechanically weathering the granite. Chemical weathering into cracks. You may have noticed the work of roots when trees
is the alteration of minerals by chemical reactions with water, or shrubs have grown next to a sidewalk for some period of time
gases of the atmosphere, or solutions. Chemical weathering (Figure 20.4).
results in the dissolving or breaking down of the minerals in The other example of mechanical weathering is believed
rocks, so it is also called decomposition. If you dissolve a sample to be caused by the reduction of pressure on rocks. As more
of limestone in a container of acid, you are chemically weather- and more weathered materials are removed from the surface,
ing the limestone. the downward pressure from the weight of the material on
Examples of mechanical weathering in nature include the the rock below becomes less and less. The rock below begins
disintegration of rocks caused by (1) wedging effects and (2) the to expand upward, fracturing into concentric sheets from the
effects of reduced pressure. Wedging effects are often caused by effect of reduced pressure. These curved, sheetlike plates fall
the repeated freezing and thawing of water in the pores and away later in the mechanical weathering process called exfolia-
small cracks of otherwise solid rock. If you have ever seen what tion (Figure 20.3B). Exfoliation is the term given to the process
happens when water in a container freezes, you know that of spalling off of layers of rock, somewhat analogous to peeling
freezing water expands and exerts a pressure on the sides of its layers from an onion. Granite commonly weathers by exfolia-
container. As water in a pore or a crack of a rock freezes, it also tion, producing characteristic dome-shaped hills and rounded
expands, exerting a pressure on the walls of the pore or crack, boulders. Stone Mountain in Georgia is a well-known exam-
making it slightly larger. The ice melts and the enlarged pore or ple of an exfoliation-shaped dome. The onionlike structure of
crack again becomes filled with water for another cycle of freez- exfoliated granite is a common sight in the Sierras, Adiron-
ing and thawing. As the process is repeated many times, small dacks, and any mountain range where older granite is exposed
pores and cracks become larger and larger, eventually forcing at the surface (Figure 20.5).

20-3 CHAPTER 20 Shaping Earth’s Surface 503