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Water freezing in crack
expands, widening crack.

Eventually,
rock is broken free by
progressive fracturing.

A

Mass of granite was originally
deeply buried and under pressure.

FIGURE 20.2 The piles of rocks and rock fragments around a As erosion exposes the mass
mass of solid rock is evidence that the solid rock is slowly of granite, it is unloaded and
uncompressed and may
crumbling away. This solid rock that is crumbling to rock fragments break apart in sheets.
is in the Grand Canyon, Arizona.

Examples of chemical weathering include (1) oxidation,
(2) carbonation, and (3) hydration. Oxidation is a reaction
between oxygen and the minerals making up rocks. The fer-
romagnesian minerals contain iron, magnesium, and other
metal ions in a silicate structure. Iron can react with oxygen
to produce several different iron oxides, each with its own
characteristic color. The most common iron oxide (hema-
tite) has a deep red color. Other oxides of iron are brownish
to yellow-brownish. It is the presence of such iron oxides that
colors many sedimentary rocks and soils. The red soils of Okla- B
homa, Georgia, and many other places are colored by the pres- FIGURE 20.3 (A) Frost wedging and (B) exfoliation are two
ence of iron oxides produced by chemical weathering. examples of mechanical weathering, or disintegration, of solid rock.

504 CHAPTER 20 Shaping Earth’s Surface 20-4