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FIGURE 20.4 Growing trees can
break, separate, and move solid rock.
(A) Note how this tree has raised the
sidewalk. (B) This tree is surviving by
growing roots into tiny joints and cracks,
which become larger as the tree grows.
A B

Carbonation is a reaction between carbonic acid and the min- Hydration is a reaction between water and the minerals of
erals making up rocks. Rainwater is naturally somewhat acidic be- rocks. The process of hydration includes (1) the dissolving of
cause it dissolves carbon dioxide from the air. This forms a weak a mineral and (2) the combining of water directly with a min-
acid known as carbonic acid (H 2 CO 3 ), the same acid that is found eral. Some minerals, such as halite (which is sodium chloride),
in carbonated soda. Carbonic acid rain falls on the land, seeping dissolve in water to form a solution. The carbonates formed
into cracks and crevices where it reacts with minerals. Limestone, from carbonation are mostly soluble, so they are eas ily leached
for example, is easily weathered to a soluble form by carbonic acid. from a rock by dissolving. Water also combines directly with
The limestone caves of Missouri, Kentucky, New Mexico, and else- some minerals to form new, different minerals. The feldspars,
where were produced by the chemical weathering of limestone by for example, undergo hydration and carbonation to produce
carbona tion (Fig ure 20.6). Minerals containing calcium, magne- (1) water-soluble potassium carbonate, (2) a chemical product

sium, sodium, potassium, and iron are chemically weathered by that combines with water to produce a clay mineral, and
carbonation to produce salts that are soluble in water. (3) silica. The silica, which is silicon dioxide (SiO 2 ), may appear
as a suspension of finely divided particles or in solution.
Mechanical weathering and chemical weathering are inter-
related, working together in breaking up and decomposing solid
rocks of Earth’s surface. In general, mechanical weathering re-
sults in cracks in solid rocks and broken-off coarse fragments.
Chemical weathering results in finely pulverized materials and
ions in solution, the ultimate decomposition of a solid rock.
Consider, for example, a mountain of solid granite, the most
common rock found on continents. In general, granite is made
up of 65 percent feldspars, 25 percent quartz, and about 10 per-
cent ferromagnesian minerals. Mechanical weathering begins
the destruction process as exfoliation and frost wedging create
cracks in the solid mass of granite. Rainwater, with dissolved
oxygen and carbon dioxide, flows and seeps into the cracks
and reacts with ferromagnesian minerals to form soluble car-
bonates and metal oxides. Feldspars undergo carbonation and
hydration, forming clay minerals and soluble salts, which are
FIGURE 20.5 Spheroidal weathering of granite. The edges and
washed away. Quartz is less susceptible to chemical weathering
corners of an angular rock are attacked by weathering from more
than one side and retreat faster than flat rock faces. The result is and remains mostly unchanged to form sand grains. The end
rounded granite boulders, which often shed partially weathered products of the complete weathering of granite are quartz sand,
minerals in onion-like layers. clay minerals, metal oxides, and soluble salts.

20-5 CHAPTER 20 Shaping Earth’s Surface 505