Geology Reference
In-Depth Information
What Would You Do
?
Soils are certainly one of our most precious natural resources.
Indeed, if it were not for soils, food production on Earth would
be vastly different and capable of supporting far fewer people.
In addition, other aspects of soils are important economi-
cally. We discussed the origin of laterite in response to intense
chemical weathering in the tropics, and we noted further that
laterite is not very productive. If the parent material is rich
in aluminum, however, the ore of aluminum called
bauxite
accumulates in horizon B. Some bauxite is found in Arkansas,
Alabama, and Georgia, but at present, it is cheaper to import
rather than mine these deposits, so both the United States and
Canada depend on foreign sources of aluminum ore.
Bauxite and other accumulations of valuable miner-
als by the selective removal of soluble substances during
chemical weathering are known as
residual concentrations.
Certainly, bauxite is a good example, but other deposits that
formed in a similar manner are those rich in iron, manga-
nese, clays, nickel, phosphate, tin, diamonds, and gold. Some
of the sedimentary iron deposits in the Lake Superior region
of the United States and Canada were enriched by chemi-
cal weathering when soluble parts of the deposits were car-
ried away. Some kaolinite deposits in the southern United
States formed when chemical weathering altered feldspars in
pegmatites or as residual concentrations of clay-rich lime-
stones and dolostones. Kaolinite is a clay mineral used in the
manufacture of paper and ceramics.
A
gossan
is a yellow to red deposit made up mostly
of hydrated iron oxides that formed by oxidation and
In the past few years, many gullies have appeared in farmers'
fi elds in your area, and residents of your area are concerned
because agriculture is the main source of jobs and tax
revenue. Obviously, a decrease in agricultural production
would be an economic disaster. You are appointed to a county
board charged with making recommendations to prevent or
at least minimize erosion on local croplands. How would you
determine what caused the problem, and what specifi c recom-
mendations would you make to reduce gullying?
leaching of sulfi de minerals such as pyrite (FeS
2
). The disso-
lution of pyrite and other sulfi des forms sulfuric acid, which
causes other metallic minerals to dissolve, and these tend to
be carried down toward the groundwater table, where the
descending solutions form minerals containing copper, lead,
and zinc. Gossans have been mined for iron, but they are far
more important as indicators of underlying ore deposits.
Mechanical and chemical weathering, erosion, transport, and
deposition are essential parts of the rock cycle (see Figure 1.14)
because they are responsible for the origin and deposition of
sediment
that may become lithifi ed, that is, transformed into
sedimentary rock
(
Figure 6.15). The term
sediment
refers to
all solid particles derived from preexisting rocks, which is
de-
trital sediment
. It also encompasses
chemical sediment
, which includes
(1) minerals derived from solutions
that contain minerals dissolved
during chemical weathering, and
(2) minerals extracted from water,
mostly seawater, by organisms to
build their shells.
Sedimentary rock
is simply rock made up of consoli-
dated sediment.
One important criterion for
classifying detrital sediment is
particle size. Particles described
as
gravel
measure more than
2 mm, whereas sand measures
1/16 -2 mm, and silt is any particle
between 1/256 and 1/16 mm. None
of these designations implies any-
thing about composition; most
gravel is made up of rock frag-
ments—that is, small pieces of
granite, basalt, or any other rock
type—but sand and silt grains are
usually single minerals, especially
quartz. Particles smaller than 1/256
mm are termed clay, but clay has
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