145
Chapter in Review
C H A P T E R
F I V E
Weathering and Soils
in Review
External processes include (1) weathering—the disintegration
and decomposition of rock at or near Earth’ s surface; (2) mass
wasting—the transfer of rock material downslope under the
influence of gravity; and (3) erosion—the removal of material by
a mobile agent, usually water, wind, or ice. They are called
external processes because they occur at or near Earth’ s surface
and are powered by energy from the Sun. By contrast, internal
processes, such as volcanism and mountain building, derive their
energy from Earth’ s interior.
Mechanical weathering is the physical breaking up of rock into
smaller pieces. Rocks can be broken into smaller fragments by
frost wedging (where water works its way into cracks or voids in
rock and upon freezing expands and enlarges the openings), salt
crystal growth, unloading (expansion and breaking due to a great
reduction in pressure when the overlying rock is eroded away),
and biological activity (by humans, burrowing animals, plant
roots, etc.).
Chemical weathering alters a rock’ s chemistry, changing it into
different substances. Water is by far the most important agent
of chemical weathering. Oxygen dissolved in water will oxidize
iron-rich minerals, while carbon dioxide (CO 2 ) dissolved in
water forms carbonic acid, which attacks and alters rock. The
chemical weathering of silicate minerals frequently produces
(1) soluble products containing sodium, calcium, potassium,
and magnesium ions, as well as silica in solution; (2) insoluble
iron oxides, including limonite and hematite; and (3) clay
minerals.
The rate at which rock weathers depends on such factors as
(1) particle size—small pieces generally weather faster than large
pieces; (2) mineral make-up—calcite readily dissolves in mildly
acidic solutions, and silicate minerals that form first from magma
are least resistant to chemical weathering; and (3) climatic factors,
particularly temperature and moisture. Frequently, rocks exposed
at Earth’ s surface do not weather at the same rate. This differential
weathering of rocks is influenced by such factors as mineral
make-up and degree of jointing.
Soil—that portion of the regolith (the layer of rock and mineral
fragments produced by weathering) that supports the growth of
Bauxite forms in rainy tropical climates. When aluminum-rich source rocks
are subjected to the intense and prolonged
chemical weathering of the tropics, most
of the common elements, including
calcium, sodium, and potassium, are
removed by leaching. Because aluminum
is extremely insoluble, it becomes concentrated in the soil (as bauxite, a hydrated
aluminum oxide). Thus, the formation of
bauxite depends on climatic conditions in
which chemical weathering and leaching
are pronounced, plus, of course, the
presence of aluminum-rich source rock.
In a similar manner, important deposits of
nickel and cobalt develop from igneous
rocks rich in silicate minerals such as
olivine.
There is significant concern regarding
the mining of bauxite and other residual
deposits because they tend to occur in
environmentally sensitive areas of the
tropics. Mining is preceded by the removal
of tropical vegetation, thus destroying
rainforest ecosystems. Moreover, the thin
moisture-retaining layer of organic matter
is also disturbed. When the soil dries out
in the hot sun, as has been mentioned, it
becomes bricklike and loses its moistureretaining qualities. Such soil cannot be
productively farmed nor can it support
significant forest growth. The long-term
consequences of bauxite mining are clearly
of concern for developing countries in the
tropics, where this important ore is mined.
Other Deposits
Many copper and silver deposits result
when weathering processes concentrate
metals that are dispersed through a lowgrade primary ore. Usually such
enrichment occurs in deposits containing
pyrite (FeS 2 ), the most common and widespread sulfide mineral. Pyrite is important
because when it chemically weathers,
sulfuric acid forms, which enables percolating waters to dissolve the ore metals. Once
dissolved, the metals gradually migrate
downward through the primary ore body
until they are precipitated. Deposition takes
place because of changes that occur in the
chemistry of the solution when it reaches
the groundwater zone (the zone beneath
the surface where all pore spaces are filled
with water). In this manner, the small
percentage of dispersed metal can be
removed from a large volume of rock and
redeposited as a higher-grade ore in a
smaller volume of rock.
C O N C E P T C H E C K 5 . 1 2
How can weathering create an ore
deposit? What important ore is an
example?
1
D I D Y O U K N O W
Bauxite is a useful indicator of past
climates because it records periods of
wet tropical climate in the geologic past.
Chapter in Review
C H A P T E R
F I V E
Weathering and Soils
in Review
External processes include (1) weathering—the disintegration
and decomposition of rock at or near Earth’ s surface; (2) mass
wasting—the transfer of rock material downslope under the
influence of gravity; and (3) erosion—the removal of material by
a mobile agent, usually water, wind, or ice. They are called
external processes because they occur at or near Earth’ s surface
and are powered by energy from the Sun. By contrast, internal
processes, such as volcanism and mountain building, derive their
energy from Earth’ s interior.
Mechanical weathering is the physical breaking up of rock into
smaller pieces. Rocks can be broken into smaller fragments by
frost wedging (where water works its way into cracks or voids in
rock and upon freezing expands and enlarges the openings), salt
crystal growth, unloading (expansion and breaking due to a great
reduction in pressure when the overlying rock is eroded away),
and biological activity (by humans, burrowing animals, plant
roots, etc.).
Chemical weathering alters a rock’ s chemistry, changing it into
different substances. Water is by far the most important agent
of chemical weathering. Oxygen dissolved in water will oxidize
iron-rich minerals, while carbon dioxide (CO 2 ) dissolved in
water forms carbonic acid, which attacks and alters rock. The
chemical weathering of silicate minerals frequently produces
(1) soluble products containing sodium, calcium, potassium,
and magnesium ions, as well as silica in solution; (2) insoluble
iron oxides, including limonite and hematite; and (3) clay
minerals.
The rate at which rock weathers depends on such factors as
(1) particle size—small pieces generally weather faster than large
pieces; (2) mineral make-up—calcite readily dissolves in mildly
acidic solutions, and silicate minerals that form first from magma
are least resistant to chemical weathering; and (3) climatic factors,
particularly temperature and moisture. Frequently, rocks exposed
at Earth’ s surface do not weather at the same rate. This differential
weathering of rocks is influenced by such factors as mineral
make-up and degree of jointing.
Soil—that portion of the regolith (the layer of rock and mineral
fragments produced by weathering) that supports the growth of
Bauxite forms in rainy tropical climates. When aluminum-rich source rocks
are subjected to the intense and prolonged
chemical weathering of the tropics, most
of the common elements, including
calcium, sodium, and potassium, are
removed by leaching. Because aluminum
is extremely insoluble, it becomes concentrated in the soil (as bauxite, a hydrated
aluminum oxide). Thus, the formation of
bauxite depends on climatic conditions in
which chemical weathering and leaching
are pronounced, plus, of course, the
presence of aluminum-rich source rock.
In a similar manner, important deposits of
nickel and cobalt develop from igneous
rocks rich in silicate minerals such as
olivine.
There is significant concern regarding
the mining of bauxite and other residual
deposits because they tend to occur in
environmentally sensitive areas of the
tropics. Mining is preceded by the removal
of tropical vegetation, thus destroying
rainforest ecosystems. Moreover, the thin
moisture-retaining layer of organic matter
is also disturbed. When the soil dries out
in the hot sun, as has been mentioned, it
becomes bricklike and loses its moistureretaining qualities. Such soil cannot be
productively farmed nor can it support
significant forest growth. The long-term
consequences of bauxite mining are clearly
of concern for developing countries in the
tropics, where this important ore is mined.
Other Deposits
Many copper and silver deposits result
when weathering processes concentrate
metals that are dispersed through a lowgrade primary ore. Usually such
enrichment occurs in deposits containing
pyrite (FeS 2 ), the most common and widespread sulfide mineral. Pyrite is important
because when it chemically weathers,
sulfuric acid forms, which enables percolating waters to dissolve the ore metals. Once
dissolved, the metals gradually migrate
downward through the primary ore body
until they are precipitated. Deposition takes
place because of changes that occur in the
chemistry of the solution when it reaches
the groundwater zone (the zone beneath
the surface where all pore spaces are filled
with water). In this manner, the small
percentage of dispersed metal can be
removed from a large volume of rock and
redeposited as a higher-grade ore in a
smaller volume of rock.
C O N C E P T C H E C K 5 . 1 2
How can weathering create an ore
deposit? What important ore is an
example?
1
D I D Y O U K N O W
Bauxite is a useful indicator of past
climates because it records periods of
wet tropical climate in the geologic past.