Chapter 4
Weathering, Erosion and Soils
All of the changes occurring in rock material as the result of exposure to air and
water. Weathering takes place anywhere that water and/or air can penetrate. It
produces altered material that is more stable at the Earth's surface than the original
parental material, and provides the material that becomes soil or sedimentary rock.
The driving forces for weathering are:
1. Solar energy - causes circulation of the atmosphere (weather) and determines
climate and vegetation.
2. Earth's internal energy - forces within the Earth uplift deeply buried rock and
expose it to weathering.
Two types of weathering are recognized:
A. Mechanical Weathering involves the physical disintegration of rock. The process
produces smaller rock particles that have the same composition as the parent material.
Mechanical weathering may occur as a result of:
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Frost Action/Frost Wedging - the 9% expansion of water during freezing in
rock fractures breaks off pieces of rock and produces talus slopes at bottom of
cliffs. It is most effective where there is:
1. abundant water
2. many cracks and pore spaces in the rock
3. a climate with temperature fluctuations across the freezing point
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Exfoliation - removal of overlying rock which causes the underlying rock to
expand and fracture due to pressure release. Fractures (sheet joints) develop
parallel to the rock surface and tend to peel off, locally producing exfoliation
domes.
Thermal Expansion and Contraction - solids expand when heated and contract
when cooled. Although such expansion due to changes in temperature from
day to night or from summer to winter may contribute to rock disintegration,
expeninents indicate it has only a minor effect.
Abrasion - particles moved by water, ice, and air can be effective in wearing
away rock.
Salt Crystal Growth - similar to frost wedging, the force of growing salt
crystals in fractures and pore spaces can pry loose grains and expand
openings. Such a process is important in arid and coastal areas.
Action of Plants and Animals - roots of plants can widen cracks and
burrowing animals can mix soil/sediment particles and allow water to
penetrate more quickly and deeply, speeding other weathering processes.
B. Chemical Weathering involves the decomposition of rock, transforms the parent
material chemically, and releases mineral grains. Chemical weathering processes
include:
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Solution - dissolving of minerals into solution. Most minerals have low
solubility in pure water, but rain contains carbonic acid, so that carbonate
minerals dissolve readily in acidic solutions.
Oxidation - atmospheric oxygen combines with metal ions to form oxides (or
hydroxides). Oxidation of pyrite, for example, produces sulfuric acid.
Hydrolysis - reaction between a mineral and water can produce a new mineral
or dissolved material. Hydrolysis of feldspar, for example, produces clay.
What Factors Control the Rate of Chemical Weathering?
The rate of weathering is controlled by:
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Surface Area (Related to particle size) controls weathering because the smaller
the particle size, the larger the surface area exposed to weathering for a given
volume of material. The presence of joints therefore increases a rock's surface
area. Spheroidal weathering, which produces rounded surfaces on weathered
rock, can be caused where:
1. Concentric shells of altered rock material (clay), produced by chemical
weathering, are easily removed.
2. Faster weathering of edges and corners of rock tends to produce
rounded surfaces, where a sphere has the least volume to surface area
ratio of any geometric form.
Climate controls weathering in that rocks experience the most thorough and
rapid chemical weathering in warm, humid climates. Weathering rates are
lowest in desert and cold climates.
Parent Material (mineral composition of rock) controls weathering processes
because minerals that fonn at high temperature and high pressure are less
stable at the Earth's surface than low temperature, low pressure minerals.
Bowen's Reaction Series can be used in reverse to predict the ease of chemical
weathering of common igneous minerals.
Presence of Plants and Animals influence weathering as chemical and physical
weathering rates are increased by the action of plants and animals.
Topography affects weathering as irregular, steep topography exposes more
rock to weathering than horizontal surfaces.
What is Soil?
Soil, a non-renewable resource, is the final product of weathering, and is defined as
that portion of the regolith (unconsolldated rock and mineral fragments covering the
land surface) which is capable of supporting plant life. Regolith can be generally
grouped as either residual (soils formed on bedrock) or transported (soil formed on
material that has been moved to its current location).
What is a Soil Profile?
Soil consists of a series of horizontal layers called horizons. A soil with welldeveloped horizons is called mature, whereas a soil with poorly developed horizons is
called immature. Soil horizons from top to bottom are as follows:
1. O Horizon — This organic zone, that contains humus (produced by bacterial
decay of organic matter) is typicallv only a few cm thick.
2. A Horizon — This horizon is a zone of leaching (topsoil), where soluble
material is leached out by rainwater. The zone has higher organic content and
biological activity than lower horizons.
3. B Horizon — This horizon represents a zone of accumulation (subsoil) where
some material from the A horizon is deposited as clay and iron oxides. It
contains the fragipan (a dense layer) which is called claypan when clay-rich or
hardpan when cemented. Common hardpan cements are caliche (calcium
carbonate) or iron oxides.
4. C Horizon — This horizon consists of partially decomposed bedrock, and can
not support plant life.
What Factors Control Soil Formation?
The factors controlling soil formation are similar to those controlling the rate of
chemical weathering:
1. Climate is most important factor influencing soil type and depth.
2. Parent material exerts some control on soil type and depth, particularly for
immature soils. The original rock particles control the fertility of the soil by
providing the inorganic nutrients.
3. Organisms contribute to soil formation and fertility.
4. Relief and Slope (Topography) Elevation affects climate and slope affects soil
formation and erosion.
5. Time affects soil maturity in that soil develops faster on unconsolidated
material than on bedrock.
How are Soils Classified?
Modern soil classifications are very complex. An older soil classification is easier to
understand because it is based on climate:
1. Pedalfer (Pedon is Greek for soil, Al = aluminum, Fe = iron) forms in humid
temperate climates. The A horizon is generally dark due to organic matter, but
most soluble materials are leached, so that clays and iron oxides are
concentrated in the B horizon.
2. Pedocal (Cal = calcium) forms in arid and semi-arid climates, where calcium
salts (caliche) accumulate in a layer. The A horizon is lighter in color due to
lower organic content and is less leached of soluble materials. Alkaline soils
can form in deserts where sodium salts are deposited in the A horizon.
3. Laterite forms in tropical rain forests, where extreme leaching removes
everything but clays and iron oxides. These soils are not fertile, despite lush
vegetation. They are mined for bauxite (aluminum ore) and iron (oxides) in
some places.
What causes Soil Degradation?
Soil degradation can be caused by any decrease in soil fertility, including erosion,
chemical deterioration, and physical deterioration:
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Erosion involves the movement (transportation) of weathered materials to a
place of deposition. About 25% of U.S. cropland is eroding faster than it is
being replaced by soil-forming processes. Improved farming practices have
significantly reduced erosion rates. Agents of erosion include gravity, water,
ice, and wind. Water erosion, for example, involves:
1. Sheet erosion where erosion is fairly evenly distributed over the
surface and removes thin layers of soil.
2. Rill erosion where erosion occurs in channels scoured by running
water, producing rills (shallow channels) and gullies (>30 cm).
The rate of erosion can be estimated by the amount of sediment (particles and
dissolved material) carried by streams in an area. Rates vary widely depending
upon cllmate, topography, rock type, and human influences (agriculture).
Differences in erosion rate are usually caused by compositional or grain-size
differences, but can also be caused by the presence/absence of fractures/joints
or by differences in sunlight intensity. Differential erosion occurs where
different portions of a rock body erode at different rates, and typically
produces
unusual
surfaces,
shapes,
and
formations.
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Chemical deterioration of soils can be caused by several factors, that include
loss of soil nutrients, pollution, or salinization. Loss of Soil Nutrients can be
caused by:
1. overuse of soil
2. clearing land of natural vegetation
3. insufficient use of fertilizer
Pollution can be caused by improper disposal of waste and overuse of
pesticides and herbicides. Salinization involves the concentration of soluble
salts in topsoil due to extensive irrigation in a semi-arid climate.
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Physical deterioration can be caused by compaction, where heavy machinery
and animals compact soil particles and reduce infiltration, or by exposure to
air and sunlight, which can bake clay-rich soil into a brick-like consistency
(laterites). These processes make it impossible to plow soil or for seeds to take
root.
How is Weathering Related to Mineral Resources?
Ore deposits can be produced or concentrated by weathering processes:
1. Residual Deposits can develop where soluble substances are removed, leaving
behind more valuable mineral resources by residual concentration.
Enrichment of low-grade deposits can occur where rock containing sulfide minerals in
low concentrations is leached of metals by weathering solutions, leaving a gossan
(hydrated ion ores formed by leaching of pyrite and other sulfides). Downward
infiltration of acid, metal-rich solutions can cause deposition of oxidized ores above
the water table and small zones of sulfide (supergene) enrichment below the water
table.