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Earth: An Introduction to
Physical Geology, 10e
Tarbuck & Lutgens
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Weathering and Soil
Earth, 10e - Chapter 6
Stan Hatfield
Southwestern Illinois College
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Earth’s External Processes
• Weathering—the physical breakdown and
chemical decomposition of rock
• Mass wasting—the transfer of rock and soil
downslope under the influence of gravity
• Erosion—the physical removal of material
by water, wind, ice, or gravity
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Weathering
•
Two types:
1. Mechanical weathering—breaking of rocks
into smaller pieces
2. Types of mechanical weathering
–
–
–
–
Frost wedging
Unloading
Thermal expansion
Biological activity
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Sheeting and Exfoliation
Due to Unloading
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Weathering
• Chemical weathering
• Breaks down rock components and internal
structures of minerals
• The most important agent is water.
– Responsible for transport of ions and molecules
involved in chemical processes
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Weathering
• Major processes of chemical weathering
• Dissolution
– Aided by small amounts of acid in the water
• Oxidation
– Any reaction when electrons are lost from one
element
• Hydrolysis
– The reaction of any substance with water.
– A hydrogen ion attacks and replaces other ions.
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Weathering
• Alterations caused by chemical weathering
• Decomposition of unstable minerals
• Formation or retention of stable materials
• Physical changes such as the rounding of
corners or edges
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Products of Weathering
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Rates of Weathering
Factors affecting weathering
• Surface area
• Rock characteristics
– Rocks containing calcite (marble and limestone)
readily dissolve in weakly acidic solutions.
– Silicate minerals weather in the same order as
their order of crystallization.
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Surface Area and
Chemical Weathering
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Rates of Weathering
• Climate
– Temperature and moisture characteristics
– Chemical weathering is most effective in areas of
warm, moist climates.
• Differential weathering
– Rocks do not weather uniformly due to regional
and local factors.
– Results in many unusual and spectacular rock
formations and landforms
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Spheroidal Weathering
of Extensively Jointed Rock
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Differential Weathering in
Bryce Canyon National Park, Utah
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Soil
• Soil = mineral and organic matter + water +
air
• Rock and mineral fragments produced by
weathering (regolith) that support the
growth of plants
• Humus (decayed animal and plant remains)
is a small, but essential component.
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Composition of a “Good” Soil
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Soil
• Factors controlling soil formation
• Parent material
– Residual versus transported soil
• Time
– Amount of time for soil formation varies
depending on geologic and climatic conditions.
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Soil
• Climate
– Most influential control of soil formation
– Key factors are temperature and precipitation.
• Plants and animals
– Influence the soil’s physical and chemical
properties
• Topography
– Steep slopes often have poorly developed soils.
– The optimum terrain is a flat-to-undulating
upland surface.
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Variations in Soil Development
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Soil
• The soil profile
• Soil-forming processes operate from the
surface downward.
• Vertical differences are called horizons,
which are zones or layers of soil.
– O horizon—organic matter
– A horizon—organic and mineral matter
» High biological activity
» O and A horizons make up the topsoil
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Soil
• Soil profile
– E horizon—little organic matter
» Zone of eluviation and leaching
– B horizon—zone of accumulation
– C horizon—partially altered parent material
• Collectively, the O, A, E, and B horizons
make up the solum, or “true soil.”
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Idealized
Soil
Profile
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Soil Profile Showing
Different Horizons
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Soil
• Classifying soils
• Variations in soil formation over time and
distances has led to a great variety of
recognized soil types.
• Groups have been established using common
characteristics.
• In the United States, a system was devised,
and called the Soil Taxonomy.
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Soil
• Classifying soils
• Soil Taxonomy
– Emphasis is placed on physical and chemical
properties.
– Six hierarchical categories exist.
– Descriptive names are derived from Latin and
Greek.
– 12 basic soil orders are recognized.
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Basic Soil Orders
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Soil
• Soil erosion
• Part of the natural recycling of all Earth
materials
• Natural rates of soil erosion depend on:
– Soil characteristics
– Climate
– Slope
– Type of vegetation
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Soil Erosion Where
Vegetation Is Lacking
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Soil
• In many regions, soil erosion is much
greater than the rate of soil formation.
• Sedimentation and chemical pollution
– Related to excessive soil erosion
– Occasionally, soil particles are contaminated
with pesticides.
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End of Chapter 6
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