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Weathering and Soil
Learning Goals: Weathering & Soils
1. Define: weathering, mass wasting, erosion, differential weathering.
2. Explain how and why rocks disintegrate to form sediment by mechanical
processes
3. Compare and contrast the major processes and effects of chemical and
mechanical weathering, and describe how they interact.
4. Explain how climate and rock type affect weathering rates.
5. List the factors influencing soil formation characteristics
6. Describe and illustrate major characteristics of common soil horizons.
7. Describe the characteristics of the three primary types of soil, and explain
important factors in the formation of each type.
8. Explain the importance of soil for human societies, and describe how human
activities affect soils.
Earth’s surface processes
Weathering – Physical breakdown and
chemical alteration of rock at Earth’s
surface
Mass wasting – transfer of rock and soil
downslope by gravity
Earth’s surface processes
Erosion – the physical removal of
material by mobile agents like water,
wind, ice, or gravity
Weathering
Two types of weathering
• Mechanical weathering – breaking of
rocks into smaller pieces
• Four types of mechanical weathering
– Frost wedging – freezing and thawing of
water in cracks disintegrates rocks
Frost Wedging Movie
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Rockfall along I-70
Weathering
• Mechanical Weathering continued
– Unloading – exfoliation of igneous rocks at
Earth’s surface due to reduction in pressure
– Like peeling layers off an onion
– Thermal expansion – alternate expansion and
contraction due to heating and cooling
– Biological activity – disintegration resulting
from plants and animals
Exfoliation
of granite
Thermal Expansion and Salt Weathering
Weathering
Chemical Weathering
• Breaks down rock and minerals
• Important agent in chemical weathering is
water (transports ions and molecules
involved in chemical reactions)
Chemical Weathering of Boulder (core stone)
Weathering
Major processes of chemical weathering
• Dissolution
– Aided by acid in water (occurs in limestone)
– Soluble ions contained in underground water
• Oxidation
– Chemical reaction where compound loses electrons
– Important in breaking down mafic minerals
– Almost like the rocks are rusting
• Hydrolysis
– Reaction of any substance with water
– Hydrogen ion replaces other positive ions
Why do rocks such as granite appear so fresh (as they originally formed) in
recently excavated quarries
A) Radioactive elements deep in the Earth cause them to melt and reform
B) Weathering occurs mostly at the surface of the Earth, where water and
atmospheric gasses exist (the weathered rocks have been removed in the
quarry to access the fresh rock)
C) Workers in the quarry purposely scrub the rocks so they appear fresh
D) Electrical charges from lightning strikes cause the weathering products
to vaporize and disappear
Why do rocks such as granite appear so fresh (as they originally formed) in
recently excavated quarries
A) Radioactive elements deep in the Earth cause them to melt and reform
B) Weathering occurs mostly at the surface of the Earth, where water and
atmospheric gasses exist (the weathered rocks have been removed in the
quarry to access the fresh rock)
C) Workers in the quarry purposely scrub the rocks so they appear fresh
D) Electrical charges from lightning strikes cause the weathering products
to vaporize and disappear
Weathering
Rates of weathering
• Mechanical weathering aids chemical
weathering by increasing surface area
Others factors affecting weathering
• Rock characteristics
– Marble and limestone easily dissolve in weak
acidic solutions
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Increase in surface area by
mechanical weathering
Calculate the amount the surface area increases
as grain size is cut in half
A) Does not change
B) Is half as large
C) Is twice as large
D) Is four times as large
E) None of the above
Increase in surface area by
mechanical weathering
Calculate the amount the surface area increases
as grain size is cut in half
A) Does not change
B) Is half as large
C) Is twice as large
D) Is four times as large
E) None of the above
Limestone weathers faster than granite
Granite Grave Marker 1888
Limestone Grave Marker 1885
Clicker Question
Limestone in humid regions often forms valleys, while in arid
regions it more commonly forms steep hills and ridges. Why is this
so?
1.
2.
3.
4.
Other kinds of rock weather very quickly under desert conditions
Frost shattering occurs more frequently under humid conditions
Limestone dissolves much more readily in humid conditions
Desert plants shield limestone from the erosive power of rain drops
Clicker Question
Limestone in humid regions often forms valleys, while in arid
regions it more commonly forms steep hills and ridges. Why is this
so?
1.
2.
3.
4.
Other kinds of rock weather very quickly under desert conditions
Frost shattering occurs more frequently under humid conditions
Limestone dissolves much more readily in humid conditions
Desert plants shield limestone from the erosive power of rain drops
Weathering
Others factors affecting weathering
• Rock characteristics continued
– Silicate minerals weather more rapidly in the
same order as their order of crystallization
(Bowens rxn)
• Climate
– Temperature and moisture most crucial factors
– Chemical weathering most effective in warm,
moist climates
Differential weathering
controlled by jointing patterns
Joint-controlled weathering
in igneous rocks
Joints in sandstone - Canyonlands and Arches Natl Parks, Utah
Why do we care about soils?
A)
B)
C)
D)
Food Production
Air Quality
Water Quality
Weathering products
Soil
(otherwise known as dirt)
Soil - combination of mineral and organic
mater, water, and air
• Portion of regolith (weathered rock and
mineral) that supports growth of plants
Components in soil
that supports plant growth
Soil
Factors controlling soil formation
• Parent material
– parent material is the underlying bedrock composition affects soil types
Soil
Factors controlling soil formation
• Time
– Soils get better developed with more time
• Climate
– Biggest control on soil formation
– Key factors are temperature and
precipitation
Soil
Factors controlling soil formation
• Plants and animals
– Organisms influence soil properties
– Also furnish organic matter to the soil
(especially plants)
• Slope
– Steep slopes have poorly developed soils (due to
faster erosion and downslope transport
– Flatter terrain accumulates soil faster
Variations in soil development
due to topography
Note location of agriculture
In the Rocky Mountain region of the United States, northfacing slopes (downhill direction is toward the north) are
typically more moist and heavily forested than south-facing
slopes. Why is this?
1. North-facing slopes receive more sunlight in the summer;
snow melts faster and more soil moisture is available for the
trees.
2. South-facing slopes receive more moisture and sunlight; rock
weathering is slower.
3. North-facing slopes receive about the same amount of
precipitation as south-facing slopes; less moisture evaporates
from north-facing slopes.
4. South-facing slopes receive less moisture, yet rock
weathering is faster
In the Rocky Mountain region of the United States, northfacing slopes (downhill direction is toward the north) are
typically more moist and heavily forested than south-facing
slopes. Why?
1. North-facing slopes receive more sunlight in the summer;
snow melts faster and more soil moisture is available for the
trees.
2. South-facing slopes receive more moisture and sunlight; rock
weathering is slower.
3. North-facing slopes receive about the same amount of
precipitation as south-facing slopes; less moisture evaporates
from north-facing slopes.
4. South-facing slopes receive less moisture, yet rock
weathering is faster
Given the
earlier
slides
on where
form best
in form
regions
of slow
Given
the
earlier
slidessoils
on where
soils
best
in regions of
erosion, slow
or deposition
wheresediments,
should a
erosion,oforfine-grained
depositionsediments,
of fine-grained
where
wheat farmer
plant
his crops
in Colorado
near
Boulder?
should
a wheat
farmer
plant his
crops
in Colorado near
Boulder?
1. In topographically high regions, such as along the Peak to
Peak Highway between Nederland and Estes Park
2. In river valleys, such as along the Boulder Creek path on the
road between Boulder and Nederland
3. In low rolling hills near Lafayette and Louisville where
Cretaceous strata are exposed
4. Along floodplains, such as along Coal Creek between
A. In
high regions, such as
along
the Peak to Peak Hwy between
Louisville/Lafayette
and
Broomfield
Nederland and Estes Park
B. In river valleys, such as along Boulder Creek on the road between
Boulder and Nederland
C. In low rolling hills near Lafayette and Louisville where shaley strata
are exposed
D. Along modern floodplains, such as along Coal Creek between
Louisville/Lafayette and Broomfield
Given the
earlier
slides
on where
form best
in form
regions
of slow
Given
the
earlier
slidessoils
on where
soils
best
in regions of
erosion, slow
or deposition
wheresediments,
should a
erosion,oforfine-grained
depositionsediments,
of fine-grained
where
wheat farmer
plant
his crops
in Colorado
near
Boulder?
should
a wheat
farmer
plant his
crops
in Colorado near
Boulder?
1. In topographically high regions, such as along the Peak to
Peak Highway between Nederland and Estes Park
2. In river valleys, such as along the Boulder Creek path on the
road between Boulder and Nederland
3. In low rolling hills near Lafayette and Louisville where
Cretaceous strata are exposed
4. Along floodplains, such as along Coal Creek between
A. In
high regions, such as
along
the Peak to Peak Hwy between
Louisville/Lafayette
and
Broomfield
Nederland and Estes Park
B. In river valleys, such as along the Boulder Creek path on the road
between Boulder and Nederland
C. In low rolling hills near Lafayette and Louisville where shaley strata
are exposed
D. Along modern floodplains, such as along Coal Creek between
Louisville/Lafayette and Broomfield
Soil Profile
The soil profile
• Soil forming processes operate from the
surface downward
• Vertical differences are called horizons –
zones or layers of soil
Soil
The soil profile
• O horizon – organic matter
• A horizon – organic and mineral matter
– High biological activity (critters live here)
– Together the O and A horizons make up topsoil
• E horizon – little organic matter
– Zone of leaching
• B horizon – zone of accumulation
• C horizon – partly altered parent material
An idealized
soil profile
A soil profile showing
different horizons
Soil types
• The characteristics of each soil type primarily depend on
the prevailing climatic conditions
Three very generic soil types
• Pedalfer
– Accumulation of iron oxides and Al-rich clays in the B
horizon
– Best developed under forest landscapes
• Pedocal
– High accumulations of calcium carbonate
– Associated with dry grasslands and brush vegetation
• Laterite
– Hot and wet tropical climates
– Intense chemical weathering
pedocal
pedalfer
laterite
Deserts
Dry and Arid
Forests
Temperate
Tropics
Hot and Wet
What sort of soil do you think forms in the
mountains and plains along the Front Range?
A. pedocal
B. pedalfer
D. Depends on where you are looking…
C. laterite
What sort of soil do you think forms in the
mountains and plains along the Front Range?
A. pedocal
B. pedalfer
D. Depends on where you are looking…
C. laterite
Why does deforestation in tropical
rainforests result in such barren-looking
and unproductive landscapes?
A) High rainfall erodes soil away
B) Cycling of organic material in tropics is rapid
C) Levels of organic material in tropics is low
D) All of the above
Soil
Soil erosion
• Recycling of Earth materials
• Natural rates of soil erosion depend on
– Soil characteristics
– Climate
– Slope
– Type of vegetation
Soil
Soil erosion
• In many regions the rate of
soil erosion is significantly
greater than the rate of soil
formation
• Farmers now level fields
with lasers to slow loss of
topsoil
06.04 In terrain with steep hill slopes, which crop and cultivation
technique will minimize soil erosion?
1. Corn; rows trending straight down the slope, frequent
cultivation.
2. Apples; land between the trees is planted in grass and not
cultivated.
3. Winter wheat; after the harvest, the field is plowed and left
idle until next fall.
4. Beans; rows are spaced wider than on a level field.
06.04 In terrain with steep hill slopes, which crop and cultivation
technique will minimize soil erosion?
1. Corn; rows trending straight down the slope, frequent
cultivation.
2. Apples; land between the trees is planted in grass and not
cultivated.
3. Winter wheat; after the harvest, the field is plowed and left
idle until next fall.
4. Beans; rows are spaced wider than on a level field.
End of Chapter 5