28/09/2010
Today’s goal
Determine why weathering and erosion are
important
Learn about mechanical and chemical weathering
Discuss erosional processes
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
Weathering
The beginning of sedimentary rocks
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Weathering
Weathering – the physical breakdown
(disintegration) and chemical alteration
(decomposition) of rock at or near Earth’s
surface


Role in engineering?

Important factors:
1.
2.


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Types of Weathering
Mechanical weathering
Mechanical weathering breaking of rocks into
smaller pieces by
physical forces
 Chemical weathering –
involves chemical
transformation of rock
into one or more new
compounds


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Frost wedging
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Mechanical Weathering
Salt weathering: crystallization of salt within a rock
that weakens the rock similar to frost wedging
Salts include:


Mechanical Weathering
Deteriorate rocks through:

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Thermal conductivity

Coefficient of thermal expansion

Range in fluctuation
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Mechanical Weathering
Moisture changes: alternating wet and dry

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Pressure and expansion
Enhanced expansion
Hydration
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

Mechanical Weathering

Unloading
Expansion and contraction
Especially prevalent with temperature changes

exfoliation as sheets of mainly igneous rocks
at the Earth’s surface, due to a reduction in
pressure (overlying rocks eroded)
Differential weathering

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Mechanical Weathering

Temperature changes

Biologic activity
 disintegration resulting
from plants (roots),
burrowing animals, and
humans
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Chemical Weathering

Aided by mechanical weathering
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Chemical Weathering


Breaks down rock components and internal
structures of minerals
Most important agents involved in chemical
weathering
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1.
2.
Chemical Weathering

What is the order from least to most soluble?
 Quartz
 Evaporite minerals
 Carbonate minerals
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Chemical Weathering


NaCl + H20  Na+ +
Dissolution
 Aided by small amounts of acid in the
water
Cl- + H2O
Chemical Weathering

Hydrolysis
 The reaction of any substance with water

2 KAlSi3O + H2CO3 + H2O
CaCO3 + H2O + CO2  Ca2+ + 2HCO32-
 K2CO3 + Al2Si2O5 (OH)4 + 4 SiO2

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Produces ________ minerals
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Chemical Weathering
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
Hydration:

CaSO4 + 2H2O ->
> CaSO4 2H2O

Anhydrite
.
Gypsum
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Chemical Weathering
Chemical Weathering

Oxidation
 Any chemical reaction in which an element
loses electrons
 Important in decomposing ferromagnesian
minerals

Fe2+ + H2O  FeO (OH) limonite (yellow)

4Fe3+ + 3O2  2(Fe2O3)
hematite (red)
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Rates of Weathering


Rocks containing silicate minerals (granite) are
__________to chemical weathering
Rocks containing calcite (marble and limestone)
__________in weakly acidic solutions
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Weathering

Silicate minerals weather what order?
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Weathering and Engineering
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Two most important factors:
1.
2.
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Weathering and Engineering

Which properties will increase, which will decrease in
a rock due to weathering?

Density
Porosityy ((open
p spaces
p
in the rock))
Strength
Elasticity
Moisture content
Density
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Erosion

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The physical removal of material by mobile agents
such as gravity, water, wind, or ice
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Erosion

wind
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Summary
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Mechanical weathering is a physical breakdown into
smaller particles
Chemical weathering involves a chemical reaction to
form new minerals/solutions
Erosion is the process by which weathered material
is transported
The action of weathering and erosion leads to
formation of sediments
Questions?
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