Ch 9. Weathering
A. Mechanical Weathering
B. Chemical Weathering
C. Soil
Weathering
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Mechanical Weathering
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Physical disintegration of rock
(with no chemical alteration)
Chemical Weathering
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Chemical alteration of minerals
within the rock
Usually softening or dissolving
the minerals
Forming clays, oxides and
solutes
Weathering
Quartz
Sediments of: Quartz1
Feldspar1
Clay2
Hematite3
Calcite4
Biotite
Na Plagioclase
Weathering, a geologic process:
1. Separates out mineral grains
2. Converts silicates to clay
3. Oxidizes iron minerals (i.e., ferromagnesians)
4. Dissolves some ions (e.g, calcium)
Erosion Transports Sediments
Weathering and
Plate tectonics

Tectonic forces lift the land up

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Compression and uplift at
convergent boundaries
Isostatic uplift due to crustal
thickening and buoyancy
Stresses fracture rocks
Weathering and Erosion break
the land down

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Mechanical and chemical
breakdown
Erosion by gravity, wind, water,
ice moves material downhill
A. Mechanical Weathering

Rocks that have been
exhumed

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Pressure release forms
sheet joints and exfoliation
Frost wedging propogates
fractures and breaks rocks
into smaller fragments
Crushing and Abrasion
break rocks down further
in tallus slopes, streams
and beneath glaciers
Mechanical
Weathering

An exfoliation
dome in
Yosemite, CA
(Half Dome)
Talus Slopes

The result of

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Mechanical
weathering
Rock falls and slides
Crushing and
abrasion (more
mechanical
weathering)
Rock Avalanches

Slopes of rock
fragments may let go
and careen downhill
as a very fast flow
Sediments from
Mechanical
Weathering

Sediments of:

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Parent rock
Mineral particles
Angular fragments
(Near source)
Weathering

Mechanical Weathering

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Frost Action
Pressure Release
Crushing and Abrasion
Chemical Weathering

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Dissolving  Dissolved ions
Oxidation  Iron in Ferromag. Minerals  Iron
Oxides (e.g., Hematite)
Formation of Clays from silicates (e.g., Feldspar)
Chemical Weathering (Table 9.1)
Hydrolysis
Oxidation: 4FeSiO3 + O2 + H2O  FeO(OH) + 4SiO2
Hydration: CaSO4 + 2H2O  CaSO4·2H2O
Soil Formation
and Weathering
Related to
Climate
Weathering
of Granite
Quartz
Sediments of:
Quartz1
Feldspar1
Clay2
Hematite3
Ions4
1. Mechanical Weathering
forms rock and mineral
fragments of parent rock
Biotite
Chemical Weathering
2. Converts silicates* to clay
3. Oxidizes iron in minerals
Na Feldspar
(i.e., ferromagnesian minerals)
4. Dissolves some ions
(e.g, Ca, Na, K, Mg)
*except quartz


Stable at High
Temperatures
Quickly Weathered
Mineral
Isolated
Silicate
Structure
Olivine
Single Chain
Structure
Pyroxene
Group
Double Chain
Structure
Amphibole
Group
Mica
Group
Sheet
Structure
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Slowly Weathered
Stable at Atmos.
Temperatures
Framework
Structure
Clay
Group
Feldspar
Group
Quartz
Converted to clay, oxides, and ions
by chemical weathering
Rates of
Weathering
Silicate
Structure
Mech. Weath.
Other factors increasing
Rates of Weathering

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Cool mountainous
regions accelerate
mechanical weath.
Warm moist climates
accelerate chem. weath.
Increased slopes
accelerate weathering

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Gravity caries sediments
away from slopes
Exposing fresh surfaces
Other factors increasing
Rates of Weathering

Joints or fractures
accelerate both mech.
and chem. weath.
Differential Weathering
John Ford Point Monument
Also See Fig. 13.7 of the
Colorado Plateau
Figure 3.8: Ship Rock Arizona
Volcanic Neck
Fig 5.3
Ship Rock, New Mexico
Volcanic Neck
John Ford Point Monument, Arizona.
Weathering forms Soils
Acidic water percolates
downward and
A. Chemically weathers
minerals in soil to form
• Clays
• Iron Oxides
• Dissolved Ions
E. Leach downward and
B. Accumulate.
C. Mechanical weathering
breaks down bedrock
Sediment: Products of Weathering
cm2
Immature
m2
Transport
Distance
History of
Weathering
Products of
Weathering
i.e., types of
sediment
cm2
Mature
Near source
Far from source
Short history of
mechanical
weathering
Long history of
mechanical & chemical
weathering
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Forms rock and
mineral fragments
Angular at first
Rounded with time
and transport

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Converts silicates to clays,
iron oxides and dissolved ions
which are transported to the
ocean by streams
leaving stable minerals,
eg. qtz. (& fld.)