Lecture Outlines
Physical Geology, 12/e
Plummer & Carlson
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Weathering and Soil
Physical Geology 12/e, Chapter 5
1) Weathering, Erosion, and
Transportation
• Rocks exposed at Earth’s surface are constantly changed
by water, air, temperature variations and other factors
• Weathering is the group of destructive processes that
change physical and chemical character of rocks at or
near Earth’s surface
• Erosion is physical picking up of rock particles by water,
ice, or wind
• Transportation is the movement of eroded particles by
water, ice, or wind
2) Weathering and Earth
Systems
• Atmosphere
– Oxygen and carbon dioxide critical to chemical weathering
– Water cycled through atmosphere is critical to chemical and
mechanical weathering processes
– Chemical weathering also removes carbon dioxide from the
atmosphere, helping keep global temperatures from soaring
2) Weathering and Earth
Systems
• Hydrosphere
– Water is necessary for chemical weathering
– Oxygen dissolved in water oxidizes iron in rocks
– Carbon dioxide dissolved in water creates carbonic acid
• Primary cause of chemical weathering
– Running water loosens and abrades particles
– Glacial ice removes and abrades particles
– Freeze/thaw cycling mechanically weathers
• Biosphere
–
–
–
–
Plant root growth widens cracks
Animal movement and human activity mechanically weather
Decaying organic matter in soils produces acidic soil moisture
Humans are AWESOME AGENTS of erosion because____________
3) Types of Weathering
• Mechanical weathering
– Physical disintegration
– Frost action, pressure-release fracturing, plant
growth, burrowing animals, salt wedging,
thermal cycling
• Chemical weathering
– Decomposition of rock from exposure to
atmospheric gases (oxygen, water vapor and
carbon dioxide)
– New chemical compounds (minerals) form
– Rate increased by increased rock surface area
Spheroidal weathering
4) Mechanical Weathering
• Frost action
– Mechanic effect of freezing (and
expanding) water on rocks
• Pressure release
– Removal of overlying rock allows
expansion and fracturing
• Plant growth
– Growing roots widen fractures
• Burrowing animals
• Extreme Temperatures
– Large temperature changes fracture rocks
by repeated expansion and contraction
5) Chemical Weathering
• Oxidation
– Chemically active oxygen from atmosphere
– Iron oxides are common result
• Soil and sedimentary rocks often stained
with iron oxides
• Acids
– Hydrogen cations replace others in minerals
– Carbonic acid from atmospheric CO2
dissolved in water
– Sulfuric, hydrofluoric acids emitted by
volcanic eruptions
– Some minerals, such as calcite, may be
totally dissolved
– Human activity, such as mining and
burning of fossil fuels, produces acids
5) Chemical Weathering
• Feldspars
– Most common minerals in crust
– Slightly acidic rain water
attacks feldspar
– Clay minerals produced
• K+, Na+, Ca++ ions released into water
• Other minerals
– Ferromagnesian minerals
• Clays, iron oxides, Mg++ ions produced
– More complex silicate bonds lead to lower weathering susceptibility
• Olivine most susceptible, quartz least
• Warm, wet climatic conditions maximize weathering
5) Chemical Weathering
• Factors Affecting Weathering:
– Moisture
– Temperature
– Climate
– Slope
6) Inorganic Carbon Cycle
• Follow the carbon!
– Present as trace gas carbon
dioxide in atmosphere
– Combines with water to
form carbonic acid
– Weathers rocks and leads to
limestone formation in
bodies of water
– Returned to mantle by
tectonic plate movement
– Released back to atmosphere
by volcanic eruptions
On a white board…
• List the types of mechanical weathering.
• List the types of chemical weathering
7) Soil
• Soil - a layer of weathered, unconsolidated
material on top of bedrock
• Average Soil:
– 45% rock
– 5% humus
– 50% pore space
– Common soil constituents:
•
•
•
•
•
Clay minerals
Quartz
Water
Pore Space
Organic matter
8) Soil Horizons
• Soil horizons
– O horizon - uppermost layer; organic material
– A horizon – dark-colored, rich in organic matter and
high in biological activity
– E horizon - zone of leaching; fine-grained
components removed by percolating water
– B horizon - zone of accumulation; clays and iron
oxides leached down from above; formation of hard
pan in wet climates
– C horizon - partially weathered bedrock
9) Factors Affecting Soil
Formation
•
•
•
•
•
Parent Material =
Slope =
Living Organisms =
Climate =
Time =
9) Factors Affecting Soil
Formation
• Parent Material = what the soil develops
from, original rock material
• Slope = thin soils on steep slopes,
bottomlands tend to have thick and
waterlogged
• Living Organisms = forms humus, supplies
nutrients
• Climate = most influential factor (temp and
precipitation)
• Time = thicker soils from more time
9) Factors Affecting Soil
Formation
• Residual soil - weathering of underlying rock
• Transported soil - brought in from elsewhere
– Floodplain deposits, etc.
– Wind-transported soil is called loess
• Soil composition
– Determined by parent rock composition
– Evolves with time and chemical weathering
• Soil thickness
– Increases with time
– Typically greater in wetter climates
– Greater in areas with low slopes
10) Soil Erosion
How
Rates
Consequences
10) Soil Erosion
How = water and wind
Rates = influenced by soil characteristics, climate,
slope, and vegetation
*type of rain (gentle vs. downpour)
*vegetation (none vs a lot)
Consequences = buildup of sedimentation in water
areas
= dust storms
= erosion at a fast rate
Analysis
Why is mechanical and chemical
weathering important for soil
development?
11) Soils and Climate
• Soil thickness and composition are
greatly affected by climate
– Temperate climates:
• More chemical weathering and thicker soils
• Great for farming
– Arid climates:
• Less chemical weathering and thinner soils
• Subsurface evaporation leads to build-up of salts
• Calcite-rich accumulation zones may form,
cementing soil together into a hardpan
– Extremely wet climates (e.g., tropical rainforest)
• Highly leached and unproductive soils (laterites)
• Most nutrients come from thick O/A horizons
Soil Classification
• World Soil Orders:
• The Twelve Orders of Soil Taxonomy
– Alfisols: Gray to brown surface horizon, common in humid forests
– Andisols: Soils formed in volcanic ash
– Aridisols: Soils formed in dry climates (low organic matter)
– Entisols: Young soils that have no horizons
– Gelisols: Weakly weathered soils with permafrost within 2 meters of
the surface
– Histosols: Wet, organic soils with little mineral material
– Inceptisols: Very young soils with weakly developed soil horizons
– Mollisols: Nearly black surface horizon rich in organic matter
– Oxisols: Heavily weathered soils (also called laterites)
– Spodosols: Acid soils low in plant nutrient ions
– Ultisols: Strongly weathered soils low in plant nutrient ions and clays
– Vertisols: Clayey soils that swell when wet and shrink whey dry
End of Chapter 5