1. Lecture Outlines Physical Geology, 12/e
Plummer & Carlson
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Weathering and Soil Physical Geology 12/e, Chapter 5

3. 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

4. 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

5. 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____________

6. 3) Types of Weathering Mechanical weathering Chemical 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

7. Analyze Types of Weathering Geo Textbook pages 115 - 123
Weathering Effect
(What we see…)
Cause/ What Happens?
Type of Weathering (Chemical/ Mechanical)
Pressure Release
Frost Wedging
Plant Growth
Animal Burrowing
Pedestal Rock
Red Sandstone
Marble vs. Slate Gravestones
Clay Particles

8. 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

9. 5) Chemical Weathering Oxidation Acids
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

10. 5) Chemical Weathering Feldspars Other minerals
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

11. 5) Chemical Weathering Factors Affecting Weathering: Moisture
Temperature
Climate
Slope

12. 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

13. On a white board… List the types of mechanical weathering.
List the types of chemical weathering

14. 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

15. 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

16. 9) Factors Affecting Soil Formation
Parent Material =
Slope =
Living Organisms =
Climate =
Time =

17. 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

18. 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

19. 10) Soil Erosion
How
Rates
Consequences

20. 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

21. Analysis
Why is mechanical and chemical weathering important for soil development? Weathering produces soils… Climate, Slope, Parent Material, Time, Living Organisms

22. 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

23. 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

24. End of Chapter 5