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

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 CO 2 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