WEATHERING VS. EROSION

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WEATHERING VS. EROSION

• Involve Shear Stress.
• Opposed By Shear Resistance.
• Weathering In Situ Decomposition And
  Disintegration Of Earth Materials.
• Erosion Removal Of Earth Materials From Original
  Location.
• Decrease In Size And Mass.
• Weathering Usually Simultaneous With Or Prior To
  Erosion.

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EXAMPLE OF WEATHERING WITHOUT EROSION
Weathering On Broad, Stable Hilltop With Little
Or No Erosion.
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EXAMPLE EROSION WITHOUT WEATHERING
Erosion In Deep, High-gradient Canyon With No
Weathering.
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TYPES OF WEATHERING

• Physical Weathering Mechanical Weathering
  Disintegration
• Change Of Rocks To Unconsolidated Debris
• Or Loosening Of Unconsolidated Materials Caused
  By Forces Set Up By Physical Processes
• (No Major Chemical Change)

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TYPES OF WEATHERING

• Chemical Weathering Decomposition
• Change In Chemical Composition Of Earth Materials
  Caused By Atmospheric Agents, Mainly Water And
  Gases
• (Chemical Reactions Occur)

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TYPES OF WEATHERING

• Usually Simultaneous Phenomena
• Dominance Controlled By Climate (4.16/97)
• Physical Weathering Increases Surface Area,
  Making Chemical Weathering More Likely To Occur.

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TYPE OF WEATHERING AND CLIMATE
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TYPE OF WEATHERING AND CLIMATE
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PROCESSES OF DISINTEGRATIONRELATIONSHIP TO
MATERIALS
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FREEZING OF WATER IN A DEPRESSION IN ROCK

• Ice - A 9 Increase In Volume Upon Freezing
• Will Force Be Transferred To Surrounding Rock?

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FREEZING OF WATER IN A DEPRESSION IN ROCK

• But, Freezing Occurs At Surface First.
• Must Overcome Tensile Strength Of Rock First.

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TENSILE STRENGTH OF ROCK

• MATERIAL
• GRANITE
• MARBLE
• LIMESTONE
• SANDSTONE
• ICE

• TENSILE STRENGTH (PSI)
• 700 to 1000
• 700 to 1200
• 500 to 800
• 100 to 400
• 200

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• So, Would The Rocks Disintegrate?
• No, Because Ice Is Usually Weaker Than The Rock.
• Rock Disintegration Therefore Requires
  Confinement Along Pre-existing Weaknesses Joints,
  Bedding Planes, Rock Cleavage, Etc.
• Listed Tensile Strengths Do Not Have To Be
  Overcome.
• Rock Disintegration By This Mechanism Usually
  Known As Frost Action Or Just Freezing-expansion

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FROST HEAVE A CRYSTAL-GROWTH AND WEDGING
PHENOMENON
SOIL
ICE
SOIL
ICE
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• Water Is Drawn To Growing Ice Mass Through
  Capillary Openings.
• Favored In Fine-grained Material. Why?
• Larger-scale Effects Seen In Permafrost
  Regions.Pingo Large Ice Mass Causing Heaving Of
  Overlying Materials (Relief 3 To 30
  M)Patterned Ground Polygonal Ground
  Patterns (May Be Sorted.) (More Later In
  Course.)

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SALT WEATHERING MECHANISMS

• Crystal Growth (See 4.15/96.)
• May Occur In Discrete Pieces Or Grain-By-grain.
• Water Uptake And Expansion, Then Loss And
  Contraction.
• Effectiveness?

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SPALLING BY CRYSTAL GROWTH OF SALTS
Spalling Rock
Ground Water Flow
Gypsum Crystals
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CLAYS AND IRON OXIDESEXPANSION - CONTRACTION

• Expansion Of Clays Varies With The Clay Mineral.
  (See T4.1/88)
• Could Break Up Rocks With Clay Matrix.
• Breaks Up Soil Aggregates, Or Peds, To Smaller
  Sizes.
• May Result In Broken Chemical Bonds
• Would Leave Unsatisfied Electrical Charges Sites
  For Chemical Weathering (Attract Ions)
• Iron Oxides Potential For Crystal Growth, Water
  Expansion Largely Unknown (Unevaluated)

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Exfoliation Spalling Sheeting

• Breaking Off Of External Surfaces Of Rocks As
  Slabs Or Sheets.
• Continuum Exfoliation Granular
  Disintegration (4.2/87 4.4/88, 4.10/92) (4.7/91)

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PRESSURE-RELEASE MECHANISM OF EXFOLIATION
Expansion
At least Several hundred feet
Joints
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Pressure-release As The Cause Of Exfoliation

• Deeply-buried RocksHigh Confining Pressurem,
  Minerals Densely Packed
• Igneous (Plutonic), Metamorphic Rocks Form There.
• Sedimentary Rocks Can Be Placed There, With
  Burial.
• Bringing Such Rocks Near The SurfaceMinerals
  Move Apart.
• Rock Expands In Direction Of Least Confining
  Pressure

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EXFOLIATION JOINTS

• Joint Spacing Increases With Depth
• Rough, Lenticular Jointing Especially Near The
  Land Surface.
• Joints Often Curve, Following Surface Topography.
• Most Effective In Formerly Massive Rocks.
• If Rock Is Bedded Or Has Previous Jointing,
  Stresses Of Expansion Are Accomodated Without
  Creating New Joints.

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CHEMICAL CHANGE AS A CAUSE OF EXFOLIATION

• Secondary Minerals Are Precipitated, Exert
  Outward Force (Occupies More Volume)

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THERMAL CHANGE AS A CAUSE OF EXFOLIATION

• Expansion With Heating, Contraction With Cooling.
• Effective With High Enough Temperature And Rapid
  Changes In Temperature. (4.2/87)
• Diurnal Or Seasonal Heating Effectiveness?
• Theory Says Yes.
• Experiments Unconvincing.
• What Factor Cannot Be Controlled?

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GRANULAR DISINTEGRATION

• Possible Causes
• Expansion Of Certain Minerals.
• Solution Of Soluble Cement (E.G. Terrigenous
  Rocks)
• Crystal Growth And/or Water Expansion.

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GEOMORPHIC FEATURES OF PHYSICAL WEATHERING

• Boulder Fields (4.14/95)
• Exfoliation Domes (4.10/92)
• Spheroidal Boulders (4.8d/91)
• Weathering Pits
• May Be Partly Biochemical Organic
• Acids By Lichens, Moss, Etc.