Weathering g110

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Weatheringg110

• Lecture prepared by Bob Hall
• Revised 1/31/2001

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Topics to be Covered in Weathering Part I

• What is Weathering?
• A Consideration of Exfoliation
• Forces in Rocks
• Frost Action
• Further Consideration What is Weathering?
• Weathering vs. Erosion
• Types of Weathering
• Role of Climate Further Considered
• Grussification

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A Consideration of Exfoliation

• What are its features?
• What are scales?
• What are the suggested processes?
• Is it the result or the cause of weathering?

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Exfoliation Spalling Sheeting

• Breaking Off Of External Surfaces Of Rocks As
  Slabs Or Sheets.

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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 -1

• Deeply-buried RocksHigh Confining Pressure,
  Minerals Densely Packed
• Igneous (Plutonic), Metamorphic Rocks Form There.
• Sedimentary Rocks Can Be Placed There, With
  Burial.

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Pressure-release As The Cause Of Exfoliation - 2

• 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 Accommodated Without
  Creating New Joints.

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Reconsideration of Exfoliation

• What are its features?
• What are scales?
• What are the suggested processes?
• Is it the result or the cause of weathering?
• Time to define weathering?
• Not yet, lets consider forces in rocks.

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Forces in Rocks

• Forces due to plate tectonics.
• Forces due to extension after uplift.
• Forces set up in weathering environment.
• Frost action
• Precipitation of salts and other minerals.
• Root growth
• Chemical reactions

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Frost Action 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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Frost Action (cont.)

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

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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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• Frost Action Conclusion?
• 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.

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• Frost Action Conclusion?
• Listed strengths do not have to be overcome.
• The rock will shatter.
• Frost action also known as freezing-expansion.

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Example Frost Action
Talus
Fig. 5.3
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What is Weathering?

• Is it?
• Fracturing due to tectonic uplift?
• Exfoliation?
• Frost action?
• Chemical precipitation in rocks?
• Thermal expansion?
• Growth of vegetation in cracks?
• Wind abrasion?

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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
• Diurnal Or Seasonal Heating Effectiveness?
• Theory Says Yes.
• Experiments Unconvincing.
• What Factor Cannot Be Controlled?

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What is Weathering? (cont.)

• Webster
• The physical and chemical effects of the forces
  of weather on rock surfaces, as in forming soil,
  sand, etc.

• Tarbuck and Lutgens
• The physical breakdown and chemical alteration of
  rocks at or near the Earths surface.

Point of difference Must weather play a
role? Webster yes Textbook - no
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What is erosion?

• Webster
• Eating into or wearing away gradually
  disintegration deterioration or decay vanishing
• Examples given are a sream eroding a gully and
  acid eroding metal.

• Tarbuck and Lutgens
• Physical removal of material by agents such as
  water, wind, or ice.

Point of difference Must material be removed
from an area? Webster yes and no Textbook -
definitely
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Weathering VS Erosion

• Weathering In Situ Decomposition And
  Disintegration Of Earth Materials.
• Erosion Removal Of Earth Materials From Original
  Location.
• Weathering Usually Simultaneous With Or Prior To
  Erosion.

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Types of Weathering - 1

• 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 - 2

• 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 - 3

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

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Type of Weathering and Climate Role of Chemical
Weathering
What would a similar diagram look
like for physical weath.?
Weak
Moderate
Strong
Mean annual rainfall
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Spheroidal Weathering
Figs. 5.2, 5.12
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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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Grus
Grus
Photos by Miller
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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.