Title: Weathering g110
1Weatheringg110
- Lecture prepared by Bob Hall
- Revised 1/31/2001
2Topics 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
3A Consideration of Exfoliation
- What are its features?
- What are scales?
- What are the suggested processes?
- Is it the result or the cause of weathering?
4Exfoliation Spalling Sheeting
- Breaking Off Of External Surfaces Of Rocks As
Slabs Or Sheets.
5Pressure-Release Mechanism of Exfoliation
Expansion
At least Several hundred feet
Joints
6Pressure-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.
7Pressure-release As The Cause Of Exfoliation - 2
- Bringing Such Rocks Near The SurfaceMinerals
Move Apart. - Rock Expands In Direction Of Least Confining
Pressure
8Exfoliation 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.
9Reconsideration 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.
10Forces 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
11Frost Action Freezing of water in a Depression
in Rock
- Ice - A 9 Increase In Volume Upon Freezing
- Will Force Be Transferred To Surrounding Rock?
12Frost Action (cont.)
- But, Freezing Occurs At Surface First.
- Must Overcome Tensile Strength Of Rock First.
13Strength of Rock
- MATERIAL
- GRANITE
- MARBLE
- LIMESTONE
- SANDSTONE
- ICE
- TENSILE STRENGTH (PSI)
- 700 to 1000
- 700 to 1200
- 500 to 800
- 100 to 400
- 200
14- 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.
15- Frost Action Conclusion?
- Listed strengths do not have to be overcome.
- The rock will shatter.
- Frost action also known as freezing-expansion.
16Example Frost Action
Talus
Fig. 5.3
17What 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?
18Chemical Change as a Cause of Exfoliation
- Secondary Minerals Are Precipitated, Exert
Outward Force (Occupies More Volume)
19Thermal 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?
20What 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
21What 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
22Weathering 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.
23Types 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)
24Types of Weathering - 2
- Chemical Weathering Decomposition
- Change In Chemical Composition Of Earth Materials
Caused By Atmospheric Agents, Mainly Water And
Gases - (Chemical Reactions Occur)
25Types of Weathering - 3
- Usually Simultaneous Phenomena
- Dominance Controlled By Climate
-
- Physical Weathering Increases Surface Area,
Making Chemical Weathering More Likely To Occur.
26Type of Weathering and Climate Role of Chemical
Weathering
What would a similar diagram look
like for physical weath.?
Weak
Moderate
Strong
Mean annual rainfall
27Spheroidal Weathering
Figs. 5.2, 5.12
28Granular Disintegration
- Possible Causes
- Expansion Of Certain Minerals.
- Solution Of Soluble Cement (E.G. Terrigenous
Rocks) - Crystal Growth And/or Water Expansion.
29Grus
Grus
Photos by Miller
30Geomorphic 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.