1. Chapter 7: Weathering and Erosion
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Learning Objectives
Weathering—The first step in the rock cycle
Contrast chemical and mechanical weathering and their place in the rock cycle.
Products of weathering
Describe the end products of weathering and the difference between soil horizons and soil profiles.
Erosion and mass wasting
Identify the sources of erosion and mass wasting.
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3. Weathering—The First Step in the Rock Cycle
Figure 7.2 How rocks disintegrate
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4. Weathering—The First Step in the Rock Cycle How Rocks Disintegrate
The chemical and physical breakdown of rock exposed to air, moisture, and living organisms
Regolith
A loose layer of fragments that covers much of Earth’s surface
Soil
The uppermost layer of regolith, which can support rooted plants
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5. Weathering—The First Step in the Rock Cycle
Mechanical weathering
The breakdown of rock into solid fragments by physical processes
Chemical composition of rock NOT altered
Chemical weathering
The decomposition of rocks and minerals by chemical and biochemical reactions
Figure 7.3 From rock to soil
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6. Weathering—The First Step in the Rock Cycle
Joint
A fracture of rock, along which no appreciable movement has occurred
Sheet jointing (exfoliation)
Frost wedging
Root wedging
Abrasion
Wearing down of bedrock by the constant battering of loose particles transported by wind, water or ice
Joint Formation
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7. Weathering—The First Step in the Rock Cycle
Joint Formation
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8. Weathering—The First Step in the Rock Cycle
Figure 7.4b Frost wedging
Figure 7.4a Sheet jointing
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9. Weathering—The First Step in the Rock Cycle
Figure 7.4c Scree slope
Figure 7.4d Root wedging
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10. Weathering—The First Step in the Rock Cycle Chemical Weathering
Dissolution
Separation of materials into ions in solution by a solvent, such as water or acid
Rainwater
Acts as weak solution of carbonic acid
Anthropogenic actions (acid rain)
Figure 7.5 (a) marble, (b) granite
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Chemical Weathering
Figure 7.6 Ion exchange
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12. Chemical Weathering Figure 7.7 Hydrolysis and oxidation
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13. Factors Affecting Weathering
Tectonic setting
Young, rising mountains weather quicker
Mechanical weathering most common
Figure 7.8 Tectonic setting
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14. Factors Affecting Weathering
Rock structure
Distribution of joints influence rate of weathering
Relatively close joints weather faster
Figure 7.8 Rock structure
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15. Factors Affecting Weathering
Topography
Weathering occurs faster on steeper slopes
Rockslides
Figure 7.8 Topography
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16. Factors Affecting Weathering
Biologic activity
Presence of bacteria can increase breakdown of rock
Figure 7.8 Biological activity
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17. Factors Affecting Weathering
Rock composition
Minerals weather at different rates:
Calcite weathers quickly through dissolution.
Quartz is very resistant to chemical and mechanical weathering.
Figure 7.8 Composition
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18. Factors Affecting Weathering
Vegetation
Contributes to mechanical and chemical weathering.
Promotes weathering due to increased water retention.
Vegetation removal increases soil loss.
Figure 7.8 Vegetation
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19. Factors Affecting Weathering
Climate
Chemical weathering is more prevalent in warm, wet tropical climates.
Mechanical weathering is less important.
Mechanical weathering is more prevalent in cold, dry regions.
Chemical weathering occurs slowly.
Figure 7.9a Climate and weathering
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20. Factors Affecting Weathering
Figure 7.9 Color corresponds to climates in Figure 7.9b
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21. Products of Weathering
Clay
Tiny mineral particles of any kind that have physical properties like those of the clay minerals
A family of hydrous alumino-silicate minerals
Figure 7.10 Kaolin: from (a) clay to (b) fine china
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22. Products of Weathering
Sand
A sediment made of relatively coarse mineral grains
Soil
Mixture of minerals with different grain sizes, along with some materials of biologic origin
Humus
Partially decayed organic matter in soil
Figure 7.11a Lunar regolith
Figure 7.11b Earth soil
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23. Products of Weathering Soil Profiles
Soil horizons
One of a succession of zones or layers within a soil profile.
Each horizon has a distinct physical, chemical, and biologic characteristic.
Soil profiles
The sequence of soil horizons from the surface down to the underlying bedrock.
Soil profiles vary, influenced by factors such as climate, topography, and rock type.
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24. Products of Weathering
Figure 7.12 Soil profile containing soil horizons (zones or layers)
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25. Products of Weathering
Figure 7.13 Climate and soils
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26. Products of Weathering
Figure 7.13 Comparison of soil from Kansas (l) and Connecticut (r)
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27. Erosion and Mass Wasting Erosion by Water
The wearing away of bedrock and transport of loosened particles by a fluid, such as water
Bed load
Sediment moved along the bottom of a stream
Saltation
A transportation mechanism in which particles move forward in a series of short jumps along arc-shaped paths
Suspended load
Sediments carried in suspension by a flowing stream of water or wind
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28. Erosion and Mass Wasting
Figure 7.14a Stream’s load
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29. Erosion and Mass Wasting Erosion by Wind
Saltation is primary mechanism for wind erosion.
Figure 7.15 Massive dust storm
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30. Erosion and Mass Wasting Erosion by Ice
Glacier
A semi-permanent or perennially frozen body of ice, consisting of recrystallized snow, that moves under the pull of gravity
Figure 7.16b Polished and grooved rock from glaciers
Figure 7.16a Debris from glaciers
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31. Erosion and Mass Wasting
Figure 7.16c Two glaciers merge
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32. Erosion and Mass Wasting Gravity and Mass Wasting
The downslope movement of regolith and/or bedrock masses due to the pull of gravity
Slope failure
Falling, slumping, or sliding of relatively coherent masses of rock
Three basic types:
Fall: vertical or near vertical drop of rock fragments
Slide: rapid displacement of rock/regolith down steep or slippery slope
Slump: rotational movement of material
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33. Mass Wasting caused for very few reasons:
*Weight, *Water, *Slope and *Geology
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35. Erosion and Mass Wasting
Figure 7.17a Repeated rockfalls
Figure 7.17b rockslide
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36. Erosion and Mass Wasting
Figure 7.17c Slump
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37. Erosion and Mass Wasting Flows
Any mass-wasting process that involves a flowing motion of regolith containing water and/or air within its pores
Slurry flows
Granular flow
Creep
The imperceptibly slow downslope granular flow of regolith
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38. Erosion and Mass Wasting
Figure 7.18 Flows
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39. Factor of Safety and Landslide Prediction
Factor of safety (FS)
The balance between destabilizing forces (shear stress) and stabilizing forces (shear strength)
Tectonics and mass wasting
Major historic landslides clustered near converging lithospheric plates
Mountains/earthquakes
Figure 7.19 Earthquakes can cause mass wasting events; Alaska 1964
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Critical Thinking
On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why?
Why are some granite bodies extensively jointed, while others are essentially joint free?
Youtube mass wasting video…
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