1. A Surface Veneer: Sediments and Soils
Interlude B

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Figure B.2
Sediment
Sediment is produced by weathering. Be sure to point out, using the image, that sediment covers bedrock in variable thickness and can be transformed into soil.
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Figure B.3
Weathering
Weathering is a combination of processes that corrode (weather) solid rock and turn it into regolith.
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Figure B.4
Jointing
Joints are natural cracks formed in rocks due to expansion of a rock undergoing exhumation and cooling and contraction.
Exfoliation occurs in exposed plutons when jointing develops in layers parallel to the surface due to pressure release.
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Figure B.5a
Frost Wedging
Water expands about 9% when frozen, exerting a force on the surrounding rocks.
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Figure B.5c
Root Wedging
As roots grow, they exert a force on the surrounding rock.
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Figure B.5b
Salt Wedging
Occurs in arid climates and coastal areas when salt dissolved in water precipitates and exerts a force on the surrounding rock.
Don’t forget to mention that there are other forms of mechanical weathering, such as thermal expansion and animal activity.
Salt wedging led to disintegration of gravestones in Whitby, England.
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Figure B.6a,b
Dissolution
Minerals dissolve into water, mostly affecting carbonate rocks and salts.
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9. Physical and Chemical Weathering
Figure B.7a,b
Physical and Chemical Weathering
Physical and chemical weathering work together to:
Increase surface area (physical)
Dissolve minerals and cements (chemical)
Alter hard minerals into soft minerals (chemical)
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10. Differential Weathering
Figure B.8a,b
Differential Weathering
Weathering occurs faster at edges and corners of a block than on a flat face.
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11. Differential Weathering
Figure B.8d
Differential Weathering
Different rocks in an outcrop weather at different rates.
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Figure B.8c
Chemical Weathering
Inscriptions on a granite headstone (left) last for centuries, but those on a marble headstone (right) may weather away in decades. These gravestones are in the same cemetery and are about the same age.
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Figure B.9
Soil Formation
Soils forms through: debris production, interaction with water, and interaction with organics.
All of these produce unique layers, called horizons, that are seen in a soil profile.
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Figure B.10a
Soil-Forming Factors
Climate includes rainfall and temperature. Vegetation can add or remove material.
Climate and vegetation
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Figure B.10b
Soil-Forming Factors
Mineral composition and resistance to weathering are important.
Substrate composition
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Figure B.10b
Soil-Forming Factors
Regolith easily washes from steeper slopes.
Wetter soils contain more organic material.
Flat soils tend to hold more moisture and develop thicker soils.
Slope steepness and wetness
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Figure B.10b
Soil-Forming Factors
Soils require time to develop.
Younger soils tend to be thinner.
Time
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Table B.2
Soil Classification
Factors used for soil classification include: texture (proportion of sand, silt, and clay), structure (how many clumps (peds) form in the soil), and color (reflects composition).
The United States uses the U.S. Comprehensive Soil Classification System
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Figure B.12
Example Soils
Aridisol–desert soil
Alfisol–temperate climate soil
Oxisol–tropical soil
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Figure B.11
Soils of the World
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Figure B.13, Another View
Soil Destruction
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Think–Pair–Share
Identical pieces of granite and gabbro are exposed in a temperate climate.
Which one will weather faster? Why?
How will the weathering products differ from each?
Recalling the chart of mineral stability (Table B.2), students should figure out that the mafic minerals in gabbro are more susceptible to chemical weathering than the felsic minerals in granite, thus the gabbro will weather first. The weathering products will be different because while both might experience some hydrolysis of feldspars (producing clay), the minerals in the granite will not undergo oxidation (producing iron oxides) as much.
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Think–Pair–Share
In a region of temperate climate, some areas are underlain by sandstone and others by limestone. Describe what the region will look like after a long period of weathering and erosion.
How would this area look if the climate was arid?
Students should figure out that in a temperate climate, limestone weathers faster than sandstone. Most will describe sandstone forming hills or ridges, and limestone wearing into valleys. Some might even mention caves in the limestone areas. In arid climates, limestone weathering will be limited and limestone could form ridges.
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24. Forming Soil Narrated Figure Video
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25. Interlude B Photo Credits
1-3 (all): Stephen Marshak
4 (top right): Visuals Unlimited;(bottom, both): Stephen Marshak
6-8 (all): Stephen Marshak
10-13 (all): Stephen Marshak
20 US Dept. of Agriculture / Natural Resources Conservation Services
21 (top): Jim Richardson/Corbis; (right both): Stephen Marshak
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