A Surface Veneer: Sediments and Soils Interlude B

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.3 Weathering Weathering is a combination of processes that corrode (weather) solid rock and turn it into regolith. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.5a Frost Wedging Water expands about 9% when frozen, exerting a force on the surrounding rocks. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.5c Root Wedging As roots grow, they exert a force on the surrounding rock. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.6a,b Dissolution Minerals dissolve into water, mostly affecting carbonate rocks and salts. Copyright © 2016 by W. W. Norton & Company, Inc.

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) Copyright © 2016 by W. W. Norton & Company, Inc.

Differential Weathering Figure B.8a,b Differential Weathering Weathering occurs faster at edges and corners of a block than on a flat face. Copyright © 2016 by W. W. Norton & Company, Inc.

Differential Weathering Figure B.8d Differential Weathering Different rocks in an outcrop weather at different rates. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.10a Soil-Forming Factors Climate includes rainfall and temperature. Vegetation can add or remove material. Climate and vegetation Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.10b Soil-Forming Factors Mineral composition and resistance to weathering are important. Substrate composition Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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 Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.10b Soil-Forming Factors Soils require time to develop. Younger soils tend to be thinner. Time Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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 Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.12 Example Soils Aridisol–desert soil Alfisol–temperate climate soil Oxisol–tropical soil Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.11 Soils of the World Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. Figure B.13, Another View Soil Destruction Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc. 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. Copyright © 2016 by W. W. Norton & Company, Inc.

Forming Soil Narrated Figure Video Links are active only in Slideshow mode. Click the image to launch the animation (requires an active Internet connection).

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 Copyright © 2016 by W. W. Norton & Company, Inc.

Copyright © 2016 by W. W. Norton & Company, Inc.