Geo-100 (Cain) Soil as a Resource Outline

Geo-100 (Cain) Soil as a Resource Outline - Chapter 11 Soil...

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Chapter 11 : Soil as a Resource Soil – Defined in different ways for different purposes: - Engineering Geologists define soil very broadly to include all unconsolidated material overlying bedrock. - Soil Scientists restrict the term soil to those materials capable of supporting plant growth and distinguishing it from regolith (which encompasses all unconsolidated material at the surface, fertile or not). Conventionally: - soil implies little transportation away from the site at which the soil formed, - sediment indicates matter that has been transported and re-deposited by wind, water, or ice. Soil-Forming Processes: Weathering - Soil is produced by weathering , a term that encompasses a variety of chemical, physical, and biological processes acting to break down rocks. Physical Mechanical Weathering – The physical breakup of rocks without changes in the rock’s composition. - In a cold climate, with temperatures that fluctuate above and below freezing, water in cracks repeatedly freezes and expands, forcing rocks apart, then thaws and contracts or flows away Crystallizing salt in cracks may have the same wedging effect. - Whatever the cause, the principal effect of mechanical weathering is the breakup of large chunks of rock into smaller ones. ( In the process, the total exposed surface area of the particles is increased .) Chemical -Involves the breakdown of minerals by chemical reaction with water, with other chemicals dissolved in water, or with gasses in the air. -Minerals differ in the kind of chemical reactions they undergo. Calcite (Calcium Carbonate) – Tends to dissolve completely, leaving no other minerals behind in its place. Calcite dissolves rather slowly in pure water but more rapidly in acidic water. Calcite dissolution is gradually eating away at the very fabric of many buildings in urban areas and where acid rain is common. Silicates – Tend to be somewhat less susceptible to chemical weathering and leave other minerals behind when they are attacked - Feldspars principally weather into clay minerals. Page 1 of 6
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Chapter 11 : Soil as a Resource - Ferromagnesian silicates leave behind insoluble iron oxides and hydroxides and some clays, with other chemical components being dissolved away - Those residual iron compounds are responsible for the reddish or yellowish colors of many soils. -A rock’s tendency to weather chemically is determined by its mineral composition:
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Geo-100 (Cain) Soil as a Resource Outline - Chapter 11 Soil...

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