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© 2011 Pearson Education, Inc.
Weathering and SoilEarth, 10e - Chapter 6
© 2011 Pearson Education, Inc.
Earth’s External Processes
• Weathering—the physical breakdown and chemical decomposition of rock
• Mass wasting—the transfer of rock and soil downslope under the influence of gravity
• Erosion—the physical removal of material by water, wind, ice, or gravity
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Weathering
• Two types:1. Mechanical weathering—breaking of rocks
into smaller pieces
2. Types of mechanical weathering– Frost wedging
– Unloading
– Thermal expansion
– Biological activity
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Sheeting and Exfoliation Due to Unloading
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Weathering
• Chemical weathering• Breaks down rock components and internal
structures of minerals
• The most important agent is water.–Responsible for transport of ions and molecules
involved in chemical processes
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Weathering
• Major processes of chemical weathering• Dissolution–Aided by small amounts of acid in the water
• Oxidation–Any reaction when electrons are lost from one
element
• Hydrolysis–The reaction of any substance with water.
–A hydrogen ion attacks and replaces other ions.
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Weathering
• Alterations caused by chemical weathering• Decomposition of unstable minerals
• Formation or retention of stable materials
• Physical changes such as the rounding of corners or edges
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Products of Weathering
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Rates of Weathering
Factors affecting weathering• Surface area
• Rock characteristics–Rocks containing calcite (marble and limestone)
readily dissolve in weakly acidic solutions.
– Silicate minerals weather in the same order as their order of crystallization.
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Surface Area and Chemical Weathering
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Rates of Weathering• Climate–Temperature and moisture characteristics
–Chemical weathering is most effective in areas of warm, moist climates.
• Differential weathering–Rocks do not weather uniformly due to regional
and local factors.
–Results in many unusual and spectacular rock formations and landforms
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Spheroidal Weatheringof Extensively Jointed Rock
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Differential Weathering in Bryce Canyon National Park, Utah
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Soil
• Soil = mineral and organic matter + water + air
• Rock and mineral fragments produced by weathering (regolith) that support the growth of plants
• Humus (decayed animal and plant remains) is a small, but essential component.
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Composition of a “Good” Soil
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Soil
• Factors controlling soil formation• Parent material–Residual versus transported soil
• Time–Amount of time for soil formation varies
depending on geologic and climatic conditions.
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Soil• Climate–Most influential control of soil formation–Key factors are temperature and precipitation.
• Plants and animals– Influence the soil’s physical and chemical
properties
• Topography– Steep slopes often have poorly developed soils.–The optimum terrain is a flat-to-undulating
upland surface.
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Variations in Soil Development
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Soil• The soil profile
• Soil-forming processes operate from the surface downward.• Vertical differences are called horizons,
which are zones or layers of soil.–O horizon—organic matter–A horizon—organic and mineral matter»High biological activity»O and A horizons make up the topsoil
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Soil• Soil profile–E horizon—little organic matter
»Zone of eluviation and leaching
–B horizon—zone of accumulation
–C horizon—partially altered parent material
• Collectively, the O, A, E, and B horizons make up the solum, or “true soil.”
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IdealizedSoil
Profile
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Soil Profile Showing Different Horizons
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Soil• Classifying soils
• Variations in soil formation over time and distances has led to a great variety of recognized soil types.
• Groups have been established using common characteristics.
• In the United States, a system was devised, and called the Soil Taxonomy.
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Soil• Classifying soils
• Soil Taxonomy–Emphasis is placed on physical and chemical
properties.
– Six hierarchical categories exist.
–Descriptive names are derived from Latin and Greek.
– 12 basic soil orders are recognized.
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Basic Soil Orders
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Soil• Soil erosion
• Part of the natural recycling of all Earth materials
• Natural rates of soil erosion depend on:– Soil characteristics
–Climate
– Slope–Type of vegetation
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Soil Erosion Where Vegetation Is Lacking
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Soil• In many regions, soil erosion is much
greater than the rate of soil formation.
• Sedimentation and chemical pollution–Related to excessive soil erosion–Occasionally, soil particles are contaminated
with pesticides.
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End of Chapter 6