© 2011 Pearson Education, Inc. Weathering and Soil Earth, 10e - Chapter 6.

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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