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Weathering and SoilEarth - Chapter 6

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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 (dissolving)–Aided by small amounts of acid in the water

• Oxidation–Any reaction when electrons are lost from one

element (rusting). • 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 WeatheringFactors 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 Weathering of Extensively Jointed Rock

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Differential Weathering, Bryce Canyon Nat. 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.