Weathering and Soils Chapter 5 Essentials of Geology, 8e · Essentials of Geology, 8e Stan Hatfield...
Transcript of Weathering and Soils Chapter 5 Essentials of Geology, 8e · Essentials of Geology, 8e Stan Hatfield...
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Weathering and Soils Chapter 5
Essentials of Geology, 8e Stan Hatfield and Ken PinzkeSouthwestern Illinois College
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Earth’s external processes
Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surfaceMass wasting – the transfer of rock and soil downslope under the influence of gravity
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Earth’s external processes
Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity
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Weathering
Two types of weatheringMechanical weathering – breaking of rocks into smaller piecesFour types of mechanical weathering– Frost wedging – alternate freezing and
thawing of water in fractures and cracks promotes the disintegration of rocks
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Frost wedging
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Increase in surface area by mechanical weathering
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WeatheringMechanical Weathering continued– Unloading – exfoliation of igneous and
metamorphic rocks at the Earth’s surface due to a reduction in confining pressure– Thermal expansion – alternate expansion and
contraction due to heating and cooling– Biological activity – disintegration resulting
from plants and animals
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Exfoliation of igneous rocks
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Joint-controlled weathering in igneous rocks
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Weathering along joints in sedimentary rocks, Bryce Canyon, Utah
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Weathering
Chemical WeatheringBreaks down rock components and the internal structures of mineralsMost important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)
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Weathering
Major processes of chemical weatheringDissolution– Aided by small amounts of acid in the water– Soluble ions are retained in the underground
water supply
Oxidation– Any chemical reaction in which a compound
or radical loses electrons
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Weathering
Major processes of chemical weatheringOxidation continued– Important in decomposing ferromagnesian
minerals
Hydrolysis– The reaction of any substance with water– Hydrogen ion attacks and replaces other
positive ions
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Weathering
Alterations caused by chemical weathering
Decomposition of unstable mineralsGeneration or retention of materials that are stablePhysical changes such as the rounding of corners or edges
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Weathering
Rates of weatheringAdvanced mechanical weathering aids chemical weathering by increasing the surface area
Others factors affecting weatheringRock characteristics– Rocks containing calcite (marble and
limestone) readily dissolve in weakly acidic solutions
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Weathering
Others factors affecting weatheringRock characteristics continued– Silicate minerals weather in the same order as
their order of crystallization
Climate– Temperature and moisture are the most
crucial factors– Chemical weathering is most effective in
areas of warm, moist climates
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Weathering
Differential weatheringMasses of rock do not weather uniformly due to regional and local factorsResults in many unusual and spectacular rock formations and landforms
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Soil
Soil is a combination of mineral and organic mater, water, and air
That portion of the regolith (rock and mineral fragments produced by weathering) that supports the growth of plants
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Typical components in a soil that yield good plant growth
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Soil
Factors controlling soil formationParent material– Residual soil – parent material is the
underlying bedrock– Transported soil – forms in place on parent
material that has been carried from elsewhere and deposited
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Soil
Factors controlling soil formationTime– Important in all geologic processes– Amount of time for soil formation varies for
different soils depending on geologic and climatic conditions
Climate–Most influential control of soil formation– Key factors are temperature and
precipitation
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Soil
Factors controlling soil formationPlants and animals– Organisms influence the soil’s physical and
chemical properties– Also furnish organic matter to the soil
Slope– Steep slopes often have poorly developed soils– Optimum terrain is a flat-to-undulating
upland surface
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Variations in soil development due to topography
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Soil
The soil profileSoil forming processes operate from the surface downwardVertical differences are called horizons –zones or layers of soil
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Soil
The soil profileO horizon – organic matterA horizon – organic and mineral matter– High biological activity– Together the O and A horizons make up the
topsoil
E horizon – little organic matter– Zone of eluviation and leaching
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Soil
The soil profileB horizon – zone of accumulationC horizon – partially altered parent material
The O, A, E, and B horizons together are called the solum, or “true soil”
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An idealized soil profile
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A soil profile showing different horizons
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Soil
Soil typesThe characteristics of each soil type primarily depend on the prevailing climatic conditions
Three very generic soil typesPedalfer– Accumulation of iron oxides and Al-rich clays
in the B horizon
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Soil
Three very generic soil typesPedalfer continued– Best developed under forest vegetation
Pedocal– High accumulations of calcium carbonate– Associated with dry grasslands and brush
vegetation
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Soil
Three very generic soil typesLaterite– Hot and wet tropical climates– Intense chemical weathering
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Soil
Soil erosionRecycling of Earth materialsNatural rates of soil erosion depend on– Soil characteristics– Climate– Slope– Type of vegetation
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Soil
Soil erosionIn many regions the rate of soil erosion is significantly greater than the rate of soil formationSedimentation and chemical pollution– Related to excessive soil erosion– Occasionally soil particles are contaminated
with pesticides