chapter 5 weathering and soil geol 101 introductory geology
TRANSCRIPT
Chapter 5
Weathering and Soil
GEOL 101
Introductory Geology
GEOL 101
Earth’s external processesWeathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface
Mass wasting – the transfer of rock and soil downslope under the influence of gravity (Chapter 9)
Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity
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Weathering
Types of weathering• Mechanical weathering • Chemical weathering• Other function together
Weathering Processes• Dissolution • Oxidation• Hydrolysis
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WeatheringMechanical & chemical
processes work together
Solid rock
Chemical weathering attacks susceptible minerals
Rock crumbles by mechanical weathering
Chemical/mechanical processes form clay and disperse minerals
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Mechanical Weathering
Increased surface area
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Types of Weathering
Mechanical weathering – breaking of rocks into smaller pieces
• Frost wedging – freezing/thawing of water in fractures, disintegration of rocks
• Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure
• Thermal expansion – expansion and contraction due to heating and cooling
• Biological activity – disintegration resulting from plants and animals
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Frost Wedging
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Unloading - Exfoliation
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Exfoliation
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Thermal Expansion
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Biological
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Chemical Weathering
Breaks down rock components and internal structures of minerals
Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)
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Chemical Weathering ProcessesDissolution
– Aided by small amounts of acid in the water– Soluble ions are retained in the underground
water supply
Oxidation– Chemical reaction where compound or
radical loses electrons– 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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Simple Dissolution
Halite (salt) dissolving in water
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Dissolution by Acidic Water
CaCO3 + 2[H+(H2O)]
Marble and limestone (calcium carbonate) decompose due to acidic water
Calcium Carbonate + Acidic Water
Ca2+ + CO2 + 3(H2O)
Soluble Calcium + Carbon Dioxide + Water
Acid rain caused by air pollution
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Oxidation
4Fe3+ + 3O22-
2Fe2O3
Iron (Fe) minerals rust when exposed to water and oxygen
Oxidation: loss of electron from element
Iron oxidation produces
Hematite (Fe2O3): reddish brown rust
Limonite [FeO(OH)]: yellowish rust
Iron + Oxygen
Hematite (iron oxide)
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Hydrolysis
Al2Si2O5(OH)4 + 2K+ + 2HCO3 -) + 4SiO2
Silicate minerals decomposed by water due to hydrolysis
Produces clay, soluble salt, silica
Potassium Feldspar + Carbonic Acid + Water
Kaolinite + Potassium + Bicarbonate + Silica(clay) (salt)
2KAlSi3O8 + 2(H+ + HCO3- ) + H2O
in solution
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Products Weathering
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Chemical Weathering Alternations
Decomposition of unstable minerals
Generation or retention of materials that are stable
Physical changes such as the rounding of corners or edges
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Weathering FactorsRates of weathering
– Advanced mechanical weathering aids chemical weathering by increasing the surface area
Rock characteristics– Rocks w/ calcite (marble and limestone) readily
dissolve in weakly acidic solutions
– Silicate minerals weather in the same order as their order of crystallization
Climate– Temperature and moisture, most crucial factors
– Chemical weathering is most effective in areas of warm, moist climates
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Chemical WeatheringRate variation by rock type
Granite Marble
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Weathering rates
organic activity - organic acidsraises wtr rates by orders of magnitude
climate - temp and amount of water
water flow through rock
bedrock composition
topography: groundwater flow
time - to equilibrium?
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Rock Characteristics
Bowen’s Reaction Series
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Weathering Processes
Reaction rates and weathering susceptibilities approximate a “backwards backwards Bowen’s Reaction SeriesBowen’s Reaction Series”
first minerals (highest P/T) to crystallize are least stable (first to weather)
last minerals (lowest P/T) to crystallize are most stable (last to weather)
Residual material is altered to a stable composition and physical form
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Differential Weathering
Masses of rock do not weather uniformly due to regional and local factors
Results in many unusual and spectacular rock formations and landforms
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Differential Weathering
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Differential weathering
Controlled by jointing patterns
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Joint-controlled weathering in igneous rocks
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Soil
Soil is a combination of mineral and organic mater, water, and air
Results from weathering
Regolith: rock and mineral fragments produced by weathering (weathered debris)
Soil supports the growth of plants
Good soil: mix of decomposed rock and humus (decay plant and animal)
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Typical soil components
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Soil
Soil Formation
Soil Profile
Soil Types
Soil Erosion
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Soil Formation
Parent 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
Time– Important in all geologic processes– Amount of time for soil formation varies for
different soils depending on geologic and climatic conditions
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Soil Formation
• Parent 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
• Time– Important in all geologic processes
– time for soil formation varies depending on geologic and climatic conditions
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Soil Formation• Climate
– Most influential control of soil formation– Key factors: temperature and precipitation
• Plants 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 flat-to-undulating upland surface
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Soil Formation
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Soil development variations due to topography
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Soil Profile
Soil forming processes operate from the surface downward
Vertical differences are called horizons – zones or layers of soil
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• O horizon – organic matter• A horizon – organic and mineral matter
– High biological activity
– O and A horizons together make up the topsoil
• E horizon – little organic matter– Zone of eluviation and leaching
– The O, A, E, and B horizons together are called the solum (“true soil”)
• B horizon – zone of accumulation• C horizon – partially altered parent material
Soil Profile
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Idealized soil profile
A
E
B
C
Parent
O
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Soil Profile showing horizons
AE
B
C
Parent
O
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Soil profile showing Solum
A
E
B
O
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Soil Development
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Soil TypesCharacteristics of each soil type primarily depend on prevailing climatic conditions
Climate conditions relate to vegetation
Three generic soil types• Pedalfer• Pedocal• Laterite
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Soil Types• Pedalfer
– Humid (>63 cm rainfall), temperate– Forest vegetation – Fe oxides and Al-rich clays in the B horizon
• Pedocal– Arid (<63 cm rainfall), temperate– Dry grasslands and brush vegetation – High accumulations of calcium carbonate
• Laterite– Tropical climates, hot and wet– Lush grasslands and trees vegetation– Intense chemical weathering
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Soil Types
Pedalfer Pedocol Laterite
Humid Arid Tropical
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Soil ErosionConstant recycling of Earth materials, part of rock cycle
Water and wind are powerful erosion forces that move soil components
Raindrop like tiny bomb to soil particles• sheet erosion: thin sheets of water
• rills: tiny channels
• gullies: deeper cuts
Sediment: soil that reaches a stream
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Soil Erosion• Natural rates of soil erosion depend on
– Soil characteristics
– Climate
– Slope
– Type of vegetation
• In many regions, rate of soil erosion is significantly > rate of soil formation
• Sedimentation and chemical pollution– Related to excessive soil erosion
– Occasionally soil particles are contaminated with pesticides, industrial pollutants etc.
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Soil ErosionGood soil development critical for agriculture and forestry
Agricultural soil conservation measuresPlanting trees as windbreaks
Plowing hill contours and terracing
Crop rotation
Forestry soil conservation measuresEliminate clear cutting (selective harvest)
Careful design of logging roads
Harvest away from drainage area