Chapter 5 Weathering and Soil

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Chapter 5 Weathering and Soil
GEOL 101 Introductory Geology
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Earths external processes

• Weathering the physical breakdown
  (disintegration) and chemical alteration
  (decomposition) or rock at or near Earths
  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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Weathering
Mechanical 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 Earths 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 Processes

• Dissolution
• 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

• Marble and limestone (calcium carbonate)
  decompose due to acidic water

Calcium Carbonate Acidic Water
CaCO3 2H(H2O)
Ca2 CO2 3(H2O)
Soluble Calcium Carbon Dioxide Water

• Acid rain caused by air pollution

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Oxidation

• 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

4Fe3 3O22- 2Fe2O3
Hematite (iron oxide)
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Hydrolysis

• Silicate minerals decomposed by water due to
  hydrolysis
• Produces clay, soluble salt, silica

Potassium Feldspar Carbonic Acid Water
2KAlSi3O8 2(H HCO3- ) H2O
Al2Si2O5(OH)4 2K 2HCO3 -) 4SiO2
Kaolinite Potassium Bicarbonate
Silica (clay) (salt)
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 Factors

• Rates 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 acids
• raises 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
Bowens Reaction Series
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Weathering Processes

• Reaction rates and weathering susceptibilities
  approximate a backwards Bowens 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 soils 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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Soil Profile

• 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

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Idealized soil profile
O
A
E
B
C
Parent
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Soil Profile showing horizons
O
A
E
B
C
Parent
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Soil profile showing Solum
O
A
E
B
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Soil Development
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Soil Types

• Characteristics 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 (gt63 cm rainfall), temperate
• Forest vegetation
• Fe oxides and Al-rich clays in the B horizon
• Pedocal
• Arid (lt63 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
Arid
Tropical
Humid
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Soil Erosion

• Constant 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 gt 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 Erosion

• Good soil development critical for agriculture
  and forestry
• Agricultural soil conservation measures
• Planting trees as windbreaks
• Plowing hill contours and terracing
• Crop rotation
• Forestry soil conservation measures
• Eliminate clear cutting (selective harvest)
• Careful design of logging roads
• Harvest away from drainage area