Download Chemical Weathering

Chapter 5
Weathering and Soil
PowerPoint Presentation
Stan Hatfield . Southwestern Illinois College
Ken Pinzke . Southwestern Illinois College
Charles Henderson . University of Calgary
Copyright (c) 2005 Pearson Education Canada, Inc.
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Earth’s External Processes
Weathering - the physical breakdown
(disintegration) and chemical alteration
(decomposition) of rock at or near Earth’s
surface
Mass wasting - the transfer of rock and soil
downslope under the influence of gravity
Erosion – the physical removal of material
by mobile agents such as water, wind, or ice
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Weathering
Two types of weathering
• Mechanical weathering - breaking of rocks
into smaller pieces by physical forces
• Chemical weathering – involves chemical
transformation of rock into one or more
new compounds
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Mechanical Weathering
• Four types of mechanical weathering
– Frost wedging – repeated freezing and thawing
of water in fractures and cracks promotes the
disintegration of rocks; causes great destruction
of Canada’s highways every spring
– Unloading – exfoliation as sheets 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 (roots) and animals
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Mechanical Weathering
Mechanical weathering increases the surface area exposing more surfaces to
chemical weathering.
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Chemical Weathering
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
Three major processes of chemical weathering
• Dissolution
– 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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Chemical Weathering
Three major processes of chemical weathering
• Oxidation 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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Chemical Weathering
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Alterations Caused by Chemical
Weathering
• Decomposition of unstable minerals
• Generation or retention of materials that
are stable
• Physical changes such as the rounding of
corners or edges caused by water flowing
through joints; called spheroidal
weathering
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Alterations Caused by Chemical Weathering
Spheroidal weathering of extensively jointed rock.
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Rates of Weathering
Rock Characteristics
• Rocks containing silicate minerals (granite) are
relatively resistant to chemical weathering
• Rocks containing calcite (marble and limestone)
readily dissolve in weakly acidic solutions
• Silicate minerals weather in the same order as their
order of crystallization (see Bowen’s Reaction Series)
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Rates of Weathering
Climate
• Temperature and moisture are the most
crucial factors affecting chemical
weathering rate
• Chemical weathering is most effective in
areas of warm, moist climates
• Chemical weathering is ineffective in polar
regions because frigid temperatures keep
moisture locked up as ice
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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, such as
the hoodoos of Drumheller, Alberta
• The number and spacing of joints can be a
significant factor
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Soil
Soil has been called the bridge between life
and the inanimate world
Soil is an important interface; a common
boundary where different parts of a system
interact
• Soil is that portion of the regolith (rock and
mineral fragments produced by weathering)
that supports the growth of plants; it is a
combination of mineral and organic matter
(humus), water, and air
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Soil
Composition (by volume) of a soil in good condition for plant growth.
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Controls of Soil Formation
Parent Material
• Residual soils – parent material is the
underlying bedrock
• Transported soils – forms in place on
parent material that has been carried from
elsewhere and deposited
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Controls of Soil Formation
Time and Climate
• Time
– 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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Controls of Soil Formation
Plants and Animals and Slope
• Plants and animals
– Organisms influence the soil’s physical and chemical
properties
– Also supply organic matter to the soil; mostly plants
– Microorganisms like fungi and bacteria play an active role
in the decay of plant and animal remains
• Slope
– Steep slopes often have poorly developed soils
– Optimum terrain is a flat-to-undulating upland surface
– South facing slope in Northern Hemisphere will receive a
great deal more sunshine, which will influence soil
character
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Controls of Soil Formation
The parent material for residual soils is the underlying bedrock; soils thin
where slopes become steeper.
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The Soil Profile
• Soil forming processes operate from the
surface downward
• Vertical differences are called horizons –
zones or layers of soil
• There are five distinct horizons designated
as O, A, E, B, and C
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The Soil Profile
• O horizon – organic matter (humus)
• A 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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The Soil Profile
• B horizon – zone of accumulation
• C horizon – partially altered parent material
The O, A, E, and B horizons together are called
the solum, or “true soil”
Well developed soil profiles are called mature
and indicate environmental conditions have
been stable for a long time; some soils lack
horizons and are called immature
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The Soil Profile
Well-developed soils show distinct layers called horizons.
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The Soil Profile
Idealized soil profile from a humid, mid-latitude climate.
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Soil Types
The characteristics of each soil type
primarily depend on the prevailing
climatic conditions
Three very generic soil types
• Pedalfer
– Accumulation of iron oxides and Al-rich clays
in the B horizon
– Organic-rich soil
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Soil Types
Three very generic soil types
• Pedalfer continued
– Best develops under forest vegetation
– Develops in humid, temperate climate
• Pedocal
– High accumulations of calcium carbonate
– Calcium carbonate cemented soils known as
caliche or hardpan
– Less clay than Pedalfer
– Associated with dry grasslands and brush
vegetation as seen in the prairies
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Soil Types
Three very generic soil types
• Laterite
– Hot and wet tropical climates
– Intense chemical weathering
– Intense leaching removes soluble material
– Concentrates iron and aluminum; the former
giving these soils a distinct red colour
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Soil Types
Generalized descriptions of Canadian soil types as recognized by the
Canadian Soil Classification System.
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Soil Types
Generalized distribution of Canadian soil types.
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Soil Types
Ancient Soils
• Paleosols
– Serve as valuable indicators of land areas
– To be preserved, paleosols must reside in an
area where it can be buried (lowland areas)
– Oldest are Proterozoic (2 billion years old)
and indicate that oxygen levels had reached a
critical level to allow the rusting of iron
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Soil Erosion
• Recycling of Earth materials; part of rock
cycle
• Natural rates of soil erosion depend on
– Soil characteristics
– Climate
– Slope
– Type of vegetation
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Soil Erosion
Rates of Erosion
• In many regions the rate of soil erosion is
significantly greater than the rate of soil
formation
• Availability of good soils is critical for
rapidly increasing world’s population
• Sedimentation and chemical pollution
– Related to excessive soil erosion
– Occasionally soil particles are contaminated
with pesticides
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End of
Chapter 5
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