Transcript Earth_Can01_ch05_Weathering/Soil Tark

Chapter 5
Weathering and Soil
PowerPoint Presentation
Stan Hatfield . SW Illinois College
PowerPoint Presentation
Ken
Pinzke. .Southwestern
SW Illinois
College
Stan Hatfield
Illinois
College
Ken
Pinzke . Southwestern
Illinoisof
College
Charles
Henderson
. University
Calgary
Charles Henderson . University of Calgary
Tark
Hamilton . Camosun College
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5-1
Earth’s External Processes
Weathering - the mechanical 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 predominant types of weathering
• Mechanical weathering - breaking of rocks into
smaller pieces by physical forces
• Chemical weathering – involves chemical
transformation of minerals in rock into one or
more new mineral compounds plus dissolved ions
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Mechanical Weathering
• Five 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 – joints & 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, forest fires,
winter, solar flares on Mercury & Moon’s Dayside!
– Bioturbation – disintegration resulting from bacteria,
plants (roots) and animals: bugs, clams, bears
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Frost Wedging!
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Frost Wedging!
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Unloading Joints, Devonian Granite
Green Point NS. (Photo by Bruce Railsback)
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Sheet Joints: Stone Mountain Georgia
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Thermal Spallation: Fires, Intrusions, Drilling
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Bioturbation: Root Wedging & Mining!
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Mechanical Weathering
Mechanical weathering increases the surface area exposing more surfaces to chemical weathering.
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to 2005
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curved rocks from original planar joints & fractures.
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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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Rates of Weathering
Climate
Temperature and moisture are the most crucial factors
affecting chemical weathering rate
Water (polar molecules) is the best solvent for ions
Chemical weathering is most effective in areas of
warm, moist climates, deep red laterite soils
In the tropics rocks are weathered over 100 m deep
This is why bauxites all form in the tropics
Chemical weathering is ineffective in alpine & polar
regions because frigid temperatures keep moisture
locked up as ice (permafrost, glaciers), regolith
Different times in Earth history, Different climate
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Chemical Weathering
Four major processes of chemical weathering
• Oxidation
– Any chemical reaction in which a compound, atom or ion loses
electrons: Fe 2+  Fe 3+ + e– Important in decomposing ferromagnesian minerals in air
• Hydrolysis
– Smaller, acidic Hydrogen ions H+ replaces other positive ions
• Hydration
– The addition of water to minerals or the solvation of their ions
• Dissolution
– Further aided by small amounts of acid in the water
– Soluble ions are retained in the underground water supply
– Ions eventually concentrate in the ocean or playa lakes
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Chemical Weathering
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Alterations Caused by Chemical Weathering
• Pyrite is the most unstable common mineral
weathering to limonite & sulphuric acid (ARD)
• Decomposition of unstable minerals (olivine, glass)
• This creates all soils and many sediments
• Generation or retention of materials that are stable
– Sheet Silicate Clays, Oxides, Hydroxides
• Physical changes such as the rounding of corners
or edges caused by water having greater access to
corners by flowing through joints; is called
spheroidal weathering
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Alterations Caused by Chemical Weathering
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Spheroidal
weathering
of extensively jointed rock.
Weathering Rind on Limestone Boulder
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Acid Rock Drainage
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Salt Wedging
Salt Wedging – A special type of mechanical plus
chemical weathering
Arid landscapes where evaporation > precipitation
Dry coastal areas where seaspray can evaporate
Evaporation concentrates salts in crack & joint tips
Dried salts are hygroscopic (pulls moisture from air)
Chemical weathering wedges between mineral crystals
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Rates of Weathering
Rock Characteristics
• Rocks containing silicate minerals (granite, gneiss,
sandstone) are relatively resistant to chemical weathering
• Evaporite deposits of halite, gypsum, anhydrite dissolve &
can form caves or cause collapse of overlying landscapes
• Rocks containing calcite (marble and limestone) readily
dissolve in weakly acidic solutions & form caves (karst)
• Shales are compacted rather than cemented so they hydrate
& fall apart readily. They weather recessively.
• Fine grained and glassy volcanic rocks weather readily &
tend to generate rich soils quickly
• Silicate minerals weather in the opposite order from their
order of crystallization (see Bowen’s Reaction Series)
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Dissolution of Joints in Anhydrite, Iowa
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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
• Mudstone versus sandstone
• Variable cementation or cement solubility
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Honeycomb Weathering
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Hoodoos & Badlands
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Soil
Soil has been called the bridge between life and
the inanimate world
• Soil is Life’s (bacteria’s) modification of rocks to
make them more useful, supply nutrients, hold
water
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
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in good condition for plant growth.
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Controls of Soil Formation
Parent Material
• Residual soils – parent material is the underlying
bedrock e.g. Regolith, Laterite, Bauxite
• Transported soils – forms in place on parent
material that has been carried from elsewhere and
deposited e.g. Alluvial, Glaciomarine
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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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Caliche: Hardpan soils, Perched water tables
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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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Laterite: Triassic-Modern, Gainesville GA
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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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Paleosol: a fossil or buried soil
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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
• Single cropping agriculture loses cm/yr!
• Swidden (slash & burn) & clearing rainforests 
• 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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The Dirty End of
Chapter 5
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