Download Weathering, Erosion and Transport

Weathering, Erosion and Transport
Weathering, Erosion, and
Transportation
• Rocks exposed at Earth’s surface are constantly changed
by water, air, temperature variations and other factors
• Weathering is the group of destructive processes that
change physical and chemical character of rocks at or
near Earth’s surface
• Erosion is physical picking up of rock particles by water,
ice, or wind
• Transportation is the movement of eroded particles by
water, ice, or wind
The Nature of Weathering
• Weathering is the physical and/or
chemical alteration of rocks and minerals
where the lithosphere, hydrosphere,
atmosphere, and biosphere meet
Weathering and Earth Systems
• Hydrosphere
– Water is necessary for chemical weathering
– Oxygen dissolved in water oxidizes iron and other metals in rocks
– Carbon dioxide dissolved in water creates carbonic acid
• Primary cause of chemical weathering
– Running water loosens and abrades particles
– Glacial ice removes and abrades particles
– Freeze/thaw cycling mechanically weathers
• Biosphere
– Plant root growth widens cracks
– Animal movement and human activity mechanically weather
– Plant roots decaying organic matter in soils pump carbon dioxide into
the soil producing acids that dissolve the rock
Products of Weathering
• Lithic (Rock) Fragments
(granite, basalt, schist, etc.)
• Dissolved Ions
(Calcium, Potassium, Sodium, etc.)
• Rust Minerals (Hematite, Goethite, etc.)
• Clay Minerals
(Bentonite, Montmorillonite, etc.)
• Residual Minerals
(Quartz, Orthoclase, Muscovite, etc.)
Two Types of Weathering
• Chemical weathering
– The decomposition of rocks and minerals as
chemical reactions alter them into new minerals
stable at the Earth’s surface
• Physical (or mechanical) weathering
– The disintegration or disaggregation of rocks by
physically breaking them apart
Weathering
• Physical and chemical weathering are
two distinct processes
• Usually work in tandem
• Chemical weathering is more significant
in warm wet low land environments;
Physical weathering is more important in
in cold areas and high elevations
Physical Weathering
Physical Weathering
• Frost action
– Mechanic effect of freezing (and
expanding) water on rocks
• Pressure release
– Removal of overlying rock allows
expansion and fracturing
• Plant growth
– Growing roots widen fractures
• Burrowing animals
• Thermal cycling
– Large temperature changes fracture rocks
by repeated expansion and contraction
But mostly physical weathering is a matter of things just
falling down. So in a sense, gravity, is the primary cause of
physical weathering.
Physical Weathering in cold high altitude
environments
Physical Weathering by running water
More on that later
Sheeting
• Release of confining pressure on rocks
formed deep within the Earth
• Development of fractures and joints
caused by expansion
• Rocks break along fractures and joints
Sheeting in granite
Ice Wedging
• Freeze - Thaw cycles are effective at
breaking apart rocks
– Water expands when it freezes
• Volume increases by 9%
– The stress of expansion breaks the rock
– Ice melts and the water percolates deeper
into the newly expanded cracks
Frost/freeze or Ice wedging
Geometry of Weathering
• Spheroidal weathering
– Corners tend to be rounded during
weathering
– Decomposition is most rapid at corners
– Rock’s shape approaches sphere
– Further weathering reduces size
Spheroidal weathering in granites
Spheroidal weathering in granites
Other Forms of Physical Weathering
• Heat
– Heat causes rocks (most solids) to expand
– Rocks are poor conductors of heat
– Outer layer of rock that expands breaks off
(spall)
• Crystal Growth
– Minerals precipitate along fractures
– Similar to ice wedging
Other Forms of Physical Weathering
• Root Growth
– Roots may exert enormous forces in growing
– Root tips pressures may exceed 10,000 kg per
square meter
– Seeds gather in cracks in rock and germinate
– Growing plant and roots slowly wedge rock apart
Geometry of Weathering
• Fractures in rock form from the reduction
in load (pressure)
– Generally form in groups
• Parallel joints
• Intersecting joints
–Cut large blocks into smaller blocks
Geometry of Weathering
• Surface Area is increased by fracturing
– The increase in surface area, increases
the rate of weathering
• Both physical and chemical
– Surface area increases exponentially
60
50
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Series1
30
Expon.
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20
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2
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Chemical Weathering
Chemical Weathering
• Minerals are destroyed or altered by
chemical reactions
–Dissolution
–Hydrolysis
–Oxidation
Chemical Weathering
• Oxidation
– Chemically active oxygen from atmosphere
– Iron oxides are common result
• Soil and sedimentary rocks often stained with
iron oxides
• Acid dissolution
– Hydrogen cations replace others in minerals
– Carbonic acid from atmospheric CO2 dissolved
in water
– Sulfuric, hydrofluoric acids emitted by volcanic
eruptions
– Some minerals, such as calcite, may be totally
dissolved
– Human activity, such as mining and burning of
fossil fuels, produces acids
Chemical Weathering
• Feldspars
– Most common minerals in crust
– Slightly acidic rain water
attacks feldspar
– Clay minerals produced
• K+, Na+, Ca++ ions released into water
• Other minerals
– Ferromagnesian minerals
• Clays, iron oxides, Mg++ ions produced
– More complex silicate bonds lead to lower weathering susceptibility
• Olivine most susceptible, quartz least
• Warm, wet climatic conditions maximize weathering
Chemical Weathering
• Most igneous and metamorphic rocks and
minerals are formed at high temperatures and
pressures
– They are in a state of equilibrium at the
Temperature (T) and Pressure (P) of formation
– At the Earth’s surface, rocks and minerals are
subject to chemical weathering
– Secondary minerals formed at the T and P
common to the Earth’s surface
Chemical Weathering (cont)
• Sedimentary Rocks:
–Limestones and Dolomites are formed in
the ocean and are easily dissolved by
water, especially if it is acidic
–Evaporites (Halite, Gypsum and
Anhydrite) are precipitated from
seawater and easily dissolved in water
even if it is not acidic
Dissolution
• Some minerals are soluble in water
– e.g., Halite - NaCl
– Minerals dissolve into constituent ions
– Ions removed with water by leaching
– Solubility of compound controls leachability
Acid Hydrolysis
• CO2 mixes with water to produce carbonic
acid, H2CO3
• Decaying organic matter produces acid
• Roots pump CO2 into the soil producing
very high concentrations of carbonic acid
• Anthropogenic sources of acid (CO2 and
SO2)
– Acid rain
Acid Hydrolysis
• H+ attacks minerals by replacing other ions in
the mineral structure
• Promotes dissolution
– Calcite hydrolysis by carbonic acid solution
CaCO3 + H2CO3  Ca+2 + 2HCO3-
Acid Hydrolysis
• New “secondary” minerals may be created
by this process
– H+ ion replaces the K+ ion in the feldspar
structure
– K+ ion goes into the water solution
– Kaolinite, a clay mineral, formed
2KAlSi3O8 + 2H2CO3 + 9H2O =
2K+ + 2HCO3- + 4H4SiO4 + Al2Si2O5(OH)4
Oxidation
• Valence state increases
– Often associated with free O2 in the
environment
• Iron is usually found as the Fe+2 ion in
silicate minerals
• Exposed to the atmosphere it will oxidize
to the Fe+3 ion
Oxidation
• Change in valence state disrupts
crystal structure
• Oxidation works in combination with
hydrolysis and dissolution
2FeSiO4 + 4H2O + O2 = 2 Fe2O3 + 2H4SiO4
Trends in Chemical Weathering
• Alkali and alkaline earth elements
removed into solution
• Al and Si are enriched in secondary
minerals
• Fe is enriched in insoluble ferric oxides
• Warm wet climates increase chemical
weathering rates
Weathering of Rocks
• Relative stability of minerals varies
widely
• Minerals composition is primary control
• Rock texture influences role of water in
weathering
Relative stability of minerals
Stability of minerals at the Earth’s surface is
predicted by Bowen’s reaction series in Reverse, i.e.,
Quartz is most stable and Olivine is least.
Inorganic Carbon Cycle
What controls CO2 concentrations on geologic timescales
– Carbon dioxide present as trace
gas in atmosphere (380ppm)
– Combines with water to form
carbonic acid (H2CO3)
– Weathers rocks and provides
CaCO3 to marine animals and
plants so they can make shells.
– Returns to the mantle during
subduction
– Released back to atmosphere by
volcanic eruptions
On geologic timescales volcanism controls CO2 concentrations.
The negative feedback mechanism on this is the rate of weathering which
increases because of warmer climates due to higher CO2 concentrations.
Climate & Weathering
• Climatic conditions strongly influence
weathering reactions
– Amount of rainfall
• Most reactions need water
– Average temperature
• Increase of 10oC doubles reaction rate
Rates of Weathering
• Rates of weathering are linked to climate
zones
– Human structures are useful gages for
measuring rates
– Thickness of soil profile is controlled by
weathering rates
Climate and weathering
From Sediment to
Sedimentary Rock
• Transportation
– Movement of sediment away from its source, typically by
water, wind, or ice
– Rounding of particles occurs due to abrasion during transport
– Sorting occurs as sediment is separated according to grain size
by transport agents, especially running water
– Sediment size decreases with increased transport distance
Distinguishing Characteristics of Clastic Sediments (cont.):
Sorting - Well-sorted sediment indicates prolonged reworking by wind or water; poorly sorted
sediment may indicate rapid deposition, or deposition by ice or mass movement.
Angularity/Roundness and Shape – Well rounded sediment also indicate prolonged reworking
by transporting agent; the shape of grains often indicates the transport system, but also may
be related to the type of mineral or rock fragment