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Weathering
What is weathering ?
Weathering is the physical, chemical and biological
decay of materials at the earth’
earth’s surface into
products which are in equilibrium with new
imposed physicophysico-chemical conditions.
Professor Paul Shaw
Weathering takes place in situ in the presence of
water.
It is not the same as EROSION
Weathering
What is being weathered ?
Rocks – minerals – elements
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What is being weathered ?
What is being produced ?
What are the processes involved ?
What are the environmental conditions
under which it takes place ?
• Elements properties : valency,
valency, ionic radius
• Mineral properties : chemistry, structure
• Rock properties : mineral suite, structure,
origins
Element properties
Properties of rocks
• Ionic radius – size of atom
• Igneous rocks
– Form under great heat and pressure, either intrusively or extrusively.
extrusively. Their
formational environment does not contain water.
– Are crystalline, with complex mineral content.
• Valency – surplus electron charge
– Anions = negative charge
– Cations = positive charge
• Sedimentary rocks
– Formed from sediments near the earth’
earth’s surface, and in the presence of
water.
– Occur as layers (strata) and may contain fossils
– Have a relatively simple chemical and physical composition
– Can form physically, organically or chemically
The valency of common elements: Si +4, Al +3, Fe +2/3
Mg +2, Ca +2, K +1, Na +1……
.O –2
+1…….O
Common anions = S, SO4, Cl,
Cl, NO3, CO3
• Metamorphic rocks
– Maintain characteristics of the parent material
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Properties of minerals
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Chemical composition
Mineral structure
Crystal size
Crystal shape
Crystal perfection
Silicate structures
Processes of chemical
Weathering
Water disassociates
To
H20, H2, O, OH
Processes of physical weathering
• Internal stress – exfoliation of ‘permissive’
permissive’ rock
• Internal disintegration through heat and hydration at crystal
level
• Solar insolation
• Fire
• Crystal growth
– Ice
– Salt
• Biophysical force
Processes of biological weathering
Essentially biophysical or biochemical.
Includes:
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Bioturbation by soil organisms
Compaction by trampling
Boring
Root pressure
Rock weathering resistance – by property
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Grain size
Classification
The end products
of weathering are
particles in a size
range
Rock weathering resistance – by representation
---change
---change in properties
Environmental factors
• All weathering takes place in the presence
of water.
• Van’
Van’t Hoft’
Hoft’s Law states that rates of
chemical activity double for every rise in
temperature of 10 degrees centigrade.
• Therefore the primary control on
weathering is climate on a large
(global/regional) scale.
The Strakhov diagram of global zonal weathering
The Peltier diagram of weathering type/intensity
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Environmental factors
• However at the local scale factors such as
slope and drainage may take precedence
Albite (NaAlSi3O8)
Anaerobic conditions = Montmorillonite
3NaAlSi3O8 +Mg + 4H2O = 2Na0.5Al1.5Mg0.5Si4O10(OH)2 + 2Na + H4SiO4
Aerobic conditions = Kaolinite
• This is particularly important in the humid
tropics, where it leads to the formation of
soil catenas
2NaAlSi3O8 + 9H2O +2H = Al3Si2O5 (OH)4 + 2Na + 4H4SiO4
Aerobic conditions with rapid drainage = Gibbsite
NaAlSi3O8 +7H2O + H = Al(OH)3 + Na + 4H4SiO4
• Different conditions may lead to different
end products from a single mineral
Environmental factors
The Soil Catena
A primary control at all scales is the rate of
removal of weathered material. If the
material is not removed, the weathering
process will eventually cease.
At a landscape scale rates of removal control
the landform suite.
Summary
• Weathering is a complex universal process,
controlled by climate and rock type.
Energy sources
and states of
geomorphological
processes
• Chemical, physical and biological weathering
takes place simultaneously, though some
processes may dominate in a given environment.
• The end products are sediments of varying sizes.
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Fluids
• Fluids are liquids (e.g. water) and gases
(air is a mixture of gases)
• Ice is a bit of an odd case
• Fluids move because of…
– gravity (e.g. streamflow)
– pressure differences (airflow)
Processes of erosion,
transport and deposition
Viscosity
Fluid flows are...
• Viscosity is the capacity of a fluid to resist
changes of shape
• Laminar
• Turbulent
Boundary Layers
Flow Separation
• A boundary layer is the depth of fluid
affected by the contact of a fluid stream
with a stationary boundary
• Logarithmic velocity profile
– “Law of the Wall”
• also sometimes known
as vortex shedding
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Particle Entrainment
• Shear stress is what gets
grains going
Why aren’t the smallest grains
the easiest to entrain?
• They rest in the zero
velocity layer
• They are hidden by
larger grains
• They stick together
(especially clays)
Modes of transport
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Suspension
Saltation
Traction
(Reptation)
(Flotation)
(Solution)
The Hjulstrom Diagram – grain size,
velocity
and fluid density
Dissolved Load
• No threshold velocity
• Same concentration
throughout the fluid depth
• Not generally visible
• Concentration declines as
discharge increases
Sediment transport
• Four modes of sediment transport
• Shear stress entrains particles
• There is a threshold shear stress at which
motion starts
• Fine sand is the most susceptible to
entrainment
• Sediment transport is roughly proportional
to the cube of flow velocity
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Sorting and grading
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Sorting and grading can be defined by
statistical parameters, using phi units
instead of measurements
You will explore this further in a practical
exercise.
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