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Weathering

The destructive process or processes bywhich earthy and rocky materials onexposure to atmospheric agents at ornear the Earths surface

changed in color, texture, composition,firmness or form, with little or notransport of the loosened or altered

material

The physical disintegration and chemicaldecomposition of rock that produces an

in-situmantle of weathered materials.

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Why weathering?

Weathering occurs because when rocks areexposed at, or located close to, the earths

surface

they are not in equilibrium with the low

temperatures, low pressures, air and waterpresent, for they were originally formed athigh temperature, high pressure and in theabsence of air and water.

As a result of the disequilibrium, rocksundergo various textural, structural,mineralogical and chemical changes to be

more in equilibrium with the newly imposedh sico-chemical conditions.

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Why weathering?

Most weathering takes place at theearth's surface, though it may alsooccur at considerable depth, as in well-jointed rocks which permit easy

penetration of atmospheric oxygen andcirculating surface waters.

Weathering is the initial stage in the

process of denudation and affects rockin situ;no transportation beinginvolved.

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Physical or Mechanical Weathering

Disintegration of rock into smaller fragments

with little or no change of its' chemicalcomposition.

1. Pressure release due to erosional unloading- leading to development of sheeting joints over

large intrusive igneous rock bodies, or theopening-up of tight or closed discontinuityplanes (as bedding, joint, fault, foliation,cleavage and other planes).

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Physical Weathering

Pressure release

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Physical or Mechanical Weathering

2. Growth of foreign crystals in rock - as in frostwedging when water freezes in cracks in rocks(with up to 9% volume expansion) or during theformation of hydrates (as gypsum from anhydratewith up to 140% volume increase).

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Physical or Mechanical Weathering

3. Thermal expansion and contraction - due

to alternate heating and cooling as very

hot days and cool nights or forest fires

and thunderstorms.

Not confirmed experimentally.

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Physical or Mechanical Weathering

4. Biological agents - as wedging action of

roots and burrowing activities of

earthworms and other organisms.

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Chemical Weathering

Decomposition of rock with a change in chemical

composition. Chemical reactions usuallyexothermic ones with new compounds being ofgreater volume and lower density than the originalminerals.

Water and air are main agents in processes ofchemical weathering which occur via severalreactions:

1. Hydration - absoprtion of water by anhydrous

minerals to form hydrates as anhydrate togypsum. Dehydration can also sometimes occur.

CaSO4 + 2H2O CaSO4.2H2O

(Anhydrite) (Gypsum)

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Chemical Weathering

2. Solution - minerals dissolve in water with no change incomposition (congrouent solution) or change in

composition (incongruent solution).

KAlSi3O8 + 4H2O + 4H+ K+ + Al3+ + 3H4SiO4 (Congruent)

(Microcline) (Silic Acid)

2KAlSi3O8 + 9H2O + 2H+ 2K+ + Al2Si2O5(OH)4

(Inongruent) (Kaolinite)

+ 4H4SiO4

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Chemical Weathering3. Carbonation - Reaction of carbonic acid

(when carbon dioxide is dissolved inwater) with minerals and rocks.Particularly important in dissolution ofcarbonate rocks, as limestones and

marbles. H2O + CO2 H2CO3 (Carbonic

Acid)

CaCO3

+ H2CO

3 Ca2+ +

2HCO3-

Very important in weathering ofcarbonate rocks Limestone & Marble

Gives rise to Karst landscape

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KARST LANDSCAPE

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Chemical Weathering

4. Oxidation - Release of electrons and change in valency.

Alteration of pyrite to goethite.

4FeS2 + 15O2 + 10H2O 4FeO.OH + 8H2SO4(Pyrite) (Goethite) (Sulfuric acid)

2FeS2 + 7O2 + 2H2O 2FeSO4 + 2H2SO4

4FeSO4 + O2 + 2H2SO4 2Fe2(SO4)3 + 2H2O

2Fe2

(SO4

)3

+ 8H2

O 4FeO.OH + 6H2

SO4

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Chemical Weathering

5. Hydrolysis - Reaction of hydrogen and hydroxyl ionswith minerals to form a weak acid and base. Importantfor breakdown of silicates, particularly feldspars.

Mg2SiO4 + 4H+ + 4OH- 2Mg+ + 4OH- + H4SiO4

(Olivine) (ions in solution) (Silic acid)

KAlSi3O8 + HOH HAlSi3O8 + KOH

(Microcline) (Al silicate)

2HAlSi3O8 + HOH

Al2Si2O5(OH)4 + 4SiO2(Al silicate) (Kaolinite)

HAlSi3O8 + HOH Al(OH)3 + 3SiO2(Al silicate)