Weathering of Rocks

Weathering of rocks are Geological process of disintegration and decomposition of rocks by means of physical, chemical and biological agencies.

Weathering of rocks are Geological process of disintegration and decomposition of rocks by means of physical, chemical and biological agencies. 

Types of weathering -

Weathering can be classified in 3 groups on the basis of factors responsible for weathering -- 

1. Physical/mechanical weathering (disintegration)

2.Chemical weathering (decomposition)

3.Biological weathering (disintegration and decomposition)

 

        1.   Physical weathering -

Physical weathering is a mechanical process causing disintegration of consolidated rocks in to smaller pieces without any chemical changes.

Physical condition of rocks -

• Permeability of rocks determines the rate of weathering
• Coarse textured sand stone > fine textured basalt
• Fine textured volcanic ash > coarse textured gravel.

 

Temperature -

• Change in day and night temperature.
• Different co-efficient expansion and contraction :- quartz > feldspar
• Dark colored rocks (Biotite) are more weatherable than the white colored (Muscovite) rocks.
• Exfoliation is peel of of surface layer of rock due to temperature differences in upper and lower layer of rocks.

 

Water -

• water has more pronounced and widespread effect on weathering of rocks.
• Act as disintegration, transportation and deposition of sand, gravel and stones determines by the speed of water.
• ↑speed, ↑weathering.
• In cold region Deposited water freeze in the cracks of rocks. Expand in size when water freezes inside the the cracks and cause weathering.

 

Wind -

• Wind has both erosive and transporting effect on sand, silt and clay.

 

Atmospheric electric phenomena -

• Lightening break rocks or widened the cracks.

 

It  causes only disintegration of rocks.

    2.    Chemical weathering -

• Transform the original mineral to form some new compounds.
• Carry out at surface of rocks.
• Pronounced in humid tropical regions.
• Less in arid region due to absence of water.
• Water is most important factor for  chemical weathering of rocks.

 Solution -

• When something dissolve in the water or acid called solution
• Solubility action ↑ when increase in CO₂, organic and inorganic acid or salts dissolved in water.
• Many rocks when come in contact with water makes solution and disintegrate.

 

NaCl (Salt in rocks) + H₂O ---- NaOH + HCl

CO₂ (Decomposition of OM) + H₂O ---- H₂CO₃

CaCO₃ less soft + H₂CO₃ ---- Ca(HCO₃)₂ more soft

Hydration -

• Chemical addition of water in any substances called hydration.
•  Minerals absorbs water and expands in volume.
• Water work as mineral swelling, loosening and softening.
• Major weathering process in secondary minerals

 

Fe2O₃ (Haematite) + H₂O ---- Fe₂O₃.H₂O (Limonite)

Al₂O₃ (Bauxite) +H₂O ----Al₂O₃.H₂O (Gibbsite)

CaSO₄ (Anhydrite) + 2H₂O ---- CaSO₄.2H₂O (Gypsum)

 

Hydrolysis -

• Hydrolysis is breakdown of water into H+ and OH-
• It results into complete disintegration of minerals.
• Major process of primary minerals.

 

KAlSi₃O₈ (Orthoclase) + H OH ---- HAlSi₃O₈ (Acid silicate clay) + KOH

2HAlSi₃O₈ + 14HOH ---- Al₂O₃.3H₂O + 6H₄SiO₄ (Silicic Acid)

 

                                                                                Oxidation -

• It occurs in excess oxygen condition at upper surface.
• Addition of oxygen and loss of electron
• Moisture adds in oxidation process
• Creates imbalance in iron containing minerals i.e. pyroxenes, amphiboles and biotite.

 

3FeO + 3O₂ ---- 3Fe₂O₃

Fe²⁺ (size ↓) ---- Fe³⁺ (electrical charge ↑)+ ^e-

 

Reduction -

• Usually occurs in water logged or stagnant water condition.
• Loss of oxygen or addition of electron.

 

3Fe₂O3----3FeO + 3O₂

Fe^{3+ +} (↓) ^e- ---- Fe²⁺ (↑) e-

Carbonation -

• Combination of CO₂ with any base.
• Involve in feldspar and carbonate minerals or rocks.

 

2KOH + CO2 ---- K₂CO₃ + H₂O

K₂CO₃ + H₂O + CO₂ ---- 2KHCO₃

It may be concluded that chemical weathering involves destruction of rocks and minerals in to simple new compounds.

        3.    Biological weathering -

• Responsible for both disintegration and decomposition unlike chemical and physical weathering.

 

Fauna -

• Human creates infrastructure like build road and dams
• Insects make holes to rock
• Termite build termateria
• Earthworm snacks

 

Flora -

• Plant roots penetrate into rocks.
• Form channel.
• Produce acid by roots.

 Physical, chemical and biological weathering occurs simultaneously in nature.

UPSC Agriculture Optional Previous Year Questions Name the important minerals found in different rocks. Describe chemical weathering of soil. 2012

Lecture of rocks and their weathering