Lecture: 5 WEATHERING OF ROCKS AND MINERALS
Lecture:
5 WEATHERING OF ROCKS AND MINERALS
Weathering:-
The breakdown of rocks and mineral by physical, chemical and biological agents.
or
A process of disintegration and decomposition of rocks and minerals by physical agents and chemical processes, leading to the formation of Regolith. It is basically a combination of transformation and synthesis.
Type of weathering:-
A. Physical weathering
B. Chemical weathering
C. Biological weathering
A. Physical weathering :- means disintegration
It is a mechanical process, causing disintegration or breakdown of consolidated massive rocks into smaller pieces without producing any new substances by physical agents i.e. Temperature, water, ice, wind, plant and animals.
I. Action of Temperature
During day time, the rocks get heated up by the sun and expand. At night, the temperature falls and the rocks get cooled and contract. This alternate expansion and contraction weakens the surface of the rock and peel off from the parent mass. This process is called Exfoliation or Onion type weathering.
II. Action of Water
⇨ Fragmentation and transport:- Water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean
⇒ Action of freezing:- In cold regions, the water in the cracks and crevices freezes into ice and the volume increases Upto 9%. The force exerted by the freezing of water is equivalent to about 1465 tonnes (Megagrams)/m2 or 150 tonnes/ft2.
⇒ Alternate wetting and Drying:- Some natural minerals increase in volume on wetting and shrink on drying. (e.g.) smectite, montmorilonite
⇨ Action of glaciers:- On moving, these exert tremendous pressure over the rock on which they pass and carry the loose materials
III. Action of wind:-
Wind has an erosive and transporting effect. The dust storm may transportons of material from one
It is an important factor causing break dplace to another.
IV. Atmospheric electrical phenomenonown
It is an important factor causing break down during rainy season and lightning breaksup rocks and or widens cracks
B. Chemical Weathering: means Decomposition
Decomposition of rocks and minerals by various chemical processes is called chemical
weathering. It is the most important process for soil formation. Chemical weathering takes
place mainly at the surface of rocks and minerals with disappearance of certain minerals
and the formation of secondary products (new materials). This is called chemical
transformation. Generally dominant in tropical region (high temperature and high rainfall).
The chemical weathering has great role in conversion of primary minerals to secondary
minerals.
Feldspar + water clay mineral + soluble cations and anions
1. Solution
Some substances present in the rocks are directly soluble in water. The soluble substances
are removed by the continuous action of water and the rock no longer remains solid and
form holes, rills or rough surface and ultimately falls into pieces or decomposes.
Ex- Halite
2. Hydration
It is the chemical combination of water molecules with a mineral to form a new mineral.
Owing to hydration, the mineral swells, looses luster, becomes softer and tend to fall apart.
This is a major chemical weathering process in secondary minerals
Ex. Formation of Limonite (Fe2O3.3H2O) from Hematite, Gypsum from CaSO4, Bauxite
(Al2O3.2H2O) from Al-oxide
3. Hydrolysis
Hydrolysis is one of the most important processes in chemical weathering and results in
complete disintegration or drastic modification (in structure and composition) of minerals.
It is a major chemical, process in primary minerals.
Hydrolysis involves the partial dissociation of water in to H+ and OH- ions, which
combine with minerals and bring about changes such as exchange, decomposition of
crystalline structure and formation of new minerals. It is also called double decomposition
process.
KAlSi3O8 + HOH→ HAlSi3O8 + 8 HOH
Orthoclase Acid silicate clay
HAlSi3O8 + KOH→ Al2O3. 3 H2O + 6 H2SiO3
Silicic acid
4. Oxidation:-
The process of combination of oxygen or losses of H+ (electron) is known
as oxidation. Moisture aids in oxidation process and this process mostly occur after
hydrolysis. Oxidation of iron reduces its size and increased the electrical charge and
creates electrical and structural imbalance in iron containing minerals such as pyroxenes,
amphiboles and biotite. This process occur where Oxygen supply is high and BOD low.
Ex. Formation of hematite (Fe2O3) from FeO
5. Reduction:-
The process of removal or loss of oxygen or gain of H+ (electron) is called reduction. It
occurs in submerged areas. In reduction reaction iron reduces to highly soluble ferrous
form. Oxidation and reduction are more common in minerals with Fe, Mn and S.
Ex. formation of orange and yellow colour mineral called Lepidocrocite.
6. Carbonation:-
Combination of carbon dioxide with water it forms carbonic acid. This carbonic acid
attacks many rocks and minerals and brings them into solution. The carbonic acid or
carbonated water has an etching effect on rocks like limestone or other rocks containing
calcium or magnesium carbonates as cementing materials leading to their breakage and
formation of new minerals
CO2 + H2O→ H2CO3
C. Biological Weathering
Unlike physical and chemical weathering, the biological or living agents are responsible
for both decomposition and disintegration of rocks and minerals.
➣ Man and Animals
o The action of man in disintegration of rocks is well known as he cuts rocks to
build dams, channels and construct roads and buildings.
o A large number of animals, birds, insects and worms, by their activities they
make holes in them and thus aid for weathering.
➣ Higher Plants and Roots
o The roots of trees and other plants penetrate into the joints and crevices of the rocks resulted weathering. The dead roots and plant residues decompose and produce carbon dioxide which is of great importance in weathering.
➽ Micro- organisms
These organisms closely associated with the decay of plant and animal remains and thus liberate CO2 and organic compounds which aid in mineral decomposition.
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