Weathering

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Weathering

Weathering is the process of breaking down of the rock materials through physical and chemical processes because of exposure at or near the Earth’s surface. Weathering occurs insitu or with no movement.

Types of weathering
1. Physical Weathering
2. Chemical Weathering
3. Biological Weathering

1. Physical weathering

Physical weathering is he process that causes disintegration of rocks without chemical changing. Physical weathering can occur due to temperature, pressure, frost action etc.
Types of Physical weathering
i)  Exfoliation
ii) Frost Action
iii) Crystal Growth

i) Exfoliation

It occurs due to two reasons
a) Temperature variation
In this case sheets of rocks split off due to differential expansion and contraction during heating and cooling regularly.

b) Unloading

Some times due to unloading of the overlying rocks the pressure of the igneous rocks decreases and due to which the rocks expand and a large scale fracture formed parallel to surface topography.

ii)  Frost Action

Water on freezing undergoes an increase in volume by about 10%. The water seeps down into fracture and due to cold climatic conditions begins to freeze. As freezing continues, the pressure exerted on the wall becomes more and more intense which results widening in the pre existing fractures and new fractures form. This is dominant mode of weathering.

iii) Crystal Growth

The soluble constitutes of the rock or minerals enter the through the fractures and joints along with water. With the evaporation of water the solution is precipitated to form crystals or crystalline aggregates and the grow. They exert large expansive stresses which help in breaking up some rocks.

2. Chemical weathering

It is a process of rocks of the crust by chemical decomposition brought about by atmospheric gases and moisture. 
The chemical weathering occurs in the following ways-

a) Oxidation
b) Hydration
c) Hydrolysis
d) Carbonation

a) Oxidation

Iron s a chief constituent of many minerals and rocks. The iron bearing minerals are especially prone to chemical weathering through the process of oxidation. 
Ferrous ion (Fe++) of mineral is oxidised to ferric ions (Fe+++) on exposure to air rich in moisture. Fe+++ ion is not stable and is further oxidised to a stable stable ferric hydroxides.
4Fe + H2O ---- 2Fe2O3
Fe2O3 + H2O ---- Fe2O3.H2O

Similarly Pyrite ( FeS2) may undergo oxidation and hydration forming sulphuric acid.
2FeS2+7O2 + 2H2O----2FeSO4 + 2H2SO4

b) Hydration

Chemical union of water molecule is termed as hydration.

CaSO4 + 2H2O ---- CaSO4.2H2O

c) Hydrolysis

The process of ionic exchange of water is called Hydrolysis.
KAlSi3O8 + H+ ---- HAlSi3O8 + K+
Which on dissilication converts to Alumina.

d) Carbonation

It is the process of weathering of rocks under the combined action of atmospheric Co2 and moisture ( H2o) which on combination form a mildy reacting carbonic acid. The acid formed exerts an especially corrosive action over a number of silicate bearing rocks and granite rocks weather into clay.
2KAlSi3O + 2H2O + Co2 ---- Al2Si2O5(OH)4 + K2Co3 + 4Sio2.

3. Biological weathering

It is of two types
a) Biophysical
b) Biochemical

a) Biophysical Weathering

Plant roots growing between joints and along minute fractures of rocks and minerals exert expansive forces tend to widen those openings and sometimes create new fractures.
On the other hand insects like earthworms, snails etc. loosen the soil cover and create suitable conditions for various external agencies to have their own action on the underlying rocks which ultimately lead to weathering.

b) Biochemical weathering

H+ ions are known to release at the roots of plants during their growth and metabolism. These ions are capable of replacing K+, Ca++, Mg++ ions from minerals surrounding the root system and make them available for use in plant growth. In this way the original minerals and rocks around the root system starts undergoing decomposition and disintegration.



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