Concept of weathering and its types

Weathering

The disintegration and decomposition of rocks and minerals are called weathering.

The process of transformation of solid rocks into soils is known as weathering.

Types of weathering

1. Physical weathering

2. Chemical weathering

3. Biological Weathering

1. Physical weathering:

The rocks are disintegrated and are broken down to comparatively smaller pieces, without producing any new substances.

Factors of physical weathering:

1. Physical condition of rocks: Coarse textured (porous) sandstone weather more readily than a fine-textured (almost solid) basalt. Unconsolidated volcanic ash weather quickly as compared to unconsolidated coarse deposits such as gravels

2. Action of Temperature:

The variations in temperature exert a great influence on the disintegration of rocks.

• 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 weaken the surface of the rock.
• The minerals within the rock also vary in their rate of expansion and contraction
•      The expansion of quartz is twice as feldspar

• Dark-colored rocks are subjected to fast changes in temperature as compared to light-colored rocks
• The differential expansion of minerals in a rock surface generates stress between the heated surface and cooled unexpanded parts resulting in fragmentation of rocks.
• This process causes the surface layer to peel off from the parent mass and the rock ultimately disintegrates. This process is called Exfoliation

3. Action of water:

Water acts as a disintegrating, transporting and depositing agent

i) Fragmentation and transport:

•   Water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean
•   Moving water has great cutting and carrying force.
• Transporting power of water varies. It is estimated that the greater the speed of water more is the transporting power and carrying capacity

4. Action of freezing:

Frost is much more effective than heat in producing physical weathering

•  In cold regions, the water in the cracks and crevices freezes into ice, and the volume increases to one-tenth
•  As the freezing starts from the top there is no possibility of its upward expansion. Hence, the increase in volume creates enormous outward pressure which breaks apart the rocks 

5. Alternate wetting and Drying:

Some natural substances increase considerably in volume on wetting and shrink on drying. (e.g.) smectite, montmorillonite

6. Action of glaciers:

• In cold regions, when snow falls, it accumulates and changes into ice sheet.
• These big glaciers start moving owing to the change in temperature and/or gradient.
• On moving, these exert tremendous pressure over the rock on which they pass and carry the loose materials

7. Action of wind:

• The wind has an erosive and transporting effect. Often when the wind is laden with fine material viz., fine sand, silt, or clay particles, it has a serious abrasive effect and the sand-laden winds itch the rocks and ultimately breaks down under its force
• The dust storm may transport tons of material from one place to another. The shifting of soil causes serious wind erosion problem and may render cultivated land degraded (e.g.) Rajasthan deserts

(2) Chemical weathering:

Decomposition of rocks and minerals takes place and producing new substances is called chemical weathering.

Chemical weathering takes place mainly at the surface of rock minerals with the disappearance of certain minerals and the formation of secondary products. This is called chemical transformation

The rate of chemical reactions increases with dissolved carbon dioxide and other solute in water, and with increases in temperature. The principal agents of chemical weathering are described below.

(a) Solution: Some substances (NaCl) present in the rock are readily soluble in water. When the soluble substances are removed by the continuous action of water, the rock no longer remains solid and falls to pieces very soon.

(b) Hydration:

Hydration means a chemical combination of molecules with a particular mineral. Soil forming minerals occurring in rocks undergo hydration when exposed to the humid conditions. e.g.

2Fe2O3 +                    3HOH →                              2Fe2O3 3H2O

Hematite (Red)            Water                          Limonite (Yellow)

Due to this reaction the increase of the mineral in volume and become soft and more readily weatherable

(c) Hydrolysis:

It is one of the most important processes in chemical weathering. Hydrolysis depends on the partial dissociation of water into H-ions and OH-ions. Increases in H-ion concentration resulting in the accelerated hydrolytic action of water. Water thus acts as a weak acid on silicate minerals, e.g.

KAlSi3O8                      + HOH →                                         HAlSi3O8 + KOH

Orthoclase                         Water                                            Acid silicate Potassium

                                                                                (dissociated) Clay hydroxide

2HAlSi3O8 + 8 HOH →                                       Al2O3. 3H2O + 6H2SiO3

     Al-oxide Clay                                                        Bauxite          Silicic acid

d) Oxidation

Oxidation means addition of oxygen to minerals. Oxidation is more active in the presence of

moisture and results in hydrated oxides. Soil forming minerals containing iron, manganese, etc. are more subjected to oxidation

(e) Reduction:

This means the removal of oxygen. Under the condition of excess water (less or no oxygen), reduction takes place e.g.

2Fe2O3                               → O2 + 4FeO

Ferric oxide                           Oxygen Ferrous

(Hematite) oxide

(f) Carbonation: Carbon dioxide dissolved in water, it forms carbonic acid:

H2O +   CO2            = H2CO3

Water    Carbon            Carbonic

              dioxide                 acid

The carbonic acid or carbonated water attacks many rocks and minerals and brings them into solution. Limestone, which is insoluble in water, is dissolved readily by carbonated water and is thus, removed from the parent rock.

3. Biological Weathering

Unlike physical and chemical weathering, the biological or living agents are responsible for both the decomposition and disintegration of rocks and minerals.

1. Man and Animals:

• The action of man in the disintegration of rocks is well known as he cuts rocks to build dams, channels and construct roads and buildings.
• A large number of animals, birds, insects, and worms, by their activities they make holes in them and thus aids for weathering.
•  In tropical and subtropical regions, ants and termites build galleries and passages and carry materials from lower to the upper surfaces and excrete acids.

2. Higher Plants and Roots:

The roots of trees and other plants penetrate into the joints and crevices of the rocks. As they grew, they exert a great disruptive force and the hard rock may break apart. (e.g.) pipal tree growing on walls/ rocks

3. Microorganisms:

•  In the early stages of mineral decomposition and soil formation, the lower forms of plants and animals like, mosses, bacteria and fungi, and actinomycetes play an important role. They extract nutrients from the rock and N from the air and live with a small quantity of water. In due course of time, the soil develops under the cluster of these micro-organisms.
• This organism closely associated with the decay of plant and animal remains and thus liberates nutrients for the use of next-generation plants and also produces CO2 and organic compounds that aid in mineral decomposition.