Rocks and Weathering and Its Classification with Complete Processes | Soil Science Notes

 

ROCKS 

Definition:- The materials of earth’s crust are nothing but rocks and minerals. These are the out come of the geological forces or processes which operate on the surface and the interior of the earth to bring about changes to produce rocks.

“Rock may be defined as a hard mass of mineral matter comprising two or more rock forming minerals.”

WEATHERING

“Weathering is the process of disintegration and decomposition of rocks and minerals, brought about by physical and chemical means, respectively, leading to the formation of regolith.”

(Unconsolidated residues of the weathering rock on the earth’s surface or above the solid rocks).

 

Types of 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 massive rocks in to smaller pieces. In this weathering rock size will be reduced without any change in chemical composition of rock. The agents responsible for physical weathering are: the physical condition of rock, changes in temperature, action of water, action of wind and atmospheric electric phenomena.

 

(a). Temperature:- As a result of diurnal temperature changes the rocks heated during day and cooled during night. Different minerals in poly-mineral rocks have different co-efficient of expansion and thus heating and cooling of rocks results in differential expansion and contraction of rocks, which eventually produce cracks in rocks, thus facilitating mechanical break down.

 

(b). Water:- Water acts as disintegrating, transporting and depositing agent. Hence the disintegration of rocks is greater near the source of a river than at its mouth. In cold regions, the water in the cracks and crevices freezes into ice, which increases the volume of water by nine percent. If the space is not available to accommodate the increased volume, it exerts pressure on rock, which leads to widening of cracks in huge boulders and dislodging of mineral grains from smaller fragments. In colder regions, the moving glaciers cause great deal of cutting and crushing of bed rocks.

 

(c). Wind:- Wind has both erosive and transporting effect. Wind when laden with fine particles (fine sand, silt and clay) has a serious abrasive effect on the rocks.

 

(d). Atmospheric Electric:- This is also an important phenomenon, during rainy season, when lightening breaks up rocks and / or widens cracks.

 

(2). Chemical Weathering:- Chemical weathering is more complex in nature and involves the transformation of the original material in to some new compounds by bringing about alteration in minerals. The plants and animals also contribute directly or indirectly to chemical weathering as they produce O2, CO2, and certain acids that react with earth materials.

 

Various chemical processes are:

(a). Solution:- Water is a universal solvent. Most of the minerals are affected by solubilizing action of water, though by varying degrees. When the soluble substances are removed by the continuous action of flowing or percolating waters, the rock no longer remains solid and develops holes, rills or rough surface and ultimately decomposes.

 

NaCl + H₂O ----à Na⁺, Cl^-, H₂O

CO₂+H₂O ----à H₂CO₃

CaCO₃ +H₂CO₃ ----à Ca(HCO₃)₂

 

(b). Hydration:- It is the chemical combination of water molecules with a mineral to form a new mineral. Due to the hydration, the mineral swells, looses luster, becomes softer and tend to fall apart. This is a major chemical weathering process in secondary minerals.

 

2Fe₂O₃ (Haematite) + 3H₂O --------à 2Fe₂O₃. 3 H₂O (Limonite)

Al₂O₃ (Bauxite) + 3H₂O -------à Al₂O₃. 3H₂O (Gibbsite)

Ca₂SO₄ (Anhydrite) + 2H₂O ------à Ca₂SO₄. 2H₂O (Gypsum)

 

(c). 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.

 

KAlSi₃O₈ (Orthoclase) + HOH -----à HAlSi₃O₈ (Acid silicate clay) + KOH

 

(d). Oxidation:- The process of combination of oxygen is known as oxidation. Moisture aids in oxidation process and this process mostly occur after hydrolysis.

 

(e). Reduction:- The process of removal or loss of oxygen 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.

 

(f). Carbonation:-  Combination of carbon dioxide with any base . This effectively decompose the minerals of rocks and organic matter accelerates this due to more CO₂ production. 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.

 

(3). Biological Weathering:- Unlike physical and chemical weathering , the biological or living agents are responsible for both decomposition and disintegration of rocks and minerals. Man, animals, higher plants, earth worms, termites and micro-organisms are responsible for biological weathering.

 

(a). FAUNA:-  

• The disintegration of rocks by cutting action of hills and rocks to build dams, roads etc., by humans
• The animals, insects live in rocks make burrows and holes in rocks leading to weakening of rocks. Their excreta and other materials lead to chemical decomposition of rocks.
• Ants and termites built galleries and passages and carries material from lower to upper horizons. Water percolate through the channels and galleries and help in weathering of materials in subsurface horizons. They secrete some acids like formic acid, which aid in decomposition of rocks. 

(b). FLORA :-  

• The roots of the plants penetrate into small cracks of rocks. As they grow they exert disruptive force due to which even hard rocks break into pieces.
• Some roots grow deep in to soil and make open channels for percolation of water in to deeper layers.
• Roots produce acids, which have solubilizing action of constituents of rocks making them weak.
• The decomposition of organic matter due to microbial activity also produces several products which play a role in chemical weathering.

The physical, chemical and biological weathering processes occur simultaneously in nature.

Thank You

Vikas Kashyap:)

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