Weathering - Formation of Soil

Weathering

Weathering is the breakdown of rocks at the Earth’s surface, by the action of rainwater, extremes of temperature, and biological activity. It does not involve the removal of rock material.

or

Weathering is the soil formation process.

Soil

Soil is major product of weathering processes. Soil is  combination of particles of Rock, Minerals, and Organic matters. Soil contains nutrition to growth of plant.

Weathering Process

Weathering processes are occurred in following major three ways or three types;

1. Physical Weathering 
2. Chemical  Weathering
3. Biological Weathering

01.Physical (mechanical) weathering

Physical weathering is the geological process of rocks breaking apart without changing their chemical composition. 

or 

Physical weathering is also refer to other things in the environment breaking down, like soil and minerals. Pressure, warm temperatures, water and ice can cause physical weathering. 

Physical weathering processes

Physical weathering of rocks is a mechanical process which is brought about by a number of factors, such as: 

1. Temperature
2. Water
3. Wind

Temperature:

It causes breakdown of rocks in the following ways:

• Differential expansion and contraction of materials
• Exfoliation
• Frost action

Water:

Water causes weathering of rocks in the following ways:

(i) Rain: 

Natural water falling either in the form of rain drops or as hail storm on the surface of rocks with beating effect bring about abrasion of massive rocks into smaller particles.

(ii) Running water:

Rapidly flowing water rolls the heavy rock masses (rock boulders) along the bottom of stream and grinds them into finer particles. 

(iii) Wave action:

It is most active in sea shores. The water waves striking with great force on the rock surface break and grind the rock into pieces. 

(iv) Glacier formation:

At mountain tops, ice formation takes place in the winter season. When the summer approaches, ice starts melting and glaciers (huge sliding masses of ice) move downward on the slopes. In the glacier movement, the rocks are corroded and finally broken into sand particles 

Wind: 

Rapid stormy wind carrying suspended sand particles causes the abrasion  of exposed rock. rocks which were subjected to wind erosion.

Note: "Product of physical weathering is Granular Material or broken fragments"

02.Chemical weathering

Olivine

Chemical weathering is the weakening and subsequent disintegration of rock by chemical reactions. Chemical Weathering Processes as follows: 

1. Oxidation
2. Reduction:
3. Hydration
4. Dehydration
5. Solution
6. Hydrolysis
7. Carbonation

(i). Oxidation: 

Oxidation is the reaction of rock minerals with oxygen

Limonite

for example,

The iron in olivine (Fe₂SiO₄) is reduced and the iron in limonite (Fe₂O₃.H₂O) is oxidized. In addition, the release of silicon and hydration makes the mineral more susceptible to physical weathering.

   

(ii). Reduction:

It means removal of oxygen from minerals,

for example,

Fe₂O₃ → Fe₂O + O₂

Reduction takes place in the deep zone where oxygen is not available.

(iii). Hydration: 

Hydration is the absorption of water into the mineral structure.

A good example of hydration is the absorption of water by anhydrite, resulting in the formation of gypsum. Hydration expands volume and also results in rock deformation.

CaSO₄+H₂O → CaSO₄.2H₂O

(iv). Dehydration:

Dehydration is the removal of water from rock or mineral structures. 

A good example of dehydration is the removal of water from limonite, resulting in the formation of hematite.

(v). Hydrolysis:

Hydrolysis is a chemical reaction caused by water. Water changes the chemical composition and size of minerals in rock, making them less resistant to weathering. 

It is essentially an exchange of constituent parts between water and rock minerals. When water reacts with strong base it produces hydroxides. The soluble products of hydrolysis are usually removed by water. Sometimes soluble products may react with insoluble ones and form clays. Hydroxides in presence of CO₂ change to carbonates and bicarbonates. Water in ionized state acts as a weak acid on siliceous matter, e.g., 

KAlSi₃O₈ + HOH → HAlSi₃O₈ + KOH 

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

mineral is completely hydrolyzed, clay minerals and quartz are produced and such elements as K, Ca, or Na are released. 

A hydrolysis reaction of orthoclase (alkali feldspar), a common mineral found in igneous rock, yields kaolinite, silicic acid, and potassium. 

2KAISi₃O₈ + 2H⁺ + 9H₂0 → H₄Al2Si₂O₉ + 4H₄SiO₄ + 2K⁺

(orthoclase + water → kaolinite + silicic acid + potassium)

(vi). Solvent:

Solvent action of water helps in the weathering of rocks. It dissolves soluble minerals of rocks. Solution helps in the removal of weathered materials but total loss is negligible Solvent action is increased in presence of CO₂ and organic acids formed by decomposing dead organic remains of plants and animals. Sodium, potassium, calcium and magnesium are easily removed from rocks in dissolved state. 

(vii).Carbanation:

It occurs simultaneously with hydrolysis. In this process, CO₂ unites with water to produce carbonic acid which is a weak acid.

The carbonic acid reacts with hydroxides of soil forming minerals and forms insoluble carbonates. Sometimes it dissolves minerals and thus weakens the rock promoting thereby the weathering. Carbonation of hydroxides results in the formation of carbonates and bicarbonates,.

CO₂ + H₂O → H₂CO₃

(carbon dioxide + water → carbonic acid)

CaCO₃ + H₂CO₃ → Ca₂+ + 2HCO₃⁻

(calcite + carbonic acid → calcium + bicarbonate)

Note: Products of Chemical Weathering are,

• Aqueous Solution
• Weathered Rock
• Clay Mineral Metals ores
• Rounding of boulders (chemical exfoliation)

03.Biological Weathering:

Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes.

for example,

• Growing plant roots can exert stress or pressure on rock. Although the process is physical, the pressure is exerted by a biological process (i.e., growing roots). Biological processes can also produce chemical weathering, for example where plant roots or microorganisms produce organic acids which help to dissolve minerals.
• Microbial activity breaks down rock minerals by altering the rock’s chemical composition, thus making it more susceptible to weathering. One example of microbial activity is lichen; lichen is fungi and algae, living together in a symbiotic relationship. Fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering.
• Burrowing animals can move rock fragments to the surface, exposing the rock to more intense chemical, physical, and biological processes and so indirectly enhancing the process of rock weathering.

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