🦀 Weathering

• Weathering is a geological process and essentially destructive in nature which leading to the formation of Regolith.
• Rock containing complex minerals weathers more easily. Basic Igneous rocks weather more than acid igneous rocks. Sedimentary rocks are more resistant to weathering than igneous and metamorphic rocks. Limestone weathers more easily than sandstone.
• There is the mechanical breakdown of the rocks to fragments, which constitutes the physical or mechanical weathering of rocks. Then chemical changes take place in the material. Finally there is addition of organic matter, the invasion of material by various organisms and thus rock is changed into soil.
• The mechanical breakdown of the rocks is referred to as weathering of rocks and the chemical and biological change taking place may be taken to constitute the “Formation of Soil”.

Parent material

• It is the regolith or at least it’s upper portion. May be defined as the unconsolidated and more or less chemically weathered mineral material from which soil are developed.
• Depending up on the agents taking part in weathering processes, it is classified into three types.

  

Physical weathering

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

• Physical condition of rocks
  • The permeability of rocks is the most important single factor. Coarse textured (porous) sand stone weather more readily than a fine textured (almost solid) basalt. Unconsolidated volcanic ash weather quickly as compared to unconsolidated coarse deposits such as gravels.
• Action of Temperature
  • The variations in temperature exert 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 weakens the surface of the rock and crumbles it because the rocks do not conduct heat easily.
  • The minerals within the rock also vary in their rate of expansion and contraction
  • The cubical expansion of quartz is twice as feldspar
  • Dark coloured rocks are subjected to fast changes in temperature as compared to light coloured rock
  • The differential expansion of minerals in a rock surface generates stress between the heated surface and cooled un-expanded 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.
• Action of Water
  • Water acts as a disintegrating, transporting and depositing agent.
  • Fragmentation and transport: Water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean
  • Moving water has the great cutting and carrying force
  • It forms gullies and ravines and carries with the suspended soil material of variable sizes.
• Action of freezing
  • Frost is much more effective than heat in producing physical weathering
  • When water freezes, its volume increases by about 9% and the force exerted is 150 tons per square foot (1 tpn = 1016 kg : 1 tonne = 1000kg).
  • As the freezing starts from the top there is no possibility of its upward expansion. Hence, the increase in volume creates enormous out ward pressure which breaks apart the rocks.
• Alternate wetting and Drying
  • Some natural substances increase considerably in volume on wetting and shrink on drying. (e.g.) smectite, montmorilonite
  • During dry summer/ dry weather – these clays shrink considerably forming deep cracks or wide cracks.
  • On subsequent wetting, it swells.
  • This alternate swelling and shrinking/ wetting or drying of clay enriched rocks make them loose and eventually breaks.
• Action of glaciers
  • In cold regions, when snow falls, it accumulates and change into an 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
  • These materials get deposited on reaching the warmer regions, where its movement stops with the melting of ice.
• Action of wind
  • 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 as degraded (e.g.) Rajasthan deserts.
• Atmospheric electrical phenomenon
  • It is an important factor causing break down during rainy season and lightning breaks up rocks and or widens cracks.

    

• Soils in early stage of development, desert soils and the soils of arctic and alpine regions are formed mainly due to the result of physical weathering. Soils evolved mainly as a result of physical weathering are called “Skeletal soils”.

Chemical Weathering

• 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.

Feldspar + Water ⇋ Clay mineral + Soluble Cations and Anions

• Chemical weathering becomes more effective as the surface area of the rock increases. Since the chemical reactions occur largely on the surface of the rocks, therefore the smaller the fragments, the greater the surface area per unit volume available for reaction. The effectiveness of chemical weathering is closely related to the mineral composition of rocks. (e.g) quartz responds far slowly to the chemical attack than olivine or pyroxene.

Chemical Processes of weathering

1. Hydration

• Chemical combination of water molecules with a particular substance or mineral leading to a change in structure.
• Soil forming minerals in rocks do not contain any water and they undergo hydration when exposed to humid conditions. Upon hydration there is swelling and increase in volume of minerals. The minerals lose their luster and become soft.
• It is one of the most common processes in nature and works with secondary minerals, such as aluminium oxide and iron oxide minerals and gypsum.

2. Hydrolysis

• Most important process in chemical weathering. It is due to the dissociation of H₂O into H⁺ and OH^- ions which chemically combine with minerals and bring about changes, such as exchange, decomposition of crystalline structure and formation of new compounds.
• Water acts as a weak acid on silicate minerals.

3. 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.
• The action is considerably increased when the water is acidified by the dissolution of organic and inorganic acids. E.g. Halites, NaCl.

4. Carbonation

• Carbon di-oxide when dissolved in water it forms carbonic acid.

2H₂O + CO₂ → H₂CO₃

• This carbonic acid attacks many rocks and minerals and brings them into solution. The carbonated water has an etching effect up on some rocks, especially lime stone. The removal of cement that holds sand particles together leads to their disintegration.

• The soluble bicarbonate is leached to the lower layers where CO₂ is liberated and the insoluble CaC0₃ is precipitated.
• The precipitation is around the CaCO₃ granules formed earlier, which serve as nuclei and promote further accretions and growth in the form of nodules. lt is common phenomenon in the tropic and the lime nodules are given the name of ‘Kankar’.
• Kankar nodules are found in most red soils and black cotton soils in south India, at 1 or 2 ft. from the surface in red soils and deeper at 3-4 ft. in black soils. Kankar is deposited in thick layers, is used as lime for construction of buildings.

5. Oxidation

• The process of addition and combination of oxygen to minerals.
• The absorption is usually from O₂ dissolved in soil water and that present in atmosphere.
• The oxidation is more active in the presence of moisture and results in hydrated oxides. E.g. minerals containing Fe and Mg.

6. Reduction

• The process of removal of oxygen and is the reverse of oxidation and is equally important in changing soil colour to grey, blue or green as ferric iron is converted to ferrous iron compounds. Under the conditions of excess water or water logged condition (less or no oxygen), reduction takes place.

2Fe₂O₃ (Haematite) - O₂ → 4FeO (Ferrous Oxide): Reduced Form

• In conclusion, during chemical weathering igneous and metamorphic rocks can be regarded as involving destruction of primary minerals and the production of secondary minerals. In sedimentary rocks, which is made up of primary and secondary minerals, weathering acts initially to destroy any relatively weak bonding agents (FeO) and the particles are freed and can be individually subjected to weathering.

Biological Weathering

• Unlike physical and chemical weathering, the biological or living agents are responsible for both decomposition (physical) and disintegration (chemical) of rocks and minerals. The biological life is mainly controlled largely by the prevailing environment.

1. Man and Animals

• The action of man in disintegration of rocks is well known as he cuts rocks to build dams, channels and construct roads and buildings. All these activities result in increasing the surface area of the rocks for attack of chemical agents and accelerate the process of rock decomposition.
• A large number of animals, birds, insects and worms, by their activities they make holes in them and thus aid for weathering.
• In tropical and sub-tropical regions, ants and termites build galleries and passages and carry materials from lower to upper surface and excrete acids. The oxygen and water with many dissolved substances, reach every part of the rock through the cracks, holes and galleries, and thus brings about speedy disintegration.
• Rabbits, by burrowing in to the ground, destroy soft rocks. Moles, ants and bodies of the dead animals, provides substances which react with minerals and aid in decaying process.
• The earthworms pass the soil through the alimentary canal and thus brings about physical and chemical changes in soil material.

2. Higher Plants and Roots

• The roots of trees and other plants penetrates into the joints and crevices of the rocks. As they grew, they exert a great disruptive force and the hard rock may broken apart.
• E.g. pipal tree growing on walls/ rocks.
• The grass roots form a sponge like mass, prevents erosion and conserve moisture and thus allowing moisture and air to enter in to the rock for further action. Some roots penetrate deep into the soil and may open some sort of drainage channel.
• The roots running in crevices in lime stone and marble produces acids. These acids have a solvent action on carbonates. The dead roots and plant residues decompose and produce carbon dioxide which is of great importance in weathering.

3. Micro-organisms

• In 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 air and live with a small quantity of water.
• In due course of time, the soil develops under the cluster of these micro-organisms. These organisms closely associated with the decay of plant and animal remains and thus liberate nutrients for the use of next generation plants and also produces CO₂ and organic compounds which aid in mineral decomposition.