Weathering

weathering

Weathering:- a process of disintegration and decomposition of rocks and minerals which are brought about by physical agents and chemical processes, leading to the formation of regolith(unconsolidated residues of the weathering rock on the weathering rock on the earth’s surface or above the solid rocks).

                                                          OR

The process by which the earth’s crust or lithosphere is broken down by the activities of the atmosphere, with the aid of the hydrosphere and biosphere and biosphere.

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 is developed.

Weathering:-

Two basic processes

1). Physical/mechanical (disintegration)

2). Chemical (decomposition)

In addition, another process: biological and all these processes are work hand in hand. 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 substance.

1.    Physical condition of soil – the permeability of rocks is the most important 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.

2.    Action of temperature – the variation in temperature exerts great influence on the disintegration of rocks.

Ø 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.

Ø This alternate expansion and contraction weakens the surface of the rock and crumbiles it because the rocks do not conduct heat easily.

Ø The minerals within the rock also vary in their rate of expansion and contraction

·       The cubical expansion of quartz in twice as feldspar

·       Dark colored rocks as subjected to fast changes in temperature as compared to light colored rocks

Ø 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.

Ø This process cause 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: water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean.

Ø Moving water has the great cutting and carrying force.

Ø It forms gullies and ravines and carries with the suspended soil material of variable sizes.

Ø Transportation power of water varies. It is estimated that the transporting power of stream varies as the sixth power of its velocity i.e. the greater the speed of water, more is the transporting power and carrying capacity.

Speed/sec                                       carrying capacity

15 cm                                              fine sand

30 cm                                               gravel

1.2 cm                                              stones (1 kg)

9.0 cm                 boulders (several tons)

The disintegration is greater near the source of river than its mouth

ii.            Action of freezing: frost is much more effective than heat in producing physical weathering.

Ø During day summer/ dry weather – these clays shrink considerably forming deep cracks or wide cracks.

Ø On subsequent wetting, it swells.

Ø This alternate swelling and shrinking/ wetting or  drying of clay enriched rocks make them loose and eventually breaks

iii.            Action of glaciers :

Ø In cold regions, when snow falls, it accumulates and changes into a ice sheet.

Ø These big glaciers start moving owing to the change in temperature and /or gradient.

Ø On moving, these exert tremendous pressure over the rock on which they pass and carry the loose materials

Ø These materials get deposited on reaching the warmer regions, where its movement stops with the melting of ice

4.    Action of wind :-

Ø 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.

Ø The dust storm any transport tons of material from one place to another. The shifting of soil cause serious wind erosion problem and may render cultivated land as degraded e.g Rajasthan deserts.

5.    Atmospheric electrical phenomenon :-

It is an important factor causing break down during rainy season and lighting breaks up rocks and or widens cracks.

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 product (new materials). This is called chemical transformation.

Feldspar + water → clay minerals + soluble cation and anion

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 tan olivine or pyroxene.

Average mineralogical composition (%)

composition	granite	basalt	shale	s.stone	l.stone
Feldspar	52.4	46.2	30.0	11.5	-
Quartz	31.3	-	2.3	66.8	-
Pyrox-amphi	-	44.5	-	-	-
FeO mineral	2.0	9.3	10.5	2.0	-
Clay mineral	14.3	-	25.0	6.6	24.0
carbonates	-	-	5.7	11.1	76.0

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. Up on 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 aluminum oxide and iron oxide minerals and gypsum.

Example:

a)    2Fe₂O₃ + 3HOH →      2Fe₂O₃.3H₂O

(haematite) (red)                  (limonite) (yellow)

b)   Al₂O₃ + 3HOH               →      Al₂O₃.3H₂O

(bauxite)                           (hyd. Aluminum oxide)      

c)    CaSO₄ + 2H₂O              →      CaSO₄.2H₂O

(Anhydrite)                           (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 being about changes, such as exchange, decomposition of crystalline structure and formation of new compounds. Water acts as a weak acid on silicate minerals.

KAlSi₃O₈ + 8 HOH    →      HAlSi₃O₈ + KOH

(orthoclase)                      (acid silt clay)

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

(Recombination)              (hyd. Alum.oxide) (silicic acid)

This reaction is important due to two reasons:

Ø Caly,bases and silicic acid – the substance formed in these reaction – are available to plants

Ø Water often containing CO₂ (absorbed from atmosphere), reacts with the minerals directly to produce insoluble clay minerals, positively charged metal ions (Ca⁺⁺,Mg⁺⁺,Na⁺,K⁺) and negatively charged ions (OH^-,HCO^-₃) and some soluble silica – all these ions are made available for plant growth.

3. solution – some substance 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, rill 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

NaCl + H₂O → Na^+’Cl^-,H₂O (Dissolved ions with water)

4.    Carbonation: carbon dioxide when dissolved in water it forms carbonic acid.

2H₂O + CO₂ → H₂CO₃

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

CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

(Calcite)                        (ca bi carbonate)

              Slightly soluble                  readily soluble

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 oxide. E.g., minerals containing Fe and Mg.

4FeO(ferrous oxide) + O₂→2Fe₂O₃(ferric oxide)

4Fe₃O₄(magnetite) + O₂→6Fe₂O₃(haematite)

2Fe₂O₃(haematite)+H₂O→2Fe₂O₃.3H₂O(limonite)

6.    Reduction – the process of removal of oxygen and is the reverse of oxidation and is equally important in changing soil color 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 palce.

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 and disintegration of rocks and minerals. The biological life is mainly controlled largely by the prevailing environment.

1.    Man and animals

Ø  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 rocks decomposition.

Ø A large number of animals, birds, insects and worms, by their activities they make holes in them and thus aid for weathering.

Ø In tropical and sub tropical regions, ants and termites build galleries and passages and carry minerals from layer 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.

Ø Rabbits, by burrowing in to the ground, destroy soft rocks. Moles, ants and bodies of the dead animals, provide substances which react with minerals and aid in decaying process.

Ø The earthworms pass the soil through the alimentary canal and thus bring about physical and chemical changes in soil material.

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.

The grass roots from 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 form 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 produce CO₂ and organic compounds which aid in mineral decomposition