What is weathering?  Discuss the different types of weathering.

  Q :- What is weathering?  Discuss the different types of weathering. 

Answer :-   The mechanical breakdown or chemical disintegration of surface rocks by various elements of weather (temperature, rainfall, wind current, atmospheric oxygen, carbon dioxide etc.) is called weathering.

1.  Mechanical Weathering :- 

When all the rocks on the surface are broken by wind, rain, snow, flowing water, wind, glaciers, etc., it is called mechanical weathering.

Mechanical weathering occurs in the desert regions of the world, cold polar regions, or dry and cool temperate climates.

Surface rocks are weathered by the processes of solar heat, wind currents, precipitation, snow, etc., dispersion of fine particles, fragmentation of rocks, weathering, calcareous formation, etc.

The main 4 processes of mechanical weathering are discussed below ----

(i) Exfoliation :-

Definition :- The word 'exfoliation' , in Bengali means 'Shalkamochon' 'Shalka' means 'Peel' and the word 'mochan' means 'Redemption'.  The mechanical weathering process in which the top layer of the rock opens up like an onion due to the increase or decrease in temperature is called Exfoliation.

Characteristics :-

(1) It is more effective in granite and gneiss rocks with concentric layers.

(2) This weathering occurs in homogenous rocks composed of similar minerals in deserts.  

(3) In this case, a thermal shield is formed from the surface of the pile inwards. 

(4) Mountain peaks are rounded due to erosion.  Namely - Tamil Nadu's 'Krishna Vennai Pandha', Ranchi or Kanke Dom. 

Process:- In hot deserts, heat-conducting rocks are concentrically layered and homogenous, with the upper layer expanding more during the day in solar heat and contracting more during cooling at night than the lower layer. The result is an uneven thermal gradient from the outside to the inside of the pile.  Over time this continuous expansion and contraction of the upper layers causes the bedrock to peel away from the body like peeling an onion.  Its other name is onion weathering. 

(ii) Block Disintegration :-

Definition:-  The mechanical weathering process in which rock strata are shaped into large cairns due to expansion and contraction caused by variations in elevation.

Characteristics :- 

(1) In this weathering the rock is separated into different columns or channels. 

(2) In this case continuous expansion and contraction of the rock takes place only in the upper layer. 

(3) It is effective in basalt lithic fragmental rocks in the desert.  Rocks are homogenous in nature composed of the same minerals.

(4) The hill tops are flattened due to its effect.

(5) It causes numerous vertical and horizontal cracks and fissures in the rock.   

Process :- Hot deserts have a wide temperature difference between day and night (15°C - 57°C).  As a result, the top layer of heat-conducting rock continuously expands during the day and contracts at night.  Over time, stress builds up in the rock beyond its tolerance limit, creating numerous vertical and horizontal cracks and fissures along the weak parts of the rock.  Later,

along the cracks, the bedrock is separated into rectangular or square columns or troughs.

(iii) Granular Disintegration :-

Definition:- The mechanical weathering process in which rock bursts into small particles as a result of compression and expansion caused by variations in texture is called granular disintegration. The word "disintegration" means breaking up.

Characteristics :- 

(1) In this mechanical weathering the rock is reduced to tiny limestone particles called sand or dust.

(2) This weathering occurs in heterogeneous rocks of different nature and color in deserts.

(3) It is effective in granite and sandstone rocks.

(4) After sunset in the desert, the sound of cracking rocks like pistol shots is heard.  

(5) Inselberg landforms are formed by this weathering.  

Process:- Due to the different color, properties and nature of the different minerals of the heterogeneous rock in deserts, the heat absorption and rejection capacity is different.  As a result, it expands and contracts differently during the day and night.  A great strain is created in the rock.  As a result, in the afternoon or afternoon, along the junction of minerals, the rock bursts into tiny particles with a sound like a pistol shot. 

(iv) Frost Weathering :-

Definition :- In the process of mechanical weathering, the water in the rock cracks in the cold region turns into ice and creates high pressure and the rock bursts into angular rock fragments, it is called snow/frost weathering or frost fragmentation.  

Characteristics :-

(1) This climate is effective in high mountain or high latitude regions with greater diurnal and seasonal variation, where daytime temperatures drop below freezing at night.

(2) It occurs due to compression and tension in the rock layer to increase the volume of ice or snow. 

(3) Fine fragmentation results in piles of sharp-edged rubble known as free or talus and felsenmar or blockspede.

(4) It occurs more in sedimentary rocks.

Process :-  Ice melt water accumulates in rock crevices during summer or during the day in frosty high mountains or high mountain ranges, or talus and mountain ice fragmentation latitudes.  When it freezes in winter or at night, the ice expands by 10% in volume and exerts tremendous pressure on the rock cracks.  When that ice melts again, the pressure is released.  Thus, when the rock is cracked by the ice, the stress and tension on the rock due to the increase in pressure drop causes the rock to break into sharp-edged rock particles.  

 2. Chemical Weathering :-

When all the rocks on the surface are decomposed by atmospheric carbon dioxide, oxygen, water vapor etc., it is called chemical weathering.

Chemical weathering occurs in warm humid equatorial climates and humid temperate climates.

The atmospheric oxygen, carbon dioxide, water vapor, etc. by oxidation, carbonation, hydrolysis, solution etc. process all the substances on the surface are separated and undergo chemical weathering.

The main 4 processes of chemical weathering are discussed below-

(i) Oxidation :-

Definition :- The chemical weathering that occurs as a result of the combination of oxygen gas (O2) in the air with the rock containing iron minerals in the presence of water or water vapor is called oxidation.

Method :- Oxidation occurs mainly in iron-rich rocks, but also in amphibole, pyroxene and biotite minerals.(1) Rocks rich in blue colored ferrous oxide (FeO) are very hard.  But in the presence of water (H2O) it is oxidized by oxygen (O2) to form a new brown or yellow compound called ferric oxide or rust (Fe2O) which makes the rock very brittle and weak.  Poor quality limonite iron ore (2Fe2O3, 3H2O) is produced. 

(2) Similarly ferrous sulfide (FeS) weakens to ferrous sulfate (FeSO4).

Reaction :-  

 (a) 4FeO + 3H2 O + O2 = 2Fe2O3 , 3H2O ; 

 (b) FeS +  2O2 = FeSO4

(ii) Carbonation :- 

Definition :- The chemical weathering that occurs as a result of carbon dioxide (CO2) in the air combined with rainwater and the carbonic acid formed by the connection with carbonate-rich rocks is called carbonation or angaryojan.  In this process characteristic landforms develop in Karst regions.

Method :- Carbonation is effective in rocks rich mainly in carbonate, besides felspar and hornblende minerals.  During rain, fresh water (H2O) mixes with atmospheric CO2 to form mild carbonic acid (H2CO3) as it falls through the atmosphere to the surface.  It is a chemical reaction with limestone or calcium carbonate (CaCO3) in Karst regions. It forms calcium bicarbonate [Ca(HCO3)2] which is easily dissolved in water and excreted. 

Reaction :- 

H2O + CO2 = H2CO3 ; 

CaCO3 + H2CO3 = Ca(HCO3)2

(iii) Hydration :- 

Definition :- When water (H2O) is added to a mineral in a rock, the chemical reaction that occurs when the bond of the mineral becomes loose or becomes a secondary mineral is called hydration.

Method :- When water molecules come in contact with rock minerals, the volume of the mineral increases.  As a

result, the binding of the minerals loosens, the secondary minerals become malleable and the rock easily

disintegrates.  Thus hematite (Fe2O3) turns into limonite (2Fe2O3 , 3H2O), anhydride (CaSO4)into gypsum (CaSO4 , 2H2 O) and olivine into serpentine.

Reaction :-  

2F2O3 + 3H2O  =  2Fe2O3 , 3H2O ; 

CaSO4 + 2H2O = CaSO4 , 2H2O

(iv) Hydrolysis :-

Definition :- The chemical reaction and separation of ionized water with rock mediated mineral molecules is called hydrolysis or hydrolysis. 

Method :- Water first ionizes (HOH) and dissociates into hydrogen ions (H+) and hydroxyl ions (OH−).  It then reacts with minerals such as orthoclase felspar (KAlSi3O8) to form alumino silicic acid (HAISi3O8) and the alkali potassium hydroxide (KOH).

Reaction :- 

KAISi3O8 + HOH = HAISi3O8 + KOH

3. Biological weathering :-

When all the rocks on the surface are crushed or decomposed by different organisms or plants, it is called biological weathering.

Biological weathering occurs in almost all climate zones of the world (except sub-desert and polar climates).

Rocks are weathered by earthworms, rats, weevils, rabbits etc. by digging holes in the soil or by rotting plants or plant roots.

      Biological weathering is done by two methods discussed below ----- 

(i) Bio-Mechanical Weathering :-

Definition :- When the material produced by mechanical crushing of rocks by different animals and plants and remains at the place of origin, it is called bio-mechanical weathering.

Method :- 

(A) By animals:- Earthworms, ants, termites, rabbits, prairie dogs, mice, foxes etc. make holes in the rock, scratch and break the rock layer.  Snails burrow into limestone.  

(B) By plants :- The roots of plants penetrate the rock layer and grow gradually.  Over time, rock cracks increase, cracking the rock and causing weathering.

(C) By human being :- Human being directly causes rock weathering in a very small area, but changes the rock very rapidly within a few moments. 

(a) Cultivation :- Rocks and soil are altered during the creation of cultivated land in hilly and plateau regions.

(b) Construction work :- Rock strata are crushed for construction of roads, dams and reservoirs. 

(c) Mineral Extraction :- During mineral extraction, man breaks the rock by blasting dynamite or using machinery. 

(ii) Bio-Chemical Weathering :-

Definition :-  The material produced by the chemical decomposition of rocks by the acids produced by the decomposition of the dead bodies of various animals and plants remains at the source.  This weathering process is called biochemical weathering.  

Method :-

(A) By human being :- Organic acids produced by decaying human remains, faeces, urine, waste products, pollution causes acid rain to disintegrate rocks.

(B) By plants:- When small plants such as moss, lichen, algae etc. growing on rocks in humid climate region die or the stems, leaves, roots of plants rot and add rain water, humic, lactic, citric, fulvic etc. organic acids are produced. That acid causes chemical weathering of rocks rich in magnesium, felspar and sulfur compounds. 

(C) By animals :-

(a) Organic acids produced by decomposition of dead bodies of animals, faeces, urine in wet and humid regions cause decomposition of rocks. 

(b) The bodies of various insects and microbes living in the rock cause chemical weathering of the rock by secreting juices and droppings.