Weathering is the disintegration and or decomposition of rocks in situ or in place

Weathering is the disintegration and or decomposition of rocks in situ or in place. This includes a group of processes that act collectively at and near the earth’s surface.

They fragment the solid rocks and convert them into clastic state. The portion of the earth’s crust which is exposed to the weathering processes in known as the zone of weathering. Rock weathering is broadly divided into three types – (i) Physical weathering (ii) Chemical weathering and (iii) Biological weathering.

Physical Weathering Processes:

These are otherwise called as mechanical weathering processes. They take place in three different ways- (i) Expansion due to unloading, (ii) Thermal expansion and (iii) Growth of crystal.

Expansion due to unloading:

Igneous rocks are formed at great depths. This leads to the formation of large scale fractures roughly parallel to the earth’s surface. These fractures give an impression of foliation on the overlying rocks known as exfoliation. Sheet structures of most granitoid rocks are formed by this process.

Thermal expansion:

Rocks are composed of different minerals which have different co-efficients of expansion. Repeated differential thermal expansion and contraction of different minerals help in disintegration of rocks. This leads to mass exfoliation or granular exfoliation.

Growth of crystals:

Freezing water in cracks and fractures form ice crystals. They exert tremendous pressure and disintegrate solid rocks. It is known as frost-wedging. A very similar process is frost – heaving where due to freezing of water, expansion results, rocks disintegrate and on inclined slopes are acted upon by gravity. Alternate freezing and thawing is much more effective in disintergation of solid rocks. In arid and semiarid regions, formation of salt crystals has also the similar effect. Another possible process of physical weathering is termed as colloid plucking by Reiche. He advocates the weathering of rocks after soil and mantle is removed by erosion just like gelatine removes portions of glass kept over it.

Chemical Weathering Processes:

The different chemical weathering processes are (i) Hydration, (ii) Hydrolysis, (iii) Oxidation (iv) Carbonation and (v) Solution.

Hydration:

The process of hydration involves adsorption of molecule such as the conversion of haematite to limonite. Hy is a process in which water is involved in the new mineral.

2Fe₂0₃ + 3H₂0 = 2Fe₂0₃ 3H₂0.

2.2.3.2. Hydrolysis: It involves the formation of hydroxyls by che reaction. Alteration of feldspars and olivine are good examples.

KA1 Si₃0₈+H₂0 = HA1 Si₃0₈ + KOH

Orthoclase + Water = Orthosilic acid + Potassium hydroxide

MgFeSi0₄ + H₂0 = Mg (OH) + H₂Si0₃ + FeO

Olivine + Water = Magnesium hydroxide + Orthosilica acid + Ferrous

Oxidation:

Ferrous oxide in the above reaction reacts with water oxygen and is oxidised to form limonite. It also involves hydration

4FeO + 3H0₂+0₂ = 2Fe_:0₃.3H₂0

Ferrous oxide + Water + Oxygen = Limonite

2.2.3.4. Carbonation: The formation of Potash carbonate by reaction potassium hydroxide with water and carbon dioxide is example carbonation

2KOH + H₂0+C0₂= K₂C0₃ + 2H₂0

Potassium Hydroxide + Water + Carobon dioxide = Potassi carbonate + Water

2.2.3.5. Solution: Calcium carbonate is dissolved in presence of water a carbon dioxide to form calcium bicarbonate which is soluble and’ removed by solution.

CaC0₃ + H₂0 + C0₂= Ca (HC0₃)₂

Calcium Carbonate + Water + Carbon dioxide = Calcium bicarbon by this process limestone, dolomite, chalk, marble are decompose.

Biological Processes:

The work of organisms including that of man aid in disintegration and decomposition of rocks. Growth of plant roots and stems expand the joints and fractures in rocks. More important is the decay of plants that produce humic acid which help in chemical weathering. Burrowing animals bring fresh rock material from the earth’s surface and help in weathering. Termites bring soil and build mounds or ant-hills. Prairie dogs and bears do the same work. The chemical and mechanical work of earthworms is note worthy. Human activity is equally important through mining and engineering constructions.

Significance of Weathering: Weathering helps in the formation of soils which is essential for humanity. It aids in mass wasting and prepares the land for erosion. Topographic surfaces are lowered down by weathering. It creates certain land forms like mantle rock (regolith), mounds, huddo rocks etc and modifies them.