Weathering and its Types Upsc

Weathering Definition

The earth materials such as rocks are subjected to the action of elements of weather and climate such as Ice, water, wind etc. These elements of weather act individually or together to affect earth materials such as rock and reduce them to fragments.

Altogether, It is defined as the decomposition of rocks by the natural process such as climate or weather by using water, ice, wind etc, through its action by mechanical disintegration and decomposition by chemical action.

Also, it is an in-situ or on-site process, that in is little or no motion takes place during the process of disintegration or decomposition of rocks. But sometimes little motion takes places due to the process of the transportation process.

The factor that supports weathering process is vegetation, topography, climatic, geological factors etc.

Climate plays an important part, as it affects the depth of the weathering mantle and it differs from one climate to another.

Weathering Upsc
Different climate and depth of weathering of mantles

Types of Weathering

  • Chemical
  • Physical
  • Biological

Chemical Weathering

Weathering by chemical action such as Solution, carbonation, hydration, oxidation, and reduction.

All these chemical action acts on rocks and this decomposes the rocks and reduce them to the fine state by water, air, and other acids individually or combined.

To speed up the chemical reaction water, air which includes oxygen, carbon-di-oxide along with heat needs to be present.

The carbon-di-oxide, in the underground, is increased by the decomposition of plants and animals, also its increase carbon dioxide in the atmosphere.

Solution

When something is dissolved in water or acids the dissolved contents with water or acid are called a solution.

The process involves the removal of solids in solution and depends on the solubility of a mineral in the weak acids or water.

Many solids disintegrate when it comes into contact with water and form as a suspension in water.

Water-soluble rock-forming minerals such as Nitrates, Sulphates, and potassium, etc are affected and easily leached out in rainy climates without leaving any residue and it gets accumulated in the dry regions.

Minerals such as Calcium Carbonate and Calcium Magnesium bicarbonate that are present in the limestone are soluble in the water containing carbonic acid.

This carbonic acid is formed when water comes in contact with Carbon dioxide. In process of Carbonic acid production is initiated by decaying organic matter along with soil is mixed in water.

Also, Sodium chloride or common salts also considered responsible for this process.

Carbonation

The reaction of carbonate and bicarbonate with minerals, that helps to break down feldspars and carbonate minerals is called Carbonation.

The water absorbs, the carbon dioxide from the atmosphere and soil to form carbonic acid.

Carbonic acid is a weak acid, which affects Calcium carbonate and Magnesium Carbonate and gets dissolved and removed in the solution.

This leaves no residue and this results in cave formation.

Hydration

The addition of water to the chemicals, minerals, etc is called Hydration.

The mineral expands after Hydration. This expansion of minerals by water increases the volume of itself inside the rock.

Gypsum is formed when water is reacted with Calcium Sulphate. This gypsum is much more unstable than Calcium Sulphate.

This process of Gypsum conversion is reversible and is continued many times and creates fatigue in the rock which leads to disintegration.

The overlying material cracks after the process of repetition of wetting and drying of clay minerals which swell and contract during the process.

Rocks are fractured by the Salts in the pores spaces which undergo rapid and repeated hydration.

The volume of salts changes during hydration which helps in physical weathering by exfoliation and granular disintegration.

Oxidation and Reduction

When a mineral is combined with oxygen to form oxides or hydroxides it is called Oxidation in Weathering.

Oxidation happens almost everywhere there is the atmosphere. And also with oxygenated water.

Iron, manganese, sulphur, etc are mineral which is commonly get involved in the oxidation process.

The rock breaks down because of disturbances caused addition of oxygen.

Iron turns to red colour when it gets oxidized as Iron oxide, also Iron oxide is further oxidized and appears as brown or yellow.

The reduction takes place when the oxidized material is placed in the absence of oxygen.

The area, where the oxygen is absent, are usually below the water table. Example stagnant water areas, waterlogged ground, etc.

The Iron oxide which is red in color turn to greenish or bluish-grey upon reduction.

The chemical weathering such as Oxidation and reduction, carbonation, and Hydration are interrelated and are responsible for weathering process.

Physical Weathering Process

Physical Weathering is also called Mechanical Weathering, and its process depends on applied forces dues gravitation forces such as pressure, load and shearing stress, expansion forces because of temperature changes, crystal growth, wetting and drying cycles by water, animal activities, etc.

The above forces are responsible for rock fracture, as when these forces are applied at the surface and within earth materials.

The Thermal expansion and pressure release are responsible for most mechanical weathering processes.

The continued fatigue caused by repeated contraction and expansion due to thermal expansion and pressure release creates great damage to rocks, even though this process is small and slow.

Unloading and Expansion

Elimination of overlying rock load because of continued erosion produces vertical (upward) pressure release with the result that the upper layers of the rock expand creating a disintegration of rock blocks.

Fractures are developed nearly parallel to the ground surface.

In the regions of the curved or arched ground surface, arched or curved fractures or cracks tend to generate massive sheets or exfoliation(peeling) slabs of rocks.

Exfoliation sheets emerging from expansion because of unloading and pressure release may average several hundreds of meters in horizontal space.

The large and smooth rounded domes are known as Exfoliation domes is end result of this process.

Exfoliation domes
Exfoliation domes By Ronnie Macdonald from Chelmsford, United Kingdom – Half Dome Trek 15, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=25809888

Temperature Changes and Expansion

Different minerals in the rocks have different limits of expansion and contraction.

The mineral expands and pushes against its neighbour when the temperature is raised, and similarly, the mineral gets contracted when the temperature falls.

Due to these diurnal changes in the temperature, this inner change between the mineral grains of the surface layers of rocks takes place frequently.

This process is most powerful in the dry climate and in the high elevations wherever the diurnal temperature variations are severe.

These variations make the rock weak due to continued fatigue, even though these changes are very small.

The outer layers of rock have higher expansion than the rocks at the depth, which leads to the creation of stress within the rock and leads to heaving and fracturing parallel to the surface.

It is because of differential heating and the following expansion and contraction of surface layers and its following exfoliation from the surface ends in the smooth rounded surface in rocks.

In granites, smooth-surfaced and rounded small to big boulders called Tors are formed due to the exfoliation.

Freezing, Thawing and Frost Wedging

This kind of weathering happens when the Ice grows within the pores and cracks the rocks by repeated cycles of freezing and melting.

Frost Weathering has the highest impact at the higher elevations in the mid-latitudes. It is because in the mid-latitudes the freezing and melting happen repeatedly.

The Glacial areas are dominated by frost wedging daily as this process, the rate of freezing is vital.

Rapid freezing of water creates sudden expansion and high pressure.

It is the result of expansion that attacks the joints, cracks, and small intergranular fractures and makes it wider till the rocks break apart.

Thawing and Frost Wedging By Stefan-Xp – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=487569

Salt Weathering

Because of the thermal action, crystallization, and hydration, the salt in the rocks expands.

Many salts such as calcium, sodium, magnesium, potassium, barium, etc have the tendency to expand.

The thermal properties and temperature are the factor of expansion.

The salt expansion is favored in deserts as the surface temperature expands between 30 to 50° C.

The salt crystals in near-surface pores produce the splitting of single grains inside the rocks that ultimately fall off.

Granular foliation or granular disintegration is the result of the process of falling off of the individual grains.

The most effective salt weathering is Salt crystallization.

In the regions where there is wetting and drying happens repeatedly, salt crystal development is supported and the nearby grains are pushed aside.

Sodium chloride and gypsum crystal in the deserts, lift up overlying layers of the materials and the consequence is, polygonal cracks form all over the heaved surface.

With salt crystal growth, chalk cracks down readily and resulted in limestone, sandstone, shale, granite, gneiss, etc.

Biological Weathering

The weathering is caused by the physical changes because of the movement and growth of organisms.

Moisture and air are penetrated into the surface of rocks and exposing it to chemical and mechanical weathering by the Burrowing and wedging actions of the earthworms, termites, rodents, etc.

Human is also part of the biological weathering, as Human disturb the vegetation by cultivation after plowing.

The production of carbonic acids, and other such acids, humic are produced by the decaying plant and animals, which form solutions that help in chemical weathering.

Also, the roots of plants and trees put great pressure on the earth’s materials and result in mechanical weathering.

Conclusion

Weathering is responsible for the breaking of rocks, which in turn forms as soil. This also helps in erosion and mass movement. By that, vegetation is flourished.

The Landform changes are the result of erosion and mass wasting, which are aided by weathering.

It also helps concentration and enrichment of ores such as Iron, Copper, etc, which is good for the countries economy.

* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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