Table of Contents
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- Exogenic (Exogenetic) processes are a direct result of stress induced in earth materials due to various forces that come into existence due to sun’s heat.
- Force applied per unit area is called stress. Stress is produced in a solid by pushing or pulling.
- Forces acting along the faces of earth materials are shear stresses (separating forces). It is this stress that breaks rocks and other earth materials.
- Earth materials become subjected to molecular stresses caused due to temperature changes.
- Chemical processes normally lead to loosening of bonds between grains.
- Thus, the basic reason that leads to weathering, erosion and deposition is development of stresses in the body of the earth materials.
- Temperature and precipitation are the two important climatic elements that control various processes by inducing stress in earth materials.
- All the exogenic geomorphic processes are covered under a general term, denudation.
- The word ‘denude’ means to strip off or to uncover.
- Weathering, mass wasting/movements, erosion and transportation are included in denudation.
- Denudation mainly depends on rock type and its structure that includes folds, faults, orientation and inclination of beds, presence or absence of joints, bedding planes, hardness or softness of constituent minerals, chemical susceptibility of mineral constituents; the permeability or impermeability etc.
- The effects of most of the exogenic geomorphic processes are small and slow but will in the long run affect the rocks severely due to continued fatigue.
- Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
- As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
- There are three major groups of weathering processes: (i) chemical; (ii) physical or mechanical; (iii) biological weathering processes.
Chemical Weathering Processes
- A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine state.
- Water and air (oxygen and carbon dioxide) along with heat speed up all chemical reactions.
- When something is dissolved in water or acids, the water or acid with dissolved contents is called solution.
- On coming in contact with water many solids disintegrate. Soluble rock forming minerals like nitrates, sulphates, and potassium etc. are affected by this process.
- So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
- Carbonation is the reaction of carbonate and bicarbonate with minerals.
- Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid on various minerals.
- Hydration is the chemical addition of water.
- Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock.
- This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.
- The volume changes in minerals due to hydration will also help in physical weathering through exfoliation and granular disintegration.
Oxidation and Reduction
- In weathering, oxidation means a combination of a mineral with oxygen to form oxides (rusting in case of iron) or hydroxides. Red soils appear red due to the presence of iron oxides.
- Oxidation occurs where there is ready access to the atmosphere and water.
- The minerals most commonly involved in this process are iron, manganese, sulphur etc.
- When oxidized minerals are placed in an environment where oxygen is absent, reduction takes place.
- Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
- Red colour of iron upon reduction turns to greenish or bluish grey.
These weathering processes are interrelated. Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.
Biological activity and weathering
- Biological weathering is removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms.
- Burrowing and wedging by organisms like earthworms, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.
- Human beings by disturbing vegetation, ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth materials.
- Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements.
- Algae utilise mineral nutrients for growth and help in concentration of iron and manganese oxides.
- Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.
Physical Weathering Processes
- Physical or mechanical weathering processes depend on some applied forces like (i) gravitational forces (ii) expansion forces due to temperature changes, crystal growth or animal activity; (iii) water pressures controlled by wetting and drying cycles.
Unloading and Expansion
- Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses.
- In areas of curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs of rock.
- Rocks composed of coarse mineral grains commonly fall apart grain by grain or undergo granular disintegration.
Exfoliation – Temperature Changes and Expansion
- With rise in temperature, every mineral expands and pushes against its neighbor and as temperature falls, a corresponding contraction takes place.
- Because of diurnal changes in the temperatures, this internal movement among the mineral grains takes place regularly.
- This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic.
- The surface layers of the rocks tend to expand more than the rock at depth and this leads to the formation of stress within the rock resulting in heaving and fracturing parallel to the surface.
- Exfoliation results in smooth rounded surfaces in rocks.
- This type of disintegration takes place in rocks with numerous joints acquired by mountain-making pressures or by shrinkage due to cooling.
- This type of disintegration in rocks can be achieved by comparatively weaker forces.
- A huge rock may undergo disintegration along weak zones to produce highly angular pieces with sharp corners and edges through the process of shattering.
Freezing, Thawing and Frost Wedging
- During the warm season, the water penetrates the pore spaces or fractures in rocks.
- During the cold season, the water freezes into ice and its volume expands as a result.
- This exerts tremendous pressure on rock walls to tear apart even where the rocks are massive.
- Frost weathering occurs due to growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting.
- Salts in rocks expand due to thermal action, hydration and crystallisation.
- Many salts like calcium, sodium, magnesium, potassium and barium have a tendency to expand.
- High temperature ranges in deserts favour such salt expansion.
- Salt crystals in near-surface pores cause splitting of individual grains within rocks, which eventually fall off. This process of falling off of individual grains may result in granular disintegration or granular foliation.
- Since gravity exerts its force on all matter, both bedrock and the products of weathering tend to slide, roll, flow or creep down all slopes in different types of earth and rock movements grouped under the term ‘mass wasting’.
Effects of Weathering
- Weathering and erosion tend to level down the irregularities of landforms and create a
- The strong wind erosion leaves behind whale-back shaped rocks in arid landscape. These are called inselberg or ruware.
- Sometimes a solid layer of chemical residue covers a soft rock. Sometimes, differential weathering of soft strata exposes the domelike hard rock masses, called tors. Tors are a common feature of South Indian landscape.
Weathering and Erosion
- Lead to simultaneous process of ‘degradation’ and ‘aggradation’.
- Erosion is a mobile process while weathering is a static process [disintegrated material do not involve any motion except the falling down under force of gravity].
Significance of weathering
- Weathering is the first step in formation of soils.
- Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc.
- Weathering helps in soil enrichment.
- Without weathering, the concentration of the same valuable material may not be sufficient and economically viable to exploit, process and refine. This is what is called enrichment.
Another important process in Exogenetic movements is erosion. We will study about erosion in Indian Geography.