Our earth is undergoing deformations imperceptibly [so slight, gradual, or subtle as not to be perceived] but continuously.
- These deformations are caused by the movements generated by various factors like
- The heat generated by the radioactive elements in earth’s interior.
- Movement of the crustal plates due to tectogenesis.
- Forces generated by rotation of the earth.
- Climatic factors like winds, precipitation, pressure belts etc.
- Isostacy ==> According to this concepts, blocks of the earth’s crust, because of variations in density would rise to different levels and appear on the surface as mountains, plateau, plains or ocean basins
- Tectonic ==>relating to the structure of the earth’s crust and the large-scale processes which take place within it.
- Geomorphic == relating to the form of the landscape and other natural features of the earth’s surface.
- The endogenic and exogenic forces causing physical and chemical changes on earth surface are known as geomorphic processes.
- Diastrophism and volcanismare endogenic geomorphic processes.
- Weathering, mass wasting, erosion and deposition are exogenic geomorphic processes.
- Geomorphic agent == mobile medium (like running water, moving ice masses, wind, waves and currents etc.) which removes, transports and deposits earth materials.
- The interaction of matterand temperature generates these forces or movements inside the earth’s crust. The earth movements are mainly of two types: diastrophism and the sudden movements.
- The energy emanating from within the earth is the main force behind endogenic geomorphic processes.
- This energy is mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth. This energy due to geothermal gradients and heat flow from within induces diastrophism and volcanism in the lithosphere.
- Diastrophism is the general term applied to slow bending, folding, warping and fracturing.
- Wrap==make or become bent or twisted out of shape, typically from the action of heat or damp; make abnormal; distort.
- All processes that move, elevate or build up portions of the earth’s crust come under diastrophism. They include:
- orogenic processes involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust;
- epeirogenic processes involving uplift or warping of large parts of the earth’s crust;
- earthquakes involving local relatively minor movements;
- plate tectonics involving horizontal movements of crustal plates.
In the process of orogeny, the crust is severely deformed into folds. Due to epeirogeny, there may be simple deformation. Orogeny is a mountain building process whereas epeirogeny is continental building process. Through the processes of orogeny, epeirogeny, earthquakes and plate tectonics, there can be faulting and fracturing of the crust. All these processes cause pressure, volume and temperature (PVT) changes which in turn induce metamorphism of rocks.
Epeirogenic or continent forming movements
- In geology, Epeirogenic movement refers to upheavals or depressions of land exhibiting long wavelengths[undulations] and little folding.
- The broad central parts of continents are called cratons, and are subject to epeirogeny.
- The movement is caused by a set of forces acting along an Earth radius, such as those contributing to Isostacy and Faulting in the lithosphere
- Epeirogenic or continent forming movements act along the radius of the earth; therefore, they are also called radial movements. Their direction may be towards (subsidence) or away (uplift) from the center. The results of such movements may be clearly defined in the relief.
- Raised beaches, elevated wave-cut terraces, sea caves and fossiliferous beds above sea level are evidences of uplift.
- Raised beaches, some of them elevated as much as 15 m to 30 m above the present sea level, occur at several places along the Kathiawar, Nellore, and Thirunelveli coasts.
- Several places which were on the sea some centuries ago are now a few miles inland. For example, Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Thirunelveli, were all flourishing sea ports about 1,000 to 2,000 years ago.
- Submerged forests and valleys as well as buildings are evidences of subsidence.
- In 1819, a part of the Rann of Kachchh was submerged as a result of an earthquake.
- Presence of peat and lignite beds below the sea level in Thirunelveli and the Sunderbans is an example of subsidence.
- The Andamans and Nicobars have been isolated from the Arakan coast by submergence of the intervening land.
- On the east side of Bombay island, trees have been found embedded in mud about 4 m below low water mark. A similar submerged forest has also been noticed on the Thirunelveli coast in Tamil Nadu.
- A large part of the Gulf of Mannar and Palk Strait is very shallow and has been submerged in geologically recent times. A part of the former town of Mahabalipuram near Chennai (Madras) is submerged in the sea.
Orogenic or the mountain-forming movements
- Orogenic or the mountain-forming movements act tangentially to the earth surface, as in plate tectonics.
- Tensions produces fissures (since this type of force acts away from a point in two directions) and compression produces folds (because this type of force acts towards a point from two or more directions). In the landforms so produced, the structurally identifiable units are difficult to recognise.
- In general, diastrophic forces which have uplifted lands have predominated over forces which have lowered them.
- These movements cause considerable deformation over a short span of time, and may be of two types.
- It occurs when the surplus accumulated stress in rocks in the earth’s interior is relieved through the weak zones over the earth’s surface in form of kinetic energy of wave motion causing vibrations (at times devastating) on the earth’s surface. Such movements may result in uplift in coastal areas.
- An earthquake in Chile (1822) caused a one-metre uplift in coastal areas.
- Anearthquake in New Zealand (1885) caused an uplift of upto 3 metres in some areas while some areas in Japan (1891) subsided by 6 metres after an earthquake.
- Earthquakes may cause change in contours, change in river courses, ‘tsunamis’ (seismic waves created in sea by an earthquake, as they are called in Japan) which may cause shoreline changes, spectacular glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.
- Volcanism includes the movement of molten rock (magma) onto or toward the earth’s surface and also formation of many intrusive and extrusive volcanic forms.
- A volcano is formed when the molten magma in the earth’s interior escapes through the crust by vents and fissures in the crust, accompanied by steam, gases (hydrogen sulphide, sulphur dioxide, hydrogen chloride, carbon dioxide) and pyroclastic material. Depending on chemical composition and viscosity of the lava, a volcano may take various forms.
- Pyroclastic==> adjective of or denoting rock fragments or ash erupted by a volcano, especially as a hot, dense, destructive flow.
Earth Movements – Exogenetic Forces
- 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 acidon 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.