GEOMORPHIC PROCESSES: Endogenetic and Exogenetic Forces and Evolution of Land forms

 GEOMORPHIC  PROCESSES

You would  like to know  the meaning of geomorphic processes. The endogenic and exogenic forces causing physical stresses and chemical actions on  earth   materials and

 bringing about  changes in the configuration of the surface  of the earth  are known as geomorphic processes. Diastrophism and volcanism are endogenic geomorphic processes. These have already been discussed in brief in the preceding unit. Weathering, mass wasting, erosion and deposition are exogenic geomorphic processes. These exogenic processes are dealt with in detail in this chapter.

Any exogenic element of nature (like water, ice, wind,  etc.,) capable of acquiring and transporting earth  materials can be called a geomorphic agent. When these elements  of nature become mobile  due to gradients, they remove the materials and transport them over slopes and deposit them at lower level. Geomorphic processes and geomorphic agents especially exogenic, unless stated separately, are one and the same.

A process is a force applied on earth materials affecting the  same.   An agent  is a mobile medium (like running water, moving ice masses, wind,  waves and currents etc.) which removes, transports and deposits  earth materials. Running water,  groundwater, glaciers,  wind,  waves and currents, etc., can be called  geomorphic agents.

Gravity besides  being  a directional force activating all downslope movements of matter also causes stresses on the earth’s materials. Indirect gravitational stresses activate wave and tide  induced currents and  winds. Without gravity and gradients there would  be no mobility and hence no erosion, transportation and deposition are possible.  So, gravitational stresses are as important as the other geomorphic processes. Gravity is the force that is keeping us in contact with the surface and it is the force that  switches on the movement  of all surface material on earth. All the movements either within the earth or on the surface of the earth  occur  due to gradients — from  higher levels to lower levels, from high pressure to low pressure areas etc.

(A) ENDOGENIC  PROCESSES

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. Due to variations in geothermal gradients and heat flow from  within, crustal thickness and strength, the action of endogenic forces are not uniform and hence the tectonically controlled original crustal surface is uneven.

Classification of Endogenic movements

Endogenic movements are divided into diastrophic movements and sudden movements.

1. Slow Movements (Diastrophic forces):

Diastrophic forces refer to forces generated by the movement of the solid material of the earth’s crust. All the processes that move, elevate or build portions of the earth’s crust come under diastrophism. Diastrophism refers to the deformation of the Earth’s crust.

• Diastrophism refers to deformation of the Earth’s crust due to diastrophic movements (deforming movements) such as folding, faulting, warping (bending or twisting of a large area) and fracturing.
• 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 (crust is severely deformed into folds) 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 (simple deformation);
• earthquakes and volcanism involving local relatively minor movements;
• plate tectonics involving horizontal movements of crustal plates.
• The most obvious evidence of diastrophic movement can be seen where sedimentary rocks have been bent, broken or tilted.

Slow movements can again be classified as vertical movements and horizontal movements.

Vertical Movements (Epeirogenic movements):

Vertical movements are mainly associated with the formation of continents and plateaus. They are also called as Epeirogenic movements

The broad central parts of continents are called cratons and are subject to epeirogeny.

They do not bring any changes in the horizontal rock strata.

While they cause upliftment of continent, they can also cause subsidence of continent.

These movements are originated from the centre of the earth.

Horizontal Movements (Orogenic Movements):

Horizontal forces acts on the earth’s crust from side to side to cause these movements.

They are also known as orogenic movements (mountain building).

They bring a lot of disruptions to the horizontal layer of strata leading to a large structural deformation of earth’s crust.

They can be classified as forces of compression and forces of tension.

2. Sudden Movements:

Sudden geomorphic movements occur mostly at the lithospheric plate margins (tectonic plate margins).

The plate margins are highly unstable regions due to pressure created by the pushing and pulling of magma in the mantle (convectional currents).

These movements cause considerable deformation over a short period.

Earthquakes

Earthquakes occur when the surplus accumulated stress in rocks in the earth’s interior due to folding, faulting or other physical changes is relieved through the weak zones over the earth’s surface in the form of kinetic energy (seismic waves).

Such movements may result in uplift or subsidence in coastal areas.

An earthquake in Chile (1822) caused a one-metre uplift in coastal areas.

An earthquake in New Zealand (1885) caused an uplift of up to 3 metres.

An earthquake in Japan (1891) caused subsidence of up to 6 metres.

Earthquakes may cause a change in contours, change in river courses, shoreline changes, glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.

Volcanoes

Volcanism includes the movement of molten rock (magma) onto or towards the earth’s surface through narrow volcanic vents or fissures.

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 etc.) and pyroclastic material (cloud of ash, lava fragments carried through the air, and vapour).

Depending on the chemical composition and viscosity of the lava, a volcano may take various forms.

(B) EXOGENIC  PROCESSES

The exogenic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors.

Gravitational force acts upon  all earth materials having a sloping surface and tend to produce  movement  of matter in  down  slope direction. Force applied per unit area is called stress. Stress is produced in a solid by pushing or pulling. This induces deformation. Forces acting  along the faces of earth  materials are shear  stresses  (separating forces).  It is this stress that breaks rocks and other earth materials. The shear stresses result in angular displacement or slippage. Besides the gravitational stress  earth  materials  become subjected to molecular stresses that  may be caused by a number of factors amongst which temperature changes, crystallisation and melting are the most common. Chemical processes normally lead to loosening of bonds between grains, dissolving of soluble minerals or cementing materials. Thus, the basic reason that leads to weathering, mass movements, and erosion is development of stresses in the body of the earth materials.

As there  are different climatic regions  on the earth’s surface the exogenic geomorphic processes vary from region to region. Temperature and precipitation are the two important climatic elements  that  control various processes.

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. The flow chart gives the denudation.

processes and their respective driving forces. It should become clear from this chart that for each process there exists a distinct driving force or energy.

As there  are different climatic regions  on the earth’s surface owing to thermal gradients created by latitudinal, seasonal and land and water spread variations, the exogenic geomorphic processes vary from region to region.   The density, type and distribution of vegetation which largely  depend  upon 

precipitation and temperature exert influence indirectly on exogenic geomorphic processes. Within different climatic regions there may be local variations of the effects of different climatic elements due to altitudinal differences, aspect variations and the variation in the amount of insolation received by north and south facing slopes as compared to east and west facing slopes. Further, due to differences in wind velocities  and directions, amount and kind of precipitation, its intensity, the relation between precipitation and evaporation, daily  range of temperature, freezing and thawing frequency, depth of frost penetration, the geomorphic processes vary within any climatic region.

Climatic factors  being equal, the intensity of action  of exogenic geomorphic processes depends upon type and structure of rocks. The term structure includes such aspects of rocks as 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. Different types of rocks with differences in their structure offer varying resistances to various geomorphic processes.  A particular rock may be resistant to one process and non- resistant to another. And, under varying climatic conditions, particular  rocks  may exhibit different degrees of resistance to geomorphic processes and hence they operate at differential rates and give rise to differences in topography. The effects of most of the exogenic geomorphic processes are small and slow and may be imperceptible in a short  time span, but will in the long run affect the rocks severely due to continued fatigue.

Finally, it boils  down to one fact that  the differences on the surface of the earth though originally related  to the crustal evolution continue to exist in some form or the other due to differences in the type and structure of earth materials, differences in geomorphic processes and in their rates of operation.

Some of the exogenic geomorphic processes have been dealt in detail here.

Source: NCERT Book

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