Introduction
It is an established fact that the development of the surface features of the globe is mainly due to the complex interaction of internal forces, atmospheric processes, rocks, ocean waters and living creatures.
The surface of the earth is a zone where the rocks uplifted by internal earth forces come into contact with the atmosphere and hydrosphere.
Thus, they are subjected to a range of processes powered by energy from the Sun. The processes which have been playing dominant roles in shapeing the surface of the earth are both constructive as well as destructive in nature. All the geological processes can be conveniently grouped into two categories viz. endogenous and exogenous processes.
1. ENDOGENOUS PROCESSES
These are also known as hypogene processes. These are the processes of internal origin. In other words, a process which originates within the earth’s crust is termed endogenous. These processes take place inside the globe and are governed by forces inherent in the earth and affected little by external influences. These processes cause phenomena, like earthquakes, emergence and development of continents, ocean troughs and mountain ridges, generation of volcanic activity, metamorphism of pre existing rocks, deformation and movement of the earths crust both Vertically and laterally etc.
The geomorphic features produced by these processes provide the setting for exogenous processes to operate upon. All features which owe their origin to an endogenous process are invariably modified by exogenous processes.
The endogenous processes are mostly caused by the thermal energy of the mantle and the crust. This thermal energy is derived front the decay and disintegration of the radioactive elements and from gravitational differentiation in the mantle. Some of the important endogenic processes and their role in the evolution of land forms are as described below :
(i) Earthquakes
It is a form of energy of wave motion transmitted through the surface layers of the earth, ranging from a faint tremor toba wild motion capable of shaking builidings apart and causing gaping fissures to open up inthe ground. The earthquakes are mostly produced due to underground dislocation of rocks.
(ii) Tectonic movements
Tectonic movement of earth’s crust are various forms and are characterised by great complexity. In the course of geological history of the earth’s crust, the rocks have been crumpled into folds, thrust over one another, broken up etc. giving rise to mountains, ridges, ocean trough and other landforms. The tectonic process of elevating or building up portions of the earth’s surface is called diastrophism, which prevents the exogenous process from ultimately reducing the earth’s land areas to sea level. It is of two types viz,
(a) Orogeny and
(b) Epeirogeny
While ‘Orogeny’ refers to mountain builiding activities with deformation of the earth’s crust, Epeirogeny refers to regional uplift with marked deformation.
The lateral displacement of the crustal blocks are manifested in the phenomenon like continental drift, ocean floor spreading etc.
(iii) Volcanism
It is the phenomenon in which matter is transferred from the earth’s interior and erupted onto its surface. It is one of the important manifestations of the dynamic nature of the earth. The process of effusion of magmatic material on to the surface of the earth, thus forming various volcanic structures and/ or flowing over the surface, is called volcanism.
Sometimes the magma on its way upward does not reach the surface and cools at various depths giving rise to magmatic bodies of irregular form, which are called Intrusive or plutons. The phenomenon is known as Intrusive magmatism Even though the intrusions are not directly responsible for topographic features their existence in the upper crust of the earth may affect,to a great extent,the topographic features of the area formed by exogenous processes.
(iv) Matamorphism
According to Turner & Verhoogen (Ignieous and Metamorphic Petrology, New York, Mc Graw Hill Book Co. 1960) “the mineralogical and structural adjustments of solid rocks to physical and chemical conditions which have been imposed at depths below the surface zones of weathering and cementations and which differ from the conditions under which the rocks in question originated” is known as metamorphism. Metamorphism involves the transformation of pre-existing rocks into new types by the action of temperature, hydrostatic as well as_ directed pressure and chemically active fluids. The main feature of the metamorphic processes is that the changes are isochemical and take place in solid state.
2. EXOGENOUS PROCESSES
These are the processes of external origin or, in other words, the processes derive their energy from sources external in relation to the cart viz.
(i) energy from the sun which causes differential heating of atmosphere ere giving rise to differences in pressure that make the wind to blow, sun's energy drives the hydrological cycle which involves the transform of moisture from water bodies to atmosphere to land again to ocean etc.
(ii) the force of gravity
(iii) the activity of organisms etc.
Thus the exogenous processes are closely linked with the role of various external agents such as weathering, blowing wind, runing water, underground water, Waves and Currents in water bodies, (seas and oceans), glaciers etc, on the surface of the earth. Since these processes are restricted to the surface of the earth, they are called epigene processes. These processes constitute a very complex sum of mutually dependent changes i.e. all the exogeneous processes are involved with each other.
The exogenous processes act on the landforms to break up the rocks (weathering), to wear down the surface and carve out valley features (erosion) and the products of destruction are either dislocated under the influence of the force of gravity or are carried away by the blowing wind, flowing waters, moving glaciers etc. to lower areas like lakes, seas, oceans etc. where deposition takes place.
The term Denudation is used for the total action of all processes by which the exposed rocks of the continents are worn away and the resulting sediments are transported to suitable areas for deposition. Thus denudation is an overall lowering of land surface.
The exogenous processes tend to remove all the unevenness on the surface of the earth. As we know, the unevenness of the earth’s surface is developed due to crustal movement, unequal erosion and deposition. The process by which the earth’s surface irregularities are removed and a level surface is created, is known as gradation. All gradation processes are directed by gravity. The processes of gradation are divisible into two major categories viz. Degradation and Aggradation.
Degradation is the process in which material from the high lands are removed by the geomorphic agents as a result of which the altitude of the highlands are reduced. Degradation of the earth’s surface ii mainly carried out through
(a) Weathering,
(b) Mass-wasting, and
(c) Erosion.
Weathering is the process of mechanical disintegration and chemical decomposition of the rocks at the earth’s surface, under the influence of factors like temprature fuctuations, water, oxygen, carbon dioxide and organic life. It is a static process and does not involve any transport of the degraded rock materials.
Mass-wasting is the process that involves the spontaneous downward movement of soil, regolith and rock under the influence of gravity. Mass-wasting is usually aided by the presence of water but without any dynamic action of it or in other words the amount of water present is not sufficient to act as a transporting medium. The process occurs at speeds ranging from those which are so slow that the moment is imperceptible to rapid flow and catastrophic slumping and rockfalls. Evidences of the down-slope movementof rock and soil is found almost universally.
Erosion is the process associated with the geomorphic agents like wind, river, glaciers etc. by which rock materials are loosened or dissolved and then transported from one place to the other. According to Arthur Holmes (Principles of Physical Geology, Second Edition, The English Language Book Society and Nelson, 1975) “all the destructive processes due to the effects of the transporting agents are described as erosion”.
Erosion consist of processes like:
(i) collecting together of the loose material produced by weathering
(ii) wearing down the surface and carve out valley features which is commonly known as abrasion or corrasion
(iii) mechanical wear and tear of the transported materials while they are in transit by the geomorphic agents, and
(iv) chemical solution through the dissolving power of the geomorphic agents like rives-water, sub-surface water etc, which is also known as Corrosion.
Aggradation is the process of deposition of sediments, As we know, under favourable conditions, when the transporting agents lose their carrying power, the transported materials get deposited, Sometimes in the sea, sometimes on the land. Thus the low lying tracts are gradually filled up through deposition of sediments by running water, ground water, wind, glaciers, wave, currents, tides in seas, Oceans etc.
Thus, in the nature the process of gradation is considered as a three fold process because the earth's surface is first decayed and eroded,secondly the products of the decay and erosion are transported and finally the transported materials are deposited in low lying areas.