What are the Two Main Geological Processes of the Development of the Surface Globe?

It is an established fact that the development of the surface globe is mainly due to the complex interaction of atmospheric processes, rocks, ocean waters and living surface of the earth is a zone where the rocks uplifted by forces come into contact with the atmosphere and hyi they are subjected to a range of processes powered by Sun.

The processes which have been playing dominant the surface of the earth are both constructive as well as nature. All the geological processes can be conveniendy 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 from 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 to a wild motion capable of shaking builidings apart and causing gaping fissures to open up in the ground. The earthquakes are mostly produced due to underground dislocation of rocks.

(ii) Tectonic movements

Tectonic movement of earth's crust are of various forms and are characterized 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 intrusives 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 (Igne­ous 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 iso-chemical 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 earth viz. (i) energy from the sun which causes differential heating of the atmosphere giving rise to differences in pressures that make the wind to blow, sun's energy drives the hydrological cycle which involves the transfer 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, running 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 arc 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 is 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 temperature fuctuqtions, water, oxygen, carbon-dioxide

(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

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.

(v) chemical solution through the dissolving power of the geo­morphic agents like river-water, sub-surface water etc, which is also known as Corrosion.

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.

The geological processes, as already indicated, play significant roles in shaping the surface of the earth. The details of the role played by each individual geomorphic agent are discussed in separate chapters of this book.