The earth is made up of a series of concentric rock layers namely crust, mantle and core. The intense heat in the earth's interior does not travel to the surface because rocks are bad conductors of heat. The internal heat of the earth therefore melts the rock and keeps the asttenosphere which is underneath the crust in a semi molten state. This leads to several conditions which cause landform development on the surface of the earth. A landform is a natural feature of earth surface.

CAUSES OF LANDFORM DEVELOPMENT ON THE SURFACE OF THE EARTH

(1) Plate Tectonics : The lithosphere is broken into several small and big plates. There plates float on the semi molten asthenosphere. The horizontal movements of there plates is very slow. The plates push, collide or slider past each other. This result in disturbance in the sedimentary rock deposit lying in between two or more plates the sedimentary rocks are compressed. This compression causes folding of the rocks. Almost all the fold mountain ranges of the world have been formed this why. The forces responsible for formation of Fold Mountains are called organic movements. The term 'orogenic is derived from the break of the word meaning 'mountain-building.' These forces act at a tangent to the surface of the earth and are primarily involved in plate tectonics.

(2) Uplift and submergence : Another set of forces operating inside the earth act along a radius from the earth's centre to the surface and are characterized by large scaleupinit or subsidence of land area. These vertical movements are known as epeirogenic movements. Such movements are a result of faulting or cracks in the earth's surface Blocks mountains, basins, rift valley are produced by there movements. 'epeiros' in Greek means 'continent' 'genic' or 'gentic' means 'building' thus the continent building actuaries occur on a very large scale

The above two force, namely orogenic and epeirogenc are together called diastrophic forces. They act horizontally and vertically and produce reliefs such a mountains, plateaus, plains lakes and big faults.

(3) Sudden Forces : Volcanic activity is a sudden movement volcanic eruptions results in the formation of volcanic mountains, extensive plateaus and also plains. Earthquakes are also termed as sudden movements. Earthquake causes formation of lakes, other fractures and creates faults on the surface of the earth. Earthquakes and volcanic activities are described as sudden force. Geologically these sudden forces are also termed as 'constructive forces' because they create relief features on the surface of the earth.

For the purpose of classification the sudden forces and the Diastrophic forces described above form two pasts of endogenic forces. These forces are active deep inside the earth. All the internal forces described above are called constructive movements.

(4) Exogenic forces : There are forces operating on the surface of the earth which cause formation of landforms. These forces are called exogenic forces. The external forces are described as destructive forces because they cause widespread destruction through weathering and erosion of existing landforms. They then form new landforms through both erosion and deposition. Landforms are produced by river erosion and deposition of sediments. Glacial erosion and deposition as well as wind erosion and deposition landforms of these types include valley alluvial and erosion plains, valleys glacial landform etc.

LANDFORMS AND THEIR TYPES

There are a large number of landforms on the surface of the earth. There are also several methods of their classification. Even after major landforms are made by internal forces of the earth, they are suspected to modification by the external forces. We will therefore mention these landforms according to there present relief features. The four major landforms are:-

• Mountains

• Plateaus

• Plains

• Valleys and Basins

(1) MOUNTAINS

A mountain is described as a very steep land or hill rising to great heights above the land surrounding it mountains are classified into three main types depending on the forces that have formed them. Three main types of mountain are fold mountains, Block mountains and Volcanic mountains.

Fold Mountains : They are a result of lateral compression of the earth's crust Fold mountains are the result of large scale earth movements caused by stresses in the earth's crust. Such stresses may be caused by weight of the overlying rocks, movements in the mantle, the expansion or contraction of some past of the earth. These stresses subject the rocks to compressive force, producing wripling or folding along the lines of weakness.

The Chief Characteristics of fold mountains wave like formation called folds. The upfolds rock strata in arch like shape are called anticlines or crest and the down folded structure is known as synclines or trough.

Fold mountains are the highest and the most range of the world. Their chief characteristics are:-

• They form irregular folds and are the youngest mountains on earth's surface.

• Most of the mountains are constituted of sedimentary rocks formed due to deposition and consolidation of sediments in shallow ocean like the Tethys, in the case of Himalayas .

• Fold mountains have greater length but are comparatively smaller in width the Himalayas about 2400Km in length.

• Most fold mountains expect the Himalayas are found on the margins of continents. They are located in two directions. Rocky and Andes are en north-south direction. The Himalayas are in west-east direction it is clear that all of them face the ocean.

• Fold mountains are associated with volcanic activity and has many active volcanoes.

The fold mountains chains that exist beneath the sea have there tops reveals as islands like in the west Indies. There are also oceanic trenches which are the results of the same force as fold mountain these forces in combination with volcanic mountain lead to the formation of ridges.

Block Mountains : The force of faulting also known as epeirogenic forces, lead to the formation of block mountain Due to tensional forces cracks or faults may occur on the earths crest. If two parallel faults occur, the land in between subsides to form a rift valley. If a block rises it forms a horst

(2) PLATEAUS

Plateaus are defined as an area of high ground, 500 feet above the ground level, these uplands have steep slope on one side. The upper past is generally flat and is called table land. Plateaus are also classified in several ways.

Intermountain Plateaus : These are the highest and most extensive types on the globe. The plateaus surrounded by hills and mountains on all sides are called intermontane plateaus they are formed along with fold fountain. The Tibetan plateau is the world's highest as well as most extensive intermontane plateau.

Piedmont Plateaus : Plateau surrounded by mountains on one side and plains on the other side are called piedmont plateau. The part facing the plains has steep slope. For example the Patagonian plateau of South America and the Appalachian Piedmont plateau which terminates in the Atlantic Coastal plains. Piedmont plateaus are residual plateaus derived from ancient fold system and are also called Residual Plateaus. Ardennes and Middle Rhine Highland are best example of residual plateaus.

Volcanic Plateaus : They are derived from lava flows from Volcanic eruptions. The plateau of Peninsular India especially the north western Deccan is an example of lava plateau. The Indian plateau covers an area of 774000 Sq km; other examples are south Africa plateau, Columbian plateau Ethiopian plateau. The peninsular plateau of India has enormous accumulation of basaltic rock. The Columbian plateau has about 20 layers of lava Suggesting that 20 different volcanic eruptions have formed this plateau.

Uplifted Plateaus : During volcanic activity the endogenic forces may uplift the middle position of the landmass in such a manner that its sides get rounded forming a dome shaped tableland Chhotanagpur Plateau is a typical example. Ozark plateau in the USA is another example.

(3) PLAINS

A plain is defined as defined as an extensive area of lowland with a level or gently, undulating surface. The formation of plains is one of the most intriguing landform development process on the surface of the earth. There are seemingly endless variation in plains. They are also an imprint of past and present processes. For example a sudden heavy downpour may wash away a standing landform or fill up a depression to form a plain. A plain is seldom formed by a single process. However three types of plains can be distinguished-structural plains, Erosional Plains and Depositional Plains.

Structural Plains : These plains are formed due to uplift or subsidence of land. Diastrophic force may cause uplift of a portion of land beneath the ocean water or may cause submergence of coastal land under ocean water. The Great plains of the USA were formed due to uplift of land submerged under water. The great plains are surrounded by Mississippi-missus plains in the east and Rockies in the west. The fact that they were submerged under water is responsible for the existence of many lakes in the vast lowland. Gulf coast is another example of an emerged coastal plain.

On the other hand, the coromandel plains in India are a result of mild subsidence followed by sedimentation. Thus, the forces of structural developments well as deposition were active in the formation of these plains.

Erosional Plains : These plains are formed by the agents of erosion after millions of years even high mountains are reduced to low undulating plains these include the following.

Pene Plains : They are landscape in which almost all hills been worn down as a result of erosion and wild plains predominate with some resistant peak remaining. Pene plain literally means almost a plain Niagara Plain in the USA Lorraine in France and southern England plains are example of these plains.

Plains of Glacial Erosion : These plains have rounded peaks and worn down ice scoured shield lands. Glaciated plains are those of Kashmir in India , northern past of North America and North western Eurasia .

Wind-eroded Plains : These are called Reg, Seris and Hamada in the Sahara desert, strong winds pick up weathered materials and deposits them elsewhere there formation are also called pediplains.

Karst Plains : There plains have corrugated and undulating surfaces produced by chemical weathering of limestone landscape by ground water for eg. Karst region of Yugoslavia .

Depositional Plains : There plains are formed by the deposition of material which have been brought by various agents of transposition.

River Deposition : The most widespread of there are alluvial plains like the northern plains of India, the flood plains like the Mississippi Plains in the USA deltaic plains in Egypt and India. At the foot hills of mountain are formed Piedmont alluvial Plains. The flood plain described above are also alluvial plains. They are found along the rivers like Yangtze, Mekong , Salveen etc. and are regarded as granaries of the world. In India flood Plains are divided into Khadar Plains and Bhangar.

Wind Deposition : Most such plains are sandy deserts like the Sahara in Africa and the desert in India. They have irregular and undulating surface made by existence of sand dunes and fallows. Loess plains in china are formed from deserts.

Marine Deposition : There plains are found in coastal regions. Examples are the east coast of India , the estuarine banks in Netherlands coast of Germany and Denmark . In Denmark we find polders and dykes. A folder is a piece of land reclaimed from the sea or lake. The submerged land is surrounded by an embankment and drained by pumping water into canals.

IMPORTANCE OF LANDFORMS

The landforms on the surface of the earth are symbol of energy of the earth they are basically a result of enormous heat generated inside the earth and climate factors operating on the surface of the earth. The earth releases its energy through tectonic activity, earthquake and volcanoes. There internal forces in addition to the surface weathering and erosional force become dominant factors in sculpturing of land the land forms have thus the following advantages:-

• Mountains, plateaus, plains, valley are beautiful sculpture of nature on the surface crust of the earth they add to the beauty of the earth.

• Landforms represent the relief feature of the earth they have and impaction climate directions of winds, precipitate as well as other atmospheric processing the Himalayas act a climatic divide shaping the distinctive climate of the Indian subcontinent.

• The mountain ranges of the world are the abode of snow and glacier. They are thus the source of perennial rivers.

• Many kinds of forest are found on the slope on mountains hills and plateaus. There are home to different species of plant and animal, thus biodiversity of plant and animal, thus biodiversity of life on earth can also be attributed to relief.

• Since such relief features also exist in oceans, the island groups and coastal regions are Ideal fishing sites.

• Major landforms are created by disturbance resulting from the forces operating in the interior of the earth. Most mineral and fossil fuel come near the surface of the earth due to these disturbances.