ICSE Class 6 Geography Chapter 5 Landforms of the Earth

Chapter Five

Landforms of the Earth

Landforms of the Earth: Mountains, plateaus, plains (definition, types and their formation)

Mountains - fold, residual and block.

Plateaus - intermontane and volcanic.

Plains - structural and depositional.

Examples from the world and India.

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 rocks and keeps the asthenosphere which is underneath the crust in a semi-molten state. This leads to several conditions which cause landform development.

Causes of Landform Development

1. Plate Tectonics

The lithosphere is broken into several small and big plates. These plates float on the semi-molten asthenosphere. The horizontal movement of these plates is very slow. The plates push, collide or slide past each other. This results in disturbances in the sedimentary rock deposits 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 way. The forces responsible for the formation of fold mountains are called Orogenic movements. The term 'orogenic' is derived from the Greek word meaning 'mountain-building'. These forces act at a tangent to the surface of the earth and are primarily involved in plate tectonics.

Fig. 5.1. Forces causing formation of landforms

2. Uplift and Submergence

Another set of forces operating inside the earth act along the radius from the earth's centre to the surface and are characterised by uplift or subsidence of land area. These vertical movements are known as Epeirogenic movements or Continent building movements. Epeiros in Greek means 'continent'; 'genic' or 'genetic' means 'building'. Such movements are the result of faulting or cracks in the earth's surface. Block mountains, basins, rift valleys, are produced by these movements. Thus, the continent building activities occur on a very large scale.

The above two forces, namely orogenic and epeirogenic, are together called Diastrophic Forces (originating deep inside the earth). They act at horizontally and vertically and produce reliefs such a mountains, plateaus, plains, lakes and big faults.

Another set of forces operating inside the earth act along the radius from the earth's centre to the surface and are characterised by uplift or subsidence of land area. These vertical movements are known as Epeirogenic movements or Continent building movements. The raising of land makes the fold mountains.

If you look at the physical map of the world you will find a number of arc shaped mountain chains. They are Andes (South America), Alps (Europe), Atlas (Africa), the Himalayas, Zagros, Elburg and Kunlun (Asia). More such mountain chains exist below the sea. Almost all the high ranges like the Himalayas and the Alps have peaks covered with snow throughout the year. Many glaciers are found on these ranges. These glaciers become the source of water for perennial rivers. Therefore, these mountains give birth to active agents of denudation and glacial erosion.

Fold mountains are the highest and the most extensive mountain ranges of the world. Their chief characteristics are:

(i) They form irregular wave-like formations called folds. The upfold rock strata in arch-like shape are called anticlines or crest and the down folded structures are known as synclines or troughs. (Fig. 5.2)

(ii) Most of these mountains are constituted of sedimentary rocks formed due to deposition and consolidation of sediments in shallow oceans like the Tethys, in the case of the Himalayas.

Teacher's Note

Fold mountains shaped our world's landscape over millions of years; understanding their formation helps us appreciate why certain regions are prone to earthquakes and have rich mineral deposits.

3. Exogenic Forces

There are forces operating on the surface of the earth, not inside the earth which cause formation of landforms. These forces are called Exogenic Forces. The external forces or Exogenic 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

There are a large number of landforms on the surface of the earth. The major landforms are: (i) Mountains; (ii) Plateaus; and (iii) Plains.

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 mountains are - Fold Mountains, Residual Mountains and Block Mountains.

Fold Mountains

They are the 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 part of the earth, etc. These stresses squeeze the rocks to compressive forces, producing wrinkling or folding along the lines of weakness. This is shown in Fig. 5.2. The raising of land makes the fold mountains.

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