Here under World Landforms, you will find reference information about world landforms.
The nature of landform in a region is shaped by two processes, which are at the work on the earth's crust. These are called the external processes and the internal processes. The external processes are those, which operate on the surface of the crust by external agents of weathering and denudation. These include the work of river, glaciers, winds and waves. The external processes are called the destructive processes, as they tend to reduce the differences in relief of the region. The higher regions are being lowered by erosion and the eroded material gets deposited in the lowland. Hence the external processes are also called gradational processes. The gradational process consists of two components, namely degradation and aggradation. Degradation results in wearing down the landmasses by erosion. Aggradation raises the level of the land by deposition.
If the external gradational processes have been operating in an area, that region has subdued relief features. The working of the external processes can be observed. They act generally slowly but they are continuous in their activity.
The Internal processes operate in the earth's interior layers. Forces build up gradually and suddenly the crust may be subjected to earth movements or volcanic activity or mountain building. Their effect is observed suddenly in the form of earthquakes or volcanic eruption. The internal processes generally raise the level of the region affected and the region has rugged relief features. These are called constructive processes. The working of these internal processes cannot be observed. They can be inferred through indirect evidences only.
The earth movements caused by the internal processes arc of two types: Epeirogenic movements or continent building movements and Orogenic movements or mountain building movements. The Epeirogenic movements take place vertically causing uplift or subsidence of the portions of the crust. These movements are called continent building movements because the present boundaries of continents are delimited by such movements. These up and down movements are caused by tensional forces. Orogenic movements are caused by forces, which are tangential to the earth's surface. They are compressional forces, which tend to compress the crust resulting in the formation of folded mountains. Compressional forces and Tensional forces act simultaneously in different parts of the crust.
Tensional forces cause breaking up of the crust along lines of weakness. Since the dawn of geological time no less than nine orogenic or mountain building movements have taken place, folding and fracturing the earth's crust. Some of them occurred in pre-Cambrian times between 600-3500 million years ago. The three more recent Orogenics are the Caledonian, Hercynian and Alpine. The Caledonian about 320 million years ago raised the mountains of Scandinavia and Scotland and is represented in North America. These ancient mountains have now worn down and no longer exhibit the striking forms that they must once have had in a later period, during the Hercynian Earth movement about 240 million years ago, have formed such ranges as the Ural mountains, the Pennines and Welsh High land in Britain, the Harz mountain in Germany, the Appalachians in North America as well as the high plateau of Siberia and China. These mountains have also been reduced in size by the various sculpturing forces.
Man is now living in an area very close to the last of the major orogenic movements of the earth, the Alpine about 300 million years ago. Young fold mountain ranges were buckled up and over-thrust on a gigantic scale. Being the most recently formed, these ranges such as the Alps, Himalayas, Andes are the loftiest and most imposing their peaks are sometimes several kilometers high. But the time will come when these lofty ranges will be lowered. From the eroded materials, new rocks will be formed, later to be uplifted to form the next generation of mountains.
The internal processes and external processes are complementary to one another. They act in opposite directions and the nature of landform at any place at a given time is the result of the balance between these two processes. Regions like the Himalayas are their present features to the dominance of the internal process of mountain building. They are the result of most recent mountain building. The external processes have not had enough time to denude them. These are old mountains ranges like the Aravallis, which are now reduced to a low level owing to the dominance of external processes of denudation.
The external processes of gradation like rivers transport materials from the higher landmasses and sit them along the coast or on the sea floor. Such transport of materials disturbs the state of balance or equilibrium.
The sea floor gets depressed by the weight of deposits. The mountain regions get uplifted as their mass gets reduced. This may be compared to a ship rising, as the materials are unloaded from a ship. The continents are considered as floating on denser layer below them. Material should flow in the mantle layer to enable slow uplift of the land.
This means that the internal processes cause earth movements, earthquakes and volcanic activity to maintain the crust in a state of balance of equilibrium. Thus the landforms are not static. They are dynamic and change, slow or sudden is a normal phenomenon.
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