Denudation Geography Notes PDF

# Denudation Denudation is the process of breaking and removing the rocks from the surface of the earth. It is wearing away of landmass by various processes like weathering, erosion mass movement and transportation. It results in lowering the level of land, rounding exposed rock surfaces and levell...

# Denudation Denudation is the process of breaking and removing the rocks from the surface of the earth. It is wearing away of landmass by various processes like weathering, erosion mass movement and transportation. It results in lowering the level of land, rounding exposed rock surfaces and levelling the peaks. ## Processes Involved in Denudation ### Weathering Weathering refers to the disintegration of rock by atmospheric agents at or near the surface of the earth. It happens due to a change in weather conditions like temperature, moisture and precipitation. ### Erosion Erosion refers to the displacement of rocks by agents like wind, water or ice. Erosion is caused by the movement of these eroding agents and the resulting smaller particles are transported to another place, whereas in weathering no such movement occurs. ### Mass Movement or Mass Wasting Mass Movement or Mass Wasting refers to the large-scale movement of loose materials (rock-waste) down the slope on account of gravity. The steeper the slope, the more rapid is the movement. ### Transportation Transportation refers to the movement of materials loosened by erosion and transported to the other place by the action of wind or water. ### Deposition Deposition refers to the laying down of material that has been weathered, eroded and transported by natural processes such as water, wind and ice. ## Agents of Denudation Agents of denudation erode, transport and deposit sediments at the earth's surface to produce erosional and depositional landforms. The four active agents of denudation are water, wind, waves and glacial ice. In this chapter, we shall study the work of rivers and wind as agents of denudation. # Work of a River Rivers are the greatest agents of erosion, transportation and deposition. The force of erosion of a river depends on the degree of inclination or gradient of its course, the volume of water it carries, the load of the sediments, velocity of water, and water discharge. ## Factors Affecting Work of a River Various factors affect erosion, transportation and deposition work of a river. Some important factors include the following: ### Head 1. **Velocity of water:** Erosion and transportation are both maximum when velocity is high. In other words, erosion and transportation are proportionate to the velocity of water. Deposition, on the other hand takes place when velocity of water is low. 2. **Volume of water:** The larger the volume of water, the greater is the power of erosion and transportation. Deposition, on the other hand, takes place better when there is reduced volume of water as well as when the load is greater. 3. **Load:** Load is the material transported by a river. The load of a river, does all the work of erosion and deposition. Mass wasting, deepening of river valleys and formation of landforms depend on the load a river carries. ## Course of a River Rivers normally originate in mountains from glaciers or from lakes. Some rivers, like those in the Deccan Plateau Region, are formed due to meeting of various streams during rainfall. The place of origin of a river is called its source. Where a river enters the sea or disappears before joining the sea is called its mouth. The streams or small rivers which join the main Hier are called its tributaries. Where a neer divides into channels or smaller niers, they are called distributaries. The path along which the river flows is known as the course of a river. From the source to its mouth, a river is divided into three main sections: Upper Course, Middle Course and Lower Course. These three sections are also called the Profile of a River. ## Upper Course In the upper course of its flow (the river is young), it is in the initial stage. At this stage, the river flows swiftly for the gradients are steep. The upper course of a river generally lies in a hilly area and the river flows a considerable distance over a steep slope. Along its course, it erodes its bed and banks; and carries large boulders of rock with it. Under the impact of rush of water and the load the river carries, erosion action is swift and effective. Therefore, in the upper course, the dominant activity of a river is erosion. Many landforms are shaped by this activity of the river. ### Landforms The significant landforms resulting from erosion in the upper course of a river are the following: 1. **V-shaped Valleys:** A V-shaped valley is a narrow valley that has a profile suggesting the form of the English alphabet "V". It is formed from a stream eroding downward, through a process, called down cutting. These valleys are formed in the initial stages of rivers and have steep slopes. **Fig. 10.3 River Valleys** In regions of hard rocks, it develops almost vertically in an I-shape. River valleys normally formed in areas of sufficient rainfall and where rocks are not very hard or resistan. 2. **Waterfall:** A waterfall is defined as a vertical fall of water of enormous volume from a great height in the long profile of a river. A waterfall is generally formed due to large differences in the rate of erosion. The smaller differences lead to formation of step like features known as rapids. The water that fall down the edge of a hard resistant rock may have at the bottom a soft rock. It is this soft rock, that gets eroded fast and creates a hollow basin called plunge pool. Some of the world's highest and best known falls are Angel Falls (986 m) in Venezuela and Yosemite falls (778 m), California, USA. Jog Falls or Gersoppa falls, on a tributary of Kaveri river, have a plunge of 60 metres. ## Middle Course The second stage of a river is known as the haturity stage. It corresponds to the middle course. At this stage, the gradient is reduced, and the river flows more slowly. The middle course of the river begins when it leaves the mountain area and enters the plains. In the middle course, the energy required to transport the materials is just enough to drag large particles. Two other important characteristics of the middle course are: - The lateral cutting s more active than down cutting, - and the volume of water in the river increases due to many tributaries joining it. As a result of these characteristics, the river carries its load in different ways. Fine particles remain suspended in water, large particles are just dragged along the bed of the river and smaller ones are rolled down in the centre. The river also gets widened as a result of great volume of water and the impact of the load. ### Landforms of The Middle Course In the Middle Course, the river's deposition work is more important than its erosion work. Therefore, the landforms of deposition are more prominent than the landforms of erosion. 1. **Meander:** River meanders are bends of longitudinal courses. S-shaped meanders which are common to most rivers result when a channel forms a curved path. These are the result of both erosion and deposition work of rivers. Meanders generally form under conditions of a gentle slope and sufficient water in rivers. The river flow is diverted by an obstruction allowing the river to do lateral erosion work. When the curvature of meander loops are made more and more circular, the curvature then becomes more sharp and the river breaks through the meander to form an oxbow lake. The Ganga in India and Mississippi in the USA are famous for their meanders. ## Lower Course of The River The third stage of a river is known as the old stage. This corresponds to its lower course. The river flows sluggishly and makes many Tandforms. In the lower course of the river, all the ideal conditions for depositional landforms exist. But the load-transporting capacity is drastically reduced due to sluggish flow as well as division of a river into many distributaries. ### Landforms of The Lower Course 1. **Delta:** The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to deposition of sediments over a large area near the mouth of rivers. Such sedimentation occurs on the sides of the stream, at the mouth in the front part as well as in the bed of the river. The deposition over a large area results in a triangular-shaped formation called delta. It resembles the Greek letter A. (Not all the rivers form deltas. There are certain conditions for the formation of deltas. These conditions include nearly calm sheltered Sea, large amount of sediment supply and a reasonable size of the river. The small rivers do not form deltas. The deltas have also their own structure and undergo upliftment or subsidence. The deltas also grow at a certain rate. For example, the Ganga-Brahmaputra delta is growing towards the sea. It is also one of the largest in the world. From here, blue pe into the direction and force of wind.) # Work of Wind The movement of air over the earth's surface is known as wind. It is almost horizontal, although there is a small vertical component. Wind picks up loose debris like sand and pebbles and transports as well as deposits them in another location. Wind can lift and carry sand grains away. When the speed of the wind slows down, the sediments brought by wind begin to drop on the ground. If this process continues, it results in a huge expanse of sand known as deserts. ## Action of Winds The wind is the most effective agent of erosion, transportation and deposition in arid regions. Since there is little vegetation or moisture to bind the loose surface materials, the effects of wind erosion are more pronounced in the desert region/on'd regions]]. Wind erosion is operative in the following ways: 1. **Deflation Hollows:** Deflation involves the lifting as well as the blowing away of these materials from the ground. The sand, lond pebbles are carried in the air or driven along the ground. The finer dust and sands always be deposited even outside the desert margins. Deflation results in the lowering of the land surface to create depressions called deflation hollows. The Qattara Depression of def sahara Desert which lies almost 450 feet below sea level is a good example of deposition. 2. **Sand Dunes:** Dunes are called hills of sand. They are formed by the movement of winds. Such hillocks may be active dunes, constantly on the move. They may also be inactive fixed dunes, rooted with vegetation. Dunes are found in deserts where the sand is being continuously moved, reshaped redeposited and. Shape and Size Dunes have heights ranging from a few metres to about 150 metres. In desert regions of the Sahara, Thar, and West Australia, there are many sand dunes. They are in different shapes and sizes (In fact, their shape and size depend upon factors such as The speed of wind, carrying the dust and sand particles. The nature and amount of sand brought by the wind. Water features e.g., lakes, streams etc., in the close-by area. The nature of vegetation on the land. The nature of the surface, where the formation of a sand dune takes place. **Shifting or Migration:** Sand dunes often move in the direction of the wind. The dunes of Trade Wind deserts take definite forms, moving in a definite direction. (Migratory sand dunes are uncertain and dangerous. When they migrate with the wind, they damage towns and villages. The shifting of a sand dune can be checked by the growth of vegetation on the wind-ward slope. Dunes are found in the tropical deserts of Asia and Africa. **Direction of Wind** A - Original Position of Dune B - Later Position Fig. 10.8 Shifting of Sand dune **(The following two types of dunes are seen in the tropical deserts:** 1. **Barchan:** These are moon-shaped dunes. They are live dunes which advance steadily before winds that blow from a particular direction. They are found in the deserts in the Sahara. Barchans are initially formed by an accumulation of sand at an obstacle, such as a heap of rocks. They occur transversely to the wind, so that their horns thin out and become lower in the direction of the wind. The windward side is convex and gently- ng while the leeward side (being sheltered) cave and steep. The crest of the sand dune is forward as more sand is accumulated. **Wind Direction** **Fig. 10.9 Barchan** The migration of the barchans is a threat to desert life. They sometimes encroach on an oasis, burying palm trees or houses. (Long-rooted trees and sand-holding grasses are planted to halt the advance of the dunes in order to prevent the fertile land from being devastated.) 2. **Longitudinal Dunes or Seif:** These are long and narrow sand-ridges which grow parallel to the direction of the prevailing wind. They are found in the interior parts of deserts. In the Libyan desert, they are known as Seifs (means 'sword' in Arabic). In the Thar Desert, they are confined to the western margin, where the force of south-west monsoon influences their formation. (An important feature of such a dune is that in its crest line there are rise and fall patterns. Seifs are found in the Sahara, Iran, Thar Desert (India) and West Australia.