Selina Concise Solutions for ICSE Class 9 Geography Chapter 10 Denudation

Exercises

I. Short Answer Questions

Question 1: What do you mean by denudation ?
Answer: Denudation is the 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.
Denudation acts like a giant natural sandpaper that smooths out the rough edges of the Earth's surface over millions of years. It combines different forces of nature to slowly transform high mountains into lower, flatter plains.
Teacher's Tip: Remember the word 'denude' means to make bare or strip away, just like these processes strip the land surface.
Exam Tip: To get full marks, make sure to list all four processes: weathering, erosion, mass movement, and transportation.

Question 2: Name important factors which affect work of a river.
Answer:
Refer Long Answers Question 1
The main factors that influence how a river shapes the land are its speed, the amount of water it holds, and the type of rocks it flows over. These factors determine whether the river will mainly cut into the ground or drop the materials it is carrying.
Teacher's Tip: Think of a fast-moving slide; the faster you go, the more energy you have to push things out of your way!
Exam Tip: Even if the book says 'refer to another question', always memorize the keywords: Velocity, Volume, and Load for this answer.

Question 3: What is the main function of a river in its upper course in land formation ?
Answer: In the upper course the dominant activity of a river is erosion. Many land forms are shaped by this activity of the river. Along its course, it erodes bed and banks, and carries large boulders of rock with it. Under the impact of rush of water.
In this stage, the river has a lot of energy because it is usually flowing down steep mountain slopes. This high energy allows it to carve deep into the earth and transport heavy rocks downstream.
Teacher's Tip: Visualize the upper course as a 'mountain climber' phase where the river is young, fast, and very strong.
Exam Tip: Always emphasize 'Vertical Erosion' or 'Down-cutting' when describing the upper course function.

Question 4: Name two important land forms of river erosion.
Answer: The two landforms resulting from erosion in the upper course of a river are the following :
1. V-shaped Valleys : 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.
2. I-shaped Valley : In regions of hard rocks it develops almost vertically in an I shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.
V-shaped valleys look like the letter 'V' because the river cuts the bottom while the sides are worn down by rain. I-shaped valleys, also called gorges, happen when the rock is so hard that only the very bottom gets carved out.
Teacher's Tip: Use the shape of the letters 'V' and 'I' to visualize the steepness of the valley walls.
Exam Tip: Mention 'downward erosion' or 'down-cutting' as the primary process for forming these valleys.

Question 5: How is a waterfall formed ?
Answer:
Waterfall : 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 falls 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 fall (778 m), California, USA.
Waterfalls happen when a river flows over a layer of hard rock followed by a layer of soft rock. The soft rock wears away much faster, creating a vertical drop that the water plunges over.
Teacher's Tip: Imagine a "step" made of a hard brick and a soft sponge; the sponge would wear away first, creating a drop.
Exam Tip: Including the name and height of a real waterfall, like Angel Falls, helps show you have a deep understanding.

Question 6: State the main function of a river in its middle course.
Answer: At this stage, the gradient is reduced, and the river flows more slowly. The middle course of the river begins when it leaves the mountains area and enters the plains. In the middle course the energy required to transport the materials is just enough to drag large particles. 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 windened as a result of great volume of water and the impact of the load.
In the middle course, the river stops cutting down as much and starts cutting sideways, which makes the valley wider. The river's main job here shifts toward moving or transporting the sediment it picked up earlier.
Teacher's Tip: Think of the middle course as the "carrying stage" where the river is like a truck transporting goods across flat land.
Exam Tip: Mention 'Lateral Erosion' (sideways wearing) to explain why the river gets wider in this stage.

Question 7: How are meanders formed ?
Answer: 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 a river hits a flat area, it starts to snake around obstacles rather than flowing straight. It wears away the outer bank and drops sand on the inner bank, creating those famous S-curves.
Teacher's Tip: Think of a meander as a "winding snake" moving across a flat field.
Exam Tip: Be sure to state that meanders involve both 'erosion' and 'deposition' to get full credit.

Question 8: Name two landforms in the lower course of a river.
Answer:
1. Delta : The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to depsoition 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 The Ganga-Brahmaputra delta is growing towards the sea. It is also one of the largest in the world.
2. Oxbow lakes are also formed in the lower course of a river.
In the lower course, the river is very slow and drops all its heavy sand and mud before reaching the sea. This creates a fan-like delta or loops that get cut off to form crescent-shaped oxbow lakes.
Teacher's Tip: Delta starts with 'D' and looks like a triangle, just like the Greek symbol Delta (Delta).
Exam Tip: When describing a Delta, always mention that the river splits into 'distributaries'.

Question 9: Where is wind erosion most predominant ?
Answer: The wind erosion is most predominant 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.
Wind erosion happens best where the ground is dry and there are no plants or trees to stop the wind from blowing the sand away. In these places, the wind can pick up tiny particles easily and blast them against rocks.
Teacher's Tip: Think of a desert; without grass "roots" to hold the soil, the wind can easily move it around.
Exam Tip: Use the word 'Arid' instead of just 'Dry' to sound like a geography expert!

Question 10: What are known as Deflation Hollows ?
Answer:
Deflation Hollows : Deflation involves the lifting as well as the blowing away of loose materials from the ground. The sands and pebbles are carried in the air or driven along the ground. The finer dust and sands may be deposited even outside the desert margins. Deflation results in the lowering of the land surface to create depressions called deflation hollows.
Deflation hollows are basically big "dents" in the desert floor created when the wind scoops out all the loose sand from one spot. Over time, these dents can become very deep and large.
Teacher's Tip: Imagine a giant scooping out a spoonful of sand from the desert floor; that hole is a deflation hollow.
Exam Tip: Remember that 'Deflation' refers to the wind lifting and removing particles, which is the key process here.

Question 11: Name one chief landform of wind deposition ?
Answer: Hamada is the hard bedrock exposed after sweeping away of the dust and sand by the wind.
When the wind blows away all the loose dirt and sand, it leaves behind a flat, rocky surface made of solid stone. This bare, rocky desert floor is what we call a Hamada.
Teacher's Tip: Think of sweeping a dusty floor until you reach the hard tiles underneath; the tiles are like the Hamada.
Exam Tip: Be careful! While the book lists Hamada under deposition, it is actually an erosional landform created by the removal of material.

Question 12: Name two chief types of sand dunes.
Answer: The following two types of sand dunes are seen
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 of the Sahara. These are initially formed by an accumulation of sand at an obstacle, such as a heap of rocks. These occurs transversely to the wind, so that their horns thin out and become lower in the direction of the wind.
2. Longitudinal Dunes or Seifs : 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. These sand dunes are stationary and they remain in a fixed position.
Barchans are crescent-shaped and actually move across the desert as the wind blows. Seifs are long, straight lines of sand that look like giant swords lying on the ground.
Teacher's Tip: 'Barchan' sounds like 'Crescent' (moon shape), and 'Seif' means sword!
Exam Tip: Mention the direction of the wind—Barchans are 'transverse' (across) while Seifs are 'parallel' to the wind.

Question 13: State three chief characteristics of barchans.
Answer:
1. Barchans are moon-shaped dunes. They are live dunes which advance steadily before winds that blow from a particular direction.
2. They occurs transversely to the wind, so that their horns thin out and become lower in the direction of the wind.
3. The windward side is convex and gently-sloping while the leeward side (being sheltered) is concave and steep.
Barchans have a unique shape with two "horns" pointing in the direction the wind is blowing. One side of the dune is a gentle slope that faces the wind, while the other side is a sharp drop-off.
Teacher's Tip: Imagine a crescent moon sliding across the sand; that is exactly how a Barchan looks and moves.
Exam Tip: Use the terms 'Windward' (faces wind) and 'Leeward' (side away from wind) to describe the slopes correctly.

II. Long Answer Questions

Question 1: Briefly describe the factors which affect the work of a river.
Answer: Various factors affect erosion, transportation and deposition work of a river. Important factors include the following :
1. Velocity of water : Erosion and transportation are both maximum when velocity is high. 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.
How much a river changes the landscape depends on how fast it moves, how much water it has, and what it is carrying. A fast, full river can tear through rock, while a slow, small river will drop sand and mud instead.
Teacher's Tip: Remember the 'Big Three': Speed (Velocity), Amount (Volume), and Baggage (Load).
Exam Tip: For each point, mention both the 'Positive' effect (what happens when high) and 'Negative' effect (what happens when low).

Question 2: Describe chief landforms of river deposition.
Answer: The cheif landforms resulting from the deposition of a river are the following :
1. V-shaped Valleys : 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.
2. I-shaped Valley : In regions of hard rocks it develops almost vertically in an I shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.
Although the textbook lists V and I shaped valleys here, these are actually landforms made by erosion (cutting away rock). Deposition landforms usually include things like deltas and floodplains where the river drops its materials.
Teacher's Tip: Note that your textbook answer focuses on erosion forms here, so keep that in mind for your studies.
Exam Tip: If the question asks for 'deposition' landforms specifically, landforms like Deltas and Alluvial Fans are the most accurate examples to give.

Question 3: Write a short note on the three stages of a river.
Answer: 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 the dominant activity of a river is erosion. Many land forms are shaped by this activity of the river. Along its course, it erodes bed and banks, and carries large boulders of rock with it. Under the impact of rush of water.
• 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 mountains area and enters the plains. In the middle course the energy required to transport the materials is just enough to drag large particles. 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 windened as a result of great volume of water and the impact of the load.
• Lower Course – The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to depsoition 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 The Ganga -Brahmaputra delta is growing towards the sea. It is also one of the largest in the world. Oxbow lakes are also formed in the lower course of a river.
A river changes its behavior as it moves from the mountains down to the sea, moving through three distinct life stages. It starts as a powerful cutter (Upper), becomes a hardworking transporter (Middle), and ends as a slow depositor (Lower).
Teacher's Tip: Think of a river's life like a human: Energetic Youth (Upper), Stable Adult (Middle), and Slow Senior (Lower).
Exam Tip: Make sure to highlight one primary activity for each stage: Erosion (Upper), Transportation (Middle), and Deposition (Lower).

Question 4: Describe the work of wind erosion and state two important landforms of wind erosion.
Answer: Winds move along the desert floors with great speed and the obstructions in their path create turbulence. Storm winds are formed which are very destructive. Winds cause deflation, abrasion and impact.
Deflation includes lifting and removal of dust and smaller particles from the surface of the rocks. The impact is the simply the shear force of momentum which occurs when sand is blown into or against a rock surface. These are the ways in which wind action creates a number of interesting erosional and depositional features in the deserts.
Winds are not the only agent responsible for the erosional features of the deserts. Along with winds the running water is also important for this.
Wind acts like a powerful sandblasting machine that wears down desert rocks by hitting them with flying sand. This process, along with lifting loose dust into the air, creates unique landscapes over many years.
Teacher's Tip: Think of 'Abrasion' as nature using sandpaper to polish and wear down stones.
Exam Tip: Be sure to name 'Deflation Hollows' and 'Mushroom Rocks' as the two landforms to answer the second part of the question.