WBBSE Class 9 Geography Solutions Chapter 5 Weathering

Detailed explanations in West Bengal Board Class 9 Geography Book Solutions Chapter 5 Weathering offer valuable context and analysis.

WBBSE Class 9 Geography Chapter 5 Question Answer – Weathering

Very Short Questions and Answers : (1 mark for each question)

Question 1.
Where does granular disintegration take place?
Answer:
Desert area.

Question 2.
Which gaseous component is needed in oxidation?
Answer:
Oxygen.

Question 3.
Combining of rock minerals with water is called?
Answer:
Hydration.

Question 4.
Which type of weathering results in rusting of rocks?
Answer:
bxidation.

Question 5.
Name a burrowing animal.
Answer:
Rat.

Question 6.
Exfoliation occurs in which type of rock?
Answer:
branite.

Question 7.
Which type of weathering occurs due to construction of houses and roads?
Answer:
Mechanical weathering.

Question 8.
Ice disintegration occurs in which climatic region?
Answer:
Tundra.

Question 9.
Which type of weathering causes changes in the structural composition of rocks?
Answer:
Chemical weathering.

Question 10.
Which type of weathering is hydrolysis?
Answer:
Chemical weathering.

Question 11.
Name the cone-shaped depositional feature formed as a result of mechanical weathering on hill slopes.
Answer:
Scree or Talus.

Question 12.
Decomposition of organic matter creates which acid?
Answer:
Humic acid.

Question 13.
Where does mass wasting occur?
Answer:
Mountainous area.

Question 14.
Give example of an easily soluble rock.
Answer:
Limestone.

Question 15.
Which two components are most important in case of weathering?
Answer:
Temperature and rainfall.

Question 16.
Mechanical weathering is predominant in which climatic regions?
Answer:
Arid and semi-arid regions.

Question 17.
Which type of weathering is prevalent in homogeneous rocks?
Answer:
Exfoliation.

Question 18.
Which type of weathering occurs in limestone areas?
Answer:
Carbonation.

Question 19.
Which type of weathering is prevalent in the rainy tropical region?
Answer:
Chemical weathering.

Question 20.
What is the loose and thin layer of finely fragmented rocks caused due to weathering known as?
Answer:
Regolith.

Question 21.
What type of chemical weathering is caused due to the reaction of iron with oxygen?
Answer:
Oxidation.

Question 22.
Ferrous oxide is converted into which compound as result of oxidation?
Answer:
Anhydrous ferrous oxide.

Question 23.
What is the process whereby small grained rocks are further fragmented known as?
Answer:
blaking.

Question 24.
Which process causes disintegration of rocks as a result of decrease in pressure of the rock layers (due to mechanical weathering)?
Answer:
Sheeting.

Question 25.
What is formed as a result of decomposition of branches, leaves, flowers and fruits of trees after they fall to the ground?
Answer:
Humus.

Question 26.
The process whereby rocks are broken apart is an example of which type of weathering?
Answer:
Mechanical weathering.

Question 27.
What occurs when rocks get heated during day time?
Answer:
They increase in volume.

Question 28.
Which type of weathering causes rocks to get fragmented in a square shape?
Answer:
Block disintegration.

Question 29.
Which type of weathering causes rock layers to come off like the peels of an onion?
Answer:
Exfoliation.

Question 30.
Who was the first to use the term ‘weathering’?
Answer:
G. K. Gilbert.

Question 31.
Which type of weathering is predominant in the Thar Desert?
Answer:
Granular disintegration.

Question 32.
In which type of weathering does gunshotlike noises occur?
Answer:
Granular disintegration.

Question 33.
In which type of weathering does both physical and chemical changes occur in rocks?
Answer:
Chemical weathering.

Question 34.
In the process of soil formation, when the minerals present in the regolith are removed from the upper to lower layers, what is this process known as?
Answer:
Eluviation.

Question 35.
What is the process by which humus is formed?
Answer:
Humification.

Question 36.
When weathering occurs as a natural process, what is it called?
Answer:
Geomorphic weathering.

Question 37.
What is the process by which the depth of gullies increases due to erosion called?
Answer:
Gully erosion.

Question 38.
How can the soil in the slopes of mountains be conserved by carving step-like structures?
Answer:
Terrace farming.

Question 39.
What is the process of conserving soil by covering it with a layer of organic material called?
Answer:
Mulching.

Short Questions and Answers : (2 marks for each question)

Question 1.
What is weathering?
Answer:
When rocks are broken down in their own places (‘in situ’) either mechanically or chemically due to the action of various elements of weather (viz., temperature, rainfall, etc.) the process is known as weathering.

Question 2.
What are the various types of weathering?
Answer:
Weathering can be of 3 types-

1. mechanical
2. chemical
3. biological weathering

Question 3.
What is mechanical weathering?
Answer:
When different elements of weather (like temperature, rainfall, etc.) break down the rocks in their own places, this process of rock fragmentation is known as mechanical weathering.

Question 4.
What is chemical weathering?
Answer:
When rocks are subject to decomposition due to reaction with oxygen, carbon dioxide, water vapour, etc., present in the atmosphere or water, the process is known as chemical weathering.

Question 5.
Where is mechanical weathering more prevalent on the earth’s surface?
Answer:
Mechanical weathering takes place mostly in high mountainous areas and in hot and dry desert regions. For example, the rocks are fragmented as a result of exfoliation, block disintegration and granular disintegration in the hot and dry desert regions.

Question 6.
Chemical weathering is prevalent in which climatic region?
Answer:
Due to high temperature and heavy rainfall, the Equational regions are more prone to chemical weathering (as a result of hydrolysis, hydration, etc.).

Question 7.
What is erosion?
Answer:
When fragmented rocks are transported from their original place of weathering to some other place by various agents like rivers, glaciers, winds, etc., this process is called erosion.

Question 8.
What is denudation?
Answer:
When the underlying rock layer of the earth’s surface is exposed as a result of the upper layer of rock being weathered and transported to some other place (erosion) by various agents, this process is called denudation (‘denude’ means to ‘lay bare’).

Question 9.
What is biological weathering?
Answer:
Rocks are sometimes broken down by plants and animals. Again, humic acid from decaying plants and animals results in breaking down of rocks. When the rocks are subject to change as a result of the decaying of plants and animals, the process is known as biological weathering.

Question 10.
What are the different types of biological weathering?
Answer:
Biological weathering is of two types 1. bio-mechanical weathering, and 2. biochemical weathering.

Question 11.
What is mass wasting?
Answer:
When fragmented rocks, pebbles, mud, etc., slide down the slope or gradient of a mountain or a highland (as a result of gravitational force), as a mass of weathered matter, the process is called mass wasting.

Question 12.
How can mass wasting be classified?
Answer:
The 2 main types of mass wasting are 1. slow movement, and 2. rapid movement.

Question 13.
What is block disintegration?
Answer:
It is a type of mechanical weathering. When the cracks in the rocks expand and contract due to differential heating (i.e., hot during daytime and cool at night), they are weakened and eventually break apart in the form of blocks. This is called block disintegration.

Question 14.
Where does mechanical weathering due to frost/ice occur?
Answer:
In the high mountainous areas and Arctic regions, frost/ice plays a major role in mechanical weathering. The ice filling up in the cracks of the rocks exerts pressure on either side and eventually breaks the rocks.

Question 15.
What is granular disintegration?
Answer:
Granular disintegration is a type of mechanical weathering found in the hot and arid desert regions. Due to of differential heating, the various minerals present in the rocks expand and contract alternately and these rates of expansion and contraction differ in case of different minerals. As a result, the rocks are broken down into small fragments. This process is known as granular disintegration.

Question 16.
What is exfoliation?
Answer:
When the intensity of temperature is high, there is a difference of temperature in the outer and inner layers of the rocks. As a result, the outer layers of the rocks come off from the underlying layers (like onion peels). This is called exfoliation. The surface of the rock assumes a rounded shape as a result of this. Exfoliation is a common occurrence in the granite-gneiss rocks of Chota Nagpur Plateau (near Ranchi) in India.

Question 17.
What is oxidation?
Answer:
Chemical reaction occurs in the rocks as a result of water (in the presence of oxygen) reacting especially with iron present in the rocks. It forms oxides and hydroxides and eventually weakens and dissolves the rocks. Brownish/yellowish stains are seen on the rocks as a result of rust formation.

Question 18.
What is carbonation?
Answer:
When water is mixed into carbon dioxide, carbonic acid is formed and it chemically reacts with calcium carbonate present in the rocks, and then dissolves the rocks. This process is known as carbonation. This is how limestones are dissolved and karst landforms are formed. For example, Borra caves near Vishakhapatnam in Andhra Pradesh.

Question 19.
What is hydration?
Answer:
When the minerals present in the rocks mix with water, they expand, create pressure on the rocks and dissolve them. This process is called hydration. This is an important process of chemical weathering.

Hydration actually causes granular disintegration and makes the rocks further susceptible to chemical weathering, especially by oxidation and carbonation. For example, the process of hydration changes feldspar into kaolinite clay, and the process known as kaolinisation.

Question 20.
What is hydrolysis?
Answer:
The chemical combination of minerals with water to form insoluble precipitates like clay is called hydrolysis. For example, clay and silicon are by-products of such a chemical reaction of granite. Another example is shown below-

Question 21.
What is the role of plants in biological weathering?
Answer:
1. When the roots of plants grow and exert pressure on the cracks in the rocks, the rocks disintegrate.
2. The rotten plant remains (viz., leaves, stems/trunks, etc.) result in the formation of humic acid, which eventually dissolves the rocks.

Question 22.
What is the role of human beings in biological weathering?
Answer:
1. Rocks are broken down into fragments as a result of agricultural practices, mining activities, etc.
2. Rocks are disintegrated and dissolved as a result of construction of houses, roads, excavation of canals, etc.

Question 23.
What is soil erosion?
Answer:
When soil particles are separated and transported elsewhere due to natural or man-made factors, it is called soil erosion. For example, soil erosion occurs as a result of run-off caused by rainfall (the top soil is washed off and transported elsewhere).

Question 24.
Where is terrace farming practiced?
Answer:
Terrace farming (or step farming) is usually practiced along the slopes of mountains. For example, it is common in the hill slopes of Darjeeling.

Question 25.
Why is weathering also known as disintegration process?
Answer:
The rocks on the earth’s surface are either disintegrated or decomposed as a result of the process of weathering and that is why it is also called a process of disintegration.

Question 26.
What is ‘colloid plucking’?
Answer:
The wet soil particles or colloids that form on the rocks, dry up eventually and exert pressure on the minerals present in the rocks. This results in weathering of rocks and this process is called ‘colloid plucking’.

Question 27.
Why is carbonation more predominant in limestone regions?
Answer:
The process of carbonation is predominant in limestone regions. When carbon dioxide mixes with water it forms carbonic acid, and when this reacts with limestone (calcium carbonate), it forms calcium bicarbonate and dissolves the limestone.

Question 28.
What is talus?
Answer:
Action of ice and snow in the cold mountainous regions, widens the cracks in the rocks and eventually breaks the rocks into angular fragments. These fragments are deposited at the foothill zones in a conelike formation known as talus or scree. These features are commonly seen in the Ladakh region. They are also known as Blackspade or Felsenmeer.

Question 29.
What type of mechanical weathering is more prevalent in the mountainous regions?
Answer:
In the high mountainous regions where the temperature falls below freezing point at night, water turns into ice and during daytime, when the temperature increases, this ice melts down into water. Due to this alternate heating and cooling process, mechanical weathering is prevalent in such areas.

Question 30.
What is humification?
Answer:
The process by which the organic matter present in the soil is decomposed by microorganisms and turned into black coloured hums, is called hunification.

Question 31.
What is eluviation and illuviation?
Answer:
1. The process by which the dissolved minerals in the upper layers of the soil are transferred to the lower layers is called eluviation.
2. The process in which the minerals present in the soil are deposited in the lower layers of the soil/rocks is known as illuviation.

Question 32.
What is Terra Rosa?
Answer:
In the karst region, the dissolved limestone is removed and accumulated in the upper layers of the soil as a result of the process of carbonation. This red coloured layer of soil deposited on the earth’s surface is called Terra Rosa (‘terra’ meaning soil and ‘rosa’ meaning red).

Question 33.
What is regolith?
Answer:
The disintegrated and decomposed rock particles lying on the upper layers of the earth’s surface are called regolith. It is from this regolith that the soil-forming process starts (a slow process).

Question 34.
What is solum?
Answer:
The soil which is formed from the original rock bed lying beneath it is called solum. The term ‘soil’ is derived from the Latin word ‘solum’.

Question 35.
Why is chemical weathering more common in the Equatorial regions?
Answer:
The Equatorial regions experience chemical weathering because-
1. the equatorial regions have high temperature and receive rainfall throughout the year
2. the leaves of the forests in this region fall on the ground and form humic acid, which facilitates chemical decomposition of the rocks.

Question 36.
Why does rust form on rocks?
Answer:
The iron-bearing minerals present in certain rocks are prone to the formation of rust. The process of oxidation converts the ferrous oxide present in the rocks into ferric acid, which results in the formation of light brown and yellow coloured rust.

Question 37.
What are tors?
Answer:
Due to differential weathering, the inner layers of the rocks are exposed to the surface when the surrounding soft regolith is removed. Sometimes the rocks that have broken apart along the points or cracks in the granite-gneiss rocks may tumble down the slope of hills. These upright hard rocks situated on the slopes or in the foothills of gently sloping hills are called tors.

Question 38.
What is desert-varnish?
Answer:
In the desert areas, the orangish-yellow coloured coating that forms on the rock layers is called desert-varnish. It is composed of clay, iron and manganese oxide. They are found mostly in basalt and quartzite rocks.

Short Questions and Answers : (3 marks for each question)

Question 1.
How does terrace farming help in conserving soli?
Answer:
The rate of flow of water along the slopes of hills and mountains can be controlled. In such areas, farming is done on steps cut along the slopes at different heights. Each step on terrace is bordered by slightly elevated mud bunds which prevent washing away of water down the slope and thus store water on these terraces. Thus, run off is reduced and infiltration of water underground is increased. These terraces are of three types-

1. Bench-like steps: The lower part of these steps are flat and on the outer sides, low bunds are constructed.
2. Successive steps: A series of steps or terraces at different altitudes (like a staircase), prevents water from flowing out and thus conserves soil.
3. Plain or flat steps: The lower part of these steps are flat and different crops are cultivated here. Thus, water is stored and soil erosion is prevented because run-off is controlled.

Question 2.
Why is granular disintegration more prevalent in hot desert areas?
Answer:
In the hot desert areas, the rocks expand on being heated up during daytime, whereas during the night, when the temperature falls, the rocks cool down and contract. This alternate process of relaxation (during heating) and tension (during cooling) results in breaking up of the rocks into smaller fragments.

Question 3.
Which type of landforms result from mechanical weathering?
Answer:

1. A soft layer of soil called regolith is formed due to mechanical weathering.
2. Due to exfoliation, rounded hills (especially in granite-gneiss rocks) are formed. For example, Ranchi dome near Ranchi lake in Jharkhand.
3. In cold climatic regions, angular rock fragments are formed as a result of weathering by ice and these accumulate in a cone-shaped manner in the foothill zones, and is called talus or scree.

Question 4.
What are the controlling factors of weathering?
Answer:
The controlling factors of weathering are as follows-

1. Structure of rocks: Weathering is accelerated in soft and jointed (with cracks and fissures) rocks.
2. Relief or topography: Weathering process is more active in steep slopes of hills and plateaus.
3. Climate: The different factors of climate (moisture in the air or humidity, rainfall, temperature) influence weathering to a great extent.
4. Biotic factors: Roots of plants and trees, flowers, fruits, man and other animals (viz., burrowing animals), are important controlling factors of weathering process.

Question 5.
How does weathering help in increasing soil fertility?
Answer:
The role of weathering in increasing soil fertility is discussed as follows –
A soft layer called regolith is formed on the upper surface of the earth as a result of weathering. The minerals present in this regolith seep underground and supply essential nutrients to the plants.

The roots of trees cause the rocks to break apart (biotic or biological weathering). Besides, burrowing animals like rabbits, rats, etc., loosen up the soil by digging into it and thereby allows air to pass below the soil which is beneficial to the plants.

Humus is formed as a result of decaying of leaves, flowers, etc., that are shed from the trees. When humus mixes with the oxygen present in the atmosphere, it enriches the various minerals found in the soil, which, in turn, increase the fertility of the soil.

Question 6.
What is the role of man in soil erosion?
Answer:
The activities carried out by human beings that lead to soil erosion, are as follows-

1. Deforestation: Due to cutting down of trees, the soil is exposed to weathering. Thus rains wash away the fertile topsoil.
2. Unscientific methods of cultivation: Shifting cultivation, intensive cultivation, etc., reduce soil fertility and increase soil erosion.
3. Increased grazing: Excessive grazing practised on the thin soil along the mountain slopes, leads to exposure of the soil to weathering and erosion.

Write the differences between the following –

Question 1.
Exfoliation and Frost action
Answer:
The differences between exfoliation and frost action are as follows-

Points of difierence	Exfoliation	Frost Action
1. Concept	The rock layers peel off and come out like onion peels as a result of high temperature in this process.	In the cold climatic regions, ice crystals formed within the rock layers aid in disintegrating the rocks.
2. Process	Due to difference of temperature there is expansion and contraction of the rocks, resulting in its weathering.	The ice crystals formed within the cracks of the rocks exert pressure and thereby break the rocks.
3. Occurrence	It usually occurs in deserts or arid and dry climatic regions.	It usually occurs in cold mountainous regions or polar regions.

Question 2.
Weathering and Mass wasting
Answer:
The differences between weathering and mass wasting are as follows-

Points of difference	Weathering	Mass Wasting
1. Concept	When climatic elements are responsible for disintegration and decomposition of rocks, it is called weathering.	When the weathered debris of rocks are carried down slopes due to gravity and are transported and deposited elsewhere, it is known as mass wasting.
2. Classification	(1) Mechanical, (2) chemical, (3) biological	(1) Slow, and (2) rapid or fast movement
3. Characteristics	This is a static process, i.e., the weathered material is deposited ‘in situ’ or in its original place.	This is a dynamic process, where weathered material is transported and deposited elsewhere.

Question 3.
Weathering and Erosion
Answer:
The differences between weathering and erosion are as follows-

Points of difference	Weathering	Erosion
1. Concept	It is the disintegration and decomposition of rocks ‘in situ’, carried out by the climatic factors (temperature, precipitation, etc.).	Erosion is the process of carrying away of rock debris by rivers, winds, etc., (operating on the earth’s surface) from one place to another.
2. Dependency	It does not depend on erosion.	It depends on weathering.
3. Rate / Pace	It is a very slow process.	It is a relatively fast process.

Question 4.
Weathering and Denudation
Answer:
The differences between weathering and denudation are as follows-

Points of difference	Weathering	Denudation
1. Concept	Weathering is the disintegration and decomposition of rocks by climatic factors (temperature, precipitation, etc.).	Denudation is the process in which the lower layers of the rocks are exposed when the upper layers are removed by weathering and erosion.
2. Dependency	It does not depend on denudation.	It depends on weathering.
3. Evolution of landforms	It is not related directly to evolution of landforms.	It has a vital role in the evolution of landforms, which is a long-term process.

Question 5.
Exfoliation and Granular disintegration
Answer:
The differences between exfoliation and granular disintegration are as follows-

Points of difference	Exfoliation	Granular Disintegration
1. Nature of rocks	This occurs in homogeneous rocks.	This occurs in heterogeneous rocks.
2. Type of weathering	In this type of weathering, the rock layers come off like peeled off onions.	Rocks are disintegrated into smaller and minute particles or grains.
3. Relief	The landforms formed as a result of this type of weathering are rounded or dome shaped.	The tops of the hills and mountains become conical as a result of this type of weathering.

Question 6.
Erosion and Mass wasting
Answer:
The differences between erosion and mass wasting are as follows-

Points of difference	Erosion	Mass Wasting
1. Concept	It is the process of carrying away of weathered rocks by natural agents from one place to another.	Mass wasting is the movement of rock debris down hill slopes due to gravity.
2. Process	Abrasion, attrition, solution, etc.	This process can be both slow and fast.
3. Visibility	It is visible everywhere.	It is visible in high relief or sloping areas.

Question 7.
Mechanical and Chemical weathering
Answer:
The differences between mechanical and chemical weathering are as follows-

Points of difference	Mechanical Weathering	Chemical Weathering
1. Place of occurrence	Rocks are disintegrated in their own places in this type of weathering.	Rocks are decomposed in their own places in this type of weathering.
2. Alteration of Minerals	It does not lead to the formation of new minerals.	It leads to the formation of new minerals.
3. Main factors	Temperature, precipitation, etc.	O2, CO2, water, minerals, etc.

Question 8.
Oxidation and Carbonation
Answer:
The differences between oxidation and carbonation are as follows-

Question 9.
Hydration and Hydrolysis
Answer:
The differences between hydration and hydrolysis are as follows-

Question 10.
Mechanical weathering and Biological weathering
Answer:
The differences between mechanical weathering and biological weathering are as follows:-

Points of difference	Mechanical Weathering	Biological Weathering
1. Concept	Disintegration of rocks by various climatic factors is called mechanical weathering.	Disintegration of rocks by various plants and animals (man, bacteria, virus) is called biological weathering.
2. Process	Rocks disintegrated only by mechanical weathering.	Rocks disintegrated by both mechanical as well as chemical weathering.
3. Areas of occurrence	This type of weathering is commonly seen in hot desert areas, cold mountainous areas and cold polar regions.	It is commonly seen in areas where life forms are abundant.

Question 11.
Chemical weathering and Biological weathering.
Answer:
The differences between chemical and biological weathering are as follows-

Points of difference	Chemical Weathering	Biological Weathering
1. Concept	The process of decomposition and disintegration of rocks by acid, CO2, O2 water is called chemical weathering.	The process of disintegration of rocks by plants and animals is called biological weathering.
2. Characteristics of rocks	Characteristics of rocks change as a result of change in their chemical composition in this process.	Physical and chemical characteristics of rocks change in this process.
3. Areas of occurrence	This type of weathering is common in Equatorial, Tropical and humid climatic regions.	This type of weathering is common in regions characterised by abundant life forms.

Give reasons for the following –

Question 1.
Mechanical weathering is predominant in desert areas.
Answer :
The factors responsible for the predominance of mechanical weathering in desert areas are as follows –
1. Range or difference of temperature is high: The daily or diurnal range of temperature (difference of temperature between day and night) is high in desert areas. Rocks expand due to heating during daytime while they contract due to cooling at night. This results in the fragmentation and mechanical disintegration of rocks.

2. Scarcity of rainfall: Chemical weathering rarely takes place due to scarcity rainfall in the desert areas. Vegetation is also scarce due to lack of water. So, when rainfall occurs all of a sudden, rocks are weathered rapidly under its influence.

3. Excessive denudation: Desert areas, being more prone to denudation, the upper layers of the rocks are removed and the rocks beneath are relieved from pressure. The underlying rocks expand consequently and cracks are formed. Subsequently, the rocks disintegrate along these cracks.

Question 2.
Rainfall has an impact on mechanical weathering.
Answer:
Rainfall influences mechanical weathering due to the following reasons-

1. Formation of ice: Rainfall is responsible for the formation of ice crystals within the cracks of rocks in colder climatic regions. These crystals subsequently disintegrate the rocks.
2. Raindrops hitting rocks: Raindrops hitting the rock surface over long periods ultimately lead to the breaking of the rocks.
3. Expansion of rocks: Rain water seeping into the rocks expand the volume of the mineral constituents within them, which, in turn, leads to the breaking apart of the rocks (by mechanical weathering).

Question 3.
Weathering and climate are interrelated.
Answer:
The interrelation between weathering and climate can be discussed if we study the following climatic regions-
1. Equatorial climatic region: Temperature and rainfall are intense throughout the year in this region. Along with chemical weathering, mechanical weathering is also rampant here due to excessive heat.
2. Hot and dry desert climatic region: Due to long periods of intense heat and almost negligible rainfall, mechanical weathering is predominant here. For example, exfoliation, granular disintegration, etc.
3. Cold mountainous climatic region / Arctic region: Due to excessive cold, formation of ice crystals along the cracks of rocks cause mechanical weathering.

Question 4.
Hot and humid climatic regions are more prone to chemical weathering.
Answer:
The hot and humid climate of the Equatorial and Tropical regions have high temperature and rainfall and this is the reason why chemical weathering is predominant here. The following are the specific reasons-
1. When rainwater falls over the earth’s surface, it mixes with the CO₂ present in the atmosphere and converts it into mild carbonic acid. When this water comes in contact with limestone (calcium carbonate), it changes into calcium bicarbonate, and thus chemical weathering is initiated.

2. Some minerals present in the rocks have higher absorbing capacity. When these minerals expand in volume, the overall compactness of the mineral is lost, subsequently giving rise to weathering.

3. In humid areas, the leaves, flowers, fruits, etc., are shed from trees, which, on decaying, form humus and subsequently humic acid. This acid facilitates chemical decomposition of the rocks.

Question 5.
Exfoliation is predominant in granite rocks.
Answer:
When the upper layers of the rocks come off from the underlying layers due to differential heating in homogeneous granitic areas, the process is known as exfoliation. Since the upper layers of the rock are more heated than the underlying layers, a thermal gradient is created. The heat affected upper layers of the rocks expand and come off like the layers of an onion. Exfoliation is thus a common occurrence in granite rocks.

Question 6.
Chemical weathering is prominent in regions having limestone rocks.
Answer:
Carbonation is a process that occurs when calcium carbonate (present in limestone) reacts with CO₂ in the atmosphere. When rain falls, the CO₂ in the atmosphere reacts with it and turns it into mild carbonic acid. This carbonic acid reacts with calcium carbonate (limestone) to form calcium bicarbonate and ultimately dissolves the limestone.

H₂ O + CO₂ → H₂ CO₃
H₂ CO₃ + CaCO₃ → Ca (HCO₃)₂

In limestone regions, limestone is dissolved by the process of carbonation and creates a host of landform features like stalactites, stalagmites, pillars, etc.

Question 7.
The hills formed of granite have rounded or dome-shaped tops.
Answer:
The regions having granite rocks that have high temperature or a dry, and desert type of climate, are found to have hills with rounded tops. This is became the rocks expand due to heat during the day while they contract at night when the temperature drops. This alternate heating and cooling process affects the upper layers of the rocks. Due to the alternate tension and compression, the upper layers come off from the underlying rock layers like the layers of an onion. This process is called exfoliation. The hills here are thus rounded in shape.

Question 8.
People residing in desert areas can hear sounds similar to gunshots.
Answer:
The temperature is very high in the desert areas and the rocks are heated over a long period during the day. However, since different minerals in the rocks have different capacities of absorbing and releasing heat, this unequal expansion and compression within the rocks exert great pressure on them and subsequently they burst, making a loud noise, that is similar to the sound of gunshots.

Long Questions and Answers : (5 marks for each question)

Question 1.
Briefly explain the concepts of erosion and denudation. [2 \(\frac{1}{2}\) + 2 \(\frac{1}{2}\)]
Answer:
The concepts of erosion and denudation are discussed below-
Erosion:
Definition: When the weathered rocks are transported from their place of origin to some other place by physical agents like rivers, glaciers, winds, etc., the process is called erosion.
Process: The main processes of erosion include attrition, friction, plucking, removal of weathered debris, etc.
Agent: Agents like flowing water, winds, moving glaciers, etc., transport the weathered material from one place to another.

Characteristics:

1. Rocks are removed
2. underlying layers of rocks are exposed after the upper layers are removed
3. dependent on weathering
4. a fast process.

Example: High velocity winds in the desert areas remove large quantities of sand and small rock fragments (by the process of erosion) to far away places.

Denudation:
1. Definition: When the lower or underlying layers of the rocks are exposed after the upper layers are removed by weathering and erosion, the process is known as denudation. The term ‘denude’ means ‘to lay bare’.

Process: The main factors responsible for denudation are-land erosion, cultivation, construction of roads and settlements, etc.

Characteristics:

• Weathering and erosion are dependent on mass wasting.
• This is a slow process.
• New rocks are exposed to the earth’s surface.

Regional differences: The rate of denudation is lower in hot dry desert areas than in the high mountainous regions. In humid regions, the rate of denudation is again higher.

Importance: Soil is formed as a result of this process and it also plays an important role in the evolution of landforms.

Question 2.
Briefly describe the concepts of weathering and mass wasting. [2 \(\frac{1}{2}\) + 2 \(\frac{1}{2}\)]
Answer:
Weathering:
1. Definition: The climatic agents (e.g., temperature, rainfall, etc.) disintegrate and decompose the rocks in their own place (‘in situ’) and this process is called weathering.

2. Nomenclature: As this is an alteration of the physical or chemical characteristics of rocks by various factors of weather/climate, the term weathering is pertinent.

3. Factors: The agents of weathering are temperature and rainfall, plant, man and other animals, landform, nature of rocks (lithological characteristics), time, etc.

4. Types: Weathering can mainly be of 3 types-

• Mechanical (rocks are disintegrated or broken into fragments)
• Chemical (rocks undergo alteration in chemical composition)
• Biological (rocks are altered by the action of plants and animals).

5. Process: Exfoliation, granular disintegration, hydration, oxidation, hydrolysis, etc.

6. Characteristics:

• Rocks are disintegrated and decomposed.
• This is a static process.
• The weathered material is not removed or transported.
• The force of weathering depends on structure and nature of the rock, climatic factors, etc.

7. Effect: Landforms like rounded or domeshaped hills, inselbergs, tors, caves, etc., are formed. Weathering also plays a significant role in soil formation.

Mass Wasting:

Definition: When the weathered materials like pebbles, boulders, etc., slide down the slopes of mountains due to gravitational pull, the process is known as mass wasting.

According to geomorphologist A. L. Bloom-“…gravitational or down slope movements of weathered rock debris is mass wasting”.

Nomenclature: As the weathered rock debris are dumped at the lower slopes of the mountains after being carried there, following the natural slope or gradient (by gravitational pull), the term ‘mass’ is thus relevant.

Factors: Slope of the land, altitude, shape, size and amount of the weathered materials, presence of vegetation, precipitation, force of gravity, etc.

Types: Mass wasting can be of 4 types-

• Slow flow
• Rapid flow
• Landslide
• Subsidence.

Process: Mud flow, soil flow, solifluction, slump, rock slide, etc.

Characteristics:

• This process is rampant in sloping lands.
• The debris are removed as a result of gravitational force.
• It can occur as a slow or rapid process.
• Physical agents of weathering and erosion (rivers, glaciers, etc.) do not have any role to play in the process of mass wasting.

Effect:

• Landslides occur in mountainous areas as a result of mass wasting.
• These cause loss of life and property.
• Landforms like steep slops, erosion on slopes, talus cones, etc., are formed.

Question 3.
What is meant by mechanical weathering? Describe the main processes involved.
Answer:
Mechanical Weathering: The process of fragmentation or disintegration of rocks in their own places (‘in situ’) by various agents of climate like temperature, rainfall, snowfall, etc., is called mechanical or physical weathering. The rocks are broken down into smaller fragments, but the chemical compositions are unaltered.

Different processes of mechanical weathering: The various processes of mechanical weathering are-
Block disintegration: When the rocks are broken into blocks or boulders, as a result of temperature difference, the process is called block disintegration.

1. Process: When there is differential heating of the outer and inner parts of rocks (the outer parts are more heated than the inner parts). There is a difference in the expansion of the components of those rocks. It is then that the vertical and horizontal cracks are formed within these rocks and they break apart along these cracks into several blocks.

2. Characteristics:

• Mostly seen in basaltic rocks
• several cracks are formed within the rocks both horizontally and vertically
• the rocks are broken in square or rectangular shapes
• the rocks are broken apart as big chunks or blocks.

3. Occurrence: Mostly occur in high temperature regions.

Exfoliation: When the layers of rocks peel off like an onion, this process is called exfoliation.

Process: Due to differential heating and cooling of the outer and inner parts of the rocks, the upper layers come off like onion peels and are prone to further weathering.

Characteristics:

1. Mostly seen in granitic rocks,
2. tops of highlands become rounded in shape as a result of this type of weathering
3. this weathering occurs in homogeneous rocks.

Occurrence: Exfoliation is most common in the hot desert regions like the Sahara and Thar deserts.

Granular disintegration: When the rocks expand and contract alternately as a result of differences (hot and cold) in temperature, they break into smaller fragments. This process is called granular disintegration.

1. Process: Rocks composed of different types of minerals absorb and release heat at varying rates and thereby do not have a uniform rate of expansion and contraction. This results in granular disintegration of these rocks.
2. Characteristics:

• This type of weathering is more common in rocks which are heterogeneous in nature.
• When the rocks burst, sounds similar to gunshots are heard.
• Formation of sand is an ultimate result of such weathering process.

3. Occurrence: This type of weathering is more commonly seen in hot desert regions.

Work of ice: In cold climatic regions, rocks are broken apart by ice crystals.

1. Process: In the cold Arctic region and other cold mountainous areas, the cracks in the rocks are filled up with water. This water freezes at night due to condensation (low temperature) and exerts pressure on either side of the cracks, ultimately breaking them apart.

2. Characteristics:

• Weathering occurs as a result of the formation of ice crystals.
• Talus cones are formed on the lower slopes of mountains.

3. Occurrence: This type of weathering occurs mostly in cold climatic regions.

Other processes: Other processes of mechanical weathering include formation of salt crystals, dirt cracking, boulder clearing, hitting by raindrops, etc.

Question 4.
What is meant by chemical weathering? Explain the main processes involved in it. [1 + 4]
Answer:
Chemical weathering: The chemical composition of rocks are altered when they react with oxygen, carbon dioxide, water vapour, etc., in their own places. This process is called chemical weathering.
Different process of chemical weathering: They are as follows –
Oxidation: When the nature of minerals within a rock alters under the influence of atmospheric oxygen and water, the process is called oxidation.
1. Process: In iron-bearing rocks, new minerals are formed and chemical decomposition takes place as a result of the chemical reactions in the presence of oxygen and water.

2. Reaction:

3. Characteristics:

1. This type of weathering takes place due to the action of water.
2. Iron-bearing rocks are subject to rusting as an effect of this.
3. Presence of oxygen is required in this type of weathering.

Carbonation: When the nature of rocks are altered as a result of the chemical reaction of water mixed with carbon dioxide, this process is called carbonation.
1. Process: When rainfall mixes with carbon dioxide (CO₂) in the atmosphere, carbonic acid is formed (CO₂ + H₂O arrow H₂CO₃). This dissolves limestone (calcium carbonate) by altering it into calcium bicarbonate (due to chemical reaction).

2. Reaction:

3. Characteristics:

• This type of weathering occurs mostly due to the action of rain water.
• It is more active in limestone rocks.

Hydration: When water reacts with the minerais present in the rocks and alters their chemical composition, this process is called hydration.
1. Process: Some minerals present in the rocks have more capacity to absorb water and they expand while undergoing chemical reactions.

2. Reaction:

3. Characteristics:

• The minerals in the rocks expand.
• As a result of absorption of water, the rocks become softer/weaker.
• This usually occurs in homogeneous rocks.

Hydrolysis: When the minerals present in the rock react with ionised water, and chemical decomposition takes place, the process is known as hydrolysis.

1. Process: When water reacts with the minerals in a rock, new minerals are formed, and the rock is decomposed.
2. Reaction:

3. Characteristics:

• It occurs with the help of ionised/charged water.
• New minerals are formed by this process.
• Specific temperature is needed for reactions to occur.

Besides, by the process of solution, gypsum, rocksalt, etc., are dissolved in water and the rocks are decomposed.

Question 5.
Classify weathering. Explain the role played by man, plants and other animals in biological weathering. [2 + 3]
Answer:

Roles of man, plants and other animals in biological weathering: Man, plants and animals aid biological weathering both directly and indirectly. The roles played by them are as follows-

1. Role of man	Construction of buildings, roads, etc., lead to disintegration of rocks.
2. Role of plants	Cultivation of land also leads to mechanical weathering.
3. Role of other animals	Mineral exploitation/mining, digging up of ponds and tanks, industrial establishments, etc., directly or indirectly cause weathering.

Question 6.
State the effects of weathering and explain how soil is formed as a result of weathering. [2+3]
Answer:
Effects of weathering:
1. Formation of landforms: Landforms like tors and rounded hills in the humid tropical regions, inselbergs in the hot dry desert regions, caves, stalactites and stalagmites in limestone areas, etc., are all formed as a result of weathering.

2. Formation of minerals: New minerals are formed as a result of chemical reactions. For example, bauxite is formed in humid tropical countries.

3. Facilitates agriculture: Due to disintegration of rocks by weathering, the porosity of soil increases, which facilitates air and water circulation and hence aids in agricultural practices.

4. Formation of regolith: Due to the disintegration and decomposition of rocks, regolith is formed, which plays a major rule in soil formation later on.

Soll Formation: The soil-forming process, as a result of weathering, may be explained by the following stages-
First stage: Various processes of mechanical weathering (for example, block disintegration, exfoliation, granular disintegration, as well as plants and animals, break down rocks into smaller fragments. This loose layer of fragmented rocks is called regolith.

Second stage: Rain water and air seeps into the various layers of the rock through these regoliths. Decomposition of these fragmented rocks also takes place as a result of the process of chemical weathering. Due to the leaching process, the nutrients of the soil seep down to the lower layers of the soil.

Third stage: The upper layer of the regolith (top soil) is rich in dead remains of plants, animals, etc. The bacteria and fungi decompose this matter, leading to the formation of humus. Humus mixes with the fragmented rocks of the regolith and makes it more soft and increases its water retention capacity.

The combined processes of regolith formation, leaching, humus formation, decomposition, etc., lead to the formation of a layer with five particles called the solum. Soil has its origin from this solum.

The upper layers of the soil have finer particles, while the deeper layers have coarser particles. Based on physical and chemical characteristics, horizontal layers of the soil can be identified. These are termed as layers A, B and C (from topdown).

Scientist Mohr has explained 5 steps of soil formation. They are-

• Primary stage/Initial stage-Unaltered parent or original rock,
• Childhood or Juvenile stage-Initiation of the process of weathering,
• Youth or Virile stage-Chemical reaction of minerals leading to decomposition,
• Old/Senile stage-Last stage of chemical weathering.
• Last/Final stageEnd stage of soil formation process.

Question 7.
Briefly explain the concept of soil erosion.
Answer:
Concept of soil erosion:
Definition: The process in which soil particles are removed from the original rock by natural or man-made factors, it is called soil erosion.
Factors: The following equation explains the process-
e = g (cl, v, t, s, h)

where, e = soil erosion, g = effectiveness, cl = climate, v = vegetative cover, t = land- form or relief, s = type of soil, h = role of man or human interference.
Causes of soil erosion: They may be classified as-natural and man-made causes.

Natural causes:

1. Rainfall: In regions with high rainfall, the raindrops loosen the soil particles from the rock layer and wash them away.
2. Wind: Winds blow with high velocity in open desert areas and coastal tracts (receiving no obstacles) and lead to soil erosion.
3. Flowing water: Currents of river water and sea waves cause soil erosion.

Man-made causes:

1. Deforestation: In deforested areas, the bare land is more prone to soil erosion. The roots of the trees bind the soil together. When the trees are cut down, the region becomes infertile.
2. Overgrazing: Excessive grazing by cattle, goats and sheep in grassland areas results in the baring of the ground, leading to soil erosion.
3.Unscientific methods of cultivation: In areas where shifting or ‘jhum’ cultivation is practised, the forest is cleared by burning it down (which is unscientific), and this leads to soil erosion.

Process of soil erosion: Soil erosion is mainly caused by flowing water and wind (natural factors).

Flowing water:

1. Sheet erosion: When rainfall or flowing water removes the soil layer by layer, it is known as sheet erosion.
2. Rill erosion: When rainwater or a flowing river (in its initial slope), flows along the slopes of a mountain as narrow channels or rills, soil erosion occurs.
3. Gully erosion: These rills become wider and deeper due to more soil erosion, and this is known as gully erosion.
4. Ravine erosion: When the gullies become further deepened and widened, the topography is converted to a ‘bedland’, where cultivation is not possible and the area becomes inaccessible too.

By work of wind: In the desert areas, winds carry sand from one area to other distant areas, leading to erosion.

Effects of soil erosion:
Effects on physical environment:

1. As a result of soil erosion, the level of water in the soil subsides,
2. hydrological cycle is affected,
3. food chain is disturbed as habitats of the decomposers (like bacteria, fungi, etc.) present in the soil are destroyed,
4. the navigability of rivers and other water bodies decrease (due to aggradation by soil deposits or siltation), and the areas become more prone to floods.

Effects on human environment:

1. Fertility of soil decreases, crop productivity is reduced,
2. decrease in navigability of rivers hampers trade and water transport,
3. ecosystem of wetlands is damaged as they get filled up with soil deposits.

Areas prone to soil erosion: Continuous flat areas, plateaus devoid of vegetation, desert areas, coastal areas, riverine tracts, etc., are more prone to soil erosion.

Question 8.
Briefly discuss the methods of soil conservation.
OR,
Discuss the preventive measures for soil erosion.
Answer:
Different methods of soil conservation: The various measures adapted for preventing soil erosion and for increasing the fertility of the soil are known as soil conservation measures.

Agricultural methods:
Afforestation: If trees are planted along the slopes of mountains, on infertile and barren lands, soil erosion can be prevented. This is because the roots of the trees bind the soil together.

Restriction on overgrazing: If overgrazing can be controlled in the grassland areas, soil erosion can be prevented.

Ban on shifting or ‘jhum’ cultivation: When the forests are burnt for clearing land, the trees are destroyed and soil erosion also increases. This can only be prevented by banning such a practice.

Slope/Terrace farming: The slopes of the mountains are carved into steps or terraces to retain rainwater on each step (to be used by the cultivated crops). Soil erosion can be prevented by practising such type of farming.

Creation of forest line/boundary: If forest boundaries are created (by planting trees) on all sides of agricultural fields, coastal regions (especially in the direction of the winds), etc., then soil erosion can be reduced.

Crop rotation: If any land is cultivated throughout the year with different types of crops (not allowing it to lay bare), then the land is not exposed to soil erosion.

Contour farming: Bunds are constructed along the contour of the land (horizontally along the slopes of mountains), water is prevented from flowing away and hence it prevents soil erosion.

Strip cropping: When crops are cultivated in long strips prepared along the slopes, they prevent soil from being carried away by flowing water along the slopes.

Land cover: If the unused parts of the crops like stems, hay, etc., are spread over any bare land, soil erosion can be prevented, especially in muddy areas.

2. Infrastructural measures:

1. Construction of artificial walls: If walls are constructed on seashores and along river banks, soil erosion by flowing water can be prevented.
2. Digging of canals: If canals are dug to facilitate drainage and several water bodies are constructed especially in the desert areas, soil erosion can be prevented.
3. Other measures: More advanced research on preventing soil erosion and increasing soil fertility, spreading awareness among the people, can help in soil conservation.

Question 9.
What is biological weathering? State the various processes of biological weathering. [1+4]
Answer:
Biological weathering: When rocks are disintegrated and decomposed by plants and animals, either directly or indirectly, the process is known as biological weathering. Such type of weathering occurs in almost all climatic regions.
Main processes of biological weathering: Biological weathering takes places mainly by two processes-

Bio-mechanical weathering:
By animals: Burrowing animals like rats, earthworms, prairie dogs, mice, etc., dig holes in the ground to live and thereby aid in bio-mechanical weathering. Termites also help in weathering by bringing the lower layers of the soil to the upper part. Moreover, the CO₂ released by the animals living underground, brings about changes in the chemical composition of rocks and soil and aids in their weathering.

Man also has a vital role in inducing weathering. For example-carrying out mining activities, construction of roads, cultivation of lands and other unplanned activities.

By plants: When the roots of trees and plants penetrate beneath the soil into the underlying rocks, they break the rocks into fragments. Roots can reach upto about 175 feet below the ground and thus aid in mechanical weathering. This is more prominent in areas with CO₂ with their exhalation, which combines with water to form carbonic acid. This dissolves the limestone rocks.

The acid formed by rotting of parts of plants like leaves, flowers, fruits, branches, etc., accelerates chemical weathering. During respiration, the tree roots release CO₂ which, after combining with water present in the soil, converts into carbonic acid. This eventually dissolves and disintegrates the rocks.