WBBSE 9th Class Social Science Solutions Geography & Environment Chapter 5 Weathering

1. Block disintegration: When the 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: (a) Mostly seen in basaltic rocks, (b) several cracks are formed within the rocks both horizontally and vertically, (c) the rocks get broken in square or rectangular shapes, (d) the rocks disintegrate as big chunks or blocks.
   3. Occurrence: This process mostly occurs in high temperature regions.
2. Exfoliation: When the layers of rocks peel off like an onion, this process is called exfoliation.
   1. 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.
   2. Characteristics: (a) Mostly seen in granitic rocks, (b) tops of highlands become rounded in shape as a result of this type of weathering, (c) this weathering occurs in homogeneous rocks.
   3. Occurrence: Exfoliation is most common in the hot desert regions like the Sahara and Thar deserts.
      
3. Granular disintegration: When the rocks expand and contract alternately as a result of difference (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: (a) This type of weathering is more common in rocks which are heterogeneous in nature. (b) When the rocks burst, sounds similar to gunshots, are heard. (c) 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.
4. 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 icemelt water in the day time. This water freezes again at night due condensation (low temperature) and exerts pressure on either side of the cracks, ultimately breaking them apart.
   2. Characteristics: (a) Weathering occurs as a result of the formation of ice crystals. (b) Talus cones are formed on the lower slopes of mountains.
      
   3. Occurrence: This type of weathering occurs mostly in cold climatic regions.
5. Other processes: Other processes of mechanical weathering include formation of salt crystals, dirt cracking, boulder clearing, hitting by raindrops, etc.

1. 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: (a) This type of weathering takes place due to the action of water. (b) Iron-bearing rocks are subject to rusting as an effect of this. (c) Presence of oxygen is required in this type of weathering.
2. 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 in the (CO₂) atmosphere, carbonic acid is formed (CO₂ + H₂O H₂CO₃). This dissolves limestone (calcium carbonate) by altering it into calcium bicarbonate (due to chemical reaction).
   2. Reaction:
      
   3. Characteristics: (a) This type of weathering occurs mostly due to the action of rain water. (b) It is more active in limestone rocks.
3. Hydration: When water reacts with the minerals 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: (a) The minerals in the rocks expand. (b) As a result of absorption of water, the rocks become softer/weaker. (c) This usually occurs in homogeneous rocks.
4. Hydrolysis: When the minerals present in the rocks 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: H₂O → H⁺ + OH^–
      
   3.  Characteristics: (a) It occurs in the presence of ionised or charged water. (b) New minerals are formed by this process. (c) Specific temperature is needed for reactions to occur.

1. Bio-mechanical weathering:

1. 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.
2. 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 dense vegetation. Respiration by the roots of plants, humus, soil moisture carbon dioxide, mild temperature, etc., aids in bio-mechanical weathering of the soil.

2. Bio-chemical weathering:

1. Lichens growing on rocks rot and form humus, which, in turn, is converted to humic acid after mixing with water. This aids in rapid chemical weathering of rocks.
2. The organisms living in limestone rocks release CO₂ with their exhalation, which combines with water to form carbonic acid. This dissolves the limestone rocks.
3. 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, changes into carbonic acid. This eventually dissolves and disintegrates the rocks.
   

Short Explanatory Answer Type Questions

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 of rainfall in the desert areas. Vegetation is also scarce due to lack of water thus, biological weathering is absent. So, only mechanical weathering plays an active part.
3. Excessive denudation: Desert areas, being more prone to denudation, the upper layers of the rocks are removed by wind action and the rocks underneath are relieved from pressure. The underlying rocks expand and consequently cracks form. Subsequently, the rocks disintegrate along these cracks.

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 leads to the fragmentation of the rocks.
3. Expansion of rocks: Rain water seeping through the rock pores and layers pand the volume of the mineral constituents within them, which, in turn, leads to the breaking apart of the rocks (by mechanical weathering).

1. All the processes of chemical weatheringcarbonation, oxidation, hydration and hydrolysis require high temperature and rainfall. Both of which are abundant in the hot and humid climatic regions. In addition to this, there is also adequate water vapour in atmosphere of these regions. All of these actively help in the process of chemical weathering.
2. When rainwater falls on the earth’s surface, it mixes with the CO₂ present in the atmosphere and forms a mild carbonic acid. When this acid water comes in contact with limestone (calcium carbonate), it changes into calcium bicarbonate, and thus chemical weathering is initiated in the karst topography regions of the equator and tropics.
3. In humid areas, the leaves, flowers, fruits, etc., shed from trees, which, on decaying, form humus and subsequently humic acid. This acid facilitates chemical decomposition of the rocks.

Short Answer Type Questions

Multiple Choice Type Questions [MCQ type]

Write the correct answer from the given alternatives

Very Short Answer Type Questions

Write the correct answer from the given alternatives

If the statement is true, write ‘T’ and if false, write ‘F’ against the following

Answer in one or two words

TOPIC – C

Effect of Weathering and Soil Erosion

Ans. Effects of weathering: The effects of weathering are-

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.

Soil Formation: The soil forming process, as a result of weathering, may be explained by the following stages-

1. First stage: Various processes of mechanical, chemical and biological weathering, break down rocks into smaller fragments and decompose them further. This loose layer of fragmented rocks on the top is called regolith, which is the first step of soil formation.
2. Second stage: Rain water and air seeps through the various layers of the rocks through these regolith layer which facilitates chemical weathering of the fragmented rocks. Biological weathering is also initiated by the micro organisms and the dead remains of plants and animals present in the top layer. As a result a black coloured substance called humus forms through the process of humification.
   
3. Third stage: Humus and other weathered particles oxidise in the presence of oxygen and form new minerals. This process is called mineralisation. During the process of soil formation iron, aluminium and other minerals transport from the top layer to the lower layers of the soil. This process is called eluviation. These minerals get accumulated in the lower layers and this process is called illuviation.

Thus, with the passage of time the components in the soil forming process change their physical and chemical characteristics to form a soft layer called soil on the surface of the Earth, with organic matter and minerals as its constituents.

2. Briefly explain the concept of soil erosion.

Ans. Concept of soil erosion: The concept of soil erosion is-

Definition: The process in which soil particles are removed from the top layer by natural or man-made factors, 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 = landform 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.

1. 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 over the adjacent land areas cause soil erosion.
2. 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 prone to soil erosion and is thus rendered 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.

1. By work of 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 river (in its youth stage), 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 ‘badland’, where cultivation is not possible and the area becomes inaccessible too.
      
2. By work of wind: In the desert areas, winds carry sand from one area to other distant areas, leading to erosion of the top layer.

Effects of soil erosion:

1. Effects on physical environment: (a) As a result of soil erosion, the groundwater level decreases, (b) hydrological cycle is affected, (c) food chain is disturbed as habitats of the decomposers (like bacteria, fungi, etc.) present in the soil are destroyed, (d) the navigability of rivers and other waterbodies decrease (due to aggradation by soil deposits or siltation), and the areas become more prone to floods.
2. Effects on human environment: (a) Fertility of soil decreases, crop productivity gets reduced, (b) decrease in navigability of rivers hampers trade and water transport, (c) 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.

3. Briefly discuss the methods of soil conservation.

Or, Discuss the preventive measures for soil erosion.

Ans. 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.

1. Agricultural methods:

1. 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.
2. Restriction on overgrazing: If overgrazing can be controlled in the grassland areas, soil erosion can be prevented.
3. Ban on shifting or jhum’ cultivation: When the forests are burnt down for clearing land, the trees are destroyed and soil erosion also increases. This can only be prevented by banning such a practice.
4. Step/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.
   
5. 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.
6. 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.
7. Contour farming: Bunds are constructed along the contour of the land (horizontally along the slopes of mountains), to prevent water from washing down and hence it prevents soil erosion.
   
8. 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.
9. 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 methods:

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 waterbodies are constructed especially in the desert areas, soil erosion can be prevented.
3. Other methods: More advanced research on preventing soil erosion and increasing soil fertility, spreading awareness among the people, can help in soil conservation.

Short Explanatory Answer Type Questions

1. What is the role of man in soil erosion?

Ans. 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 rainwater washes 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.

2. How does terrace farming help in conserving soil?

Ans. 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 or terrace is bordered by slightly elevated mud bunds which prevent washing away of water down the slope and thus stores 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 washing down 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 seeps underground and soil erosion is prevented because run-off is controlled.

3. How does weathering help in increasing soil fertility?

Ans. The role of weathering in increasing soil fertility is discussed as follows-

1. 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 (in solution with water) and supply essential nutrients to the plants.
2. 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 allow air circulation in soil, which is beneficial to the plants.
3. 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.

4. What are the landforms developed due to the weathering?

Ans. Weathering is a process in which surface rocks are disintegrated or decomposed and remain in its original place or ‘in situ’. Weathered materials are transported through the process of erosion. So, the process of weathering and erosion together develop various types of landforms, namely-

1. Round shaped topography develops over granite by the process of exfoliation;
2. flattened topography develops on basalt by the process of block disintegration;
3. residual hills like inselbergs form in the hot deserts. Sometimes, these are transformed in mesas, buttes due to excessive weathering and erosion;
4. in humid tropical climate, tors develop;
5. in limestone regions, caves, stalactites, stalagmites etc. form due to carbonation and solution processes. Sometimes, rivers that flows over a karst region may disappear and enters underground through shallow holes or sink holes which develop blind or dry valleys on the surface.
6. Felsenmeer forms through deposition of talus or scree along the foothills.

Short Answer Type Questions

1. What is soil erosion?

Ans. When soil particles from the top layer 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 surface run-off due to heavy rainfall (the top soil is washed off and transported elsewhere).

2. Where is terrace farming practiced?

Ans. Terrace farming (or step farming) is usually practiced along the slopes of mountains. For example, it is common in the hill slopes of Darjeeling.

3. What is talus?

Ans. 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.

4. What is humification?

Ans. The process by which the organic matter present in the soil is decomposed by microorganisms and turned into a black coloured semi-solid substance, is called humification.

5. What is eluviation and illuviation?

Ans. The process by which the dissolved minerals in the upper layers of the soil are transferred to the lower layers is called eluviation.

The process in which the minerals present in the soil are deposited in the lower layers of the soil/rocks is known as illuviation.

6. What is terra rosa?

Ans. In the karst region, the dissolved limestone gets 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).

7. What is regolith?

Ans. The layer formed of disintegrated and decomposed rock particles lying on the upper layers of the earth’s surface is called regolith. The soil-forming process starts from the formation of regolith (a slow process).

8. What is solum?

Ans. The humus-rich layer formed in the process of soil formation lying on the parent rock is called solum. The term ‘soil’ is derived from the Latin word ‘solum’.

9. What are tors?

Ans. Due to differential weathering, the hard inner layers of the rocks are exposed to the surface when the surrounding soft regolith gets removed. Sometimes the rocks that have broken apart along the points or cracks may tumble down the slope of hills. The remaining upright hard rocks situated on the slopes or in the foothills of gently sloping hills are called tors.

10. What is humus?

Ans. Dead plant, animal remains, fallen leaves, flowers, fruits, etc. get deposited on the regolith layer as raw organic matter. The organic matter is decomposed by microorganisms and turned into deep brown or black coloured complex matter. This complex matter is known as humus. The soil that is rich in humus is fertile in nature. Water holding capacity of humús-rich soil is greater.

Multiple Choice Type Questions [MCQ type]

Write the correct answer from the given alternatives

1. The first stage of soil formation is-

A. solum

B. regolith

C. batholith

D. monolith

Ans. B

2. Conical rocks formed due to frost action-

A. talus or scree

B. erg

C. pediment

D. bajada

Ans. A

3. Terrace farming is prevalent in-

A. hilly areas

B. deserts

C. plain areas

D. coastal areas

Ans. A

4. Soil erosion in desert areas takes place-

A. due to river action

B. due to glacial action

C. due to wind action

D. due to human beings

Ans. C

5. Layer of soil formed due to weathering in limestone regions is called-

A. terra rosa

B. talus

C. blocksped

D. pediment

Ans. A

6. The process of humus formation is called-

A. regolith

B. humification

C mineralisation

D. alluviation

Ans. B

7. Which process of weathering leads to the formation of karst landforms? 

A. oxidation

B. carbonation

C. hydrolysis

D. block disintegration

Ans. B

8. One of the man-made reasons of soil erosion is-

A. water flow

B. farming

C. extension of root

D. digging holes in soil

Ans. B

9. The process of weathering that forms inselbergs is known as-

A. exfoliation

B. carbonation

C. hydrolysis

D. oxidation

Ans. A

10. The layer of disintegrated rock particles that is formed due to weathering is known as-

A. hardpan

B. batholith

C. regolith

D. lopolith

Ans. C

11. Covering the land by straw, root of the crops, etc. is a process of soil conservation, which is called-

A. mulching

B. terracing

C. strip cropping

D. contour cropping

Ans. A

12. The process that results in removal of minerals from the upper layer of soil, is-

A. humification

B. mineralisation

C. calcification

D. eluviation

Ans. D

13. …….. forms due to weathering.  

A. Mountain

B. Plateau

C. Soil

D. Desert

Ans. C

14. Maximum eluviation occurs in-

A. desert areas

B. cold areas

C. rainy areas

D. temperate areas

Ans. C

Very Short Answer Type Questions

Fill in the blanks with suitable words

1. The layer of loose and unconsolidated materials lying over the bedrock is called ………

Ans. regolith

2. The most important effect of weathering is the formation of ……….

Ans. soil

3. ………. cultivation should be practised on hill slopes in order to prevent soil erosion.

Ans. Step

4. Deposition of ………. helps in the weathering of rock beds,

Ans. groundwater

5. The erosion of soil by raindrops is a ………. cause.

Ans. natural

If the statement is true, write ‘T’ and if false, write ‘F’ against the following

1. Weathering helps in the storage of ground water in the rock beds.

Ans. T

2. The disintegrated layer of the rocks as a result of weathering is called regolith.

Ans. T

3. Rock particles formed by the disintegration due to ice crystals are called boulders.

Ans. F

4. Regolith is formed in the last stage of soil formation.

Ans. F

5. An important influence of weathering is the formation of soil.

Ans. T

6. Afforestation controls soil erosion.

Ans. T

7. Weathering helps in increasing soil fertility.

Ans. T

8. Soil erosion increases due to the excesive grazing.

Ans. T

Answer in one or two words

1. What is the loose and thin layer of finely fragmented rocks due to weathering known as?

Ans. Regolith.

2. 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?

Ans. Eluviation.

3. What is the process called by which humus is formed?

Ans. Humification.

4. When weathering occurs as a natural process, what is it called?

Ans. Geomorphic weathering.

5. What is the process called by which the depth of gullies increases due to erosion?

Ans. Gully erosion.

6. How can the soil in the slopes of mountains be conserved?

Ans. By terrace farming.

7. What is the process of conserving soil by covering it with a layer of organic material called?

Ans. Mulching.

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