Weathering: WBBSE Class 9 Geography solutions

Summary

Our Earth’s surface is like a place where two different natural powers are always working. One set of powers, from inside the Earth, pushes land up and makes it uneven. Another set of powers, from outside, like flowing water, wind, and glaciers, works to wear down these raised parts and make the surface level. These outside powers change the Earth’s relief through weathering and erosion. The whole process of wearing away rocks and leveling the land is called denudation. This happens in two main ways: weathering and erosion.

Weathering is when rocks break down into smaller pieces or change their composition right where they are, on or near the Earth’s surface. It doesn’t involve moving the broken material far away. Think of it as rocks getting old and crumbly due to natural agents. This process turns hard, solid rock into finer material. Weathering is like the first step in changing the land, as it prepares rock bits for other forces to carry away. It’s a still process, meaning the rock breaks down in its place.

Erosion is the carrying away of these weathered rock materials from one spot to another. Agents like running water, ice, wind, or ocean waves pick up and move the broken rock. So, erosion is a moving process. Erosion depends on weathering because there needs to be broken material for erosion to act upon. Weathering is the start of denudation, while erosion is a later stage.

Mass wasting is when gravity pulls rock and soil down a slope. This happens when bedrock is broken by weathering. These broken pieces, called rockwastes, move downhill. Sometimes, large piles of this debris, called scree or talus, gather at the bottom of hills. Denudation, then, is the combined effect of weathering, erosion, and mass wasting, all working together to strip the land bare. Gradation is a related idea, where erosion, transport, and then the dropping of rock bits (deposition) work to bring land surfaces to a common level. This can involve lowering high areas (degradation) or building up low areas (aggradation).

Weathering can happen in different ways. Mechanical or physical weathering breaks rocks into smaller pieces without changing what they are made of. This can be caused by changes in temperature, where rocks expand when hot and shrink when cold, eventually cracking. This is common in deserts. Block disintegration is when rocks break into block-like pieces. Exfoliation is like an onion peeling, where outer layers of rock flake off. Granular disintegration happens when different minerals in a rock expand and contract at different rates, causing the rock to crumble into grains. Frost action occurs when water gets into cracks, freezes, expands, and wedges the rock apart.

Chemical weathering changes the chemical makeup of rocks, causing them to decompose. Oxidation is when minerals react with oxygen, like iron rusting. Carbonation happens when carbon dioxide dissolved in water forms a weak acid that reacts with rocks like limestone, creating features like caves and sinkholes. Hydration is when minerals absorb water and expand. Hydrolysis is a chemical reaction between water and minerals. Climate affects these processes; mechanical weathering is more common in very cold or very dry places, while chemical weathering is more common in hot and wet places.

Biological weathering involves living things. Plant roots can grow into cracks and split rocks. Burrowing animals can loosen rock and soil. Decomposing plants and animals can produce acids that chemically weather rocks.

Weathering has several effects. It helps create soil, which is vital for plants. It can expose useful minerals. However, it can also lead to soil erosion if the loose material is washed or blown away. Soil forms when weathering breaks down rock into small particles, and then organic matter from plants and animals mixes in over a long time. This process is called soil genesis.

Soil erosion is the wearing away of the top layer of soil. This can be caused by heavy rain, strong winds, steep slopes, or when land is left bare without plants. To protect soil, we can plant trees (afforestation), prevent too much animal grazing, and use farming methods like terracing on slopes. Keeping soil fertile is also important, by adding manure or rotating crops.

Textual Questions, Answers (Oriental)

Multiple Choice Questions

[a] Which one of the following is an example of physical weathering?

A. solution
B. frost action
C. carbonation

Answer: B. frost action

[b] Which one of the following is an example of chemical weathering

A. block disintegration
B. exfoliation
C. oxidation

Answer: C. oxidation

[c] The most important factor of soil formation is-

A. weathering
B. folding
C. tectonic movement

Answer: A. weathering

[d] Granular disintegration is caused by-

A. release of pressure on rockbeds
B. thermal change
C. crystal growth

Answer: B. thermal change

[e] The principal natural agent of mechanical weathering is-

A. sun
B. rain
C. wind

Answer: A. sun

[f] The type of weathering which is predominant in the hot desert is-

A. biological weathering
B. chemical weathering
C. mechanical weathering

Answer: C. mechanical weathering

[g] Rock gets rusted as iron oxide due to the process of-

A. carbonation
B. oxidation
C. hydration

Answer: B. oxidation

[h] Limestones deposit which grow upward from the floor of the cave are called-

A. stalactites
B. stalagmites
C. column

Answer: B. stalagmites

True or false

(a) Chemical weathering is the process of mechanically disintegration or break-up of rocks

Answer: False

Explanation: Chemical weathering involves the decomposition of rocks through chemical reactions, whereas mechanical disintegration refers to physical weathering without any change in chemical composition.

(b) Exfoliation occurs in the rock consisting of homogeneous minerals.

Answer: True

Explanation: Exfoliation happens when rocks with homogeneous minerals, such as granite, expand during the day and contract at night, causing surface layers to peel off like onion skins.

(c) Rusting on rocks is caused by carbonation.

Answer: False

Explanation: Rusting is caused by oxidation, a chemical reaction between minerals (especially iron) and oxygen, not by carbonation, which involves carbon dioxide and water reacting with minerals like limestone.

(d) Carbonation takes place in limestone region.

Answer: True

Explanation: Carbonation is most active in limestone regions where carbonic acid (formed from carbon dioxide and water) reacts with calcium carbonate in rocks to form soluble calcium bicarbonate.

(e) Mechanical weathering is very favourable in the desert region.

Answer: True

Explanation: Deserts experience extreme temperature fluctuations, which lead to repeated expansion and contraction of rocks, making mechanical weathering highly effective.

Answer in very short

1. (a) What is weathering?

Answer: Weathering refers to the process by which rocks get broken down to small fragments when they are exposed on the surface of the land; it is the disintegration and decomposition of rocks in situ. According to B. Sparks, “Weathering is the mechanical fracturing and chemical decomposition of rocks, in situ, by natural agents at the surface of the earth.”

1. (b) What are the two main types of weathering?

Answer: Weathering is mainly of two types: Mechanical or Physical Weathering and Chemical Weathering. There is also a Biological Process.

1. (c) Describe the different processes involved in mechanical weathering.

Answer: Mechanical weathering processes include:

Block Disintegration: Rocks in arid regions develop joints and cracks from successive expansion and contraction, breaking into smaller blocks, also aided by pressure release. Shattering is the further disintegration into angular pieces.
Exfoliation (Onion Peeling): Thin surface layers of homogeneous rocks peel off due to differential expansion and contraction between the surface and inner layers.
Granular Disintegration: Rocks break into small fragments as different minerals within them expand and contract at different rates.
Frost Action: Water in rock cracks freezes, expands, widens cracks, and breaks the rock, common in cool, temperate, or high-altitude regions.

2. What is chemical weathering? Describe the different processes of chemical weathering. In which climatic zone does such weathering predominate?

Answer: Chemical weathering is the decomposition and disintegration of rocks due to chemical reactions. According to Arther Holmes, it is “The alternation and solution of rock material by chemical processes.”

The main processes are:

Oxidation: Minerals in freshly exposed rocks take up additional oxygen, forming oxides or hydroxides, like iron rusting.
Carbonation: Atmospheric carbon dioxide mixed with water forms carbonic acid, which reacts with minerals, especially calcium carbonate in limestone, forming soluble bicarbonates.
Hydration: Minerals absorb water and expand, causing stress and disintegration, like feldspar converting to kaolinite.
Hydrolysis: A chemical reaction between mineral ions and water ions (H⁺ or OH⁻) breaks down rocks, often forming clay.

Chemical weathering is important in:

I. Equatorial regions: High temperature and rainfall make chemical weathering most favourable.
II. Tropical Regions: Humid and dry conditions invite chemical weathering, especially during rainy seasons. It is also most active in limestone rocks.

3. (a) Describe the processes of mechanical weathering in brief.

Answer: The processes of mechanical weathering involve:

(a) Temperature changes causing rocks to expand and contract, leading to breakage.
(b) Block Disintegration, where rocks break into smaller blocks due to temperature changes and pressure release.
(c) Exfoliation, the peeling off of surface layers of rocks.
(d) Granular Disintegration, where rocks break into fragments due to differential mineral expansion.
(e) Frost Action, where freezing water expands in rock cracks, breaking them apart.

3. (b) Which type of weathering is dominant in humid tropics? Classify intrusive rocks with examples.

Answer: In humid tropics, characterized by high temperature and rainfall, chemical weathering is most favourable and its rate is highest.

Intrusive igneous rocks (also called plutonic rocks) are formed when magma cools and solidifies beneath the Earth’s surface. They are classified based on their texture (crystal size) and mineral composition.

Classification by Mineral Composition (simplified):

Felsic Rocks: Rich in feldspar and silica (quartz). Generally light-colored. Example: Granite (coarse-grained, composed mainly of quartz, feldspar, and mica).
Intermediate Rocks: Composition between felsic and mafic. Example: Diorite (coarse-grained, composed mainly of plagioclase feldspar, amphibole, and pyroxene; less quartz than granite).
Mafic Rocks: Rich in magnesium and iron (ferromagnesian minerals). Generally dark-colored. Example: Gabbro (coarse-grained, composed mainly of pyroxene and plagioclase feldspar).
Ultramafic Rocks: Very rich in magnesium and iron, low silica content. Very dark-colored. Example: Peridotite (coarse-grained, composed mainly of olivine and pyroxene).

Classification by Texture (related to cooling rate):

Intrusive rocks are generally coarse-grained (phaneritic texture) because the magma cools slowly underground, allowing large crystals to form. The examples above (Granite, Diorite, Gabbro, Peridotite) all typically exhibit this coarse-grained texture.

Answer in short questions

1. Differentiate between weathering and erosion.

Answer: The distinction between weathering and erosion is as follows:

Weathering	Erosion
1. It is the decay or disintegration of rocks on the surface of the earth.	1. Removing or wearing away of disintegrated rocks.
2. Main agents of weathering are temperature, humidity and precipitation.	2. Main agents of erosion are river, glacier, wind and waves.
3. Weathering is a static process.	3. Erosion is a dynamic process.
4. Weathering is independent. It does not depend on erosion.	4. Erosion depends on weathering.
5. Weathering is the initial stage of denudation.	5. Erosion is the last stage of denudation.
6. It is not involved with transportation of weathered products except gravity.	6. Erosion includes removal or transportation of the weathered rock material from one place to another.

2. What is Mechanical weathering?

Answer: When disintegration of rocks takes place without any change in chemical composition of minerals, it is called Mechanical or Physical weathering. This is a physical process in which the rock is broken into smaller pieces, each retaining the characteristic of the original rock. The end result is many small pieces from a single large one. The chemical nature and composition of the rock remains the same. According to C. D. Ollier (1969), “Weathering is the breakdown and alternation of minerals near the earth’s surface to products that are more in equillibrium with newly imposed physio-chemical conditions.” According to W. D. Thornbury, “Weathering may be defined as the disintegration or decomposition of rock in place.”

3. Under what type of climatic condition, mechanical weathering is most effective?

Answer: Mechanical weathering is predominant in hot and cold deserts, cool climate and dry temperature regions, polar and tundra region, taiga regions etc. Mechanical or physical weathering is important in Polar regions, where weathering is mainly due to freeze-thaw. Most of the regions are snow covered, and mechanical weathering, particularly frost action, plays the vital role. Chemical actions become weak due to low temperature. Thus, in cold areas like Arctic, physical weathering is dominant. In dry deserts, physical weathering is considered to be dominant due to wide ranges of temperature. Dearth of water in dry and desert regions offers there little scope for chemical weathering. Rocks get cracks and fractures very easily for heavy compression and tension and it shatters bare rocks into pieces.

4. In which type of climate mechanical weathering is more prevalent? State with reasons.

Answer: Mechanical weathering is largely found in hot desert regions, glaciated high mountainous regions and frozen cold temperate regions.

In Polar regions, weathering is mainly due to freeze-thaw. Most of the regions are snow covered, and mechanical weathering, particularly frost action, plays the vital role. Chemical actions become weak due to low temperature. Thus, in cold areas like Arctic, physical weathering is dominant.

In dry deserts, physical weathering is considered to be dominant due to wide ranges of temperature. Dearth of water in dry and desert regions offers there little scope for chemical weathering. Rocks get cracks and fractures very easily for heavy compression and tension and it shatters bare rocks into pieces.

5. Why is the chemical weathering most active in limestone or chalk area?

Answer: Chemical weathering, especially the carbonation process is most active in limestone rocks (CaCO₃). When rain water (H₂O) falls, sometimes the atmospheric carbon dioxide (CO₂) combines with it and turns it into weak acid which is known as carbonic acid (H₂CO₃) [water (H₂O) + carbon dioxide (CO₂) = carbonic acid (H₂CO₃)]. The chemical reaction of this acid (H₂CO₃) on limestone or calcium carbonate (CaCO₃) produces a component, called calcium bi-carbonate [CaCO₃ + H₂CO₃ = Ca(HCO₃)₂]. In limestone regions, the carbonic acid (H₂CO₃) forms varieties type of landforms. When this carbonic acid fed rain water falls on limestone rocks (CaCO₃), the calcium carbonate present in the rock absorbs carbon dioxide from rain water and becomes calcium bicarbonate. Calcium bicarbonate is more easily soluble and is removed as the water flows on the rock or seeps below the surface.

6. What is soil erosion?

Answer: Soil erosion means the wearing away of the upper part of the soil. It sometimes, happens where bare soil is left exposed on the ground. Rain water, running water and wind carry away the loose particles of soil. It is a great problem, particularly to agriculture and landuse.

Write short notes

(i) Solution

Answer: The process of carbonation is known as ‘solution’ wherein atmospheric carbon dioxide after mixing with water forms carbonic acid (CO₂ + H₂O = H₂CO₃). When this carbonic acid fed rain water falls on limestone rocks (CaCO₃), the calcium carbonate present in the rock absorbs carbon dioxide from rain water and becomes calcium bicarbonate. Calcium bicarbonate is more easily soluble and is removed as the water flows on the rock or seeps below the surface. The solution process is accelerated when water (H₂O) falls over limestone rocks, leading to the formation of long furrows with vertical walls, known as Karren or Clint. Deep narrow grooves between clint are called grikes.

(ii) Hydration

Answer: When minerals form new compounds by chemically reacting with water, the process is called Hydration. Thus, Hydration is a process by which certain types of mineral expand as they take up water, causing additional stresses in the rock. Many of the decomposition products of rockforming minerals are subject to hydration, thereby accelerating the disintegration of the rock in itself. Hydration is a mechanical effect, but it occurs intimately with hydrolysis in such a manner that it is difficult to draw any hard and fast line between mechanical and chemical weathering. When carbon dioxide is dissolved in water, the chemical action takes place at a fast rate. For example, Feldspar mineral through hydration is converted into Keolinite, a process known as ‘Kaolinization’. An example of hydration is: 2 Fe₂O₃ (Hematite) + H₂O (water) = 2 Fe₂O₃. 3H₂O (Limonite).

(iii) Exfoliation

Answer: When a rock consists of homogeneous minerals like granite, the thin surface layer of rocks absorbs heat during the day and expands. During the night, this surface layer contracts more than the inner ones and is peeled off one after another like an onion. This is known as Exfoliation or Onion Peeling.

Rocks are bad conductors of heat. When the rock consists of homogeneous minerals, it expands due to high temperature at daytime and contracts at night when the temperature falls to near freezing point. Thus, a rock layer close to the surface is subject to repeated alternate expansion and contraction, but rocks below it do not take part in it. As a result, the surface layer is pulled apart and peels off like layers of onion. This process is called exfoliation. N. H. Horrocks comments, “Exfoliation is partly produced by frost action, even in hot deserts where freezing of dew at night can produce frost heaving.”

Many granitic batholiths exposed above the ground surface are continuously affected by exfoliation weathering. For instance, Kanke Dam near Ranchi city exhibits a fine example of such weathering. The effect of this process is that a round shaped hill top is formed.

(iv) Block disintegration

This process occurs due to temperature change: in desert areas, bare rock surfaces are highly heated during the day, and the outer layers expand considerably. During the night, the temperature often falls close to freezing point, resulting in contraction. The alternate expansion and contraction develop a series of joints both parallel and perpendicular to the exposed surface, and the rock tends to break into blocks. This is appropriately termed Block Disintegration. It should also be pointed out that block disintegration should not be considered the result of only temperature changes; rather, unloading of superincumbent load or release of confining pressure also helps in this process.

The block disintegration process mainly acts on Basalt type of rocks. Results include: (i) Steps like stairs formed by this process; Deceantrans are formed by this mechanical process. (ii) Flat shaped hill tops are formed by this type of mechanical process. Shattering is the further disintegration of the products of block disintegration, involving the disintegration of rock along new surfaces of breakage, producing highly angular pieces with sharp corners and edges.

(v) Oxidation

Answer: The process of oxidation occurs when minerals in freshly exposed rocks take up additional oxygen. Deep-buried clays are often blue or grey in colour as long as air is excluded from them, but on exposure, they are oxidised and turn red or brown as ferric compounds are formed.

The chemical process of oxidation simply means a reaction of atmospheric oxygen to form oxides. When water is mixed with oxygen, its reaction with the minerals of the rocks forms hydroxide. The process of oxidation occurs when minerals in freshly exposed rocks take up additional oxygen. The chemicals break down rocks as they react with oxygen. For example, iron contained in rocks is rusted in the presence of oxygen. The rust expands, breaking up the iron-bearing rocks. An example is: 4 FeO + O₂ + 3H₂O → 2Fe₂O₃H₂O (Ferrous oxide + Oxygen + Water → Hydrous oxide [Limonite]).

(vi) Biological weathering

Answer: Biological weathering, or Organic weathering, refers to the disintegration, break up, and decomposition of rock masses by plants, animals, and activities of man. This type of weathering includes both (a) Mechanical disintegration and (b) Chemical decomposition.

(a) Mechanical disintegration includes:

(i) Plant-roots: The roots of trees and plants enter the cracks, joints, and hollows and loosen the rocks from inside.
(ii) Animals: Burrowing of animals such as rodents, insects, and worms loosen particles in rocks. Charles Darwin had calculated that insects in the British gardens upturn 254 kg of soil per hectare per year.
(iii) Man: Many activities of man, such as mining, quarrying, and agricultural practices, add to the weathering of rocks.

(b) Chemical decomposition includes:

(i) Plants: Decomposed leaves, stems, flowers, etc., form humus which, mixed with water, forms acid to react with the rock and leads to chemical weathering.
(ii) Animals: Decomposition of animals results in the formation of organic acids which may react with minerals present in rocks, leading to chemical weathering.

(vii) Soil conservation

Answer: Conservation of soil means protection for soil from erosion and preservation of soil fertility. Soil is a natural resource, so it should be conserved. The conservation of soil can be done in two ways: (1) By checking soil erosion and (2) By increasing the fertility of the soil.

Measures to Check Soil Erosion include:

(a) Preservation of soil erosion by afforestation: New plants should be planted in regions where soil has become exposed, in order to check rain wash and surface run off. Around agricultural land, trees should be planted to control soil erosion.
(b) Restriction of Overgrazing: Overgrazing should be avoided so that new plants can grow there. If vegetative cover on land is saved, soil erosion can be checked naturally.
(c) Introduction of contour cultivation/terraced cultivation: On mountain slopes, running water rushes down turbulently and carries away mud, silt, and clay. To check the force of running water and rain wash, a series of steps or terraces can be made on the land. This will check soil erosion.
(d) Prohibition of Jhum Cultivation: In ‘Jhum’ cultivation (also known as ‘shifting cultivation’), forested lands are first destroyed by fire to clear the forest, and cultivation is introduced. After a few years, when soil fertility decreases, the land is discarded, and a new plot is taken for cultivation. The abandoned land remains bare and open, becoming an arena of soil erosion. This type of cultivation should be prohibited to check soil erosion.

Measures to Conserve Soil fertility include:

(i) Soil fertility should be maintained by supplying adequate manure and water to the soil.
(ii) Soil should be used according to soil character. Care should be taken to preserve its fertility and productivity. The cultivation of soil-exhausting plants and crops should be avoided or proper precautions should be taken.
(iii) Crop rotation should be introduced to regenerate the fertility of the soil; sometimes green manure revives productivity.
(iv) Care should be taken to convert infertile soil into fertile ones by applying chemical fertilizers.
(v) Agricultural research can bring about new thoughts on soil character, new methods for its preservation, and new ideas to increase its productivity.

Answer the following questions briefly

(I) Describe how does the Mechanical weathering take place.

Answer: Mechanical weathering takes place through various processes:

Temperature changes: In very hot and dry regions like deserts, extreme temperature differences cause rocks to break. During the day, rocks, especially on the surface, get very hot and expand. At night, rapid cooling takes place causing the rocks to contract. This continuous expansion and contraction causes rocks to crack and eventually break up.
Block Disintegration: In arid regions, rocks develop joints and cracks through successive expansion and contraction and ultimately breakdown into smaller blocks. This occurs due to temperature changes where bare rock surfaces are highly heated during the day causing outer layers to expand, and at night, temperatures fall causing contraction. Alternate expansion and contraction develop joints, and the rock breaks into blocks. Unloading of superincumbent load or release of confining pressure also helps in this process.

Exfoliation or Onion Peeling: When a rock consists of homogeneous minerals like granite, the thin surface layer absorbs heat during the day and expands. During the night, this surface layer contracts more than the inner ones and peels off like an onion. This is because rocks are bad conductors of heat, and the surface layer is subject to repeated alternate expansion and contraction while rocks below are not.

Granular Disintegration: Different minerals present in rocks have different rates of expansion and contraction. Dark coloured minerals absorb more heat and expand more than light coloured minerals. This alternating expansion and contraction between day and night leads to the break up of rock into small fragments.

Frost Action: In humid, cool temperate regions or high altitudes, water collected in cracks and pores of rocks at daytime freezes at night. Ice expands by about 1/10th of its volume when it freezes, exerting enormous pressure on the walls of cracks. As a result, cracks are widened, and the rock breaks into pieces. This disintegration occurs due to freeze and thaw of water between rock particles and in crevices and pore spaces.

(2) Describe the various processes of Chemical weathering.

Answer: The most important processes of chemical weathering are:

Oxidation: This process occurs when minerals in freshly exposed rocks take up additional oxygen. It is a reaction of atmospheric oxygen to form oxides. When water is mixed with oxygen, its reaction with rock minerals forms hydroxide. For example, iron in rocks rusts in the presence of oxygen, and the rust expands, breaking up the iron-bearing rocks.
Carbonation: This refers to the absorption of carbon dioxide from the atmosphere by minerals present in the rocks. It is also known as ‘solution’. Atmospheric carbon dioxide mixes with water to form carbonic acid (CO₂ + H₂O = H₂CO₃). When this carbonic acid rain falls on limestone rocks (CaCO₃), the calcium carbonate absorbs carbon dioxide from rainwater and becomes calcium bicarbonate, which is more easily soluble and is removed by water.
Hydration: This is a process by which certain types of mineral expand as they take up water, causing additional stresses in the rock. Minerals form new compounds by chemically reacting with water. Many decomposition products of rock-forming minerals are subject to hydration, thereby accelerating rock disintegration. For example, feldspar mineral through hydration is converted into Keolinite (‘Kaolinization’).
Hydrolysis: This is a chemical reaction between mineral and water, specifically between hydrogen ions or hydroxyl (OH) ions, and the ions of the mineral. It is the chemical breakdown of rocks as they react with water. Some rock minerals react with water to form clay, weakening the rock’s structure. Silicate minerals are most affected by hydrolysis.

(3) Distinguish between Mechanical and Chemical weathering.

Answer: The distinction between Mechanical Weathering and Chemical Weathering is as follows:

Mechanical Weathering

(i) It is a physical process in which the rock is broken into smaller pieces.
(ii) There is no change in the composition of minerals present in the rocks.
(iii) Physical weathering takes place in all regions where there are wide changes in temperature.
(iv) Disintegration of rock is associated with the mechanical weathering.
(v) Block disintegration, exfoliation, granular disintegration, shattering etc. are the important processes of mechanical weathering.
(vi) Mechanical weathering largely found in hot desert regions, glaciated high mountainous regions and frozen cold temperate regions.

Chemical Weathering

(i) The weathering of rock begins through chemical process.
(ii) Chemical weathering results in changing composition of minerals present in the rocks.
(iii) Chemical weathering takes place in all regions where there is rain.
(iv) Decomposition of rock is associated with the chemical weathering.
(v) Carbonation, oxidation, hydration, hydrolysis, solution etc. are the important processes of chemical weathering.
(vi) Chemical weathering largely occurr in equatorial humid regions, tropical wet regions, and limestone regions.

(4) What measures should be adopted for the conservation of soil?

Answer: The conservation of soil can be done in two ways: (1) By checking soil erosion and (2) By increasing the fertility of the soil.

Measures to Check Soil Erosion:

(a) Preservation of soil erosion by afforestation: New plants should be planted in regions where soil has become exposed, in order to check rain wash and surface run off. Around the agricultural land, trees should be planted to control soil erosion.
(b) Restriction of Overgrazing: Overgrazing should be avoided so that new plant can grow there. If vegetative cover on land is saved soil erosion can be checked naturally.
(c) Introduction of contour cultivation/ terraced cultivation: On mountain slopes, running water rushes down turbulently and carries away mud, silt and clay. To check the force of running water and rain wash a series of steps or terraces can be made on the land. This will check soil erosion.
(d) Prohibition of Jhum Cultivation: In ‘Jhum’ cultivation (shifting cultivation), forested lands are destroyed by fire, and cultivation is introduced. After a few years, when soil fertility declines, the land is discarded, becoming bare and prone to erosion. This type of cultivation should be prohibited.

Measures to Conserve Soil fertility:

(i) Soil fertility should be maintained by supplying adequate manure and water to the soil.
(ii) Soil should be used according to soil character. Care should be taken to preserve its fertility and productivity. The cultivation of soil-exhausting plants and crops should be avoided or proper precautions should be taken.
(iii) Crop rotation should be introduced to regenerate the fertility of the soil ; sometimes green manure revives the productivity.
(iv) Care should be taken to convert the infertile soil into fertile ones by applying chemical fertilizers.
(v) Agricultural researches can bring about new thoughts on soil character, new method for its preservation and new ideas to increase its productivity.

Extras

Additional MCQs

1. What are the two opposing forces responsible for shaping the earth’s surface?

A. Volcanic and tectonic forces
B. Erosion and precipitation
C. Endogenetic and exogenetic forces
D. Temperature and wind

Answer: C. Endogenetic and exogenetic forces

2. What is the process of wearing away of rocks and levelling the surface of the earth called?

A. Gradation
B. Weathering
C. Erosion
D. Denudation

Answer: D. Denudation

3. What is the final stage of denudation?

A. Weathering
B. Mass wasting
C. Soil formation
D. Erosion

Answer: D. Erosion

4. Which of the following is a static process?

A. Erosion
B. Denudation
C. Weathering
D. Transportation

Answer: C. Weathering

5. Who defined weathering as the disintegration or decomposition of rocks in situ by natural agents?

A. Arthur Holmes
B. B. Sparks
C. W. D. Thornbury
D. C. D. Ollier

Answer: B. B. Sparks

6. Which process is not involved in the transportation of weathered material?

A. Erosion
B. Weathering
C. Mass wasting
D. Gradation

Answer: B. Weathering

7. What is the movement of rock debris down a slope under the influence of gravity called?

A. Erosion
B. Denudation
C. Mass wasting
D. Gradation

Answer: C. Mass wasting

8. What is the conical deposit of large boulders at the base of a hill called?

A. Regolith
B. Talus cone
C. Scree
D. Clint

Answer: B. Talus cone

9. What does the term “denudation” literally mean?

A. To construct
B. To elevate
C. To level
D. To strip bare

Answer: D. To strip bare

10. Which of the following is the correct equation for denudation?

A. Weathering + Transportation
B. Erosion + Deposition
C. Weathering + Erosion + Mass wasting
D. Erosion + Mass wasting

Answer: C. Weathering + Erosion + Mass wasting

11. Which of the following represents the process of gradation?

A. Erosion + Weathering
B. Transportation + Mass wasting
C. Erosion + Transportation + Deposition
D. Weathering + Deposition

Answer: C. Erosion + Transportation + Deposition

12. What is block disintegration primarily caused by?

A. Freezing water
B. Temperature changes
C. Carbonic acid
D. Organic matter

Answer: B. Temperature changes

13. Which type of rock is mainly affected by block disintegration?

A. Granite
B. Chalk
C. Basalt
D. Limestone

Answer: C. Basalt

14. What process involves the peeling of surface rock layers like an onion?

A. Frost action
B. Granular disintegration
C. Exfoliation
D. Shattering

Answer: C. Exfoliation

15. What type of weathering causes round shaped hill tops?

A. Block disintegration
B. Granular disintegration
C. Shattering
D. Exfoliation

Answer: D. Exfoliation

16. What is granular disintegration caused by?

A. Uniform minerals
B. Frost action
C. Differential thermal expansion
D. Pressure release

Answer: C. Differential thermal expansion

17. What is the process of water entering rock cracks and freezing to break the rock called?

A. Hydration
B. Carbonation
C. Frost action
D. Oxidation

Answer: C. Frost action

18. Which chemical process causes rocks to turn red or brown due to reaction with oxygen?

A. Hydration
B. Carbonation
C. Oxidation
D. Hydrolysis

Answer: C. Oxidation

19. What is formed when carbon dioxide in rainwater reacts with limestone?

A. Calcium carbonate
B. Calcium bicarbonate
C. Calcium oxide
D. Calcium chloride

Answer: B. Calcium bicarbonate

20. What landform is formed by the downward dripping of limestone-rich water from cave ceilings?

A. Stalagmite
B. Stalactite
C. Pillar
D. Karren

Answer: B. Stalactite

21. What is the process called when minerals form new compounds by reacting with water?

A. Oxidation
B. Hydration
C. Hydrolysis
D. Carbonation

Answer: B. Hydration

22. What is the chemical breakdown of rocks by reaction with water called?

A. Hydration
B. Carbonation
C. Hydrolysis
D. Oxidation

Answer: C. Hydrolysis

23. What is the end product of potassium feldspar after hydrolysis?

A. Limonite
B. Silica
C. Clay
D. Kaolinite

Answer: D. Kaolinite

24. In which regions is chemical weathering most active?

A. Polar
B. Tundra
C. Desert
D. Equatorial

Answer: D. Equatorial

25. Which mineral reaction causes rusting in rocks?

A. Hydration
B. Oxidation
C. Carbonation
D. Hydrolysis

Answer: B. Oxidation

26. What type of weathering is common in limestone regions?

A. Hydration
B. Hydrolysis
C. Carbonation
D. Oxidation

Answer: C. Carbonation

27. Which activity is not associated with mechanical weathering?

A. Exfoliation
B. Frost action
C. Granular disintegration
D. Hydrolysis

Answer: D. Hydrolysis

28. What is biological weathering caused by?

A. Water
B. Ice
C. Plants and animals
D. Wind

Answer: C. Plants and animals

29. What term describes the surface layer of weathered rock debris on earth?

A. Regolith
B. Talus
C. Scree
D. Humus

Answer: A. Regolith

30. What is the term for the formation of true soil from weathered material?

A. Denudation
B. Humification
C. Pedogenesis
D. Gradation

Answer: C. Pedogenesis

31. What is the principal source of humic acid in soil formation?

A. Wind erosion
B. Decomposed rock
C. Decayed plants
D. Glacial movement

Answer: C. Decayed plants

32. What type of erosion forms finger-like rills on the surface?

A. Sheet erosion
B. Gully erosion
C. Rill erosion
D. Slip erosion

Answer: C. Rill erosion

33. What type of erosion removes soil in channels down a slope?

A. Slip erosion
B. Sheet erosion
C. Gully erosion
D. Wind erosion

Answer: C. Gully erosion

34. What method helps conserve soil on mountain slopes?

A. Crop rotation
B. Afforestation
C. Jhum cultivation
D. Terrace farming

Answer: D. Terrace farming

35. What is Jhum cultivation also known as?

A. Monoculture
B. Slash and burn
C. Mixed farming
D. Terrace cultivation

Answer: B. Slash and burn

36. What is the process of enriching soil by growing specific crops in sequence?

A. Contour farming
B. Afforestation
C. Crop rotation
D. Manuring

Answer: C. Crop rotation

37. What type of weathering includes both mechanical and chemical processes caused by organisms?

A. Mechanical
B. Chemical
C. Biological
D. Organic

Answer: C. Biological

38. What substance is formed by the mineralisation of dead organic matter in soil?

A. Gravel
B. Limonite
C. Humus
D. Regolith

Answer: C. Humus

39. What type of erosion is caused by uniform removal of topsoil over a large area?

A. Rill erosion
B. Sheet erosion
C. Slip erosion
D. Gully erosion

Answer: B. Sheet erosion

40. What soil conservation technique involves planting trees around fields?

A. Shelter belt
B. Terrace farming
C. Crop rotation
D. Strip farming

Answer: A. Shelter belt

Additional Questions, Answers

1. What is weathering?

Answer: Weathering refers to the process by which rocks get broken down to small fragments when they are exposed on the surface of the land. It may be described as the disintegration or decomposition of rocks in situ by natural agents at or near the surface of the earth. According to B. Sparks, weathering is the mechanical fracturing and chemical decomposition of rocks, in situ, by natural agents at the surface of the earth. According to W. D. Thornbury, weathering may be defined as the disintegration or decomposition of rock in place.

2. Define erosion.

Answer: The transportation of the weathered rock materials from one place to another is called erosion. It is the process of removing or wearing away of disintegrated or weathered rocks.

3. What is meant by mass wasting?

Answer: Everywhere on the earth’s surface, gravity pulls continually downward on all materials. Disintegrated and fragmented rock materials formed due to weathering processes are called rockwastes, which tend to move down the hillslope. This is called mass movement of rockwaste or simply mass wasting. Thus, mass movement is the detachment and downslope transport of soil and rock material under the influence of gravity. The collective term for gravitational or downslope movements of weathered rock debris is mass-wasting.

4. Define denudation.

Answer: The activity of wearing away of rocks and levelling the surface of the earth is called denudation. Denudation means laying bare and comes from the Latin word denudare which means to ‘strip bare.’ Denudation is the wearing away of rocks so that the underlying rocks are laid bare. Actually, denudation is the combined action of weathering process and erosional process and it also includes mass wasting. Thus, denudation is a group of processes which act collectively to wearing away of rocks or surface whereby the underlying rocks are laid bare. Denudation equals Weathering plus Erosion plus Mass wasting.

5. What does gradation consist of?

Answer: Gradation consists of both degradation and aggradation.

6. What is degradation?

Answer: When landforms of high elevation are reduced in height, the process is known as degradation; degrade means to lower.

7. Define aggradation.

Answer: When landforms of lower elevation are increased in height, the process is known as aggradation; ‘aggregate’ means add to.

8. What is mechanical weathering?

9. Define block disintegration.

Answer: In arid regions, rocks develop joints and cracks through successive processes of expansion and contraction and ultimately breakdown into smaller blocks. This type of landform is known as block disintegration. In desert areas, the bare rock surfaces are highly heated during the day and the outer layers expand considerably, but during the night the temperature often falls close to freezing point with resulting contraction. The alternate expansion and contraction develop a series of joints both parallel and perpendicular to the exposed surface and the rock tends to break into blocks. This process may be appropriately termed Block Disintegration. Unloading of superincumbent load or release of confining pressure also helps in this process.

10. What is exfoliation?

11. What is granular disintegration?

Answer: Different minerals present in the rocks may have different rates of expansion and contraction. For example, dark coloured minerals will absorb more heat and expand to a greater extent than light coloured minerals. Such alternating expansion and contraction between day and night leads to a break up of rock into small fragments. This is called granular disintegration. The coarse-grained rocks are more affected by shattering process in those hot deserts which are characterised by high range of daily temperature. The different parts of the same rock mass receive and absorb different amount of solar radiation, as a result the different parts of the rocks are affected by differential expansion and contraction which cause stresses within the rocks due to which they are disintegrated into smaller particles.

12. Define frost action.

Answer: In humid, cool temperate region or in high altitude, water is collected in cracks and pores of rocks at daytime and freezes at night. Ice expands 1/10 th of the volume of water it freezes. Thus the frozen water exerts enormous pressure on walls of cracks. As a result the cracks are widened and ultimately the rock breaks into pieces. This is called Frost Action. Disintegration of rocks into large size blocks due to freeze and thaw of water is of common occurrence in the temperate and cold climatic regions. Frost action weakens the rocks due to freeze and thaw of water between the particles of the rocks and due to freeze and thaw of water in the crevices and pore spaces.

13. What is chemical weathering?

Answer: According to Arthur Holmes, the alternation and solution of rock material by chemical processes is referred as chemical weathering. Thus, decomposition and disintegration of rocks due to chemical reactions is called chemical weathering.

14. Define oxidation.

Answer: The process of oxidation occurs when minerals in freshly exposed rocks take up additional oxygen. The chemical process of oxidation simply means a reaction of atmospheric oxygen to form oxides. When water is mixed with oxygen its reaction with the minerals of the rocks forms hydroxide. The chemicals break down rocks as they react with oxygen. For example, iron contained in rocks is rusted in presence of oxygen. The rust expands, breaking up the iron-bearing rocks.

15. What is carbonation?

Answer: When the carbon dioxide mixed in water affects the rocks, it is known as carbonation. Thus carbonation refers to the absorption of carbon dioxide from the atmosphere by minerals present in the rocks. Carbonation is the reaction of carbonate or bicarbonate ions with minerals. The process of carbonation is known as ‘solution’ wherein atmospheric carbon dioxide after mixing with water forms carbonic acid (CO₂ + H₂O = H₂CO₃). When this carbonic acid fed rain water falls on limestone rocks (CaCO₃), the calcium carbonate present in the rock absorbs carbon dioxide from rain water and becomes calcium bicarbonate. Calcium bicarbonate is more easily soluble and is removed as the water flows on the rock or seeps below the surface.

16. Define hydration.

Answer: When minerals form new compounds by chemically reacting with water, the process is called Hydration. Thus Hydration is a process by which certain types of mineral expand as they take up water, causing additional stresses in the rock. Many of the decomposition products of rockforming minerals are subject to hydration, thereby accelerating the disintegration of the rock in itself.

17. What is hydrolysis?

18. What do you understand by biological weathering?

Answer: Biological weathering refers to disintegration, break up and decomposition of rock masses by plants, animals and activities of man. This type of weathering includes both (a) Mechanical disintegration, as well as (b) Chemical decomposition. Mechanical disintegration includes processes like plant roots entering cracks and loosening rocks, and animals burrowing and loosening particles. Chemical decomposition involves decomposed plant matter forming humus which mixes with water to form acid reacting with rock, and decomposition of animals forming organic acids reacting with minerals. Thus biological weathering may be mechanical and chemical in nature.

19. What is soil erosion?

Answer: Soil Erosion means the wearing away of the upper part of the soil. It sometimes happens where bare soil is left exposed on the ground. Rain water, running water and wind carry away the loose particles of soil. It is a great problem, particularly to agriculture and landuse.

20. Define soil conservation.

Answer: Conservation of soil means protection for soil from erosion and preservation of soil fertility. Soil is a natural resource so its should be conserved.

21. What is slip erosion?

Answer: A slip is a landside in which rocks filled water move down in one mass. This slip is due to the pressure of water absorbed by soil during heavy rain. When it is unable to percolate down a greater mass of overlying soil on steep land slips down.

22. Define rill erosion.

Answer: Rill Erosion is the removal of loose surface soil by small rill channel which may progressively join up to form gullies. In this stage, the small streams which are formed by joining the water channels, start downward cutting of soil leaving behind finger like rills over the surface.

23. What is gully erosion?

Answer: Gully Erosion is when soil is removed by water flowing along definite paths down the slope in channels.

24. What is humus?

Answer: Humus is formed from decomposed leaves, stems, flowers etc., which mixed with water forms acid to react with the rock and leads to chemical weathering. In soil formation, bacteria and other micro-organisms act on the remains of plants and organic material, forming humus and releasing minerals vital to plant growth.

25. Define regolith.

Answer: Due to weathering, the softened and loose rock mixture formed on the bed rock is called Regolith.

26. What is pedogenesis?

Answer: During weathering, rocks are broken down into smaller particles. The weathered material undergoes further changes, which is a complex process known as pedogenesis or soil development. Whereas in weathering mostly physical and chemical factors are involved, pedogenesis is largely a biological phenomenon involving living organisms and the addition of organic matter.

27. Explain the difference between weathering and erosion.

Answer: Weathering is the decay or disintegration of rocks on the surface of the earth, whereas erosion involves removing or wearing away of disintegrated rocks. The main agents of weathering are temperature, humidity and precipitation, while the main agents of erosion are river, glacier, wind and waves. Weathering is a static process, but erosion is a dynamic process. Furthermore, erosion depends on weathering, but weathering is independent and does not depend on erosion. Weathering is the initial stage of denudation, while erosion is the last stage of denudation. Weathering is not involved with transportation of weathered products except gravity, whereas erosion includes removal or transportation of the weathered rock material from one place to another.

28. Differentiate between mechanical and chemical weathering.

Answer: Mechanical weathering is a physical process in which the rock is broken into smaller pieces, whereas the weathering of rock in chemical weathering begins through a chemical process. In mechanical weathering, there is no change in the composition of minerals present in the rocks; however, chemical weathering results in changing the composition of minerals present in the rocks. Physical weathering takes place in all regions where there are wide changes in temperature, while chemical weathering takes place in all regions where there is rain. The disintegration of rock is associated with mechanical weathering, whereas the decomposition of rock is associated with chemical weathering. Block disintegration, exfoliation, granular disintegration, shattering etc. are the important processes of mechanical weathering; conversely, carbonation, oxidation, hydration, hydrolysis, solution etc. are the important processes of chemical weathering. Mechanical weathering is largely found in hot desert regions, glaciated high mountainous regions and frozen cold temperate regions, while chemical weathering largely occurs in equatorial humid regions, tropical wet regions, and limestone regions.

29. State the agents of mechanical weathering.

Answer: Important physical or mechanical weathering agents are:

(i) Temperature,
(ii) Frost,
(iii) Moisture and water,
(iv) Wind,
(v) Plants and Trees,
(vi) Animals.

30. What causes block disintegration?

Answer: In arid regions, rocks develop joints and cracks through successive processes of expansion and contraction due to temperature changes, and ultimately breakdown into smaller blocks. In desert areas, the bare rock surfaces are highly heated during the day causing outer layers to expand considerably, but during the night the temperature often falls close to freezing point resulting in contraction. The alternate expansion and contraction develop a series of joints both parallel and perpendicular to the exposed surface and the rock tends to break into blocks. Unloading of superincumbent load or release of confining pressure also helps in this process.

31. Briefly explain exfoliation.

Answer: When a rock consists of homogeneous minerals like granite, the thin surface layer absorbs heat during the day and expands. During the night, this surface layer contracts more than inner ones. Thus a rock layer close to the surface is subject to repeated alternate expansion and contraction but rocks below it do not take part in it. As a result, the surface layer is pulled apart and peels off like layers of an onion. This process is known as Exfoliation or Onion Peeling.

32. Mention two important landforms created by thermal changes.

Answer: Physical or mechanical weathering due to changes in temperature results in some important landforms in desert areas. Two examples are:
(i) Steps like stairs formed by the block disintegration process.
(ii) Round shaped hill tops formed by the exfoliation process of weathering.

33. Briefly describe granular disintegration.

Answer: Granular disintegration occurs because different minerals present in rocks may have different rates of expansion and contraction. For example, dark coloured minerals will absorb more heat and expand to a greater extent than light coloured minerals. Such alternating expansion and contraction between day and night leads to a break up of rock into small fragments. The coarse-grained rocks are more affected by this process in hot deserts characterized by a high range of daily temperature. Different parts of the same rock mass receive and absorb different amounts of solar radiation, resulting in differential expansion and contraction which cause stresses within the rocks, leading to their disintegration into smaller particles.

34. How does frost action cause rock disintegration?

Answer: Frost action causes rock disintegration in humid, cool temperate regions or at high altitudes. Water collects in cracks and pores of rocks during the daytime and freezes at night. Since ice expands 1/10th of the volume of water it freezes, the frozen water exerts enormous pressure on the walls of cracks. As a result, the cracks are widened, and ultimately the rock breaks into pieces. This disintegration of rocks into large size blocks due to the freeze and thaw of water weakens the rocks in two ways: (i) due to freeze and thaw of water between the particles of the rocks and (ii) due to freeze and thaw of water in the crevices and pore spaces. The diurnal freeze and thaw cycle causes alternate expansion and contraction, introducing tension and stresses that disintegrate rocks into small particles.

35. What happens during oxidation of rocks?

Answer: During oxidation, minerals in freshly exposed rocks take up additional oxygen. This chemical process involves a reaction of atmospheric oxygen to form oxides. When water is mixed with oxygen, its reaction with the minerals of the rocks forms hydroxide. The chemicals break down rocks as they react with oxygen. For example, iron contained in rocks is rusted in the presence of oxygen. The rust expands, breaking up the iron-bearing rocks.

36. Describe the carbonation process briefly.

Answer: Carbonation is the reaction of carbonate or bicarbonate ions with minerals. It is also known as ‘solution’. In this process, atmospheric carbon dioxide mixes with water to form carbonic acid (CO₂ + H₂O = H₂CO₃). When this carbonic acid-fed rainwater falls on limestone rocks (CaCO₃), the calcium carbonate present in the rock absorbs carbon dioxide from the rainwater and becomes calcium bicarbonate. Calcium bicarbonate is more easily soluble and is removed as the water flows on the rock or seeps below the surface.

37. What is hydration in weathering?

Answer: Hydration is a process by which certain types of mineral expand as they take up water, causing additional stresses in the rock. It occurs when minerals form new compounds by chemically reacting with water. Many of the decomposition products of rock-forming minerals are subject to hydration, thereby accelerating the disintegration of the rock in itself.

38. How does hydrolysis affect rocks?

Answer: Hydrolysis is the chemical breakdown of rocks as they react with water. Some rock minerals react with water to form clay, which weakens the structure of the rock. Silicate minerals are most affected by hydrolysis, and this reaction starts immediately when a mineral comes in contact with water.

39. List the causes of soil erosion.

Answer: The major causes of soil erosion are:

(a) Character of the soil: The loose and fragile soil easily wear away by the action of rain water, running water and wind.
(b) Character of the land slope: The slope of the land determines the force of action. The erosional agents act upon soil vigorously on the steep slope.
(c) Bare Land: The roots of the plants bind the soil. Bare lands easily wear away by the action of rain water and running water.
(d) Unwise and uncontrolled use: This includes (i) removal of vegetation cover which aggravates the loosening of the soil, (ii) Overgrazing by animals, and (iii) Faulty method of cultivation.

40. Mention four types of soil erosion.

Answer: Generally there are four types of soil erosion:

(a) sheet erosion
(b) slip erosion
(c) Rill erosion
(d) Gully erosion.

41. How can afforestation prevent soil erosion?

Answer: Afforestation helps in the preservation of soil from erosion. New plants should be planted in regions where soil has become exposed, in order to check rain wash and surface run off. Around the agricultural land, trees should be planted to control soil erosion.

42. Briefly explain contour farming.

Answer: On mountain slopes, running water rushes down turbulently and carries away mud, silt and clay. Contour farming involves making a series of steps or terraces on the land to check the force of running water and rain wash. This method will check soil erosion.

43. What is slip erosion and how does it occur?

Answer: Slip Erosion is a landslide in which rocks filled with water move down in one mass. This slip is due to the pressure of water absorbed by soil during heavy rain. When the soil is unable to percolate down a greater mass of overlying soil on steep land, it slips down.

44. Describe the constructive effects of weathering.

Answer: The constructive effects of weathering are:

(1) The effect of weathering is to produce a layer of loose material on the surface of the earth.
(2) Weathering contributes to soil formation which promotes agriculture.
(3) Chemical weathering produces various new minerals which prove helpful in several economic activities.
(4) Weathering results in the availability of building materials like cement and limestone.
(5) The weathering process sometimes exposes the minerals which become easier to be exploited.

45. What are the destructive effects of weathering?

Answer: The destructive effects of weathering are:

(1) Weathering sometime dissolves the soluble components of the soil and makes the upper surface pitted and furrowed. It causes difficulty in agriculture.
(2) Weathering leads to soil erosion which is a great menace for farming.
(3) In mountain areas, weathering some times causes icebergs to slide down the slopes into the valleys. They obstacle in sailing of boats and steamers.

46. Discuss the stages in soil formation.

Answer: Soil is formed by the physical, chemical and biological changes that take place continuously in the soil layer. The stages are:

(i) Stage-1 : Bare rock begins to break up near the surface as a result of weathering. Due to weathering, the softened and loose rock mixture formed on the bed rock is called Regolith.
(ii) Stage-2: The organic layer develops. Bacteria and other micro-organisms act on the remains of plants and organic material, forming humus and releasing minerals vital to plant grown.
(iii) Stage-3: Water percolating through the material, distributes minerals, creating soil.

47. How do plants and animals cause biological weathering?

(a) Mechanical disintegration is caused by:

(I) Plant-roots: The roots of trees and plants enter the cracks, joints and hollows and loosen the rocks from inside.
(ii) Animals: Burrowing of animals such as rodents, insects and worms loosen particles in rocks. Charles Darwin had calculated that insects in the British gardens upturn 254 kg of soil per hectare per year.

(b) Chemical decomposition is caused by:

(i) Plants : Decomposed leaves, stems, flowers etc. form humus which mixed with water forms acid to react with the rock and leads to chemical weathering.
(ii) Animals: Decomposition of animals results in the formation of organic acids which may react with minerals present in rocks. This leads to chemical weathering.

48. Explain the mechanism of exfoliation with an example.

Answer: When the rock consists of homogeneous minerals like granite, the thin surface layer of rocks absorbs heat during the day and expands. During the night, this surface layer contracts than inner ones and are peeled off one after another like an onion. This is known as Exfoliation or Onion Peeling. Rocks are bad conductor of heat. When the rock consists of homogeneous minerals, the rock expands due to high temperature at daytime and contracts at night when temperature falls to near freezing point. Thus a rock layer close to the surface is subject to repeated alternate expansion and contraction but rocks below it do not take part in it. As a result of it, the surface layer is pulled apart and peels off like layers of onion. This process is called exfoliation. Kanke Dam near Ranchi city exhibits a fine example of such weathering process.

49. Explain the importance of oxidation in chemical weathering.

50. Describe the role of carbonation in shaping limestone regions.

Answer: Carbonation is the reaction of carbonate or bicarbonate ions with minerals. The process of carbonation is known as ‘solution’ wherein atmospheric carbon dioxide after mixing with water forms carbonic acid (CO₂ + H₂O = H₂CO₃). When this carbonic acid fed rain water falls on limestone rocks (CaCO₃), the calcium carbonate present in the rock absorbs carbon dioxide from rain water and becomes calcium bicarbonate. Calcium bicarbonate is more easily soluble and is removed as the water flows on the rock or seeps below the surface. In limestone regions, the carbonic acid (H₂CO₃) forms varieties type of landforms. Solution process is accelerated when water (H₂O) falls over limestone rocks. Thus long furrows are formed which have vertical walls, known as Karren or Clint, with deep narrow grooves between called grikes. Underground caves known as Caverns are formed. Hundreds of small holes called sink holes form, through which water percolates down. Inside the caverns, water containing limestone in solution drips through joints, leaving deposits of dissolved limestone. Deposits hanging from the ceiling are called stalactite, and deposits growing upwards from the floor are called stalagmite. Sometimes stalactites and stalagmites may join together to form a column or pillar. Chemical weathering, especially the carbonation process is most active in limestone rocks (CaCO₃).

51. Explain how hydration contributes to weathering.

Answer: Hydration is a process by which certain types of mineral expand as they take up water, causing additional stresses in the rock. When minerals form new compounds by chemically reacting with water, the process is called Hydration. Many of the decomposition products of rockforming minerals are subject to hydration, thereby accelerating the disintegration of the rock in itself. Hydration is a mechanical effect, but it occurs intimately with hydrolysis. For example, Feldspar mineral through hydration is converted into Keolinite, the process being known as ‘Kaolinization’. Another example is 2 Fe₂O₃ (Hematite) + H₂O (water) = 2 Fe₂O₃. 3H₂O (Limonite).

52. Describe hydrolysis and give one example.

Answer: According to the geomorphologist C.D. Olier (1969), Hydrolysis is a chemical reaction between mineral and water, that is between hydrogen ions or hydroxyl (OH) ions, and the ions of the mineral. Thus hydrolysis is the chemical breakdown of rocks as they react with water. Some rock minerals react with water to form clay. This weakens the structure of the rock. Silicate minerals are most affected by hydrolysis. This reaction starts immediately when a mineral comes in contact with water.
Example-1: The hydrolysis of magnesium silicate minerals (Mg₂SiO₄) in contact with 4 ionized water molecules (4H₂O → 4H⁺ + 4OH⁻) takes place in following manner— Mg₂SiO₄ + 4 H⁺ + 4OH⁻ hydrolysis → 2 Mg⁺⁺ + 4OH⁻ + H₄SiO₄ (Silica acid in solution).

Example-2: The mechanism of the hydrolysis of potassium feldspar is given below— 2 KAlSi₃O₈ + H₂CO₃ + 9 H₂O → Al₂Si₂O₅ (OH)₄ (Orthoclase + Carbonic acid + Water → Kaolinite, (a clay mineral)).

53. Discuss the role of climate in weathering processes.

Answer: Climate has a close relation with weathering. The process of weathering differs from one climatic region to another.

(a) Mechanical weathering: Mechanical or physical weathering is important in:

I. Polar regions: Weathering in polar regions is mainly due to freeze-thaw. Most of the regions are snow covered, Mechanical weathering, particularly, frost action, plays the vital role. Chemical actions become weak due to low temperature. Thus, in cold areas like Arctic, physical weathering is dominant.
II. Dry and desert regions: In dry deserts, physical weathering is considered to be dominant due to wide ranges of temperature. Dearth of water in dry and desert regions offers there little scope for chemical weathering. Rocks get cracks and fractures very easily for heavy compression and tension and it shatters bare rocks into pieces. Mechanical weathering is largely found in hot desert regions, glaciated high mountainous regions and frozen cold temperate regions.

(b) Chemical weathering: Chemical weathering is important in:

I. Equatorial regions: In equatorial region there is high temperature as well as high rainfall throughout the year. For this hot and humid climatic condition, chemical weathering is definitely most favourable in this region. Rocks are as deep as 61 metres from the surface. CO₂ of air, humic acid from vegetation and nitric acid from thunder of clouds help in the chemical weathering in rocks. The rate of chemical weathering is the highest for certain rocks in this region.
II. Tropical Region: Here the range of temperature is wide in between summer and winter. This region experiences humid as well as dry weather condition. All these invite physical and chemical weathering to act upon. During rainy season, chemical weathering comes in action. Chemical weathering largely occurr in equatorial humid regions, tropical wet regions, and limestone regions.

54. Explain the processes involved in mechanical weathering and their resultant landforms.

Answer: Mechanical weathering is a physical process in which rock is broken into smaller pieces, each retaining the characteristic of the original rock, without any change in the chemical composition of minerals. Important physical or mechanical weathering agents are Temperature, Frost, Moisture and water, Wind, Plants and Trees, and Animals.
Mechanical weathering can occur in many ways:

Temperature: In very hot and dry regions like deserts, extreme temperature differences cause rocks to break. During the day, rocks, especially on the surface, get very hot and expand. At night, rapid cooling takes place causing the rocks to contract. This continuous expansion and contraction causes rocks to crack and eventually break up. Mechanical weathering is predominant in hot and cold deserts, cool climate and dry temperature regions, polar and tundra regions, taiga regions etc.
Important Landforms by thermal change include:

Block Disintegration: In arid regions, rocks develop joints and cracks through successive processes of expansion and contraction and ultimately breakdown into smaller blocks. In desert areas, bare rock surfaces are highly heated during the day causing outer layers to expand considerably, while at night, contraction occurs as temperatures fall. This alternate expansion and contraction develops a series of joints both parallel and perpendicular to the exposed surface, causing the rock to break into blocks. Unloading of superincumbent load or release of confining pressure also helps in this process. The block disintegration process mainly acts on Basalt type of rocks. Resultant landforms include steps like stairs and flat-shaped hilltops. Shattering is the further disintegration of the products of block disintegration, producing highly angular pieces with sharp corners and edges.

Exfoliation: When the rock consists of homogeneous minerals like granite, the thin surface layer absorbs heat during the day and expands. During the night, this surface layer contracts more than inner ones and layers are peeled off one after another like an onion. This is known as Exfoliation or Onion Peeling. Since rocks are bad conductors of heat, a rock layer close to the surface is subject to repeated alternate expansion and contraction, but rocks below it do not take part. As a result, the surface layer is pulled apart and peels off like layers of onion. Many granitic batholiths exposed above the ground surface are affected by exfoliation weathering. The effect is a round-shaped hill top.

Granular disintegration: Different minerals present in rocks may have different rates of expansion and contraction. Dark coloured minerals absorb more heat and expand more than light coloured minerals. Such alternating expansion and contraction between day and night leads to a break up of rock into small fragments. Coarse-grained rocks are more affected, especially in hot deserts with a high range of daily temperature. Differential expansion and contraction within the rock mass cause stresses, leading to disintegration into smaller particles.

Frost Action: In humid, cool temperate regions or in high altitude, water collected in cracks and pores of rocks at daytime freezes at night. Ice expands 1/10th of the volume of water it freezes, exerting enormous pressure on the walls of cracks. This widens the cracks, and ultimately the rock breaks into pieces. Disintegration of rocks into large size blocks due to freeze and thaw of water is common in temperate and cold climatic regions. Frost action weakens rocks due to freeze and thaw of water between rock particles and within crevices and pore spaces. The diurnal freeze and thaw cycle causes alternate expansion and contraction, introducing tension and stresses that disintegrate rocks into small particles.

55. Describe the process and effects of chemical weathering with suitable examples.

Answer: Chemical weathering involves the decomposition and disintegration of rocks due to chemical reactions. The most important processes are Oxidation, Carbonation, Hydration, and Hydrolysis.

Oxidation: This occurs when minerals in freshly exposed rocks take up additional oxygen, often turning red or brown as ferric compounds are formed. It is a reaction of atmospheric oxygen to form oxides. When water is mixed with oxygen, its reaction with rock minerals forms hydroxide. For example, iron contained in rocks is rusted in the presence of oxygen. The rust expands, breaking up the iron-bearing rocks.

Example: 4 FeO (Ferrous oxide) + O₂ + 3H₂O → 2Fe₂O₃.3H₂O (Hydrous oxide [Limonite])

Carbonation: This refers to the absorption of carbon dioxide from the atmosphere by minerals present in the rocks, often when carbon dioxide is mixed in water. It is the reaction of carbonate or bicarbonate ions with minerals, also known as ‘solution’. Atmospheric carbon dioxide mixing with water forms carbonic acid (CO₂ + H₂O = H₂CO₃). When this carbonic acid rain falls on limestone rocks (CaCO₃), the calcium carbonate absorbs carbon dioxide from the rainwater and becomes calcium bicarbonate, which is more easily soluble and is removed as water flows or seeps. Chemical weathering, especially carbonation, is most active in limestone rocks (CaCO₃).

Example: CaCO₃ (Limestone) + H₂CO₃ (carbonic acid) = Ca(HCO₃)₂ (calcium bi-carbonate)

Hydration: This is a process by which certain types of mineral expand as they take up water, causing additional stresses in the rock. Minerals form new compounds by chemically reacting with water. Many decomposition products of rock-forming minerals are subject to hydration, accelerating rock disintegration. Hydration is a mechanical effect but occurs intimately with hydrolysis. Feldspar mineral through hydration is converted into Keolinite (‘Kaolinization’).

Example: 2 Fe₂O₃ (Hematite) + 3H₂O (water) = 2 Fe₂O₃. 3H₂O (Limonite)

Hydrolysis: This is the chemical breakdown of rocks as they react with water. It is a chemical reaction between mineral and water, specifically between hydrogen ions (H+) or hydroxyl (OH-) ions, and the ions of the mineral. Some rock minerals react with water to form clay, weakening the rock structure. Silicate minerals are most affected.

Example: Hydrolysis of magnesium silicate minerals (Mg₂SiO₄) in contact with ionized water (4H₂O → 4H⁺ + 4OH⁻): Mg₂SiO₄ + 4H⁺ + 4OH⁻ → 2Mg⁺⁺ + 4OH⁻ + H₄SiO₄ (Silica acid in solution)

56. Explain biological weathering, mentioning both mechanical and chemical aspects.

Answer: Biological weathering, or Organic weathering, refers to the disintegration, break up, and decomposition of rock masses by plants, animals, and activities of man. This type of weathering includes both mechanical disintegration and chemical decomposition.

(a) Mechanical disintegration:

(I) Plant-roots: The roots of trees and plants enter cracks, joints, and hollows and loosen the rocks from inside.
(ii) Animals: Burrowing of animals such as rodents, insects, and worms loosens particles in rocks. Charles Darwin calculated that insects in British gardens upturn 254 kg of soil per hectare per year.
(iii) Man: Many activities of man, such as mining, quarrying, agricultural practices etc., add to the weathering of rocks.

(b) Chemical decomposition:

(i) Plants: Decomposed leaves, stems, flowers etc. form humus which, mixed with water, forms acid that reacts with the rock and leads to chemical weathering.
(ii) Animals: Decomposition of animals results in the formation of organic acids which may react with minerals present in rocks, leading to chemical weathering.
Thus, biological weathering may be mechanical and chemical in nature.

57. Discuss the main agents and types of soil erosion.

Answer: The agents of soil erosion are powerful and transport soil. The main agents causing soil erosion are rain water, running water, and wind. The erosional agents act upon soil vigorously on steep slopes and less destructively on flat land. Generally, there are four types of soil erosion:

(a) Sheet erosion: When a layer of soil on the surface is removed from over a large area by running water, it is called sheet erosion.
(b) Slip Erosion: A slip is a landslide in which rocks filled with water move down in one mass. This slip is due to the pressure of water absorbed by soil during heavy rain. When the soil is unable to percolate down, a greater mass of overlying soil on steep land slips down.
(c) Rill Erosion: It is the removal of loose surface soil by small rill channels which may progressively join up to form gullies. In this stage, the small streams formed by joining water channels start downward cutting of soil, leaving behind finger-like rills over the surface.
(d) Gully Erosion: When soil is removed by water flowing along definite paths down the slope in channels, it is called gully erosion.

58. What measures can be adopted to conserve soil fertility?

Answer: Soil fertility should be maintained by supplying adequate manure and water to the soil. Soil should be used according to soil character. Care should be taken to preserve its fertility and productivity. The cultivation of soil-exhausting plants and crops should be avoided or proper precautions should be taken. Crop rotation should be introduced to regenerate the fertility of the soil; sometimes green manure revives the productivity. Care should be taken to convert the infertile soil into fertile ones by applying chemical fertilizers. Agricultural research can bring about new thoughts on soil character, new methods for its preservation, and new ideas to increase its productivity.

59. How is soil degradation prevented through agricultural practices?

Answer: Soil erosion, a form of soil degradation, can be checked through various agricultural and land management practices. These include:

(a) Preservation of soil erosion by afforestation: New plants should be planted in regions where soil has become exposed, in order to check rain wash and surface run off. Around agricultural land, trees should be planted to control soil erosion. Planting trees and grass can make soil erosion lesser by obstructing rainfall. Shelter belts also help.
(b) Restriction of Overgrazing: Overgrazing should be avoided so that new plants can grow there. If vegetative cover on land is saved, soil erosion can be checked naturally.
(c) Introduction of contour cultivation/ terraced cultivation: On mountain slopes, running water rushes down turbulently. To check the force of running water and rain wash, a series of steps or terraces (terrace farming) can be made on the land. Contour farming, following the land’s contours, also checks soil erosion.
(d) Prohibition of Jhum Cultivation: In ‘Jhum’ or ‘shifting cultivation’, forested lands are destroyed by fire, and cultivation is introduced. After a few years, when soil fertility declines, the land is abandoned, remaining bare and prone to erosion. This type of cultivation should be prohibited to check soil erosion.

60. Explain block disintegration and exfoliation processes, highlighting differences in formation.

Answer: Block Disintegration: This process occurs in arid regions where rocks develop joints and cracks through successive expansion and contraction due to temperature changes, ultimately breaking down into smaller blocks. Bare rock surfaces heat intensely during the day causing expansion, and cool rapidly at night causing contraction. This develops joints parallel and perpendicular to the surface, breaking the rock into blocks. It mainly acts on Basalt type rocks and can also be aided by pressure release. The result can be steps like stairs or flat-topped hills.

Exfoliation: This process occurs when a rock consists of homogeneous minerals, like granite. The thin surface layer absorbs heat and expands during the day, then contracts at night more than the inner layers. Because rock is a poor conductor of heat, only the surface layer experiences significant temperature fluctuations. This differential expansion and contraction causes the surface layer to be pulled apart and peel off like the layers of an onion. This is also known as Onion Peeling. The result is often a round-shaped hill top.

Differences in Formation:

Rock Structure/Composition: Block disintegration is often associated with rocks that already have or develop joints and cracks (like Basalt), breaking along these lines of weakness into angular blocks. Exfoliation typically occurs in homogeneous rocks (like granite) without pre-existing joint patterns dictating the breakage, resulting in the peeling of curved layers parallel to the surface.
Mechanism: While both involve thermal expansion and contraction, block disintegration emphasizes breaking along intersecting joints, whereas exfoliation emphasizes the peeling off of surface-parallel layers due to temperature gradients between the surface and the interior.
Resultant Shape: Block disintegration leads to angular blocks and features like steps or flat tops, while exfoliation leads to curved sheets peeling off and often results in rounded landforms.

61. Discuss the influence of climate on mechanical and chemical weathering processes.

Answer: Climate has a close relation with weathering, and the dominant process differs from one climatic region to another.

(a) Mechanical weathering:

I. Polar regions: Weathering here is mainly due to freeze-thaw (frost action). Most regions are snow-covered, so mechanical weathering, particularly frost action, plays the vital role. Chemical actions are weak due to low temperature. Thus, in cold areas like the Arctic, physical weathering is dominant.
II. Dry and desert regions: In dry deserts, physical weathering is considered dominant due to wide ranges of temperature. The dearth of water offers little scope for chemical weathering. Rocks get cracks and fractures easily from thermal stress and shatter. Mechanical weathering is largely found in hot desert regions.
Mechanical weathering is also largely found in glaciated high mountainous regions and frozen cold temperate regions.

(b) Chemical weathering:

I. Equatorial regions: Here, there is high temperature and high rainfall throughout the year. This hot and humid condition makes chemical weathering definitely most favourable. Rocks can be weathered as deep as 61 metres. CO₂ of air, humic acid from vegetation, and nitric acid from thunder clouds aid chemical weathering. The rate of chemical weathering is highest for certain rocks in this region. Chemical weathering largely occurs in equatorial humid regions.
II. Tropical Region: Here the range of temperature is wide between summer and winter. This region experiences humid as well as dry conditions. Both physical and chemical weathering act upon rocks. During the rainy season, chemical weathering comes into action. Chemical weathering largely occurs in tropical wet regions.
Chemical weathering is also active in limestone regions.

62. What measures should be adopted to control soil erosion?

Answer: Soil conservation means protection for soil from erosion and preservation of soil fertility. Soil is a natural resource, so it should be conserved. The conservation of soil can be done by checking soil erosion. Soil can be conserved in many ways:

(a) Preservation of soil erosion by afforestation: New plants should be planted in regions where soil has become exposed, in order to check rain wash and surface run off. Around the agricultural land, trees should be planted to control soil erosion.
(b) Restriction of Overgrazing: Overgrazing should be avoided so that new plants can grow there. If vegetative cover on land is saved, soil erosion can be checked naturally.
(c) Introduction of contour cultivation/ terraced cultivation: On mountain slopes, running water rushes down turbulently. To check the force of running water and rain wash, a series of steps or terraces can be made on the land. This will check soil erosion.
(d) Prohibition of Jhum Cultivation: In ‘Jhum’ or ‘shifting cultivation’, forested lands are destroyed, and after a few years, the land is abandoned, becoming bare and prone to erosion. This type of cultivation should be prohibited to check soil erosion.

63. Explain the importance of soil as a dynamic entity and describe the process of soil genesis.

Answer: Soil is the upper weathered layer of the earth’s crust. It is not a lifeless matter but is considered to be a link between the living and non-living. Soil is a dynamic entity which is always undergoing physical, chemical, and biological changes. According to F. J. Monkhouse, “Soil is the thin surface-layer on the earth, comprising mineral particles formed by the break-down of rocks, decayed organic material, living Organism, soil water and soil atmosphere”. According to Dokuchaiev, “Soil is an independent, subaerial, evolutionary natural body on the earth’s surface occurring at dynamic equilibrium with environment”.

Soil development, called Soil genesis, is the final product of weathering and soil development processes occurring simultaneously.

During weathering, rocks are broken down into smaller particles. This weathered material undergoes further changes through pedogenesis or soil development, which is largely a biological phenomenon. Living organisms (algae, fungi, bacteria etc.) secrete organic acids, produce CO₂, and add organic matter after death. These geochemical, biochemical, and biophysical processes convert weathered rock debris into true soils, consisting of a complex mineral matrix associated with organic compounds and a rich micro-organism population. Dead organic matter is mineralized, and these minerals are gradually added to different layers (horizons) of the developing soil, forming a soil profile.

The stages in Soil Formation are:

(i) Stage-1: Bare rock begins to break up near the surface as a result of weathering. The softened and loose rock mixture formed on the bedrock is called Regolith.
(ii) Stage-2: The organic layer develops. Bacteria and other micro-organisms act on the remains of plants and organic material, forming humus and releasing minerals vital to plant growth.
(iii) Stage-3: Water percolating through the material, distributes minerals, creating soil.

Ron'e Dutta

Ron'e Dutta is a journalist, teacher, aspiring novelist, and blogger who manages Online Free Notes. An avid reader of Victorian literature, his favourite book is Wuthering Heights by Emily Brontë. He dreams of travelling the world. You can connect with him on social media. He does personal writing on ronism.

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