Which soil is best for cotton cultivation?

Black soil (Regur soil) is ideal for cotton because it is clayey and retains moisture well.

What is the difference between soil conservation and soil erosion?

Soil conservation protects soil from erosion to maintain fertility, while soil erosion is the removal of the top soil layer by natural forces or human activity.

Why are igneous rocks called primary rocks?

Because they were the first rocks formed on Earth from the cooling and solidification of molten magma.

Samacheer Kalvi 8th Social Science Rocks and Soils Exercise Solutions

Exercise Solutions Available

Step-by-step answers for all textbook questions in this chapter

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I. Choose the Correct Answer (MCQs)

1. Which is known as the sphere of rocks?

a) Atmosphere   b) Biosphere   c) Lithosphere   d) Hydrosphere

Lithosphere refers to the rocky outer layer of the Earth.

2. World Soil Day is observed on:

a) 15th August   b) 12th January   c) 15th October   d) 5th December

World Soil Day is celebrated on 5th December every year to raise awareness about soil conservation.

3. Fossils are found in:

a) Sedimentary rocks   b) Igneous rocks   c) Metamorphic rocks   d) Plutonic rocks

Sedimentary rocks form from deposited layers that can trap and preserve organic remains over time.

4. The top layer of soil is called:

a) Organic layer or humus   b) Topsoil   c) Subsoil   d) Bedrock

The A-horizon or topsoil is the uppermost fertile layer rich in organic and mineral matter.

5. Ideal soil for growing cotton:

a) Red soil   b) Black soil   c) Alluvial soil   d) Mountain soil

Black soil is clayey and retains moisture, making it ideal for cotton cultivation.

6. The major component of soil is:

a) Rocks   b) Minerals   c) Water   d) All the above

Minerals form about 45% of soil composition, making them the major component.

7. The most widespread and productive category of soil:

a) Alluvial soil   b) Black soil   c) Red soil   d) Mountain soil

Alluvial soil, found in river valleys and flood plains, is the most productive soil for agriculture.

8. Laterite soil is formed by the process of:

a) Weathering of igneous rocks   b) Deposition of silt   c) Leaching   d) Erosion

Laterite soil forms in tropical regions where heavy rainfall causes leaching, washing away soluble minerals from the upper layers.

9. Which of the following is an example of the rock cycle in action?

a) Sand forming dunes   b) Lava cooling to form basalt, which weathers to soil, which compacts into sedimentary rock   c) Water evaporating from oceans   d) Wind carrying pollen

The rock cycle involves one rock type transforming into another: magma cools to igneous rock, which weathers into sediment that becomes sedimentary rock, which under heat/pressure becomes metamorphic rock.

10. The percentage of mineral matter in soil composition is approximately:

a) 25%   b) 45%   c) 5%   d) 10%

Mineral matter constitutes about 45% of soil, making it the largest single component by proportion.

II. Fill in the Blanks

1. Scientific study of rocks is called Petrology. 2. Black soil is highly suitable for millets cultivation. 3. The "Skin of earth" is Soil. 4. White marble is the kind of metamorphic rock using which Taj Mahal was built. 5. Igneous rock is known as the primary rocks. 6. The B-Horizon is called the zone of accumulation where iron, clay, and aluminium compounds leached from above are deposited. 7. Laterite soil is formed by intense leaching in tropical regions with alternating wet and dry seasons. 8. Red soil gets its colour from the presence of iron oxide and is found in semi-arid regions of India. 9. Desert soil is porous and saline, making agriculture difficult without proper irrigation. 10. The scientific study of rocks — petrology — helps us understand the composition, texture, and structure of different rock types.

III. True or False

1. Igneous rocks are called primary rocks.

AnswerTrue 2. Slate is formed from shale. — True 3. Red soil is formed by the process of leaching. — False (Laterite soil, not red soil, is formed by leaching.) 4. M-sand is used as an alternative for natural sand in construction. — True 5. Volcanic mountains are covered with sedimentary rocks. — True 6. All sedimentary rocks contain fossils. — False (Only organic sedimentary rocks typically contain fossils; chemical and mechanical sedimentary rocks generally do not.) 7. Laterite soil is highly fertile and ideal for all types of crops. — False (Laterite soil is generally infertile due to leaching; it becomes suitable for tea and coffee plantations only after manuring.)

IV. Match the Following

Set 1:

No.	Column A	Answer
1	Granite	2 — Igneous rock
2	Fossils	1 — Found in sedimentary rocks
3	Basalt	4 — Extrusive igneous rock
4	Marble	3 — Metamorphic rock

Set 2:

No.	Column A	Answer
1	O-Horizon	3 — Humus layer
2	A-Horizon	1 — Topsoil
3	B-Horizon	4 — Subsoil (zone of accumulation)
4	C-Horizon	2 — Partially weathered parent material

Set 3:

No.	Column A	Answer
1	Alluvial soil	4 — River valleys and flood plains
2	Black soil	1 — Cotton cultivation
3	Laterite soil	5 — Formed by leaching
4	Red soil	2 — Iron oxide content
5	Desert soil	3 — Porous and saline

Set 4:

No.	Column A	Answer
1	Limestone	3 — Changes to marble
2	Shale	1 — Changes to slate
3	Granite	4 — Changes to gneiss
4	Sandstone	2 — Changes to quartzite

V. Choose the Incorrect Statement

Q1:

a) Igneous rocks are called the primary rocks.
b) Soil is the product of weathering of rocks.
c) Sedimentary rocks are the hardest ones.
d) Deccan plateau is the region of Igneous rocks.

Answer(c) — Sedimentary rocks are not the hardest; igneous and metamorphic rocks are harder.

Q2:

a) Soil erosion decreases its fertility.
b) Dynamic metamorphism is caused by high temperature.
c) Soil is a renewable source.
d) Humus is a part of the top layer of soil.

Answer(c) — Soil is not a renewable source in the short term; it takes thousands of years to form.

VI. Consider the Statements

Statement (1): Sedimentary rocks consist of many layers. Statement (2): Sedimentary rocks are formed by sediments deposited at different points of time.

a) 1 and 2 are correct and 2 explains 1   b) 1 and 2 are correct but 2 does not explain 1   c) 1 is correct but 2 is incorrect   d) 2 is correct but 1 is incorrect

Both statements are correct, and statement 2 explains why sedimentary rocks have layers — they are deposited at different times.

VII. Give Reasons

1. Chemical sedimentary rocks are found in the beds of reservoirs. Chemical sedimentary rocks form by precipitation of minerals from water, typically through evaporation of chemical-rich solutions in lake beds and reservoirs.

2. Igneous rocks are found in the regions of volcanoes. Igneous rocks form when molten magma flows out of volcanoes and solidifies. Hence, volcanic regions naturally contain igneous rocks.

VIII. Distinguish the Following

Metamorphic Rock vs. Sedimentary Rock

Feature	Metamorphic Rocks	Sedimentary Rocks
Formation	Altered from igneous/sedimentary rocks by heat and pressure	Formed from deposited sediments cemented over time
Types	Thermal and Dynamic Metamorphism	Organic, Mechanical, and Chemical
Fossils	Rare (destroyed by heat/pressure)	Common (preserved in layers)
Example	Marble, Slate, Gneiss	Sandstone, Shale, Chalk

Soil Conservation vs. Soil Erosion

Feature	Soil Conservation	Soil Erosion
Meaning	Protecting soil from erosion to maintain fertility	Removal of topsoil by wind, water, or human activity
Effect	Increases fertility and productivity	Decreases fertility and agricultural productivity
Methods	Afforestation, crop rotation, wind breaks, dams	Caused by deforestation, overgrazing, improper farming

IX. Short Answer Questions

1. How are igneous rocks formed? Igneous rocks are formed by the solidification of molten magma originating from deep within the Earth. When magma reaches the surface through volcanic eruptions and cools rapidly, it forms extrusive igneous rocks such as basalt, which have a fine-grained texture. When magma cools slowly deep inside the Earth's crust, it forms intrusive igneous rocks such as granite, which have a large-grained texture. Igneous rocks are also called primary rocks because they were the first rocks formed on Earth.

2. Describe the composition of soil. Soil consists of approximately 45% mineral matter, 5% organic matter (humus), 25% water, and 25% air. The mineral component comes from weathered parent rock and provides structural support as well as essential nutrients. Organic matter, or humus, is formed from decomposed plant and animal material and improves soil fertility and water retention. The proportions of these components vary by location and time depending on climate, vegetation, and the type of parent rock from which the soil is derived.

3. Define 'rock'. A rock is a naturally occurring aggregate of one or more minerals found in the solid state on the Earth's surface. Rocks vary widely in hardness, colour, and texture depending on their mineral composition and the conditions under which they formed. For example, granite is a hard rock composed of quartz, feldspar, and mica, while chalk is a soft sedimentary rock made primarily of calcium carbonate.

4. State the types of soils. The six major soil types found in India are: Alluvial soil (found in river valleys and flood plains, most productive), Black soil or Regur soil (clayey, ideal for cotton), Red soil (formed by weathering of crystalline rocks, contains iron oxide), Laterite soil (formed by leaching in tropical regions), Mountain soil (found on slopes, thin and acidic), and Desert soil (porous and saline, found in arid regions). Each type supports different kinds of vegetation and agricultural activity depending on its mineral composition and water-retention capacity.

5. What is soil conservation? Soil conservation is the process of protecting soil from erosion and degradation in order to maintain its fertility and productivity for future use. Common methods include afforestation (planting trees to bind the soil), controlled grazing to prevent overgrazing, constructing dams and bunds to reduce water runoff, practising crop rotation to preserve nutrients, and building wind breaks to reduce wind erosion. Soil conservation is essential because soil takes hundreds of years to form even a thin layer, making it practically a non-renewable resource on a human timescale.

X. Detailed Answer Questions

1. Explain the three types of rocks with examples.

Answer

Rocks are classified into three major types based on their mode of formation: igneous, sedimentary, and metamorphic.

Igneous Rocks (Primary Rocks): These are the first rocks formed on Earth, created by the solidification of molten magma. They are classified into two subtypes based on where the magma cools:

Extrusive Igneous Rocks: Magma reaches the surface (as lava) and solidifies quickly, resulting in a fine-grained texture. Example: Basalt, which forms the Deccan Plateau.
Intrusive Igneous Rocks: Magma cools slowly deep inside the crust, resulting in a large-grained texture. Deep-seated intrusive rocks are called Plutonic rocks (e.g., Granite, Diorite); shallow-depth ones are called Hypabyssal rocks. Igneous rocks do not contain fossils because the extreme heat destroys any organic matter.

Sedimentary Rocks (Stratified Rocks): These form from sediments deposited by water, wind, or ice over long periods, which are then compacted and cemented together. They are classified into three subtypes:

Organic: Formed from decomposition of dead plants and animals; they contain fossils. Examples: Chalk, Talc.
Mechanical: Formed from disintegration of existing rocks, deposited by erosion agents. Examples: Sandstone, Shale.
Chemical: Formed by precipitation of minerals from water through evaporation. Example: Rock Salt. Sedimentary rocks are characterised by distinct layers (strata) and are generally softer than igneous and metamorphic rocks.

Metamorphic Rocks: These form when existing igneous or sedimentary rocks are subjected to high temperature (thermal metamorphism) or high pressure (dynamic metamorphism), or both, deep within the Earth. The original rock undergoes physical and chemical changes without melting. Key transformations include: Granite changes to Gneiss; Shale changes to Slate; Limestone changes to Marble; Sandstone changes to Quartzite. Metamorphic rocks are harder than their parent rocks and rarely contain fossils, as the heat and pressure usually destroy organic remains.

2. Give an account of different layers (horizons) of soil.

Answer

Soil is arranged in distinct horizontal layers called horizons, collectively forming a soil profile from the surface down to the unweathered bedrock. Each horizon has unique characteristics:

O-Horizon (Humus): The uppermost layer dominated by organic material such as fallen leaves, twigs, and decomposing plant and animal matter. This layer is dark in colour and rich in nutrients, playing a vital role in supporting microbial life and providing organic material to the layers below.

A-Horizon (Topsoil): Composed of a mixture of organic matter and mineral particles, this is the most fertile layer where plant roots primarily grow. It is also called the eluvial layer because some minerals are washed downward from it by percolating water.

E-Horizon (Elevated/Eluviation Layer): This layer is significantly leached of clay, iron, and aluminium oxides by percolating water, leaving behind a concentration of sand and silt particles with ore minerals. It is lighter in colour than the layers above and below.

B-Horizon (Subsoil / Zone of Accumulation): Iron, clay, aluminium, and organic compounds accumulate here after being leached downward from the O, A, and E horizons above. This layer is denser and less fertile than the topsoil.

C-Horizon: Contains partially weathered parent material that is still in the process of transitioning into soil. It retains some of the characteristics of the original rock but shows signs of physical and chemical breakdown.

R-Horizon: Consists of unweathered bedrock — the solid parent material from which all the above soil horizons eventually develop through the processes of weathering over long periods of time.

3. Classify and explain the types of soil found in India.

Answer

India has six major soil types, each with distinct characteristics and agricultural suitability:

Alluvial Soil: Found in river valleys, flood plains (especially the Indo-Gangetic plains), and coastal regions. It is formed by silt deposited by running water over centuries. Alluvial soil is the most widespread and productive soil type in India. It is highly fertile and suitable for growing sugarcane, jute, rice, wheat, and other food crops, making it the backbone of Indian agriculture.

Black Soil (Regur Soil): Formed by the weathering of igneous rocks, particularly basalts, and found mainly in the Deccan Plateau region. It is clayey in texture and has a high capacity to retain moisture, which makes it ideal for cotton cultivation. It is also suitable for growing millets, pulses, and oilseeds. Black soil develops deep cracks during the dry season, which helps in natural aeration.

Red Soil: Formed by the weathering of metamorphic and crystalline rocks, it is found in semi-arid regions of India. The soil appears brown-red due to the presence of iron oxide. It is generally not very fertile but can support the cultivation of millets, pulses, and tobacco with proper irrigation and fertilisation.

Laterite Soil: Formed by intense leaching in tropical regions with alternating wet and dry seasons. Heavy rainfall washes away soluble minerals, leaving behind iron and aluminium compounds, giving the soil a reddish appearance. It is generally infertile but becomes suitable for tea, coffee, and cashew nut plantations after manuring and proper treatment.

Mountain Soil: Found on mountain slopes across the Himalayan and other hill regions. It is thin, acidic, and varies in nature with altitude. It supports the growth of tea, coffee, and spices in the lower altitudes, while higher altitudes support coniferous forests.

Desert Soil: Found in hot desert regions such as western Rajasthan. It is porous, saline, and generally infertile. The organic matter content is very low, and agriculture is not successful without irrigation. When irrigated, it can support some hardy crops like millets and fodder grasses.

Common Mistakes

1. Confusing red soil with laterite soil: ❌ Mixing up their formation process — red soil is formed by weathering of crystalline rocks; laterite soil is formed by leaching in tropical regions. ✅ Remember: laterite = leaching; red = weathering of crystalline rocks with iron oxide. 2. Calling sedimentary rocks the hardest: ❌ Assuming sedimentary rocks are the hardest rock type. ✅ Igneous and metamorphic rocks are harder than sedimentary rocks, which are generally softer due to their layered formation. 3. Calling soil a renewable resource: ❌ Classifying soil as a quickly renewable resource. ✅ Soil takes 200-300 years to form just 1 cm, making it practically non-renewable on a human timescale. 4. Confusing intrusive and extrusive igneous rocks: ❌ Mixing up which type cools quickly vs. slowly. ✅ Extrusive rocks cool quickly on the surface (fine-grained, e.g., basalt); intrusive rocks cool slowly inside (large-grained, e.g., granite). 5. Confusing thermal and dynamic metamorphism: ❌ Swapping the causes of thermal and dynamic metamorphism. ✅ Thermal metamorphism is caused by high temperature; dynamic metamorphism is caused by high pressure. 6. Misidentifying the top layer of soil: ❌ Thinking the O-horizon (humus) is the topsoil. ✅ The A-horizon is the topsoil; the O-horizon is the humus layer above it. 7. Assuming all sedimentary rocks contain fossils: ❌ Believing every sedimentary rock preserves fossils. ✅ Only organic sedimentary rocks typically contain fossils; chemical and mechanical sedimentary rocks generally do not. 8. Confusing leaching with weathering: ❌ Treating leaching and weathering as the same process. ✅ Weathering is the physical and chemical breakdown of rocks into smaller particles, while leaching is the specific process of washing away soluble minerals from the upper soil layers by percolating water. 9. Assuming laterite soil is fertile: ❌ Thinking laterite soil naturally supports abundant crop growth. ✅ Laterite soil is generally infertile due to heavy leaching; it becomes productive only after manuring and treatment, particularly for plantation crops like tea and coffee.

Exam Checklist

[ ] Can identify the three types of rocks and give examples of each
[ ] Know the difference between extrusive and intrusive igneous rocks
[ ] Can explain how sedimentary rocks are formed and their subtypes (organic, mechanical, chemical)
[ ] Understand thermal vs. dynamic metamorphism with examples
[ ] Can name and describe all six soil horizons (O, A, E, B, C, R)
[ ] Know the six types of soil in India and their characteristics
[ ] Can distinguish between soil conservation and soil erosion
[ ] Remember that World Soil Day is on 5th December
[ ] Can explain the rock cycle and how one rock type transforms into another
[ ] Understand the process of leaching and which soil type it produces
[ ] Know the approximate composition of soil (45% minerals, 5% organic matter, 25% water, 25% air)
[ ] Can identify key rock transformations: limestone to marble, shale to slate, granite to gneiss, sandstone to quartzite

Internal Links

[Class 8 Social Science Guide](/class/8/social)
[Rocks and Soils — Chapter Guide](/class/8/social/rocks-and-soils)
[Hydrologic Cycle — Exercise Solutions](/class/8/social/solutions/hydrologic-cycle-exercise-solutions)
[Weather and Climate — Exercise Solutions](/class/8/social/solutions/weather-and-climate-exercise-solutions)

Frequently Asked Questions

Igneous, sedimentary, and metamorphic rocks. Igneous form from cooling magma, sedimentary from deposited layers, and metamorphic from heat and pressure changes.