8th Std Social Science Term 1 Solution | Lesson.5 Rocks and Soils

Lesson.5 Rocks and Soils

Lesson.5 Rocks and Soils

I. Choose the correct answer

1. Which of the following is known as sphere of rocks

1. Atmosphere
2. Biosphere
3. Lithosphere
4. Hydrosphere

Ans : Lithosphere

2. World soil day is observed on

1. 15th August
2. 12th January
3. 15th October
4. 5th December

Ans : 5th December

3. Fossils are found in

1. Sedimentary rocks
2. Igneous rocks
3. Metamorphic rocks
4. Plutonic rocks

Ans : Sedimentary rocks

4. The first layer of soil is called as

1. Regur
2. Regolith
3. Unweathered rock
4. Partially weathered rock

Ans : (*) Horizon

5. Ideal soil for growing cotton is

1. Red soil
2. Black soil
3. Alluvial soil
4. Mountain soil

Ans : Black soil

6. The major component of soil is

1. Rocks
2. Gas
3. Water
4. Minerals

Ans : Minerals

7. Which one of the following is the most widespread most and productive category of soil

1. Alluvial soil
2. Black soil
3. Red soil
4. Mountain soil

Ans : Alluvial soil

II. Fill in the Blanks.

1. Scientific study of rocks is called ________.

Ans : Petrology

2. _________ soil is highly suitable for cotton cultivation.

Ans : Black

3. The “skin of earth” is _________.

Ans : Soil

4. _________ is the kind of metamorphic rock using which Taj Mahal was built.

Ans : White Marble

5. _________ is known as the primary rocks.

Ans : Igneous rock

III. Match the following

1.

     a  b  c  d
A.  2  1  4  3
B.  2  1  3  4
C.  4  3  2  1
D.  3  4  2  1

Ans : 2,  1,  4,  3

2.

     a  b  c  d
A.  2  4  1  3
B.  2  4  3  1
C.  3  1  2  4
D.  3  1  4  2

Ans : 2,  4,  1,  3

IV. State true or false :

1. Igneous rocks are called primary rocks.

Ans : True

2. Slate is formed from shale.

Ans : True

3. Red soil is formed by the process of leaching.

Ans : False

Correct statement: Laterites soils is formed by the process of leaching.

4. M-sand is used as alternative for natural sand in construction.

Ans : True

5. Volcanic mountains are covered with sedimentary rocks.

Ans : False

Correct statement: Sedimentary rocks are formed from broken pieces of rocks.

V. Choose the incorrect statement from the following

1. 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.

Ans : Sedimentary rocks are the hardest ones.

2. 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.

Ans : Dynamic metamorphism is caused by high temperature.

VI. Consider the following statements and choose the right option from the given ones:

I) Statement (1): Sedimentary rocks consist of many layers.

Statement (2): Sedimentary rocks are formed by the sediments deposited at different points of time.

1. 1 and 2 are correct and 2 explains 1
2. 1 and 2 are correct but, 2 does not explain 1
3. 1 is correct but, 2 is incorrect
4. 2 is correct but, 1 is incorrect.

Ans : 1 and 2 are correct and 2 explains 1

VII. Give reasons for the following:

1. Chemical sedimentary rocks are found in the beds of reservoirs.

Chemical Sedimentary rocks are formed by precipitating of minerals from water. It is formed usually through evaporation of chemical rich solutions.

2. Igneous rocks are found in the regions of volcanoes.

The igneous rocks are formed by the solidification of molten magma

VIII.Distinguish the following:

1. Metamorphic rock and sedimentary rock.

2. Soil conservation and Soil erosion.

IX. Give short answers. 

1. How are igneous rocks formed?

The igneous rocks are formed by the solidification of molten magma.

2. Describe about the composition of soil.

The basic components of soil are mineral, organic matter, water and air. It consists of about 45% mineral, 5% organic matter, 25% of water and 25% air. It is only a generalized fact. The composition of soil varies from place to place and time to time.

3. Define ‘rock’.

• The rocks are the solid mineral materials forming a part of the surface of the earth and other similar planets.
• A rock is an aggregate of one or more minerals. Rock is an important natural resource and is found in solid state.
• It may be hard or soft in nature.

4. State the types of soils.

Alluvial soil, Black soil, Red soil, Laterite soil, Mountain soil, Desert soil.

5. What is soil conservation?

Soil conservation is the process of protecting the soil from erosion to maintain its fertility

X. Give detailed answer for the following:

1. Explain the process of soil formation.

• Soil is a mixture of organic matter, minerals, gases, liquids and organisms that together support life.
• Soil minerals form the basis of soil. It forms on the surface of the earth. It is known as the ‘skin of the earth’.
• Soils are produced from rocks (parent material) through the processes of weathering and natural erosion.
• Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material. It leads to the formation of loose material.
• In course of time, they further break down into fine particles. This process release the minerals locked in the rock fragments.
• Later on, the vegetative cover which develop in that region forms humus content in the soil. This way the soil gets matured gradually.

2. Classify and explain the rocks.

Igneous:

• The igneous rocks are formed by the solidification of molten magma. Also called Primary or Parent Rocks.
• Types:
  1. Extrusive Igneous Rocks.
  2. Intrusive Igneous Rocks

1. Extrusive Igneous Rocks:

Molten magma which comes out from the interior of the earth’s surface, gets solidified and forms such rocks. Fine grained and glassy in nature.

Eg., Basalt, found in north western peninsular India.

2. Intrusive Igneous rocks:

Molten magma cools down deep inside the earth’s crust, becomes solid and forms such rocks. Form large grains. Deep seated rocks are plutonic rock and ones at shallow depths – Hypabysal rocks.

Example Granite, Diorite.

Sedimentary:

• Formed by the sediments derived and deposited by various agents. Also called stratified rocks.
• Types:
  1. Organic sedimentary rocks
  2. Mechanical sedimentary rocks,
  3. Chemical sedimentary rocks

1. Organic sedimentary rocks:

Formed due to decomposition of dead Plants and Animals. Contains fossils.

Example Chalk, Talc

2. Mechanical sedimentary rocks:

Formed from the disintegration of Igneous and metamorphic rocks. Get deposited due to erosion by natural agents. Get cemented after a long time to form rocks.

Example Sandstone, Shale.

3. Chemical sedimentary rocks:

Formed by precipitating of minerals from water. Formed due to evaporation of chemical rich solution.

Example Rock Salt.

Metamorphic:

• Formed when Igneous and sedimentary rocks are subjected to high temperature and pressure.
• Types:
  1. Thermal
  2. Dynamic

1. Thermal Metamorphic:

If the change in rocks is caused by high temperature.

2. Dynamic Metamorphic:

If the change in rock is caused by high pressure.

Formed from Igneous rocks. Eg., Granite into gneiss Formed from Sedimentary rocks.

Formed from Sedimentary rocks. Eg., Shale into Slate

3. Give an account on different layers of soil.

O-Horizon or Humus

This layer is dominated by organic material (leaves, needles, twigs, moss and lichens).

A- Horizon or Top Soil

It is a part of top soil, composed of organic matter mixed with mineral matter.

E-Stands for elevated layer.

E-Stands for elevated layer. This layer is significantly leached of clay, iron, and aluminum oxides, which leaves a concentration of ore

B- Horizon or Sub-soil

This layer reflects the chemical or physical alteration of parent material. Thus iron, clay,  aluminum and organic compounds are found accumulated in this horizon.

C- Horizon or Parent Rock

Partially weathered parent material accumulates in this layer.

R- Horizon Parent Rock

This layer consists of unweathered part of bed rock

4. Classify and explain the soil.

Definition:

Soil is a mixture of Organic matter, minerals, gases, liquids and organisms that together support life.

Classification of soils:

Soils are classified on the basis of their formation, colour, physical and chemical properties. Based on these, soil is classified into six major types. They are: Alluvial soil, Black soil, Red soil, Laterite soil, Mountain soil, Desert soil.

Alluvial soil:

• Found in the regions of river valleys, flood plains and coastal regions.
• Formed by the deposition of silt by the running water.
• Most productive of all soils.
• Suitable for the culitivation of sugarcane, jute, rice, wheat.

Black soils:

• Formed by weathering of igneous rocks.
• Clayey in nature.
• Retains moisture.
• Ideal for growing cotton.

Red Soils:

• Formed by weathering of metamorphic rocks and crystalline rocks.
• Found in semi – arid regions.
• Not a fertile.
• Is soil brown red in colour due to the presence of iron oxide
• Suitable for millet cultivation.

Laterite soils:

• Formed by the process of leaching.
• Found in trophical regions, which experienced alternate wet and dry condition.
• Infertile soil.
• Suitable for plantation of tea and coffee.

Mountain soils:

• Found in slopes of mountains.
• Thin and acidic in nature.
• Nature of soil differs based on the altitude.

Desert soils:

• Found in hot desert region.
• Porous and saline in nature.
• Infertile in nature.
• Agriculture not successful

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