Samacheer Class 8 Social Science - Rocks and Soil

(c) Lithosphere
2. World soil day is observed on
a) 15th August
b) 12th January
c) 15th October
d) 5th December
(d) 5th Decembe
3. Fossils are found in
a) Sedimentary rocks
b) Igneous rocks
c) Metamorphic rocks
d) Plutonic rocks
(a) Sedimentary rocks4. The top layer of soil is called as
a) organic layer or humas
b) topsoil
c) subsoil
d) bedrock
(b) topsoil
5. Ideal soil for growing cotton is
a) Red soil
b) Black soil
c) Alluvial soil
d) Mountain soil
(b) Black soil
6. The major components of soil is
a) Rocks
b) Minerals
c) Water
d) All the above
(b) Minerals
7. Which one of the following is the most widespread most and productive category of soil
a) Alluvial soil
b) Black soil
c) Red soil
d) Mountain soil
(a) Alluvial Soil

Petrology
2. ……………. soil is highly suitable for millets cultivation.
Black
3. The “Skin of earth” is
Soil
4. ……………. is the kind of metamorphic rock using which Taj Mahal was built.
White marble
5. ……………. is known as the primary rocks.
Igneous rock
True or False:
1. Igneous rocks are called primary rocks.
True
2. Slate is formed from shale.
True
3. Red soil is formed by the process of leaching.
False
4. M-sand is used as alternative for natural sand in construction.
True
5. Volcanic mountains are covered with sedimentary rocks.
True

d) Deccan plateau is the region of Igneous rocks.
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.
c) Soil is a renewable source.

c) Taj Mahal in India was built with white marbles of Sedimentary rocks.

(a) 1 and 2 are correct and 2 explains 1

Chemical Sedimentary rocks are formed by the precipitating minerals from water. It is formed usually through the evaporation of chemical-rich solutions.
2. Igneous rocks are found in the regions of volcanoes
The formation of igneous rocks takes place when there is an outflow of molten magma.

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.
The composition of soil varies from place to place and from 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
Laterites 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.

Soil and its Formation:Igneous:
(i) The igneous rocks are formed by the solidification of molten magma. Also called Primary or Parent Rocks.
(ii) Types:
Extrusive Igneous Rocks.
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 northwestern 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:
(i) Formed by the sediments derived and deposited by various agents. Also called stratified rocks.
(ii) Types:
Organic sedimentary rocks
Mechanical sedimentary rocks,
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:
(i) Formed when Igneous and sedimentary rocks are subjected to high temperature and pressure.
(ii) Types:
Thermal
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. Eg., Shale into Slate3. Give an account of different layers of soil.
Layers of soil:
O – Horizon or humus:
This layer is dominated by organic material eg: leaves
A – Horizon or Topsoil:
It is a part of topsoil composed of organic matter mixed with mineral matter.
E – Horizon or Elevated layer:
E – stands for the elevated layer. This layer is significantly leached of clay, iron and aluminium oxides, which leaves a concentration of ore.
B – Horizontal or subsoil:
This layer reflects the chemical or physical alternation or parent material. Thus iron, clay, aluminium and organic compounds are found accumulated in the Horizon.
C – Horizon or Parent Rock:
Partially weathered parent material accumulates in this layer.
R – Horizon or Parent Rock:
This layer consists of an unweathered part of the bedrock.
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 cultivation 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 the alternate wet and dry conditions.
Infertile soil.
Suitable for plantation of tea and coffee.
Mountain soils:
Found in slopes of mountains.
Thin and acidic in nature.
The nature of soil differs based on the altitude.
Desert soils:
Found in the hot desert region.
Porous and saline in nature.
Infertile in nature.
Agriculture is not successful.

(c) Greek
2. The soils found in the desert regions are …………..
a) Mountain Soils
b) Desert Soils
c) Black Soil
d) Red Soil
(b) Desert Soils
3. Which Soils are formed by the weathering of metamorphic rocks?
a) Red Soils
b) Rock Soils
c) Laterite Soils
d) Mountain Soils
(a) Red Soils
4. …………….. is a part composed of organic matter mixed with mineral matter.
a) Humus
b) Top Soil
c) Elevated layer
d) Sub-soil
(b) Top Soil
5. The time to form one cm of soil is
a) 100 years
b) 200 years
c) 500 years
d) 1000 years
(b) 200 years
6. …………….is known as the zone of accumulation.
a) Humus
b) Top Soil
c) Elevated layer
d) Sub-soil
(d) Sub-soil
7. In the soil composition ………….. are minerals.
a) 5%
b) 10%
c) 25%
d) 45%
(d) 45%
8. In the soil composition is air.
a) 5%
b) 10%
c) 25%
d) 40%.
(c) 25%
9. ……………. soils are formed by the deposition of silt by the running water.
a) Black
b) Red
c) Alluvial
d) Desert
(c) Alluvial
10. the Soil composition is organic matter.
a) 5%
b) 10%
c) 25%
d) 45%
(a) 5%

one
2. There are ………….. different types of minerals found on the earth Surface.
20003. Rocks are classified in to ………….. types.
three
4. The two types of Intrusive Igneous rocks are ………….., …………..
Plutonic rocks, Hypabysal rocks
5. Igneous Rocks are ………….. rocks
Primary
6. ……………. into slate caused by thermal metamorphism.
Basalt
7. Soil is a basic requirement for the growth of ………….. and …………..
Plants, animals
8. To become a well-matured soil, it takes about ………….. years.
3000
9. Parent Rock is converted to loose Rock by ……………
soil
10. Laterite soil is suitable for plantation crops like ………….., …………..
tea, coffee

False
2. Lava comes out from a volcano.
True
3. The word Ignis means rock.
False
4. Oldest Sedimentary rocks of the world has been identified in Green land.
True
5. Soil supports ecosystem and play an important role in land management.
True

c) 1 is correct but, 2 is incorrect