Samacheer Class 8 Social Science - Rocks and Soil

The scientific study of rocks is called petrology, which is the branch of geology that examines the composition, structure, and formation of rocks. Black soil is highly suitable for millets cultivation due to its fertility and water retention capacity. The soil is often referred to as the skin of the earth because it forms a thin layer covering the earth's surface and is essential for supporting life. White marble is the kind of metamorphic rock using which the Taj Mahal was built, making it one of the most famous examples of marble architecture in the world. Igneous rocks are known as the primary rocks because they are formed directly from the cooling and solidification of molten magma. Igneous rocks are indeed called primary rocks because they are the first rocks to form from the earth's molten material. Slate is formed from shale through the process of metamorphism, where heat and pressure transform the original rock. Red soil is not formed by the process of leaching; rather, it is formed due to the weathering of rocks in tropical and subtropical regions. M-sand, which is manufactured sand, is used as an alternative for natural sand in construction to address the shortage of natural sand and reduce environmental impact. Volcanic mountains are covered with sedimentary rocks that have accumulated over time from the weathering and erosion of volcanic materials.

Chemical sedimentary rocks are found in the beds of reservoirs and are formed by the precipitating minerals from water. These rocks form usually through the evaporation of chemical-rich solutions, where dissolved minerals in water become concentrated and eventually crystallize out to form solid rock. Examples include rock salt and gypsum, which form when mineral-rich water bodies evaporate. Igneous rocks are found in the regions of volcanoes and in areas of intense heat beneath the earth's surface. The formation of igneous rocks takes place when there is an outflow of molten magma from the earth's interior. When this magma cools and solidifies either on the earth's surface as lava or beneath the surface as intrusive magma, it forms igneous rocks. Volcanic regions are particularly rich in igneous rocks because of the frequent eruptions and the continuous flow of molten material.

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.

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

It is false that a well-matured soil takes about 2000 years to form; soil formation is an extremely slow process that typically requires thousands of years, often much longer than 2000 years depending on climate and other factors. Lava comes out from a volcano, which is true; lava is the molten rock that erupts from a volcano and flows on the earth's surface. The word ignis does not mean rock; it is false that ignis means rock. The oldest sedimentary rocks of the world have been identified in Greenland, which is true; these ancient rocks provide evidence of the earth's geological history. Soil supports ecosystems and plays an important role in land management, which is true; soil is essential for plant growth, provides habitat for organisms, and is crucial for sustainable land use and agricultural productivity.