Class 7 Social Science Inside Our Earth Exam Notes

Class 7 Social Science Inside Our Earth Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Class 7 Social Science – Inside Our Earth (Exam Notes)

Introduction

The earth is the only planet in the solar system which supports life. It is a dynamic planet in which changes are constantly taking place. The three realms of our earth are land (lithosphere), water (hydrosphere) and air (atmosphere). What lies in the interior of the earth has always been a mystery for most of us.

Structure of the earth

The earth is made up of concentric layers, one inside another (just like an onion). The three main layers of the earth are the crust, the mantle and the core.

1. The crust

1. It is the outermost and thinnest of all the layers.
2. It is also called the lithosphere, as it is a sphere of solid rocks.
3. Its thickness is about 35–40 km on the continental masses and only about 5–10 km on the ocean floors.
4. The continental mass is made up of rocks whose main mineral constituents are silica and alumina (SIAL).
5. The main constituents of the rocks of the oceanic crust are silica and magnesium (SIMA).
6. The crust forms less than one per cent of the radius of the earth.

2. The mantle

1. It lies just below the crust and extends up to a depth of about 2900 km below the crust.
2. At a depth of about 100–250 km, the mantle is partially molten.
3. This region is known as the asthenosphere.
4. The temperature in the mantle increases with depth and ranges between about 870°C (upper layer) and about 2200°C (lower layer).
5. The density of the mantle varies between 3 g per cm³ at the top and 4.5 g per cm³ at the bottom.

3. The core

1. It is the innermost layer of the earth and has a radius of about 3500 km.
2. It is the most inaccessible layer, lying around the centre of the earth.
3. It is the heaviest part of the earth and is made up of nickel (Ni) and iron (Fe). Thus, it is also called NIFE.
4. The estimated temperature is about 2200°C to 5000°C.
5. The layer is thought to be in a molten state, but the very high pressure keeps it firm and solid.
6. The density varies from 5 g per cm³ to about 13.9 g per cm³.

Rocks and minerals

The lithosphere, or the crust of the earth, is made up of a large variety of rocks. A rock is any natural mass of mineral matter that makes up the crust of the earth. Rocks differ from one another in colour, structure, texture, mode of occurrence and so on. Scientists have recognised more than 2000 minerals in the rocks, but only a few are important. The crust of the earth contains more of silicon and aluminium.

According to the mode of formation, the rocks of the earth's crust are of three major types:

1. igneous rocks
2. sedimentary rocks
3. metamorphic rocks

The igneous rocks

1. The term "igneous" has been derived from the Latin word ignis, which means fire.
2. Igneous rocks are associated with volcanic eruptions.
3. The hot molten material, called magma, is found at great depths in the interior of the earth.
4. The magma is brought to the surface of the earth during volcanic eruptions.
5. When the molten magma (now called lava) cools and solidifies, igneous rocks are formed.
6. These rocks were the first to be formed and are called the primary rocks.
7. Igneous rocks are hard, smooth, fine-grained and compact, or have large crystals.
8. These rocks do not have fossils of animals or plants.
9. Igneous rocks have silicate minerals, and most of the metals, such as iron, aluminium and gold, are found in them.
10. Igneous rocks are of two types: (A) extrusive igneous rocks and (B) intrusive igneous rocks.

(A) Extrusive igneous rocks

1. They are formed by the cooling of molten lava on the surface of the earth.
2. The hot lava cools down rapidly and becomes solid.
3. These rocks are smooth and have a fine-grained structure, such as basalt.
4. The Deccan Plateau of India is made up of basalt rocks.

(B) Intrusive igneous rocks

1. They are formed when the hot molten magma cools down and solidifies deep inside the crust of the earth.
2. The slow rate of cooling forms coarse-textured, hard rocks with large crystals, such as granite.
3. Grinding stones are generally made of granite.

The sedimentary rocks

1. When igneous rocks are exposed on the earth's surface, they are broken down by the agents of weathering into small fragments. These small fragments, called sediments, are carried away and deposited by the agents of gradation, such as rivers, glaciers, wind and sea waves.
2. The loose deposited sediments are compressed and hardened by the weight of new layers of sediments. The sediments are cemented together to form sedimentary rocks. Sedimentary rocks are the secondary rocks, as they are formed by the solidification of sediments derived from igneous, sedimentary or metamorphic rocks.
3. Sedimentary rocks are softer than igneous rocks and may also contain fossils of plants, animals and other organisms that once lived on them.
4. These rocks are mostly formed under water and have horizontal layers.
5. Sedimentary rocks are the most widespread and cover about 75 per cent of the surface area of the earth.
6. These rocks can create extensive landforms.
7. Coal and petroleum are the most important products derived from sedimentary rocks.
8. Sandstone is made from grains of stone.

The metamorphic rocks

1. The term "metamorphic" has been derived from "metamorphose", which means change in form. Rocks that were once igneous or sedimentary can change under great heat and pressure into metamorphic rocks.
2. The minerals in the rocks may change their composition and texture under high temperature and pressure.
3. For example, limestone can change into marble and clay can change into slate.
4. Metamorphic rocks are generally hard and have a high specific gravity.

Significance of rocks

1. Rocks are of great use, as they are composed of various minerals.
2. Soils are derived from the weathering of rocks.
3. Almost all types of building materials needed for making roads, houses, buildings and bridges are derived from rocks.
4. Rocks are a source of metals like iron, copper, gold and silver.
5. Some minerals are used as fossil fuels, such as coal and petroleum.
6. They are also used in industries, for example iron and aluminium.

Rock cycle

1. The change of one type of rock into another type under certain conditions, in a cyclic manner, is called the rock cycle.
2. The hot molten magma cools and solidifies to form igneous rocks.
3. The different agents of weathering break the igneous rocks into small fragments.
4. These small fragments of igneous rocks are carried and deposited by the agents of gradation to form sedimentary rocks.
5. The igneous and sedimentary rocks thus formed can be subjected to great heat and pressure.
6. This can change them into metamorphic rocks.
7. At many places, the great heat and pressure can melt the metamorphic rocks to form molten magma.
8. This molten magma can cool down to form igneous rocks again.

The rock cycle derives energy from inside the earth and also from the Sun. The rock material of the earth is never lost in the rock cycle. The rock cycle on the earth's surface has neither a beginning nor an end.

Flow chart of the rock cycle

Rock distribution in India

1. Granite is found in Rajasthan, Madhya Pradesh, the Chota Nagpur Plateau, Karnataka, Tamil Nadu, Andhra Pradesh and the Deccan Trap region (Maharashtra).
2. Sandstone occurs in the Vindhyachal and Aravali ranges. Limestone occurs widely in peninsular India. Coal mines occur in the basins of the rivers Godavari, Mahanadi and Damodar. Marble is found in Rajasthan. The Indo-Gangetic plain is of sedimentary accumulation.
3. Rocks have great economic value. The cotton soil found in the Deccan Trap is a product of extrusive igneous rocks. Other soils, like alluvium, are also the result of disintegrated rocks. Stones used in the construction of houses, roads and dams are all rock products. Rocks are a source of precious and useful metals. Iron ore, bauxite, copper, lead, zinc and gold are found in igneous rocks. Non-metallic minerals include sulphur, which is used for medicinal purposes. Coal and petroleum are important sources of energy.