What are the 3 layers of the Earth? Concept notes for CBSE Class 7 Geography

The Earth is round in shape. It looks like a tennis ball. But if we open it up, will it be hollow from the inside like a tennis ball? No. Earth comprises three parts — the geosphere, the hydrosphere and the atmosphere. The geosphere is the solid part of the Earth composed of minerals, rock formations, and the soil where we walk. 

The hydrosphere contains all the water on the Earth. Approximately 70% of Earth is covered with water. This water is found in a liquid state in the form of streams, seas, lakes, oceans, and rivers; and in a solid-state in the form of icebergs. It is further divided into layers — the stratosphere, the troposphere, the mesosphere, the thermosphere, and the exosphere. 

It is important for life on Earth because it regulates the temperature, stores the air we breathe and protects us from ultraviolet rays and intense sunlight. The atmosphere, together with the hydrosphere, regulates the water cycle. 

When we analyse the structure of the geosphere from the inside, we find out that it is made up of three layers — crust, mantle and core.

Layers of the earth

Suppose we open up a boiled egg. We find that it has different layers – the outermost layer is the hard outer shell made of calcium, then comes the white layer called albumin and the innermost layer is  the yellow yolk. 

Similarly, earth is made up of several concentric layers. 

1. Crust: It is the outermost layer of the Earth on which we live. It is thin, hard and strong. Earth involves land as well as oceans. The outer part of  the earth in the land is called the continental crust, while the outer part of the earth involving the oceans is called the oceanic crust. The size of the oceanic and continental crust is different from each other. The continental crust is around 35 km deep, while the oceanic crust is only 5 km deep. Oceanic crust is  compressed; it is formed of silica and magnesium. So, it is called sima (si-silica and ma-magnesium). Even though the oceanic crust  is smaller than the continental crust, it is heavier because its material is dense and heavy. It mainly contains silica and alumina. So, it is called sial (si-silica and al-aluminium). Crust constitutes 1% of the earth by volume. It is further divided into three layers — sedimentary layer, granitic layer, and basaltic layer. Conrad discontinuity is found between the granitic and basaltic layers. The Earth’s crust is thinnest in valleys and thickest under mountain ranges.
2. Mantle: It is the second and the thickest layer of the earth. It contains molten magma, so it is not solid. It is in a semi-liquid state as it is viscous material. The material inside the mantle is more viscous than tar, used in building roads. Magma is contained within the magma chamber. The temperature in different parts of the mantle varies depending on how close it is to the core. The mantle extends up to a depth of 2900 km below the crust. It is further divided into — the outer mantle and the inner mantle. Mantle constitutes 84% of the earth by volume. The speed of earthquake waves in the mantle is 7.9 to 8.1 km. The outer mantle has a higher viscosity than the inner mantle. The space between the crust and mantle is called Moho’s discontinuity. The interior of the mantle moves, but very slowly. This movement is responsible for moving the tectonic plates, and when they collide, earthquakes occur. 
3. Core: It is the deepest layer of the Earth. It is spherical. The length of the core is about 3500 km. It is made up of nickel and iron. So, it is called nife (ni-nickel and fe-ferrous/iron). Core constitutes 15% of the earth by volume and 32% of the earth by mass. It is further divided into— the outer core and the inner core. The outer core lies 2900 km to 5100 km deep to the earth’s surface. It contains some electrically charged particles. It is the liquid part of the core. The inner core lies 5100 km to 6378 km deep to the earth’s surface. The density of this part ranges between 11 to 13.6. It is solid. It is mainly made of iron. The temperature in the inner core ranges from 5000 to 6000 degrees celsius. You might be wondering why is the inner core solid despite such high temperatures? The pressure in the inner core is so high that the material gets solidified. 

Mechanical divisions of the Earth

On this basis, the Earth is divided into three layers.

1. Lithosphere: The term litho means rock. It is the outermost layer. It consists of crust (continental crust and oceanic crust) and the upper part of the outer mantle.
2. Asthenosphere: It is the second layer of the Earth. It consists of the lower part of the outer mantle.
3. Mesosphere: It consists of the inner mantle.
4. Barysphere: It  consists of the core of the Earth.

Rocks and minerals of the Earth

Rocks the natural mass of mineral matter. There are three major types of rocks on the earth which form the earth’s crust:

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

1. Igneous rocks: The name igneous is derived from the Latin word ignis, meaning fire. The eruption of volcanoes produces lava in the form of  molten magma. When molten magma cools and deposits, it becomes solid and forms igneous rocks. Igneous rocks are also called primary rocks. This is  because these are the first rocks to be formed. Sedimentary and metamorphic rocks are derived from it. There are two types of igneous rocks – intrusive rocks and extrusive rocks. When the lava erupts, molten magma is released. This molten magma comes on the Earth’s surface, cools down and solidifies. Rocks formed in this way are called extrusive igneous rocks. They have a very fine, grained structure. For example, Basalt. The Deccan plateau in India is made up of such rocks. If the molten magma doesn’t come out on the Earth’s surface, rather it cools down deep inside the earth’s crust, intrusive rocks are formed. Since they cool down slowly, they form larger grains. For example, Granite
2. Sedimentary rocks: The name sedimentary is  derived from the Latin word sedimentum, meaning settle down. When igneous rocks roll down, crack and hit each other, they are broken down into small fragments called sediments. These loose sediments are  compressed and hardened to form layers of rocks called sedimentary rocks. Sediments are transported and deposited by wind and water. Sedimentary rocks contain fossils of plants, animals, and other microorganisms that once lived on them. For example, sandstone is made from grains of sand. 
3. Metamorphic rocks: The name Metamorphic is derived from the Greek word metamorphose, meaning change of form. Under heat and pressure, igneous and sedimentary rocks can change into metamorphic rocks. For example, clay changes into slate, and limestone changes into marble.

Uses of rocks:

1. Our roads, houses and buildings are made from rocks. Taj Mahal is made of white marble.
2. Rocks contain minerals. Minerals are naturally occurring substances with certain physical properties and definite chemical compositions. These minerals are useful to us. For example, coal, natural gas, and petroleum are fuels. Iron, aluminium, gold, and uranium are used as raw materials in industries.

Fun facts about the Earth

• The inner and the outer core together are the size of planet Mars
• The core as a structural element was discovered in 1906 by R.D Oldham during his study of earthquake wreckage. It was the first such internal element of the earth to be identified 
• The Earth is continuously moving, but the movement is so slow that we don’t notice it!
• Earth is the only planet in the universe that has all the favourable factors to support life
• 70% of the Earth’s surface is covered by water

The inside of our Earth is dynamic, ever-changing and mysterious. It is important to know facts about the planet we live on to understand it better and find ways to avoid natural disasters and keep ourselves safe. 

FAQs

1. What is the hottest layer of the Earth?

The innermost layer of the Earth, called the core, is the hottest layer of the Earth. The temperature in the inner core ranges from 5000 to 6000 degrees celsius.

2. How do the layers of Earth differ from each other?

The layers of Earth differ from each other in terms of their thickness, composition, temperature and colour.

1. Crust: This is a relatively thin layer of the Earth. It is mainly composed of silica and magnesium. It is green in colour.
2. Mantle: It is the thickest. It is composed of a semi-solid material called magma. Its temperature is high but lower than the core. The outer mantle is red, and the inner mantle is orange.
3. Core: It is the deepest layer of the Earth and is mainly composed of nickel and iron. It has the highest temperature. The outer core is yellow, and the inner core is white.

3. How were the different layers of the Earth formed?

The Earth is made up of minerals of different densities. If we immerse mud and stones in a bucket of water, the stones being denser, settle down at the bottom, while the mud forms a layer above it. Similarly, the minerals or metals with different densities came to settle at different levels in the Earth during evolution, giving rise to the formation of different layers of the Earth. The heaviest metals settled down at the bottom to constitute the core of the Earth. The lightest ones rose to the top to form the crust.

4. Why do we need to study the layers of the Earth?

• The layers of the Earth keep moving gradually, which is normal. However, if the layers of the Earth move suddenly, earthquakes occur, leading to death and destruction. The study of the layers of the Earth helps the geologists to predict the onset of earthquakes so that we can be prepared for those 
• The study of the layers of the Earth helps us to decipher the location of oil, coal, and other valuable natural resources 
• It helps us to predict the weather

5. What is an earthquake?

The movement of the lithospheric plates causes vibrations on the surface of the Earth due to the movement of the hot, molten magma that lies underneath it. When these vibrations are sudden and violent, earthquakes occur, leading to death and destruction. The study of Earth’s layers helps geologists to predict earthquakes so that we can be prepared for them.

6. Why is Earth the only planet where life exists?

Earth is the only planet that supports life. Following are the factors that make life favourable on Earth.

• The Earth lies at a right distance from the sun. The sun is far enough so that we don’t burn, and it is near enough to provide us with heat and light
• The Earth contains plants that produce food and oxygen
• The Earth is covered by a protective lining called the ozone layer. This layer protects us against the harmful ultraviolet radiations of the sun
• The air on the Earth contains the elements that help us survive. It contains the right amount of oxygen that helps us to breathe 
• Earth contains water in abundance. Without water, life is not possible 

7. What is a rock cycle?

It is the formation of different types of rocks from each other. There are mainly three types of rocks— igneous, metamorphic, and sedimentary. The igneous rocks are also called primary rocks, as those were the first type of rocks formed during evolution. Metamorphic rocks and sedimentary rocks are derived from igneous rocks. The rain, wind, and other natural forces regulate the rock cycle.