Maharashtra Board Class 8 Geography Chapter 2 Interior Of The Earth PDF Download

Interior Of The Earth

Let's Recall

In the previous classes, you have been introduced to rock types, volcanoes and earthquakes. Answer the following questions on the basis of that.

When earthquake occurs, what happens exactly?

How are igneous rocks formed?

What is a volcano?

Which materials come out during a volcanic eruption?

In which state of matter are these materials?

Are these materials cool or hot? Why?

Try This

Boil half a liter of milk. When the milk comes to a boil, switch off the gas. Keep a lid on the vessel. (The latter part of the experiment is very important. You have to use your observational skills and arrive at some conclusions.)

After ten minutes, remove the lid, take it to a side and tilt it. Observe what happens. What can you see on the milk? In what form do you see the layer of matter on the milk? Remove this layer of matter. Take note of the difference in temperature of this matter and the milk and answer the following questions.

When the milk was kept for boiling, in which state was it?

When the milk was boiling, what did you observe?

What had accumulated over the lid of the vessel?

Tell whether the matter accumulated on the milk is liquid or solid?

Was it cooler or hotter than the milk below?

On which other substances can similar experiments be carried out?

Geographical Explanation

Before boiling, the milk was in the liquid state. After it came to a boil, steam started coming out from it. After some time, a thick layer of cream develops over the milk. The temperature of the cream is lesser than the milk below. Thus, it can be implied that the creamy layer cooled faster than the milk below it, which remained hot and in a liquid form. A similar process occurred when the earth cooled.

The scientists unanimously believe that the Earth was formed out of the solar system itself. Initially, earth was a gaseous hot balloon. It cooled down as it rotated around itself. The process of cooling took place from the outer surface towards the earth's center. As a result, the outer layer (crust) of the earth become cooler and solid but as we move from surface to the core, heat increases and at certain depths, the interior of the earth is semi liquid.

Man has always been inquisitive about what lies in the interior of the earth. A direct observation of the earth's interior has not been possible as yet. Through various methods, the scientists have tried to estimate about the same. To infer about these, study of the materials coming out of a volcanic eruption and the seismic waves is most important.

Matter coming out of volcanic eruptions consists of hot magma, gases, steam, etc. When lava cools and solidifies, igneous rocks are formed. Through the estimates of the study of temperature, density, gravitational force and pressure the interior of the earth was understood. For instance, when we go deep into a mine, we feel an increase in temperature. Similarly, the magma coming out of volcanoes from the earth's interior is hot. Such estimations were made by the geologists. Several earthquakes occur in various places on the earth every year. Seismic waves are generated. These waves travel through the interior of the earth.

Teacher's Note

The earth cooled like milk cooling when you leave it to sit. Just like hot milk forms a cream layer on top, the earth's outer layer (crust) cooled first while the inside stayed hot.

Exam Trick

Remember: Earth cooled from outside to inside, like a hot samosa cooling from the outside but remaining hot inside. The outer crust cooled first and became solid.

Points To Remember

Earth was a hot gaseous ball that cooled down over time.
The crust is the solid outer layer that cooled first.
Heat increases as we go deeper into the earth.
Volcanic eruptions and seismic waves help us learn about the earth's interior.
Scientists cannot dig to see the earth's interior directly.

The Composition Of The Earth's Interior

Try This

Carry out the following activity by dividing the students into two groups. Use the pictures on the back cover for reference.

Take clay balls of 3 colors - red, yellow and blue. (As available in the market)

Make the red ball bigger.

Roll out the yellow colour ball. You will get a flat roti-like structure. Place the red colour ball inside the yellow one like you fill puran in a puranpoli. Give it the shape of a sphere.

Now roll out the blue ball and fill the yellow one in it as done earlier. Make a sphere out of this too.

Show the continents in yellow on this sphere like a globe. Now your globe made from clay is ready.

To see the interior of the earth, cut the sphere exactly in half. You can see various layers of colors inside like the layers of the earth's interior. Name these layers.

Think About It

Can we dig deep from one side of the earth and come out from the other side? Write your imaginations in your notebooks and discuss in class. (Note: Teachers should listen to the ideas of the students and direct the topic towards the earth's interior)

Do You Know?

Our earth was formed approximately 4.6 billion years ago. Initially, the earth was in a gaseous state. It started cooling through the process of radiation. The earth then liquefied. With time, the outermost part of the earth cooled first and became solid. This outermost layer of the earth is called the crust. Even today outer planets of the solar system are in a gaseous state.

Geographical Explanation

While going from the earth's surface to its core, major changes occur in temperature and density. With respect to these two elements, the earth's interior can be divided into following layers.

Teacher's Note

The earth has many layers like an onion. Each layer is different in temperature, density, and the materials found in it, like how different layers of an onion are different.

Exam Trick

Remember: CRUST is on top (where we live), MANTLE is in the middle, CORE is at the center. Think of it like the seed inside a mango - seed at center, flesh in middle, skin on top.

Points To Remember

The crust is the solid outer layer where we live.
The mantle is below the crust and is very hot.
The core is at the center of the earth.
Temperature increases as we go deeper into the earth.
Density also increases as we go deeper inside.

The Crust

The uppermost layer of the earth's surface is in solid state and is called the crust. The thickness of the crust is not the same everywhere. On an average, it is 30 to 35 km thick. The thickness of the crust below the continents is from 16 to 45 km, 40 km under the mountain ranges and less than 10 km under the oceans.

As we move deeper under the surface, temperature increases. After that, the rate of increase in temperature reduces and again increases in the core. At the centre of the earth, the temperature is around 5500° to 6000° C.

The crust is the thinnest of all the layers when compared to the mantle and the core. It can be divided into two sub-layers.

Continental Crust

The continents are mainly composed of Silica (Silica is the compound of Silicon Elements) and Aluminum. Because of their high proportion in this layer, the layer was earlier known as Sial. The density of continental crust is 2.65 to 2.90 gm/cm³. The thickness of this layer is approximately 30 kilometers. Granite rocks are mainly found in this layer.

Oceanic Crust

This is the second layer of the crust. It is made up of silica and magnesium. It was earlier called Sima. This layer is approximately 7 to 10 km thick. The density of this layer is 2.9 gm/cm³ to 3.3 gm/cm³. In this layer, basalt and gabbro rocks are mainly found.

Do You Know?

There is a discontinuity between the continental crust and oceanic crust. It was first deduced by a scientist called Conrad and is named after him as Conrad discontinuity.

There is discontinuity between crust and mantle too. This discontinuity was deduced by a scientist called Mohorovicic. It has been named after him as Moho discontinuity.

There is discontinuity between mantle and core too. It has been named after Gutenberg, a scientist, who discovered it.

Teacher's Note

The crust has two parts: continental crust (where continents are) and oceanic crust (under the oceans). They are made of different materials and have different thicknesses, like how different types of soil are found in different places in India.

Exam Trick

Remember: Continental crust = Sial (Silica + Aluminum), Oceanic crust = Sima (Silica + Magnesium). Continental crust is thicker, oceanic is thinner.

Points To Remember

The crust has two layers: continental and oceanic.
Continental crust is made of silica and aluminum.
Oceanic crust is made of silica and magnesium.
Continental crust is thicker than oceanic crust.
There are boundaries called discontinuities between different layers.

This is a preview of the first 3 pages. To get the complete book, click below.