Interior of the Earth – Detailed Summary | Samacheer Kalvi 7th Social Science | Term 1 Geography

Interior of the Earth – Detailed Summary

Book: Samacheer Kalvi 7th Social Science

Term: Term 1

Subject: Geography

Unit: Unit 1

Lesson Name: Interior of the Earth

About the Lesson

“Interior of the Earth” is an important Geography lesson in the Samacheer Kalvi 7th Social Science book. This lesson explains the inner structure of the earth, lithospheric plates, tectonic movements, earthquakes and volcanoes.

The earth is a dynamic planet. Its surface has mountains, plateaus, plains, valleys, oceans and continents. The surface of the earth changes due to internal and external forces.

Learning Objectives

To know about the interior structure of the earth.
To understand the three main layers of the earth.
To learn about crust, mantle and core.
To understand SIAL, SIMA and Nife.
To know the role of lithospheric plates.
To understand tectonic movements.
To learn about endogenic and exogenic forces.
To understand earthquakes and their effects.
To learn about volcanoes and their types.
To know about the Pacific Ring of Fire.

Detailed Summary of the Lesson

1. Introduction

The earth is our homeland. It is not a fixed or motionless planet. It is a dynamic planet where changes happen continuously.

The surface of the earth has lofty mountains, high plateaus, large plains and deep valleys. These features are formed and changed by forces acting inside and outside the earth.

2. Interior of the Earth

The interior of the earth means the inner part of the earth below the surface.

The structure of the earth may be compared to that of an apple. Like an apple, the earth also has layers. These layers are identified mainly through the study of earthquake waves.

3. Three Layers of the Earth

The earth has three main concentric layers:

Crust
Mantle
Core

4. Crust

The crust is the outermost layer of the earth.

Its thickness varies from place to place. It is thicker under continents and thinner under oceans. The average thickness of the crust varies from 5 km to 30 km.

5. Continental Crust

Continental crust is the crust found under continents.

It is about 35 km thick. It is less dense than oceanic crust because it is made of both light and dense rock types.

6. Oceanic Crust

Oceanic crust is the crust found under ocean floors.

It is only about 5 km thick. It is denser than continental crust because it is mostly made of basaltic rocks.

7. SIAL

The upper part of the crust consists of granite rocks and forms the continents.

It contains silica and alumina. So it is called SIAL.

8. SIMA

The lower part of the crust is made of denser basaltic rocks and forms the ocean floors.

It contains silica and magnesium. So it is called SIMA.

9. Mantle

The mantle is the layer found below the crust.

It is about 2,900 km thick. It is divided into upper mantle and lower mantle.

10. Mohorovicic Discontinuity

The boundary between the crust and mantle is called Mohorovicic discontinuity.

It separates the outer crust from the mantle below it.

11. Core

The core is the innermost layer of the earth.

It is also called barysphere. The core has very high temperature and pressure.

12. Outer Core

The outer core is rich in iron and is in liquid state.

It extends from about 2,900 km to 5,150 km.

13. Inner Core

The inner core is solid in state.

It is composed of Nickel and Ferrous. This composition is called Nife.

14. Weichart-Gutenberg Discontinuity

The boundary between the mantle and the core is called Weichart-Gutenberg discontinuity.

This is an important textbook-back fill-in-the-blank point.

15. Radius of the Earth

The radius of the earth is about 6371 km.

The crust forms only a small part of the earth’s volume, while the mantle and core form the major part.

16. Blue Planet

Earth is called the Blue Planet because about 71% of the earth’s surface is covered by water.

Oceans, seas, lakes and rivers make the earth appear blue from space.

17. Lithosphere

The lithosphere is the solid outer part of the earth.

It includes the crust and the uppermost part of the mantle.

18. Lithospheric Plates

The lithosphere is broken into a number of plates called lithospheric plates.

These plates may be oceanic or continental. They move independently over the asthenosphere.

19. Asthenosphere

The asthenosphere is the part of the mantle that flows and moves the plates of the earth.

It plays an important role in plate movement.

20. Tectonic Movements

The movement of the earth’s lithospheric plates is called tectonic movement.

These movements are caused by the internal heat of the earth.

21. Plate Movements

Lithospheric plates move in different directions and at different speeds.

Some plates move away from each other.
Some plates move towards each other and collide.
Some plates slide past each other.

22. Formation of Mountains

When two continental plates converge, neither plate may be forced under the other.

Instead, folds may be created. Great mountain ranges like the Himalayas were formed in this way.

23. Endogenic Forces

The forces that act in the interior of the earth are called endogenic forces.

These forces produce changes on the earth’s surface from within.

24. Exogenic Forces

The forces that work on the surface of the earth are called exogenic forces.

Wind, water, waves and glaciers are examples of external forces that shape the earth’s surface.

25. Earthquake

An earthquake is a sudden movement of a portion of the earth’s crust which produces shaking or trembling.

Earthquakes may cause widespread damage to life and property.

26. Focus

The point where earthquake vibrations originate is called the focus of the earthquake.

It is located inside the earth.

27. Epicentre

The point on the earth’s surface directly above the focus is called the epicentre.

Earthquake damage is usually severe near the epicentre.

28. Seismic Waves

Earthquake vibrations travel in different directions in the form of seismic waves.

The study of these waves helps scientists understand the interior of the earth.

29. Types of Earthquake Waves

There are three types of earthquake waves:

P waves or longitudinal waves
S waves or transverse waves
L waves or surface waves

30. Seismograph

A seismograph is an instrument used to record earthquake waves.

This is an important textbook-back fill-in-the-blank question.

31. Richter Scale

The magnitude of an earthquake is measured by the Richter scale.

The numbers on this scale range from 0 to 9.

32. Causes of Earthquakes

Sudden slipping of earth’s crust along fractures or faults.
Movement of molten rocks below the surface.
Volcanic activity.
Sudden shifting of landmass.

33. Effects of Earthquakes

Changes in the earth’s surface.
Landslides in mountainous regions.
Falling of buildings.
Fire accidents.
Disturbance of underground water system.
Tsunami when earthquake occurs under or near the sea.
Loss of life and property.

34. Tsunami

Tsunami is a Japanese term.

It refers to huge waves caused in the sea by an earthquake below or near the sea.

35. Earthquake Distribution

Earthquakes are commonly found along the same regions where volcanoes occur.

The Circum-Pacific belt, also called the Pacific Ring of Fire, is one of the most earthquake-prone and volcanic regions in the world.

36. Earthquakes in India

In India, the Himalayan region and the Ganga-Brahmaputra valley are prone to earthquakes.

The Deccan Plateau was once considered comparatively safe, but it has also experienced earthquakes such as Koyna and Latur.

37. Volcano

A volcano is a vent or opening in the earth’s crust through which hot magma erupts from deep below the surface.

When magma reaches the surface, it is called lava.

38. Magma and Lava

Magma is molten rock found below the earth’s surface.

Lava is molten rock that reaches the earth’s surface during a volcanic eruption.

39. Vent

The narrow pipe through which magma flows out is called a vent.

This is an important textbook-back one-mark question.

40. Crater

The top of the cone of a volcanic mountain has a depression known as a crater.

If the crater is very large and basin-shaped, it is called a caldera.

41. Fissure Eruption

When magma rises slowly to the surface and spreads over a vast area, it is known as fissure eruption.

This is an important fill-in-the-blank point from the textbook-back exercise.

42. Explosive Eruption

If magma rises quickly to the surface, lava is thrown high into the atmosphere.

Ash, steam, gases and pieces of rocks are also thrown out. This type of eruption is known as explosive eruption.

43. Lava Cones

Lava cones are mountains of accumulation.

They are formed by the accumulation of lava and volcanic materials.

44. Types of Volcanoes Based on Activity

Volcanoes are classified based on their activity into:

Active volcanoes
Dormant volcanoes
Extinct volcanoes

45. Active Volcanoes

Active volcanoes are volcanoes that erupt frequently.

Examples include Mt. Stromboli in the Mediterranean Sea, St. Helens in the USA and Pinatubo in the Philippines.

46. Dormant Volcanoes

Dormant volcanoes have shown no sign of activity for many years but may become active at any time.

They are also called sleeping volcanoes. Examples include Mt. Fujiyama in Japan and Mt. Krakatoa in Indonesia.

47. Extinct Volcanoes

Extinct volcanoes are volcanoes that are not expected to erupt again.

Examples include Mt. Popa in Myanmar, Mt. Kilimanjaro and Mt. Kenya in Africa.

48. Shield Volcano

A shield volcano is formed by quiet eruption of lava with low silica content.

Volcanoes of the Hawaii Islands are examples of shield volcanoes.

49. Composite Volcano

Composite volcanoes are made of alternate layers of lava, cinder and ash.

They are also called stratovolcanoes. St. Helens in the USA is an example.

50. Pacific Ring of Fire

The Pacific belt is known as the Ring of Fire.

It includes the volcanoes of the eastern and western coastal areas of the Pacific Ocean. It includes about two-thirds of the world’s volcanoes.

51. Major Zones of Volcanic Activity

There are three major zones of volcanic activity in the world:

The Circum-Pacific belt
The Mid-Continental belt
The Mid-Atlantic belt

52. Constructive and Destructive Nature of Volcanoes

Volcanoes can be both constructive and destructive.

They are constructive because cooled lava forms fertile soil rich in minerals. They are destructive because lava, ash and gases can destroy houses, trees, crops and lives.

Central Idea of the Lesson

The central idea of the lesson is that the earth has three main layers: crust, mantle and core. The lithosphere is broken into plates, and their movements cause major changes on the earth’s surface. Earthquakes and volcanoes are sudden movements caused by internal forces. Understanding the interior of the earth helps us understand mountains, earthquakes, volcanoes and natural disasters.

Interior Layers of the Earth

Layer	Position	Important Points
Crust	Outermost layer	Thin layer; divided into SIAL and SIMA.
Mantle	Middle layer	About 2,900 km thick; divided into upper and lower mantle.
Core	Innermost layer	Also called barysphere; has outer core and inner core.

SIAL, SIMA and Nife

Term	Composition	Location / Meaning
SIAL	Silica and Alumina	Upper part of the crust; forms continents.
SIMA	Silica and Magnesium	Lower part of the crust; forms ocean floors.
Nife	Nickel and Ferrous	Composition of the inner core.

Important Boundaries Inside the Earth

Boundary	Separates
Mohorovicic Discontinuity	Crust and Mantle.
Weichart-Gutenberg Discontinuity	Mantle and Core.

Earth Movements

Term	Meaning
Lithospheric Plates	Broken pieces of lithosphere that move over the asthenosphere.
Tectonic Movements	Movement of the earth’s lithospheric plates.
Endogenic Forces	Forces that act in the interior of the earth.
Exogenic Forces	Forces that work on the surface of the earth.
Asthenosphere	Part of the mantle that flows and moves the plates.

Earthquake – Important Points

Topic	Important Point
Earthquake	Sudden movement of a portion of the earth’s crust causing shaking or trembling.
Focus	Point inside the earth where vibrations originate.
Epicentre	Point on the earth’s surface directly above the focus.
Seismic Waves	Earthquake vibrations travelling in different directions.
Seismograph	Instrument used to record earthquake waves.
Richter Scale	Scale used to measure earthquake magnitude.
Tsunami	Huge sea wave caused by earthquake under or near the sea.

Types of Earthquake Waves

Wave	Other Name
P Waves	Longitudinal waves.
S Waves	Transverse waves.
L Waves	Surface waves.

Volcano – Important Points

Term	Meaning
Volcano	Vent or opening in the earth’s crust through which magma erupts.
Magma	Molten rock below the earth’s surface.
Lava	Magma that comes out to the earth’s surface.
Vent	Narrow pipe through which magma flows out.
Crater	Depression at the top of a volcanic cone.
Caldera	Large basin-shaped crater.
Fissure Eruption	Magma rises slowly and spreads over a vast area.

Types of Volcanoes Based on Activity

Type	Meaning	Example
Active Volcano	Erupts frequently.	St. Helens, Stromboli, Pinatubo.
Dormant Volcano	Inactive for many years but may become active again.	Mt. Fujiyama, Mt. Krakatoa.
Extinct Volcano	Not expected to erupt again.	Mt. Kilimanjaro, Mt. Kenya, Mt. Popa.

Shield Volcano and Composite Volcano

Shield Volcano	Composite Volcano
Formed by quiet eruption of lava with low silica content.	Made of alternate layers of lava, cinder and ash.
Broad and gently sloping.	Steep-sided cone.
Example: Volcanoes of Hawaii Islands.	Example: St. Helens in the USA.

Major Zones of Volcanic Activity

Zone	Important Point
Circum-Pacific Belt	Known as the Pacific Ring of Fire; includes about two-thirds of world volcanoes.
Mid-Continental Belt	Includes Alpine mountain chains, Mediterranean Sea and fault zone of Eastern Africa.
Mid-Atlantic Belt	Located along mid-Atlantic ridges; many volcanoes are fissure eruption type.

Important Exam Points from the Lesson

Question Focus	Answer
Nife is made up of	Nickel and Ferrous.
Earthquake and volcanic eruption occur near edges of	Plates.
Magnitude of earthquake measured by	Richter scale.
Narrow pipe through which magma flows out	Vent.
Lava cones are	Mountains of accumulation.
Top depression of volcanic cone	Crater.
Belt known as Ring of Fire	Pacific belt.
Core separated from mantle by	Weichart-Gutenberg discontinuity.
Earthquake waves recorded by	Seismograph.
Magma rising and spreading over a vast area	Fissure eruption.
Seismology is the study of	Earthquake.
Outermost layer of earth	Crust.
SIAL	Upper part of crust; silica and alumina.
Movement of lithospheric plates	Tectonic movements.
Point where earthquake vibrations originate	Focus.
Point directly above focus	Epicentre.
Radius of the earth	6371 km.
Barren Island situated in	Andaman Sea.
Stromboli known as	Lighthouse of the Mediterranean Sea.
India earthquake-prone region	Himalayan region and Ganga-Brahmaputra valley.

Vocabulary to Remember

Interior – inner part of something.
Crust – outermost layer of the earth.
Mantle – middle layer below the crust.
Core – innermost layer of the earth.
Barysphere – another name for the core.
SIAL – silica and alumina; upper part of crust.
SIMA – silica and magnesium; lower part of crust.
Nife – nickel and ferrous; composition of inner core.
Lithosphere – solid outer part of the earth.
Asthenosphere – flowing part of mantle that moves plates.
Tectonic Movement – movement of lithospheric plates.
Endogenic Forces – forces acting inside the earth.
Exogenic Forces – forces acting on the surface of the earth.
Earthquake – sudden shaking of the earth’s crust.
Focus – point where earthquake vibrations originate.
Epicentre – point on surface above the focus.
Seismograph – instrument recording earthquake waves.
Richter Scale – scale measuring earthquake magnitude.
Tsunami – huge sea wave caused by earthquake.
Volcano – opening in crust through which magma erupts.
Magma – molten rock below the surface.
Lava – molten rock on the surface.
Vent – narrow pipe through which magma flows out.
Crater – depression at the top of volcanic cone.
Caldera – large basin-shaped crater.
Fissure Eruption – slow spread of magma over a vast area.
Pacific Ring of Fire – major volcanic and earthquake belt around the Pacific.

TET / Exam Focus Areas

Very Important Focus: These are the most expected textbook-based areas from this lesson.

What are the three layers of the earth?
What is crust?
What is mantle?
Write a note on the core of the earth.
What are SIAL and SIMA?
What is Nife?
What is tectonic movement?
What are endogenic and exogenic forces?
Define earthquake.
What are focus and epicentre?
What is seismograph?
What is Richter scale?
Write the effects of earthquakes.
What is tsunami?
Define volcano.
What are magma, lava, vent and crater?
Differentiate active and dormant volcanoes.
Differentiate shield and composite volcanoes.
Name the major volcanic zones of the world.
Why is the Pacific belt called the Ring of Fire?

Short Answer Questions and Answers

1. Name the three layers of the earth.

The three layers of the earth are crust, mantle and core.

2. What is the crust?

The crust is the outermost layer of the earth. It is thicker under continents and thinner under oceans.

3. What is SIAL?

SIAL is the upper part of the crust. It contains silica and alumina and forms the continents.

4. What is SIMA?

SIMA is the lower part of the crust. It contains silica and magnesium and forms the ocean floors.

5. What is the mantle?

The mantle is the middle layer of the earth found below the crust. It is about 2,900 km thick.

6. Write a note on the core of the earth.

The core is the innermost layer of the earth. It is also called barysphere. It has outer core and inner core.

7. What is Nife?

Nife is the composition of the inner core. It is made up of Nickel and Ferrous.

8. What is tectonic movement?

The movement of the earth’s lithospheric plates is called tectonic movement.

9. What are endogenic and exogenic forces?

Forces that act inside the earth are called endogenic forces. Forces that work on the surface of the earth are called exogenic forces.

10. Define earthquake.

A sudden movement of a portion of the earth’s crust which produces shaking or trembling is called an earthquake.

11. What is focus?

Focus is the point inside the earth where earthquake vibrations originate.

12. What is epicentre?

Epicentre is the point on the earth’s surface directly above the focus.

13. What is seismograph?

Seismograph is an instrument used to record earthquake waves.

14. What is Richter scale?

Richter scale is used to measure the magnitude of an earthquake.

15. Name the three types of earthquake waves.

The three types of earthquake waves are P waves, S waves and L waves.

16. What is tsunami?

Tsunami is a huge sea wave caused by an earthquake below or near the sea.

17. What is a volcano?

A volcano is a vent or opening in the earth’s crust through which hot magma erupts from deep below the surface.

18. What is magma?

Magma is molten rock found below the earth’s surface.

19. What is lava?

Lava is magma that comes out to the earth’s surface during a volcanic eruption.

20. What is a vent?

Vent is the narrow pipe through which magma flows out.

21. What is a crater?

Crater is the depression found at the top of the cone of a volcanic mountain.

22. What is fissure eruption?

When magma rises slowly to the surface and spreads over a vast area, it is called fissure eruption.

23. Name the three types of volcanoes based on activity.

The three types are active volcanoes, dormant volcanoes and extinct volcanoes.

24. What is an active volcano?

An active volcano is a volcano that erupts frequently.

25. What is a dormant volcano?

A dormant volcano has shown no activity for many years but may become active at any time.

26. What is an extinct volcano?

An extinct volcano is a volcano that is not expected to erupt again.

27. Name the major zones of volcanic activity.

The major zones are the Circum-Pacific belt, Mid-Continental belt and Mid-Atlantic belt.

28. Which belt is known as the Ring of Fire?

The Pacific belt is known as the Ring of Fire.

29. Why is the continental crust less dense than oceanic crust?

The continental crust is less dense because it is made of both light and dense rock types, while oceanic crust is mostly made of dense basaltic rocks.

30. Why can no one take samples from the interior of the earth?

No one can take samples from the interior of the earth because the central core has very high temperature and pressure.

How to Write This in Exam

For a short answer, write the direct definition first. For example, if the question asks “What is an earthquake?”, write “A sudden movement of a portion of the earth’s crust which produces shaking or trembling is called an earthquake.”

For a long answer, divide the answer into headings such as layers of the earth, SIAL and SIMA, plate movements, earthquakes, volcanoes and volcanic zones.

Sample Exam Answer

“Interior of the Earth” is an important Geography lesson in 7th Social Science. The earth is a dynamic planet. Its surface has mountains, plateaus, plains and valleys. The interior structure of the earth can be compared to that of an apple. The earth has three main layers: crust, mantle and core.

The crust is the outermost layer of the earth. It has two parts: SIAL and SIMA. SIAL is made of silica and alumina and forms the continents. SIMA is made of silica and magnesium and forms the ocean floors. The mantle lies below the crust and is about 2,900 km thick. The core is the innermost layer of the earth and is also called barysphere. The inner core is made of Nickel and Ferrous, called Nife.

The lithosphere is broken into lithospheric plates. These plates move over the asthenosphere. The movement of lithospheric plates is called tectonic movement. The forces acting inside the earth are called endogenic forces, while the forces acting on the surface are called exogenic forces.

An earthquake is a sudden movement of a portion of the earth’s crust which produces shaking or trembling. The point where earthquake vibrations originate is called focus. The point on the surface directly above the focus is called epicentre. Earthquake waves are recorded by seismograph, and earthquake magnitude is measured by Richter scale.

A volcano is an opening in the earth’s crust through which hot magma erupts. Magma that reaches the surface is called lava. The narrow pipe through which magma flows out is called vent. The depression at the top of a volcanic cone is called crater. Volcanoes are classified as active, dormant and extinct. The Pacific belt is known as the Ring of Fire because many volcanoes occur there.

One Mark Important Points

Three Layers of Earth – Crust, Mantle, Core
Outermost Layer – Crust
Middle Layer – Mantle
Innermost Layer – Core
Core Also Called – Barysphere
SIAL – Silica and Alumina
SIMA – Silica and Magnesium
Nife – Nickel and Ferrous
Earth’s Radius – 6371 km
Earth Called – Blue Planet
Earth Covered by Water – 71%
Crust-Mantle Boundary – Mohorovicic Discontinuity
Mantle-Core Boundary – Weichart-Gutenberg Discontinuity
Lithospheric Plate Movement – Tectonic Movement
Asthenosphere – Part of mantle that moves plates
Endogenic Forces – Forces inside the earth
Exogenic Forces – Forces on the earth’s surface
Earthquake – Sudden shaking of earth’s crust
Focus – Point where earthquake vibrations originate
Epicentre – Point directly above focus
Earthquake Waves Recorded By – Seismograph
Earthquake Magnitude Measured By – Richter Scale
Types of Earthquake Waves – P, S and L waves
Tsunami – Japanese term
Volcano – Opening through which magma erupts
Narrow Pipe of Volcano – Vent
Top Depression of Volcano – Crater
Large Basin-shaped Crater – Caldera
Lava Cones – Mountains of accumulation
Pacific Belt – Ring of Fire
Seismology – Study of earthquakes
Volcanology – Study of volcanoes
Volcanologists – People who study volcanoes

Short Conclusion

“Interior of the Earth” teaches us about the inner structure of the earth and the forces that change the earth’s surface. The earth has three layers: crust, mantle and core. The lithosphere is divided into plates, and their movement causes tectonic changes. Earthquakes and volcanoes are caused by internal forces. This lesson is very important for exams because many direct questions can be asked from crust, mantle, core, SIAL, SIMA, Nife, tectonic movements, earthquakes, seismograph, Richter scale, volcanoes, vent, crater and Pacific Ring of Fire.