Earth’s Structure: ICSE Class 9 Geography

Summary

Our Earth has different layers inside, much like an onion. Scientists learn about these hidden layers not by digging all the way to the center, but in clever ways. They study earthquake waves, also called seismic waves. Some waves, P-waves, travel through all parts of Earth, while S-waves cannot go through liquids. This helps scientists understand what the Earth’s inside is like. They also look at hot, molten rock from volcanoes and study meteorites from space, as Earth is thought to be made of similar materials. Heat from deep inside also gives clues about internal temperatures.

The Earth is surrounded by a blanket of gases called the atmosphere. The Earth’s outermost solid layer is the crust, also called the lithosphere. It includes the land and the ocean floor. The water on Earth, such as oceans and seas covering parts of the crust, is known as the hydrosphere. The crust is the thinnest layer and is made of rocks rich in substances like silicates. It has two main parts: SIAL, made from Silica and Aluminium, which forms continents; and SIMA, from Silica and Magnesium, which forms ocean floors. SIAL is lighter, so it gently rests on top of SIMA. All known life exists on the crust.

Below the crust is a much thicker layer called the mantle. It is the largest layer by volume. The mantle has an upper and a lower part. The upper mantle is cooler and solid. A part of the upper mantle, called the asthenosphere, is partially melted, like very thick honey. The lower mantle is hotter and denser but stays mostly solid because of very high pressure. Temperatures in the mantle are very high and increase with depth.

At Earth’s center is the core. The core is mainly made of Nickel (Ni) and Iron (Fe), so it is often called NIFE. It is the densest part of our planet. The core has two sections: an outer core and an inner core. The outer core is liquid, as shown by S-waves not passing through it. The inner core, though even hotter, is solid because of immense pressure. The core creates Earth’s magnetic field, which acts like a giant magnet.

Boundaries exist between these main layers. The Mohorovicic Discontinuity, or Moho, is between the crust and mantle. The Gutenberg Discontinuity is between the mantle and core. At these boundaries, earthquake waves change their speed. The Earth’s crust is not still. It changes very slowly over millions of years due to strong forces, causing continents to move and mountains to form.

Textbook solutions (Morning Star/Total)

Multiple-choice questions

1. The molten material thrown up by volcanoes rises to the surface of the earth from the ______

(a) Magnum chamber
(b) Magma chamber
(c) Magnum
(d) Magma

Answer: (b) Magma chamber

2. Meteors are rocks from outer space. They hold special significance for scientists because

(a) they contain radioactive materials.
(b) the earth is made of same materials.
(c) they give an idea of other plants.
(d) they pass through the atmosphere of earth.

Answer: (b) the earth is made of same materials.

3. The earth is made up of several concentric layers. The correct order of the layers from the centre to the surface is

(a) Crust – Mantle – Core
(b) Core – Mantle – Crust
(c) Mantle – Crust – Core
(d) Mantle – Core – Crust.

Answer: (b) Core – Mantle – Crust

4. Name the two broad divisions the earth crust is divided depending on chemical composition.

(a) Lithosphere, hydrosphere
(b) SIAL, SIMA
(c) Core, Mantle
(d) Crust, Asthenosphere

Answer: (b) SIAL, SIMA

5. Continents : SIAL :: ______ : SIMA

(a) Mountains tops
(b) Ocean floor
(c) Plains
(d) Volcanoes

Answer: (b) Ocean floor

6. This is the densest part of the earth believed to be composed of iron and nickel which is responsible for earth’s magnetism. Identify it.

(a) Mantle
(b) Core
(c) Crust
(d) None of the above

Answer: (b) Core

7. Which of the following is the correct order as we move towards the interior of the earth.

(a) SIAL – SIMA – NIFE
(b) NIFE – SIMA – SIAL
(c) NIFE – SIAL – SIMA
(d) None of the above.

Answer: (a) SIAL – SIMA – NIFE

8. Which of the following is incorrect about the inner core?

(a) It is in a molten state.
(b) It is a region of high density.
(c) It is a region of high pressure.
(d) It is composed of iron and nickel.

Answer: (a) It is in a molten state.

9. At the depth of around 100-410 km, mantle is partially molten and is known as:

(a) Asthenosphere
(b) Mantlenosphere
(c) Moltenosphere
(d) None of the above

Answer: (a) Asthenosphere

10. What is the temperature variation in the mantle in °C?

(a) 1500 to 3700
(b) 1500 to 2500
(c) 1000 to 3700
(d) 1000 to 4000

Answer: (c) 1000 to 3700

11. Name the mantle-core interface where primary seismic waves slow down and secondary seismic waves disappear completely.

(a) Gutenberg Discontinuity
(b) Mantlic Discontinuity
(c) Core Discontinuity
(d) Mohorovicic Discontinuity

Answer: (a) Gutenberg Discontinuity

12. Name the boundary between the crust and the mantle which has characteristics of both layers.

(a) Gutenberg Discontinuity
(b) Crust Discontinuity
(c) Mantlic Discontinuity
(d) Mohorovicic Discontinuity

Answer: (d) Mohorovicic Discontinuity

13. What life sustaining unit contains only one per cent of the earth’s mass?

(a) biosphere
(b) lithosphere
(c) crust
(d) mantle

Answer: (c) crust

14. Mohorovicic discontinuity is found at about ______ beneath the oceans and ______ beneath the continents.

(a) 8 km, 32 km
(b) 8 km, 10 km
(c) 10 km, 60 km
(d) 10 km, 40 km

Answer: (a) 8 km, 32 km

15. Extending upwards, the earth is enveloped by a blanket of gases. Name it.

(a) Lithosphere
(b) Hydrosphere
(c) Atmosphere
(d) Chamber

Answer: (c) Atmosphere

16. The entire core is about ______ km in diameter.

(a) 5000
(b) 6000
(c) 7000
(d) 8000

Answer: (c) 7000

17. Name the layer of the earth that is fragile as compared to the rest of the layers of the earth. Different theories suggest that this layer undergoes changes to give earth a new shape.

(a) Mantle
(b) Core
(c) Crust
(d) Asthenosphere

Answer: (c) Crust

Short answer questions

1. In which part of the earth is NIFE found? What is it composed of?

Answer: NIFE is found in the core of the earth, deep inside the earth. It is made up of nickel (Ni) and iron (Fe).

2. What are the consequences of the pressure and temperature in the interior of the earth?

Answer: As the pressure increases towards the interior of the earth, the density also increases. The temperature also gradually starts rising towards the interior. For example, the solid state of the inner core is due to high density and pressure which have compressed the molten rock material. The lower mantle is solid due to high pressure, even though it is hotter than the upper mantle. At depths of around 100-410 km in the mantle, the asthenosphere is partially molten, while the temperature in the mantle varies from about 1000°C to around 3,700°C, and in the core, it ranges from about 4400°C to about 6000°C.

3. How do the meteorites help scientists to know about the interior of the earth?

Answer: Scientists study meteorites which fall on the surface of the earth. The earth is said to be constituted of the same materials as the meteorites. This study, keeping in mind the theories of the origin of the earth, helps scientists to know about the materials that constitute the interior of the earth.

4. What is the composition of the earth’s crust?

Answer: The earth’s crust, or Lithosphere, is made up of rocks with a large content of silicates, feldspar, mica etc. The Lithosphere can be divided into two broad divisions: the SIAL and SIMA. The name Sial is taken from Silica (Si) and Aluminium (Al), and this layer forms the continents. Sima is named after Silica (Si) and Magnesium (Mg), and this layer makes up the Ocean floor.

5. What is the lithosphere?

Answer: The lithosphere is the earth’s crust, which is the outer layer of the earth. It is also described as the outermost crust of the earth and is the thinnest layer. The crust is called Lithosphere because it is a sphere of solid rocks (Lithos meaning stones or rocks). The Lithosphere is only 1% of earth’s mass and contains all known life in the universe.

6. Name the three layers of the earth’s interior. Name their boundaries.

Answer: The three layers of the earth’s interior, based on chemical composition, are the Core, Mantle, and Crust.

The boundary between the Crust and the Mantle is known as the ‘Moho Discontinuity’.
The boundary between the mantle and the core, or the mantle-core interface, is known as the Gutenberg Discontinuity.

7. Describe the mantle. State its two chief characteristics.

Answer: The mantle is the layer that lies below the crust. Its average thickness is 2900 km and it makes up 84 per cent of the earth’s total volume. The mantle is divided into two parts—the upper mantle and the lower mantle. The upper mantle extends from the Moho Discontinuity to a depth of 700 km; it is cooler and made of solid rocks. The lower mantle extends from 700 km to 2900 km and is hotter and denser than the upper mantle; it is solid due to high pressure. At the depth of around 100-410 km, the mantle is partially molten and is known as asthenosphere. The temperature in the mantle varies between about 1000°C to around 3,700°C. As the pressure increases towards the interior of the earth, the density of the mantle also increases.

Two chief characteristics of the mantle are:
(i) It makes up 84 per cent of the earth’s total volume and has an average thickness of 2900 km.
(ii) It is divided into the upper mantle (which is cooler and made of solid rocks, containing the partially molten asthenosphere) and the lower mantle (which is hotter, denser, and solid due to high pressure).

8. Where is asthenosphere found? In which form does it exist?

Answer: The asthenosphere is found in the mantle at a depth of around 100-410 km. It exists in a partially molten form.

9. What happens to the P and S waves at the Gutenberg Discontinuity.

Answer: At the Gutenberg Discontinuity, there is an abrupt change in the seismic waves. Primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely.

10. Why is the earth’s interior in most part found in a solid state despite great heat and pressure?

Answer: Certain parts of the Earth’s interior remain in a solid state despite great heat due to immense pressure. For instance, the lower mantle is solid due to high pressure. The solid state of the inner core is due to high density and pressure which have compressed the molten rock material. The upper mantle is cooler and made of solid rocks. Thus, high pressure in deeper layers can keep materials solid even at very high temperatures by compressing them.

Distinguish between the following

(i) Crust and Core.

Answer: The Crust, also known as the Lithosphere, is the outermost and thinnest layer of the Earth. It is a sphere of solid rocks, making up only 1% of the Earth’s mass and containing all known life. The crust’s thickness varies, being about 60 km below high mountains and 5-10 km below the oceans. It is composed of rocks with a large content of silicates, feldspar, mica, etc., and can be divided into SIAL and SIMA. The temperature of the upper oceanic crust is estimated at 0°C, gradually rising towards the interior, and the weight and pressure of rocks increase with depth.

The Core, on the other hand, is the most inaccessible and densest part of the Earth, located at its centre. It is called NIFE, being made up of Nickel (Ni) and Iron (Fe), which are heavy metals with high density. The entire core is about 7000 km in diameter and is divided into an outer and inner core. Its temperature ranges from about 4400°C to 6000°C. The core is believed to be responsible for Earth’s magnetism. The inner core is solid due to high density and pressure, while the S-waves’ disappearance in the outer core suggests it is in a molten state. The core is dense, likely due to a greater percentage of iron.

(ii) Sial and Sima.

Answer: Sial and Sima are the two broad divisions of the Lithosphere based on chemical composition.

Sial forms the continents. Its name is derived from Silica (Si) and Aluminium (Al). Sial appears to float above Sima because it has less density.

Sima makes up the Ocean floor. Its name is derived from Silica (Si) and Magnesium (Mg). Sima is denser than Sial.

(iii) Gutenberg Discontinuity and Mohorovicic Discontinuity.

Answer: The Gutenberg Discontinuity is the boundary between the mantle and the core, also known as the mantle-core interface. It begins at a depth of 2800 km. At this discontinuity, there is an abrupt change in seismic waves: Primary seismic waves (P waves) decrease in velocity, while secondary seismic waves (S waves) disappear completely.

The Mohorovicic Discontinuity, also known as ‘Moho Discontinuity’, is the discontinuity between the Crust and the Mantle. It was discovered by Yugoslavian scientist Andreaja Mohorovicic in 1909. It forms the boundary between the crust and the mantle and does not exist at a uniform depth; it is found at about 8 km beneath the oceans and about 32 km beneath the continents. The Mohorovicic Discontinuity has the characteristics of both the upper layer (the crust) and its bottom layer (the Mantle).

Structured questions

1. (a) Describe the layers of the interior of the earth and their chemical composition.

Answer: The earth’s interior is divided into three layers: Core, Mantle, and Crust, based on chemical composition.

The Crust, also known as the Lithosphere, is the outermost and thinnest layer of the earth. It is made up of rocks with a large content of silicates, feldspar, mica, etc. The Lithosphere can be divided into two broad divisions: SIAL and SIMA. Sial, composed of Silica (Si) and Aluminium (Al), forms the continents and appears to float above Sima because it has less density. Sima, named after Silica (Si) and Magnesium (Mg), makes up the Ocean floor. The crust is a relatively thin layer of rocks with a thickness of 60 km below high mountains and just 5-10 km below the oceans.

The Mantle lies below the crust. Its average thickness is 2900 km and makes up 84 per cent of the earth’s total volume. The mantle is divided into two parts: the upper mantle and the lower mantle. The upper mantle extends from the Moho Discontinuity to a depth of 700 km; it is cooler and made of solid rocks. The lower mantle extends from 700 km to 2900 km and is hotter and denser than the upper mantle. At the depth of around 100-410 km, the mantle is partially molten and is known as asthenosphere. The lower mantle is solid due to high pressure.

The Core is the most inaccessible part lying at the centre of the earth and is the densest part. It is divided into an outer and inner core, with the entire core being about 7000 km in diameter. The core is called NIFE, as it is made up of Nickle (Ni) and Iron (Fe). Since these are heavy metals with high density, they are found deep inside the earth. It is generally believed that the core’s composition of iron and nickel is responsible for Earth’s magnetism.

1. (b) There are two transitional zones between the two consecutive layers of the interior of the earth. Name them and state their chief characteristics.

Answer: The two transitional zones between consecutive layers of the interior of the earth are the Mohorovicic Discontinuity and the Gutenberg Discontinuity.

The Mohorovicic Discontinuity, also known as ‘Moho Discontinuity’, is the discontinuity between the Crust and the Mantle.

Its chief characteristics are:
(i) It forms the boundary between the crust and the mantle.
(ii) It does not exist at a uniform depth; it is found at about 8 km beneath the oceans and about 32 km beneath the continents.
(iii) It has the characteristics of both the upper layer (the crust) and its bottom layer (the Mantle).

The Gutenberg Discontinuity is the boundary between the mantle and the core, or the mantle-core interface.

Its chief characteristics are:
(i) It begins at the depth of 2800 km.
(ii) At this discontinuity, there is an abrupt change in the seismic waves; Primary seismic waves (P waves) decrease in velocity, while secondary seismic waves (S waves) disappear completely.

1. (c) Give a geographical reason for each of the following:

(i) Temperature starts rising gradually towards the interior of the earth.

Answer: Temperature starts rising gradually towards the interior of the earth because the heat given out by the earth comes from radioactive elements deep inside the earth.

(ii) The asthenosphere is in a semi-molten state.

Answer: The asthenosphere is in a semi-molten state because at its depth of around 100-410 km, the mantle is partially molten; this occurs because while the temperature is high, the pressure at this depth is not as great as in the lower mantle, which remains solid due to very high pressure.

(iii) The inner core of the earth is in a solid state.

Answer: The inner core of the earth is in a solid state due to high density and pressure which have compressed the molten rock material.

1. (d) Look at the figure on the side and answer the questions:

(i) Label the parts: (1), (2), (3), (4) and (5).

Answer: The parts are:
(1) Crust
(2) Mantle
(3) Outer Core
(4) Inner Core
(5) Atmosphere

(ii) Name the state (solid, liquid or gas) in which each part exists.

Answer: The states are:
(1) Crust: Solid
(2) Mantle: Predominantly solid (the upper mantle is made of solid rocks, the lower mantle is solid, and the asthenosphere within the upper mantle is partially molten)
(3) Outer Core: Molten (liquid)
(4) Inner Core: Solid
(5) Atmosphere: Gas

(iii) Which part of the earth is suitable for human habitation? Why?

Answer: The crust is the part of the earth suitable for human habitation. This is because the crust contains all known life in the universe.

2. (a) State two chief characteristics of the earth’s crust.

Answer: Two chief characteristics of the earth’s crust are:

(i) It is the thinnest layer of the earth, a relatively thin layer of rocks with a thickness of 60 km below high mountains and just 5-10 km below the oceans.
(ii) It is a sphere of solid rocks and contains all known life in the universe.

2. (b) Which layer is responsible for earth’s magnetic field? Why?

Answer: The core is the layer responsible for earth’s magnetic field. This is because it is generally believed that the core is composed of iron and nickel, which is responsible for earth’s magnetism.

2. (c) Give a geographical reason for each of the following:

(i) NIFE is so called.

Answer: The core of the earth is called NIFE because it is made up of Nickle (Ni) and Iron (Fe).

(ii) The core of the earth is the most inaccessible part of the earth.

Answer: The core of the earth is the most inaccessible part of the earth because it is lying at the centre of the earth.

(iii) The earth’s crust undergoes many changes.

Answer: The earth’s crust undergoes many changes because of the continuous changes due to forces of nature. The Theory of Continental Drift and the Theory of Plate Tectonics suggest that the earth’s crust undergoes structural changes, and lateral movements also give the earth a new shape very slowly over millions of years.

2. (d) Draw a well labelled diagram to show the interior of the earth.

Answer:

Earth’s Structure icse class 9 geography new

Thinking skills

1. The earth’s crust is only one percent of earth’s mass yet it is a large reservoir of resources for mankind. Justify this statement with examples.

Answer: The statement is justified because, despite constituting only 1% of the Earth’s total mass, the crust is the layer that contains all known life in the universe. It is the most accessible layer for humans and provides essential resources.
For example:

The crust is made up of rocks rich in silicates, feldspar, and mica, which are fundamental components for various industries and construction.
The continental part of the crust, known as SIAL, is rich in Silica (Si) and Aluminium (Al). Aluminium is a widely used metal.
The oceanic part of the crust, SIMA, is composed of Silica (Si) and Magnesium (Mg). These elements and the minerals they form are vital.
The materials thrown up by volcanoes, originating from deeper within the Earth but passing through and depositing on the crust, also contribute to the mineral resources available.

2. What would happen if the earth’s core no longer remain hot and the magma cools and hardens inside the earth? Give reasons to support your answer.

Answer: If the Earth’s core were to cool significantly and magma were to harden inside the Earth, several critical geological processes would cease or change dramatically:

Cessation of Volcanic Activity: The molten material (magma) that rises from the magma chamber deep inside the Earth to cause volcanic eruptions would solidify. Without molten magma, volcanoes would become inactive, and the process of new land formation and mineral deposit through volcanic activity would stop.
Loss or Weakening of Earth’s Magnetic Field: The core, composed of iron and nickel, is believed to be responsible for Earth’s magnetism. The S-waves’ disappearance in the outer core suggests it is in a molten state. If the core, particularly the outer core, cooled and solidified, the mechanism generating the magnetic field would likely be disrupted, leading to a significantly weaker or absent magnetic field. This field protects the planet from harmful solar radiation.
End of Plate Tectonics and Continental Drift: The asthenosphere, a partially molten layer in the upper mantle (at depths of 100-410 km), facilitates the movement of tectonic plates. This movement is responsible for continental drift, the formation of mountains, earthquakes, and the overall structural changes the Earth’s crust undergoes. If magma cooled and the asthenosphere solidified, these tectonic movements would halt, leading to a geologically inactive Earth surface.
Changes in Internal Heat Flow: The heat given out by the Earth, partly from radioactive elements deep inside, contributes to the planet’s internal temperature. A cooling core would mean a significant reduction in this internal heat, altering the thermal dynamics of the planet’s interior.

Project/Practical Activity

Suppose you were given the responsibility to explore the interior of the earth. Draw a project report describing the steps you would take, your objectives and the estimation of the costs of the project, etc.

Answer: Do it yourself. Here are the instructions:

Title of Project Report: Exploring the Interior of the Earth
Objective:
- To understand the structure and composition of the Earth’s interior.
- To study the properties of different layers like the crust, mantle, and core.
- To learn how scientists estimate temperature, density, and composition using indirect methods.
Steps to Follow:
1. Introduction Section
  - Write a short introduction about the earth’s structure: crust, mantle, and core.
  - Mention that direct observation is not possible, and we depend on indirect methods.
2. Methods of Exploration
  - List and explain the tools and methods used:
    - Seismic wave study (P-waves and S-waves).
    - Study of volcanic materials.
    - Analysis of meteorites.
    - Understanding temperature and density from radioactive heat inside the earth.
  - You may use labelled diagrams to show seismic waves and earth layers.
3. Plan of Action
  - Imagine a team of scientists planning a mission.
  - Mention setting up observation stations to record seismic activity.
  - Propose computer modelling and laboratory analysis of rocks and lava.
  - Suggest creating educational models or simulations to explain findings.
4. Estimated Cost
  - Keep it simple. You can break it into:
    - Equipment and instruments: ₹5 lakh
    - Data analysis and software: ₹3 lakh
    - Team and training: ₹2 lakh
    - Miscellaneous: ₹1 lakh
    - Total Estimated Cost: ₹11 lakh
5. Expected Outcome
  - Better understanding of Earth’s layers.
  - Insight into natural disasters like earthquakes and volcanoes.
  - Improved knowledge for future resource exploration.
6. Conclusion
  - Summarise the importance of studying the Earth’s interior.
Presentation Tips:
- Use headings and subheadings clearly.
- Add simple diagrams (e.g., the structure of the Earth).
- Keep your language simple and factual.
Optional: You may include printed snapshots of the YouTube videos or draw sketches in your notebook.

Extras

Additional MCQs

1. Which method is NOT used to study the Earth’s interior?

A. Excavations
B. Seismic waves
C. Volcano ejecta
D. Meteorites

Answer: A. Excavations

2. Secondary seismic waves do not pass through which medium?

A. Liquids
B. Solids
C. Gases
D. Vacuum

Answer: A. Liquids

3. What is the name of the discontinuity between the crust and the mantle?

A. Mohorovicic discontinuity
B. Gutenberg discontinuity
C. Asthenosphere
D. Lithosphere

Answer: A. Mohorovicic discontinuity

4. What is the boundary between the mantle and the core called?

A. Gutenberg discontinuity
B. Mohorovicic discontinuity
C. Asthenosphere
D. Lithosphere

Answer: A. Gutenberg discontinuity

5. Which layer of the Earth is composed mainly of nickel and iron?

A. Core
B. Mantle
C. Crust
D. Asthenosphere

Answer: A. Core

6. Which sphere is made up of rocks with large content of silicates, feldspar and mica?

A. Lithosphere
B. Asthenosphere
C. Hydrosphere
D. Atmosphere

Answer: A. Lithosphere

7. Which division of the crust floats above the other due to lower density?

A. Sial
B. Sima
C. Nife
D. Asthenosphere

Answer: A. Sial

8. What do the initials “SIAL” stand for?

A. Silica and Aluminium
B. Silica and Magnesium
C. Silicates and Aluminium
D. Nickel and Iron

Answer: A. Silica and Aluminium

9. What are the main chemical constituents of the SIMA?

A. Silica and Magnesium
B. Silica and Aluminium
C. Silicates and Feldspar
D. Nickel and Iron

Answer: A. Silica and Magnesium

10. What is the approximate diameter of the Earth’s core?

A. 7000 km
B. 2900 km
C. 60 km
D. 5–10 km

Answer: A. 7000 km

11. What is the temperature range in the Earth’s core?

A. 4400–6000 °C
B. 1000–3700 °C
C. 1500–2500 °C
D. 0–100 °C

Answer: A. 4400–6000 °C

12. Which seismic waves disappear completely in the outer core?

A. Secondary waves
B. Primary waves
C. Surface waves
D. Body waves

Answer: A. Secondary waves

13. What is the average thickness of the Earth’s mantle?

A. 2900 km
B. 700 km
C. 100 km
D. 36 km

Answer: A. 2900 km

14. At what depth does the partially molten layer of the mantle occur?

A. 100–410 km
B. 700–2900 km
C. 0–100 km
D. 2800–2900 km

Answer: A. 100–410 km

15. What term is used for the partially molten zone of the upper mantle?

A. Asthenosphere
B. Lithosphere
C. Mantlesphere
D. Hydrosphere

Answer: A. Asthenosphere

16. Which layer contains about 84 per cent of the Earth’s total volume?

A. Mantle
B. Crust
C. Core
D. Atmosphere

Answer: A. Mantle

17. Why is the inner core solid despite extremely high temperature?

A. High pressure
B. Low temperature
C. Low density
D. High fluidity

Answer: A. High pressure

18. What describes the lower mantle compared to the upper mantle?

A. Hotter and denser
B. Cooler and lighter
C. Partially molten
D. Low density

Answer: A. Hotter and denser

19. Which sphere of the Earth is made up of gases enveloping the planet?

A. Atmosphere
B. Hydrosphere
C. Lithosphere
D. Biosphere

Answer: A. Atmosphere

20. Which sphere consists of parts of the crust submerged under oceans?

A. Hydrosphere
B. Lithosphere
C. Asthenosphere
D. Biosphere

Answer: A. Hydrosphere

21. What percentage of the Earth’s mass is represented by the crust?

A. 1 per cent
B. 10 per cent
C. 25 per cent
D. 50 per cent

Answer: A. 1 per cent

22. What is the typical thickness of the continental crust beneath high mountains?

A. 60 km
B. 5–10 km
C. 32 km
D. 2900 km

Answer: A. 60 km

23. At approximately what depth is the Mohorovicic discontinuity found beneath the continents?

A. 32 km
B. 8 km
C. 700 km
D. 2800 km

Answer: A. 32 km

24. At what depth is the Mohorovicic discontinuity found beneath the oceans?

A. 8 km
B. 32 km
C. 5 km
D. 60 km

Answer: A. 8 km

25. Which discontinuity begins at around 2800 km depth?

A. Gutenberg discontinuity
B. Mohorovicic discontinuity
C. Asthenosphere
D. Lithosphere

Answer: A. Gutenberg discontinuity

26. What happens to the velocity of primary seismic waves at the Gutenberg discontinuity?

A. Decrease
B. Increase
C. Remain constant
D. Cease completely

Answer: A. Decrease

27. Which layer is responsible for generating the Earth’s magnetic field?

A. Core
B. Mantle
C. Crust
D. Asthenosphere

Answer: A. Core

28. Which property increases as depth towards the Earth’s centre increases?

A. Density
B. Porosity
C. Permeability
D. Evaporation

Answer: A. Density

29. Which process helps estimate the interior temperature of the Earth?

A. Study of radioactive heat
B. Core sampling
C. Satellite imaging
D. Surface erosion

Answer: A. Study of radioactive heat

30. Meteorites are studied to understand the composition of which part of the Earth?

A. Interior
B. Crust
C. Atmosphere
D. Oceans

Answer: A. Interior

31. Which of the following is NOT part of the lithosphere?

A. Core
B. Sial
C. Sima
D. Rocks

Answer: A. Core

32. Which layer of the Earth is suitable for human habitation?

A. Crust
B. Mantle
C. Core
D. Asthenosphere

Answer: A. Crust

Additional Assertion and Reason

1. Assertion (A): Secondary seismic waves (S-waves) are unable to traverse through liquid sections of the Earth’s interior.
Reason (R): The behavior of S-waves, specifically their inability to pass through certain zones, helps scientists delineate liquid layers within the Earth.