Earth's Structure - Questions & Answers

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

2. Meteors are rocks from outer space. They hold special significance for scientists because
(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
(b) Core - Mantle - Crust

4. Name the two broad divisions the earth's crust is divided, depending on chemical composition.
(b) SIAL, SIMA

5. Study the figure on the right and complete the analogy:
Continents : SIAL :: Ocean floor : SIMA

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.
(b) Core

7. Assertion (A): Sial appears to float above Sima.
Reason (R): Sial has lower density than Sima.
(a) Both A and R are true and R is the correct explanation of A.

8. Which of the following is the correct order as we move towards the interior of the earth.
(a) SIAL - SIMA - NIFE

9. Which of the following is incorrect about the inner core?
(a) It is in a molten state.

10. At the depth of around 100-410 km, mantle is partially molten and is known as:
(a) Asthenosphere

11. What is the temperature variation in the mantle in °C?
(c) 1000 to 3700

12. Name the mantle-core interface where primary seismic waves slow down and secondary seismic waves disappear completely.
(a) Gutenberg Discontinuity

13. Name the boundary between the crust and the mantle which has characteristics of both layers.
(d) Mohorovicic Discontinuity

14. Assertion (A): The thickness of the lithosphere is very fragile compared to the nature and composition of the earth.
Reason (R): There are continuous changes in the earth's crust due to forces of nature.
(a) Both A and R are true and R is the correct explanation of A.

15. What life sustaining unit contains only one per cent of the earth's mass?
(c) crust

16. Mohorovicic discontinuity is found at about 8 km beneath the oceans and 32 km beneath the continents.

17. Extending upwards, the earth is enveloped by a blanket of gases. Name it.
(c) Atmosphere

18. The diameter of the core is about 7000 km.

19. 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.
(c) Crust

1. In which part of the earth is NIFE found? What is it composed of?
NIFE is found in the core of the earth. It is composed of Nickel (Ni) and Iron (Fe).

2. What are the consequences of the pressure and temperature in the interior of the earth?
As the pressure increases towards the interior of the earth, the density also increases. Due to this extremely high density and compressive pressure, the inner core is kept in a solid state despite the extremely high temperatures.

3. How do the meteorites help scientists to know about the interior of the earth?
The earth is believed to be constituted of the same materials as the meteorites that fall on its surface. By studying the composition of these meteorites, scientists can estimate the materials that make up the interior of the earth.

4. What is the composition of the earth's crust?
The earth's crust is made up of rocks with a large content of silicates, feldspar, and mica. It is divided into two broad divisions based on composition: SIAL (Silica and Aluminium) and SIMA (Silica and Magnesium).

5. What is the Lithosphere?
The Lithosphere is the outermost solid crust of the earth. The term comes from "Lithos" meaning stones or rocks. It is a thin sphere of solid rocks that makes up only 1% of the earth's mass and contains all known life in the universe.

6. Name the three layers of the earth's interior. Name their boundaries.
The three layers are the Crust, Mantle, and Core. The boundary between the Crust and Mantle is the Mohorovicic Discontinuity, and the boundary between the Mantle and Core is the Gutenberg Discontinuity.

7. Describe the mantle. State its two chief characteristics.
The mantle is the layer lying below the crust, with an average thickness of 2900 km, making up 84% of the earth's total volume. Its two chief characteristics are: (1) It is divided into the upper mantle (cooler solid rocks) and lower mantle (hotter, denser solid rocks), and (2) it contains a partially molten zone called the asthenosphere between 100-410 km in depth.

8. Where is asthenosphere found? In which form does it exist?
The asthenosphere is found in the upper mantle at a depth of around 100-410 km. It exists in a partially molten (semi-molten) state.

9. What happens to the P and S waves at the Gutenberg Discontinuity?
At the Gutenberg Discontinuity, 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?
The earth's interior, such as the lower mantle and inner core, remains in a solid state because the extremely high density and compressive pressure prevent the rock material from melting, completely counteracting the effects of the great heat.

(i) Crust and Core.
The Crust is the outermost, thinnest layer of the earth (making up only 1% of the earth's mass) composed of solid rocks like silicates, feldspar, and mica. In contrast, the Core is the innermost, densest, and most inaccessible part of the earth, composed of heavy metals like iron and nickel (NIFE).

(ii) Sial and Sima.
SIAL forms the continents, has less density, and is rich in Silica (Si) and Aluminium (Al). SIMA makes up the ocean floor, is denser, and is rich in Silica (Si) and Magnesium (Mg). SIAL appears to float above SIMA.

(iii) Gutenberg Discontinuity and Mohorovicic Discontinuity.
The Gutenberg Discontinuity is the boundary separating the mantle and the core, beginning at a depth of 2800 km. The Mohorovicic Discontinuity is the boundary between the crust and the mantle, varying in depth from about 8 km beneath oceans to 32 km beneath continents.

1. (a) Describe the layers of the interior of the earth and their chemical composition.
The earth is divided into three concentric layers:
1. Crust (Lithosphere): The outermost layer composed of rocks rich in silicates, feldspar, and mica. It is divided into SIAL (Silica and Aluminium) and SIMA (Silica and Magnesium).
2. Mantle: The middle layer composed of dense solid rocks, containing a partially molten layer called the asthenosphere.
3. Core: The densest innermost part composed of heavy metals, specifically Nickel (Ni) and Iron (Fe), which is why it is called NIFE.

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.
1. Mohorovicic Discontinuity: Located between the crust and the mantle. It does not exist at a uniform depth (8 km under oceans, 32 km under continents) and shares characteristics of both the upper layer (crust) and bottom layer (mantle).
2. Gutenberg Discontinuity: Located between the mantle and the core at a depth of 2800 km. It is marked by an abrupt change in seismic waves, where P-waves decrease in velocity and 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.
This is because heat is given out by radioactive elements deep inside the earth, alongside immense weight and pressure of overlying rocks which generate heat.

(ii) The asthenosphere is in a semi-molten state.
At a depth of 100-410 km in the upper mantle, the balance of heat and pressure is such that the rocks become partially molten.

(iii) The inner core of the earth is in a solid state.
The extreme density and compressive pressure at the very centre of the earth severely compress the rock material, preventing it from melting despite temperatures reaching up to 6000°C.

1. (d) Look at the figure on the side and answer the questions:
(i) Label the parts: (1), (2) and (3).
(1) Crust, (2) Mantle, (3) Core.

(ii) Name the state (solid, liquid or gas) in which each part exists.
The Crust exists in a solid state. The Mantle exists mostly in a solid state (with a partially molten asthenosphere). The Core consists of a liquid (molten) outer core and a solid inner core.

(iii) Which part of the earth is suitable for human habitation? Why?
The Crust (Lithosphere) is suitable for human habitation because it is a solid sphere of rocks that contains all known life. It provides a stable surface, moderate temperatures at the top, and is covered by life-sustaining components like the hydrosphere (oceans/seas) and atmosphere.

2. (a) State two chief characteristics of the earth's crust.
1. It is the thinnest outer layer, constituting only 1% of the earth's mass and varying from 5 to 60 km in thickness.
2. It is highly fragile and undergoes continuous structural changes due to lateral plate movements and forces of nature.

2. (b) Which layer is responsible for earth's magnetic field? Why?
The Core is responsible for the earth's magnetic field. This is because it is composed of heavy magnetic metals, specifically iron and nickel, whose properties and movements create a magnetic field oriented towards the North and South Poles.

2. (c) Give a geographical reason for each of the following:
(i) NIFE is so called.
The core is called NIFE because it is made up largely of the heavy metals Nickel (Ni) and Iron (Fe).

(ii) The core of the earth is the most inaccessible part of the earth.
It lies at the very centre of the earth under 2900 km of solid rock, enduring extreme high pressure and blistering temperatures ranging from 4400°C to 6000°C.

(iii) The earth's crust undergoes many changes.
The earth's crust is very thin and fragile compared to the rest of the earth's interior. It undergoes continuous structural changes and lateral movements due to powerful natural forces from within, as explained by the Theories of Continental Drift and Plate Tectonics.

2. (d) Draw a well labelled diagram to show the interior of the earth.
[Students should draw a cross-section of the Earth labeling the Crust (Lithosphere), Mantle (including Asthenosphere), Outer Core, Inner Core, Mohorovicic Discontinuity, and Gutenberg Discontinuity.]

1. The earth's crust is only one per cent of earth's mass yet it is a large reservoir of resources for mankind. Justify this statement with examples.
Despite being incredibly thin, the crust provides the essential environment for all known life. It supplies solid land for habitation, agriculture, and construction. Additionally, it is heavily composed of valuable minerals and building materials like silicates, feldspar, mica, iron, magnesium, and aluminium, which mankind extracts for technological and societal development. The depressions in the crust also hold the hydrosphere (water), which is critical for survival.

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.
If the earth's core cooled and hardened completely, the earth would lose its magnetic field, as magnetism is generated by the molten heavy metals (iron and nickel) in the core. Additionally, the internal heat drives volcanic activities and the movement of tectonic plates (Plate Tectonics). Without this heat, the crust would cease to undergo the structural changes that recycle minerals and shape the continents, fundamentally changing the dynamic nature of our planet.

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.
[This is a practical creative assignment. Students are expected to draft a hypothetical project report utilizing their understanding of seismic waves, volcanic analysis, deep-earth drilling machinery concepts, and safety measures required to study high-pressure and high-temperature environments.]