Selina Concise Solutions for ICSE Class 9 Geography Chapter 4 Earths Structure

Exercises

I. Short Answer Questions

Question 1. Name the sources of information about forces operating inside the earth.
Answer: Information about forces operating inside the earth is taken by the study of seismic waves, materials thrown up by volcanoes and the evidence from the theories of the origin of the earth.
In simple words: Scientists can't travel to the center of the Earth, so they study things that come out of it, like lava from volcanoes, and listen to how vibrations from earthquakes (seismic waves) travel through the ground to figure out what's inside.

📝 Teacher's Note: Explain that seismic waves act like a "sonar" or "ultrasound" for the planet, allowing us to see layers we cannot physically reach.

🎯 Exam Tip: "Seismic waves" and "Volcanic materials" are the two most important technical terms to include in your answer.

Question 2. In which part of the earth is NIFE found ? What it is composed of ?
Answer: NIFE is found in the inner core of die earth. NIFE is composed of Nickel (Ni) and Iron (Fe), being heavy metals and having high density, these are found deep inside the earth.
In simple words: "NIFE" is just a nickname for the Earth's center. It's made by combining the chemical symbols for Nickel (Ni) and Iron (Fe). These metals are very heavy, so they sank to the very middle of the Earth.

📝 Teacher's Note: Use the name Ni-Fe to help students memorize the chemical symbols for Nickel and Iron simultaneously.

🎯 Exam Tip: Remember that NIFE belongs to the Core, not the crust or mantle.

Question 3. What are the consequences of the pressure and temperature in the interior of the earth ?
Answer: Due to the extreme temperature of \( 2200^{\circ}\text{ C} \), every matter is in liquid and gaseous state in the interior core, the temperature in the mantle ranges from \( 870^{\circ}\text{ C} \) to \( 2200^{\circ}\text{ C} \), so things are in semi - liquid to liquid state, the crust, a layer of 60 km, breadth, is made of solid rocks with a density of \( 2 - 3\text{ g/cm}^3 \). The density goes on increasing to \( 3-4\text{ g/cm}^3 \) in the mantle and \( 10-13\text{ g/cm}^3 \) in the core.
In simple words: As you go deeper, it gets much hotter and the weight (pressure) of everything above gets heavier. This heat melts rock into liquid or "goop," but the extreme pressure at the very center can actually squash things back into a solid.

📝 Teacher's Note: This is a great place to discuss phase changes. While heat wants to melt things, pressure wants to keep them solid.

🎯 Exam Tip: Be sure to mention that density increases as you move from the crust to the core.

Question 4. What is the lithosphere ?
Answer: The crust is called Lithosphere made of solid rocks with a thickness of 60 km. below high mountains and 6-12 km below the oceans.
In simple words: The lithosphere is the "hard shell" of the Earth. It's the part we walk on and build houses on, made of solid rock.

📝 Teacher's Note: "Lithos" is Greek for stone. This layer includes the crust and the topmost solid part of the mantle.

🎯 Exam Tip: Mention the varying thickness: it is much thicker under mountains than under the ocean floor.

Question 5. Name the three layers of the earth’s interior.
Answer: The three layers of earth’s interior are:
1. Core
2. Mantle
3. Crust
In simple words: Think of the Earth like a hard-boiled egg. The shell is the crust, the white part is the mantle, and the yolk is the core.

📝 Teacher's Note: Always teach these in order, starting from the outside (Crust) to the inside (Core) or vice versa to help students visualize the structure.

🎯 Exam Tip: Listing them correctly is a simple way to score marks in basic descriptive questions.

Question 6. State two chief characteristics of the earth’s crust.
Answer: The crust is made of solid rocks and divided into oceans and continents.
In simple words: The crust is the Earth's outer skin. It is solid and is split into the land we live on (continents) and the ground under the sea (oceans).

Question 7. Describe the mantle. State its two chief characteristics.
Answer: Mantle lies between 60 - 2900 km. depth. It is divided into two parts namely upper mantle and lower mantle, which are in the form of solid rocks and semi-molten rocks respectively.
In simple words: The mantle is the thickest layer of Earth. The top part is solid rock, but the bottom part is hot and soft, like thick honey or plastic.

Question 8. Where is asthenosphere found ? In which form does it exist ?
Answer: At the depth of 100 - 250 km. The mantle is partially molten and known as asthenosphere, with a temperature of \( 1100^{\circ}\text{C} \).
In simple words: The asthenosphere is a "soft" layer in the upper mantle. Because it is hot and squishy, the solid plates of the Earth's surface can slide slowly on top of it.

📝 Teacher's Note: This layer is critical for understanding plate tectonics. Without this squishy layer, continents wouldn't be able to move.

Question 9. Write one difference between Moho Discontinuity and Gutenberg Discontinuity.
Answer: Moho Discontinuity is the boundary between crust and mantle and Gutenberg Discontinuity is the boundary between mantle and core.
In simple words: These are "border lines." Moho separates the top shell from the middle layer, and Gutenberg separates the middle layer from the center.

🎯 Exam Tip: Use the mnemonic: Moho is at the Middle-top (Crust/Mantle), Gutenberg is at the Ground-center (Mantle/Core).

Question 10. Why is the earth’s interior in most part found in a solid state despite great heat and pressure ?
Answer: The solid state of the inner core is due to high density and pressure which have compressed molten rock material and keep this layer firm and solid in some parts due to high pressure inspite of the temperature of \( 5000^{\circ}\text{C} \).
In simple words: Even though it's hot enough to melt everything, the pressure from the weight of the whole world is so strong that it squashes the atoms together into a solid block.

Question 11. Name two types of earth movements.
Answer: Two types of movements are due to isostasy and tectonic plates. Isostasy is the process of natural balance between different landforms and the sliding movement of the tectonic plates of the earth’s crust.
In simple words: Earth moves in two ways: one is like a boat balancing in water (isostasy), and the other is like giant pieces of a puzzle sliding around (tectonic plates).

Question 12. What is Geology ?
Answer: Geology is the science dealing with the origin and types of rocks found in the interior of the earth.
In simple words: Geology is the study of rocks and how the Earth was made.

II. Give reasons for each of the following

Question 1. The study of meteorites helps scientists to know about the interior of the earth.
Answer: At the time of the origin of the earth every planet and meteorites were floating in the space and the materials of the earth were same as that of the meteorites. So, the scientists can calculate the composition of rocks of the earth by studying the materials found in the meteorites.
In simple words: Space rocks (meteorites) were made from the same "space dust" as Earth. Since we can't look at the center of Earth, we look at these rocks to guess what Earth's core is made of.

📝 Teacher's Note: Explain that meteorites are like "leftover building blocks" from when the solar system was being built.

Question 2. Temperature starts rising gradually towards the interior of the earth.
Answer: Due to enhancing density and pressure the temperature goes on increasing gradually towards inside of the interior of the earth.
In simple words: Think of a crowded room. As you pack more people (density) and push them together (pressure), it gets hotter. The Earth's center is packed extremely tight, which generates huge heat.

Question 3. The asthenosphere is in a semi-molten state.
Answer: At the depth of 100 – 250 km the mantle is partially molten and known as asthenosphere due to the temperature around \( 1100^{\circ}\text{C} \) along with greater pressure and density.
In simple words: This layer is like hot wax—it's not quite solid rock, but not quite liquid lava either. The heat is just enough to make it soft.

Question 4. The inner core is in a solid state.
Answer: The high pressure in the interior core keeps this layer firm and solid in some parts, in spite of the temperature as high as \( 5000^{\circ}\text{C} \).
In simple words: The pressure at the center of the Earth is so immense that it forces the liquid metal to turn into a solid, even though it's incredibly hot.

Question 5. The continents are placed above the oceans.
Answer: The density of continents is lesser than the layer supporting the ocean beds, so the continents came floating upwards at the time of the formation and solidification of the earth, as the lighter things come upwards floating over the heavier things i. e. Sial is lighter than Sima.
In simple words: Continents are like pieces of wood floating on water. Because the rocks making up the land (Sial) are lighter than the rocks under the sea (Sima), the land stays higher up.

📝 Teacher's Note: Use the "iceberg" or "boat" analogy to explain why lighter rocks float higher in the mantle than denser ones.

III. Long Answer Questions

Question 1. Look at the figure on the side and answer the questions:

(a) Label the parts : (1), (2), (3), (4) and (5).
(b) Name the state (solid, liquid or gas) in which each part exists.
(c) What part is suitable for human habitation? Why?
Answer:
(a) Labels:
1. Atmosphere
2. Lithosphere (Crust)
3. Mantle
4. Core
5. Hydrosphere

(b) States of Matter:
1. Atmosphere — Gas
2. Lithosphere — Solid
3. Mantle — Semi-solid
4. Core — Molten state or liquid (outer) and Solid (inner)
5. Hydrosphere — Liquid (water)

(c) The outer part of the earth (Crust/Lithosphere) is suitable for human habitation due to favourable conditions for survival, i.e. atmosphere for air, Lithosphere for settlement due to ideal temperature and land, hydrosphere for hydrological cycle for providing rainfall and fresh water bodies on the earth and oceans for navigation and trade routes etc.
In simple words: Humans live on the outer "shell" because that's where we have breathable air, liquid water to drink, and solid ground that isn't hot enough to melt our shoes!

📝 Teacher's Note: When labeling diagrams, ensure students connect the part of the diagram to the specific number provided. Point out that the Core actually has two parts with different states (outer liquid, inner solid).

Question 2. Describe the layers of the interior of the earth and their chemical composition.
Answer: The interior of the earth is divided into three major parts i.e. crust, mantle and core.

• Crust (SIAL): Consists majority of silica and aluminium.
• Mantle (SIMA): Consists majority of silica and magnesium. It makes the bed of oceans.
• Core (NIFE): Consists of excess of Nickel and Iron (Ni + Fe).

In simple words: Earth's layers have chemical nicknames based on what they are made of. SIAL (silica + aluminum) is the top land, SIMA (silica + magnesium) is the heavy layer underneath the oceans, and NIFE (nickel + iron) is the heavy metal center.

Question 3. 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 are:
1. Moho Discontinuity: The transitional zone between crust and mantle. It is the dividing zone between solid and semi-solid state of rocks due to the increasing temperature as the depth increases.
2. Gutenberg Discontinuity: The boundary between mantle and core. Below this zone both density and temperature go on increasing. The density is more than 13 and temperature is more than \( 2200^{\circ}\text{C} \).
In simple words: These are imaginary borders where the rock changes its nature. Moho is where rock starts getting squishy, and Gutenberg is where rock turns into metal.

Question 4. Explain the layers of the interior of the earth with reference to the following : (a) Depth, (b) Temperature, (c) Density.
Answer:
Crust :
(a) Depth: 35-50 km below continents and 6-12 km. below the oceans.
(b) Temperature: Less than \( 870^{\circ}\text{C} \)
(c) Density: \( 2.7\text{ g/cm}^3 – 5.5\text{ g/cm}^3 \).

Mantle :
(a) Depth: 50 – 2900 km.
(b) Temperature: \( 1500^{\circ}\text{ C} – 2200^{\circ}\text{C} \)
(c) Density: \( 3\text{ g/cm}^3 – 4.5\text{ g/cm}^3 \)

Core :
(a) Depth: 2900 km – 3500 km (radius of core is about 3471 km)
(b) Temperature: \( 2200^{\circ}\text{C} – 5000^{\circ}\text{C} \)
(c) Density: \( 10.0\text{ g/cm}^3 – 13.6\text{ g/cm}^3 \).
In simple words: As you travel toward the center, the Earth gets much deeper, much hotter, and the material gets much heavier and more packed (denser).

Question 5. Study the figure on the side and answer the questions:

(a) What is known as Sial ? How deep is the area marked by Sial ?
(b) What role does Sima play ?
(c) Why is the expression ‘Nife’ so called ?
(d) Which layer is responsible for earth’s magnetic field ? Why?
(e) What happens to the continents if there is an earthquake?
Answer:
(a) Sial is the upper layer or crust of the earth. The name Sial is based on the excess of Silica and Aluminium. The depth of Sial is 60 km.
(b) Sima is the second layer of earth which gives support to the ocean beds. There is the excess of Silica and Magnesium.
(c) ‘Nife’ stands for Nickel (Ni) and Iron (Fe). It is so called because of the majority of Nickel and Iron in the core.
(d) The Core of earth is responsible for earth’s magnetic field because it is composed of iron and nickel which are magnetic. The magnetic field is oriented towards North and South Poles.
(e) Several drastic changes may occur during the earthquake. If intensity is 8 or more buildings may break up, casualties occur and major changes on the landforms can happen, such as broad breaking gaps, or the origin and disappearing of several small islands.
In simple words: These terms help us understand what the ground is made of. The heavy metal at the center acts like a giant magnet, which is why a compass always points North!

🎯 Exam Tip: For (d), always mention Iron as the main reason for magnetism.

Practice Questions (Solved)

Question 1. Which are the two most abundant chemical elements in the Earth’s crust ?
Answer: Oxygen and Silicon.
In simple words: Most of the ground under your feet is actually made of oxygen and silicon "locked" together in rocks.

Question 2. Why does the Sun not rise at the same time everywhere in the world ?
Answer: If the Earth were flat, the whole world would have the sunrise and sunset at the same time. As the Earth is spherical and rotates from West to East, places in the East see the Sun earlier than the places in the West.
In simple words: Because the Earth is a ball that spins, the Sun "hits" one side before the other. That's why people in Japan see the morning before people in New York!

Question 3. “The whole of the approaching ship is not visible at one time.” Why ?
Answer: The Earth has a spherical shape. Along its curved surface, the appearance of a ship is gradual. We see first the smoke, then the mast and then the hull. If the Earth were flat, the entire ship would be seen all at once.
In simple words: Imagine a ship coming up over a very long, low hill. You see the top first and then the rest as it comes closer. The "hill" is actually the curve of the Earth.

Question 4. “Even when the Earth is spherical, it appears to be flat.” Discuss.
Answer: The actual shape of the Earth is spherical. The curvature of the Earth is small as compared to its big size. For a small area upto 100 sq. miles, this curvature is negligible. Therefore, it appears flat.
In simple words: The Earth is so massive that the tiny bit we see around us looks flat, just like a tiny ant on a giant beach ball might think the ball is a flat floor.

Question 5. Why is the Earth slightly flattened at the poles ?
Answer: The Earth is not a perfect sphere. It is slightly flattened at both the poles. It is due to the centrifugal force produced by the rotation of the Earth.
In simple words: Because the Earth spins so fast, it "bulges" out at the middle and "squashes" down at the top and bottom, like a balloon being gently squeezed.

Question 6. Explain briefly the structure of the earth. OR Discuss the structure of the earth giving details about each of its layers and arguments in support of your contention.
Answer: The structure of the earth means the interior of the earth. The entire earth is composed of three zones:
1. Lithosphere (Crust): Outermost solid layer, 8 to 60 km thick. It has two layers: a lower basaltic (SIMA) layer and an upper granitic (SIAL) layer.
2. Mesosphere (Mantle): Beneath the crust, about 2900 km thick. It contains silicate layers and mixed metals.
3. Barysphere (Core): The central nucleus made of dense rock materials like Nickel and Iron (NIFE). It has a liquid outer part and a solid inner part.
In simple words: The Earth has a thin rocky skin, a very thick "goopy" middle layer, and a heavy metal heart.

Question 7. Where is Mantle located in the Earth ?
Answer: The Mantle or Mesosphere is located between 2850 – 2900 km beneath the earth crust (Note: Mantle extends from the base of the crust down to 2900 km).
In simple words: The mantle starts right under the thin crust and goes down almost halfway to the center of the Earth.

Question 8. Describe any three experiments to prove the Spherical Shape of the Earth.
Answer:
1. Sighting a Ship: When a ship approaches, the mast is seen first, then the hull. This proves the surface is curved.
2. Bedford Level Experiment: Fixing three poles of equal length at equal distances on the ground (Bedford Canal) shows the middle pole is higher, proving the Earth's surface is curved.
3. Circular Horizon: From a high point, the horizon always appears circular. This circular view widens as you go higher, which only happens on a sphere.
In simple words: We can prove Earth is round by watching ships, measuring sticks in a straight line, and looking at the circular "edge" of the world from a plane.

📝 Teacher's Note: The Bedford Level experiment was a famous test in 1870. Remind students that "curvature" is the key word here.

🎯 Exam Tip: Being able to describe the "mast-first" appearance of a ship is a classic answer for spherical proof.