NCERT Solutions for Class 11 Geography Chapter 3: Interior of the Earth (NCERT 2026–27)

These Class 11 Geography Chapter 3 solutions cover Interior of the Earth from Fundamentals of Physical Geography, the NCERT textbook continued for the 2026–27 session. The chapter explains how scientists learn about the earth’s deep interior through direct and indirect sources, how earthquakes and seismic waves (P-waves, S-waves and surface waves) reveal the layered structure, why a shadow zone forms, and how the crust, mantle and core are arranged. It also describes volcanoes and volcanic landforms, both extrusive and intrusive. Below you get step-by-step, exam-ready answers to every NCERT exercise question, plus key terms, extra practice, MCQs, Assertion–Reason questions and FAQs.

Class: 11 Subject: Geography Book: Fundamentals of Physical Geography Chapter: 3 Topic: Interior of the Earth Session: 2026–27
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Class 11 Geography Chapter 3 – Overview

Since no one can reach the earth’s centre (its radius is about 6,378 km), our knowledge of the interior rests mainly on estimates and inferences. Direct sources include surface rocks, deep mines (gold mines in South Africa, 3–4 km), deep drilling projects (the Kola hole, ~12 km) and magma from volcanic eruptions. Indirect sources include the rise of temperature, pressure and density with depth, meteors, gravitation, the magnetic field and, most importantly, seismic (earthquake) waves. Earthquakes release energy along a fault, producing body waves (faster P-waves that pass through solids, liquids and gases, and slower S-waves that pass only through solids) and the most destructive surface waves. Because S-waves cannot cross the liquid outer core, a shadow zone appears between 105° and 145° from the epicentre, proving the core is partly liquid. The interior is layered into the crust (5 km oceanic, ~30 km continental, up to 70 km in the Himalayas), the mantle (to 2,900 km, containing the weak asthenosphere — source of magma), and the core (liquid outer core, solid inner core, made of nickel and iron – the ‘nife’ layer). The chapter ends with types of volcanoes and the intrusive landforms (batholiths, laccoliths, lapoliths, phacoliths, sills, dykes).

Key Terms & Concepts

Direct sources: materials available for actual observation — surface rocks, rocks from mines, samples from deep drilling projects (Deep/Integrated Ocean Drilling Projects, Kola deep drill ~12 km), and magma ejected during volcanic eruptions.

Indirect sources: evidence inferred from properties of matter — rise of temperature, pressure and density with depth, meteors, gravitation and gravity anomalies, magnetic surveys, and seismic (earthquake) waves.

Earthquake & fault: an earthquake is the shaking of the earth caused by the release of energy along a fault (a sharp break in crustal rocks) when locked rock blocks suddenly slide past one another.

Focus (hypocentre) & epicentre: the focus is the point inside the earth where energy is released; the epicentre is the point on the surface directly above the focus and the first to feel the waves.

Body waves: waves generated at the focus that travel through the body of the earth — P-waves (primary, fastest, like sound waves, travel through solid, liquid and gas, vibrate parallel to direction of travel) and S-waves (secondary, slower, travel only through solids, vibrate perpendicular to direction of travel).

Surface waves: waves generated when body waves interact with surface rocks; they travel along the surface, are the last to be recorded, and are the most destructive.

Shadow zone: the band of the earth’s surface where particular earthquake waves are not recorded — the zone beyond 105° receives no S-waves, while P-waves are absent in a band between 105° and 145° from the epicentre.

Richter & Mercalli scales: magnitude (energy released) is measured on the Richter scale (0–10); intensity (visible damage) is measured on the Mercalli scale (1–12).

Crust, mantle, core: the crust is the outermost brittle solid layer (5 km oceanic, ~30 km continental, up to 70 km in the Himalayas); the mantle extends from the Moho discontinuity to 2,900 km and holds the weak asthenosphere; the core (beyond 2,900 km) has a liquid outer part and a solid inner part of nickel and iron (the ‘nife’ layer).

Lithosphere & asthenosphere: the lithosphere is the crust plus the uppermost solid mantle (about 10–200 km thick); the asthenosphere is the weak, partly molten upper-mantle zone (up to ~400 km) that is the chief source of magma.

Magma & lava: molten rock in the upper mantle is magma; once it starts moving towards or reaches the surface it is called lava.

Volcano types: shield, composite, caldera, flood basalt province and mid-ocean ridge volcanoes, classified by the nature of eruption and the form developed at the surface.

Intrusive forms: shapes formed when lava cools within the crust — batholiths, laccoliths, lapoliths, phacoliths, sills/sheets and dykes.

NCERT “Exercises” — Full Solutions

All questions below are reproduced verbatim from the NCERT textbook’s end-of-chapter Exercises section. Answers are original, written in exam-ready style.

1. Multiple choice questions.

(i) Which one of the following earthquake waves is more destructive? (a) P-waves    (b) S-waves    (c) Surface waves    (d) None of the above

ANSWER (c) Surface waves. Surface waves are generated when body waves interact with surface rocks and travel along the earth’s surface. They are the last to be recorded on the seismograph but cause the greatest displacement of rocks, leading to the collapse of structures — hence they are the most destructive.

(ii) Which one of the following is a direct source of information about the interior of the earth? (a) Earthquake waves    (b) Volcanoes    (c) Gravitational force    (d) Earth magnetism

ANSWER (b) Volcanoes. During a volcanic eruption, molten material (magma) is thrown onto the surface and becomes available for direct laboratory analysis. Earthquake waves, gravitational force and earth magnetism are all indirect sources.

(iii) Which type of volcanic eruptions have caused Deccan Trap formations? (a) Shield    (b) Flood    (c) Composite    (d) Caldera

ANSWER (b) Flood. The Deccan Traps, which cover most of the Maharashtra plateau, are a flood basalt province formed by highly fluid lava that flowed over long distances, building up thick basalt sheets in successive flows.

(iv) Which one of the following describes the lithosphere: (a) upper and lower mantle    (b) crust and upper mantle    (c) crust and core    (d) mantle and core

ANSWER (b) crust and upper mantle. The lithosphere is made up of the crust together with the uppermost (solid) part of the mantle, with a thickness ranging from about 10 to 200 km.

2. Answer the following questions in about 30 words.

(i) What are body waves?

ANSWER Body waves are earthquake waves generated due to the release of energy at the focus. They move in all directions, travelling through the interior (body) of the earth, and are of two types — P-waves and S-waves.

(ii) Name the direct sources of information about the interior of the earth.

ANSWER The direct sources are surface rocks, rocks obtained from mining areas, samples collected through deep drilling projects (such as the Kola deep drill), and magma brought to the surface during volcanic eruptions.

(iii) Why do earthquake waves develop shadow zone?

ANSWER A shadow zone develops because earthquake waves are reflected or refracted when they meet materials of different densities. S-waves cannot pass through the liquid outer core, so they are blocked, while P-waves are bent, leaving zones where waves are not recorded.

(iv) Briefly explain the indirect sources of information of the interior of the earth other than those of seismic activity.

ANSWER Apart from seismic activity, indirect sources include the increase of temperature, pressure and density with depth; meteors, whose material resembles the earth’s; and gravitation (gravity anomalies) and magnetic surveys, which reveal the distribution of mass and minerals within the crust.

3. Answer the following questions in about 150 words.

(i) What are the effects of propagation of earthquake waves on the rock mass through which they travel?

ANSWER As earthquake waves propagate, they set up vibrations in the rocks through which they pass, and different waves affect the rock mass in different ways. P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of propagation, creating density differences — that is, alternate stretching and squeezing (compression) of the rock material. S-waves and the other transverse waves vibrate perpendicular to the direction of propagation. The vibration of S-waves is in the vertical plane, so they generate troughs and crests in the material through which they pass. Surface waves, which develop when body waves strike the surface rocks, are the most damaging. They cause the displacement of rocks, and hence lead to the collapse of structures on the surface. In addition, the velocity and direction of all these waves change as they travel through materials of different densities — the denser the material, the higher the velocity — and the waves are reflected and refracted at boundaries between layers.

(ii) What do you understand by intrusive forms? Briefly describe various intrusive forms.

ANSWER Intrusive forms are the landforms created when lava cools and solidifies within the crustal portions of the earth instead of reaching the surface. Rocks formed this way are plutonic (intrusive) igneous rocks, and they take on different shapes depending on where the lava settles. The chief intrusive forms are: Batholiths: large domes of magmatic, granitic material that cool at great depth. They are exposed at the surface only after denudation removes the overlying rocks, and they represent the cooled bases of magma chambers. Laccoliths: large dome-shaped intrusive bodies with a level base, connected by a pipe-like conduit from below; they resemble surface composite-volcano domes but lie at deeper levels (e.g. the granite domal hills of the Karnataka plateau). Lapoliths: when intruding lava spreads horizontally and develops a saucer shape, concave to the sky, it forms a lapolith. Phacoliths: wavy masses of intrusive rock found at the base of synclines or the crest of anticlines in folded country, with a definite conduit to a magma chamber beneath. Sills and sheets: near-horizontal sheets of intrusive rock — the thicker deposits are called sills and the thinner ones sheets. Dykes: when lava solidifies within near-vertical cracks and fissures, it forms wall-like structures called dykes (very common in western Maharashtra, where they fed the Deccan Trap eruptions).

Extra Practice Questions

Short Answer Type Questions

Q1. Differentiate between the focus and the epicentre of an earthquake.

ANSWERThe focus (or hypocentre) is the point inside the earth, along the fault, where the energy of an earthquake is released. The epicentre is the point on the earth’s surface lying directly above the focus; it is the first to experience the seismic waves.

Q2. How are earthquakes measured?

ANSWEREarthquakes are scaled either by magnitude or intensity. The Richter scale (0–10) measures the magnitude, that is, the energy released during the quake, expressed in numbers. The Mercalli scale (1–12) measures the intensity, based on the visible damage caused.

Q3. Why are S-waves important in the study of the earth’s interior?

ANSWERS-waves travel only through solid materials. Because they cannot pass through the liquid outer core, their absence beyond 105° from the epicentre helped scientists infer that the outer core is in a liquid state, revealing the layered structure of the interior.

Q4. What is the asthenosphere and why is it important?

ANSWERThe asthenosphere is the upper, weaker portion of the mantle (‘astheno’ means weak), extending to about 400 km. It is important because it is the main source of magma that finds its way to the surface during volcanic eruptions.

Q5. What is the ‘nife’ layer?

ANSWERThe ‘nife’ layer is another name for the earth’s core. It is made up of very heavy material constituted mostly by nickel (Ni) and iron (Fe), from which the name ‘nife’ is derived; its outer part is liquid and inner part solid.

Long Answer Type Questions

Q1. Describe the layered structure of the earth’s interior.

ANSWERThe earth’s interior is divided into three main layers. The crust is the outermost, brittle solid part; the oceanic crust has a mean thickness of about 5 km, the continental crust about 30 km, rising to as much as 70 km in mountain regions like the Himalayas. Beyond the crust, separated from it by the Moho discontinuity, lies the mantle, which extends down to a depth of 2,900 km. The upper part of the mantle is the asthenosphere (up to ~400 km), a weak zone that is the main source of magma; the crust and the uppermost solid mantle together form the lithosphere (10–200 km thick), while the lower mantle is solid. The innermost layer is the core, beyond the core-mantle boundary at 2,900 km; its outer part is liquid and its inner part solid. The core is made of very heavy material, chiefly nickel and iron, and is therefore called the ‘nife’ layer.

Q2. Explain the different types of volcanoes on the basis of the nature of eruption and the form developed at the surface.

ANSWERVolcanoes are classified by their eruptions and the forms they build. Shield volcanoes are the largest; built of very fluid basaltic lava, they have gentle, non-steep slopes and are usually low in explosivity (e.g. the Hawaiian volcanoes), though they become explosive if water enters the vent, forming cinder cones. Composite volcanoes erupt cooler, more viscous lava along with large amounts of pyroclastic material and ash; the layers accumulate near the vent, building tall, layered cones, and eruptions are often explosive. Calderas are the most explosive; they are so violent that on eruption they collapse into themselves, forming large depressions called calderas, indicating a huge nearby magma chamber. Flood basalt provinces outpour highly fluid lava that travels long distances, building thick basalt sheets over thousands of square kilometres (e.g. the Deccan Traps). Mid-ocean ridge volcanoes occur along the 70,000-km-long system of mid-ocean ridges, whose central portions experience frequent eruptions.

MCQs & Assertion–Reason

1. The radius of the earth is approximately:

(a) 3,478 km    (b) 6,378 km    (c) 9,500 km    (d) 12,756 km

2. Which earthquake waves can travel through solids, liquids and gases?

(a) S-waves    (b) Surface waves    (c) P-waves    (d) None of these

3. The point within the earth where the energy of an earthquake is released is called the:

(a) epicentre    (b) focus    (c) fault    (d) shadow zone

4. The shadow zone for both P-waves and S-waves lies between:

(a) 0° and 105°    (b) 105° and 145°    (c) 145° and 180°    (d) beyond 180°

5. The boundary between the crust and the mantle is known as the:

(a) shadow zone    (b) core-mantle boundary    (c) Moho discontinuity    (d) asthenosphere

6. The mantle extends up to a depth of about:

(a) 400 km    (b) 2,900 km    (c) 5,000 km    (d) 6,378 km

7. The intensity of an earthquake is measured on the:

(a) Richter scale    (b) Mercalli scale    (c) Beaufort scale    (d) Kelvin scale

8. The core of the earth is mainly composed of:

(a) silica and aluminium    (b) silica and magnesium    (c) nickel and iron    (d) basalt and granite

9. The largest of all volcanoes on the earth (barring basalt flows) are:

(a) composite volcanoes    (b) shield volcanoes    (c) calderas    (d) cinder cones

10. A wall-like intrusive form that solidifies almost perpendicular to the ground is called a:

(a) sill    (b) laccolith    (c) dyke    (d) lapolith

Answer key: 1-(b), 2-(c), 3-(b), 4-(b), 5-(c), 6-(b), 7-(b), 8-(c), 9-(b), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Surface waves are the most destructive earthquake waves.

Reason: Surface waves travel along the surface and cause the displacement of rocks, leading to the collapse of structures.

A-R 2. Assertion: S-waves help prove that the outer core is in a liquid state.

Reason: S-waves can travel only through solid materials and are blocked by the liquid outer core.

A-R 3. Assertion: Volcanoes are a direct source of information about the earth’s interior.

Reason: During eruptions, magma is thrown onto the surface and becomes available for laboratory analysis.

A-R 4. Assertion: P-waves arrive at the surface after S-waves.

Reason: P-waves are primary waves and are the fastest of all earthquake waves.

A-R 5. Assertion: The continental crust is thicker than the oceanic crust.

Reason: The continental crust is thickest in areas of major mountain systems, reaching up to about 70 km in the Himalayan region.

Answer key: 1-(A), 2-(A), 3-(A), 4-(D), 5-(B).

Exam Tips & Common Mistakes

How to score full marks in this chapter

Keep a clear two-column comparison of P-waves vs S-waves (speed, medium, direction of vibration) and of direct vs indirect sources ready for objective and short questions. For the shadow zone, remember the figures — no S-waves beyond 105° and the P-wave shadow band between 105° and 145°. Memorise the key depths: crust 5/30/70 km, mantle to 2,900 km, asthenosphere ~400 km, lithosphere 10–200 km. For the 150-word intrusive-forms answer, name and define all six forms (batholith, laccolith, lapolith, phacolith, sill/sheet, dyke) with one Indian example each (Deccan Traps, Karnataka granite domes).

Common mistakes to avoid

  • Swapping P-waves and S-waves — remember P (primary) are faster and pass through all media; S (secondary) are slower and pass only through solids.
  • Confusing the focus (inside the earth) with the epicentre (on the surface).
  • Mixing up the Richter scale (magnitude/energy, 0–10) with the Mercalli scale (intensity/damage, 1–12).
  • Describing the lithosphere as the crust alone — it is the crust plus the uppermost mantle.
  • Calling volcanoes an indirect source — they are a direct source of information.
  • Confusing extrusive forms (cooling at the surface) with intrusive forms (cooling within the crust).

Frequently Asked Questions

What is Chapter 3 of Class 11 Geography (Fundamentals of Physical Geography) about?

Chapter 3, Interior of the Earth, explains the direct and indirect sources of information about the earth’s interior, how earthquakes and seismic waves (P, S and surface waves) reveal its layered structure, why a shadow zone forms, the arrangement of crust, mantle and core, and the types of volcanoes and intrusive volcanic landforms.

What is the difference between P-waves and S-waves?

P-waves (primary waves) are the fastest, vibrate parallel to the direction of travel and can pass through solids, liquids and gases. S-waves (secondary waves) are slower, vibrate perpendicular to the direction of travel and can pass only through solid materials, which is why they are blocked by the liquid outer core.

How many questions are in the Class 11 Geography Chapter 3 exercise?

The NCERT Exercises for Chapter 3 have three questions: one multiple-choice question with four parts, four short-answer questions (about 30 words each) and two long-answer questions (about 150 words each) — all solved step by step on this page.

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