Interior of the Earth, Class 11 Geography: Earthquakes, Structure and Volcanoes

What Interior of the Earth is about

Weightage: This chapter is part of Unit II, The Earth, of Fundamentals of Physical Geography, which carries 9 of the 70 theory marks in the school annual exam together with The Origin and Evolution of the Earth and Distribution of Oceans and Continents. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Direct and indirect sources of information

NCERT section Sources of Information about the Interior

Direct sources (material we can actually examine):

• Surface rocks and mine rocks: gold mines in South Africa reach 3-4 km; deeper is too hot.
• Deep drilling projects: the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project; the deepest drill at Kola (Arctic Ocean) has reached 12 km.
• Volcanic eruptions: magma thrown onto the surface can be analysed in the laboratory, but the depth of its source is hard to know.

Indirect sources (inferences):

• Temperature, pressure and density all increase with depth (known from mining); knowing their rate of change and the earth's thickness, scientists estimate values at each depth.
• Meteors: not material from the earth's interior, but solid bodies formed of the same or similar material, with similar structure.
• Gravitation: g is greater at the poles, less at the equator, because the equator is farther from the centre. g also depends on the mass of material; the difference between expected and observed readings is a gravity anomaly, which reveals the distribution of mass in the crust.
• Magnetic surveys: show the distribution of magnetic materials in the crust.
• Seismic activity: the most important source; seismic waves give a complete picture of the layered interior.

Worked example

Why are volcanic eruptions a direct source but an imperfect one? (2 marks)

Try it first, then show the solutionSolution

1. They bring actual molten material (magma) to the surface where it can be tested in a laboratory: a direct source.
2. But we cannot know exactly from what depth the magma came, so we cannot tie the sample to a particular layer.

Answer: Direct because real material is obtained; imperfect because the depth of its source is uncertain.

Earthquakes: fault, focus and epicentre

NCERT section Earthquake

Earthquake: shaking of the earth; a natural event caused by the release of energy that sends waves in all directions.

Why the earth shakes:

• A fault is a sharp break in crustal rocks; rocks on either side tend to move in opposite directions.
• The weight of overlying strata presses them so friction locks them together.
• Eventually the tendency to move overcomes friction; blocks deform and then slide past one another abruptly, releasing energy.

Terms:

• Focus (hypocentre): the point where energy is released.
• Epicentre: the point on the surface nearest to the focus, directly above it; the first to experience the waves.

Where: all natural earthquakes occur in the lithosphere (up to 200 km depth). The seismograph records waves reaching the surface.

Worked example

An earthquake's focus lies 30 km below a village. Where is its epicentre, and why is that village shaken first? (2 marks)

Try it first, then show the solutionSolution

1. The epicentre is the surface point directly above the focus, i.e. at the village.
2. It is the nearest surface point to the focus, so waves reach it before any other place.

Answer: At the village directly above the focus; it is closest to the focus.

Earthquake waves and their propagation

NCERT section Earthquake Waves

Two main groups:

• Body waves: generated at the focus, travel in all directions through the body of the earth.
• Surface waves: generated when body waves interact with surface rocks; travel along the surface.

	P-waves (primary)	S-waves (secondary)	Surface waves
Arrival	First (fastest)	After a time lag	Last
Medium	Solid, liquid and gas (like sound waves)	Only solids	Along the surface
Vibration	Parallel to direction of travel: squeezing and stretching (density differences)	Perpendicular in the vertical plane: crests and troughs	Perpendicular to travel
Damage	Less	Moderate	Most destructive: displace rocks, collapse structures

Velocity and direction: velocity rises with density of material; waves reflect (rebound) or refract (change direction) when they meet materials of different density. These changes, read from seismograph records, reveal the layers.

Why S-waves matter: because they cannot pass through liquid, their absence beyond a certain distance revealed a liquid outer core.

Worked example

A seismograph records three wave sets: first, then after a lag, then a last, high-amplitude set. Identify them in order and state which causes buildings to collapse. (2 marks)

Try it first, then show the solutionSolution

1. First: P-waves (fastest); second: S-waves (arrive after a lag); last: surface waves.
2. Surface waves are the most destructive: they displace rocks and cause structures to collapse.

Answer: P, S, surface; surface waves cause collapse.

Shadow zones

NCERT section Emergence of Shadow Zone

Shadow zone: an area where seismographs do not record a particular earthquake wave. Each earthquake has its own shadow zone, which can be drawn if the epicentre is known.

Observations:

• Within 105° of the epicentre: both P and S-waves are recorded.
• Beyond 145°: P-waves are recorded, S-waves are not.
• Between 105° and 145°: neither wave arrives, so this band is the shadow zone for both types.
• The entire zone beyond 105° gets no S-waves.

Comparison:

• P-wave shadow zone: a band around the earth between 105° and 145° from the epicentre.
• S-wave shadow zone: much larger: everything beyond 105°, a little over 40 per cent of the earth's surface.

Why it forms (reasoning): S-waves cannot pass through the liquid outer core, so they vanish beyond 105°; P-waves are refracted at the core-mantle boundary because of the density change, so they miss the 105°-145° band and reappear beyond 145°.

Worked example

Seismograph X is 120° from an epicentre and Y is 160° away. Which waves does each record? (2 marks)

Try it first, then show the solutionSolution

1. 120° lies in the 105°-145° band: neither P nor S-waves arrive at X.
2. 160° is beyond 145°: P-waves arrive at Y, but S-waves do not, since nothing beyond 105° receives S-waves.

Answer: X: no P or S-waves; Y: P-waves only.

Types, measurement, effects and frequency of earthquakes

NCERT section Types of Earthquakes; Measuring Earthquakes

Types (five):
1. Tectonic: the most common; sliding of rocks along a fault plane.
2. Volcanic: a special class of tectonic quake, confined to areas of active volcanoes.
3. Collapse: minor tremors when roofs of underground mines collapse in intense mining areas.
4. Explosion: caused by chemical or nuclear explosions.
5. Reservoir induced: in areas of large reservoirs.

Measurement:

Scale	Measures	Range
Richter	Magnitude: energy released	0-10
Mercalli (Italian seismologist)	Intensity: visible damage	1-12

Effects (twelve): ground shaking, differential ground settlement, land and mud slides, soil liquefaction, ground lurching, avalanches, ground displacement, floods from dam and levee failures, fires, structural collapse, falling objects, tsunami. The first six bear on landforms; the rest concern life and property.

• A tsunami occurs only if the epicentre is under ocean water and the magnitude is high enough; a tsunami is a wave generated by the tremor, not an earthquake itself.
• Shaking lasts only seconds but is devastating if magnitude is above 5 on the Richter scale.

Frequency: not all parts of the globe get major shocks. Quakes of 8+ are rare (about once in 1-2 years); 'tiny' quakes occur almost every minute.

Worked example

A tremor near a newly filled large dam is felt in nearby villages. Name the type of earthquake and the scale that would record how much damage it caused. (2 marks)

Try it first, then show the solutionSolution

1. Earthquakes in areas of large reservoirs are reservoir induced earthquakes.
2. Visible damage is measured by intensity, on the Mercalli scale (1-12).

Answer: Reservoir induced earthquake; Mercalli intensity scale.

Structure of the earth: crust, mantle and core

NCERT section Structure of the Earth

Crust

• Outermost solid part; brittle.
• Oceanic crust is thinner (mean 5 km); continental crust about 30 km, thicker under major mountains: up to 70 km in the Himalayan region.

Mantle

• From the Moho discontinuity to 2,900 km depth; denser than the crust.
• Upper part: asthenosphere ('astheno' = weak), up to about 400 km; the main source of magma for eruptions.
• Lithosphere = crust + uppermost mantle, 10-200 km thick.
• Lower mantle, beyond the asthenosphere, is solid.

Core

• Earthquake wave velocities revealed its existence.
• Core-mantle boundary at 2,900 km.
• Outer core: liquid; inner core: solid.
• Made of very heavy material, mostly nickel and iron: the nife layer.

Worked example

Using the radius of 6,378 km and the core-mantle boundary at 2,900 km, find the radius of the core. Which wave behaviour shows that its outer part is liquid? (2 marks)

Try it first, then show the solutionSolution

1. Core radius = 6,378 - 2,900 = 3,478 km.
2. S-waves, which travel only through solids, are not recorded beyond 105° from the epicentre: they do not pass through the outer core.

Answer: About 3,478 km; the S-wave shadow zone shows the outer core is liquid.

Volcanoes and their types

NCERT section Volcanoes and Volcanic Landforms

Volcano: a place where gases, ash and/or molten rock (lava) escape to the ground. Active if material is being released or was released in the recent past.

Magma and lava: molten material in the upper mantle (asthenosphere) is magma; once it moves towards the crust or reaches the surface it is lava. Erupted material: lava flows, pyroclastic debris, volcanic bombs, ash and dust, and gases (nitrogen and sulphur compounds, minor chlorine, hydrogen and argon).

Types (by nature of eruption and surface form):

Type	Lava and eruption	Form	Example
Shield	Very fluid basalt; low explosivity (explosive if water enters the vent)	Largest (barring basalt flows), not steep; lava fountain can build a cinder cone	Hawaii
Composite	Cooler, more viscous lava; explosive; much pyroclastic material and ash	Layers accumulate near the vent	-
Caldera	Most explosive; huge magma chamber close below	Collapse on themselves forming a depression (caldera) rather than a tall cone	-
Flood basalt provinces	Highly fluid lava over long distances; flows can exceed 50 m thick and run hundreds of km	Thousands of sq km of basalt	Deccan Traps (most of the Maharashtra plateau)
Mid-ocean ridge	Frequent eruptions in the central ridge	Ridge system over 70,000 km long through all ocean basins	Oceans

Worked example

Why are shield volcanoes not steep, while composite volcanoes are? (2 marks)

Try it first, then show the solutionSolution

1. Shield volcanoes erupt very fluid basalt that spreads far before solidifying, giving gentle slopes.
2. Composite volcanoes erupt cooler, more viscous lava with pyroclastic material that piles up in layers close to the vent, building steeper cones.

Answer: Fluid basalt spreads (gentle shield); viscous lava and ash pile up near the vent (steep composite).

Volcanic landforms: intrusive forms

NCERT section Volcanic Landforms

Igneous rocks form from cooling lava: volcanic rocks cool at the surface; plutonic rocks cool in the crust. Lava that cools inside the crust forms intrusive forms (Figure 3.4).

• Batholith: large body of magma cooled deep in the crust as a large dome; exposed only after denudation removes the cover; covers large areas, can be several km deep; granitic; the cooled portion of magma chambers.
• Laccolith: large dome-shaped body with a level base, connected to the source by a pipe-like conduit; like a surface dome of a composite volcano but deeper; a localised source of lava. Many exfoliated granite domal hills of the Karnataka plateau are laccoliths or batholiths.
• Lapolith: saucer-shaped, concave to the sky.
• Phacolith: a wavy mass at the base of synclines or top of anticlines in folded igneous country, with a conduit to a magma chamber below.
• Sill and sheet: near-horizontal intrusive bodies; thick = sill, thin = sheet.
• Dyke: lava rising through cracks and fissures solidifies almost perpendicular to the ground as a wall-like structure; the most common intrusive form in western Maharashtra, regarded as feeders of the eruptions that built the Deccan Traps.

Worked example

Distinguish between a sill and a dyke. (2 marks)

Try it first, then show the solutionSolution

1. A sill is a near-horizontal sheet of intrusive igneous rock (thick; a thin one is a sheet).
2. A dyke is a wall-like body that solidified almost perpendicular to the ground in cracks and fissures.

Answer: Sill: horizontal; dyke: vertical, wall-like.

Interior of the Earth in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Layers of the Earth, Volcanoes and Intrusive Landforms
• Earthquakes: Waves, Shadow Zones, Types and Measurement

Interior of the Earth: key facts

Depth records

Earth radius about 6,378 km; South African gold mines 3-4 km; Kola drill 12 km

Direct sources reach only a tiny fraction of the radius.

Gravity

$ggreateratpoles,lessatequator;observedg-expectedg=gravityanomaly$

Anomalies show mass distribution in the crust.

Wave order

1. 1.P (first, all media)
2. 2.S (lag, solids only)
3. 3.surface (last, most destructive)

P vibrates parallel; S perpendicular in vertical plane.

Shadow zone

Both waves within 105°; P absent 105°-145°; S absent beyond 105°

S shadow zone: a little over 40% of the earth's surface.

Scales

$Richter=magnitude(energy),0-10;Mercalli=intensity(damage),1-12$

Devastating if magnitude above 5.

Layer depths

• •Crust: oceanic 5 km, continental 30 km, Himalaya 70 km
• •lithosphere 10-200 km
• •asthenosphere to 400 km
• •mantle to 2,900 km
• •core below

Outer core liquid, inner core solid, nickel and iron (nife).

Earthquake types

Tectonic, volcanic, collapse, explosion, reservoir induced

Tectonic is the most common.

Intrusive forms by shape

$Domedeep=batholith;domewithlevelbase=laccolith;saucer=lapolith;wavy=phacolith;horizontal=sill/sheet;verticalwall=dyke$

Volcanic rocks cool at the surface, plutonic rocks in the crust.

Volcano types

Shield	fluid basalt, Hawaii
composite	viscous, layered
caldera	most explosive, collapse
flood basalt	Deccan Traps
mid-ocean ridge	over 70,000 km

Classified by eruption nature and surface form.

Interior of the Earth: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Interior of the Earth

The question patterns to expect, and the method that works for each.

Tricks to remember Interior of the Earth

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Wave order

  'P-S-L: Please Stay Low': P first, S second, surface (L) last and most damaging.

  Use it when: Order and damage of waves.

• Shadow zone numbers

  'One-oh-five, S can't survive; one-four-five, P's alive'.

  Use it when: Degrees for P and S shadow zones.

• Earthquake types

  'TV CER': Tectonic, Volcanic, Collapse, Explosion, Reservoir induced.

  Use it when: Listing the five types.

• Layer depths

  '5-30-70, 200-400, 2900': oceanic, continental, Himalayan crust; lithosphere, asthenosphere; mantle base.

  Use it when: Structure diagram and MCQs.

• Intrusive shapes

  'Big Loaf, Low Plate, Phat Wave, Sill Sits, Dyke Divides': batholith, laccolith, lapolith, phacolith, sill, dyke.

  Use it when: Intrusive landforms.

Interior of the Earth: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Interior of the Earth

The slips examiners see most often, and what to write instead.

• ✗Mistake: Mixing up focus and epicentre

  Both are 'the centre' in everyday speech.

  Wrong working

  The epicentre is the point inside the earth where energy is released.

  Right working

  The focus (hypocentre) is where energy is released; the epicentre is the surface point directly above it.

  ✓Do this: Epi = upon (on the surface).

  Usually costs: 1 mark in a definition; can spoil a whole 3-mark answer.

• ✗Mistake: Saying S-waves travel through liquids

  Students generalise from P-waves.

  Wrong working

  Both P and S-waves pass through all materials.

  Right working

  P-waves pass through solids, liquids and gases; S-waves only through solids.

  ✓Do this: S = Solid only.

  Usually costs: 1-2 marks; it breaks the shadow-zone explanation.

• ✗Mistake: Swapping Richter and Mercalli

  Both measure earthquakes.

  Wrong working

  Mercalli measures energy on a 0-10 scale.

  Right working

  Richter: magnitude (energy), 0-10. Mercalli: intensity (visible damage), 1-12.

  ✓Do this: Mercalli = Mess (damage).

  Usually costs: 1 mark per MCQ.

• ✗Mistake: Giving wrong shadow-zone degrees

  Numbers 105 and 145 get swapped.

  Wrong working

  S-waves are absent between 105° and 145° only.

  Right working

  S-waves are absent everywhere beyond 105°; P-waves are absent between 105° and 145°.

  ✓Do this: S starts at 105° and never comes back.

  Usually costs: 1 mark in a 3-mark answer.

• ✗Mistake: Calling sills vertical

  Sills and dykes are confused.

  Wrong working

  Sills are wall-like vertical bodies.

  Right working

  Sills are near-horizontal (thin ones are sheets); dykes are vertical walls.

  ✓Do this: A window sill is horizontal.

  Usually costs: 1 mark per form described.

NCERT exercise guide: Interior of the Earth

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Interior of the Earth

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Rock samples brought up from a deep mine and lava erupted from a volcano have one thing in common as evidence. What is it?
   (a) Both are indirect sources
   (b) Both are direct sources of material from inside the earth
   (c) Both are seismic evidence
   (d) Both are gravity anomalies

   Hint

   Can you hold the material in your hand?

   Answer

   (b) Both are direct sources: actual material from below the surface.

2. Q2 · 1 mark · MCQExam level

   A plate of the lithosphere slides over a weak, partly molten layer. Which pair correctly names the plate and the layer below it?
   (a) Mantle; outer core
   (b) Crust and uppermost mantle; asthenosphere
   (c) Crust alone; lower mantle
   (d) Crust and core; mantle

   Hint

   Astheno means weak.

   Answer

   (b) The lithosphere is the crust with the uppermost mantle; it rests on the asthenosphere (up to about 400 km), the main source of magma.

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Gravity is greater at the poles than at the equator.
   Reason (R): The density of crustal rock is greater at the poles.
   Choose (a)-(d).

   Hint

   What reason does the chapter give?

   Answer

   (c). A is true, but the reason is that the equator is farther from the centre, not polar rock density.

4. Q4 · 2 marks · Short answerExam level

   Why is the asthenosphere important for volcanoes? (2 marks)

   Hint

   'Astheno' means weak.

   Answer

   The asthenosphere is the weak upper part of the mantle, extending to about 400 km. It is the main source of magma that reaches the surface in eruptions.

5. Q5 · 2 marks · Short answerHarder exam level

   The mean continental crust is 30 km thick and the crust under the Himalayas is 70 km. By what percentage is the Himalayan crust thicker than the mean? Give one reason the chapter implies. (2 marks)

   Hint

   (70 - 30) / 30 × 100.

   Answer

   (70 - 30) / 30 × 100 = 133% thicker. The chapter says continental crust is thicker in areas of major mountain systems.

6. Q6 · 3 marks · Short answerHarder exam level

   Compare shield and caldera volcanoes on lava type, explosivity and form. (3 marks)

   Hint

   One is the gentlest, one the most explosive.

   Answer

   Shield: very fluid basalt, low explosivity (unless water enters the vent), large gently sloping cones, e.g. Hawaii. Caldera: most explosive, fed by a huge magma chamber close below; they collapse on themselves to form a depression (caldera) instead of a tall cone.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a country wants to locate a new nuclear power plant. Using the chapter, which three earthquake effects should its planners study most carefully, and why? (3 marks)

   Hint

   Choose effects on structures and on landforms.

   Answer

   Any three justified, e.g. ground shaking and ground displacement (can crack the reactor building), soil liquefaction or differential ground settlement (foundations sink unevenly), tsunami if coastal (flooding), floods from dam failures if downstream of a reservoir.

8. Q8 · 5 marks · Long answerHarder exam level

   Describe the structure of the earth. (5 marks)

   Hint

   Crust, mantle, lithosphere, asthenosphere, core.

   Answer

   Crust: brittle, oceanic 5 km, continental 30 km, up to 70 km in the Himalayas. Mantle: from the Moho to 2,900 km; asthenosphere to 400 km is weak and the main magma source; lower mantle solid. Lithosphere: crust plus uppermost mantle, 10-200 km. Core: boundary at 2,900 km; outer core liquid, inner core solid; nickel and iron (nife). Wave velocities revealed the core.

9. Q9 · 3 marks · Short answerHOTS / competency

   A tremor is felt in a city 150° from the epicentre of a distant earthquake. A student says 'S-waves must have reached us first'. Correct the student with reasons. (3 marks)

   Hint

   Two facts: order of arrival, and S-wave shadow.

   Answer

   P-waves always arrive before S-waves because they are faster. Moreover, at 150° (beyond 105°) no S-waves arrive at all, because they cannot cross the liquid outer core. So the city receives P-waves (refracted through the core), and later surface effects, but never S-waves.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.How deep has the Kola drill reached?Show answer

   12 km.

2. 2.What is a fault?Show answer

   A sharp break in crustal rocks along which rocks tend to move in opposite directions.

3. 3.Which waves are similar to sound waves?Show answer

   P-waves.

4. 4.What share of the earth's surface is the S-wave shadow zone?Show answer

   A little over 40 per cent.

5. 5.What is the range of the Mercalli scale?Show answer

   1-12.

6. 6.Why is the core called nife?Show answer

   It is made mostly of nickel (Ni) and iron (Fe).

7. 7.When is magma called lava?Show answer

   Once it starts moving towards the crust or reaches the surface.

8. 8.Which intrusive form fed the Deccan Traps eruptions?Show answer

   Dykes.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Radius about 6,378 km; mines 3-4 km; Kola drill 12 km.
• ☐Indirect sources: temperature-pressure-density, meteors, gravity anomaly, magnetic surveys, seismic waves.
• ☐Fault → friction locks → abrupt slip → energy at focus → epicentre first.
• ☐P: fastest, all media, parallel vibration. S: solids only, perpendicular. Surface: last, most destructive.
• ☐Shadow: both within 105°; P absent 105°-145°; S absent beyond 105° (over 40% of surface).
• ☐Types: tectonic, volcanic, collapse, explosion, reservoir induced.
• ☐Richter magnitude 0-10; Mercalli intensity 1-12; devastating above 5; 8+ rare, tiny every minute.
• ☐Crust 5/30/70 km; lithosphere 10-200 km; asthenosphere to 400 km; mantle to 2,900 km; core nife, outer liquid, inner solid.
• ☐Volcanoes: shield, composite, caldera, flood basalt (Deccan Traps), mid-ocean ridge (over 70,000 km).
• ☐Intrusive: batholith, laccolith, lapolith, phacolith, sill/sheet, dyke.

Interior of the Earth: frequently asked questions