Earth's Structure ICSE Class 9: Crust, Mantle and Core

What Earth's Structure is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Earth's Structure is topic 2(a), Structure of the Earth, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

How we know about the interior

Direct evidence is limited: deep mines and boreholes reach only a few kilometres. They show that temperature rises with depth, roughly 1 °C for every 32 m in the upper crust.

Indirect evidence:
1. Seismic waves: earthquake waves travel at different speeds through materials of different density. P-waves (primary) pass through solids and liquids; S-waves (secondary) pass only through solids. S-waves do not pass through the outer core, which shows it is liquid. Sudden changes in wave speed mark boundaries between layers.
2. Volcanoes bring up molten material (magma) from below the crust.
3. Meteorites, believed to be made of material like that of the earth's interior, are rich in iron and nickel.
4. The earth's average density (about 5.5 g/cm³) is much greater than that of surface rocks (about 2.7 g/cm³), so the interior must contain much denser material.

Worked example

(3 marks) State three sources of information about the interior of the earth.

Try it first, then show the solutionSolution

1. Seismic waves: S-waves do not pass through the liquid outer core - 1 mark.
2. Volcanic eruptions bring up molten material from inside - 1 mark.
3. Meteorites / mines and boreholes / average density of the earth - 1 mark.

Answer: (1) Earthquake waves change speed and direction in different layers; S-waves cannot cross the liquid outer core. (2) Volcanoes bring up magma from below the crust. (3) Meteorites, rich in iron and nickel, and the earth's high average density show the interior is much denser than the surface rocks.

The crust

Meaning: the crust is the thin, outermost solid layer of the earth, on which we live.

Extent: it is the thinnest layer, forming less than 1 per cent of the earth's volume. It is thicker under continents (about 30-40 km on average, and up to about 70 km under high mountains) and thinner under oceans (about 5-10 km).

Composition and types:

• Continental crust (SIAL): made of lighter rocks rich in silica (Si) and alumina (Al), mainly granite; density about 2.7 g/cm³.
• Oceanic crust (SIMA): made of heavier rocks rich in silica (Si) and magnesium (Ma), mainly basalt; density about 3.0 g/cm³. SIMA also lies beneath the SIAL of the continents.

The lighter continental crust 'floats' on the denser layers below.

The lower boundary of the crust is the Mohorovičić discontinuity (Moho), where seismic waves speed up sharply.

Worked example

(3 marks) Distinguish between SIAL and SIMA.

Try it first, then show the solutionSolution

1. SIAL: silica and alumina; SIMA: silica and magnesium - 1 mark.
2. SIAL forms the continents (granite); SIMA forms the ocean floor and lies below SIAL (basalt) - 1 mark.
3. SIAL is lighter (about 2.7 g/cm³); SIMA is denser (about 3.0 g/cm³) - 1 mark.

Answer: SIAL is rich in silica and alumina, forms the continental crust (mainly granite) and is lighter; SIMA is rich in silica and magnesium, forms the ocean floor and the layer beneath the continents (mainly basalt) and is denser.

The mantle

Meaning: the mantle is the thick layer between the crust and the core.

Extent: from the base of the crust (the Moho) to a depth of about 2,900 km. It forms about 84 per cent of the earth's volume, the largest layer.

Composition: dense silicate rocks rich in iron and magnesium (such as peridotite). Density increases with depth, roughly from 3.3 to 5.5 g/cm³.

Parts:

• Upper mantle: its top part, together with the crust, forms the rigid lithosphere. Below this is the asthenosphere (roughly 100-250 km deep), a weak, partly molten, plastic zone on which the lithospheric plates move; it is a source of magma for volcanoes.
• Lower mantle: solid, because the very high pressure keeps the rock solid despite high temperatures.

Slow convection currents in the mantle are thought to help move the plates of the lithosphere.

The lower boundary of the mantle is the Gutenberg discontinuity, at about 2,900 km.

Worked example

(3 marks) Describe the mantle with reference to its extent, composition and the asthenosphere.

Try it first, then show the solutionSolution

1. Extent: from the Moho to about 2,900 km; largest layer by volume - 1 mark.
2. Composition: dense silicate rocks rich in iron and magnesium - 1 mark.
3. Asthenosphere: partly molten zone in the upper mantle; plates move over it; source of magma - 1 mark.

Answer: The mantle extends from below the crust to about 2,900 km and is the largest layer. It is made of dense silicate rocks rich in iron and magnesium. Its upper part contains the asthenosphere, a partly molten plastic zone on which crustal plates move and from which magma rises.

The core

Meaning: the core is the central part of the earth.

Extent: from about 2,900 km depth to the centre at about 6,371 km.

Composition: mainly nickel (Ni) and iron (Fe), hence the name NIFE. It is the densest part, with density up to about 13 g/cm³ near the centre, and the hottest, with temperatures estimated at about 5,000-6,000 °C.

Parts:

• Outer core (about 2,900-5,100 km): liquid. S-waves cannot pass through it. Movement of molten iron here is believed to produce the earth's magnetic field.
• Inner core (about 5,100-6,371 km): solid, despite the great heat, because the pressure is enormous.

Worked example

(3 marks) Why is the core called NIFE? Why is the inner core solid although it is extremely hot?

Try it first, then show the solutionSolution

1. Composed mainly of nickel (Ni) and iron (Fe) - 1 mark.
2. Temperature is very high (about 5,000-6,000 °C) - 1 mark.
3. Enormous pressure prevents the material from melting, so the inner core is solid - 1 mark.

Answer: The core is called NIFE because it is made mainly of nickel and iron. The inner core is extremely hot, but the enormous pressure at the centre of the earth prevents its material from melting, so it remains solid.

Discontinuities and changes with depth

Boundaries between layers are called discontinuities, because the speed of seismic waves changes suddenly there.

• Mohorovičić discontinuity (Moho): between the crust and the mantle; named after the Croatian scientist Andrija Mohorovičić.
• Gutenberg discontinuity: between the mantle and the core, at about 2,900 km; named after Beno Gutenberg.

With depth:

• temperature increases (heat from radioactive decay and from the earth's formation);
• pressure increases because of the weight of overlying rocks;
• density increases, from about 2.7 g/cm³ in the crust to about 13 g/cm³ in the inner core.

The heaviest materials (iron and nickel) lie at the centre and the lightest (SIAL) at the surface, a pattern explained by heavier materials sinking when the young earth was molten.

Worked example

(2 marks) Name the discontinuities that separate (i) the crust from the mantle, (ii) the mantle from the core.

Try it first, then show the solutionSolution

1. (i) Mohorovičić discontinuity (Moho) - 1 mark.
2. (ii) Gutenberg discontinuity - 1 mark.

Answer: (i) The Mohorovičić (Moho) discontinuity. (ii) The Gutenberg discontinuity, at about 2,900 km.

Lithosphere and the importance of the layers

Lithosphere: the rigid outer shell made of the crust and the uppermost solid part of the mantle, about 100 km thick. It is broken into large pieces called plates, which move slowly over the asthenosphere. Movements of these plates cause earthquakes, volcanoes and the formation of fold mountains.

Importance of each layer:

• Crust: provides land, soil, minerals, fossil fuels and the base for all life on land.
• Mantle: its partly molten upper part supplies magma for volcanoes and allows plate movement.
• Core: produces the earth's magnetic field, which protects life from harmful charged particles from the sun and helps navigation with a compass.

Worked example

(2 marks) What is the lithosphere? Why is it important in the study of landforms?

Try it first, then show the solutionSolution

1. Rigid outer shell: crust plus the uppermost mantle - 1 mark.
2. It is broken into plates whose movement causes earthquakes, volcanoes and fold mountains - 1 mark.

Answer: The lithosphere is the rigid outer shell of the earth, made of the crust and the uppermost mantle. It is broken into plates whose movements cause earthquakes, volcanoes and fold mountains.

Earth's Structure 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: Crust, Mantle and Core

Earth's Structure: key facts

Crust thickness

Continental ≈ 30-40 km (up to ≈ 70 km); oceanic ≈ 5-10 km

Lower boundary: Moho.

SIAL and SIMA

$SIAL=silica+alumina(\approx 2.7g/c{m}^3);SIMA=silica+magnesium(\approx 3.0g/c{m}^3)$

Granite; basalt.

Mantle

Moho to ≈ 2,900 km; ≈ 84% of volume

Asthenosphere ≈ 100-250 km.

Core (NIFE)

≈ 2,900 km to centre (≈ 6,371 km)

Outer liquid, inner solid; ≈ 5,000-6,000 °C.

Densities

Surface rock ≈ 2.7; earth average ≈ 5.5; inner core ≈ 13 g/cm³

Temperature gradient

≈ 1 °C per 32 m in the upper crust

How the school exam asks Earth's Structure

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

Tricks to remember Earth's Structure

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

• SIAL on top, SIMA below, NIFE inside

  Light Al at the top, heavier Mg below, heaviest Ni-Fe at the centre.

  Use it when: Order of composition.

• M before G

  Moho is shallow, just under the crust; Gutenberg lies at Great depth, about 2,900 km.

  Use it when: Discontinuities.

• Egg model

  Shell = crust, white = mantle, yolk = core.

  Use it when: Picturing proportions.

Earth's Structure: 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 B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 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 Earth's Structure

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

• ✗Mistake: Saying the whole core is liquid

  Students remember 'molten core'.

  ✓Do this: Outer core liquid; inner core solid.

  Usually costs: 1 mark.

• ✗Mistake: Swapping SIAL and SIMA

  Similar names.

  ✓Do this: SIAL = Al (alumina), light, continents; SIMA = Ma (magnesium), dense, oceans.

  Usually costs: 1 mark.

• ✗Mistake: Giving the mantle as the thinnest layer

  Confusion about thickness.

  ✓Do this: Crust is thinnest; mantle is thickest by volume.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up Moho and Gutenberg

  Two discontinuities.

  ✓Do this: Moho: crust-mantle; Gutenberg: mantle-core at 2,900 km.

  Usually costs: 1 mark.

Practice questions on Earth's Structure

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

   The boundary between the mantle and the core is called the:
   (a) Moho
   (b) Gutenberg discontinuity
   (c) asthenosphere
   (d) lithosphere

   Hint

   At 2,900 km.

   Answer

   (b) Gutenberg discontinuity.

2. Q2 · 1 mark · MCQHarder exam level

   Which layer forms the largest part of the earth's volume?
   (a) Crust
   (b) Mantle
   (c) Outer core
   (d) Inner core

   Hint

   About 84 per cent.

   Answer

   (b) Mantle.

3. Q3 · 1 mark · Very short answerExam level

   What is the main rock of the continental crust?

   Hint

   SIAL.

   Answer

   Granite.

4. Q4 · 2 marks · Very short answerExam level

   Expand SIAL, SIMA and NIFE.

   Hint

   Elements.

   Answer

   SIAL: silica + alumina; SIMA: silica + magnesium; NIFE: nickel + iron.

5. Q5 · 2 marks · Very short answerHarder exam level

   What is the asthenosphere? Why is it important?

   Hint

   Upper mantle.

   Answer

   A weak, partly molten, plastic zone in the upper mantle (about 100-250 km deep). Lithospheric plates move over it, and it is a source of magma for volcanoes.

6. Q6 · 2 marks · Very short answerHOTS / competency

   The average density of the earth is about 5.5 g/cm³, but surface rocks are about 2.7 g/cm³. What does this suggest?

   Hint

   Something heavy inside.

   Answer

   The interior must contain material much denser than surface rocks, such as the iron and nickel of the core, to raise the average density to 5.5.

7. Q7 · 2 marks · Very short answerHarder exam level

   Why can we not obtain direct information about the core?

   Hint

   Depth, heat, pressure.

   Answer

   The core lies about 2,900 km below the surface; the deepest boreholes reach only a few kilometres, and the extreme heat and pressure make drilling deeper impossible.

8. Q8 · 3 marks · Short answerExam level

   Describe the core of the earth.

   Hint

   Extent, composition, parts.

   Answer

   The core extends from about 2,900 km to the centre (about 6,371 km). It is made mainly of nickel and iron (NIFE), with very high density and temperatures of about 5,000-6,000 °C. The outer core is liquid and the inner core solid.

9. Q9 · 3 marks · Short answerHOTS / competency

   If there were no liquid outer core, what difference might it make to life on earth? Explain.

   Hint

   Magnetic field.

   Answer

   The earth's magnetic field is believed to come from the movement of molten iron in the outer core. Without it, the field might not exist; harmful charged particles from the sun would reach the atmosphere more easily, and compasses would not work, affecting navigation and possibly life.

10. Q10 · 3 marks · Short answerHarder exam level

    Draw a labelled description (in words) of the earth's interior, giving the depth of each boundary.

    Hint

    Four layers, two discontinuities.

    Answer

    Crust (surface to about 35 km under continents, 5-10 km under oceans) → Moho → mantle (to about 2,900 km, with asthenosphere at about 100-250 km) → Gutenberg discontinuity → outer core (liquid, about 2,900-5,100 km) → inner core (solid, about 5,100-6,371 km).

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.Depth of the mantle's lower boundary?Show answer

   About 2,900 km.

2. 2.Radius of the earth?Show answer

   About 6,371 km.

3. 3.Main rock of the oceanic crust?Show answer

   Basalt.

4. 4.Which waves cannot pass through the outer core?Show answer

   S-waves.

5. 5.What is NIFE?Show answer

   The core, made of nickel and iron.

6. 6.What makes up the lithosphere?Show answer

   The crust and the uppermost mantle.

Quick revision checklist

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

• ☐Evidence: seismic waves (S-waves blocked by outer core), volcanoes, meteorites, mines, density.
• ☐Crust: continental 30-40 km (SIAL, granite); oceanic 5-10 km (SIMA, basalt).
• ☐Moho: crust-mantle boundary.
• ☐Mantle: to 2,900 km; iron-magnesium silicates; asthenosphere partly molten.
• ☐Gutenberg discontinuity: mantle-core at 2,900 km.
• ☐Core (NIFE): outer liquid, inner solid; 5,000-6,000 °C; density up to about 13 g/cm³.
• ☐Temperature, pressure and density increase with depth.

Earth's Structure: frequently asked questions