GeographyICSE Class 9 · Earth's Structure

Layers of the Earth: Crust, Mantle and Core (ICSE Class 9 Geography)

Updated 17 October 2026 · Draft, not yet fact-checked

The earth has three main layers: the thin rocky crust (continental sial and oceanic sima), the thick mantle of dense rock and the iron-nickel core with a liquid outer part and a solid inner part. Temperature, pressure and density increase with depth. The crust and the uppermost solid mantle together form the lithosphere.

Understand Layers of the Earth: Crust, Mantle and Core

1. How we know the interior. Direct sources are mines, deep drilling and volcanic lava; indirect sources are *seismic waves* (P and S waves change speed and direction in different layers; S waves do not pass through liquid), meteorites (believed to be similar in composition) and the earth's magnetic field and gravity.

2. The crust. The thin outer layer, about 5 to 10 km thick under the oceans and up to about 35 km or more under the continents (up to about 70 km under high mountains). *Continental crust* is mainly granite, rich in silica and aluminium, called *sial*. *Oceanic crust* is mainly basalt, rich in silica and magnesium, called *sima*, and is denser.

3. The mantle. Lies below the crust to about 2,900 km depth. It is made of dense rock rich in silicates of iron and magnesium; the upper part is partly molten and flows slowly (the *asthenosphere*), supplying magma to volcanoes.

4. The core. Radius about 3,470 km, mainly iron and nickel (*nife*). The *outer core* is liquid; the *inner core* is solid because of the enormous pressure.

5. Discontinuities. The *Mohorovicic* discontinuity separates crust and mantle; the *Gutenberg* discontinuity separates mantle and core.

6. The lithosphere. The crust plus the rigid upper mantle forms the lithosphere, which is broken into plates. Temperature rises with depth, reaching about 5,000–6,000 °C in the core.

Layers of the Earth: Crust, Mantle and Core: key facts

Layers: $C r u s t ∣ M an tl e ∣ C or e$
Crust types: $C o n t in e n t a l : s ia l, g r ani t e, l i g h t er ∣ O ce ani c : s ima, ba s a l t, d e n ser$
Core: Outer: liquid
Inner: solid
Iron and nickel
Boundaries: Moho: crust and mantle | Gutenberg: mantle and core

Layers of the Earth: Crust, Mantle and Core in real life

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

Boiled egg model

Suppose a student cuts a boiled egg: a thin shell, a thick white and a yolk.

Match the egg to the earth.

Try it first, then show the solutionSolution

Shell: crust.
White: mantle.
Yolk: core.

Answer: Shell is crust, white is mantle, yolk is core.

Memory hook: Shell, white, yolk.

Tricks to remember Layers of the Earth: Crust, Mantle and Core

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

SiAl above SiMa
Sial floats on sima.
Use it when: Crust.
NIFE
Nickel and iron in the core.
Use it when: Core composition.

Layers of the Earth: Crust, Mantle and Core: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Name layers · 3 marks · Short answerBoard level

Name (a) the outermost layer, (b) the largest layer by volume, (c) the liquid layer. [3]

Try it first, then show the solutionSolution

(a) Crust.
(b) Mantle (about 84% of the earth's volume).
(c) Outer core.

Answer: (a) Crust (b) Mantle (c) Outer core.

Example 2: Sial and sima · 3 marks · Short answerHarder board level

Give three differences between sial and sima. [3]

Try it first, then show the solutionSolution

Location: sial in continents; sima under oceans.
Rock: granite versus basalt.
Density: sial lighter, sima denser.

Answer: Sial: continental, granite, lighter; sima: oceanic, basalt, denser.

Example 3: S waves · 3 marks · Short answerHarder board level

A set of stations on the far side of the globe fail to record one type of wave from a quake. Name the wave, give the reason and state the inference. [3]

Try it first, then show the solutionSolution

The missing wave is the S wave, which cannot travel through liquids.
It is stopped at the outer core.
So the outer core must be liquid.

Answer: S waves; they are stopped by liquid, showing that the outer core is liquid.

Example 4: Why solid inner core · 3 marks · Short answerHarder board level

The inner core is hotter than the outer core yet solid. Explain. [3]

Try it first, then show the solutionSolution

Pressure increases with depth.
Enormous pressure keeps the inner core solid.
Lower pressure lets the outer core remain liquid.

Answer: Very high pressure keeps the inner core solid despite high temperature.

Example 5: Case: deepest mine · 6 marks · Case-basedHOTS / competency

Case. A miner finds temperature rising as he goes deeper into a mine. (i) What does this show about the earth? [1] (ii) Why can we not go to the mantle by drilling? [2] (iii) Name an indirect source of information. [1] (iv) Why is the mantle partly molten? [2]

Try it first, then show the solutionSolution

(i) Temperature increases with depth.
(ii) The depth and heat are too great for equipment.
(iii) Seismic waves.
(iv) High temperature melts some rocks but pressure varies.

Answer: (i) Heat rises with depth (ii) Too deep and hot (iii) Seismic waves (iv) High temperature in the upper mantle.

Where students lose marks in Layers of the Earth: Crust, Mantle and Core

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

Mistake: Writing that the whole interior is molten
Only outer core is liquid.
Do this: State the solid mantle and inner core.
Usually costs: 1 mark.
Mistake: Calling sial the oceanic crust
Names swapped.
Do this: Sima is oceanic.
Usually costs: 1 mark.
Mistake: Confusing lithosphere and crust
Not the same.
Do this: Lithosphere includes upper mantle.
Usually costs: 1 mark.

Exam tips

Draw a labelled cross section of the earth.
Use a table of layer, thickness, state and composition.
Name the discontinuities.
Explain seismic evidence in sentence form.

Practice questions on Layers of the Earth: Crust, Mantle and Core

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

Q1 · 1 mark · MCQBoard level
The boundary between the crust and mantle is the:
(a) Gutenberg discontinuity
(b) Mohorovicic discontinuity
(c) Conrad discontinuity
(d) Lehmann discontinuity
Hint
Moho.
Answer
(b) Mohorovicic.
Q2 · 2 marks · Very short answerBoard level
Name two metals in the core. [2]
Hint
NIFE.
Answer
Nickel and iron.
Q3 · 2 marks · Very short answerBoard level
Define lithosphere. [2]
Hint
Crust plus.
Answer
The crust together with the rigid upper mantle.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The oceanic crust is thinner than the continental crust.
Reason (R): The oceanic crust is made of denser basalt.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Thickness versus density.
Answer
(b).
Q5 · 3 marks · Short answerHarder board level
State three changes in conditions as one goes from the crust to the core. [3]
Hint
Increase.
Answer
Temperature, pressure and density increase with depth.
Q6 · 3 marks · Short answerHarder board level
Why do volcanoes provide direct evidence of the interior? Mention a limitation. [3]
Hint
Lava.
Answer
Lava and magma bring material from below; the material changes in composition on rising, so it only shows the upper mantle.
Q7 · 2 marks · Very short answerBoard level
Why are meteorites used to understand the earth's core? [2]
Hint
Same origin.
Answer
They are believed to be from the same material that formed the earth, rich in iron and nickel.
Q8 · 3 marks · Short answerHarder board level
Compare the crust and the core with respect to state, thickness and composition. [3]
Hint
Table.
Answer
Crust: solid, thin, silicates; core: liquid outer and solid inner, large, iron and nickel.
Q9 · 3 marks · Long answerHOTS / competency
Case. An earthquake wave recorded across the world fails to reach a belt of stations on the far side. Explain how this helps scientists find the liquid core. [3]
Hint
Shadow zone.
Answer
S waves stop at the liquid outer core, leaving a shadow zone; the pattern shows the core is liquid.
Q10 · 3 marks · Long answerHOTS / competency
Case. A student says the earth's interior is molten because lava comes out. Evaluate her statement. [3]
Hint
Partial.
Answer
Lava comes from pockets of magma in the crust or upper mantle; most of the mantle is solid and only the outer core is liquid.

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.Core composition?Show answer
Iron and nickel.
2.Which layer is the largest by volume?Show answer
Mantle – about 84% of the earth's volume.
3.Continental crust is called?Show answer
Sial.
4.Moho separates?Show answer
Crust and mantle.
5.Outer core is?Show answer
Liquid.

Layers of the Earth: Crust, Mantle and Core: frequently asked questions

What is magma?

Molten rock beneath the surface.

Which wave cannot pass through liquid?

S wave.

Is the crust the same everywhere?

No, it is thicker under continents.