GeographyISC Class 11 · Earth's Interior, Earthquakes and Volcanoes

Structure of the Earth's Interior and Seismic Waves (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

The earth's radius is about 6,371 km. It has a thin crust, a mantle to about 2,900 km, a liquid outer core to about 5,150 km and a solid inner core. Seismic waves reveal this: S-waves cannot cross the liquid outer core, so a shadow zone forms beyond about 105 degrees from an earthquake.

Understand Structure of the Earth's Interior and Seismic Waves

Layers. The earth's mean radius is about 6,371 km, and density rises with depth.

Crust: oceanic crust averages about 5 km and is mainly basalt (sima); continental crust averages about 30 to 35 km, reaching about 70 km under the Himalayas, and is mainly granitic (sial). Its base is the Mohorovicic (Moho) discontinuity.
Mantle: from the Moho to about 2,900 km, most of the earth's volume. The asthenosphere in the upper mantle is weak and partly molten; the crust plus the rigid top of the mantle form the lithosphere. The mantle-core boundary is the Gutenberg discontinuity.
Core: outer core (about 2,900 to 5,150 km) is liquid iron and nickel and is thought to generate the magnetic field; inner core (about 5,150 km to the centre) is solid because of pressure.
Temperature rises with depth, roughly 25 to 30 degrees C per km in the upper crust (the geothermal gradient); density rises from about 2.7 to 3 g/cm3 in the crust to over 10 g/cm3 in the core, with an average of about 5.5 g/cm3.

Sources of information. Direct: mines, deep drilling (the deepest borehole, at Kola in Russia, reached about 12 km), volcanic material, ocean drilling. Indirect: temperature and pressure trends, meteorites, gravity anomalies, magnetism and above all seismic waves.

Seismic waves. P-waves are fastest, compress and expand along the direction of travel and pass through solids, liquids and gases. S-waves are slower, shake at right angles and pass only through solids. Surface waves are slowest and cause most damage. Waves speed up, slow down and refract at boundaries, showing that the earth is layered.

Shadow zones: within about 105 degrees of the epicentre both P and S arrive; between about 105 and 145 degrees neither arrives directly; beyond about 145 degrees only P arrives. S-waves are absent beyond 105 degrees, so the outer core is liquid.

Structure of the Earth's Interior and Seismic Waves: key facts

Layer depths: Crust 5-70 km
Mantle to 2,900 km
Outer core to 5,150 km
Inner core to 6,371 km; Moho and Gutenberg boundaries.
Shadow zones: $B o t h P an d S : w i t hin 105 d e g r ees ∣ N e i t h er : 105 - 145 d e g r ees ∣ P o n l y : b ey o n d 145 d e g r ees$
Distance from S-P lag: $d = l a g / (1/ v S - 1/ v P) ∣ l a g in s, s p ee d s ink m / s$
Temperature with depth: $T = T (s u r f a ce) + g r a d i e n t x d e pt h ∣ G r a d i e n t ab o u t 25 - 30 d e g r ees C p er k m$

How to answer Structure of the Earth's Interior and Seismic Waves questions

Questions mix description (4 marks) with short calculations (3 to 4 marks).

1For layer questions, write crust, mantle, core with depth, state and composition.
2For waves, compare P, S and surface waves in speed, motion and media.
3For shadow zones, quote 105 and 145 degrees and link S-wave absence to a liquid outer core.
4For sums, write the formula, substitute with units and check the answer is sensible.

Structure of the Earth's Interior and Seismic Waves 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.

Distance of a quake

Suppose a station records a lag of 12 s between P and S arrivals, with P at 7 km/s and S at 4 km/s.

How far is the epicentre?

Try it first, then show the solutionSolution

1/4 - 1/7 = 3/28 s per km.
d = 12 / (3/28) = 112 km.

Answer: 112 km.

Memory hook: Bigger lag, farther quake.

Why a hotter mine

Suppose a mine shaft is 3 km deep with surface temperature 20 degrees C and a gradient of 25 degrees C per km.

Estimate the rock temperature at the bottom.

Try it first, then show the solutionSolution

Rise = 25 x 3 = 75 degrees C.
Temperature = 20 + 75 = 95 degrees C.

Answer: 95 degrees C.

Memory hook: Each kilometre adds about 25 degrees.

Tricks to remember Structure of the Earth's Interior and Seismic Waves

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

Layer order
Crust, Mantle, Outer core, Inner core: C-M-O-I.
Use it when: Order and states.
P and S
P = Pushes through everything; S = Solids only.
Use it when: Wave media.

Structure of the Earth's Interior and Seismic Waves: solved examples

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

Example 1: Boundary names · 1 mark · MCQBoard level

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

Try it first, then show the solutionSolution

Mantle-core boundary.

Answer: (c) Gutenberg discontinuity.

Example 2: Core dimensions · 3 marks · Short answerBoard level

Using a radius of 6,371 km, outer core to 5,150 km and mantle base at 2,900 km, find the thickness of the outer core and the radius of the inner core.

Try it first, then show the solutionSolution

Outer core thickness = 5,150 - 2,900 = 2,250 km.
Inner core radius = 6,371 - 5,150 = 1,221 km.

Answer: Outer core 2,250 km thick; inner core radius 1,221 km.

Example 3: S-P lag · 4 marks · Long answerHarder board level

Seismic waves from an earthquake reach a station with S-waves arriving 20 s after P-waves. If vP = 6 km/s and vS = 3.5 km/s, find the distance to the epicentre.

Try it first, then show the solutionSolution

Per km lag = 1/3.5 - 1/6 = 2/7 - 1/6 = 5/42 s.
d = 20 / (5/42) = 20 x 42/5 = 168 km.
Check: P takes 168/6 = 28 s; S takes 168/3.5 = 48 s; difference 20 s.

Answer: 168 km.

Example 4: Geothermal gradient · 4 marks · Long answerHarder board level

The surface temperature is 15 degrees C and the gradient is 25 degrees C per km. Find (a) the temperature at 12 km and (b) the depth at which rock reaches 215 degrees C.

Try it first, then show the solutionSolution

(a) 15 + 25 x 12 = 15 + 300 = 315 degrees C.
(b) Rise needed = 215 - 15 = 200 degrees C.
Depth = 200 / 25 = 8 km.

Answer: (a) 315 degrees C (b) 8 km.

Example 5: Read the shadow zone · 5 marks · Case-basedHOTS / competency

Case (invented): Three stations lie at 70, 120 and 160 degrees from an epicentre. (a) State the waves each receives directly. (b) Explain the absence of S-waves beyond 105 degrees. (c) Say what the zone at 120 degrees tells us about the core.

Try it first, then show the solutionSolution

(a) 70 degrees: both P and S. 120 degrees: neither directly (within the 105-145 degree zone). 160 degrees: P only.
(b) S-waves cannot pass through liquid, so the outer core blocks them.
(c) The core has a liquid outer layer that bends P-waves, creating the P shadow zone.

Answer: (a) Both, none, P only (b) S-waves cannot cross liquid (c) the outer core is liquid.

Where students lose marks in Structure of the Earth's Interior and Seismic Waves

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

Mistake: Writing that S-waves pass through liquids
Waves are confused.
Wrong working
S-waves pass the outer core
Right working
S-waves cannot pass the liquid outer core
Do this: S-waves travel only through solids; P-waves go through all states.
Usually costs: 1 to 2 marks.
Mistake: Mixing the boundaries
Moho and Gutenberg swapped.
Do this: Moho: crust-mantle; Gutenberg: mantle-core.
Usually costs: 1 mark.
Mistake: Calculation without units
km, s or degrees C missing.
Do this: Write units at each step.
Usually costs: 1 mark.

Exam tips

Give depths with km for every layer.
State the shadow-zone angles as 105 and 145 degrees.
Show the S-P lag formula before substituting.
Link a liquid outer core to missing S-waves.

Practice questions on Structure of the Earth's Interior and Seismic Waves

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
Which seismic waves travel fastest?
(a) P-waves
(b) S-waves
(c) Love waves
(d) Surface waves
Hint
Primary.
Answer
(a) P-waves.
Q2 · 4 marks · Long answerHarder board level
A station records S-waves 12 s after P-waves. If vP = 7 km/s and vS = 4 km/s, find the distance to the epicentre.
Hint
Use 1/vS - 1/vP.
Answer
1/4 - 1/7 = 3/28 s per km; d = 12 / (3/28) = 12 x 28/3 = 112 km. Check: P 112/7 = 16 s; S 112/4 = 28 s; lag 12 s.
Q3 · 3 marks · Short answerHarder board level
Distinguish between the lithosphere and the asthenosphere.
Hint
Rigid versus weak and partly molten.
Answer
The lithosphere is the crust plus the rigid top of the mantle (about 10-200 km thick); the asthenosphere is the weak, partly molten upper mantle zone below it, from which magma can rise.
Q4 · 4 marks · Long answerHarder board level
Compare oceanic and continental crust on thickness, composition and name.
Hint
Sima and sial.
Answer
Oceanic crust averages about 5 km, is mainly basalt and called sima; continental crust averages about 30-35 km (up to about 70 km under the Himalayas), is mainly granitic and called sial.
Q5 · 4 marks · Case-basedHOTS / competency
The deepest borehole reaches about 12 km. (a) Express this as a percentage of the radius 6,371 km. (b) Explain why scientists depend on indirect evidence. (c) Name two indirect sources.
Hint
12 / 6,371 x 100.
Answer
(a) 12/6,371 = 0.00188, about 0.19 per cent. (b) Direct sampling reaches only a tiny fraction of the depth. (c) Seismic waves, meteorites, gravity or magnetism.

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.Moho separates?Show answer
Crust and mantle.
2.State of the outer core?Show answer
Liquid.
3.Slowest waves?Show answer
Surface waves.
4.Deepest borehole depth in the chapter?Show answer
About 12 km.
5.Upper crust gradient?Show answer
About 25-30 degrees C per km.

Structure of the Earth's Interior and Seismic Waves: frequently asked questions

Why is the inner core solid?

Enormous pressure keeps it solid despite the heat.

Where do P-waves arrive beyond 145 degrees?

They arrive; S-waves do not.

What generates the magnetic field?

Movement of the liquid outer core is thought to.