CDS Geography · Earth's Structure, Landforms and Geological Time
Interior, Plates and Earthquakes
The layers of the Earth and what the crust is made of, the plates that carry the continents, and the earthquake waves that let us see inside.
Why this matters
Eleven questions, and five of them come from the papers of 2025 and 2026. Most test one clean fact: which wave cannot cross the outer core, what the Richter scale measures, which plate an island sits on.
Concept 1 of 4: Layers of the Earth and the elements of the crust
Nobody has drilled through the crust, so the inside of the Earth is known mostly from how earthquake waves travel through it. The picture is three shells: a thin rocky crust, a thick mantle and a metal core. The crust itself is mostly oxygen and silicon locked together in rock.
Definition
The layers, from the outside in:
- Crust: the thin outer shell. About 5 km thick under the oceans and about 30 km under the continents, more under big mountains.
- Mantle: from the base of the crust down to about 2,900 km. Its upper part, the asthenosphere, is soft and slowly flowing. The crust and the uppermost mantle together make the rigid lithosphere.
- Outer core: liquid, mostly iron and nickel.
- Inner core: solid, also iron and nickel, at the centre about 6,370 km down.
- By weight the crust is about 46.6% oxygen, 27.7% silicon, 8.1% aluminium, 5.0% iron and 3.6% calcium, then sodium, potassium and magnesium.
| Element in the crust | Share by weight | Rank |
|---|---|---|
| Oxygen | About 46.6% | 1st |
| Silicon | About 27.7% | 2nd |
| Aluminium | About 8.1% | 3rd |
| Iron | About 5.0% | 4th |
| Calcium | About 3.6% | 5th CDS 2025 (II): in ascending order the four ran calcium, aluminium, silicon, oxygen. |
| Sodium, potassium, magnesium | About 2 to 3% each | 6th to 8th |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q89Moderate
Example 1 · Earth's Structure, Landforms and Geological Time · Interior, Plates and Earthquakes
Arrange the following major elements present in the Earth's crust, in ascending order, in weight percentage :
1. Aluminium
2. Calcium
3. Silicon
4. Oxygen
Select the answer using the code given below :
Aluminium, not iron, is the commonest metal in the crust
After oxygen and silicon comes aluminium (about 8%), then iron (about 5%). Iron is the main metal of the core, not of the crust.
The lithosphere is more than the crust
The rigid lithosphere is the crust plus the uppermost mantle. The soft layer below it is the asthenosphere.
Concept 2 of 4: Plates and the theories behind them
The lithosphere is broken into large plates that move a few centimetres a year over the soft asthenosphere. Every island and country rides on one plate, so a question that names an island is really asking which plate is under it.
Definition
The plates and the ideas that led to them:
- NCERT lists seven major plates: the Antarctic (with the ocean around it), North American, South American, Pacific, India-Australia-New Zealand, African and Eurasian plates.
- Minor plates include the Cocos, Nazca, Arabian (mostly the Saudi Arabian landmass), Philippine, Caroline and Fuji plates.
- Alfred Wegener proposed continental drift in 1912. Harry Hess proposed sea-floor spreading in the 1960s, and plate tectonics followed in 1967.
- Older theories CDS asks about: Chamberlin (with Moulton), the planetesimal hypothesis of the Earth's origin; Harold Jeffreys, the thermal contraction theory of mountains; Leopold Kober, the geosynclinal theory of mountains; Reginald Daly, the sliding continents hypothesis.
| Plate | Major or minor | Land on it |
|---|---|---|
| Pacific | Major | Most of the Pacific Ocean floor, Hawaii |
| North American | Major | North America, Greenland, Baffin Island CDS 2025 (II): Baffin Island is on the North American Plate. |
| South American | Major | South America |
| Eurasian | Major | Europe, most of Asia, the Andaman Islands |
| African | Major | Africa, Madagascar, Comoros |
| India-Australia-New Zealand | Major | India, Sri Lanka, Australia, New Zealand |
| Antarctic | Major | Antarctica and the ocean around it |
| Arabian | Minor | Mostly the Saudi Arabian landmassQ CDS 2020 (II): the Saudi Arabian plate was the one that is not a major plate. |
| Nazca and Cocos | Minor | Pacific floor off South and Central America |
| Theories | Not a plate | Planetesimal: Chamberlin. Thermal contraction: Jeffreys. Geosyncline: Kober. Sliding continents: DalyQ |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q90Moderate
Example 2 · Earth's Structure, Landforms and Geological Time · Interior, Plates and Earthquakes
Match List-I with List-II and select the answer using the code given below the Lists :
Code :
| List-I (Island / Nation) | List-II (Tectonic Plate) |
|---|---|
| A. Comoros | 1. North American Plate |
| B. Baffin Island | 2. African Plate |
| C. Sri Lanka | 3. Eurasian Plate |
| D. Andaman Islands | 4. Indo-Australian Plate |
The Arabian plate is minor
The seven major plates are the Antarctic, North American, South American, Pacific, India-Australia-New Zealand, African and Eurasian plates. The Arabian plate is a minor one.
Sri Lanka and the Andamans are on different plates
Sri Lanka rides the India-Australia plate. The Andaman Islands sit on the Eurasian side, where the Indian plate sinks beneath it.
Concept 3 of 4: P-waves, S-waves and surface waves
An earthquake sends out two kinds of body wave. P-waves push and pull like sound, so they pass through anything. S-waves shake the rock sideways, and a liquid cannot pass on a sideways shake. That one difference told scientists the outer core is liquid.
Definition
The three kinds of wave:
- P-waves (primary): longitudinal, like sound waves. The fastest, so they arrive first. They travel through solids, liquids and gases.
- S-waves (secondary): transverse. The rock vibrates at right angles to the direction of travel, in the vertical plane, so they make crests and troughs. Slower than P-waves, and they travel only through solids.
- Because S-waves do not cross the outer core, scientists concluded that the outer core is liquid. The area where no S-waves arrive is the shadow zone.
- Surface waves arrive last and do the most damage. S-waves do more damage than P-waves.
| Wave | Kind of motion | Travels through | Arrives |
|---|---|---|---|
| P-wave | Longitudinal, like sound | Solids, liquids and gases | FirstQ |
| S-wave | Transverse, at right angles, makes crests and troughs | Solids only | SecondQ CDS 2024 (II): 'S-waves are less destructive to buildings' was the false statement. |
| Surface wave | Rolls along the surface | The crust near the surface | Last, most damaging |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q1Moderate
Example 3 · Earth's Structure, Landforms and Geological Time · Interior, Plates and Earthquakes
Consider the following statements with reference to Secondary waves (S-Waves) generated during an earthquake :
1. They create troughs and crests in the material through which they pass.
2. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane.
Which of the statements given above is/are correct ?
P-waves cross liquids too
P-waves pass through solids, liquids and gases. It is the S-wave that travels only through solids.
S-waves are slower but more damaging than P-waves
S-waves arrive after P-waves, but their sideways shaking does more damage to buildings and bridges, not less.
Concept 4 of 4: Focus, epicentre and measuring earthquakes
An earthquake starts at one point under the ground and its shaking spreads out from there. Two numbers describe it: how much energy it let out (magnitude) and how hard it shook a given place (intensity).
Definition
Key terms:
- The focus or hypocentre is the point inside the Earth where the rock breaks and energy is released.
- The epicentre is the point on the surface directly above the focus. It can lie under the sea as easily as on land.
- Magnitude is the energy released, on the Richter scale (0 to 10, each step about 30 times more energy).
- Intensity is the shaking and damage at a place, on the Mercalli scale (1 to 12). It falls as you move away from the epicentre.
- Most earthquakes are tectonic, along plate boundaries, above all converging ones. Collapse earthquakes (roofs of mines and caves falling in) are small and rare.
- A tsunami is a sea wave set off by an undersea earthquake. It is not an earthquake itself.
| Term | What it means |
|---|---|
| Focus or hypocentre | Point inside the Earth where the quake startsQ |
| Epicentre | Point on the surface right above the focus |
| Richter scale | Magnitude: the energy releasedQ CDS 2025 (II): 'Richter is the intensity scale' was the false statement. |
| Mercalli scale | Intensity: the shaking and damage felt at a place |
| Collapse earthquake | Small, rare quake when an underground roof falls in |
| Where quakes cluster | Plate boundaries, mostly converging onesQ |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q33Moderate
Example 4 · Earth's Structure, Landforms and Geological Time · Interior, Plates and Earthquakes
Consider the following statements with reference to earthquakes :
1. Collapse earthquakes are most common
2. Tsunamis are waves generated by the tremors and not an earthquake in itself
3. High magnitude earthquakes i.e. 8+ are quite rare whereas 'tiny' types occur almost every minute
Which of the statements given above are correct ?
Richter is magnitude, not intensity
The Richter scale measures the energy released. The intensity of shaking at a place is measured on the Mercalli scale.
The epicentre is not always on land
The epicentre is simply the surface point above the focus. Many lie under the ocean, which is how undersea quakes raise tsunamis.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
Layers of the Earth and the elements of the crust6 rows
| Element in the crust | Share by weight | Rank |
|---|---|---|
| Oxygen | About 46.6% | 1st |
| Silicon | About 27.7% | 2nd |
| Aluminium | About 8.1% | 3rd |
| Iron | About 5.0% | 4th |
| Calcium | About 3.6% | 5th CDS 2025 (II): in ascending order the four ran calcium, aluminium, silicon, oxygen. |
| Sodium, potassium, magnesium | About 2 to 3% each | 6th to 8th |
Plates and the theories behind them10 rows
| Plate | Major or minor | Land on it |
|---|---|---|
| Pacific | Major | Most of the Pacific Ocean floor, Hawaii |
| North American | Major | North America, Greenland, Baffin Island CDS 2025 (II): Baffin Island is on the North American Plate. |
| South American | Major | South America |
| Eurasian | Major | Europe, most of Asia, the Andaman Islands |
| African | Major | Africa, Madagascar, Comoros |
| India-Australia-New Zealand | Major | India, Sri Lanka, Australia, New Zealand |
| Antarctic | Major | Antarctica and the ocean around it |
| Arabian | Minor | Mostly the Saudi Arabian landmassQ CDS 2020 (II): the Saudi Arabian plate was the one that is not a major plate. |
| Nazca and Cocos | Minor | Pacific floor off South and Central America |
| Theories | Not a plate | Planetesimal: Chamberlin. Thermal contraction: Jeffreys. Geosyncline: Kober. Sliding continents: DalyQ |
P-waves, S-waves and surface waves3 rows
| Wave | Kind of motion | Travels through | Arrives |
|---|---|---|---|
| P-wave | Longitudinal, like sound | Solids, liquids and gases | FirstQ |
| S-wave | Transverse, at right angles, makes crests and troughs | Solids only | SecondQ CDS 2024 (II): 'S-waves are less destructive to buildings' was the false statement. |
| Surface wave | Rolls along the surface | The crust near the surface | Last, most damaging |
Focus, epicentre and measuring earthquakes6 rows
| Term | What it means |
|---|---|
| Focus or hypocentre | Point inside the Earth where the quake startsQ |
| Epicentre | Point on the surface right above the focus |
| Richter scale | Magnitude: the energy releasedQ CDS 2025 (II): 'Richter is the intensity scale' was the false statement. |
| Mercalli scale | Intensity: the shaking and damage felt at a place |
| Collapse earthquake | Small, rare quake when an underground roof falls in |
| Where quakes cluster | Plate boundaries, mostly converging onesQ |
Watch out for (8)
- Aluminium, not iron, is the commonest metal in the crust→ Layers of the Earth and the elements of the crust
- The lithosphere is more than the crust→ Layers of the Earth and the elements of the crust
- The Arabian plate is minor→ Plates and the theories behind them
- Sri Lanka and the Andamans are on different plates→ Plates and the theories behind them
- P-waves cross liquids too→ P-waves, S-waves and surface waves
- S-waves are slower but more damaging than P-waves→ P-waves, S-waves and surface waves
- Richter is magnitude, not intensity→ Focus, epicentre and measuring earthquakes
- The epicentre is not always on land→ Focus, epicentre and measuring earthquakes
Test yourself on Earth's Structure, Landforms and Geological Time
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.