NDA Geography · Earth's Structure, Landforms and Geological Time

Earth's Interior, Crust and Plate Tectonics

The Earth is built in shells — crust, mantle, core — separated by discontinuities, and its rigid outer shell is broken into plates whose movements build mountains, open oceans and ring the Pacific with fire.

All Earth's Structure, Landforms and Geological Time notes NDA Geography strategy

Why this matters

15 PYQs and the densest-HARD subtopic in the chapter. Two ideas earn most of the marks: the layer order and what each layer is made of (inner core SOLID, outer core LIQUID), and the three plate-boundary types. Get the lithosphere definition and the major-vs-minor plate list cold.

Concept 1 of 6

The layers of the Earth and the lithosphere

Intuition

Cut the Earth open and you find concentric shells, like an onion. From outside in: a thin CRUST, a thick MANTLE of hot silicate rock, then the CORE — a liquid outer core and a solid inner core. The word lithosphere does NOT mean the crust alone: it means the rigid outer shell that the tectonic plates are made of — the crust PLUS the topmost, rigid part of the mantle.

Definition

The shells, outside in:

Crust — the thin outermost skin (oceanic 5–10 km, continental 30–70 km).
Mantle — silicate rock down to ~2900 km. Its upper part includes the asthenosphere, a partially-molten, slowly-flowing layer that the plates ride on.
Outer core — LIQUID iron-nickel; its motion generates Earth's magnetic field.
Inner core — SOLID iron-nickel (solid despite being hottest, because the pressure is enormous).

Lithosphere = crust + uppermost SOLID mantle (the rigid plate layer). It sits ON TOP of the soft asthenosphere. The lithosphere is thickest under the great mountain belts (deep crustal roots) and thinnest under the oceans.

Worked example

Arrange these four layers from the Earth's surface inward: outer core, crust, inner core, mantle.

The crust is the outermost skin you stand on.
Below it lies the mantle, the thick silicate shell.
Below the mantle is the liquid outer core.
At the centre is the solid inner core.

Answer:Crust → Mantle → Outer core → Inner core.

Practice this conceptself-check · 3 quick reps

Try it yourself

The lithosphere consists of which parts of the Earth?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which is solid — the inner core or the outer core?
2.
Lithosphere = crust + ?
3.
Where is the lithosphere thickest?

From the bank · past-year question

Example 1Earth's Structure, Landforms and Geological TimeEASY

Lithosphere consists of

[Q131 · Sep · 2019]

Lithosphere is NOT just the crust

A favourite trap offers 'crust and core' or 'upper and lower mantle' as the lithosphere. Neither is right — the lithosphere is the crust + uppermost solid mantle, the rigid layer the plates are cut from.

Hottest is not always liquid

The inner core is the hottest part of the Earth yet it is solid, because the crushing pressure at the centre keeps the iron from melting. The cooler outer core is the one that is liquid.

Drill 2 more on the layers of the earth and the lithosphere

Concept 2 of 6

Discontinuities between the layers

Intuition

The boundaries where one shell gives way to the next are called discontinuities — places where the speed of seismic waves jumps because the material changes. Each one has a name, and the NDA loves to test which boundary sits where.

Definition

The named boundaries, surface inward:

Conrad — within the continental crust (upper 'sial' over lower 'sima').
Mohorovicic (Moho) — separates the crust from the mantle.
Repetti — within the mantle (upper / lower mantle).
Gutenberg — separates the mantle from the outer core.
Lehmann — separates the outer core from the inner core (the innermost discontinuity).

Discontinuity	Separates	Position
Conrad	Upper / lower crust	Shallow
Mohorovicic (Moho)	Crust / mantle	Base of crust NDA 2022 — Moho is THE crust-mantle boundary.
Repetti	Upper / lower mantle	Mid-mantle
Gutenberg	Mantle / outer core	Deep
Lehmann	Outer core / inner core	Innermost NDA 2023 — Lehmann is the deepest, in the innermost part of the Earth.

Practice this conceptself-check · 3 quick reps

Try it yourself

Which discontinuity is found in the innermost part of the Earth?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which discontinuity separates the crust from the mantle?
2.
Which discontinuity lies between the mantle and outer core?
3.
Name the innermost discontinuity.

From the bank · past-year question

Example 2Earth's Structure, Landforms and Geological TimeEASY

Which one of the following 'discontinuities' separates the Earth's crust from the mantle?

[Q92 · Sep · 2022]

Moho vs Gutenberg

Both are mid-Earth boundaries, but Moho is shallow (crust ↔ mantle) and Gutenberg is deep (mantle ↔ outer core). Don't swap them.

Drill 1 more on discontinuities between the layers

Concept 3 of 6

Oceanic vs continental crust

Intuition

There are two kinds of crust and they behave very differently. Oceanic crust is thin, dense and basaltic; continental crust is thick, light and granitic. Because oceanic crust is heavier, it is the one that sinks (subducts) when the two meet. And the whole crust is brittle — it cracks rather than flows, which is why it breaks into plates and snaps in earthquakes.

Definition

Key contrasts:

Oceanic crust — thin (~5–10 km), DENSE, basaltic ('sima', rich in silica + magnesium). Being heavier, it subducts beneath continental crust.
Continental crust — thick (~30–70 km), LIGHT, granitic ('sial', rich in silica + aluminium).
The crust is brittle, not plastic — it fractures, which is why plate edges and faults exist.
Most of the Earth's internal heat is stored in the mantle; convection in the MANTLE (not the crust) drives the plates.

Worked example

Two slabs of crust collide: one oceanic, one continental. Which one slides underneath, and why?

Compare densities: oceanic crust is basaltic and dense; continental crust is granitic and light.
The denser slab cannot ride over the lighter one — it sinks.
So the oceanic slab subducts beneath the continental slab.

Answer:The oceanic crust subducts, because it is denser (heavier) than continental crust.

Practice this conceptself-check · 3 quick reps

Try it yourself

Statement: 'The Earth's crust is brittle.' Is this correct?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which crust is denser, oceanic or continental?
2.
Continental crust is rich in which two elements (sial)?
3.
Where is most of the Earth's internal heat stored?

From the bank · past-year question

Example 3Earth's Structure, Landforms and Geological TimeHARD

Which of the following statements is/are correct? 1. The Earth's crust is brittle in nature. 2. The mean thickness of the oceanic crust is 15 km, whereas that of the continental crust is around 30 km. Select the correct answer using the code given below.

[Q129 · Sep · 2022]

Convection is in the MANTLE, not the crust

A multi-statement trap claims the convective cells that move plates circulate 'in the crust'. They circulate in the mantle (the asthenosphere). The crust is too thin and brittle to convect.

Oceanic crust is thinner, not thicker

Don't be talked into '30 km oceanic crust'. Oceanic crust is the THIN one (~5–10 km); the thick (~30–70 km) crust is continental.

Drill 1 more on oceanic vs continental crust

Concept 4 of 6

What the crust is made of

Intuition

The crust is overwhelmingly silicate minerals. By abundance, feldspars and quartz dominate, followed by the ferromagnesian minerals (pyroxene, amphibole, mica). Knowing the rough abundance order, and that the crust's pore space controls how water moves through it, covers the odd composition question.

Definition

The continental crust is mostly silicate minerals — feldspar (~half the crust) and quartz are the most abundant; pyroxene, amphibole and mica are the common dark (ferromagnesian) minerals.
A rough crustal-abundance order among the dark minerals tested by the NDA: mica < amphibole < pyroxene.
Crust also stores groundwater in its pore spaces. Whether rainfall becomes groundwater depends on precipitation amount, evaporation rate, and the ground's ability to let water infiltrate — not on how far the site is from the sea.

Worked example

Three minerals — mica, amphibole, pyroxene — are listed. Put them in ascending order of how much of the crust they form.

Mica is comparatively minor among the three.
Amphibole is somewhat more abundant.
Pyroxene is the most abundant of the three.

Answer:Mica < Amphibole < Pyroxene.

Practice this conceptself-check · 3 quick reps

Try it yourself

Which of these does NOT affect how much rainfall becomes groundwater: precipitation amount, evaporation rate, ground's infiltration ability, distance from the sea?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which mineral makes up about half the Earth's crust?
2.
Order by crustal abundance: mica, amphibole, pyroxene.
3.
Name one factor that does NOT control groundwater recharge.

From the bank · past-year question

Example 4Earth's Structure, Landforms and Geological TimeHARD

Which of the following is the correct ascending order of the given minerals in terms of their presence in the Earth's crust? 1. Amphibolite 2. Mica 3. Pyroxene Select the answer using the code given below:

[Q87 · Apr · 2025]

Drill 1 more on what the crust is made of

Concept 5 of 6

Tectonic plates and their boundaries

Intuition

The lithosphere is cracked into about seven MAJOR plates and several MINOR ones, all drifting on the asthenosphere. Where plate edges meet, three things can happen: they collide (convergent), they pull apart (divergent), or they grind past each other (transform). Almost every dramatic feature on Earth — mountains, ocean ridges, the Ring of Fire — sits on one of these three boundary types.

Definition

Major plates (7): Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic. Minor plates include Cocos, Nazca, Caroline, Philippine, Arabian, Juan de Fuca. The three boundary types:

Convergent (collide): oceanic-continental → SUBDUCTION + volcanic arc + deep trench (Andes); continental-continental → fold mountains (Himalayas); oceanic-oceanic → island arcs (Japan).
Divergent (separate): mid-ocean ridges, sea-floor spreading, new land (Mid-Atlantic Ridge — Iceland sits on it, giving geothermal energy, new land and tourism).
Transform (slide past): strike-slip faults (San Andreas).

The Ring of Fire circles the Pacific — a belt of convergent subduction zones, hence an active seismic AND volcanic zone with deep trenches.

Worked example

Iceland sits astride the Mid-Atlantic Ridge. Name the boundary type and one benefit the islanders get from it.

The Mid-Atlantic Ridge is where two plates pull apart — a divergent boundary.
Rising magma there gives geothermal heat, builds new land, and draws tourists.

Answer:A divergent boundary; benefits include geothermal energy (also new land and tourism).

Practice this conceptself-check · 4 quick reps

Try it yourself

Which one is a MAJOR plate: Cocos, Arabian, Pacific, Philippine?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Name the boundary type that builds the Himalayas.
2.
What forms at a divergent boundary in an ocean?
3.
The San Andreas Fault is which boundary type?
4.
Is the Antarctic Plate major or minor?

From the bank · past-year question

Example 5Earth's Structure, Landforms and Geological TimeHARD

Which of the following statements about Ring of Fire is/are correct? 1. It is a zone of convergence plate boundaries. 2. It is an active seismic and volcanic zone. 3. It is associated with deep trench. Select the correct answer using the code given below:

[Q109 · Apr · 2018]

Antarctic is a MAJOR plate

A 'which is NOT a minor plate' question hides the Antarctic Plate among Cocos/Nazca/Caroline. Antarctic is one of the seven majors, so it is the odd one out.

Ring of Fire ≠ purely convergent

The Ring of Fire is dominated by convergent subduction, but it also includes transform segments, so 'a zone of convergent boundaries' is treated as not fully correct. What IS always true: it is an active seismic + volcanic zone with deep trenches.

Drill 4 more on tectonic plates and their boundaries

Concept 6 of 6

Folds and crustal deformation

Intuition

When plates push together, layered rock buckles into folds instead of breaking. The shape of a fold is described by its axial plane — the imaginary surface that splits the fold symmetrically. As compression increases, folds tilt from upright to overturned to recumbent (lying down).

Definition

Fold types by how far compression has tilted the axial plane:

Symmetrical / upright — axial plane vertical.
Asymmetrical — axial plane inclined.
Overturned — one limb pushed past vertical.
Recumbent — axial plane virtually HORIZONTAL (extreme compression, the fold lies on its side).
Isoclinal — both limbs parallel (dipping the same way).

Folding at convergent boundaries is how fold mountains (Himalayas, Alps) are raised.

Worked example

In which fold is the axial plane virtually horizontal?

Upright = vertical axial plane; asymmetrical/overturned = inclined.
As compression lays the fold flat, the axial plane approaches horizontal.
That extreme is the recumbent fold.

Answer:A recumbent fold.

Practice this concept2 quick reps

Practice — Level 1 (2 reps)

Quick reps to lock in the method. Try each, then check.

1.
A fold whose axial plane is horizontal is called?
2.
Folding builds which class of mountains?

From the bank · past-year question

Example 6Earth's Structure, Landforms and Geological TimeMODERATE

In which one of the following folds is the axial plane found to be virtually horizontal?

[Q115 · Sep · 2019]

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 (1)

Discontinuities between the layers5 rows

Discontinuity	Separates	Position
Conrad	Upper / lower crust	Shallow
Mohorovicic (Moho)	Crust / mantle	Base of crust NDA 2022 — Moho is THE crust-mantle boundary.
Repetti	Upper / lower mantle	Mid-mantle
Gutenberg	Mantle / outer core	Deep
Lehmann	Outer core / inner core	Innermost NDA 2023 — Lehmann is the deepest, in the innermost part of the Earth.

Watch out for (7)

Lithosphere is NOT just the crust
→ The layers of the Earth and the lithosphere
Hottest is not always liquid
→ The layers of the Earth and the lithosphere
Moho vs Gutenberg
→ Discontinuities between the layers
Convection is in the MANTLE, not the crust
→ Oceanic vs continental crust
Oceanic crust is thinner, not thicker
→ Oceanic vs continental crust
Antarctic is a MAJOR plate
→ Tectonic plates and their boundaries
Ring of Fire ≠ purely convergent
→ Tectonic plates and their boundaries

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Earth's Structure, Landforms and Geological TimeEASY

Which one of the following best describes the Lithosphere?

[Q66 · Sep · 2021]

Example 2Earth's Structure, Landforms and Geological TimeHARD

Which one of the following is found in the innermost part of the Earth?

[Q133 · Apr · 2023]

Example 3Earth's Structure, Landforms and Geological TimeMODERATE

Consider the following statements: I. The continental crust is thinner under fold mountains. II. The mean thickness of ocean crust is 30 km. III. The Earth's crust is brittle in nature. Which are correct?

[Q104 · Apr · 2026]

Example 4Earth's Structure, Landforms and Geological TimeMODERATE

Which one of the following factors does not affect the distribution of groundwater?

[Q55 · Sep · 2021]

Example 5Earth's Structure, Landforms and Geological TimeMODERATE

Which of the following statements regarding Earth's internal structure is/are correct ? 1. The oceanic crust is heavier than the continental crust. 2. Most of the Earth's internal heat is contained within the mantle. 3. Large convective cells in the crust circulate heat and drive plate-tectonic processes. Select the answer using the code given below :

[Q115 · Apr · 2024]

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