Interior of the Earth – NDA Geography Notes

The Earth’s surface looks permanent and solid to us. But below that surface lies a world we have never directly seen: layers of rock, liquid metal, and solid iron extending thousands of kilometres to the Earth’s centre. We know about this hidden interior not by digging into it (the deepest hole humans have ever drilled is only about 12 kilometres) but by studying how earthquake waves travel through it.

NDA tests this chapter through direct questions about which layer is which, what the lithosphere consists of, what the discontinuities are called, and where the crust is thinnest. These are factual but logical. Once you understand the structure clearly, the answers become obvious.

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Why the Earth Has Layers

When the Earth first formed about 4.6 billion years ago, it was extremely hot, so hot that the entire interior was molten rock. In this molten state, materials separated by density. Heavier materials sank toward the centre, lighter materials rose to the surface. This process is called differentiation.

Heavy elements like iron and nickel sank to form the core. Lighter silicate rocks rose to form the mantle and crust. This is why the Earth has a layered structure today. Physics and gravity caused heavy things to sink and light things to float.

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The Crust

The crust is the outermost solid layer of the Earth, the thinnest of all layers, like the skin of an apple compared to the fruit inside.

Continental Crust forms the landmasses. It is about 30–50 km thick on average, thicker under mountain ranges (up to 70 km) and thinner elsewhere. It is made mostly of lighter rocks rich in silicon and aluminium, called SIAL (Silicon + Aluminium).

Oceanic Crust lies under the oceans. It is much thinner, only about 5–10 km. It is made of heavier rocks rich in silicon and magnesium, called SIMA (Silicon + Magnesium).

Feature	Continental Crust	Oceanic Crust
Composition	SIAL (Silicon + Aluminium)	SIMA (Silicon + Magnesium)
Thickness	30–50 km (up to 70 km under mountains)	5–10 km
Density	Lighter	Heavier (denser)

The Earth’s crust is brittle in nature. It breaks and fractures under stress rather than flowing. [NDA 2022-II]

The oceanic crust is heavier (denser) than the continental crust. This is why when oceanic and continental plates collide, the oceanic plate sinks beneath the continental plate. [NDA 2024-I]

Where is the crust thinnest? The crust is thinnest at the ocean bottoms: where thin oceanic crust forms the floor of the deep sea. Under mountain ranges like the Himalayas, the crust is thickest: mountains have deep roots extending into the mantle. [NDA 2009-II]

The oceanic crust’s mean thickness is about 5–10 km: NOT 15 km as is sometimes incorrectly stated. [NDA 2022-II]

Most abundant element in the Earth’s crust by weight = Oxygen (~46%). Silicon is second (~28%). Magnesium is present in much smaller quantity.

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The Mantle

The mantle is the thick layer between the crust and the core. It makes up about 84% of Earth’s volume, the largest layer by far. The mantle extends from the base of the crust down to about 2,900 km depth.

The mantle is solid, but it behaves like a very thick, slow-flowing liquid over millions of years. Both temperature and pressure increase as you go deeper into the Earth.

Most of Earth’s internal heat is contained within the mantle. [NDA 2024-I]

Convection currents in the mantle: hot material rises, spreads sideways, cools, and sinks again in slow circular movements. These convection currents drive plate tectonics, pulling and pushing the tectonic plates on the surface. The convective cells driving plate tectonics are in the mantle: NOT in the crust. The crust is too thin and rigid to generate convective cells. [NDA 2024-I]

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The Core

The core is the innermost layer, extending from about 2,900 km depth to the centre at 6,371 km. It is divided into two parts.

Outer Core: from about 2,900 to 5,100 km depth. It is liquid: made of molten iron and nickel. The flow of this liquid metal generates Earth’s magnetic field.

Inner Core: from about 5,100 km to the centre. Despite temperatures over 5,000°C, it is solid: because the pressure at this depth is so enormous that it cannot melt.

The core is made mostly of iron and nickel, the heaviest common elements, which sank to the centre during Earth’s early molten phase.

Layer	Depth	State	Composition	Key Feature
Crust	0–35 km (avg)	Solid	SIAL / SIMA	Thinnest layer
Mantle	35–2,900 km	Solid (slowly flows)	Silicate rock	84% of Earth’s volume; most heat
Outer Core	2,900–5,100 km	Liquid	Iron + Nickel	Generates magnetic field
Inner Core	5,100–6,371 km	Solid	Iron + Nickel	Hottest; solid due to pressure

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Discontinuities: The Boundaries Between Layers

The boundaries between the Earth’s layers are called discontinuities, named after the scientists who discovered them by studying how earthquake waves change speed and direction at these boundaries.

Discontinuity	Depth	Separates
Conrad Discontinuity	Within continental crust	Upper crust from lower crust
Mohorovičić Discontinuity (Moho)	Base of crust (~35 km avg)	Crust from Mantle
Gutenberg Discontinuity	~2,900 km	Mantle from Outer Core
Lehmann Discontinuity	~5,100 km	Outer Core from Inner Core

Order from shallowest to deepest: Conrad → Moho → Gutenberg → Lehmann.

★ IMPORTANT  The Lehmann Discontinuity is the deepest, separating the liquid outer core from the solid inner core at ~5,100 km. [NDA 2023-I] The Gutenberg Discontinuity separates the mantle from the outer core at ~2,900 km. The Moho separates the crust from the mantle. The Conrad is within the crust itself, the shallowest.

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The Lithosphere and Asthenosphere

Lithosphere

The lithosphere is NOT just the crust. It is the crust PLUS the uppermost solid part of the mantle, together forming a rigid, brittle outer shell of the Earth. [NDA 2015-II | NDA 2019-II | NDA 2021-I | NDA 2021-II]

This definition has appeared in NDA four times in different forms, always testing the same fact. The answer is always: lithosphere = crust + upper mantle.

The lithosphere is broken into several large pieces called tectonic plates (covered in the chapter on Plate Tectonics).

The maximum depth (thickness) of the lithosphere is found under the Himalayan Mountains: because the crust there is thickest. [NDA 2021-I]

Asthenosphere

Just below the lithosphere, in the upper mantle, is a zone called the asthenosphere. Unlike the lithosphere which is rigid, the asthenosphere is partially molten and can flow slowly. The tectonic plates of the lithosphere float on and move over the asthenosphere.

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Sources of Information About the Earth’s Interior

Direct sources:

Earthquake waves: seismic waves travel through the Earth and reveal its internal structure through changes in speed and direction at different boundaries.

Volcanoes: they bring material from deep inside the Earth (the mantle) to the surface, giving us direct rock samples.

Indirect sources: Gravitational force and Earth’s magnetism give us indirect clues about the interior but do not directly sample or pass through it.

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