Interior of the Earth – NDA Geography Notes

Introduction

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.

This chapter covers the layers of the Earth, the boundaries between them, the composition of the crust, and the concept of the lithosphere. 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.

Understand the Topic

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 — not by design, but because physics and gravity caused heavy things to sink and light things to float.

The Layers of the Earth

The Earth has three main layers — the crust, the mantle, and the core. Each has very different properties in terms of composition, temperature, pressure, and density.

1. The Crust

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

There are two types of crust:

Continental Crust — the crust that 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 — so it is called SIAL (Silicon + Aluminium).

Oceanic Crust — the crust that 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).

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 the thin oceanic crust forms the floor of the deep sea. Under mountain ranges like the Himalayas, the crust is actually the thickest — mountains have deep roots extending down into the mantle. [NDA 2009-II]

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

Elements in the crust:

The most abundant element in the Earth’s crust by weight is oxygen (about 46%). Silicon is second (about 28%). Among the elements commonly asked about — oxygen is highest and magnesium is in much smaller quantity. [NDA tested via CDS]

2. 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 — similar to how glass appears solid but flows extremely slowly over centuries.

Both temperature and pressure increase as you go deeper into the Earth. [NDA tested via CDS]

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

Convection currents in the mantle — hot material in the mantle rises, spreads sideways, cools, and sinks again in a slow circular movement called convection. These convection currents are what drives plate tectonics — they pull and push the tectonic plates on the surface. The convective cells driving plate tectonics are in the mantle — not in the crust. [NDA 2024-I]

3. The Core

The core is the innermost layer. It extends from about 2,900 km depth to the centre at 6,371 km.

The core 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 at 6,371 km. 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.

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	Location	What it Separates
Conrad Discontinuity	Within the continental crust	Upper crust from lower crust
Mohorovičić Discontinuity (Moho)	Base of the crust	Crust from Mantle
Gutenberg Discontinuity	~2,900 km depth	Mantle from Outer Core
Lehmann Discontinuity	~5,100 km depth	Outer Core from Inner Core

The Lehmann Discontinuity is the deepest of all — it separates the liquid outer core from the solid inner core, at about 5,100 km depth. [NDA 2023-I]

The Gutenberg Discontinuity separates the mantle from the outer core at about 2,900 km — the second deepest.

The Moho (Mohorovičić Discontinuity) separates the crust from the mantle — the most famous discontinuity.

The Conrad Discontinuity is within the crust itself — the shallowest of the four.

From deepest to shallowest: Lehmann → Gutenberg → Moho → Conrad.

The Lithosphere

The lithosphere is one of the most frequently tested terms in this chapter — and it is also one of the most frequently confused.

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

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

The lithosphere is broken into several large pieces called tectonic plates — covered in Chapter 8.

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

The 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.

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
• Volcanoes — bring material from deep inside the Earth to the surface, giving us direct rock samples

Indirect sources:

• Gravitational force
• Earth’s magnetism

Earthquake waves and volcanoes are the direct sources. Gravitational force and Earth magnetism are indirect sources. [NDA tested via CDS]

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