Maarg Manthan · Topic 2.3

Crust, Mantle and Core – Internal Structure of the Earth

Geography › Interior of the Earth and Magnetic Field · Topic 2.3

The Earth has three main layers: the thin solid crust, the mantle that extends to about 2,900 km, and the core that extends to the centre at about 6,371 km. The crust and the uppermost solid mantle form the lithosphere, the mantle includes the plastic asthenosphere, and the core has a liquid outer part and a solid inner part made mainly of iron and nickel.

6 min read · UPSC Prelims · Geography · Interior of the Earth and Magnetic Field

At a Glance

Crust 5 to 70 km thick; oceanic about 5 km, continental about 30 km
Mantle From the crust to about 2,900 km
Outer core Liquid; about 2,900 to 5,150 km
Inner core Solid; about 5,150 to 6,371 km
Core composition Iron and nickel (nife)
Exam link UPSC Prelims, Geography: Interior of the Earth and Magnetic Field

Where Does It Fit?

This is the third topic of the category on the interior of the Earth and its magnetic field. The previous topic explained seismic waves, and this one uses them to describe the layers. The next topic covers the discontinuities between the layers. Previous topic: Seismic Waves and Their Types. Next topic: Seismic Discontinuities of the Earth.

What Are the Main Layers of the Earth?

In short: the Earth is built like an egg with a thin crust, a thick mantle and a dense core.

Studies of seismic waves show that the interior is arranged in concentric layers that differ in density and state. The three major layers are the crust, the mantle and the core, and each is subdivided. The temperature and pressure rise with depth, and the density increases from about 3 g per cm cubed at the surface to about 13 g per cm cubed at the centre.

Layer	Approximate depth	State	Main materials
Crust	0 to 5-70 km	Solid	Silicates; granite and basalt
Mantle	To about 2,900 km	Solid, with a plastic upper part	Iron and magnesium silicates
Outer core	About 2,900 to 5,150 km	Liquid	Iron and nickel
Inner core	About 5,150 to 6,371 km	Solid	Iron and nickel

What Is the Crust?

The crust is the outermost solid layer and the thinnest, forming about 1 per cent of the volume and 0.5 per cent of the mass of the Earth. Its thickness varies: oceanic crust is about 5 km thick, while continental crust averages about 30 km and can reach about 70 km below high mountain ranges such as the Himalayas. The continental crust is rich in silica and aluminium and is called sial, and its rocks are mainly granitic with a density of about 2.7 g per cm cubed. The oceanic crust is richer in silica and magnesium and is called sima, with basaltic rocks of a density of about 3 g per cm cubed. Oceanic crust is also younger, no more than about 200 million years old, while continental rocks may be billions of years old.

Oxygen (about 46.6 per cent), silicon (27.7), aluminium (8.1), iron (5), calcium (3.6), sodium (2.8), potassium (2.6) and magnesium (2.1) make up the bulk of the crust by weight.

What Is the Mantle?

The portion of the interior below the crust up to a depth of about 2,900 km is the mantle. It forms about 84 per cent of the volume and about 68 per cent of the mass of the Earth. The upper mantle extends to about 700 km, and the upper part of it, with the crust, forms the rigid lithosphere, whose thickness ranges from about 10 km to 200 km. Below the lithosphere is the asthenosphere, a weak, plastic and partly molten layer at about 100 to 400 km. Magma that reaches the surface through volcanoes originates in this layer, and the lithospheric plates move over it. The lower mantle is solid and denser, as pressure there is very high, and the average density of the mantle is about 3.4 g per cm cubed. The origin of plate movement is in Plate Tectonics and Types of Plate Boundaries.

What Is the Core?

The core lies at a depth of about 2,900 km below the surface and extends to the centre at about 6,371 km. It forms about 16 per cent of the volume but about 32 per cent of the mass of the Earth, because it is very dense. It is composed mainly of iron and nickel, often called nife. The core has two parts. The outer core, about 2,200 km thick, is liquid, as shown by the absence of S waves in it. The inner core, about 1,200 km in radius, is solid despite temperatures of about 5,000 to 6,000 degrees Celsius, because the enormous pressure keeps the metal from melting. The density at the centre is about 13 g per cm cubed. The motion of the liquid outer core generates the magnetic field, as explained in Dynamo Theory and the Magnetic Field of the Earth.

How Do the Layers Compare?

The crust is thin and light, the mantle is thick and intermediate, and the core is small in volume but heavy. The difference between lithosphere and asthenosphere is based on strength, whereas the difference between crust and mantle is based on composition, so the lithosphere includes both crust and uppermost mantle. As an analytical point, this distinction explains why plates are made of crust together with rigid upper mantle, and not of crust alone. The boundaries between the layers are discussed in Seismic Discontinuities of the Earth.

Why Is This Topic Important?

Foundation Basis of plate tectonics
Factual Thickness and composition asked
Concept Lithosphere and asthenosphere

What Came Next?

The next topic describes the seismic discontinuities of the Earth. Next topic: Seismic Discontinuities of the Earth.

Key People and Terms

Sial Silica and aluminium; continental crust
Sima Silica and magnesium; oceanic crust
Lithosphere Crust plus rigid upper mantle
Asthenosphere Weak plastic layer in the upper mantle
Nife Iron and nickel of the core
Outer core Liquid layer

Exam Corner

Points to Remember

Oceanic crust is about 5 km; continental about 30 km.
Mantle extends to about 2,900 km.
The outer core is liquid; the inner core is solid.
Core forms 16 per cent of volume but about 32 per cent of mass.
Lithosphere = crust plus upper mantle.

Do Not Confuse With

Crust and lithosphere Crust alone versus crust plus rigid upper mantle
Sial and sima Continental versus oceanic crust
Outer and inner core Liquid versus solid

Memory Hook

Crust thin, mantle thick, core heavy; outer flows, inner stays solid.

Mains Angle

Questions ask for the structure and composition. These points add depth.

Layers Differ in composition and state
Lithosphere Plates are lithospheric
Core Source of magnetism

A Question You May Face

An original practice question, not a past paper question.

“Describe the internal structure of the Earth and explain the difference between the crust and the lithosphere.”

How to Answer

Layers Crust, mantle and core.
Composition Sial, sima and nife.
State Solid and liquid parts.
Lithosphere Strength-based layer.

GS Relevance

Prelims: layers, thickness, composition, lithosphere. GS Paper 1: interior of the Earth.

Frequently Asked Questions

What are the three main layers of the Earth?

The three main layers of the Earth are the crust, the mantle and the core. The crust is the thin outer solid layer, the mantle extends to about 2,900 km, and the core extends to the centre at about 6,371 km and is made of iron and nickel.

What is the difference between sial and sima?

Sial is the continental crust, rich in silica and aluminium, with granitic rocks of density about 2.7 g per cm cubed. Sima is the oceanic crust, rich in silica and magnesium, with denser basaltic rocks of density about 3 g per cm cubed.

What is the lithosphere?

The lithosphere is the rigid outer shell of the Earth, made of the crust and the uppermost solid part of the mantle. Its thickness ranges from about 10 km to 200 km, and the tectonic plates are pieces of the lithosphere moving over the asthenosphere.

What is the asthenosphere?

The asthenosphere is a weak, plastic and partly molten layer of the upper mantle, at roughly 100 to 400 km below the surface, lying beneath the lithosphere. It is the source of the magma that reaches the surface and the layer over which plates move.

Why is the inner core solid although it is hot?

The inner core has a temperature of about 5,000 to 6,000 degrees Celsius, but the pressure at the centre of the Earth is enormous, and it keeps the iron and nickel in a solid state. The outer core, with lower pressure, remains liquid.

PYQ Practice — Statement Analysis

1 The continental crust is thicker than the oceanic crust.

True

About 30 km against about 5 km.

2 The outer core of the Earth is solid.

False

It is liquid; the inner core is solid.

3 The lithosphere includes the crust and the upper part of the mantle.