Maarg Manthan · Topic 1.4

Layered Structure of the Earth (Five Layers)

Geography › Earth's Origin, Interior and Motions · Topic 1.4

The Earth is made of layers that differ in composition, density and state. By composition, the five layers are the crust, upper mantle, lower mantle, outer core and inner core. By physical strength, the outer part is divided into the rigid lithosphere and the weak asthenosphere, above the stronger mesosphere and the two core layers. The crust is the thinnest, the mantle the thickest, and the core the hottest and densest, with the outer core liquid and the inner core solid.

5 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

Crust Solid; thin; about 1 per cent of volume
Mantle To about 2,900 km; about 84 per cent of volume
Outer core Liquid; 2,900 to 5,150 km
Inner core Solid; 5,150 to 6,371 km
Centre temperature About 5,000 to 6,000 °C
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic gives the overview of the five layers that the Earth separated into after its formation. It prepares the ground for the detailed topics on seismic waves, discontinuities and the internal structure later in this macro. Previous topic: The Solar System and the Evolution of the Earth. Next topic: Evolution of the Lithosphere, Atmosphere and Hydrosphere.

What Are the Five Layers of the Earth?

In short: the Earth has a thin crust on top, a thick mantle in the middle and a metallic core at the centre, and each of the mantle and core is split into two.

Layer	Approximate depth	State	Main material
Crust	0 to about 35 km (5 to 70 km)	Solid	Silicate rocks; granite on land, basalt under the oceans
Upper mantle	Crust to about 660 km	Mostly solid, with a weak partly molten zone	Peridotite (iron and magnesium silicates)
Lower mantle	About 660 to 2,900 km	Solid but hot and slowly flowing	Dense silicates and oxides
Outer core	About 2,900 to 5,150 km	Liquid	Iron and nickel with lighter elements
Inner core	About 5,150 to 6,371 km	Solid	Iron and nickel

The mantle forms about 84 per cent of the Earth’s volume and about two-thirds of its mass. The core makes up about a third of the mass but only about 15 per cent of the volume, because it is so dense. The crust is less than 1 per cent of the volume.

How Do Density, Temperature and Pressure Change with Depth?

Density: the crust has a density of about 2.7 to 3.0 g per cm cubed, the mantle about 3.4 to 5.6, the outer core about 10 to 12 and the inner core about 13. The average density of the whole Earth is about 5.5 g per cm cubed, which is why a heavy core must exist.
Temperature: it rises with depth, from the surface to perhaps 5,000 to 6,000 °C at the centre, roughly as hot as the surface of the Sun.
Pressure: it rises with the weight of the rock above, to millions of times atmospheric pressure at the centre. High pressure keeps the inner core solid even though it is very hot.

The heat comes from the energy left over from the Earth’s formation and from the decay of radioactive elements.

How Are the Layers Divided by Strength?

Layers can also be named by how they behave under stress. This view is used in plate tectonics.

Mechanical layer	Includes	Behaviour
Lithosphere	Crust and the uppermost rigid mantle, to about 100 km	Rigid; broken into plates
Asthenosphere	Upper mantle below the lithosphere	Weak and ductile; partly molten
Mesosphere	Lower part of the mantle	Stronger and stiffer
Outer core	Liquid iron-nickel layer	Flows; makes the magnetic field
Inner core	Solid iron-nickel ball	Rigid

The two ways of dividing the Earth overlap but do not coincide. The lithosphere, for example, is thicker than the crust because it includes the top of the mantle. The full details are given in Internal Structure of the Earth: Crust, Mantle, Outer Core, Inner Core and Lithosphere and Asthenosphere.

How Do We Know the Layers Exist?

No drill has gone beyond the crust, so the layers are known from indirect evidence, mainly the way earthquake waves speed up, slow down, bend or disappear at certain depths, as in Seismic Waves: Body Waves and Surface Waves. The sharp boundaries between layers, the seismic discontinuities, are covered in Seismic Discontinuities (Conrad, Mohorovicic, Repetti, Gutenberg, Lehmann). The evidence for a liquid outer core comes from the absence of one kind of earthquake wave, the S wave, behind the core.

As an analytical point, the five-layer model is a simplification. In reality, the boundaries are zones where properties change over a short distance, and the mantle shows variations in temperature and composition from place to place, which drive the movement of plates.

Why Is This Topic Important?

Foundation Needed for plates, volcanoes and earthquakes
Facts Depths and states are asked
Concept Two ways of dividing the Earth

What Came Next?

The next topic explains how the Earth’s surface layers, the atmosphere and the oceans, came into being over time. Next topic: Evolution of the Lithosphere, Atmosphere and Hydrosphere.

Key People and Terms

Crust Outermost solid layer
Mantle Thick layer between crust and core
Outer core Liquid metallic layer
Inner core Solid metallic centre
Peridotite Main rock of the upper mantle
Mesosphere Strong lower part of the mantle

Exam Corner

Points to Remember

The mantle is about 84 per cent of the Earth’s volume.
The outer core is liquid; the inner core is solid.
Core depth starts at about 2,900 km.
The Earth’s average density is about 5.5 g per cm cubed.
The lithosphere includes the crust and the top of the mantle.

Do Not Confuse With

Crust and lithosphere Crust alone versus crust plus rigid upper mantle
Mesosphere (Earth) Lower mantle layer, not the atmospheric mesosphere
Composition layers and strength layers Two different ways of dividing the same Earth

Memory Hook

Crust, upper, lower, outer, inner: five from the skin to the centre.

Mains Angle

Questions ask for the layers and their role in tectonics. These points add depth.

Layers Composition versus strength
Heat Source of interior heat
Link Plate movement and the magnetic field

A Question You May Face

An original practice question, not a past paper question.

“Describe the layered structure of the Earth and distinguish between layers based on composition and layers based on strength.”

How to Answer

Layers Name five with depths.
Properties Density, heat, pressure.
Strength Lithosphere and asthenosphere.
Evidence Indirect, from seismic waves.

GS Relevance

Prelims: names, depths and states of the layers, lithosphere and asthenosphere. GS Paper 1: interior of the Earth.

Frequently Asked Questions

What are the five layers of the Earth?

By composition, the Earth has the crust, upper mantle, lower mantle, outer core and inner core. The crust and mantle are rocky, while the core is made of iron and nickel. The outer core is liquid and the inner core is solid.

How thick is the Earth's crust?

The crust is the thinnest layer, ranging from about 5 km under the oceans to about 70 km under high mountains, with an average continental thickness of about 30 to 35 km. It makes up less than one per cent of the Earth's volume.

Which is the largest layer of the Earth?

The mantle is the largest layer. It extends from the base of the crust to a depth of about 2,900 km and forms about 84 per cent of the volume of the Earth and roughly two-thirds of its mass.

Why is the inner core solid?

The inner core is extremely hot, at about 5,000 to 6,000 °C, but the pressure at the centre of the Earth is so enormous that the iron and nickel there cannot melt. The high pressure keeps the inner core solid, while the outer core above it is liquid.

What is the average density of the Earth?

The average density of the Earth is about 5.5 g per cm cubed, which is nearly twice the density of surface rocks. This high value shows that the interior must contain a very dense core made of iron and nickel.

PYQ Practice — Statement Analysis

1 The outer core of the Earth is in a liquid state.

True

S waves do not pass through it.

2 The mantle is thinner than the crust.

False

The mantle is the thickest layer.

3 The lithosphere consists of the crust only.