Maarg Manthan · Topic 1.18

Lithosphere and Asthenosphere

Geography › Earth's Origin, Interior and Motions · Topic 1.18

The lithosphere is the rigid outer shell of the Earth, made of the crust and the uppermost part of the mantle, and it is broken into plates. Its thickness is about 100 km on average, thinner under the oceans and thicker under the continents. Below it lies the asthenosphere, a weaker, hotter and partly molten layer of the upper mantle, on which the plates move. The two layers are defined by how they flow, not by what they are made of, and their behaviour is the basis of plate tectonics and isostasy.

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

At a Glance

Lithosphere Crust plus rigid uppermost mantle; about 100 km
Asthenosphere Weak, partly molten upper mantle layer
Boundary Marked by the slow-wave zone
Role Plates move over the asthenosphere
Related concept Isostasy
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic follows the one on the internal structure and takes the two outer mechanical layers in detail. Everything in the next macros, from plate boundaries to volcanoes and earthquakes, depends on the relation between the rigid lithosphere and the weak asthenosphere. Previous topic: Internal Structure of the Earth: Crust, Mantle, Outer Core, Inner Core. Next topic: Seismic Discontinuities (Conrad, Mohorovicic, Repetti, Gutenberg, Lehmann).

What Is the Lithosphere?

In short: the lithosphere is the cool, strong outer shell of the Earth, and it is made of the crust and the top of the mantle.

Make-up: it includes the oceanic or continental crust and the uppermost, rigid part of the mantle.
Thickness: it averages about 100 km. Oceanic lithosphere is thin at ridges, where it is new, and thickens to about 100 km and more as it ages and cools. Continental lithosphere is usually 100 to 200 km thick and is thicker, up to about 250 km or more, under old stable cores called cratons.
Behaviour: it is cool and rigid, and it breaks when stressed, causing earthquakes. It is divided into about seven major and many minor plates.
Not the same as the crust: the crust is defined by its chemistry, while the lithosphere is defined by its strength.

The Indian cratons, such as the Dharwar craton, have deep, cold lithospheric roots that have survived for billions of years, which helps to explain the stability of the Peninsular shield.

What Is the Asthenosphere?

The asthenosphere (from the Greek word for ‘weak’) lies below the lithosphere in the upper mantle. It begins at roughly 100 km below the oceans and somewhat deeper below the continents and extends down to about 400 km, and some definitions carry it to 660 km.

Hot and weak: the temperature is close to the melting point of mantle rock, at about 1,300 °C near its top.
Partial melting: a small part of the rock, probably 1 to 2 per cent, is melted, which lets the layer flow slowly like a very thick fluid over millions of years while still being solid.
Low velocity zone: seismic waves slow down in it, which is how it was identified, as noted in P waves Pass through solids and liquids; fastest S waves Pass through solids only Key clue Change of wave…">Understanding the Earth’s Interior Through Seismic Waves.
Source of magma: where the asthenosphere rises or its pressure falls, as under mid-ocean ridges and at hotspots, it melts further and feeds volcanoes.

How Do the Two Layers Compare?

Feature	Lithosphere	Asthenosphere
Position	Outermost layer	Below the lithosphere, in the upper mantle
Depth	0 to about 100 km (up to 250 km under cratons)	About 100 to 400 km
Behaviour	Rigid, brittle	Weak, ductile, flows slowly
Temperature	Cool; rises with depth	Near the melting point
Movement	Broken into moving plates	Slow convective flow
Earthquakes	Common	Rare because rock flows instead of breaking
Seismic waves	Fast	Slower (low velocity zone)

The boundary between them is not a sharp line but a gradual change, and its depth depends on the local temperature.

Why Do These Layers Matter: Isostasy and Plate Movement?

Because the lithosphere is lighter than the dense asthenosphere beneath, it floats on it like a block of wood on water. This balance is called isostasy. Two models explain it: the Airy model of 1855 assumes that blocks of the same density float with different heights and have roots of different depths, so high mountains have deep roots, while the Pratt model of 1854 assumes that blocks float to the same depth, but have different densities.

Isostatic adjustment: when an ice sheet melts, the land below slowly rises, which is still going on in Scandinavia and Canada. Heavy loads, such as thick sediments in deltas, press the crust down.
Plate motion: plates glide on the weak asthenosphere, driven by mantle convection and by the pull of sinking slabs, as in Plate Tectonics: Major and Minor Lithospheric Plates.
Volcanism and earthquakes: melting in the asthenosphere and breaking of the lithosphere are the root of both.

As an analytical point, the contrast in strength, not in chemistry, between these two layers is what allows plate tectonics to operate on the Earth.

Why Is This Topic Important?

Core idea Basis of plate tectonics
Concept Isostasy and mountain roots
Confusion Crust versus lithosphere

What Came Next?

The next topic lists the seismic discontinuities, the boundaries between the layers of the Earth. Next topic: Seismic Discontinuities (Conrad, Mohorovicic, Repetti, Gutenberg, Lehmann).

Key People and Terms

Lithosphere Rigid outer shell
Asthenosphere Weak layer below it
Isostasy Floating balance of the crust
Craton Ancient stable continental core
Partial melt Small share of liquid in rock
Convection Slow circulation of heated rock

Exam Corner

Points to Remember

The lithosphere = crust + rigid upper mantle.
The asthenosphere is weak and partly molten.
Average lithosphere thickness: about 100 km.
Airy (roots) and Pratt (density) explain isostasy.
Plates move on the asthenosphere.

Do Not Confuse With

Lithosphere and crust Strength-based shell versus chemical layer
Asthenosphere and mesosphere Weak upper layer versus stiffer lower mantle
Airy and Pratt Different depth of roots versus different densities

Memory Hook

Lithos is stone, rigid and thin; Astheno is weak, where plates begin.

Mains Angle

Questions ask for the role of these layers in plate movement. These points add depth.

Contrast Strength, not composition
Isostasy Balance of crustal blocks
Link Plates, magma and earthquakes

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between the lithosphere and the asthenosphere and explain their importance for plate tectonics.”

How to Answer

Define Both layers with depths.
Compare Strength, heat and motion.
Isostasy Floating balance.
Link Plates and volcanoes.

GS Relevance

Prelims: definition, thickness and role of lithosphere and asthenosphere, isostasy. GS Paper 1: interior of the Earth and plate tectonics.

Frequently Asked Questions

What is the lithosphere?

The lithosphere is the rigid, cool outer shell of the Earth, consisting of the crust and the uppermost part of the mantle. It averages about 100 km in thickness, is thinner under the oceans and thicker under the continents, and is broken into the plates that move over the weaker layer below.

What is the asthenosphere?

The asthenosphere is the weak, hot and partly molten layer of the upper mantle below the lithosphere, extending from about 100 km to roughly 400 km depth. It is solid but flows very slowly, which lets the lithospheric plates move over it. It is also the main source of magma.

How is the lithosphere different from the crust?

The crust is the outermost layer defined by its rock composition, while the lithosphere is defined by its strength. The lithosphere includes the crust and also the rigid uppermost mantle, so it is always thicker than the crust and acts as one mechanical unit.

What is isostasy?

Isostasy is the state of balance between the lithosphere and the denser asthenosphere, in which blocks of crust float at levels that depend on their thickness and density, much like icebergs in water. High mountains have deep roots, and land rises or sinks as loads such as ice are added or removed.

Why is the asthenosphere important for plate tectonics?

The asthenosphere is weak and can flow slowly, so the rigid lithospheric plates can glide over it. Its slow convective currents and the pull of sinking plates drive plate motion, and its melting produces magma at ridges and hotspots, linking it to volcanoes and earthquakes.

PYQ Practice — Statement Analysis

1 The lithosphere includes the crust and the uppermost part of the mantle.

True

It is a layer defined by strength.

2 The asthenosphere is a rigid layer.

False

It is weak and partly molten.

3 The Airy model of isostasy assumes blocks of different density.