Lithosphere

Geography glossary

Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

The lithosphere ("litho" = rock) is the rigid outer shell of the earth. It is made of the crust plus the uppermost part of the mantle and is 10-200 km thick: thinner under the oceans and thicker under the continents. It matters because it is broken into tectonic plates. The way these plates move explains where earthquakes, volcanoes and fold mountains form.

Explanation

Two ways to divide the outer earth

By rock type (chemistry): crust → mantle. The Moho (Mohorovicic discontinuity) is the boundary between them.
A discontinuity is a sharp boundary where the speed of seismic waves (earthquake waves) suddenly changes.
By strength (how rigid or soft the rock is): lithosphere (rigid) → asthenosphere (weak and plastic).
These two ways do not match up.
The lithosphere holds all of the crust and the top part of the mantle.
So the base of the lithosphere lies below the Moho, not at it.
The Moho lies almost entirely inside the lithosphere. It meets the base of the lithosphere only beneath mid-ocean ridges, where the rigid layer is very thin [2].

Parts and thickness

Crust (top part of the lithosphere): solid and brittle, so it breaks rather than bends.
Oceanic crust: about 5 km thick (range 5-10 km). It is made of basalt (a dark, heavy volcanic rock) and is called sima (silica + magnesium).
Continental crust: about 30 km thick, and up to 70 km under the Himalayas. It is made of granite (a light-coloured, lighter rock) and is called sial (silicate + aluminium).
Uppermost mantle (lower part of the lithosphere): rigid rock that is welded to the crust above it. The crust and this layer move together as one shell.
Thickness of the whole lithosphere:
Under the oceans: thinner, 5-100 km. It is thinnest at mid-ocean ridges.
Under the continents: thicker, about 200 km.
Below it lies the asthenosphere ("astheno" = weak). This is a weak, plastic, partly molten zone in the upper mantle that goes down to about 400 km (NCERT).
Plastic means the rock is solid but can flow slowly over very long periods, like soft toffee.

How the lithosphere works: plates, earthquakes and volcanoes

The rigid shell is broken into plates.
These pieces are called tectonic plates, for example the Indian, Eurasian, Pacific, Arabian and Anatolian plates.
They move slowly over the soft asthenosphere, which lets them slide.
Earthquakes start in the lithosphere.
Plates push together, pull apart or slide past each other.
The brittle rock bends under the stress until it suddenly breaks.
The energy that is released is an earthquake.
Volcanoes form at certain plate edges.
Where one plate sinks under another, or where plates move apart, magma rises.
This magma comes mainly from the asthenosphere and forms volcanoes.
The sea floor is made and destroyed.
New basalt rises and cools at mid-ocean ridges (underwater mountain chains where plates move apart).
Old, heavy sea floor sinks back into the mantle at trenches (deep, narrow cuts in the ocean floor).
Result: no oceanic crust is older than about 200 million years. Light continental crust cannot sink, so it survives for about 4 billion years (NCERT: over 3,200 million years).
Mountains have deep roots.
Big mountains like the Himalayas have a deep "root" of light crust pushed down into the mantle.
This root holds the high land up, the way the large hidden part of an iceberg holds it up. This balance is called isostasy.

Where it occurs

Everywhere on the earth: the lithosphere covers the whole planet as one broken shell of plates.
Himalayas (India): the Indian plate keeps pushing into the Eurasian plate. The crust here is about 70 km thick, and the lithosphere is under constant stress. The Himalayan belt and North-East India are among the most quake-prone parts of India.
Bhuj (Gujarat, India), 2001: an earthquake inside the Indian plate, far from any plate edge. It shows that stress can build up anywhere in the brittle lithosphere.
Tohoku (Japan), 2011: the Pacific plate sinks under the plate carrying northern Japan. This is a subduction zone (a place where one plate dives under another). The quake set off a huge tsunami.
Turkey-Syria, 2023: the Anatolian and Arabian plates slide past each other along the East Anatolian Fault.
Mid-ocean ridges: this is where the oceanic lithosphere is thinnest and new sea floor forms. It is the only place where the Moho forms the base of the lithosphere [2].
Pacific "Ring of Fire": volcanoes form here where plates sink, fed by magma from the asthenosphere below the lithosphere.

Don't confuse with

Crust: the crust is only the top part of the lithosphere and is divided by rock type. The lithosphere is crust + uppermost mantle and is divided by rigidity.
Asthenosphere: it lies below the lithosphere and is weak, plastic and partly molten (down to about 400 km). The lithosphere is rigid and brittle. Plates are made of lithosphere and move over the asthenosphere.
Moho: the Moho is the crust-mantle boundary, not the base of the lithosphere. It lies almost entirely inside the lithosphere [2].
Lower mantle: it is solid because very high pressure stops the rock from melting, but it is deep below the asthenosphere and is not part of the lithosphere.

Prelims Hooks

Lithosphere = crust + uppermost mantle. It is NOT crust + core.
Thickness: 10-200 km overall. 5-100 km under the oceans (thinner) and about 200 km under the continents (thicker).
Order from the surface downward: crust → Moho → lithospheric mantle → asthenosphere (to about 400 km) → lower mantle → core boundary at 2,900 km.
Trap: the Moho is not the base of the lithosphere. The two meet only beneath mid-ocean ridges [2].
Lithosphere is rigid and brittle, so earthquakes start in it. The asthenosphere is the main source of magma.
Bhuj 2001 = quake inside a plate (intraplate). Tohoku 2011 = subduction quake. Turkey-Syria 2023 = East Anatolian Fault (Anatolian and Arabian plates sliding past each other).

Mains Points

Plate tectonics depends on the lithosphere and asthenosphere working together.
A rigid lithosphere, broken into plates, moves over a weak, partly molten asthenosphere.
Stress builds up at the plate edges and breaks the brittle rock, causing earthquakes, while rising magma forms volcanoes.
Examples: the subduction quake at Tohoku 2011, the strike-slip quake (plates sliding past each other) in Turkey-Syria 2023, and the intraplate quake at Bhuj 2001.
India's earthquake risk (GS-III, disasters):
The Indian plate keeps pushing into the Eurasian plate, so the Himalayan lithosphere is under constant stress.
Result: the Himalayan belt and North-East India are highly quake-prone. Bhuj (2001) showed that even "stable" peninsular India can have strong quakes.
What India needs: earthquake-safe building rules, seismic zoning and a good network of earthquake-recording stations.
Oceanic and continental lithosphere behave differently.
Young, heavy oceanic lithosphere sinks at trenches. This makes subduction zones the sites of deep earthquakes, volcanic arcs and tsunamis.
Light, old continental lithosphere stays at the surface and crumples into fold mountains such as the Himalayas.

Related concepts

Structure of the earth I: crust, mantle, lithosphere and asthenosphere

Sources

1Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 4 "Distribution of Oceans and Continents" (primary)
2Britannica, "Mohorovičić discontinuity (Moho)"britannica.com · tier 3