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

Detail

1. The layered earth: the main idea

• The earth is built in layers, like an onion. From the outside in, they are the crust, the mantle and the core.
• The layers are told apart by a sudden change in the speed of seismic waves (earthquake waves). Such a sharp boundary between two layers is called a discontinuity.
• There are two ways to divide the outer part of the earth:
• By rock type (chemistry): crust → mantle. The Moho is the boundary between them.

• By strength (how rigid or soft the rock is): lithosphere (rigid) → asthenosphere (weak and plastic).

• The two ways do not match up. The lithosphere holds all of the crust plus the top part of the mantle.

2. The crust

• Crust: the outermost layer of the earth. It is solid and brittle, so it breaks rather than bends. It is the thinnest layer.
• It comes in two types:

Feature	Oceanic crust	Continental crust
Mean thickness	About 5 km (range 5-10 km)	About 30 km. It reaches 70 km under the Himalayas
Main rock	Basalt: dark, heavy volcanic rock	Granite: light-coloured, lighter rock
Old name for the material	Sima (silica + magnesium)	Sial (silicate + aluminium)
Density	Higher (heavier)	Lower (lighter)
Age of oldest rocks	Nowhere older than about 200 million years	About 4 billion years (NCERT: over 3,200 million years)
Why	It is always made at mid-ocean ridges and destroyed at trenches	It is too light to sink back into the mantle, so it survives

• Why oceanic crust stays young:
• New basalt rises and cools at mid-ocean ridges. These are underwater mountain chains where plates move apart.
• The sea floor spreads outward from the ridge, like a slow conveyor belt.
• At trenches (deep, narrow cuts in the ocean floor), the old, heavy sea floor sinks back into the mantle.

• Result: no oceanic crust is older than about 200 million years.

• Why continental crust is thick under mountains:

• 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 a floating iceberg is held up by the large part below water. This balance is called isostasy.
• Result: the crust is about 70 km thick under the Himalayan region, against about 30 km on average.

3. Conrad discontinuity

• Conrad discontinuity: a boundary inside the continental crust. It separates:
• the upper crust: granitic and lighter (sial);

• the lower crust: basaltic and heavier (sima).

• At this boundary, seismic waves suddenly speed up.

• It is not found everywhere, even under the continents. It is not found in oceanic crust, because oceanic crust has no granite layer.

4. Mohorovicic discontinuity (Moho)

• Moho: the boundary between the crust and the mantle.
• Discovery: the Croatian seismologist Andrija Mohorovicic found it in 1909.
• He studied earthquake waves recorded at different distances.
• Some waves reached far-away stations faster than expected. They had travelled through a deeper, faster layer.

• Result: he worked out that a sharp boundary lies under the crust.

• What happens at the Moho: seismic wave speeds jump sharply (rise quickly) [2].

• Depth:
• 5-10 km below the ocean floor. Britannica gives about 7 km below oceanic crust [2].
• 30-70 km below the continents. Britannica gives about 35 km as the usual depth [2].

• It is deepest under high mountain ranges such as the Himalayas.

• Link to the lithosphere: the Moho lies almost entirely inside the lithosphere. It forms the lithosphere-asthenosphere boundary only beneath mid-ocean ridges, where the rigid layer is very thin [2].

5. Mantle

• Mantle: the layer below the crust. It runs from the Moho down to 2,900 km.
• Size: it is the biggest layer. It holds about 84 per cent of the earth's volume and about two-thirds of its mass.
• Material: silicate rock rich in iron and magnesium. It is denser (heavier for its size) than the crust.
• Parts of the mantle:
• Uppermost mantle: rigid. It is welded to the crust, and together they form the lithosphere.
• Asthenosphere: weak and plastic, in the upper mantle.

• Lower mantle: below the asthenosphere. It is solid because the very high pressure keeps the rock from melting.

• The mantle ends at the boundary with the core at 2,900 km. (The core is covered in Structure of the earth II.)

6. Asthenosphere

• Asthenosphere ("astheno" = weak): a weak, plastic, partly molten zone in the upper mantle. It reaches down to about 400 km (NCERT).
• Plastic means it is solid, but over very long periods it can flow slowly, like soft toffee or putty.

• Partly molten means a small share of the rock is melted.

• Why it matters:

• Main source of magma. Melted rock from here rises to feed volcanoes. Examples are volcanoes along the Pacific "Ring of Fire" and at mid-ocean ridges.

• The plates move over it. Its soft rock lets the rigid plates above it slide.

• The lower mantle below it is solid again.

7. Lithosphere

• Lithosphere ("litho" = rock): the crust plus the uppermost mantle. It forms one rigid outer shell, 10-200 km thick.
• Under the oceans it is thinner: 5-100 km.

• Under the continents it is thicker: about 200 km.

• It is broken into plates. These pieces are called tectonic plates (for example, the Indian, Eurasian, Pacific, Arabian and Anatolian plates).

• How plates cause earthquakes and volcanoes:
• The plates move slowly over the soft asthenosphere.
• Where plates push together, pull apart or slide past each other, the brittle rock bends until it suddenly breaks. The released energy is an earthquake.

• Where one plate sinks under another, or where plates move apart, magma rises. This forms volcanoes.

• Examples (present day):

• Bhuj (Gujarat, India), 2001: a quake 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.

• Exam trap: the lithosphere is "crust and upper mantle", NOT "crust and core".

8. Quick comparison: lithosphere and asthenosphere

Feature	Lithosphere	Asthenosphere
What it is made of	Crust + uppermost mantle	Part of the upper mantle
Depth / thickness	10-200 km thick	Below the lithosphere, down to about 400 km
Nature	Rigid and brittle	Weak, plastic, partly molten
Role	Broken into plates; earthquakes start here	Plates move over it; main source of magma

Prelims Hooks

• Order from the surface downward: crust → Moho → mantle (lithospheric part → asthenosphere → lower mantle) → core boundary at 2,900 km.
• Moho = crust-mantle boundary. Found by Andrija Mohorovicic (Croatia), 1909. It lies 5-10 km below the ocean floor and 30-70 km below the continents.
• Conrad discontinuity = the boundary between the granitic upper and basaltic lower continental crust. It is absent in oceanic crust and is not found everywhere.
• Sial = silica + aluminium = continental crust (granite). Sima = silica + magnesium = oceanic crust (basalt).
• Oceanic crust is thinner (about 5 km), denser and younger (under about 200 million years). Continental crust is thicker (about 30 km, up to 70 km under the Himalayas), lighter and older (about 4 billion years; NCERT: over 3,200 million years).
• The mantle makes up about 84 per cent of the earth's volume and reaches down to 2,900 km.
• The asthenosphere ("weak sphere") reaches about 400 km (NCERT). It is the main source of magma.
• Trap: the lithosphere = crust + uppermost mantle, not crust + core. It is 10-200 km thick, thinner under the oceans.
• Trap: the Moho is not the base of the lithosphere. The Moho lies almost entirely inside the lithosphere and meets its base only beneath mid-ocean ridges [2].

Mains Points

• Seismic waves are our only "window" into the interior. No one can drill or dig to the mantle. The jumps in wave speed at the Conrad and Moho discontinuities show where the layers are. This is why the layered model rests on earthquake records.
• Plate tectonics depends on the lithosphere and asthenosphere working together. A rigid, brittle lithosphere broken into plates moves over a weak, partly molten asthenosphere. This explains where earthquakes and volcanoes happen. Examples: the subduction quake at Tohoku 2011, the strike-slip quake (plates sliding past each other) in Turkey-Syria 2023, and the quake inside the Indian plate at Bhuj 2001.
• India's disaster risk (GS-III):
• The Indian plate keeps pushing into the Eurasian plate.
• This has given the Himalayas a crust about 70 km thick and a lithosphere under constant stress.
• Result: the Himalayan belt and North-East India are among the most quake-prone parts of the country. Even "stable" peninsular areas can have strong quakes, as Bhuj (2001) showed.

• So India needs earthquake-safe building rules, seismic zoning and a good network of earthquake-recording stations.

• Oceanic and continental crust behave differently. Young, heavy oceanic crust sinks at trenches, which makes subduction zones the sites of deep earthquakes, volcanic arcs and tsunamis. Light, old continental crust stays at the surface and crumples into fold mountains such as the Himalayas.