Internal Structure of Earth: Crust, Mantle & Core, Discontinuities

Structure of Earth is divided into four major components: Crust, Mantle, Outer core, inner core. Each layer has a unique chemical composition, physical state, and can impact life on Earth’s surface. Large size, non-uniform structure, high density and temperature make Earth’s structure very complex.

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  • Outermost solid part of Earth and is brittle in nature. 
  • Average density of outer and lower crust is 2.7 and 3.0, respectively. 
  • Oceanic crust is thinner (5 Km) as compared to continental crust (30 Km) 
  • Continental crust is thicker in areas of major mountain systems. It is as much as 70 km thick in the Himalayan region.7
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  • Portion of interior beyond the crust is called mantle. 
  • Extends from Moho’s discontinuity to a depth of 2,900 km.
  • Upper portion of mantle (lower part of Lithosphere) is called asthenosphere which is partially molten in condition whereby molten magma is in motion. 
  • It is extending up to 400 km. 
  • The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. 
  • Previously mantle was divided into two zones: (i) Upper Mantle from Moho’s discontinuity to 1000 km depth (ii) Lower Mantle from 1000 to 2900 km depth. But now on the basis of discovery by International Union of Geodesy and Geophysics, Mantle is divided into three zones; 

Zone 1: Moho’s discontinuity to 200 km depth

Zone 2: 200 – 700 km depth

Zone 3: 700 – 2900 km depth

  • Velocity of seismic waves relatively slows down in the uppermost zone of upper mantle for a depth of 100-200 km. This is called as zone of low velocity. 
  • The lower mantle extends beyond the asthenosphere. It is in solid state.


  • Core-mantle boundary is located at the depth of 2,900 km. 
  • Outer core is in liquid state while inner core is in solid state. S-Waves disappear in outer core (5150 km is the boundary line).
  • Composition of Core: Core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.
  • Importance of core: (i) Core is responsible for generation of Earth’s magnetic field (ii) Contains information regarding earliest history of accretion of planet (iii) Thermal and compositional features established when core formed largely controlled subsequent evolution of core and influence the evolution of mantle, crust and atmosphere. 


Unique layers within the Earth are there according to their characteristics. All those layers are separated from each other through a transition zone. These transition zones are called discontinuities.

They are marked by clearcut variations in density of material, velocity of seismic waves, temperature and pressure conditions. 

There are five discontinuities inside the earth.

  • Conrad Discontinuity: Transition zone between SIAL and SIMA.
  • Mohorovic Discontinuity: Transition zone between the Crust and Mantle.
  • Repiti Discontinuity: Transition zone between Outer mantle and Inner mantle.
  • Gutenberg Discontinuity: Transition zone between Mantle and Core.
  • Lehman Discontinuity: Transition zone between Outer core and Inner core.
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