Layers of the Earth

The Earth can be divided into one of two ways – mechanically or chemically. Mechanically – or rheologically, meaning the study of liquid states – it can be divided into the lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. But chemically or by composition, which is the more popular of the two, it can be divided into the crust, the mantle (which can be subdivided into the upper and lower mantle), and the core – which can also be subdivided into the outer core, and inner core.

Compositional layers of the Earth:

Core, mantle, and crust are divisions based on composition. The crust makes up less than 1 percent of Earth by mass, consisting of oceanic crust and continental crust is often more felsic rock. The mantle is hot and represents about 68 percent of Earth’s mass. Finally, the core is mostly iron metal. The core makes up about 31% of the Earth.

 

Compositional Layers of Earth

 

Crust:

  • It is the outermost solid part of the earth, normally about 8-40 kms thick.
  • It is brittle in nature.
  • Nearly 1% of the earth’s volume and 5% of earth’s mass are made of the crust.
  • The thickness of the crust under the oceanic and continental areas are different. Oceanic crust is thinner (about 5kms) as compared to the continental crust (about 30kms).
  • Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL(Sometimes SIAL is used to refer Lithosphere, which is the region comprising the crust and uppermost solid mantle, also).
  • The mean density of the materials in the crust is 3g/cm3.
  • The discontinuity between the hydrosphere and crustis termed as the Conrad Discontinuity.

Mantle:

  • The portion of the interior beyond the crust is called as the mantle.
  • The discontinuity between the crust and mantleis called as the Mohorovich Discontinuity or Moho discontinuity.
  • The mantle is about 2900kms in thickness.
  • Nearly 84% of the earth’s volume and 67% of the earth’s mass is occupied by the mantle.
  • The major constituent elements of the mantle are Silicon and Magnesium and hence it is also termed as SIMA.
  • The density of the layer is higher than the crust and varies from 3.3 – 5.4g/cm3.
  • The uppermost solid part of the mantle and the entire crust constitute the Lithosphere.
  • The asthenosphere (in between 80-200km) is a highly viscous, mechanically weak and ductile,  deforming region of the upper mantle which lies just below the lithosphere.
  • The asthenosphere is the main source of magma and it is the layer over which the lithospheric plates/ continental plates move (plate tectonics).
  • The discontinuity between the upper mantle and the lower mantleis known as Repetti Discontinuity.
  • The portion of the mantle which is just below the lithosphere and asthenosphere, but above the core is called as Mesosphere.

Core:

    • It is the innermost layer surrounding the earth’s centre.
    • The core is separated from the mantle by Guttenberg’s Discontinuity.
    • It is composed mainly of iron (Fe) and nickel (Ni) and hence it is also called as NIFE.
    • The core constitutes nearly 15% of earth’s volume and 32.5% of earth’s mass.
    • The core is the densest layer of the earth with its density ranges between 9.5-14.5g/cm3.
    • The Core consists of two sub-layers: the inner core and the outer core.
    • The inner core is in solid state and the outer core is in the liquid state (or semi-liquid).
    • The discontinuity between the upper core and the lower core is called as Lehmann Discontinuity.
    • Barysphere is sometimes used to refer the core of the earth or sometimes the whole interior.

Mechanical Layers of the Earth:

The structure of the Earth can also be defined and divided based on how the insides of the planet behavior. Thereby, the mechanical layers correspond to the physical or mechanical properties of these layers.

Below are brief overviews of the five mechanical layers of the Earth:

Mechanical Layers of Earth

 

 

Lithosphere:

    • The lithosphere is the outermost layer of the Earth that consists of the entire crust and the top-most portion of the mantle.
    • The average thickness is ~70km, but ranges widely: It can be very thin, only a few km thick under oceanic crust or mid-ocean ridges, or very thick, 150+ km under continental crust, particularly mountain belts.
    • Depth- 0-100 km
    • Furthermore, they are divided into pieces called tectonic plates.
    • The movements of these plates are responsible for mountain-building, oceanic trench formation, earthquakes, and volcanic eruption.

Asthenosphere:

    • The asthenosphere includes the soft layer of the mantle on which the lithosphere moves.
    • Depth- 100km to 350 km .
    • It is made of solid silicate materials, but the high temperature allows it to flow on very long timescales.
    • The lithosphere-asthenosphere boundary is where geophysicists mark the difference in ductility between the two layers.

Mesosphere:

    • The mesosphere is the layer below the asthenosphere but above the outer core. It is essentially the lower mantle.
    • Average depth-350-2900km
    • Despite its high temperature, the intense pressure in this region restricts the movements of the molecules of the silicate material despite being under high temperature, thus making it extremely rigid.

 Outer Core:

    • The outer core extends from the bottom of the mesosphere or the lower mantle and surrounds the inner core.
    • Composed of iron and nickel, the extreme temperature allows these metals to remain in their liquid phases.
    • It is the only layer of the Earth that is a true liquid.
    • Furthermore, its movement is responsible for generating the magnetic field.

 Inner Core: 

    • The inner core is also made of iron and some nickel.
    • However, unlike the outer core, it is a solid ball.
    • The solidity is due to the intense pressure from the upper layers.
    • Hence, although it is as hot as the surface of the Sun, there is speculation that the inner core is slowly growing as the liquid outer core at the boundary with the inner core cools and solidifies due to the gradual interior cooling.