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We all know that geography is the description of the earth. So at first, we need to know about the interior structure of earth. Interior of the earth cannot be known by direct observation. Along with the advancement of science and technology, internal structure of earth can be known through the analysis of seismic waves. There are three types of seismic waves – primary or P-wave, secondary or S-wave, and surface or L-waves. P-wave can go through rigid and liquid medium, and S-wave can go through only rigid medium. Hence, analysing the velocity of these waves, it is possible to speculate on the state of matter and density of the earth's interior.


Discontinuities and the Interior of the Earth

In the year 1909, Yugoslavian scientist Mohorovich found that the movement and distraction of seismic waves in the interior of earth, in between 80 to 200 km. from earth's surface. Thus he came to understand that there must be a change in density of substance from where the waves deflected. This discontinuity was named after him as Mohorovich Discontinuity or Moho discontinuity. The idea of discontinuity was developed from a study of German seismologist Emil Johann Wiechert, about the movement and angular measurement of distraction of seismic waves.

Likewise, Beno Gutenberg, an American-German seismologist, invented a discontinuity 2900 km. deep from the surface. It is known as Wiechert-Gutenberg Discontinuity.

Similarly a discontinuity is found in the continental region at an depth of around 15 to 20 km., named after seismologist Victor Conrad as Conrad Discontinuity. It is not found in the oceanic regions.

Based on this discontinuities, earth's interior has divided into three main parts : Earth Crust, Mantle, and Centrosphere.


1. Earth Crust

The outermost surface part of earth is called earth crust. It is also known as Lithosphere. It is subdivided into three parts, viz., Continental Crust, Oceanic Crust, and Crust of transitional type.
  • Continental Crust is the part of continental region of the earth surface. Main rock forming substances of continental crust are Silica and Aluminium, hence it is called 'Sial'. Density of substances in this region varies around 2.55 gm./cc. Depth of this layer from surface is 15 to 70 km.
  • Oceanic Crust refers the part of oceanic region under the continental crust, with a depth of about 5 to 7 km. Main rock forming substances of oceanic crust are Silica and Magnesium for which it is also called as 'Sima'. The upper portion of oceanic crust is developed by sedimentary rocks, and the lower portion is with Basalt.
  • Crust of transitional type refers to an intermediate region of aforementioned two divisions, having a mixed characteristics of both of that. This includes the continental shelf regions, islands etc.
Conrad Discontinuity separates the Sial and Sima.

Fig. : Interior Structure of the Earth : Discontinuities and Layering

2. Mantle

Mantle is the middle part of earth crust and centrosphere. Moho discontinuity denotes the boundary between lithosphere and mantle. From the Mohorovich discontinuity, mantle is situated around up to 2,900 km. It is divided into two parts, e.g. Upper Mantle, and Lower Mantle.
  • Upper Mantle : It is the upper portion of mantle that extends from Moho discontinuity to approximately 1,000 km. It is important to note that Asthenosphere, a highly viscous, ductile region, is a part of upper mantle at an depth of about 80 to 150 km. The tectonic plates are lying upon this layer. Mantle is formed with Chromium (Cr), Iron (Fe), Silica (Si), Magnesium (Mg). Hence it is called 'Crofesima'. Upper mantle is further subdivided into two layers : Gutenberg Layer (150 km.–410 km.), and Golitsyn Layer (410 km.–1,000 km.).
  • Lower Mantle : It extends from 1,000 km. to 2,900 km. The main substances are Nickel (Ni), Iron (Fe), Silica (Si), Magnesium (Mg). So this layer is called 'Nifesima'. Density of this layer is about 5 gm./cc. mainly due to high pressure and temperature.

3. Centrosphere

It is the core of earth. Wiechert-Gutenberg Discontinuity is separating the centrosphere from the mantle. It extends from the utmost of mantle at 2,900 km. to the center of earth. Centrosphere occupies the 17% to total volume, and 34% of total mass of earth. Centrosphere is divided into two parts : Outer Core, and Inner Core.
  • Outer Core is liquid in nature. Density of this region varies around 10 gm./cc. The main substances of outer core are Chromium (Cr), and Iron (Fe). Thenceforth it is called 'Crofe'.
  • Inner Core is quite rigid due to extreme temperature and pressure. Density of inner core is about 14 gm./cc. Nickel (Ni), and Iron (Fe) are main substances of the center of earth, hence it is called 'Nife'.
It is important to mention that in the year 1936, Inge Lehmann, a famous Danish seismologist, invented the discontinuity between outer and inner core. It is named after him as Lehmann Discontinuity.

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