structure of earth
Source: knowledge on geography
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Nebular theory of Laplace

  • According to this theory, there was a nebula (a cloud of hydrogen and helium) rotating with great speed.
  • The nebula radiated heat. It cooled down due to loss of heat, started contracting and gradually converted to a solid form.
  • But while it was rotating at a great speed (before converting into a solid form), a ring got separated from its equator.
  • This ring broke into nine parts, which were in the gaseous form.
  • These parts rotated, got compressed, cooled down and solidified to become the nine planets.
  • In the case of Earth, the heavier elements got accumulated in the core and the lighter elements got accumulated at the surface (property of a rotating body). This means that as we go from the surface to the core, the density will increase.
  • Temperature increases with increase in depth (because of the presence of residual energy in the core).

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Structural study of the Earth’s interior

  1. Direct evidence is provided by mines and volcanic eruptions but up to a few Km only.
  2. Indirect evidence is provided by temperature and pressure inside the Earth, the density of its different shells, the behavior of earthquake waves, evidence from meteorites, gravitation and magnetic field.

Edward Suess, an Austrian geologist, said that the core is made up of Nickel and Iron, the mantle and oceanic crust are made up of SIMA (Silica & Magnesium) and the outer layer or the continental crust is made up of SIAL (Silica & Aluminium).

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Seismic wave studies of the earth’s interior

  • All natural earthquakes occur in the Lithosphere (crust and the upper mantle).
  • Seismic waves provide an indirect evidence of the structure of the earth.
  • Seismic waves or earthquake waves are mechanical waves which require a medium for propagation.
  • Their behavior changes with the change of medium.
  • Seismic waves are of three types:
  1. Primary (P) wave: It is a longitudinal body wave. It can travel in all the three mediums viz., solids, liquids, and gases. But its speed is highest in solids, followed by liquids and the least in gases.
  2. Secondary (S) wave: It is a transverse body wave. This wave can travel through solids and gases only. 
  3. Surface (L) wave: It is also called the Long wave. It is the last to report on the seismograph and is the most destructive.

Read in Hindi: धरती की संरचना का भूकंपीय अध्ययन

  • Upon experimenting with P and S waves, it is found that there is a change in the course and velocity of the waves on crossing the boundaries of different zones inside the earth.
  • The velocity of ‘P’ and ‘S’ waves increases with depth but only up to 2,900 Km.
  • After this, S-waves passing across the direction of their movement do not pass.
  • P-waves traveling in the direction of their movement generally pass at a reduced velocity.
  • The Long (L) waves do not pass and do not go deeper in the earth.
  • The velocity of P-waves passing through inner core again increases as compared to their passage through the outer core.

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  1. The speed of both the waves increase in the Lithosphere, indicating that it is in the solid state.
  2. The speed of P-waves decreases in the asthenosphere. But the asthenosphere is not in the liquid state because the S-waves pass through it with reduced speed. This means that the asthenosphere is in the semi-molten state.
  3. The mantle rocks are dense beyond asthenosphere because the speed of both types of waves increases.
  4. The outer core is in the liquid state because P-waves traverse it with reduced speed and the S-waves are absent.
  5. The inner core is in the solid state because the speed of the P-waves here increases tremendously.

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