World Geography Part – 4 Pressure, Density & Earth quake waves

In this article, we will understand various concept related to the earth’s interior such as Pressure, Density and earthquake waves.


  • It also increases from the surface towards the centre of the earth due to the huge weight of overlying rocks.
  • In deeper portion pressure is tremendously high, its 3 to 4 million times near the centre as compared with the pressure of the atmosphere at sea level.
  • Due to high pressure, the molten material beneath earth acquires the property of a solid and is probably in the plastic state.


  • It increases due to increase in pressure and presence of heavier materials towards the earth’s centre
  • The outermost layer is composed of sedimentary rock; thickness varies from (0.8km to 1.6 km)
  • The second layer is of crystalline rocks, the density of which ranges from 3.0 to 3.5 at different places.
  • The average density of the earth is about 5.5, so, without a doubt, we can say that it will be more than 5.5 at the core.

Natural sources to understand the Interior of the earth

We can know about the interior of the earth with the help of the following sources

  • Vulcanicity
  • Seismology (EQ waves)


  • On the basis of upwelling of and spread of hot and liquid lava on the earth surface during the volcanic eruption, we can say that there is such a layer which is in the liquid state.
  • Such molten chamber has been termed as magma chamber which supplies magma and lava during volcanic eruptions.

Seismology (Earthquake waves)

  • The place of the occurrence of the earthquake is called focus, and place which experiences the seismic evens first called epicentre.
  • It is located on the earth’s surface and is always perpendicular to the focus.
  • The focus is always inside the earth, the deepest focus has been measured at the depth of 700km from the earth’s surface.
  • There are three broad categories of seismic waves

Primary waves

  • Longitudinal or compressional waves or P waves are analogous to sound waves, particles move both to and fro from the line of the propagation of the ray.
  • It travels with the fastest speed through solid materials.

Secondary waves

  • Also called transverse waves or distortion wave or simply S waves.
  • These are analogous to water ripples or light waves, wherein the particles move at right angles to the rays.
  • It cannot pass through liquid materials

Surface waves

  • Also called long period waves or simply L waves. These waves generally affect the surface of the earth and die out at smaller depth.
  • This wave covers the longest distances of all the seismic waves.
  • Their speed is slower than P and S waves but these are most violent and destructive.

How seismic waves are helpful in determining the earth’s interior

  1. So we can say that the nature and properties of the composition of the interior of the earth may be successfully obtained on the basis of the study of these waves.
  2. The main aspects such as velocity and travel paths of these waves passing through a homogeneous solid body.
  3. But these waves reflect or refract while passing through a body having the heterogeneous composition
  4. In reality, the recorded seismic waves denote the fact that these waves seldom follow the straight line paths.
  5. Thus it becomes clear that earth is not made of homogenous materials rather there is the variation of density inside the earth

Evidence from the theories of the origin of the earth

Several attempts to explain the origin and evolution of the continent and ocean basin have been registered, the scientifically recognized are

  • Planetesimal hypothesis
  • Tidal hypothesis
  • Nebular hypothesis

Planetesimal hypothesis

  • The earth was originated due to accretion and aggregation of solid dust particles.
  • Based on this corollary the core of the earth should in a solid state

Tidal hypothesis       

  • The core of the earth should be in the liquid state.
  • According to this the earth have been formed from the tidal material ejected from the primitive sun

Nebular hypothesis

  • It was coined by Laplace, and according to this theory, the core of the earth should be in the gaseous state.

But from the above discussion we have already proved that these hypotheses have no place in the current scenario, and now we have solid evidence to prove earth’s origin.

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