LECTURE -1 (INTERIOR OF EARTH )

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INTERIOR OF EARTH 

Human life is largely influenced by the physiography of the region. The physiography, in turn, is influenced by various endogenic processes operating in the interior of the earth. This makes it necessary for us to understand the interior structure of the earth.

The earth’s radius is 6370 km. Going beyond a limit is not possible as it is very hot at this depth. In such conditions, the knowledge about the interior of the earth is mainly based on estimates and inferences.

The knowledge about the interior of the earth is obtained through direct sources and indirect sources. Direct sources include rock materials from mining areas and molten magma from volcanic eruptions. Besides, scientists are working on “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”. These projects have provided information about the interior of Earth through analysis of materials collected at different depths.

Indirect sources include meteors that at times reach the earth, gravitation, magnetic field, and seismic activity. Most of our information about earth’s interior has come from seismic studies.

The sources which provide knowledge about the interior of the earth may be classified into

  • Artificial sources
  • Natural sources
  • Evidence from the theories of the origin of the earth

 

Artificial sources to understand Interior of the earth

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

  • Temperature
  • Pressure
  • Density

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Temperature

  • It increases with the depth, as observed in the mines and deep wells
  • The molten lava erupted from the earth’s interior also support this fact.
  • It’s not uniform from the surface towards the earth’s center, in beginning its increases at an average rate of 1 degree Celsius for every 32 meters increase in depth.
  • With such a speed it will be 300-degree celcius at a depth of 10 km, and 1200 degree Celsius at 40 km.
  • But its interior is not in molten state, because rocks buried under the pressure of several km thicknesses of overlying rocks melt at the higher temperature.

Pressure

  • It also increases from the surface towards the center of the earth due to huge weight of overlying rocks.
  • In deeper portion pressure is tremendously high, its 3 to 4 million times near 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.

Density

  • It increases due to increase in pressure and presence of heavier materials towards the earth’s center
  • 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 doubt, we can say that it will be more than 5.5 at the core.

Natural sources to understand Interior of the earth

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

  • Vulcanicity
  • Seismology (EQ waves)

 Vulcanicity

  • On the basis of upwelling 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 Epicenter.
  • It is located on the earth’s surface and is always perpendicular to the focus.
  • The focus is always on 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 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?

  • 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.
  • The main aspects such as velocity and travel paths of these waves passing through a homogeneous solid body.
  • But these waves reflect or refract while passing through a body having the heterogeneous composition
  • In reality, the recorded seismic waves denote the fact that these waves seldom follow the straight line paths.
  • Thus it becomes clear that earth is not made of homogenous materials rather there is a variation of density inside the earth.

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  • Crust – The outermost layer, or shell, of the earth (or any other differentiated) planet. Earth’s crust is generally defined as the part of the Earth above the Moho discontinuity. It represents less than one of Earth’s total.

 

  • Mantle – The zone of earth’s interior between the baseline of the crust  (Moho) discontinuity) and the core. The mantle has an average density of about 3.3g/cm3 and accounts for about 68 percent of earth’s mass. The mantle lies between the crust and the core of the earth. The lower layers of mantle are floored by the Gutenberg discontinuity.
  • Core – The central part of the earth below the depth of 2900 Km.
  • Mohorovivic (Moho) Discontinuity – The first global seismic discontinuity below the surface of the Earth. It lies at a depth varying from about 5 to 10 km beneath the ocean floor to about 35 km beneath the continents, commonly referred to as the Moho.

 

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