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Internal structure of earth and plate tectonics

 

Internal Structure of the Earth and Plate Tectonics

Division of Earth

  • The internal structure of earth is divided into:

    (a) Crust

     (b) Mantle and

     (c) Core

 The division is based on physical and chemical properties,  thickness, depth, density, seismic data, pressure-temperature and availability of metals.


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Types of Seismic Waves

  • seismological data shows us that the earth has a layered structure.

  • Seismic waves with varying velocities that depend on the type of wave and physical properties of medium for the wave travelling.

  • Therefore, it is necessary to understand different types of waves.



 

Wave type

Description

1. Body wave (2 types)

 

 

(a) Primary wave/ P-wave/Transverse wave         





(b) Secondary wave/Shear wave/Longitudinal wave

  • Body waves travel through the body of the earth.


  • The particles move forward and backward to the direction in which the wave is travelling.

  • Can travel in all media(solid, liquid and gas).

  • Short wavelength and high frequency.

  • The particles move forward and backward at right angles to the path at which waves is travelling.

  • Can travel in solid media only.

  • Short wavelength and high frequency.

2. Surface wave 


  


  Rayleigh wave or Love wave

-They travel along paths nearly parallel to the surface.


-These are transverse wave.

  • They are responsible for most of the destructive earthquakes.

  • They have low frequency, long wavelength and low velocity.



Figure 1: Internal Structure of Earth

Internal Structure of Earth-Crust

  • It is the uppermost shell of the earth covering the rocks of the interior thinly (Figure 1).

  • Average thickness of crust: 33 kms

  • Thickness of Crust in Oceanic areas: 5 to 10 kms

       in Continental areas: 35 kms

       in Orogenic belts: 55-70 kms

  • The Mohorovicic discontinuity separates crust from mantle.

  • Crust can be divided into two layers, the upper layer is called Sial and the lower one is called Sima. The boundary between the sial and sima is called Conrad Discontinuity.



Crust-Sial and Sima

 

Sial

Sima

  • Also called upper continental crust.

  • Rich in silica and aluminum.

  • Rock of this layer are of granitic to grano dioritic in composition.

  •  Thickness: 11 kms (extends up to Conrad discontinuity) 



-Also called lower continental crust.

-Rich in silica and magnesium.

-Rocks of this layer are basaltic in composition.


  • Thickness: 22 kms (extends from Conrad discontinuity to Mohorovicic discontinuity)

  • Outer sima ( 11-19kms) has rocks of intermediate composition.

  • Inner sima( 19-33kms) has basic to ultrabasic rocks)


Mantle

  • Second major part of the earth.

  • Mantle extends from below the Mohorovicic discontinuity upto a depth of 2900 kms ( Thickness: 2865 kms).

  • Forms 83% of the earth by volume and 68% by mass.

  • Composition of Mantle: Olivine- Pyroxene complex in solid state.

  • Mohorovicic discontinuity (at the depth of 33 kms) separates mantle from crust.

  • Gutenberg-Weichert discontinuity ( at the depth of 2900 kms) separates mantle from core.


Core


  • Innermost part of the earth.

  • Separated from the mantle by Gutenberg- Weichert discontinuity and extends upto the center of the earth.

  • Comprises of about 17% of the volume and 34% of the mass of the earth.

  • Outer core( 2900-4982 kms) is said to be inn fluid state as it doesn’t transmit S-waves.

  • Middle core (4982- 5121 kms) is in a fluid to semi fluid state.

  • Inner core (5121-6371 kms) is said to be in solid state.


 Some Facts

  • Temperature of the earth at it’s center: 6000◦ C

  • Pressure of the earth at it’s center : 3 million atmosphere

  • There is a sharp change in the density from about 5.5*103 kg m-3 in the mantle to about 10.6* 103 kg m-3 in the core.

  • While at the center of the core the density increases to 12 or 13* 103 kg m-3. 


Plate Tectonics

What is plate tectonics?

The  movement of  lithospheric plates  lying bodily over the plastic asthenosphere  is called plate tectonics. 


Introduction- Plate tectonics

  • Crust of the earth ( Oceanic and Continental) together with the uppermost portion of mantle constitutes the lithosphere.

  • Part of the mantle comprises of the layer Asthenosphere which behaves plastically due to increased temperature and pressure.

  • The lithosphere is capable of moving bodily over the asthenosphere.



Plates

  • A plate is a large, rigid slab of rock moving slowly over the asthenosphere and they are usually of continental dimensions.

  • According to Le Pichon, there are 6 major plates and smaller plates can be incorporated within theses 6 plates. They are: 

(a) Pacific Plate 

(b) The American Plate 

(c) The African Plate

(d) The Eurasian Plate/ Tibetan Plate

(e) The Indian Plate

(f) The Antarctic Plate

  • Of the six plates, 5 contain part of continents in the lithosphere and only Pacific plate is made mostly of ocean floor.


Some Facts on Plate

  • Plates are separated by faults and thrusts.

  • Motion of the plates may be towards or away from each other.

  • Two plate margins meet at a common plate boundaries and where three plate boundaries meet, it is called Triple- Junction.

  • When two plates diverge, we find extensional features called ridges.

  • When two plates converge and one is thrust beneath the other, we find the island arcs.

When two plates slide past each other, there occurs transcurrent faults.

Plate Boundaries

  • Plate boundaries are the sites of intense geological activities which are mainly due to the movement of plates.


  • As the big listhospheric plates move by diverging  or converging along their boundaries, tremendous energies are released.


  • Based on the movement of plates, there are 3 types of plate boundaries:

   1. Constructive Boundaries

  2.Destructive Boundaries

3. Conservative Boundaries

Constructive Boundaries:


  • Also called diverging plate boundaries.

  • This is a zone along which two plates are in motion away from each other.

  • As a result of the movement, a fissure develops allowing hot molten materials to come out from the mantle and to form new plate materials.

  • Since new material or crust is created by the materials from the mantle, this type of plate boundary is known as constructive or divergent margin.

  • Mid-oceanic ridges are formed during oceanic divergence.



Divergent.jpg
Figure 2: Divergent plate boundary



Destructive Boundaries


  • Also called converging plate boundaries.

  • These are the zones along which two plates are in motion towards each other.

  • Along such boundaries, crust is destroyed and recycled back into the interior of the earth as one plate dives under another.

  • This process of entrance of one plate( denser) under another(rarer) is called subduction.

  • There are 3 types of convergence processes:

     (a) Ocean-Ocean Convergence

      b) Ocean-Continent Convergence

      (c) Continent-Continent Convergence


Ocean-continent.jpg

Fig 4: (b) Ocean-continent convergence

Continent-Continent.png

Fig: (c) Continent-Continent convergence


Ocean- Ocean Convergence ( Figure 4 (a))


  • Subduction of plate and formation of oceanic trenches start.

  • Island-arcs are formed by the upwelling of melted lavas due to the friction during subduction.

  • Also results in formation of undersea volcanoes.


Ocean-Continent Convergence( Figure 4 (b))


  • Oceanic crust is subducted under a continent due to the density.

  • Magma rises from the subduction zone forming Volcanic arcs.


Continent-Continent Convergence( Figure 4 (c))


  • It is also called continental collision where two continents converge.

  • When two continents collide, new mountains are formed.



Conservative Boundaries

  • These are boundaries where two continents converge.

  • These boundaries are also called transform boundaries.

  • Such boundaries have neither loss nor gain of surface areas so it is called conservative boundaries.

  • They are represented by faults.

  • The faults are of transcurrent or transform type ( Figure 4).

  • Responsible for shallow earthquakes.

  • San Andreas Fault of California is an example.



       

Figure 4: Conservative Plate Boundary (transform Fault)





How was Himalaya formed?


  • About 55 million years ago, the collision of the Indian Plate took place with Tibetan (Eurasian)  plate.


  • Many scientists believed that during collision  the northward moving Indian plate first touched the southern edge of the Tibetan plate.


  • There was Tethys sea (oceanic crust) in between the Indian plate and Eurasian plate which disappeared with collision.


  • The mountain building process continued with the collision and is still in progress.


  • We can observe that indian plate is shifting towards north by 5cm/yr.



Evolution of Himalaya-Diagram


 
Figure 5: Evolution of Himalayas

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