Structure of the earth s interior
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Structure of the Earth’s Interior. Do you REALLY believe? ? ?. All the planets of the solar system came out of the Sun!! Humans have been able to peep into the mantle!! All the Interior of the Earth is in molten state!! The Earth has only one North Pole!!. How was Earth formed?.

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Structure of the Earth’s Interior

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Structure of the Earth’s Interior

Do you REALLY believe???

  • All the planets of the solar system came out of the Sun!!

  • Humans have been able to peep into the mantle!!

  • All the Interior of the Earth is in molten state!!

  • The Earth has only one North Pole!!

How was Earth formed?

Big Bang Theory

Solar Nebular Theory

The birth and evolution of the universe

-14 billion years ago,

-huge explosion -the Big Bang.

-the universe was very hot and dense and filled with energy.

-As space expanded, the temperature began to fall and the first particles of matter formed from the energy.

-After more than 1 billion years, the first stars and galaxies had formed. --The Solar System, including our Earth, formed about 4.6 billion years ago.


Solar Nebular Theory

Solar Nebular Theory

  • About 15 billion years ago,

  • The sun was a protostar

  • a huge, fast moving nebula happened to brush past it.

  • Due to the gravity of the nebula, some part of the sun was scraped off in the form of a large mass of dust particles and tiny planetary matter.

  • The planetary matter followed the nebula for a distance but could not keep pace with the very fast moving nebula and was hence suspended in space.

  • Initial motion given to the planetary matter was now controlled by the gravity of the sun because of which the material started revolving around the primitive sun.

  • Because of the revolution of the planetary material around the sun, there was a churning effect resulting in the accretion (getting together) of the material forming the early meteorites.

Due to continuous accretion, the meteorites grew larger in size. Several meteorites got together and the formed the early asteroids.

  • As the asteroids were attracted towards each other due to gravity, they collapsed into each other forming the early planetoids

  • As a result of tremendous impact velocities of the asteroids, a lot of friction and heat was generated because of which the early planetoids were in a molten state.

The earth in a molten state

Molten earth started cooling down

What is the current state of Earth?

Outer layer became the solid crust

Heavier materials sank to the core

Molten material got trapped in the mantle between the crust and the core

The gases would be confined near the surface of the earth to form the atmosphere and at a later stage, the hydrosphere.

It can therefore be concluded that the interior of the earth would consist of the crust, mantle and the core.

Verification of this assumption is very difficult. The deepest hole dug into the earth is only 16 km deep. Hence direct methods cannot be used to find out about the interior of the earth.


We therefore have to rely upon results obtained from study of the behaviour of seismic waves inside the earth.

  • A wave is a series of disturbances caused in the particles of any medium due to the propagation of energy.

  • Seismic waves are generated due to the release of seismic energy during an earthquake. However, they can also be created artificially using an explosion.

  • There are four kinds of Seismic waves – P or primary waves, S or secondary waves, Love waves (named after the British physicist Augustus Love) and Rayleigh waves (named after the British physicist John William Rayleigh).

  • Out of these P and S waves provide maximum information about the earth’s interior.

Watch Video:

The following table describes the properties of P waves and S waves

Watch Video:

How to determine the width of the layers inside the earth using P and S waves?

Using P waves to determine the width of the crust

Following is a description of the experiment conducted by E Mohorovicic, a Dutch physicist who proved the existence of the crust and the mantle.

  • Consider the figure above. E is a point on the earth’s surface where an artificial explosion is created. As a result of this, seismic P waves travel in all directions. One such P wave travels along the path EB. Upon reaching the crust-mantle boundary (the Mohorovocic Discontinuity, or simply, the Moho), it is partly refracted into the mantle and partly reflected back to the surface of the earth where it is recorded at a seismic recording station R.


ER distance = 160 km

Explosion at-09.00.00 am

1st set of P waves reach R at 09.00.32 am

2nd set of P waves reach R at 09.00.40 am

Speed of waves= 160/32= 5 km/sec

Time taken to reach from EB to BR=

40 seconds * 5 km= 200 km

EB= BR= 100 km


  • To prove that EAB is congruent to ABR

  • Angle EAB= angle RAB

  • AB= AB ( Normal )

  • Angle EBA = Angle RBA( Law of reflection)

    Therefore by ASA, proved.

    According to Pythagoras Theorem, EB2=EA2+AB2

    = 10000= 6400+AB2

    = 3600 kms = 60 km ( Thickness of the crust)

  • Several such experiments indicate that the thickness of the continental crust is 70 to 80 km while the thickness of the oceanic crust is in the range of 10 to 20 km. On an average, the crust can be said to be about 60 km in thickness.

Using S waves to determine the width of the mantle and the core





Consider the figure below.

An explosion is created at point E on the earth’s surface due to which S waves travel in all directions. Most of them are refracted by the mantle and are received at points R1, R2, R3 and R4. Some of these S waves are blocked by the core, which suggests that it is in a liquid state. This is why no S waves reach the seismic recording station R5.


The part of the Earth between the points R3 and R4 (including R5) is called the S wave shadow zone and the angle formed between these points at the centre of the earth (angle R3CR4) is always equal to 150º.





Watch Video:

A point to be noted here is that while S waves are blocked by the core (outer core), P waves are received at R5 and all other stations in the shadow zone. P wave velocities are not uniform throughout the core, which indicates that there is a solid inner core (where the velocity of P waves increases).





  • The result that we get using this method is that the radius of the core is 3500 km. Since the radius of the Earth is 6400 km, the thickness of the mantle would be 2900 km.

  • Experiments conducted at various places on the earth indicate that the radius of the inner core is 1200 km.

What is the composition of the crust?

  • The most abundant rocks in the earth’s crust are granite and basalt. All other rocks within the crust are derivatives of these two basic rocks. Granite consists of silica and aluminium while basalt consists of silica and magnesium.

  • Granite is present on the continental crust, which is why the upper crust is also called sial (from silica and aluminium). Basalt is present on the oceanic crust, which is why the lower crust is also called sima (from silica and magnesium).

What is the composition of the mantle ?

  • The composition of the mantle cannot be determined directly. Therefore certain indirect methods are used to determine the composition of the mantle.

    1. The thickness of the oceanic crust is in the range of 10 to 20 km. Therefore magma from the mantle can directly erupt through submarine volcanoes. A chemical analysis of this lava gives us the composition of the mantle.

2. The velocity of P waves in the mantle is about 8 km/s. Scientists passes P waves through several rocks present on the earth and found that the velocity of P waves equals to 8 km/s in only three rocks, viz. Peridotite, Dunite and Eclogite. It is interesting to note that these rocks are found mostly in the vicinity of submarine volcanoes.

3. It is believed that the primitive earth formed as a result of several meteoritic impacts. It is possible that the composition of the mantle is similar to the meteorites falling on the earth even today. A chemical analysis of meteorites gives results similar to the two methods discussed above.

What is the composition of the crust, mantle and core?

  • The core of the earth is thought to be made up of mostly two elements – iron and nickel. It is believed that iron and nickel sank to the centre of the earth’s gravity form the molten mass of the primitive earth.

  • The outer core is liquid and inner core is solid

Which phenomenon can be associated with the fact that the earth’s outer core is a liquid?

The Earth’s


The Earth’s Magnetism

  • In 1600,

  • the English doctor and natural philosopher William Gilbert

  • Demonstrated that the entire Earth behaves as an enormous magnet.

  • This dynamo theory is the most widely accepted. This theory suggests that convection currents consisting of positively charged ions of iron and nickel and free electrons within the liquid core behave like the individual wires in a dynamo, which, aided by the earth’s rotation, set up a gigantic magnetic field.

The Earth’s Magnetism

  • The magnetic poles of the Earth do not correspond with the geographic poles, which are the ends of its rotational axis. The north magnetic pole is presently located in the Canadian Northwest Territories, almost 1,090 km south of the geographic North Pole. The south magnetic pole is presently situated at the edge of the Antarctic continent.

  • These magnetic poles are not constant. In fact, the earth’s magnetic field has changed polarity over geological time. At several periods during the earth’s geological history, these poles were also reversed.

  • It is postulated that the solid inner core rotates more slowly than the outer core, thus accounting for the changing polarity of the earth’s magnetic field.

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