As all of us have studied the earth’s interior in our school days .
Now lets look at it it again . Earlier the earth was molten . Then the lighter elements like silica , oxygen started coming to the top and the heavier elements like nickel and iron started going towards the centre .
The layers at the top cooled off . But the interior still remains hot.
The earth now has 3 layers :
1) The crust : this is where we live . It extends upto 30 Km deep .
It is cool and brittle . It tends to break during earth quakes .
2) The Mantle : This extends from 30 kilometres to 2900 kms below the
earth . It accounts for most of the earth’s volume . It is composed of
Iron , Magnesium , silicon . All earthquakes , volcanoes orignate from
this layer . It is semisolid . It has a temperature of about 1000 degrees.
3) The core : The core has two parts :
a) The outer core : This is liquid . consists of molten iron and nickel.
It has a temperature of about 3700 degrees .
b) The inner core : Because of intense pressure this is a solid ball of
iron . It has a temperature of > 4300 degrees and is about 5200 Kms
How do we know what is inside the earth ??
Well there are various ways to find out . When waves generated by siesmic activity ( such as earthquakes ) goes from one layer to another it bends ( like light bends on hitting a surface with different refractive index ) .
another way is to study the insides of the earth is to study the two types of siesmic waves : The compressional P waves and the shear S waves .
The compressional P waves can travel both in solids and liquids . The shear S waves can travel only in solids ( in liquids if something is pushed sideways there is very little resistance ) .
So suppose an earthquake occurs in New York and you observe the waves on the other end of the earth . Scientists observed that till a seperation angle of 105 degrees you get both P waves and the S waves thru the earth . After a seperation angle of 105 degrees the S waves ( Shear waves) disappeared completely . That means that the waves are passing thru liquid . By some trignometrical calculations u can find the radius of the liquid inside the earth .
The earth’s Magnetic field can also be explained by the movement of the liquid in the core . The moon and Mars have no magnetism .
To account for the magnetic field in the earth the convection has to be very rapid . The liquid iron seems to be charged and its rapid movement causes the magnetism .
We can also learn a lot about our magnetic field by observing the field of Sun which is similar but stronger . The magnetic lines of force twist arouund the core and then they rise to the top( cooler layers) .
Here they become cyclonically twisted till a release happens in form of sun spots . The sun’s Magnetic field reverses its polarity every 22 years .
If you apply the same equations to the earth then we see that it should change its polarity ( south becomes north and vive versa ) every 250,000 years .
Scientists when studying old rocks have observed that the orientation in which the rocks are laid/frozen does show us that about 300,000 years ago the earth’s magnetic field was indeed opposite to what it is now .