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What should you understand about the interior of the earth?
It is not possible to know about the earth’s interior by direct observations because of the huge size and the changing nature of its interior composition.
It is an almost impossible distance for the humans to reach till the centre of the earth (The earth’s radius is 6,370 km).
Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of few kilometers.

Sources of Information about the interior of the earth
Direct Sources:
Rocks from mining area
Volcanic eruptions
Indirect Sources
By analyzing the rate of change of temperature and pressure from the surface towards the interior.
Meteors, as they belong to the same type of materials earth is made of.
Gravitation, which is greater near poles and less at the equator.
Gravity anomaly, which is the change in gravity value according to the mass of material, gives us information about the materials in the earth’s interior.
Magnetic sources.
Seismic Waves: the shadow zones of body waves (Primary and secondary waves) give us information about the state of materials in the interior.


structure of the interior of the earth

Crust
It is the outermost solid part of the earth, normally about 8-40 kms thick.
It is brittle in nature.
Nearly 1% of the earth’s volume and 0.5% of earth’s mass are made of the crust.
The thickness of the crust under the oceanic and continental areas are different. Oceanic crust is thinner (about 5kms) as compared to the continental crust (about 30kms).
Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL (Sometimes SIAL is used to refer Lithosphere, which is the region comprising the crust and uppermost solid mantle, also).


Mantle
The portion of the interior beyond the crust is called as the mantle.
The discontinuity between the crust and mantle is called as the Mohorovich Discontinuity or Moho discontinuity.
The mantle is about 2900kms in thickness.
Nearly 84% of the earth’s volume and 67% of the earth’s mass is occupied by the mantle.
The major constituent elements of the mantle are Silicon and Magnesium and hence it is also termed as SIMA.
The density of the layer is higher than the crust and varies from 3.3 – 5.4g/cm3.
The uppermost solid part of the mantle and the entire crust constitute the Lithosphere.
The asthenosphere (in between 80-200km) is a highly viscous, mechanically weak and ductile, deforming region of the upper mantle which lies just below the lithosphere.
The asthenosphere is the main source of magma and it is the layer over which the lithospheric plates/ continental plates move (plate tectonics).
image of tectonic plates of the lithosphere


Core
It is the innermost layer surrounding the earth’s centre.
The core is separated from the mantle by Guttenberg’s Discontinuity.
It is composed mainly of iron (Fe) and nickel (Ni) and hence it is also called as NIFE.
The core constitutes nearly 15% of earth’s volume and 32.5% of earth’s mass.

Temperature
A rise in temperature with increase in depth is observed in mines and deep wells.
These evidence along with molten lava erupted from the earth’s interior supports that the temperature increases towards the centre of the earth.
The different observations show that the rate of increase of temperature is not uniform from the surface towards the earth’s centre. It is faster at some places and slower at other places.
In the beginning, this rate of increase of temperature is at an average rate of 10C for every 32m increase in depth.

Pressure
Just like the temperature, the pressure is also increasing from the surface towards the centre of the earth.
It is due to the huge weight of the overlying materials like rocks.
It is estimated that in the deeper portions, the pressure is tremendously high which will be nearly 3 to 4 million times more than the pressure of the atmosphere at sea level.
At high temperature, the materials beneath will melt towards the centre part of the earth but due to heavy pressure, these molten materials acquire the properties of a solid and are probably in a plastic state.