The earth’s radius is 6,370 km. It is impossible to reach the centre of the earth and make observations or collect samples of the material. Therefore, the study of the interior of the Earth is based on direct and Indirect Sources. 

Direct Sources

The important direct source that gives information about the interior of the Earth are Underground mining, Deep Drilling projects, Volcanic eruption etc. 

Underground mines

• A large part of the earth’s surface is composed of rocks derived from within the earth’s body.
• The most important direct source about the interior of the earth is the solid earth material is surface rock or the rocks we get from mining areas. Eg.: Certain Gold mines in South Africa are as deep as 3 – 4 km. 
• Since the temperature increases with the depth, it is not possible to go beyond more depth. 

Deep Drilling Project

• There are a number of projects to penetrate deeper depths to explore the conditions in the crustal portions. It provided a large volume of information through the analysis of materials collected at different depths.
• ‘Deep Ocean Drilling Project’ and ‘Integrated Ocean Drilling Project’ are the two major Deep Drilling Projects. The deepest drill is at Kola, in the Arctic Ocean, that has reached a depth of 12 km. 

Volcanic eruption

• The molten material called magma is thrown out at the time of Volcanic eruption
• These are available for laboratory analysis. It indicates that there is a layer which is either molten or in a semi- molten state. 

INDIRECT SOURCES

• Analysis of properties of matter indirectly provides information about the interior of Earth. 
• The important source that provides details about the interior of the earth are information from the meteors, density, gravitation, magnetic field, and seismic activity. 

Meteors 

• The material and the structure observed in the meteors are similar to that of the earth. 
• Hence, meteors are an important source of information about the interior of the earth.

Density

• The density and temperature of the material increases with depth. 
• The average density of earth was calculated as 55 g/cm3 by using the spherical shape, mean radius and mass of the earth. 
• The density of the Earth is higher at core than all other parts of the earth.
• With the help of rate of change of density and temperature and the total thickness of the earth, it is possible to estimate the values of temperature, pressure and the density of materials at different depths.

Pressure 

• Even though the density increases with Pressure, there is a critical limit beyond which its density cannot be increased.
• The high density in the core is the result of the presence of heavy metallic material of high density there. 

Temperature

• Heat flows outwards from the interior of the Earth in the form of thermal convective currents. 
• The temperature increases by 1⁰C for every 32m of depth. 
• This rate of change of temperature is not uniform while going down the surface of the Earth. Relatively higher temperatures are found in tectonically active regions.

Gravity

• The gravitational force (g) is different at different latitudes on the surface. Since the distance from the centre to the equator is greater than that from the poles, the gravitational force is greater near the poles and less at the equator. 
• The gravity values also differ according to the mass of material. The reading of the gravity at different places differs from the expected values. Such a difference is called a gravity anomaly.
• Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.

Magnetic field 

• Magnetic surveys provide information about the distribution of magnetic materials in the crustal portion. 
• This provides information about the distribution of materials in this part.

Seismic Activity 

• Seismic waves are generated during earthquakes. 
• There are various types of earthquake waves. The nature of these earthquake waves and their travel speeds reveal the internal layered structure of the earth.