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. The interior becomes hotter with depth, which is convincingly indicated by the volcanic eruption. Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of a few kilometers. Apart from seismological studies, other important data sources such as meteorite analysis, etc., though indirect, logically prove that the earth’s interior comprises different layers. These layers are distinguished by their physical and chemical properties, particularly in the thickness, depth, density, temperature, metallic content, etc.
The study of the propagation of earthquake waves through the earth’s interior has provided significant information about the layers of the earth. There are three types of earthquake waves:
Longitudinal, Primary, or P-waves:Â They are similar to sound waves in which the particle moves to and fro in the direction in which the wave is traveling. These waves travel in solid, liquid, and gaseous media. They have short wavelengths and high frequency.
Transverse, Secondary, S-waves, or Sheer waves: These waves are like waves that run along a string that is fastened at one end, stretched fairly tight, and shaken at the other end. In such waves, the particle moves to and fro at a right angle to the path of the wave. These waves travel only in solid medium. In comparison to the primary waves, they are slow in motion. They also have short wavelengths and high frequencies.
Surface, L-waves, Rayleigh, or R-waves:Â These are transverse waves and are confined to the outer skin of the crust. They are responsible for most of the destructive forces of earthquakes. They have low frequency, long wavelength, and low velocity.
The velocities of P and S waves change with depth and each can be related to the change in materials. Each range of changing velocity demarcates a zone of discontinuity. Based on seismic investigation, the earth’s interior has been broadly divided into three major parts- the Crust, Mantle, and Core.
It can also be inferred that-
The crust, mantle, and core are separated by two sharp breaks, usually known as major discontinuities. The crust is composed of heterogeneous material and has an average thickness of about 33 km.
The second major segment of the earth, i.e., the mantle extends from below the crust to a depth of 2900 km.Â
The third major segment of the earth i.e., the core extends from below the mantle up to the center of the earth.
