The study of the earth's interior is the subject of Geology, Since, the interior part of the earth is not visible directly, knowledge about it is gathered through indirect means. The earth as a whole has been divided into 3 broad zones - Crust, Mantle and Core.
The Average thickness of this upper most layer of the earth is about 30 km. The average density of the upper crust is 2.8 and that of the lower crust is 3.0. This difference in density is believed to be due to the pressure. This discontinuity in density between upper crust and lower crust is known as 'conrad discontinuity'. Silica and Allumunium are the main constituent elements of the crust. Therefore, it is also know as the SIAL.
At the lower end of the crust the speed of the seismic waves increases suddenly and reaches upto 7.9 to 8.1 km/sec. This results into a discontinuity between the upper mantle and the lower crust, which shows a change in the density of the rocks. This discontinuity was discovered in 1909 by a Russian scientist A. Mohorvicie, therefore, it is named after him as 'Mono-discontinuity'. The mantle extends UPPER MANTLE" upto a depth of about 2900 km from the Moho-discontinuity,. Silica and Magnesium are the major constitutiing elements of this layer, hence it is also called SIMA.
Between 100 km to 200 km depth of the upper mantle, the velocity of the seismic waves is slowed down to 7.8 km/sec. Therefore, this zone is is known as the 'Zone of Low Velocity'. The discontinuity in density between upper mantle and lower mantle is known as Repetti discontinuity.
At the lower end of the lower mantle the velocity of the P waves suddenly increases to 13.6 km/sec. This shows as sudden change in the density of the rocks resulting into a type of discontinuity, known as 'Weicher Gutenber Dscontinuity'. It is divided into two parts Outer core and inner core. S waves cannot penetrate into the outer core. The density of the core is more than the double of the density of the mantle. In the inner core where the density is at its maximum, the velocity of the P waves is 11.23 km/sec because of its relatively liquid or semiliquid state due to excessive temperature but because of too much pressure it is in semi-liquid or plastic state. It's main constituens are nickel and iron, so it is also known as NIFE.
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