Interior structure of the Earth // Crust, mantle and Core of the earth

Interior structure of the Earth  :

1. The Crust :-

                          The crust is the outermost layer of the earth. The thickness of the crust is varies under oceanic and continental areas. The thickness of the oceanic crust is around 5 km, where as the thickness of the continental crust is around 30 km. It should be note here that the crust is thicker under mountainous areas. It is around 70 km thicker under the Himalayan region. The average density of the outer and lower crust is 2.8 and 3.0 respectively. The difference of density between the upper and lower crust is because of the pressure of supperincumbent load. The average velocity of P waves in the upper crust is 6.1 km per second while it becomes 6.9 km per second in the lower crust.

2. The mantle :-

                           The portion of the interior beyond the crust is called the mantle layer. The mantle extends from Moho's Discontinuity to a depth of 2900 km. Moho's Discontinuity is boundary or zone between the crust and the mantle. The upper portion of the mantle is called Asthenosphere. The mantle part has higher density than the crust. The mantle having density of 4.6 g cm -3 extends for a depth of 2900 km inside the earth. It may be mentioned that the thickness of the mantle is less than half of the radius of the earth (6370 km) but it contains 83 percent of the total volume and 68 percent of the total mass of the earth.The crust and the upper layer of the mantle is called Lithosphere. The material in upper layer of mantle is called Magma. Once it starts moving towards the crust or it reaches the surface of the earth, it is referred to as lava.
     It should be note here that recently mantle is divided on the basis of information received from the discovery of the International Union of Geodesy and Geophysics into 3 sub zones viz. (a) first zone extending from Moho Discontinuity to 200 km depth, (b) second zone extending from 200 km to 700 km depth and (c) third zone extending from 700 km to 2900 km depth.

3. The core :-

                            The earthquake wave velocities helped in understanding the existence of the core of the earth. The core-mantle boundary is located at a depth of 2900 km. The core has two part- a. Outer Core, and b. Inner Core. The outer core is in liquid state and the inner core is in solid state. The density of inner core is 13.3 to 13.6. P waves travel through this zone with the speed of 11.23 km per second. It is generally believed that the core is composed of iron and nickel but according to second view point the core may be formed of silicates. It is also believed that after disintegration on high pressure the electronic structures have changed into heavy metallic materials, thus the density of the core has increased. According to third view point initially the core was composed of hydrogen but later on hydrogen was transformed into metallic materials due to excessive pressure.
Most of present day geophysicist and geochemists believe that the core is made of metallic materials mainly iron and nickel. Thus it is sometime referred to as the nife layer.