LITHOSPHERE
Earth’s crust and the interior of the earth: The earth is almost like a sphere. Its radius is about 6,400 kilometres. There is no scope to observe the interior of the earth by piercing the hard external covering. The geologists by observing the characteristics of the seismic waves, gravitation and the characteristics of the magnetic field have developed some idea about the elements and structure of the earth’s interior.
Depending on the worldwide collected information, the earth from the surface to the centre has been divided into three strata. These are (a) the earth’s crust, (b) barysphere and (c) centrosphere.
(a) The earth’s crust: The hard stratum of the rock covering the outer shell of the whole earth is known as the earth’s crust. The thickness of the earth’s crust is the lowest in comparison to the other interior layers of the earth. On an average, it is about 20 kilometres. The thickness of the earth’s crust on an average is about 35 kilometres beneath the continents and only 5 kilometres beneath the ocean. Generally, the continental earth’s crust is known as SIAL which is composed of Silicon (Si) and of Aluminium (Al). On the other hand, the earth’s crust beneath the ocean is composed of basalt which is heavier than that of Sial layer and its major elements are Silicon (Si) and Magnesium (Mg) and are generally known as SIMA. It is inferred that probably this basalt stratum has been existing throughout the world as a coating beneath the Sial and in the bottom of the seas and oceans. The temperature beneath the earth’s crust increased by 30° Celsius for every kilometre of depth.
(b) Barysphere: The stratum beneath the earth’s crust is the barysphere. But, there is a thin layer between the earth’s crust and the barysphere. The Yogoslavian Seismographer Mohorovicic in 1909 first discovered this layer of separation. So, the layer has been known as Mohorovicic Discontinuity. The layer of barysphere is 2,885 kilometres thick which is extended from Mohorovicic upto the outskirts of the centrosphere. The rocks of the upper part of the centrosphere are hard and brittle and this layer is about 100 kilometres deep. The earth’s crust and the upper part of the barysphere is 100 kilometres thick and this collectively known as rocksphere or lithosphere.
The rocks beneath the hard layer of barysphere being close to the melting point are partially soft and this layer extends downwards for about 300 kilometres. This layer is known as Asthenosphere. This hard layer of rock can move slowly over the Asthenosphere. The upper part of the barysphere is made of silicate which is primarily composed of iron and magnesium. It is assumed that the lower part of the barysphere (700 to 2,885 kilometres) is constituted of the minerals composed of complete mixture of iron oxide (FeO), Magnesium Oxide (MgO) and the Silicon dioxide (SiO2). The temperature of the barysphere at 100 kilometres depth ranges from about 1,100° Celsius to 1,200° Celsius which increases to 1,900° Celsius at a depth of 700 kilometres and to 3,000°Celsius at the outskirts of the centrosphere.
(c) Centrosphere: The centrosphere is a layer which is extended from the lower part of the barysphere upto the centre of the earth. The thickness of this layer is about 3,486 kilometres. The centrosphere occupies about 16 per cent of the total area of the earth and its weight is about one-third of the total weight of the earth. The pressure of this layer is several lakh times more than the air pressure and the temperature ranges from 3,000° Celsius to 5,000° Celsius. It is learnt from the seismic waves that the centrosphere has a liquid cover which is about 2,770 kilometres thick and has also a hard inner cover which is about 1,216 kilometres thick. The major two elements of the centrosphere is Ferrous (Fe) and Nickel (Ni) which is combinedly known as NiFe.
