The solid component of the earth is called lithosphere. It consists of three parts: crust, mantle and core.
The outermost shell A-C of lithosphere composed of virgin rocks that are tough is known as crust of the earth. The crust varies from 64 to 96 km. About 70 per cent of the crust is immersed in oceans and seas.
The shell A-C can be divided into two parts:
(i) A-B (upper layer):
This lighter upper layer is composed mainly of granite. It is also called SLAL as the rock forming minerals are mostly compounds of silica (Si) and aluminum (Al). The specific gravity of this layer is about 2.7.
(ii) B-C (layer following upper layer):
This layer is composed mainly of denser rocks like basalt and is also known as SIMA as the chief constituent of the rocks is silica (Si) and magnesium (Mg). The specific gravity of this layer is about 3.0.
The imaginary line passing through B where the two layers, namely, SIAL and SIMA meet with each other is known as ‘Conrad Discontinuity’.
The lithosphere is divided into several sections called plates. There are continental plates as well as oceanic plates. The plates float and drift on the asthenosphere due to the difference in densities as well as the heat generated by the earth’s interior.
When the two plates drift apart, a deep rack or rift is created. At other places where two plates come closer and collide, sedimentary rocks are folded to form mountains. Volcanoes and earthquakes mostly occur along the edges of these plates.
For simplicity A-B layer is considered as one plate and B-C another plate. These plates can be called as tectonic plates. The movements of these tectonic plates can be called tectonic movements.
Below the crust (A-C) of lithosphere lies the mantle (C-E). The thickness of mantle is about 2880 km. The mantle can be divided into two parts:
(i) C-D (upper layer):
This is the lighter upper part of the mantle and is called “asthenosphere’. This layer is mainly composed of chromium (Cr), iron (Fe), silicon (Si) and magnesium (Mg) and is also called Chrofesima. The specific gravity of this layer varies from 3 to 3.5. It is from this part of the mantle that the molten rocks of magma find their way to the earth’s surface through cracks and faults.
(ii) D-E (lower layer):
This is the lower part of the mantle and is called ‘mesosphere’. The specific gravity of this layer varies from 3.5 to 4.5. This layer is mainly composed of nickel (Ni), iron (Fe), silicon (Si) and magnesium (Mg) and is also called Nifesima.
In mantle too, the specific gravity increases with increase in the depth. An imaginary line passing through ‘C’ where the crust meets the mantle is known as’ Mohorovicic discontinuity’ or simply ‘Moho discontinuity’.
The innermost part of the lithosphere is the core, also known as bar sphere, the sphere of weight. This layer surrounds, the centre of the earth, its thickness is about 3500 km. This is the hottest, heaviest and densest layer of the earth’s interior and is composed mainly of nickel (Ni) and iron (Fe) and is named NIFE.
The other components are sulphur (S), silica (Si) and cobalt (Co). It is believed that the earth acts as a magnet due to the presence of the two metals nickel (Ni) and iron (Fe). The core can be divided into two parts:
(i) E-F (outer layer):
It is the outer core containing molten metal and thus, is the molten metallic core. The thickness of this layer is about 2250 km.
(ii) F-G (inner layer):
It is the inner core containing relatively more solidified, compact metallic minerals. The thickness of this layer is about 1250 km. The imaginary line passing through ‘F’ where the mantle meets the core is known as ‘Gutenberg discontinuity’.
The specific gravity of the core is about 13 and temperature of the core is 5000-5500°C.The core radiates heat in much smaller amount compared to the earth’s surface and hence, has such a high temperature. Such intense heat would normally melt all materials, but the tremendous high pressure of the overlying rocks keeps the material in a semi molten or viscous state.