Good knowledge of the interior of the earth is necessary to crack the UPSC Prelims. The UPSC wants its candidates to have a good knowledge of geography and hence the interior of the earth.
As we cannot go directly inside the earth and see what are the components the earth is made of. so how do we get the information about the interior of the earth? There are two sources viz. Direct and Indirect sources.
The most easily available solid earth material is surface rock or the rocks we get from mining areas. Gold mines in South Africa are as deep as 3-4 km. Besides mining, scientists have taken up several projects to penetrate deeper depths to explore the conditions in the crustal portions. Scientists world over are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project” The deepest drill at Kola, in the Arctic Ocean, has so far reached a depth of 12 km.
Volcanic eruptions are the other direct sources. When magma is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis.
The indirect sources include gravitational, magnetic field, and seismic activity. The gravitational force is different at different latitudes. It is greater at the poles and less near the equator.
Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
Seismic activity is one of the most important sources of information about the interior of the earth. Seismic activity is caused due to earthquake. The earthquake generates two types of waves viz. body waves and surface waves. The earthquake and these waves are explained in detail in our separate post. (Click here to read)
From all these direct and indirect sources, scientists have given the model of Structure of the Earth. The interior of the earth is distributed in layers. There are three main layers viz. crust, mantle and core.
It is the outermost solid part of the earth. The part on which we walk. The thickness of continental crust and oceanic crust is different. The mean thickness of the oceanic crust is 5 km and that of the continent is around 30 km. The continental crust is thicker in some major mountains systems. It is as much as 70 km thick in the Himalayan region. The density of continental crust is 3 g/cm³ and the density of oceanic crust is 2.7 g/cm³.
The portion of the interior beyond the crust is called the Mantle. The mantle is up to 2,900 km inside the earth. This boundary of 2,900 km is called the Moho’s discontinuity. The mantle further can be divided into two parts i.e. the upper mantle and the lower mantle. The density of mantle is higher than that of the crust. Its density is 3.4 gm/cm³
The core starts where the mantle ends i.e. deep inside 2,900 km of earth. The core is also divided into two parts. There is an outer core, which is in liquid form and inner core, which is in solid form. The density of the material at the mantle-core boundary is 5 gm/cm³ and at the centre of the earth at 6,300 km, the density is around 13 gm/cm³. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.
The above three layers are major layers. There are two more layers of the earth that are minor but important.
The uppermost part of the mantle is called ‘Aesthenosphere’. This aesthenosphere is in the liquid form and the main source of lava/magma.
This layer of the earth is made up of the crust and uppermost part of the mantle. This thickness of this layer is from 0-200 km. When we talk about the tectonic plates which are divided into many parts, we are generally talking about lithosphere. This lithosphere swims on the layer of molten magma i.e. asthenosphere. The movement of the lithosphere causes phenomena like earthquake and formation of volcanoes etc.