Human life is largely influenced by the physiography of the region. The phisiography, in turn, is influenced by various endogenic processes operating in the interior of the earth. This makes it necessary for us to understand the interior structure of the earth.
The earth’s radius is 6370 km. Going beyond a limit is not possible as it is very hot at this depth. In such conditions the knowledge about the interior of the earth is mainly based on estimates and inferences.
The knowledge about interior of the earth is obtained through direct sources and indirect sources. Direct sources include rock materials from mining areas and molten magma from volcanic eruptions. Besides, scientists are working on “Deep Ocean drilling Project” and “Integrated Ocean Drilling Project”. These projects have provided information about interior of Earth through analysis of materials collected at different depths.
Indirect sources include meteors that at times reach the earth, gravitation, magnetic field, and seismic activity. Most of our information about earth’s interior has come from seismic studies.
Seismic Studies and the Earth's Interior
Seismic waves produced during an earthquake are recorded by a seismograph when these waves reach the surface of the earth.
There are two types of seismic waves- body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.
There are two types of body waves- P waves (Primary waves) and S waves (Secondary waves). P waves move faster and are similar to sound waves. They travel through gaseous, liquid and solid materials. S waves can travel only through solid materials.
Earthquake waves get recorded in seismographs located at far off locations. However there exist some specific areas where the waves are not reported. These are called shadow zones.
The variations in direction, velocity, reflection, refraction, dispersion, shadow zones and other propagation characteristics of different waves have been studied to get a detailed picture of the earth’s interior.
Structure of Earth’s Interior
The inferences drawn from the study of the seismic waves, we have been able to understand the nature of Earth’s interior. A diagram depicting the layered structure of Earth is given below.
As we can see, the earth’s interior is divided into three concentric layers. The outermost layer is the crust which is solid in nature. The mean thickness of oceanic crust is 5 km whereas the mean thickness of continental crust is around 30 km. Major constituents elements of crust are Silica (Si) and Aluminium(Al). Its mean density is 3gm/cm3.
The layer below the crust is mantle. It extends up to a depth of 2900 km. It is mainly made up of Silica (Si) and Magnesium (Mg). Its mean density is 3.4 gm/cm3. The upper portion of mantle is called asthenosphere. It is main source of magma during volcanic eruptions. The crust and uppermost part of the mantle is called lithosphere. Its thickness ranges from 10-200 km.
The innermost layer of the earth is called the core. It extends from 2900 km to the centre of the earth at 6370 km. The outer core is in liquid state and the inner core is in solid state. The density of material at the mantle-core boundary is around 5 g/cm3 while at the centre of the earth it is around 13 gm/cmm3. The core is mostly constituted of Nickel (Ni) and Iron (Fe).
Temperature inside the earth increases with increase in depth. But the rate of increase of temperature beneath the surface decreases towards the centre. While in the upper 100 km, the increase in temperature is at the rate of 12°C per km, in the next 300 km it is 20°C per km but is only 10°C per km below it.
The pressure also increases from the surface towards the centre of the earth due to huge weight of the overlying rocks. Due to increase in pressure and presence of heavier materials towards the earth’s center, the density of earth’s layers also goes on increasing.