Interior of the earth
The interior of the earth can be understood through direct and indirect sources. The direct sources such as surface rocks and volcanic eruptions and indirect sources such as gravitation, magnetic field and seismic activity reveal the structure of the earth.
Seismic wave or Earthquake wave studies
These waves are also called mechanical waves. It means that they need a medium for propagation which might be solid, liquid or gas. They cannot pass through the vacuum. Their behaviour changes with the changes in the medium. These waves are classified into two categories namely body waves and surface waves. The body waves are generated due to the release of energy at the focus and that energy travels in all directions through the body of the earth. Hence, they are called body waves. And when these body waves interact with the surface rocks and generate a new set of waves these new waves are called surface waves.
On the basis of these behaviours, four types of waves have been found. These are P waves, S waves, L waves and R waves.
The P and S waves are body waves and they are important to study. The P waves travel through all three mediums (solid, liquid and gas) but the speed of P waves is highest in solids and slowest in gases (S>L>G). they are called as ‘primary waves.’ They are similar to sound waves.
And the S waves pass through only solid medium. They are called as ‘secondary waves.’ Reflection causes waves to rebound whereas refraction makes waves move in different directions.
However, when the p waves and s waves are not reported on the seismograph, such a zone is called the ‘shadow zone’.
Hence these waves have helped scientists to understand the structure of the interior of the earth.
The structure of the Earth
The earth is divided into three parts namely crust, mantle and core. The asthenosphere is semi-molten and beyond the asthenosphere, the mantle rocks are dense. And the outer core is in liquid form whereas the inner core is in solid form.
The Crust
It is the outermost layer of the earth. There are two types of crust namely continental crust and oceanic crust. The oceanic crust is thinner as compared to the continental crust. The oceanic crust is 5 km whereas the continental crust is 30 km. Moreover, the continental crust is thicker in mountain ranges. It is around 70 km thick in the Himalayan region.
The Mantle
The layer between the crust and the core is called the mantle. The mantle extends from Moho’s discontinuity to a depth of 2900 km. The upper part of the mantle is called the asthenosphere. It is the main source of magma. It has higher density than the earth’s crust. The crust and the uppermost part of the mantle are called the lithosphere. Its thickness ranges from 10 to 200 km. The lower mantle extends beyond the asthenosphere. It is in a solid state.
The core
The outer core is in a liquid state while the inner core is in a solid state. The core is made up of heavy material mostly constituted by nickel and iron(Nife).
Composition of interiors of the earth
| Earth as a whole | Earth’s Crust |
| Iron (Fe): Highest | Oxygen (O2): Highest |
| Oxygen (O2) | Silicon (Si) |
| Silicon (Si) | Aluminium (Al) |
| Magnesium (Mg) | Iron (Fe) |
| Sulphur (S) | Calcium (Ca) |
| Nickel (Ni) | Magnesium (Mg) |
| Calcium (Ca) | Sodium (Na) |
| Aluminium (Al): Lowest | Potassium (K): Lowest |