Since we can not directly observe the interior of the earth, we have to depend upon the evidences of interior of the earth. Scientists have always wondered about the structure, composition, temperature and the state of matter in the interior of the earth. The deepest borewell in the history of human beings is Kola Superdeep Borehole which is 12.2 Kms deep. Beyond this depth, we have not observed the interior of the earth directly. Therefore, the scientists depend on the other natural and artificial sources of evidence of the structure of the earth’s interior. We will not be discussing the evidence of interior of the earth based on the theories of origin of earth because those theories are merely hypotheses and largely rejected.
These Evidences are divided into Natural and Artificial Evidences.
The Artificial Evidence is based on deduction and logic. This evidence explains the interior of the earth indirectly.
- The density of rocks in the top layer of the lithosphere is 2.7 grams/CM3.
- Whereas, the physicists, based on the mass and gravity of earth, conclude that the density of earth is 5.5. g/CM3.
- It means, the density of the rocks deeper in earth must be greater and should increase with depth.
- The earth’s rocks exert pressure on any object which is deeper inside earth.
- Therefore, as we dig deeper, the pressure of rocks on any object keeps on increasing.
- So it is deduced that the rocks keep on getting compact with depth due to the incumbent pressure of rocks.
- Therefore, the earth might be solid from within.
- Due to an increase in pressure on an object, its temperature also rises.
- The heat generated at the time of earth’s formation is also trapped inside.
- The radioactive decay of uranium, plutonium etc. also generate heat inside earth.
- While drilling borewells, it has been observed that the rate of increase of temperature in the crust is about 15-30°C per kilometre but this rate slows down after reaching the asthenosphere (depth of 400 Kms.).
- It is estimated that the temperature at the core of earth is around 6000°C which may lead to complete melting of rocks, hence, the earth’s interior might be in liquid state.
4. Magnetic Field of Earth
- The scientists also deduce that the magnetic field of our earth is only possible if earth functions like a dynamo.
- To work as dynamo, the earth must have a metallic and solid core surrounded by a liquid layer.
- The Scientists from European Space Agency through the study of magnetic field of earth through Swarm Satellites’ Constellation also conclude that the liquid in the outer core flows like a meandering jet stream.
These sources provide contrary information to each other if seen separately. When studied in relationship to each other, they provide a better understanding of the interior of earth.
Relationship between Pressure, Density and Temperature inside Earth
The pressure leads to increase in density but pressure also increases temperature of the object on which pressure is being exerted. Therefore, the increasing pressure leads to melting of rocks with depth in the asthenosphere (400 Kms deep). The rocks stay in viscous semi-molten state until they reach the boundary of the inner core (5400 Kms). In the inner core, the pressure and density of rocks is so high that the rocks cannot stay in liquid state despite high temperature.
Natural Evidence can be collected through raw observation and scientific instruments.
1. Volcanic Eruption
The volcanic eruption brings lava from the interior of the earth to the surface. So, it is certain that one of the layers inside earth’s interior is in liquid or semi-molten state.
It is assumed that the meteorites are remnants of rocks which were left revolving around the Sun at the time of the formation of the planet. So, it is deduced that one of the layers in earth’s interior has the same composition as the meteorites. The meteorites are mostly formed of metallic elements such as iron and nickel.
3. Earthquakes and Seismology
Earthquake is defined as the shaking of the earth. When the earth shakes, it produces waves of different types. The waves’ frequency, wavelength and speed is measured by seismograph. Seismologists are specialist scholars of earthquakes.
1. Primary Wave (P-Waves)
- These are fastest travelling waves at the rate of approximately 6.0-7.2 Kms/s.
- They travel through solid as well as liquid matter.
- The matter in the medium moves forward (to) and backward (fro).
2. Secondary Waves (S-Waves)
- These waves travel at the rate of approximately 3-5 Kms/s. depending on the density of medium.
- They travel through solids only and disappear in liquids.
- The matter in the medium moves up and down i.e. perpendicular to the direction of propagation of waves.
3. Surface/Longitudinal Waves (L-Waves)
- These waves are slowest
- They travel only in the top layer of earth and are most destructive.
- There are two types of L wave i.e. Love Waves and Rayleigh Wave.
- Love Waves travel only the surface of earth and the rocks’ matter moves horizontally.
- Rayleigh Waves travel both horizontally and vertically leading to a circular motion in the layer of rocks.
Seismologists after study of these waves found that P-Waves and S-Waves deflect at the depth of 400 Kms. and their speed also increases. P-Waves experience sudden increase in speed in the outer core at 2900 kms. depth whereas the S-Waves disappear completely. At the depth of 5400 kms, the speed of P-Waves increases further. This pattern of waves’ speed, deflection of waves and lack of S-Waves in certain layers suggests that density of earth’s rocks keep on increasing with depth. But the disappearance of the S-Waves at 2900 Kms depth conveys the presence of a liquid layer. The further increase in speed of P-Waves from 8.1 kms/s in outer core to 11 kms/s in the inner core concludes that the layer from 5400 Kms. to the centre of the earth is in solid state because the speed of P-Waves can not be so high in a liquid matter.
Kulwinder Singh is an alumni of Jawaharlal Nehru University, New Delhi and working as Assistant Professor of Geography at Pt. C.L.S. Government College, Kurukshetra University. He is a passionate teacher and avid learner.