Interior of the Earth
The study of Earth's interior structure has evolved from early theories like Sir Eduard Suess's three-layer model to modern understanding based on seismic wave analysis and other scientific methods.
Sir Suess proposed an early model of Earth's interior divided into three distinct layers:
- SIAL
- SIMA
- NIFE
SIAL Layer
→ It is dominated by Silica and Aluminium.
→ Average density is 2.7 gm/cm³.
→ Thickness is 30 to 50 km over continent & 0 to 5 km over ocean.
→ Continental crust is made up of SIAL and it is dominated by Granitic rocks.
SIMA
→ Made up of denser material, having average density of 3.0 gm/cm³.
→ Heavy minerals such as iron and magnesium.
→ It forms oceanic crust and has dominance of Basaltic rocks.
NIFE
→ It is most dense layer which forms core.
→ Average density is 11 gm/cm³.
→ Dominance of Nickel and Iron.
Question: Physical conditions of the Interior of the Earth.
Question: Layer structure of the interior of the Earth.
Question: Role of Seismic waves in studying the interior of the earth.
METHODS TO STUDY THE INTERIOR OF EARTH
To study the interior of earth there are some artificial sources as well as some natural sources.
The Artificial Sources Are:
1. Digging a hole or Mining Industry
2. Studying density, pressure and temperature differences
3. Deep ocean Drilling process.
The Natural Sources Are:
1. Vulcanicity
2. Seismology
3. Meteors which fall on the earth
PHYSICAL CONDITIONS OF EARTH'S INTERIOR
Temperature
There is a general rule that the temperature will keep on increasing as we go inside the earth at the rate of 1°C for every 32 meters. Thus increase in the temperature is also proved by presence of hot spring and volcanic activity. It is said that temp. inside the earth is around 4000°-6000°C. If we take a general look at temp. increases at rate of 1°C per 32 meters then the temp. in the core should be around 200000°C (2 lakh °C).
This is because numerous studies have shown that temp. increases at very fast rate only till top 100km, after that the rate of temp. decreases.
It's estimated that the temp. below the crust is around 3000°C and the temp. in the lower mantle and core is around 4000°C. Therefore the zone below the earth's crust up to 200km is the zone of weak rocks known as Asthenosphere, and is source of magma or lava which is given out during Volcano.
Astheno → Weak
Sphere → Zone
Pressure
As we go below the earth the pressure goes on increasing due to the weight of overlying rocks. The pressure of the rocks is around 1 million Atmosphere at the depth of 500km. Hence, the pressure increases to such an extent there is increase in the melting point of the rock, and hence majority of the earth's interior is solid in nature.
Density
We can conclude that as pressure goes on increasing the density should also go on increasing. The earth is denser because as we go deeper inside the earth the density of the material increases.
The core due to presence of extremely dense iron and Nickel has highest density while the crust is made up of lightest rock.
This can also be proved by the study of meteors whose core is exposed when they reached earth's surface and these cores are made up of Nickel and iron.
STUDY OF SEISMIC WAVE
Natural Sources:
1. Vulcanicity
2. Meteors
3. Seismic waves
Vulcanicity
When lava or magma comes from below the earth, it gives an idea of the material that are present inside the earth. But most of the Volcano originate at the depth of 40 to 50 km, and do not originate below 200 km and therefore study of volcano do not give complete understanding of interior of the earth.
Meteors
When the meteor reached the earth their outer layer is burned exposing the core and thus we can have indirect idea of how the core was formed and the constituents of the core but again the study of meteors do not give a complete understanding.
SEISMIC WAVES
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| Note:- AI-generated schematic map for educational use only |
Seismology is the branch of science which studies various aspects of seismic wave i.e the waves that are generated during the earthquake.
By studying the body wave we can have a complete understanding of the interior of Earth.
During an earthquake there are two kinds of waves are generated i.e the Body wave and Surface wave.
The body waves are further divided into:
1. P-Waves
2. S-Waves
TYPES OF SEISMIC WAVES
P-Waves or Primary Waves
→ The first wave to be generated during the earthquake.
→ Can travel through solid, liquid and gaseous medium.
→ Travels with more speed if the density is more.
→ Are longitudinal waves i.e. materials vibrate parallel to wave propagation.
S-Waves or Secondary waves
→ Secondary waves are similar to light waves in which particles vibrate perpendicular to the wave direction.
→ Travel with half the speed of P-wave and S-waves are more destructive than P-waves.
→ Can pass only through solid.
Surface Wave
These are the most destructive and slowest of all the wave. As they can pass only up to smaller depth, they can give idea only about Earth's Crust.
RELEVANCE OF SEISMIC WAVE
1. The other natural or artificial method have limitations that they do not give a complete understanding of earth's interior.
2. The seismic wave which pass through the interior of earth through their change in velocity, direction and shadow zones have contributed in understanding the earth.
3. Velocity - Velocity of p-wave and s-wave goes on increasing as it goes interior. The sudden change in the velocity is experienced at 30km below which is known as Moho Discontinuity / Crust-Mantle Boundary.
The velocity is greater in the mantle but upper 200km experience low velocity as it is the Zone of weak rock (Asthenosphere).
The velocity further decreases in the outer core as well as S-wave do not pass through outer core and therefore we can conclude that the outer core is liquid in Nature.
4. Direction - As the waves are refracted we can conclude that the earth is made up of material of different density.
5. Shadow Zone - These are the zones where the seismic wave are not recorded.
As the s-wave is not recorded beyond the 105° from the focus, it's proved that s-wave cannot pass through outer core due to its liquid nature.
Similarly the P-wave also experience a smaller shadow zone due to refraction in the outer core.
Conclusion
Thus the modern view of the interior of the earth is an outcome of complete understanding provided by the seismic wave as shown in the diagram below-
Why This Topic is Important for UPSC
Understanding the interior of the Earth is crucial for UPSC examination as it forms the foundation for topics like plate tectonics, earthquakes, volcanoes, and mineral distribution. Questions on Earth's interior structure, seismic waves, and physical conditions frequently appear in both Prelims and Mains (GS-I) examinations.
📘 Related Geography Content:
Salisbury’s Classification of Landforms
Fundamental Concepts of Geomorphology
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