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Interior of earth | Internal structure of the earth | Physical conditions of the earth's interior

Why the interior of the earth needs to study?
  • It helps to deduce the cause of natural phenomena such as earthquakes, volcanoes, and tsunamis. In the future, it may help to predict natural hazards like earthquakes.
  • It helps us in developing in an understanding of the endogenic forces
  • It helps us to understand the earth's magnetism.
  • It helps us to understand the movement of tectonic plates.
Sources of information about the Earth's interior:
There are two major sources of information that give us information about the interior of the Earth.
  • direct source.
  • indirect source.

Each source gives us a piece of information about the Earth's interior. It is only after analyzing both sources of information that we get a complete picture of the interior of the Earth.

Direct sources of information about the Earth's interior:
The following are the major direct sources of information about the Earth's interior:
  • Deep Mining
  • Deep Ocean Drilling Project and Integrated Ocean Drilling Project
  • source of volcano
  • Surface Rock Studies

Deep mining tells us that the deeper we go into the Earth, the temperature [thermal evidence] increases, the density of the material increases as well as the chemical properties of the material change. Changes in temperature, changes in density, and the chemical properties of our Earth's interior are all part of the geological evidence of Earth's interior. Gold mines in South Africa are about 3 to 4 km deep from the earth's surface. Due to the rising temperature, it is not possible to go deeper than this.

The Deep Ocean Drilling Project and the Integrated Ocean Drilling Project tell us that oceanic materials are denser than continental materials. The deepest drill at Kola in the Arctic Ocean has reached a depth of about 12 km. It provides a lot of information about the density of the material at different depths.

Volcanic material comes from the asthenosphere, providing us with information about the composition of the material at such depths. Although it is very difficult to identify the depth of volcanic material from how deep it comes, it is certain that matter inside the Earth at a certain depth is not in solid form.
Analyzing different parts of the Earth's surface as well as rocks from deep mines gives us a lot of geological evidence about the history and internal structure of the Earth's surface.

Indirect sources of information regarding the internal structure of the earth:
The following are some important indirect sources of information that are not directly collected from the interior of the earth.
  • Meteors Analysis
  • Earth's Gravitational force
  • Earth's Magnetic Field
  • Seismic activities or Earthquake
As we know that all planets and meteors formed at the same time and both are made of similar material, so the internal composition of meteor and earth will be similar. Meteor analysis provides us with information about the layers and chemical properties of the Earth's interior.

The gravitational force of the Earth is not the same on the Earth's surface at different latitudes. It is more near the poles and less at the equator. This uneven distribution of Earth's gravitational forces tells us that the Earth is bulged at the equator (the distance between the Earth's surface and the Earth's center is greater at the equator than at the Earth's poles) and flattened at the poles.

The Earth acts like a giant magnet, which means that the Earth must have a layer of molten iron (the outer core) that generates magnetic energy.

Seismic Activities or Earthquakes:
Seismic activities are one of the important indirect sources of information about the Earth's interior. It gives us a complete picture of the different layers of the Earth.

As we know that earthquake means the shaking of the earth. An earthquake releases energy in the form of waves by breaking the crust of a large part of the earth. The point from which energy is released is called the focus or hypocenter. The point closest to the focus on the Earth's surface is called the epicenter.

All earthquakes of the earth are originated from the lithosphere (up to the depth of 200 km from the earth's surface). A seismograph is an instrument that records the earthquake's wave.
Earthquake waves are generally two types:
  • Body wave
  • Surface wave.
Body waves are released from focus. Body waves travel in all directions of the earth's body. That is why it is called the body's wave.
The body wave interacts with surface rock and generates a new type of wave, called surface waves.
The velocity of waves is different in different mediums and materials. The higher the density of the material, the greater the velocity of the waves.

Body waves are of two types -P wave ( primary wave) and S wave (secondary wave). 

P waves are faster and the first-to-arrive earth's surface which is why it is also called the primary wave. P waves are similar to sound waves means they can travel in all three mediums-solid, liquid, and gas. The speed of p waves is fastest in the solid medium and slowest in the gaseous medium. These characteristics of P waves help us to identify the solid, semi-liquid, and liquid layers of the earth's interior.
p wave
s waves
S waves arrive at the earth's surface with some lag of time which is why it is called secondary waves. S waves can only travel in a solid medium. It can not travel in a liquid or gaseous medium. This fact helps us to identify the liquid layer of the earth.

The emergence of the shadow zone:
In some specific areas where waves are not reported, such a zone is called the Shadow zone.
As we know that S waves do not travel in a liquid medium and P waves travel in both medium solid and liquid. 
It was observed that P and S waves are recorded in all places within 105 degrees from the epicenter. However, only P waves are recorded beyond 145 degrees from the epicenter and S waves are not recorded beyond 105 degrees from the epicenter. 
Both P and S waves make the shadow zone between 105 degrees to 145 degrees from the epicenter. But the entire zone beyond 105 degrees from the epicenter is the shadow zones of S waves as this area does not receive the S waves.
From the emergence of shadow zones of the P and S between 105 degrees and 145 degrees from the epicenter, it is clear that there is a liquid layer ( outer core) in the earth's interior as it prevents the S waves to travel in the liquid layer.

The internal structure of the earth:
After analyzing both the direct and indirect sources of information on geological evidence [ rocks densities, chemical properties, rock types, rocks ages, etc] and thermal evidence[ temperature changes], we came to know the following characteristics of the internal structure of the earth: 
Earth has three layers:
  • The crust
  • Mantle
  • The core

पृथ्वी के आंतरिक भाग
The crust:
the following are the main features of the crust:
  • The crust is the outermost solid part of the earth. 
  • This layer is brittle in nature.
  • The thickness of the crust varies under oceanic and continental areas. The ocean crust is thinner than the continental crust however it is heavier than the continental crust. Within the continental crust, thickness is not uniform, it is thicker in the mountainous areas as compared to plains and plateau regions. As per estimates crust thickness in the Himalayan region is around 70 km. The average thickness of the oceanic crust is around 5km and the average thickness of the continental crust is about 30 km. 
  • The main chemical compositions of continental crust are silica and alumina. The Continental crust is thus called sial( si-silica and al-alumina).
  • The main chemical compositions of the oceanic crust are silica and magnesium. Thus, the oceanic crust is also called sima ( si-silica and ma-magnesium).
  • The mean density of crust density is about 2.7 g/cm3.
  • As for as the volume of the crust is concerned with respect to the earth's volume, the crust makes up only 1 percent of the volume of the earth.
The following are the primary chemical structures of the crust:
  • Oxygen (the largest, it is about 46.6% by weight)
  • Silicon (the second largest, 27.7% by weight)
  • Aluminum (8.1%)
  • Iron (5%)


The mantle:
The following are the main features of the mantle:
  • The mantle is the second and middle layer of the earth. Moho's discontinuity separates crust and mantle. The mantle extends from Moho's discontinuity to a depth of 2,900 km.
  • Whole mantle layers are further divided into three layers-upper solid mantles ( very thin and solid form), asthenosphere ( semi-liquid form), and lower mantle( solid form).
  • The lithosphere is comprised of the crust and the uppermost solid part of the mantle. Lithosphere plate float above the asthenosphere.  The thickness of the lithosphere varies from 10 km to 200 km.
  • The middle part of the mantle is called the asthenosphere which is plastic or semi-liquid in nature. The depth of the asthenosphere is approx. 400 km, it is the main source of Volcanoes.
  • The average density of the mantle is higher than the crust. The average density of the mantle is around 3.49 g/cm3.
  • As far as the volume of the mantle is concerned with respect to the earth's volume, the mantle consists of 84 percent of the volume of the earth.

The core:

The following are the main characteristics of the core
  • The core is the third and innermost layer of the earth. The core starts from 2900km from the earth's surface and reaches the earth's center( 6371 km). 
  • The core has further divided into two sub-layers-outer core( which is in liquid form) and the inner core( which is in solid form).
  • The density of the core varies from 5g/cm3 to 13g/cm3. The highest-density materials are in the center of the earth.
  • The main chemical constituents of the core are Nickel and Iron. That is why it is called the nife layer(ni-nickel and fe-ferrous (Iron).

Try to solve the following questions:
  • Account for the geological and thermal evidence to explain the internal structure of the earth. ( UPPSC 2022 geography)
  • Give an account of the different layers of the interior of the earth. (UPPSC 2017 geography )
  • Analyze the structure of the earth's interior. ( UPPSC 2000 Geography)
  • Write short notes on the Interior of Earth. ( UPPSC 1996 geography)
  • Highlight the internal structure of the earth on the basis of seismological evidence. ( UPPSC 1994 geography) 
You may like also:
  • पृथ्वी का आंतरिक भाग | पृथ्वी की आंतरिक संरचना

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