Lecture 3 & 4 : Geography

Topics Covered in this Lecture

  1. Shape of the Earth
  2. Latitudes and Longitudes
  3. Various Motions of the Earth
  4. Time and Date Lines

Topics Covered in this Lecture

Sources to study the interior

Direct sources

  1. By digging through the earth
  2. deep mines
  3. projects to penetrate deeper depths such as “deep” ocean drilling projects and integrated ocean drilling projects.
  4. volcanic eruptions

The earth’s radius is 6371 km, therefore it is impossible to reach the center of the earth to collect samples. The deepest mine in the world( in South Africa) is just 3.9km deep. In the ocean drilling project, we have just reached up to 12km.

Indirect sources

  1. Studying pressure and temperature
  2. studying density inside the earth
  3. meteors that reach the earth
  4. study of seismic waves

Seismic waves

  • Seismic waves are the waves which are generated during earthquake.
  • these are recorded with the help of instrument known as seismograph.
  • Earthquake is shaking of earth, during which energy waves are generated that travel in all directions.
  • The point where earthquake occurs is known as focus or hypocenter which is always inside the earth.
  • The point on the surface, nearest to the focus, which is first one to experience the waves is called epicenter.

Types of seismic waves

  • Earthquake waves are basically of two types- body waves and surface waves.
  • Body waves are generated due to the release of energy at the focus and travel through the body of the earth.
  • Body waves after interacting with surface rocks generate surface waves.
  • There are two types of body waves called primary (P-waves) and secondary waves(S-waves)
  • P-waves are similar to sound waves. Means they can travel through gaseous. Liquid and solid materials. As they travel with faster speed they are first waves to arrive at surface. P-waves vibrate parallel to the direction of the wave leading to stretching and squeezing of the material.
  • S-waves are next waves to arrive at the surface. Interestingly, S-waves can travel only through solids. They vibrate perpendicular to the direction of propagation creating crest and troughs in materials through which they possess.
  • Surface waves (L-waves) are slowest of all seismic waves. Those are most violent and destructive of all waves.

Interior of Earth

Simpler concept

  • Earth is made up of concentric zones.
  • The outer part is known as Lithosphere- this Lithosphere has two parts, the upper part is known as SIAL, while the upper part is known as SIMA.
  • Below the lithosphere lies the middle layer known as mesosphere or mantle.
  • The innermost layer of Earth is known as core or Barysphere or NIFE.

Role of Seismic waves-

  • Best way to know the interior of the earth is to study the seismic waves.
  • Three waves: P, S, and L generates earthquake.
  • The velocity of these waves are changes while passing through different mediums. Denser the medium, greater will be the velocity.
  • They reflect of refract as they go through medium of different densities. Hence, their direction also changes.


  • P and S waves pass through both the outer layer of the earth.
  • P-waves pass through the inner layer but S-waves do not pass through the inner layers of the earth.
  • There is change in direction of waves while passing through different layers of the earth.
  • There are some areas where the seismic waves are not reported, known as Shadow Zones

Shadow Zone

Areas where seismic waves are not located are known as Shadow zones. These are of two types-

  1. P-waves shadow zone
  2. S wave shadow zone

P-waves shadow zone appears as a band around the earth between 105-degree and 145-degree away from the epicenter.

The shadow zone of S-waves is beyond 105-degree.

Through seismic waves, we come to know about the properties of the waves. Thus, we divide earth into-

  1. Crust- Continental and oceanic
  2. Mantle- upper and lower
  3. core- outer and inner


Crust is the outermost thin layer covering below the layers of sedimentary rocks. It is divided into two parts- oceanic and continental.

Continental crust– It is made up of Sialic or granitic rock. It is a comparatively less dense 2.7 gm cm cube as compared to the oceanic crust. Its average thickness of around 30 to 35 km. but thickness is greater below mountainous region in fact, the Himalayas the thickness of the crust is around 70 km.

Oceanic Crust– With an average thickness of 5 km, it is composed of Basaltic rocks (SIMA). It is more denser as compared to the continental crust.


The crust-mantle boundary is “Moho Discontinuity”.

The uppermost part of the mantle and crust is known as the Lithosphere. This lithosphere slides/glides on the Asthenosphere.

It is the zone of weak rocks which is the source of magma or Lava. The mantle is further divided into upper and lower mantle. Mantle-Core boundary is “Gutenberg discontinuity” The mantle occupies maximum volume of the earth.


Divided into two parts-

Outer core- It is liquid in nature or in the molten state. Temperature is very high in the outer core.

Inner core– It is the densest layer of the earth because the velocity of P-waves is maximum here. It is dominated by Nickle and Iron. The presence of a liquid outer core is responsible for the magnetic property of earth that is rotation.

Practice questions-

  1. why seismic waves is the best way to study interior of the earth? (250 words)
  2. Give a brief of interior of the earth. (250 words)
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