Question

Which discontinuity lies between SIAL and SIMA?
1. Gutenberg discontinuity
2. Conrad Discontinuity
3. Mohorovicic Discontinuity
4. None of the above

Answer 1

Correct Answer - Option 2 : Conrad Discontinuity

The correct answer is Conrad Discontinuity.

• The border at which the density of SIAL and SIMA changes is known as Conrad discontinuity.
• SIAL indicates the composition of Silica and Aluminium whereas SIMA indicates the composition of Silica and Magnesium.

• SIAL( Silica+ Aluminium):
  • SIAL is the topmost layer.
  • It forms the outermost crust of the earth commonly called the lithosphere.
  • It is the lightest of all the layers of the entire interior composition of the earth.
  • The specific gravity of Sial is between 2.7 to 2.9 gm/cm³.
  • It is average thickness is about 20 km.
  • This layer is dominated by silica and Aluminium.
  • It is made up of granite rock.
• SIMA(Silica+ Magnesium):
  • ​SIMA is found beneath the SIAL.
  • It is heavier than the SIAL layer.
  • SIMA indicates the composition of Silica and Magnesium.
  • SIMA extends totally up to 42 km depth.
  • It is specific gravity between 2.9 to 4.7 gm/cm³.
  • It is made of Basalt rocks.

• Gutenberg Discontinuity: 
  • ​The discontinuity between Mantle and Core is known as Gutenberg discontinuity.
  • This discontinuity is located inside the earth at a depth of about 2900 kilometres below the surface.
  • A famous Seismologist Beno Gutenberg studied the inner layers of the earth and observed that at a certain depth of the earth's surface, primary waves of earthquake slowed down dramatically.
• Mohorovicic Discontinuity:
  • It is the boundary between the Crust and the Mantle in the earth.
  • This is a depth where seismic waves change velocity and there is also a change in chemical composition.
  • The boundary is between 25 and 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor.