NCERT-GEOGRAPHY-CLASS-07-CHAPTER-02

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Concentric layers

 

Layers we found around our earth centre.


   

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Crust


  • This is the solid, the uppermost portion of the earth. Or we can say the solid uppermost concentric layer of the Earth. 
  • The crust has two divisions
  1. Continental and
  2. Oceanic.
  • The thickness of the crust is on average 35 kilometres on the continental portion and on average 5 kilometres on the ocean.
  • The crust is about 1% total mass of the Earth.    

 

SiAl

  • The continental crust portion is made up of
  1. Silica &
  2. Aluminium is called SiAl.
  • The thickness of the sial is about 35 kilometres.

 

SiMa

  •  On the other hand, the oceanic crust is made up of

  1. silica, and
  2. Magnesium is known as SiMa.

  • The thickness of Sima is about 5 kilometres.


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Mantle

     
  • The structure of the Earth, starting from the centre up to the crust, is divided into 3 major portions.
  • The innermost portion is the core, while the outermost portion is the crust.
  • The portion in the middle of the core and the crust, which is up to the depth of  2900 km known as the mantle.
  • It is -
  1. The most active portion in the internal structure of the Earth, 
  2. Having around 84% total mass of the Earth.
  • Mental is made up of "pyroclastic material", which has silica and magnesium.
Note- Pyro is a Greek word which means "fire". It means a zone full of and operated by fire with temperature and density fluctuation, which is evident from the formation of a "magma plume".


" Magma Plume"- a rising column of hot molten or semi-molten material from the deeper part of the Earth. 
It shows that the Earth’s interior is not static, but active and dynamic due to heat energy.


Core

  • The innermost portion of the Earth has around 15% mass of the Earth's mass.


 

NiFe

 

  • In the structure of the core, the prominent constituent

  1. Nickel about 80%, and
  2. Ferrous about 20%.

  • Cumulatively meaning called NiFe.

 

Mineral


Those -

  1. naturally occurring inorganic substances 
  2. have certain physical and definite chemical properties known as minerals.

   

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Rock

      



·          Rock is a kind of mineral.

·          Any natural mass of mineral matter that makes up the Earth's crust is known as rock.

 

Magma

     

  • Molten or semi-molten material, which, in its mixture, has
  1. molten rocks,
  2. gases, and
  3. Water vapour is known as magma.
  • It is formed in the mantle portion -Asthenosphere- of the Earth and is a product of heat and pressure.
  • The route through which Magma comes to the earth, the surface is known as a "vent".




Lava

      
  • With the eruption of the magma gases, led by water vapour and sulphur dioxide, they escape to the atmosphere.
  • Now, in pyroclastic material, we are resting with semi-solid extremely hot material which cools down on the crust portion, to solidify, known as Lava.

 

Igneous rocks

  • Rocks formed out of cooling down magma in the crust portion of the earth known as igneous rocks.

 

Primary rock

  • When we studied the Rock cycle, we came to know that igneous rocks are the first to form, and without the igneous rocks, the Rock cycle couldn't be completed. That's why igneous rocks are also known as primary rocks.


 

Extrusive igneous rock

  • Igneous rocks that are formed above the crust surface are known as extrusive igneous rocks.

 

Intrusive igneous rock

  • Igneous rocks are formed beneath the crust surface then known as intrusive igneous rocks.

 

Sediments

  • Igneous rocks, metamorphic rocks, and even sedimentary rocks 

  1. undergo the process of weathering, 
  2. rolling down, 
  3. cracking, and 
  4. hitting each other, 

  • so they break down into small particles known as sediments.

 

Sedimentary rocks

  •  When sediments are transported and deposited by the vectors, such as

  1. running water,
  2. wind
  3. groundwater
  4. river, and
  5. A glacier, in the presence of moisture, is deposited in compressed and hardened form as concentric layers of rocks known as sedimentary rocks.

  • Sedimentary rocks are also known as "layered rocks".

 

Fossil

  • The organic remains of the plant Kingdom and the animal kingdom, which are trapped in the layers of rocks known as fossils.

 

Metamorphic rocks

  •  Those sedimentary rocks and igneous rocks, 

  1. under great heat and pressure, 
  2. through the transformation of chemical structure 
  3. transformed into a new type of Rock known as a metamorphic rock.


Rock cycle



  • The rock cycle is a cyclic process in which rock formation and denudation take place simultaneously.
  • In the first stage of the cycle, igneous rocks are formed through magma.
  • Now, in the next stage under the denudational process, involving the weathering process over igneous & Metamorphic rock, new kinds of rock- "sedimentary rocks"- formed.
  • Under tremendous heat and pressure, both igneous rocks & sedimentary rocks could be transformed into a new kind of rock known as "metamorphic rocks".
  • Metamorphic rocks can also be formed through re-metamorphism of metamorphic rocks.
Note- And that's how, aided by subduction and radioactive decay, in the cyclic process, the rejuvenation process of the rock cycle is still going on.


 

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