Inside Our Earth

Rocks: The earth's crust is made up of rocks of various types. A rock is a natural piece of mineral matter that makes up the earth's crust. Rocks can be of differnt:

  • Colour
  • Texture
  • Size

Types of Rocks

  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks

Igneous rocks

The word igneous is derived from the Latin word Ignis which means fire. The rocks formed by the solidification on the cooling of molten magma, are called igneous rocks. Depending on where the molten magma cools, they are of the following types:

  • Intrusive Rocks
  • Extrusive Rocks

Intrusive Igneous Rocks

When the molten magma cools deep inside the earth's crust, intrusive igneous rocks are formed. They

  • Cool down slowly
  • Form large grains

Granite is intrusive igneous rock.

Grinding stones used to prepare paste/powder of spices and grains are made of granite.

Extrusive Igneous Rocks

When the molten magma (lava) comes on the earth's surface, it rapidly cools down and becomes solid. Rocks formed in such a manner on the crust are called extrusive igneous rocks. They have a very fine grained structure. Basalt is an example of extrusive igneous rocks.

The Deccan Plateau is made up of basalt rocks.

Sedimentary Rocks

These rocks get their name from the Latin word sedimentum which means settle down. These rocks are formed by the settling down of sediments. Sediments are the smaller particles/fragments that are formed by breaking down of rocks when they roll down, crack and hit each other. These sediments are transported by wind, water, etc. These sediments (when compressed and hardened) form sedimentary rocks.

Sandstone is an example of sedimentary rock. It is made up of grains of sand. Sedimentary rocks may also contain fossils of plants, animals and other micro–organisms that once lived on them.

Metamorphic rocks

The Greek word metamorphose which means change of form, is the root of the word metamorphic. In other words, these are the rocks that are formed when igneous and sedimentary rocks change their form under following two circumstances:

  • Great heat and
  • Great pressure

The following are the examples of metamorphic rocks:

  1. Slate: Clay becomes slate after metamorphosis.
  2. Marble: Limestone becomes marble after metamorphosis.

Uses of rocks

  • Hard rocks are used for making roads, houses, factories, buildings, etc.
  • Red sandstone and marble are used in making buildings.
  • Many rocks are used as gemstones.
  • Stones are used in many games such as, Seven stones (pitthoo), Hopscotch (stapu) and Five stones (gitti)
Rock Cycle

A cyclic manner is followed in transformation of one type of rock to another type under certain conditions. This process of transformation of rock from one to another is called rock cycle.

rock cycle

Fig: Rock Cycle


Rocks are made up of different minerals. Minerals

  • Are naturally occurring substances
  • Have certain physical properties
  • Have a definite chemical composition
Uses of minerals
  • They are used as fuels, for example: coal, petroleum and natural gas.
  • They are used in industries for making medicines, fertlisers and many other items. Iron, bauxite, mica, gold, silver, etc. are examples of minerals.

Lava: Fiery red molten magma coming out from the interior of the earth on its surface.

Fossils: The remains of dead plants and animals trapped in the layers of rocks.

Do You Know?
  • The deepest mine (4 km deep) is in South Africa. Engineers have dug a hole about 6 km deep in search of oil.
  • A 6000 km deep hole in the ocean floor needs to be dug to reach the centre of the earth (which is not possible!).
  • The radius of the earth is 6371 km.
  • In the descending order of the volume of the earth (thickness) at its three layers, we first have the core (83 %), then the mantle (16 %) and finally the crust (0.5 %).

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