Earth’s Rocks

Materials of Earth’s Crust

The outermost part of lithosphere is called crust. This is the most significant part of the earth because it is occupied by humans. The material of the crust is made up of rocks.

The rocks are of different types. They are hard like granite, soft like clay and loose like gravel. Rocks have a great variety of colour, weight and hardness.

  • Rocks are composed of minerals. They are aggregates or physical mixture of one or more minerals.
  • Minerals on the other hand are made up of two or more elements in a definite ratio. They have a definite chemical composition.
  • Crust is made up of more than 2000 minerals, but out of these, 6 are the most abundant and contribute the maximum to this uppermost part of the earth. These are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.

Granite is a rock and its constituent minerals bound together are quartz, feldspar and mica which make it a hard rock. Change in the ratio of these minerals give rise to granites of different colours and hardness. The minerals containing metals are called metallic minerals. Haematite, a major iron ore is a metallic mineral. Ores are metallic minerals which can be profitably mined. Rocks are of immense economic importance to us.


Types of Rocks

Rocks differ in their properties, size of particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types –


Economic significance of Rocks




Igneous rocks are formed by the solidification of hot molten material called magma or lava.
Extrusive igneous rocks are formed by cooling of lava on the earth’s surface e.g. basalt, gabbro.
Intrusive igneous rocks are formed by solidification of magma below the earth’s surface, e.g. granite.
Sedimentary rocks are formed by the successive deposition of sediments.
These rocks have layered structure, therefore they are also known as stratified rocks.
Fossil is the solid part or an impression of a prehistoric animal or plant embedded in sedimentary rocks in which they are buried.
Metamorphic rocks are formed by the effect of heat or pressure on sedimentary or igneous or even metamorphic rocks.
Thermal metamorphism is the process by which a rock under-goes change as a result of great heat.
Dynamic metamorphism is the modification of rock, by tremendous pressure during extensive earth movements.



Bibliography : NIOS – Geography

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