Classification of rocks

Rocks are formed by the composition of minerals. They may be formed by the deposition of soil materials or by cooling the molten lava on the earth’s surface during the volcanic eruption. The classification of rocks are done in three types. They are:

1) Geological classification

2) Physical classification

3) Chemical classification

1.Classification of rocks based on Geology:

1) Igneous rock

2) Sedimentary rock

3) Metamorphic rock

Igneous rock:

fig: Igneous Rocks

Igneous rock is formed by cooling the molten lava on the earth’s surface during the volcanic eruption. The portion of the lava which comes outside the surface cools quickly and forms. The rocks of non-crystalline nature called as trap or basalt. The rest of the portion which remains inside the earth undergoes cooling at a slow rate result in the formation of rock of crystalline nature known as granite.


Sedimentary rock:

fig: Sedimentary Rocks

Sedimentary rock is formed by the gradual deposition of disintegrated rocks ( due to the atmospheric action such as rain, wind, and temperature), vegetable matters and clay at the bottom of the river and lake. These rocks are also called a stratified( distinguishable layer) because these rocks are formed by the gradual deposition of disintegrated rocks. Limestone and Sandstone are examples of sedimentary rock.

Metamorphic rock:

fig: Metamorphic Rocks

  When Sedimentary rock or Igneous are subject to great heat and pressure inside the eartha new variety of rock is formed which is known as a metamorphic rock. The change of structure is called metamorphism. Eg Limestone converts into marble, slate converts into gneiss.  

fig: Metamorphism

2.Classification based on the physical structure:

 Physically rocks are classified into 3 types:

 1) Stratified rock

2) Unstratified rock

3) foliated rock

 1) Stratified rock:

The rock that shows a distinct layer (distinguishable layer) along which it can be split into the slab. Eg sedimentary rocks are the stratified rock.

2) Unstratified rock :

These rocks do not distinct layer along which it cannot be split into a slab. Eg all Igneous rock are unsatirical rocks.

 3) foliated rock:

These rocks have tended to be split in a definite direction only. E.g. all Metamorphic rocks are foliated rocks except marble and quartzite.

 3.Classification of rocks based on Chemical Composition:

Chemically rocks are classified into three groups:

i) Silicious rocks

ii) Argillaceous rocks

iii) Calcareous rocks

i) Silicious rocks:

These rocks mainly consist of sand or silica as in quartize or granite. These are hard and durable and not easily affected by the weathering agencies.


ii) Argillaceous rocks:

These rocks mainly consist of clay or alumina as in slate and laterite. These stones are hard and durable but brittle.


iii) Calcareous rocks:

These rocks mainly consist of lime or Calcium Carbonates as in limestone and marble.



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