Classification of Rocks

 There are mainly three types of classification 

   a) Geological 

   b) Physical 

   c) Chemical

a) Geological classification:

Classification based on the formation of rocks 

          I. Igneous rocks: These rocks are formed by cooling of Magma. 

They are classified into two 

i) intrusive rocks : These are formed inside the earth surface 

Hypabyssal rocks: Stones formed at shallow depth from Earth's surface. E.g.. Dolorite 

Plutonic rock: Stones formed at considerable depth from the Earth's surface. E.g. Granite

ii) extrusive rocks : These are formed outside the Earth's surface. E.g. Basalt, Trap

            II. Sedimentary rocks : These are formed by the depositions of weathering products of pre existing rocks.

Eg. Sandstone, Limestone, gypsum, Gravel

            III. Metamorphic rocks: These rocks are formed by changing the character of pre existing rocks 

Eg. Quartzite, slate, marble, Gneiss

b) Physical classification 

           I. Stratified rocks: These rocks have definite plane of stratification 

Eg. All sedimentary rocks 

         II. Unstratified rocks: These rocks have no specific stratification. They are crystalline, granular or compact structure 

Eg. All igneous rocks

        III. Foliated rocks: These rocks have a tendency to split along a definite direction.

Eg. All metamorphic rocks 

c) Chemical classification 

           I. Siliceous rock: silica is the predominant constituents in this type of rocks. They are hard, durable and not easily affected by weathering

Eg. Granite, Quartzite 

          II. Argillaceous rock: clay is the predominant constituents. These are dense, compact or maybe soft

E.g.. slate, laterite

          III. Calcareous rock: calcium carbonate is the predominant constituent. 

Eg . Limestone, marble