Rocks are made up of individual solid substance called minerals. A rock as any natural mass of mineral matter that makes up the earth's crust. On the basis of the formation, rocks are classified into :
- Igneous Rocks
- Sedimentary Rocks
- Metamorphic Rocks
These rocks are formed by cooling, solidification and crystallization of hot and molten magma found below the earth crust. These are granular and crystalline rocks. These are also known as 'Primary or Ba'sic Rocks' because of the fact that these rocks were first to be formed and supply raw material for other types of rocks to be formed. No layering is found in these rocks and fossils are also not found. Nearly 90% of the crust is made up of igneous rocks. For example Granite Basalt Dolerite and Magetite
Rocks formed on the surface of the earth due to the erosion and depositon of igneous and metamorphi rocks are known as sedimentary rocks. Sedimentary rocks are found over about 75% area of the crust, but they contribute only 5% in the formation of the crust. These rocks contain fossils. On the basis of the nature of the sediments, sedimentary rocks are classified into :
- Mechanically formed sedimentary rocks, such as : Sandstones, Conglomerates, Clay rocks, Shale and Loess.
- Organically formed sedimentary rocks, such as : Limestones, Coal and Peat.
- Chemically formed rocks, such as : Chalk rocks, Gypsum and Salt rock.
Metamorphic rocks are formed due to complete alternation in the appearance and constitution of pre-exising rocks due to change in mineral composition and texture through temperature and pressuer. These are the hardest rocks and do not contain fossils.
Metamorphic rocks formed through the sedimentary rocks. (Meta - Sedimentary of Para - Metamorphic Rocks.)
- Slate from Shale
- Marble from Limestone
- Quartzite from chalk & dolomite
Metamorphic rocks formed through the Igneous rocks. (Meta - Igneous or Ortho-Meta morphic rocks)
- Gneisses from Granites
- Amphibolite from Basalt
- Schist from Basalt
Metamorphic rocks formed by the further metamorphosis of metamorphic rocks.
- Phyllite from Slate
- Schist from Phyllite
- Serpentine from Gabbro