Rocks can be classified in following three ways.
- Geological Classification
- Chemical Classification
- Physical Classification
1. Geological Classification of Rocks
Rocks can be subdivided into three categories as per their geology.
Igneous Rocks (Primary, Un-stratified, Eruptive)
It is cooled down molten lava – also known as magma. These rocks form deep inside the core of earth where they continuously face extremely high temperatures. When near the core, they are in molten form but convert to solid when present in relatively cooler regions. Final form of rock is solidified form of magma. Example: Granite and Basalt.
Sedimentary Rocks (Aqueous Stratified)
Gradually deposited disintegrated rocks. These rocks take years to sediment in form of layers over layers. Surface of earth is subjected to various chemical agents which are brought by rain, frost and various other weather conditions. These agents break up the surface which is carried away by further rains to rivers and other water channels in which they continue to travel. As the velocity of the river decreases, matter starts depositing in form of debris and ultimately form the rock. Example: Sand stone, Lime stone.
Due to changes in structure of earth, igneous and sedimentary rocks find their way to high temperature zones inside the core. Continuous exposure to high temperature and pressure changes either the mineral composition or texture and at times both to produce an entirely new form. Such produced rocks are called Metamorphic rocks. Example: Granite changes to Gneisses, Lime stone to Marble and Shale to slate.
2. Chemical Classification of Rocks
Rocks can be subdivided in three different categories basing on their chemical composition.
Rocks containing Alumina(Al2 O3) and Clay as main components. Example: Slate, Laterite.
Rocks mainly containing Sand or Silicates(SiO2). Example: Granite, Basalt, Trap, Quartzite.
Rocks containing Calcium Carbonate and Lime. Example: Limestone, Marble, Dolomite.
3. Physical Classification of Rocks
Rocks having distinct layers which are separable. Cleavage plane for split is easily visible. Example: Slate, Sandstone, Limestone.
Cannot be easily stripped into slabs. No sign of distinct strata. Example: Granite, Basalt, Trap.
Having tendency to split up only in a definite direction.