Classification of rocks

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Rocks can be classified based on geological classification, physical properties and chemical composition.

Geological Classification of Rocks

Based on the origin and formation of rocks, they are classified into following types:

• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks

Igneous Rocks

• The word ‘Igneous’ is derived from the Latin word ‘Ignis’ which means ‘Fire’.
• Igneous rock are formed as a result of cooling of molten lava followed by the hardening of cooled lava.
• These rocks do not contain any fossils or shells
• Igneous rocks include both harder and softer rocks.
• They are durable, hard, massive and stronger than other stones.
• They are crystalline in nature and do not possess planes of stratification.
• These rocks have the ability to take good polish and hence they are popular for face work.
• Example – Granite, Basalt, Trap, Andesite, Rhyolite, Diorite, Gabbro, pumice, Granodiorite, Dacite.

Classification of Igneous rocks

Based on the condition of cooling and crystallization, igneous rocks are classified into plutonic, volcanic and hypabyssal rocks.

Intrusive (or) Plutonic Igneous Rocks

• Rocks formed by the cooling of molten magma within the earth surface are called as intrusive or plutonic rocks.
• These rocks are of crystalline nature.
• The process of cooling of molten magma is slow thus giving time for the crystals to grow larger. Hence, the size of the grains are larger.  
• Example – Granite, Syenite, Diorite, Gabbro, Labradorite

Hypabyssal rocks

• If the magma solidifies at a relatively shallow depth (i.e., upto 2 to 3 km below the earth surface), then those rocks are called as hypabyssal rocks.
• These rock possesses a finely grained crystalline structure.
• Example – Dolerite.

Extrusive or Volcanic rocks

• Rocks formed by the cooling of molten magma above the surface of the earth are called extrusive or volcanic rocks.
• Extrusive igneous rocks cool rapidly and so there is less time for the formation of crystals.
• These rocks are non-crystalline, fine grained and possess glassy texture.
• The texture of these rock is highly influenced by the rate of cooling of the magma.
• Example – Trap, Basalt, Obsidian, Prophyrite, Tuff, Pumice

Based on the composition, igneous rocks are classified into ultramafic igneous rocks, mafic igneous rocks, intermediate igneous rocks and felsic igneous rocks.

Ultramafic Igneous Rocks 

• These igneous rocks are rich in iron and magnesium.
• It contains lesser proportion of silica.
• Ultramafic rocks does not contain quartz. 

Mafic Igneous Rocks

• Mafic igneous rocks contain calcium feldspar.
• These rocks are dark coloured.
• The density of mafic rocks are higher.

Intermediate igneous rocks 

• Intermediate igneous rocks consist of iron, magnesium silicates, sodium and aluminium rich minerals, lesser amounts of quartz.

Felsic igneous rocks 

• Felsic igneous rocks are rich in potassium feldspar, aluminium rich mica and quartz.
• These rocks are light coloured.
• The density of felsic rocks are lesser

Based on the silica content, igneous rocks are classified into acid rocks, intermediate rocks, basic rocks and ultra-basic rocks.

Type of igneous rock	Silica content	Example
Acid rocks	70 – 80	Granite, Rhyolite
Intermediate rocks	60 – 70	Syenite, Andesite
Basic rocks	45 – 60	Gabbro
Ultra-basic rocks	30 – 45	Peridotite

Sedimentary Rocks

• The word ‘sedimentary’ is derived from the Latin word ‘Sedimentum’ which means ‘Settle down’.
• Most sedimentary rocks are formed through the process of erosion (particles carried away from their source by water or wind), deposition (particles deposited as loosely packed sediments), compaction (particles compacted together under pressure) and cementation (particles glued together).
• As a result of weathering action of wind, water and frost on igneous rocks, the existing rock disintegrates. This disintegrated rock pieces are called as sediments. The sediments are carried away by wind and water and gets deposited in other locations. Due to the action of high pressure and heat or by chemical agents, these materials gets consolidated in layers.
• The sedimentary rocks formed are uniform, fine grained, compact and soft.
• In general, sedimentary rocks represent a bedded or stratified structure.
• Most sedimentary rocks are characterized by parallel or conflicting beds. The beds reflect the variations in either the rate of deposition of the material or the nature of the matter that is deposited.
• The properties of the sedimentary rocks varies depending on the nature of the sediment and type of bond.
• Example – Limestone, Dolomite, Sandstone, Mudstone, Shale.

Formation of Sedimentary Rocks

The formation of sedimentary rocks can take place in any of the following ways:

• Chemical Sedimentary rocks – Sedimentary rocks resulting from the precipitation of salts in drying water basin (chemical deposits) are classified as chemical sedimentary rocks. Example –  Gypsum, Anhydrite, Magnesite, Dolomite.
• Organic sedimentary rocks – Sedimentary rocks resulting from the accumulation of plant or animal remains (such as corals, shells or bone) are termed as organic sedimentary rocks. Example – Chalk, Diatomite, Tripoli, Coal.
• Clastic sedimentary rocks – The sedimentary rocks resulting from the deterioration of massive igneous rocks are classified as clastic sedimentary rocks. Example – Sandstone, Laterite, Conglomerate, Breccia, Siltstone, Mudstone, Shale.

Metamorphic Rocks

• The igneous and sedimentary rocks after transforming physically and chemically under the action of pressure, temperature, earth movements etc., are classified as metamorphic rocks. This process of formation of metamorphic rocks is termed as metamorphism.
• These changes occur in the rocks when they remain solid (and not in melted state) because each mineral is stable only over a specific range of temperature and pressure. When a mineral gets heated beyond its stability range, it breaks down to form another mineral.
• Metamorphic rocks can be of either foliated or non-foliated nature.
• The most popular metamorphic rock is marble.
• Example – Gneiss, Quartzite, Marble, Slate, Phyllite, Schist.

Examples of few metamorphic rocks along with their parent rock are as follows:

Original rock	Metamorphic rock	Original rock	Metamorphic rock
Granite	Gneiss	Conglomerate	Gneiss, Schist
Syenite	Gneiss	Shale	Slate, Schist
Sandstone	Quartzite	Dolomite	Marble
Limestone	Marble, Schist	Dolerite, Basalt	Schist
Mudstone	Slate

Physical Classification of Rocks

            Based on the physical structure of the rocks, they are classified as follows:

• Stratified rocks
• Unstratified rocks
• Foliated rocks

Stratified Rocks

• Stratified rocks are made up of different layers. These layers are separated by the planes of stratification or cleavage planes. Stratified rocks can be easily split along these planes.
• Sedimentary rocks are stratified rocks and metamorphic rocks are either stratified or unstratified.
• Example  – Sandstone, Limestone, Slate

Unstratified Rocks

• These rocks are not stratified. Hence they possess crystalline or compact granular structure.
• Unstratified rocks cannot be split in to thin sheets.
• Igneous rocks are unstratified rocks.
• Example – Granite, Trap, Marble.

Foliated Rocks

• Foliated rocks have a layered or band like structure which is due to the exposure of the rock to pressure and heat. These rocks have a tendency to split along one particular direction only. This direction may or may not be parallel to each other as in case of stratified rocks.
• Except Quartzite and Marble, most metamorphic rocks belong to this classification.
• Example – Gneiss, Schist, Shale.

Chemical Classification of Rocks

            Based on the chemical composition of rocks, rocks are classified as siliceous rocks, argillaceous rocks and calcareous rocks.

Siliceous Rocks

• Silica is the principal constituent of these rocks.
• Siliceous rocks are very hard and are also durable. Thus, the weathering agents like water, wind, salts, etc., have negligible effect on siliceous rocks.
• Example – Granite, Sandstone, Quartzite, Gneiss, Basalt, Trap, Syenite, Chert.

Argillaceous rocks

• The word ‘argil’ means ‘clay’. Thus, the rocks whose principal constituent is clay (or alumina) are termed as argillaceous rocks. 
• These stones are hard, dense, compact and durable but brittle in nature. These rocks cannot withstand shock.
• All clay stones are argillaceous.
• Example – Slate, Laterite, Kaolin, Shale.

Calcareous Rocks

• These rocks contain carbonates of lime (calcium carbonate) as the predominant constituent.
• Calcareous rocks are hard. But, the durability of these rocks depends upon the atmospheric constituents as they are react easily even with dilute hydrochloric acids.
• Example – Limestone, Marble, Kankar, Dolomite.