Updated By: LatestGKGS Desk
Types and sub-types of the rocks, division, formation, mechanism, properties
Types of rocks- igneous, sedimentary, and metamorphic and their properties
There are mainly three types of rocks:
- Igneous Rocks— solidified from magma and lava.
- Sedimentary Rocks— the result of deposition of fragments of rocks.
- Metamorphic Rocks— formed out of existing rocks undergoing recrystallization.
Igneous Rocks
- Formed out of magma and lava and are known as primary rocks.
- If the molten material is cooled slowly at great depths, mineral grains may be very large.
- Sudden cooling (at the surface) results in small and smooth grains.
- Granite, gabbro, pegmatite, basalt, etc.are some of the examples of igneous rocks.
- There are two types of igneous rocks: intrusive rocks (Granite) and extrusive rocks (Basalt-Deccan Traps).
Plutonic Rocks or intrusive rocks
- Sometimes, the molten matter is not able to reach the surface and instead cools down very slowly at great depths. Granite is a typical example. These rocks appear on the surface only after being uplifted and denuded.
Lava or Volcanic Rocks or Extrusive rocks
- These are formed by rapid cooling of the lava thrown out during volcanic eruptions. Basalt is a typical example. The Deccan traps in the peninsular region are of basaltic origin.
Sedimentary Rocks
- Sedimentary or detrital rocks.
- Formed as a result of denudation (weathering and erosion).
- These deposits through compaction turn into rocks. This process is called
- Cover 75 percent of the earth’s crust but volumetrically occupy only 5 percent.
Depending upon the mode of formation, they are classified into
- mechanically formed — sandstone, conglomerate, limestone, shale, loess etc.
- organically formed — geyserite, chalk, limestone, coal etc.
- chemically formed — chert, limestone, halite, potash etc.
Metamorphic Rocks
- The word metamorphic means ‘change of form’.
- Form under the action of pressure, volume and temperature (PVT) changes.
- Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks.
- Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
On the basis of the agency of metamorphism, metamorphic rocks can be of two types
Thermal Metamorphism
- The change of form or re-crystallization of minerals of sedimentary and igneous rocks under the influence of high temperatures is known as thermal metamorphism.
- There may be various sources of the’high temperatures—hot magma, hot gases, vapors and liquids, geothermal heat etc.
Dynamic Metamorphism
- This refers to the formation of metamorphic rocks under the stress of pressure.
- Sometimes the high pressure is accompanied by high temperatures and the action of chemically charged water.
- The combination of directed pressure and heat is very powerful in producing metamorphism because it leads to more or less complete recrystallization of rocks and the production of new structures. This is known as dynamothermal metamorphism.
- Under high pressure, granite is converted into gneiss; clay and shale are transformed into schist.
