CLASSIFICATION OF ROCKS

 Rocks are defined as natural solid massive aggregates of minerals forming the crust of the earth.  Petrology is the branch of geology dealing with various aspects of rocks such as their formation, classification and occurrence.

 For a geologist, rocks are said to be the “documents of geology”.  A professional civil engineer has to deal with rocks during most of his work.  For a mining engineer the rocks form the primary source of all the mineral wealth.  Its is also important for a town planner and an architect.

IGNEOUS ROCKS SEDIMENTARY ROCKS METAMORPHIC ROCKS

 Igneous rocks is formed through the cooling and solidification of magma or lava.  They are formed with or without crystallization.  They are formed below the surface as intrusive or plutonic rocks and are formed on the surface as extrusive or volcanic rocks.

GRANITE: An intrusive igneous rock formed. BASALT: An extrusive igneous rock. Light coloured tracks show the direction of lava flow.

 Sedimentary rocks are types of rocks that are formed by the deposition of material at the earth’s surface and within bodies of water.  The sedimentary rock cover of the continents of the Earth’s surface is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.

 Mechanically formed rocks  Chemically formed rocks  Organically formed rocks

MECHANICAL PROCESS CHEMICAL PROCESS ORGANIC PROCESS

 Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism.  The original rock is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (1500 atm), causing profound physical and/or chemical change.  The original rock may be sedimentary, igneous or another older metamorphic rock.  Metamorphic rocks make up a large part of the Earth’s crust.

 Metamorphism means “change in form”.  There are two types of metamorphism by which rocks are formed. They are, Contact Metamorphism Regional Metamorphism

 Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock.  The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it.

 Regional metamorphism, also known as dynamic metamorphism, is the name given to changes in great masses of rock over a wide area.  Rocks can be metamorphosed simply by being at great depths below the Earth's surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above.  The process of metamorphism may have destroyed the original features.

Metamorphosed quartz sandstone Recrystallization under high pressures and temperatures causes the grains to grow larger and impinge on each other.

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