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Chapter: 11th 12th std standard Geography earth space Higher secondary school College Notes

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Classification of Rocks : Igneous, Sedimentary, Metamorphic rocks

The nature of the rock is determined by the presence of its minerals. Rocks can be classified into three types based on their formation. 1.Igneous rocks, 2.Sedimentary rocks, 3.Metamorphic rocks.

Mountains, plateaus and plains are all part of lithosphere and they are made up of many types of rocks. Some rocks are white and some others are black. A few rocks are brittle and others are hard; some are like sand, are permeable, others, such as clay, are non-permeable. Rocks are composed of many minerals such as silica, aluminum, iron and magnesium. The nature of the rock is determined by the presence of its minerals. Rocks can be classified into three types based on their formation.

 

Igneous rocks :

 

The earth is about 4,600 million years old. The oldest rocks that have been found were created by volcanic eruptions over hundreds of millions of years. These rocks are still made every time a volcano erupts. These rocks are formed from molten rock that has slowly cooled underground or erupted to the surface of the earth through a volcano. While molten rock is underground, it is called magma. Molten rock that has erupted from a volcano, by contrast, is called lava. These rocks are usually very hard. Igneous rocks have crystals. The size of these crystals may depend on how quickly the molten rock has cooled. If it cooled slowly, the crystals are large; if it cooled quickly, they are small. Today igneous rocks represent 95 per cent of the Earth's crust.

 

Sedimentary rocks :

 

Sedimentary rocks are formed by the accumulation and cementation of mud, silt, or sand derived from weathered igneous rock fragments. Sedimentary rocks represent less than 5 per cent of the Earth's crust but 75 per cent of the Earth's land surface.


Metamorphic rocks :

 

Metamorphic rocks are igneous or sedimentary rocks that have been altered by heat and / or pressure, either because they have been buried and folded deep in the crust, or because they have come into contact with molten igneous rock. Metamorphism can result in the formation of completely new minerals. It can also destroy original structures such as sedimentary layering or fossils. Intense pressure causes the realignment of minerals, forming new layers. About one per cent of rocks in the crust are metamorphic.

 

The above said rocks on the Earth crust are constantly being created, worn down and redeposited in a slow rock cycle. In a Rock Cycle, weathering is the first step for a number geomorphic and biogeochemical processes and is fundamental to many other aspects of the hydrosphere, lithosphere, and biosphere. Let us now examine what is weathering and what are the products of weathering.


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