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Igneous Rocks

Igneous rocks are rocks that have formed due to the cooling followed by solidification of molten matter. Molten matter or rocks that have come up from Earth’s interior mostly from upper mantle in form of magma. Magma after reaching the Earth’s surface solidifies after it comes in contact with atmosphere and water. Magma after coming to the surface is termed as lava. It cools down and become solid by forming crystals. 

The molten magma from the Earth’s upper mantle solidifies after it comes in contact with air and water and slowly crystallizes. The igneous rock which forms beneath Earth surface after the magma cooling is classified as intrusive rock. The lava which cools on top of Earth surface is classified as extrusive rock.

The igneous forms of rocks are identified by their texture, colour, density and the mineral composition. The texture basically depends upon the size, shape and the manner in which the crystals are arranged in the rock. The texture of igneous rock depends upon the time period it takes to cool down and solidify. 

Igneous Rocks

The rock forms that cools down slowly usually forms very large crystals. The igneous rocks are usually formed when the intrusive igneous form that slowly cools deep inside the Earth’s crust. The best example would be granite stone which is nothing but intrusive igneous rock which forms due to slow cooling of magma beneath Earth crust. The crystals that form within granite could be observed clearly in the rock form.

The rocks that cools down quickly usually does not have any crystals at all and even though there are crystals present would be of very small size. Basalt rock is one such rock form which cools down quickly and has small size crystals present. 

Some of the extrusive rock forms that are formed beneath ocean cools very rapidly and develop a glassy texture all across. Obsidian is a volcanic glass that is formed when lava cools down rapidly. Yet another type of extrusive rocks which cools down so rapidly that gas molecules are trapped within and provides a porous texture to the rock form. These are essentially called as vesicular rock forms. The best example for such rock forms are pumice stone which has finely grained texture and resembles a sponge.



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The definition of igneous rock is as follows: These are rocks that have crystallized from molten rock or magma beneath Earth crust. The definition also clearly clarifies what kind of crystallization we are looking into. Whether the crystallization is rapid or slow or has it occurred under ocean or in air. Has it crystallized beneath the crust or above? All these factors lead to the observed characteristic that usually follows the igneous definition.

How Does It form?

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Igneous rocks form due to either rapid solidification or gradual crystallization of the molten matter. These crystallization or solidification can occur either beneath the Earth crust or above the crust. The time duration that usually gives us the various types of igneous form depends upon where exactly these processes are taking place. 

If the molten matter rolls down the ocean floor or is coming out from crevices beneath ocean floor then it solidifies rapidly. 

In case the matter is coming out of crust in open air then process is relatively slow. It takes even longer period to cool down if it solidifies under the Earth crust. 

How Are They Classified?

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Igneous rocks are classified on the basis of crystallization, amount of mineral deposit, and the place of crystallization. The occurrence of mineral deposit gives them the texture, colour and overall appearance whereas the layering of minerals, amount of gas trapped and concentration of these minerals provides the further classification. Commercial viability also depends upon these features and more the variation of vesiculation and mineral deposit makes it more appealing and hence escalation of price.


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1) The igneous form of rocks does not have any fossil deposit and even if there are chances of fossil deep inside the crust, by the time it erupts out of Earth its completely destroyed due to sheer heat these rocks produce.

2) Most of the igneous forms have one or two mineral deposit in them. 

3) These are either glassy or coarse in appearance. 

4) Usually do not react with acids.

5) The mineral deposits are in patches and of different size.


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Based on the type of cooling the igneous form of rock has been classified as either extrusive or intrusive. Intrusive forms of igneous rocks are formed when the magma solidifies below the Earth crust. Whereas the extrusive form of igneous rocks are formed when the magma comes out from Earth crevice on surface in lava form and cools down. 

Based on the manner, place and time taken for cooling process the igneous rocks are classified further into different rock types. For intrusive igneous form the Diorite stone, Gabbro stone, Granite stones and Pegmatite stones are some of the stone types. 

For extrusive forms of igneous rocks the Andesite stone, Basalt rocks, Obsidian rocks, Pumice stones, Rhyolite stones and Scoria rocks are some of the examples. 


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The examples of igneous rocks are mainly based on composition, time of cooling and density of crystals in rocks. The common stones of intrusive form of igneous are granite; gabbro and diorite while for extrusive form of igneous rocks are basalt, pumice and rhyolite.

Intrusive Igneous Examples

Granite: These are coarse-grained stones formed beneath the Earth crust. These are usually light in colour with deep streaks of mineral deposit. This form of intrusive igneous rock contains quartz in parts and feldspar minerals in rock layers.

Diorite: These igneous rocks are coarse-grained formed beneath the Earth crust. These intrusive igneous rock forms contain a mix of feldspar along with streaks of pyroxene, layers of hornblende and in some layers even quartz is also observed.

Gabbro: These are igneous rocks formed beneath the crust and are coarse-grained dark in colour. These intrusive igneous rocks contain layers of feldspar along with augite and some layers even olivine are also available.

Extrusive Igneous Examples

Andesite: These forms of extrusive rocks solidify above crust and are fine-grained. The extrusive igneous rock contains mainly plagioclase along with different other minerals such as hornblende, pyroxene and even biotite in some regions.

Basalt: These forms of extrusive rock forms above the crust and are fine-grained dark in colour. These extrusive igneous rock forms have components of plagioclase and pyroxene.

Pumice: These forms of extrusive igneous are light in colour. Have vesicular regions all across. These igneous rocks form by rapid solidification of the molten lava. The texture of vesicular form is mainly due to gas particles which are trapped in the molten matter during solidification process.

Obsidian: These are dark in colour volcanic glass usually forms by very rapid cooling of molten magma. As it starts cooling rapidly there is no formation of crystals.
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