Igneous+Rocks

= = **Igneous Rock Formation ** By: Hee Ho Han and Mukyo Abe = = = = = = Igneous rocks are one of the three major rock types. They are basically made of magma. Igneous rocks are often called fire rocks. This is because the word "igneous" comes from a Latin word for, "of fire." Igneous rocks are the oldest type of rocks. All types of rocks begin as an igneous rock. Often igneous rocks are formed different ways. Therefore, they are classified in different groups.

** Magma ** Igneous rocks are made of magma, which I already mentioned. Mostly, magma comes from deep below the Earth's surface. Since magma is less dense, therefore it's forced upwards through the surface. Magma becomes lava when it reaches Earth’s surface. The picture beside explains how the igneous rocks form. It is also called the rock cycle.



Intrusive Igneous Rocks ** Intrusive igneous rocks are the rocks which cool down really slowly under the surface. All igneous rocks doesn’t cool the same way. That’s the reason why they don’t look all the same too. Intrusive igneous rocks have large crystals. The granite stone is the example of intrusive igneous rock. As you can see by the picture below, [|Gabbro] is a coarse-grained and dark colored rock. It mainly contains feldspar, augite and sometimes olivine. [|Granite] is a very light colored rock. It is mostly composed of quartz and feldspar minerals. The [|Diorite] is a coarse-grained rock, which contains a mixture of [|feldspar], [|pyroxene], [|hornblende] and sometimes [|quartz].


 * [[image:Gabbro.jpg width="209" height="175" caption="Gabbro"]] || [[image:granite-400.jpg width="221" height="174" caption="Granite"]] || [[image:diorite.jpg width="188" height="175" caption="Diorite"]] ||

**Extrusive** **Igneous Rocks** Extrusive igneous rocks forms when magma erupted from a volcano. Or it reaches Earth’s surface through long cracks. When lava cools slowly and forms small rocks with crystals, this is called the extrusive igneous rock. Basalt and Obsidian rocks are an example of an extrusive igneous rock. The picture below shows different kinds of extrusive igneous rocks. [|Obsidian] is a dark colored volcanic glass. The reason why it doesn't form any crystal minerals is because it cools down so rapidly that it doesn't have any time to form a crystal. The [|Andesite] is a fine-grained extrusive igneous rock. Mainly it's composed of [|plagioclase] with other minerals such as hornblende, pyroxene and [|biotite]. The [|Basalt] is a fine-grained, dark-colored rock which mainly contains plagioclase and pyroxene.


 * < [[image:Obsidian.jpg width="164" height="182" caption="Obsidian"]] ||< [[image:andesite.jpg width="197" height="179" caption="Andesite"]] ||< [[image:basalt.jpg width="203" height="183" caption="Basalt"]] ||

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