Igneous Rocks 101

What is an Igneous Rock?

An Igneous rock is the product of cooled solidified magma/lava. As magma/lava cools it goes through the process of crystallization which turns it into an igneous rock. The amount of time it takes for an igneous rock to form depends on its location and its composition. There are two types of igneous rocks; Extrusive and Intrusive.

What is the difference between an Extrusive and Intrusive igneous rock?

The difference between an Extrusive and Intrusive igneous rock is the way in which they cool. An Extrusive igneous rock cools very fast on the surface and is created by lava. Since the cooling process is very fast extrusive igneous rocks have very small crystals (fine grained). On the other hand an Intrusive igneous rock cools very slowly beneath the surface and is created by magma. Since the cooling process is very slow intrusive igneous rocks have very large crystals (coarse grained). In some instances there is also a third type of igneous rock. Technically it is an extrusive rock, but it resembles glass. This glass type of igneous rock forms when magma/lava is instantly cooled.

What are the properties used to identify Igneous rocks?

Properties used to identify igneous rocks include mineral composition, texture, and color. There are four different mineral compositions; Felsic, Intermediate, Mafic, and Ultramafic. The texture of an igneous rock is determined by its grain size. There are eight terms relating to the type of texture an igneous rock has. They are pegmatitic, phaneritic, porphyritic, aphanitic, glassy, vesicular, frothy, and pyroclastic.


Why does the type of mineral composition matter when identifying an igneous rock?

By looking at what type of mineral composition an igneous rock has you can determine what type of magma made it and what environment it was made in.


What is the difference in composition between Felsic, Intermediate, Mafic, and Ultramafic igneous rocks?

Igneous rocks contain the following common minerals: Plagioclase feldspar, Olivine, Potassium feldspar, Pyroxene, Quartz, Amphibole, Biotite, and Muscovite. The percentage of these minerals present in an igneous rock determines whether it is felsic, intermediate, mafic, or ultramafic. Felsic rocks are those that are light in color and are mostly made up of feldspars and silicates. Mafic rocks are darker colored and are mostly made up of magnesium and iron. Intermediate rocks fall somewhere in between being Felsic and Mafic. Ultramafic rocks are very dark colored. For more detailed descriptions of each type click on its name to be redirected to the glossary.


What determines what type of texture an igneous rock has?

Texture is determined by the grain size of igneous rock which is determined by the rate of cooling. For example a smooth igneous rock was cooled very fast because it is has small crystals and is therefore an extrusive rock. The texture of this rock would be called fine grained or aphanitic. In the reverse situation an igneous rock that cooled very slowly and formed large grains would be described as having a coarse grained or phaneritic texture. To see the meaning of each type of texture click on its name to be redirected to the glossary (Pegmatitic, Phaneritic, Porphyritic, Aphanitic, Glassy, Vesicular, Frothy, Pyroclastic).


Where are igneous rocks found?

The most common igneous rock Basalt (mafic) makes up most of the oceanic plates and is founds at divergent plate boundaries. In fact most igneous rock activity occurs at divergent plate boundaries. However, intermediate to felsic igneous rocks are most commonly found along continental margins.


Igneous Rock Flow Chart (requires at least Acrobat 4 to open)

Igneous Rock Flow Chart

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