How Igneous Rocks are Formed

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Igneous rocks are formed by the cooling of magma or lava either on the surface of the earth or below it.  Each igneous rock formed will have similar or different elements contained within it.   During cooling crystallization will occur as the rock becomes solid forming the many different igneous rocks on earth.

Igneous rocks will crystalize at varying temperatures.  As time goes on under crystallization, minerals will form in many different shapes and sizes depending on environment and chemical composition.  Igneous rocks may contain only one mineral throughout it or it can have many.

How Igneous Rocks are Classified

Example of the igneous rock obsidian, an extrusive rock.

Igneous rocks are classified by their mineral composition when formed.  That means there are varying amounts of mineral content contained within the rock that put them in a specific group.  Classification of igneous rocks are also by their texture or grain size.

Igneous rocks formed within the earth will tend to be more course grained.  The crystals that are created will be visible to the eye.  Igneous rocks formed on the surface of the earth will be fine grained and you cannot see the crystallization.

Examples of course grained igneous rocks will include peridotite, gabbro, diorite, and granite. Examples of fine grained igneous rocks will include basalt, andesite, and rhyolite.

A great example of a glass igneous rock is obsidian, which cools quickly on the surface of the earth after erupting from a volcano.  The quick cooling gives this rock a glassy look.

Four Classifications of Igneous Rocks 

The four classifications of igneous rocks are ultramafic, mafic, intermediate, and felsic. 

  • Ultramafic rocks are rocks that are predominantly consist of olivine and/or pyroxene.  These rocks will have a very low silica content below 45%.  These rocks will commonly have a greater than 90% mafic mineral content.
  • Mafic rocks consist mainly of plagioclase and pyroxene.  They can also have amphibole, biotite, lesser amounts of olivine.  These rocks are high in magnesium and iron.  These rocks are more on the darker side due to these types of minerals.
  • Intermediate rocks are fairly even mixtures of felsic type minerals such as plagioclase and also mafic type minerals such as mainly hornblende, pyroxene, and biotite.  Intermediate rocks have very little or no quartz in it.
  • Felsic rocks are for the most part feldspar with at least 10% quartz available, and less that 15% of mafic type minerals.  For an igneous rock to be a felsic rock it needs to have 75% felsic type minerals.
Classification of igneous rocks to their grain size, category, and formation temperature with silica content.

Two Categories of Igneous Rocks

Intrusive and extrusive rocks are the two categories of formed igneous rocks.  The intrusive category means igneous rocks formed within the earth.  The extrusive category means igneous rocks formed close to or above the surface of the earth.

Igneous Intrusive Rocks

The common characteristic of intrusive igneous rocks is they are cooled slowly below the earth’s surface.  Magma will push its way up out of the melting heat of liquid magma into the mantle where it can cool.  As it cools, intrusive igneous rocks will cool slowly. 

Slow cooling allows for more course grained igneous rock and larger mineral crystals to form. Intrusive rocks are also often called plutonic rocks. Plutonic means the rock was formed by solidification at considerable depth beneath the earth’s surface.

Igneous Extrusive Rocks

The common characteristic for extrusive igneous rocks is they are cooled near or on the surface of the earth.  As lava violently erupts from a volcano it will cool very quickly.  Quick cooling forms a very fine grained or even a glassy appearance, which is another common characteristic of extrusive igneous rocks. 

The quick cooling does not allow for crystal growth visible by the eye, but it is there. If you are interested in more information on igneous rocks you may find this article helpful. “What are Igneous Rocks”

There are many different types of igneous rocks on our earth.  Some people may find it interesting to see some examples of igneous rocks available on our earth.  You may find this article interesting as it lists out some common examples of igneous rocks. “Several Examples of Igneous Rocks”

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