What are Igneous Rocks – Intrusive and Extrusive

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Igneous rocks are rocks that are created beneath the earth in the earths mantle or above on the earth’s surface. An igneous rock is either an intrusive or extrusive rock and can have one or a multiple variety of minerals within it.

The Forming of Igneous Rocks

Igneous rocks are formed by the heating and cooling of molten rock.  They are sometimes called Magmatic rock.  The cooling of igneous rock will happen under or above the earth’s surface. 

The molten rock will crystalize and solidify giving the two basic groups of igneous rocks called Intrusive and Extrusive.

The difference between intrusive and extrusive igneous rocks is intrusive igneous rocks will form below the earth with the cooling of magma.  Extrusive igneous rocks will form above or close to the surface of the earth with the cooling of lava.

Igneous rocks will consist of crystals due to the process of cooling down.  The slower the cooling process, the longer the crystals have time to form.  For more information on how igneous rocks are formed you may want to read this article “How Igneous Rocks are Formed“

Intrusive Igneous Rocks

Intrusive igneous rocks are rocks that form from magma that is trapped deep below the earth’s surface.  Also called plutonic rock, intrusive igneous rock will slowly cool over many years until it becomes solid rock.

With the slower cooling, the minerals that are within intrusive igneous rock will have more time to get larger in size.  Minerals are generally formed by crystallization through heat and precipitation which can lead to some very large crystals.

The common characteristic of an intrusive igneous rock is that they are a coarser grained rock.  This characteristic makes up a majority of intrusive igneous rocks we would see today.  The coarse grains of minerals in the rock can commonly be identified by visual inspection.

As a great example of intrusive igneous rocks you might consider clicking over to our “All about Diorite” page where you can see a good example of a coarser grained rock.  Diorite, a popular block in minecraft as some may know it, is also an excellent example of an intrusive igneous rock.

Extrusive Igneous Rocks

Extrusive igneous rocks are volcanic rocks that are formed as the magma exits the volcano.  The rapid cooling of the extrusive igneous rock happens on the surface or very near the surface of the earth.   

The lava that erupts from the volcano will cool and solidify quickly in the contrasting temperature at the surface of our planet.  Due to this quick cooling, the minerals in the rock do not have much time to form very large crystals. 

The common characteristic of an extrusive igneous rock is that they are a fine grained rock.  Due to this characteristic, some fine grained extrusive igneous rocks have a very glossy like texture and can be visually identified by a smooth and sometimes glassy surface.

Obsidian is an extrusive igneous rock, not an intrusive igneous rock. Obsidian is a great example of a fine grained extrusive igneous rock. This rock is an example of glassy surface when lava quickly cooled on the earth’s surface.

For more information on obsidian visit this article “All about Obsidian”. This article will give you a visual with additional information on that extrusive igneous rock.

Classifying Igneous Rocks

Igneous rocks are classified by, chemical composition, texture, mineralogy, how they occurred, and geometric distribution.

  • Chemical Composition – chemical composition is the ratio and type of atoms in their molecular structure and how they are arranged in the igneous rock. 
  • Texture – The texture of the igneous rock can come in a few forms.  Types of igneous texture are:
    • Phaneritic – Phaneritic crystals are large enough to see.  Some examples of this texture are granite and diorite. Visit our article on diorite “All about Diorite“, to see an example of this rock. Or view this quick 20 second YouTube of diorite showing off its sparkling crystals.
    • Aphanitic – Aphanitic crystals are too small to see.  Some examples of this texture are andesite and rhyolite. Visit our article on rhyolite “All about Rhyolite“, to see an example of this rock. Or view this quick 20 second YouTube of Rhyolite and it texture, also showing some other minerals.
    • Porphyritic – Porphyritic crystals can be seen, and some crystals cannot be seen.  Magma will be in a slower cooling process, with larger crystal formations, and then be pushed to the surface for quick cooling.  
    • Glassy – Glossy with no crystals.  The rock cools very quickly for this texture.
    • Pyroclastic – Glassy but fragmented.  More violent volcanic eruptions cause the fragmentation.
    • Pegmatitic – Minerals grow extremely large.  The magma cools very slowly to allow larger crystals.
  • Mineralogy – Mineralogy is the study of the chemistry, crystalline structure, and physical properties of minerals.
  • Igneous Rock Occurrence – How the igneous rocks occurred can be attributed to where in the earth, what surroundings and conditions created them.
  • Geometric Distribution – The geometric distribution deals with the probability and distribution of elements within the igneous rock itself.

What Are Some Types of Igneous Rocks?

Some types of igneous rocks will include andesite, basalt, diorite, gabbro, granite, dacite, obsidian, pumice, pegmatite, peridotite, rhyolite, scoria, and tuff.   There are a good amount of different types of igneous rocks you can find, in fact there are hundreds. 

Some igneous rocks may be harder to locate and require you to go to a special location, and others may be common igneous rocks sitting right in your back yard.

You can find some great examples of common igneous rocks in this article “Several Examples of Igneous Rocks” This article will list out several examples, with pictures, and a little information on each igneous rock.

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