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Longview - Minerals and Rocks

A guide devoted to the study of the earth's composition

What is an Igneous Rock?

 Igneous rock [( ig -nee-uhs)] 
 

Rocks formed by the cooling and solidifying of molten materials. Igneous rocks can 

form beneath the Earth 's surface from magma, or on its surface, from lava.

Types of Igneous Rocks

Identification of Igneous Rocks

Igneous rocks are different than minerals in that they are identified by the mineral composition, their texture (size of crystals within) and their color.  Igneous rocks are divided into two large groups; intrusive and extrusive igneous rocks.  Intrusive igneous rocks are ones that cooled within the Earth's surface.  Extrusive igneous rocks are ones that cooled on the Earth's surface.  The area where they cool causes them to have very different properties.  Because intrusive igneous rocks cooled beneath the Earth's surface, they cooled very slowly, which allowed for the growth of large crystals.  In many cases, you can see the crystals contained within the rock.  Extrusive rocks cool on the Earth's surface, which causes them to cool very quickly. Because of that, crystals do not have time to form, so the texture of extrusive rocks is generally very fine.

This link, Igneous Rocks, provides a great description of the characteristics used to identify igneous rocks.

Igneous Rock Formations

Magma moving beneath the Earth's surface and either cooling below the surface or exiting to the surface creates a variety of igneous features.