July 20, 2011
CheckPoint: Igneous Rocks
The answer to how igneous rocks are formed is given in their name; Igneous is Latin for “fire”. This basically means igneous rocks are cooled fire rocks from magma or lava, depending on where they cooled and formed. There are two categories of igneous rocks that also depend on where each was formed. Intrusive igneous rocks are igneous rocks that form when magma cools slowly and becomes solid underground. Extrusive igneous rocks are igneous rocks that quickly cool becoming solid at the crust surface due to partial melting of rocks from the mantle and crust. Extrusive igneous rocks cool and become solid much quicker than intrusive igneous rocks.
Each category of igneous rocks has their own distinguishing characteristics to help a person distinguish one category from the other. Intrusive igneous rocks can be distinguished by crystals that interlock during the cooling off period. The slow cooling off process allows the larger crystals to form. Extrusive igneous rocks have a fine grainy texture from the quicker cooling process that doesn’t allow time for crystals to grow. Intrusive igneous rocks with visible crystals all about the same size are referred to as a phanerictic texture while extrusive igneous rocks with their fine grained texture are referred to as aphanites rocks with the most common of them being basalt.
Coffey, J..(December 6,2010). How Are Igneous Rocks Formed. Universe Today. Retrieved July 19, 2011 from: http://www.universetoday.com/82009/how-are-igneous-rocks-formed/