Igneous rocks are the group of rocks that constitute over 90% of the Earth's crust. However, a large portion of these rocks go unnoticed because they are covered by sedimentary rocks (Levin). These rocks make up most of the mountains that we see today. Igneous rocks are produced through the cooling of magma, in fact the Latin word ignis means fire (Zoch). There are two main groups that igneous rocks are put into: intrusive and extrusive.

Intrusive rocks, or plutonic rocks, are rocks that are formed by magma cooling within the Earth (Thompson). Intrusive rocks, due to their slower cooling time, have a coarse texture and larger, more visible grains. One example of an intrusive igneous rock is granite. In granite, it is very easy to see the various mineral components with just the naked eye.

A micrograph of a granite sample. This image was taken from: Atlas of Rocks, Minerals, and Textures

The other group of igneous rocks are extrusive, or volcanic rocks. They are called this due to the fact that they form on the surface of the earth, which means they can only form through volcanic activity (Thompson). Because large amounts of magma are thrust upon the surface of the earth, the lava cools much faster. This causes extrusive rock to have a smooth texture and very small mineral particles. In fact, most extrusive rocks' mineral structures must be viewed with the aid of a microscope (Levin). One of the best examples of such a rock is obsidian. This rock is a very dark black and is very smooth, in fact it is what is known as a volcanic glass (Levin).

A micrograph of nepheline basalt, another volcanic rock. This image was taken from: Atlas of Rocks, Minerals, and Textures

In order to decide which group of igneous rocks a particular specimen belongs to, scientists use the texture and grain size of the rock. There are also varying degrees of each igneous rock and each has its own classification. For example, if a rock's mineral, or grain size, is larger than five millimeters, it is known as a coarse grained igenous rock. This would then mean that it is an intrusive rock. If they grain size is up to one millimeter, then it is classified as a fine grained igneous rock, making it an extrusive rock (Thompson).

In conclusion, igneous rocks are a group of rocks formed exclusively by magma that is either in the earth, or on the surface of the earth. They can be very smooth and glass-like, to very porous. All in all, igneous rocks are very easy to classify due to the simplicity of the classification system.

Works Cited

Levin, Harold L. The Earth through Time. Saunders College Publishing: Fort Worth; 1996.

Thompson, Graham R. and Jonathan Turk. Earth Science and the Environment. Saunders College Publishing: Fort Worth; 1992.

Zoch, Lesley. Personal Interview. 30 March 1998.

This page was created on 3/30/98. E-mail to The_Duke@pathfindermail.com

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