Identification of
Igneous Rock

Igneous rocks are those rocks that have cooled from magma either deep within the Earth or near or on Earth’s surface. The depth at which a magma cools will have a number of effects on the resulting rock. If magma cools deep within the Earth, or intrusively, the surrounding material will insulate the magma, much like a comforter insulates a person on a bed. This will cause the magma to cool at a much slower rate and allow individual mineral crystals to grow larger and larger. If, on the other hand, a magma cools on or nearer to the surface, or extrusively, there will be less insulating material and therefore the magma will cool more rapidly. This will result in smaller mineral crystals.

To begin Identifying an igneous rock first determine its color index and select the appropriate menu option on the left

Color Index

The color of igneous rocks is a very useful indicator of the types of minerals present in the rock and therefore the specific type of rock. The color of an igneous rock is measured by a color index. The color index is a measure of the ratio of dark colored, or mafic, minerals to light colored, or felsic, minerals.

Bowen Reaction Series

According to Bowen’s Reaction series, minerals crystallize at different temperatures depending on their chemical composition. At high temperatures only minerals that have structures stable under those conditions will be able to crystallize. Typically those minerals are olivine, pyroxene, and plagioclase feldspar. These minerals, as seen in the earlier mineral identification lab, are typically dark in color (ranging from a dark green to a dark gray). Therefore igneous rock that forms at high temperature would be composed of these minerals and hence be dark or mafic in color.

On the other hand, rocks that have formed from magma at relatively low temperatures would be composed of the lighter colored quartzes, potassium feldspar, and muscovite mica. These rocks are termed felsic because of their typically high content of both feldspar and silica.

To begin identifying your igneous rock determine its color index based on the scale below and click on the appropriate link in the menu on the left

Felsic
Intermediate
Maffic


©2006 Sean Tvelia