How can igneous rocks be identified?
Igneous rocks are formed through the solidification or crystallization of molten magma.
The type of igneous rocks that form from magma is a function of three factors: the chemical composition of the magma; temperature of solidification; and the rate of cooling which influences the crystallization process. Magma can vary chemically in its composition. For example, the amount of silica (SiO 2 ) found in magma can vary from 75% to less than 45%.This website provides background on igneous rocks and their formation.
However, in general, igneous rocks will have:
- inter grown crystals
- minerals which have grown or fused together
There are two types of igneous rocks, intrusive and extrusive.
- underground crystals
- cools slowly
- coarse crystals
- above ground crystals
- cools quickly
- fine crystals
- vesicular (contains holes or air pockets). Example: pumice
- and often glassy or shiny in looks and texture. Example: obsidian