How can igneous rocks be identified?

1 Answer
Jan 3, 2015

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