Igneous rocks are classified according to their mineral content: Ultramafic rocks are dominated by olivine and/or pyroxene .

Mafic rocks are dominated by plagioclase and pyroxene (even if you can't see them with the naked eye) and smaller amounts of olivine.

Intermediate rocks are roughly even mixtures of felsic minerals (mainly plagioclase ) and mafic minerals (mainly hornblende , pyroxene, and/or biotite). There is little or no quartz.

Felsic rocks are mostly feldspar (especially K-feldspar), at least 10% quartz, and less than 15% mafic minerals (biotite, hornblende). Within these four groups, rocks are named depending on whether they display coarse-grained or fine-grained textures. The coarse-grained textures indicate intrusive rocks whereas the fine-grained textures generally indicate extrusive rocks. Extrusive rocks defined mainly by texture are discussed under rock textures: obsidian, scoria, pumice, tuff, and volcanic breccia.



