• 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.