1.6 Igneous Rock Naming Guide

The International Union of Geological Sciences (IUGS) developed a series of ternary diagrams to use in the naming of igneous rocks based on the volume proportions (mode) of minerals present. Two ternary diagrams are used together to make each of the QAPF diamond diagrams. Click on any of the diagrams to see a larger version.










The following sequence of questions will help you identify which of the IUGS ternary diagrams to use name a particular igneous rock. Your answers will lead to one of the diagrams. (This guide will not lead to the IUGS name if you have a rock that belongs to one of the special categories, which are not common.) Answer the questions by selecting the appropriate checkbox.

Is the igneous rock a coarse-grained (plutonic/intrusive) rock, or a fine-grained (volcanic/extrusive) rock?

coarse-grained rock.
fine-grained rock.

According to the IUGS (Le Maitre p.23), a plutonic rock is "...an igneous rock with a phanerititc texture, i.e. a relatively coarse-grained (>3 mm) rock in which the individual crystals can be distinguished with the naked eye...." The IUGS states that a volcanic rock is "...an igneous rock with an aphanitic texture, i.e. a relatively fine-grained (less than mm rock) in which most of the individual crystals cannot be distinguished by the naked eye...." Phaneritic rocks are likely to be slowly cooled (intrusive) and aphanitic rocks are likely to be quickly cooled (extrusive), but the name is based on the grain size (which is known -- you can see it) and not the origin (which may be speculative).