1.5 Petrologic Compromises

Over time, many rock names were introduced by geologists, sometimes for the same rock type or with defintions that overlapped with other rock types. This led to uncertainty and miscommunication, rather than the efficiency of a well-known name. Rock name politics came into play, as preferences for a particular name might be based on a place or a naming petrologist. A compromise solution to the petrological conflicts was developed by the Subcommission on the Systematics of Igneous Rocks of the International Union of Geological Sciences (IUGS). The recommendations of this group were summarized in a paper (Le Bas and Streckeisin, 1991) and published as a book (Le Maitre and others, 2005).

The IUGS recommendations are embodied in a series of diagrams based on the mineralogic content (modes) of coarse-grained (plutonic) igneous rock and on modes or whole rock chemistry of fine-grained (volcanic) rocks. Most of the IUGS naming diagrams are available as interactive diagrams in the Rock Library. To use the IUGS diagrams, you need to be able to read ternary diagrams. If you are uncertain about using ternary diagrams, click here:

The sequence of questions on the next page (or use the button) will guide you to the correct IUGS diagram to use to name your rock using the mode of the rock.    

If you do not have modal data for your rock but you do have major element geochemical data, you can use a Total Alkalis vs. Silica scatter plot (TAS Diagram) to give the rock a name based on its chemistry. Choose either the or the .

To see examples of igneous rocks in the Rock Library, choose one of these lists:

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