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Article, pp. 4-10 | Full Text | PDF (2MB)

A More Informative Way to Name Plutonic Rocks

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Allen F. Glazner

Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA

John M. Bartley

Dept. of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA

Drew S. Coleman

Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA


The International Union of Geological Sciences (IUGS) system for rock classification, introduced more than 40 years ago, has served geologists well but suffers from the problem of dividing a continuum of rock compositions into arbitrary bins. As a result, closely related rocks can be given unrelated names (e.g., granodiorite and tonalite), and the names themselves, which were generally derived from the names of places or people, rarely contribute to understanding the processes that generate the diversity of igneous rocks. Here we propose a quantitative modification to the IUGS system that reduces the number of distinct names but more effectively communicates the inherent variability of plutonic rocks. The system recognizes that mapped plutonic rock units are characterized by recognizable textures and mineral assemblages, but that mineral proportions within those units can be highly variable. Adding quantitative data to rock names is an important step toward moving geologic field observations into quantitative digital form and preparing them for advanced data mining and analysis.

One thing quarks do have going for them: all their names are simple—something chemists, biologists, and especially geologists seem incapable of achieving when naming their own stuff. —Neil deGrasse Tyson, Astrophysics for People in a Hurry

Manuscript received 11 Sept. 2018. Revised manuscript received 27 Nov. 2018. Manuscript accepted 30 Nov. 2018. Posted 9 Jan. 2019.

© The Geological Society of America, 2019. CC-BY-NC.