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A More Informative Way to Name Plutonic Rocks






          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; and
          Drew S. Coleman, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA


          ABSTRACT                           a formidable entry barrier to students of   granodiorites (Fig. 1). Thus, any classifi-
            The International Union of Geological   the field. In a recent undergraduate text-  cation based on discrete categories will
          Sciences (IUGS) system for rock classifi-  book, Winter (2010, p. 32) lists 157 com-  split continuously variable rock composi-
          cation, introduced more than 40 years   mon igneous rock names, many of them   tions at arbitrary boundaries.
          ago, has served geologists well but suffers   unknown to practicing petrologists. Say   An international effort to systematize
          from the problem of dividing a continuum   “kugdite” to a geologist and you will   the nomenclature of plutonic igneous
          of rock compositions into arbitrary bins.   likely get a puzzled stare.  rocks was started in the 1960s under the
          As a result, closely related rocks can be   Classification of igneous rocks has   leadership of Swiss petrologist Albert
          given unrelated names (e.g., granodiorite   occupied and irritated petrologists for   Streckeisen, and summaries of this work
          and tonalite), and the names themselves,   centuries. Unlike biological classifica-  (e.g., Streckeisen, 1974, 1976; LeBas and
          which were generally derived from the   tions, which can place organisms into   Streckeisen, 1991) are the standard refer-
          names of places or people, rarely contrib-  discrete categories, rock classifications   ences for current nomenclature. The prin-
          ute to understanding the processes that   place sharp boundaries between objects   cipal classification is based on a double
          generate the diversity of igneous rocks.   that are completely gradational. A biolo-  triangle (Fig. 2); this diagram, appropriate
          Here we propose a quantitative modifica-  gist can classify something definitively    for rocks with 10% or more quartz or
          tion to the IUGS system that reduces the   as a dog or cat, knowing that there are no   feldspathoid minerals plus feldspars, uses
          number of distinct names but more effec-  doggish cats or cattish dogs, but a petrolo-  the modal (volume) proportions of quartz
          tively communicates the inherent vari-  gist cannot do so—there are plenty of   (Q), alkali feldspar (A), plagioclase (P),
          ability of plutonic rocks. The system rec-  granodioritic granites and granitic   and feldspathoids (F) to name rocks.
          ognizes that mapped plutonic rock units
          are characterized by recognizable tex-
          tures and mineral assemblages, but that   Cathedral Peak
          mineral proportions within those units   Granodiorite
          can be highly variable. Adding quantita-
          tive data to rock names is an important
          step toward moving geologic field obser-
          vations into quantitative digital form and
          preparing them for advanced data mining
          and analysis.
          One thing quarks do have going for them: all                                        Figure 1. Outcrop photo-
          their names are simple—something chemists,                                          graphs of the Cathedral
          biologists, and especially geologists seem                                          Peak Granodiorite and El
          incapable of achieving when naming their own                                        Capitan  Granite, Yosemite
                                                                                              National Park, California,
          stuff. —Neil deGrasse Tyson, Astrophysics for                                       USA. In spite of largely
          People in a Hurry                                                                   equivalent mineral propor-
                                              El Capitan                                      tions, one is termed a gran-
          INTRODUCTION                        Granite                                         ite and the other a granodi-
            Why do we bother to name rocks? One                                               orite. This confusion is
                                                                                              lessened if name boundar-
          answer among many is that rock catego-                                              ies are considered fuzzy
          ries can efficiently convey important                                               rather than sharp. Pennies
                                                                                              are 2 cm in diameter.
          information about a geologic setting just                  G384A
          as biological categories can convey the
          same for ecosystems. Say “zebra” to a
          biologist and they will likely think
          “African savanna”; say “granodiorite”
          to a geologist and they will likely think
          “subduction zone.” However, the sheer
          number of igneous rock names presents

          GSA Today, v. 29, https://doi.org/10.1130/GSATG384A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.

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