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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

4 GSA Today | February 2019

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