Page 58 - gt1505

Geology—Past & Future

REVISITED

Editor’s note: The following is the ninth installment of our encore presentation of articles that highlighted the 10th anniversary
of the first issue of Geology, as published in Geology in Dec. 1983 [v. 11, no. 12, p. 679–691, doi: 10.1130/0091-7613(1983)11

<679:GAF>2.0.CO;2]. Each section was written by a different author (author affiliation notations are as originally published
in 1983). See the August 2013 GSA Today (v. 23, no. 8, p. 18–19) for the first installment and table of contents. In this issue:
article 18: “Igneous rocks,” by Alexander R. McBirney; and article 19: “Tectonics,” by E.M. Moores.

GSA TODAY | MAY 2015 Igneous rocks plume, “gravitational anchor,” or “emulp” (plume spelled back-
wards) has plummeted from sight. Few question the time-distance
Alexander R. McBirney, Department of Geology, University of relations in island chains, but does anyone have a convincing
Oregon, Eugene, Oregon 97403 explanation of what is going on?

Ten years ago, two topics dominated the published work on Ten years ago, it seemed patently obvious that andesites are
igneous rocks: lunar magmatism and plate tectonics. Whether primary magmas generated in Benioff zones by melting of
these were the most important issues or not, they certainly subducted sediments. Today, most would agree that the dominant
accounted for the largest number of papers and the greatest magma of arcs is basaltic; evidence that subducted sediments
expenditures of time and money. Whatever one’s stand on contribute anything to calc-alkaline magmas is at best ambig-
magma oceans or the europium content of KREEP, all would uous. Indeed, it would be hard to find a consensus that sediments
agree that the impact of the lunar program on petrology was are even subducted.
profound. It set new standards of research and led to vastly
improved analytical and experimental techniques. In 1973 plate If our mission in 1973 was to relate igneous rocks to plate
tectonics was the “unifying theory” to which we sought to relate tectonics, trace-element geochemistry was the means by which we
all magmatic processes. Given the fervor of the times, disillusion aimed to do this. Geochemists in search of the magic component
was inevitable. All exciting new ideas, once they are fashionable, turned first to more and more obscure elements and then to
are doomed to become old-fashioned. So it is still too early to isotopes of those elements, and in time they came upon
expect an objective appraisal. Nevertheless, it is worth consid- samarium-neodymium, which is said to be the most important
ering what has happened to our views of igneous activity in each new tool of the decade.
of the major tectonic settings—spreading axes, subduction
zones, and intraplate “hot spots.” The potentialities of the system are indeed impressive. I wonder,
though, whether these tools of geochemistry may not have
Thanks to refined geological and geophysical surveys of the outstripped our understanding of the physical processes to which
deep oceanic ridges, detailed geochemical and petrologic studies we apply them. How many times in the past decade have we seen
of ophiolites, and core samples of the deep ocean floor, we have an highly sophisticated and costly analytical work interpreted in
elegantly documented model for magma chambers under ocean terms of absurdly inadequate physical models of magmatic
ridges. But geophysicists tell us they find no seismic evidence that evolution?
these magma chambers exist. Try as they will, they simply cannot
find them! The increasing awareness of this deficiency one finds in current
literature makes the task of forecasting the direction of research
The mantle plumes that in 1973 were proliferating at such an for the coming decade an easy one. When the next review of this
astonishing rate have since been decimated. The descending kind is written, igneous geologists will be as concerned with heat
and mass transfer as they are today with REE plots. As a result, I
predict that our present confusion about igneous rocks will rise to
undreamed-of levels of sophistication.

Send brief comments to gsatoday@geosociety.org. Should this article spark a longer comment, please consider writing a GSA Today Groundwork or science article;
learn more at www.geosociety.org/gsatoday/.

58