Potassium-Argon/Argon-Argon Dating Methods

  • Robert C. Walter
Part of the Advances in Archaeological and Museum Science book series (AAMS, volume 2)


During the latter half of this century anthropological surveys in East Africa have made significant contributions to understanding how the human species has evolved. In the past two decades, particularly, discoveries of our fossil ancestors have been made in unprecedented numbers and diversity. Detailed studies of these fossils provide new insights into human evolution, such as the origin of locomotion and cultural activity, and the evolution of the brain, among many other complex features that have come to define humanity. Even during the time this manuscript was written, new hominid discoveries in Ethiopia and Kenya were announced that trace our earliest ancestors further back into the Pliocene. The ages assigned to these fossils have been obtained through radiometric dating of volcanic rocks interbedded with the fossiliferous sediments. Such numerical calibrations are crucial to understanding rates and timing of evolutionary change.

K-Ar dating has played a key role in unraveling the temporal patterns of hominid evolution as far back as the first significant discovery of East African australopithecines at Olduvai Gorge in 1959. It was in large part due to the desire to understand the age of the Olduvai hominid remains that pioneering attempts were made to date geologically young materials using the K-Ar method. Yet even this seminal K-Ar dating study was plagued by the seemingly insurmountable problem of contamination. The principal materials for dating East Africa hominid sites are volcanic ashes, yet many of these ashes are not deposited as primary air fall tephras (Greek for ash). Rather, most are reworked by stream action and are redeposited into the sedimentary environment. In the process of reworking, these ashes can pick up pre-existing detrital grains that, by definition, are older than the juvenile ash. If during K-Ar analyses these detrital grains are not recognized and eliminated then they can cause the measured ages to be systematically too old.

Recent advances in K-Ar geochronology, specifically the 40Ar/39Ar variant of the K-Ar method, have helped to establish a precise and accurate temporal framework for hominid evolution in East Africa. Single-crystal laser-fusion 40Ar/39Ar dating has been a major factor in this success. This grain-discrete method now permits precise and accurate ages to be measured on single grains and, thus, contaminating grains can be eliminated. The laser-fusion 40Ar/39Ar technique has had a profound impact on geochronology by enabling reliable ages to be obtained where none were possible before.


Fission Track Planetary Science Letter Pumice Clast Hominid Evolution Auckland Volcanic Field 
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Copyright information

© Springer Science+Business Media New York 1997

Authors and Affiliations

  • Robert C. Walter
    • 1
  1. 1.Institute of Human OriginsBerkeleyUSA

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