Even metaphysical time is measured by means of geophysical processes. Ages calculated from measurements of processes such as radioactivity and magnetic-field variations have gained such prominence in archaeology that they threaten to eclipse the more fundamental stratigraphic method. Their claims to accuracy, however, have proven unreliable. It is essential that archaeologists understand the weaknesses as well as the strengths of these esoteric chronometric methods. The application of sound, careful stratigraphic methods of observation and recording in the field can help control for the grosser errors of radiometric and magnetic dating methods by calling attention to discrepancies that require special attention and interpretation.

CHRONOMETRY BASED ON RADIOACTIVE DECAY

Elemental atoms may have one or more unstable isotopic forms with different atomic weights, subject to loss of alpha (α) or beta (β) particles by spontaneous emission. A radioactive isotope has a characteristic half-life, the time during which half of all the radioactivity will be spent. The rate at which various materials emit particles, therefore, can be used to estimate the passage of time from a defined beginning point. Counting apparatus counts particle emissions; over a short span of time average emission rates can be recalculated as portions of half-lives. The emission of beta particles by radioactive carbon, and of alpha particles by uranium and its radioactive “daughter” products in decay series, are the basis of several chronometric methods that have redefined the reach and potential of the historical geosciences and archaeology.