Radiometric dating of rocks and minerals to constrain the age of the Earth, timing of geological events and paleobiological histories has its roots in the works of nuclear physicists of the early Nineteenth Century during the period of discovery of radioactivity and investigations into the nature of the atom. The intervening years since have seen great progress in using the long-lived radioactive elements to constrain the origin and evolution of the Earth and to place the rock and fossil record into a consistent, numerically quantifiable temporal framework.U-Th-Pb and40Ar/39Ar dating methods have emerged as the primary tools for calibrating most of Earth history. It is important for all geoscientists to appreciate the physical basis underlying these methods and to have the ability to evaluate dates by means of currently accepted practices of data presentation. This introduction, along with the accompanying chapters, is intended to help the consumers of radiometric dates to understand better the uses and limitations of radiometric dating methods in an effort to tailor methods and techniques to address specific geochronologic needs, including calibration of the geologic time scale.The ultimate goal of a fully calibrated rock record remains an on-going endeavor. The 2004 ICS geologic time scale is the latest compilation of those efforts. The numerical age calibration is constrained by only 213 radiometric dates, the vast majority of which are U-Pb and40Ar/39Ar dates. Radiometric age control is not evenly distributed through geologic time. There are virtually no radiometric dates in the late Cenozoic where magnetostratigraphy and cyclostratigraphic methods are more precise and applicable. Radiometric dating efforts are concentrated on biostratigraphically important segments of the rock record such as the Permian-Triassic and Cretaceous-Paleocene boundary events, and this is reflected in high-precision calibration of these boundaries. Large segments of geologic time, however, are constrained by either a few radiometric dates per chronostratigraphic unit (most of the Paleozoic) or none at all (Upper Triassic). The current status of radiometric age control on the rock record largely reflects real, underlying scientific issues in biostratigraphy and geochronology, and thus can help point the way to fruitful lines of collaboration between paleontologists, stratigraphers, and geochronologists.
