ANALYTICAL METHODS/Geochronological Techniques 77 Geochronological Techniques E A Eide, Geological Survey of Norway, Trondheim, Norway ß 2005, Elsevier Ltd All Rights Reserved Introduction Geochronology is the study of time as it relates to Earth history As a distinct discipline within the natural sciences, geochronology emerged fully during the late nineteenth and early twentieth centuries with the discovery of radioactivity and the advent of radiometric dating methods Importantly, the appearance of modern geochronology was the result of a strong interest in Earth history and the development of relative methods to estimate the age of Earth, both of which had been aspects of natural science research since at least the seventeenth century The human fascination with studying time and marking its passage can be traced to ancient cultures, exemplified through the precise astronomical calendars produced by numerous early civilizations (Figure 1) These calendars were based on calculations of the movements of celestial bodies relative to Earth and helped to raise speculations about the position and motion of Earth within this celestial system These speculations led to efforts to understand Earth’s origin and calculate its age, which today is generally agreed to be 4.5–4.6 billion years (By), and is the starting point for the geological time-scale (GTS) (Figure 2) The GTS is an iterative solution between ‘absolute’ and ‘relative’ ages determined by absolute and relative geochronological techniques The formal distinction between absolute and relative ages has its roots in ancient calendars for which the passage of time was calculated from astronomical events linked to the solar year Broadly, an absolute age is one that is based on processes affected only by the passage of time and which may thus be valid worldwide In a strict sense, an absolute age should have direct correspondence to the absolute time-scale, determined on the basis of the solar year (Table 1) Relative ages are applicable to a restricted geographic area and usually pertain to a limited geological time period Relative ages place the formation of different rock units or their physical features (faults, unconformities, etc.) in a relative chronological order Though knowledge of the exact formation ages of different rock units is useful, numerical (absolute) ages are not prerequisite for establishing their relative chronology Nonetheless, relative ages must eventually be calibrated against independently established (absolute) time-scales if they are to be extrapolated globally The framework for the GTS is based on relative ages, represented by the established, sequential subdivisions of geological time (Figure 2) The nomenclature of this framework was developed largely through the studies of natural scientists in the eighteenth and nineteenth centuries (see Famous Geologists: Sedgwick; Murchison; Darwin; Smith; Cuvier; Hutton) During the twentieth century, absolute age determinations for rocks around the globe allowed refinement of the GTS and adjustments were made to the initially imprecise or disputed boundaries between the geological systems The absolute ages were derived using radiogenic isotope geochronological techniques Calculating an age for a rock or mineral using these techniques combines precise measurement of naturally occurring, radioactive isotopes and their stable decay products with the physical principle that the radioactive decay of the isotopes occurred at a constant, known rate Because radiogenic ages are ‘absolute’ in the sense that the decay of a radioactive isotope primarily depends only on the passage of time, radiogenic ages for rocks found in one area of the world should, in principle, be directly comparable ‘in time’ to other rocks dated with similar methods in other areas of the world Regardless of the geochronological technique used, the combination of relative and absolute ages has yielded the opportunity not only to generate geological time-scales, but also to determine the ages of rocks and geological structures, the timing of geological ‘events’, and, importantly, the rates at which geological processes occur Today, the primary techniques for relative dating of geological materials include biostratigraphy, palaeomagnetism and magnetostratigraphy, and chemostratigraphy (see Palaeomagnetism, Magnetostratigraphy, Analytical Methods: Fission Track Analysis) Of the absolute dating methods, radiogenic isotope geochronology, astronomical time calibrations, and dendrochronology (see Dendrochronology) are the most widely used However, it is the rock type that usually dictates the geochronological technique appropriate for obtaining the rock’s age Thus, basic knowledge of the relative and absolute geochronological techniques is useful not only to select the appropriate method to date the rock, but also to interpret the age(s) produced, and to give a higher degree of confidence to comparisons made between geological ages and the processes to which they are linked