To reconstruct the history of the Earth we need to know what happened and when – events and their dates – and we should like to know how it happened and why – processes and their rates. To date a historical event we need a timescale for reference – a calendar – and a means of placing events in this timescale – a clock. Direct access to the primary physical calendar, of time measured in years by means of elemental radiometry as clock, is possible in only a minority of geological problems. By far the richest historical source in the Phanerozoic Eon has been the stratigraphical analysis of sedimentary rocks by means of fossils, the approach pioneered by William Smith. The succession of fossil biotae found in the rocks is used to construct the calendar of relative time, the familiar geological calendar defining the standard chronostratigraphical timescale still in process of refinement today. Rocks are then dated through time correlations with this scale by means of their guide fossils (yon Buch) as clocks. The power to measure the rates of geological processes then depends on the time resolution achievable by means of fossils, the time intervals between distinguishable events, the finesse of the calendar.
The present-day state of play is reviewed, both in the refinement of the geological calendar and the finesse that has been attained. Comparison of the geological calendar with our familiar human historical calendar reveals some illuminating parallels as well as some important differences. Illustrative examples are taken from the Jurassic Period (170 Ma bp) and its ammonites as clocks.
Figures & Tables
The Age of the Earth: From 4004 BC to AD 2002
The age of the Earth has long been a subject of great interest to scientists from many disciplines, particularly geologists, biologists, physicists and astronomers. This volume, The Age of the Earth: from 4004 BC to AD 2002, brings together contributors from these different subjects, along with historians, to produce a comprehensive review of how the Earth’s age has been perceived since ancient times. Touching on the works of eminent scholars from the seventeenth to nineteenth centuries, it describes how concepts of the Earth’s history changed as geology slowly separated itself from religious orthodoxy to emerge as a rigorous and self-contained science. Fossils soon became established as useful markers of relative age, while deductions made from geomorphological processes enabled the discussion of time in terms of years. By the end of the nineteenth century biologists and geologists were fiercely debating the issue with physicists who were unwilling to give them the time needed for evolution or uniformitarianism.
With the discovery of radioactivity, attempts to calculate the Earth’s age entered a new era, although these early pioneers in radiometric dating encountered many difficulties, both technical and intellectual, before the enormity of geological time was fully recognized. This effort affected both the theory and practice of geology. Geochronology was largely responsible for it maturing into a professional scientific discipline, as increasingly refined techniques measured not only the age of the rocks, but the rate of processes which now elucidate many aspects of the Earth’s evolution.
Even today the Earth’s chronology remains a contentious topic — particularly for those dating the oldest rocks — and it is implicated in debates surrounding our hominid ancestors, the origins and development of life, and the age of the universe.
The Age of the Earth: from 4004 bc to AD 2002 will be of particular interest to geologists, geochemists, and historians of science, as well as astronomers, archaeologists, biologists and the general reader with an interest in science.