Prior to the discovery of radioactivity, time and rock correlation were determined by the fossil record, the assumption being that rock units with similar fauna were the same age. Unique events, such as volcanic eruptions, tsunamis, or floods, also served as markers for correlation. The use of annual growth tree rings, or dendrochronology, to determine the age of events proved useful for dating the recent past, as was the interpreting and understanding of ancient history calendars (see Dendrochronology). However, it was not until the end of the nineteenth century with the discovery of X-rays by Wilhelm Rcintgen, the promotion of the use of radioactivity by Marie and Pierre Curie, and development of radiocarbon dating by Willard Libby that a procedure for using absolute dates was developed and used (see Dating Methods, Overview).
Numerical (absolute) dates are obtained by analyzing the radioactivity decay of minerals contained in the rocks and are absolute only as far as conditions are optimum and the analysis is precise. The method entails determining the ratio between the ‘parent’ and the ‘daughter’ of the decaying mineral, for example, uranium and thorium to lead, rubidium to strontium, or potassium to argon. These methods are for determining long time durations because of their slow decay constants. By the twenty-first century, the age of the Earth had been pushed back farther and farther to the age accepted today.
World History
