Using this established record, geologists have been able to piece together events over the past 635 million years, or about one-eighth of Earth history, during which time useful fossils have been abundant.The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute isotopes has been improved to the point that for rocks 3 billion years old geologically meaningful errors of less than ±1 million years can be obtained.Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence.Although with clever detective work many complex time sequences or relative ages can be deduced, the ability to show that objects at two separated sites were formed at the same time requires additional information.A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself.Just as the use of the fossil record has allowed a precise definition of geologic processes in approximately the past 600 million years, absolute ages allow correlations back to Earth’s oldest known rocks formed more than 4 billion years ago.In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated.
Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface.
Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated.
Some method of correlating rock units must be found.
The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils.
To achieve this precision, geochronologists have had to develop the ability to isolate certain high-quality minerals that can be shown to have remained closed to migration of the radioactive parent atoms they contain and the daughter atoms formed by radioactive decay over billions of years of geologic time.
In the ideal case, the geologist will discover a single rock unit with a unique collection of easily observed attributes called a marker horizon that can be found at widely spaced localities.