To date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques.
Any feature, including colour variations, textures, fossil content, mineralogy, or any unusual combinations of these can be used.
It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced.
In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated.
Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record.
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.
The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils.
Radiocarbon dating is only effective for objects and fossils that are less than 50,000 years old.
However, scientists can look at the decay of other elements in these objects allowing them to date them up to 2.2 billion years.
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.
Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated.
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.