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.
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 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.
Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents.
These units, called in their molten form, constitute major crustal additions.
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.
It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced.
In addition, because sediment deposition is not continuous and much rock material has been removed by from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.
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.
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.