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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.
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.
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.
This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the rocks or soil.
For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built.
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Try out a dating site for alternatives to surround yourself with out-of-the-box people who totally get where you’re coming from.Our editors will review what you’ve submitted and determine whether to revise the article.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work!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.Any feature, including colour variations, textures, fossil content, mineralogy, or any unusual combinations of these can be used.
In addition, they have had to develop special techniques with which to dissolve these highly refractory minerals without contaminating the small amount (about one-billionth of a gram) of contained lead and uranium on which the age must be calculated.