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The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere.
Some method of correlating rock units must be found.
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.
These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.
Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present.
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.