What is the role of isotopes in radiometric dating
Geologists analyze geologic time in two different ways: in terms of relative geologic age, and in terms of absolute (or numeric) geologic age.
The combination of these two types of geologic ages makes a complete record of earth's geologic history in terms of the order of events and in terms of how many years ago each event occurred.
Relative geologic age is established, based on such evidence as the order in which layers of sediment are stacked, with the younger layer originally on top.
For example, deep in the Grand Canyon in Arizona, there are places where a layer of rock of Devonian age is right on top of a layer of rock of Cambrian age.
That means that tens of millions of years of geologic time lapsed between those two rock layers forming, and there is no sedimentary rock, no rock record, to record the details of what happened during that geologic time.
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The main thing is that it's consistent with other forms of dating.