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The best evidence is contained in the Earth’s incomplete and complex but accurate stratigraphic record — a record that has been the subject of nearly two centuries of study.Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1.Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age.
The first is to search for and date the oldest rocks exposed on the surface of the Earth.
These oldest rocks are metamorphic rocks with earlier but now erased histories, so the ages obtained in this way are minimum ages for the Earth.
Because the Earth formed as part of the Solar System, a second approach is to date extraterrestrial objects, i.e., meteorites and samples from the Moon.
The rocks in these shields are mostly metamorphic, meaning they have been changed from other rocks into their present form by great heat and pressure beneath the surface; most have been through more than one metamorphism and have had very complex histories.
A metamorphic event may change the apparent radiometric age of a rock.
Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock.