Uranium to lead dating
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.
All the major continents contain a core of very old rocks fringed by younger rocks.
Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age.
Not all metamorphisms completely erase the radiometric record of a rock’s age, although many do.
The abundance and variety of fossils in Phanerozoic rocks have allowed geologists to decipher in considerable detail the past 600 million years or so of the Earth’s history.
In Precambrian rocks, however, fossils are rare; thus, the geologic record of this important part of the Earth’s history has been especially difficult to decipher.
Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets.
The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites.
Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock.
Moreover, many of the oldest dated rocks intrude still older but undatable rocks.
Before reviewing briefly the evidence for the age of the Earth, I emphasize that the formation of the Solar System and the Earth was not an instantaneous event but occurred over a finite period as a result of processes set in motion when the universe formed.
It is, therefore, more correct to talk about formational intervals rather than discrete ages for the Solar System and the Earth.
A particularly fascinating question about the history of the Earth is “When did the Earth begin?