Present evidence indicates, however, that these intervals were rather short (100-200 million years) in comparison with the length of time that has elapsed since the Solar System formed some 4 to 5 billion years ago.
Thus, the ages of the Earth, the Moon, and meteorites as measured by different methods represent slightly different events, although the differences in these ages are generally slight, and so, for the purposes of this chapter they are here treated as a single event.
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.
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.
Although early stratigraphers could determine the relative order of rock units and fossils, they could only estimate the lengths of time involved by observing the rates of present geologic processes and comparing the rocks produced by those processes with those preserved in the stratigraphic record.
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?
” The answer to this question was provided by radiometric dating and is now known to within a few percent.