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.

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.

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.

Nonetheless, stratigraphy and radiometric dating of Precambrian rocks have clearly demonstrated that the history of the Earth extends billions of years into the past.

Radiometric dating has not been applied to just a few selected rocks from the geologic record.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.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.This was done by observing the relative age sequence of rock units in a given area and determining, from stratigraphic relations, which rock units are younger, which are older, and what assemblages of fossils are contained in each unit.