| EON |
ERA |
Based on the great changes in the record of life, geologic
time has been subdivided into four major intervals of time, eons. These
eons have been further subdivided into smaller units, eras, periods and
epochs. This categorization system is referred to as the Geologic
Time Scale, and it is this time scale to which geologists refer when
discussing the history of the earth. Remember that although such a scale
is often labeled with absolute ages for the sake of information, it is
based on fossils.
|
| Phanerozoic |
Cenozoic 66
MY |
| Mesozoic 250
MY |
| Paleozoic 540
MY |
| Proterozoic |
Late 1.3-0.54
BY |
| Middle 1.6-1.3
BY |
| Early 2.5-1.6
BY |
| Archean |
Late 3.0-2.5
BY |
| Early 3.9-2.0
BY |
| Hadean |
No Hadean Eras >3.9
BY |
Fossils are indeed powerful tools; and in many ways they are the only reliable
ones available to a geologist when it comes to unraveling the history of
the earth. Still, it bears re-emphasizing that despite the fact that we
may be able to ascribe an extremely accurate relative age to a fossil,
a layer or an event, the date is a relative date and we cannot know how
many thousands or millions of years ago such an event took place. Such
determinations are done by using techniques which involve changes which
occur at known rates, techniques of absolute dating.
|
