| 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 |
This
portion of geologic
time is often informally
called the
PREPALEOZOIC
ERA |
| 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. |
|