Geologists and paleontologists usually represent geologic time vertically.
This arrangement is derived from the vertical succession of rock strata
in Earth itself. Because of the Principle of Superposition, geologists
know that new layers can be laid down only on top of pre-existing
older strata, and therefore older rocks lie below younger ones. The
vertical time scale mimics this stratigraphic arrangement by placing
older time periods below younger ones, with the present day at the
By itself, this vertical arrangement reveals only the relative ages
of rocks, not how old the layers are. Geologists and paleontologists,
however, also want to know the specific ages of rock layers and of
the fossils within them. They use a variety of absolute dating methods
to determine precisely when particular rocks were formed. Many different
rocks from different strata around the world have been dated this
Once numerical ages have been obtained, they can be combined with
the vertical time succession to create a true geologic time scale.
This works because some of the strata that have been dated are the
same ones that were used to create the relative vertical time scale.
The result is a global, comprehensive time scale that lists all the
ages of Earth’s history, with the oldest ages placed at the
bottom and the youngest at the top. Any rock layer can be correlated
with this global time scale and given a real, rather than a relative,
age. Scientists have also developed a system of standard names and
colors for all the time periods.
Geologists are especially interested in the ages of the boundaries
between different eras, periods, and epochs because these often mark
important events in Earth’s history. As geologists refine the
numerical dates and acquire new fossil evidence, these boundary dates
can shift. In fact, the combined absolute/relative time scale is
always being revised in order to produce an ever more precise picture
of Earth history.
View the Time Scales
Geologic Time Scale - American
Geologic Time Scale - European