The Geologic
Time Scale
Based on their
interpretations of the
rock record,
geologists have
divided Earth’s 4.56
billion-year history
into units that
represent specific
amounts of time.
Taken together,
these time spans
make up the
geologic time scale.
The geologic time scale
is broken up into
•Eons
Longest
expanse of geologic
time
•Eras
Next longest
expanse of geologic
time
•Periods Third
Longest expanse of
geologic time
•Epochs Shortest
expanse of geologic
time
Eons represent the
greatest expanses of
time. Eons are
divided into eras.
Each era is
subdivided into
periods. Finally,
periods are divided
into still smaller units
called epochs.
The eon that
began about
540 million
years ago is the
Phanerozoic.
Rocks
deposited in the
phanerozoic
contain
abundant
fossils that
document major
changes in life
forms over time.
(recent life)
(middle life)
(ancient life)
There are 3 eras
within the
Phanerozoic. The
paleozoic, which
means :ancient life,”
the Mesozoic, which
means “middle life,
and the Cenozoic,
which means
“recent life.”
Each era change comes at profound life
changes or extinction events.
• Climate Changes
• Disease
• Meteorite Impact
• Volcanic Eruptions
Each era is
subdivided into
periods, each
of which is
characterized
by somewhat
less profound
change in life
forms as
compared with
eras.
The periods of the Cenozoic are divided into still
smaller units called epochs. Epochs of other
periods are named early, middle, and late.
In Precambrian
time (older than
540 million years
ago) there isn’t as
much detail of the
geologic time
scale. Scientists
have divided the
Precambrian into
Eons into the
Proterozoic,
Archaean, and
Hadean.
Mya
During the Precambrian time, there were
fewer life forms. These life forms are more
difficult to identify and the rocks have been
disturbed.
There are some difficulties when trying to define
the divisions of the geologic time scale. For
example, sedimentary rock may contain particles
that contain radioactive isotopes, but these
particles are not the same age as the rock in
which they occur.
Radiometric dating of metamorphic rocks
may also be difficult. Sometimes the
particular mineral may not represent when
the rock formed but rather when it was
metamorphosed. Sometimes in
sedimentary layers geologists hope to find
fossils or ash layers that can be dated.