The Geologic Time Scale

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The Geologic Time Scale
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Based on their interpretations of the rock
record, geologists have divided Earths 4.56
billion-year history into units that represent
specific amounts of time. Taken together, these
time spans make up the geologic time scale.
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• 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

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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.
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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.
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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.
(recent life)
(middle life)
(ancient life)
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Each era change comes at profound life changes or
extinction events.

• Climate Changes
• Disease
• Meteorite Impact
• Volcanic Eruptions

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Each era is subdivided into periods, each of
which is characterized by somewhat less profound
change in life forms as compared with eras.
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The periods of the Cenozoic are divided into
still smaller units called epochs. Epochs of
other periods are named early, middle, and late.
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In Precambrian time (older than 540 million years
ago) there isnt as much detail of the geologic
time scale. Scientists have divided the
Precambrian into Eons into the Proterozoic,
Archaean, and Hadean.
Mya
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During the Precambrian time, there were fewer
life forms. These life forms are more difficult
to identify and the rocks have been disturbed.
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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.
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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.