The Geologic The Geologic Time ScaleTime 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.

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)

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.