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Lesson 9-02 Geologic Time Scale

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Lesson 9.02 – Geologic Time Scale


1. c. Students know the evidence from geological studies of Earth and other planets suggests that the early Earth was very different from Earth today.

(Please refer to Section 12.4 in your textbook)



Historians divide human history into certain periods, such as the Renaissance and the Industrial Revolution, based on human events. Thus you can produce a timeline of human history. Geologists have done something similar. 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 shown in Figure 17. The major units of the time scale were described during the nineteenth century, principally by scientists working in Western Europe and Great Britain. Because radiometric dating was unavailable at that time, the entire time scale was created using methods of relative dating. It was only in the twentieth century that radiometric dating permitted numerical dates to be added.




Structure of the Time Scale

As shown in Figure 17, the geologic time scale is divided into eons, eras, periods, and epochs. 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, a term derived from Greek words meaning “visible life.” It is an appropriate description because the rocks and deposits of the Phanerozoic Eon contain abundant fossils that document major changes in life-forms over time.



There are three eras within the Phanerozoic. The Paleozoic, which means “ancient life,” the Mesozoic, which means “middle life,” and the Cenozoic, which means “recent life.” As the names imply, the eras are bounded by profound worldwide changes in life forms. Each era is subdivided into periods, each of which is characterized by a somewhat less profound change in life forms as compared with the eras.



The periods of the Cenozoic are divided into still smaller units called epochs. The epochs of other periods, however, are not usually referred to by specific names. Instead, the terms early, middle, and late are generally applied to the epochs of these earlier periods.



Precambrian Time

Notice that the detail of the geologic time scale doesn’t begin until the start of the Cambrian Period, about 540 million years ago. The more than 4 billion years prior to the Cambrian is divided into eons, as shown in Figure 17. The common name for this huge expanse of time is the Precambrian.


Figure 17 The Geologic Time Scale The numerical dates were added long after the time scale had been established using relative dating techniques.

Although it represents about 88 percent of Earth history, the Precambrian is not divided into nearly as many smaller time units as is the Phanerozoic eon. The reason is simple. Precambrian history is not known in great enough detail. The amount of information that geologists have acquired about Earth’s past decreases substantially the farther back in time you go. During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.








1.    Take notes on the above information.

2.    Click here (http://videos.howstuffworks.com/hsw/17896-historical-geology-geologic-time-scale-video.htm) to view a video about the Geologic Time Scale.

3.    Click here (http://www.youtube.com/watch?v=-2YEJBK9yJM&feature=related) to watch a video about Early Earth.




1.    Turn in your notes.

2.    Take the Lesson 9-02 Quiz in Quia.




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