Lesson 4: The Geologic Time Scale.

Lesson 4: The Geologic Time Scale.

Why do geologists use the geologic time scale?
Because of the time span of Earth’s past is so great, geologist use geologic time scale to show Earth’s history.
What is the geologic time scale?
The geologic time scale is a record of the geologic events and the evolution of life forms as shown in the fossil record.
How do scientists develop the geologic time scale?
Scientists first developed the geologic time scale by studying rock layers and index fossils worldwide. With this information,scientists placed Earth’s rocks in order by relative age. Later, radioactive dating helped determine the absolute age of the divisons in the geologic time scale.
What do the divisons of geologic time scale depend on?
The divisons of the geologic time scale depend on events in the history of life on Earth.
How does geologic time begin?
Geologic time begins with a long span of time called Precambrian time.
When did precambrian time end?
Precambrian time,which covers about 88 percent of Earth’s history, ended 542 million years ago (few fossils survive from this time period)
How do geologists divide the time?
Geologists divide the time between Precambrian time and the present into three long units of time called eras.
What are the eras?
They are the Paleozoic eras, the Triassic period, and the Cenozoic era.
How are eras divided?
Eras are subdivided into units geologic time called periods.
Where do the names of the geologists periods come from?
The names of many of the geologist periods come from places around the world where geologists first described the rocks and fossils of that period.
How is geologic time divided?
Geologic time is divided into eras and periods to show the long span of Earth’s history.