Key Concepts:• Why is the geological time scale used to show Earth’s history?

• What are the different units of the geological time scale?

Key Terms:• geological time scale• era• period

Section 10.4 – The Geological Time Scale

The Geological Time Scale

Because the time span of Earth’s past is so great, geologists use the geological time scale to show Earth’s history.

The geological time scale ______________________________________________________ is a record of the life formsand geological events in Earth’s history.

Scientists first developed the geological time scale bystudying rock layers and index fossils worldwide. With this information, scientists placed Earth’s rocksin order by relative age.Later, radioactive dating helped them determine theabsolute age of the divisions in the geological time scale.

Divisions of Geological Time

Geological time begins with a long span of time called the Precambrian Time. Precambrian timecovers about 88% of Earth’s history, ended 544 million years ago.

After Precambrian Time, the basic units of the geological time scale are eras and periods.

Geologist divide the time between Precambrian Timeand the present into three long units of time called_____. eras

They are the Paleozoic Era, the Mesozoic Era, and Cenozoic Era.

Eras are subdivided into units of geological time called________. You can see that the Mesozoic Era includesthree periods: the Triassic Period, the Jurassic Period,And the Cretaceous Period.

periods

The names of many of the geological periods come from places around the world where geologists firstdescribed the rock and fossils of that period. Forexample, the name Cambrian refers to the Cambria,the old Roman name for Wales.

10.4 Assessment questions

1a. What is the geological time scale?

b. What information did geologists use in developing the geological time scale?

2a. What the basic units into which the geological time scale is dived?

A record of life forms and geological events in Earth’s history.

Scientists studied rock layers and fossils worldwide and placed Earth’s rocklayers in order by relative age. Later, radioactivedating helped them determine the absolute ages of thedivisions.

Eras and periods

2b. Study Fig. 14. Which major period of geological time was the longest? When did it begin and end?

c. Place the following in the correct order from earliest to latest: Tertiary, Jurassic, Quaternary, Triassic, Cretaceous.

Precambrian time;Began 4,600,000,000 or 4.6 billion years ago and ended 544,000,000 or 544 million years ago

Triassic, Jurassic, Cretaceous, Tertiary, Quaternary

Classwork: Complete Young Canyon Made of Old Layers

HW: Complete worksheet 2 of your Brochure/Time Period. Bring material to work in class for the rest of the week. As time allow, you can work on it. You may use my computer, also. Read 10.5 & Do Assessment questions.