Geology 12 - Historical Geology
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Absolute Dating and the Geologic Time Scale

Handouts: Download and bring to class

Reading: Chapter 5 & 4

Read the chapter in this order: Radioactivity, First Dating of Minerals, Concept of Decay (p. 92-94), Modern Isotopic Dating (p. 94-98), Absolute Measures of Time (p. 91) Modern Relative Time Scale (p. 71-73), Isotopic Time Scale (p. 98), Additional Relative Time Scales (p. 83-85), Memorable Dates (p. 99-100). You can skim the sections on the development of the time scale (p.70) and early attempts at absolute dating (p. 90-92); it's not a topic we're going to explicitly deal with.

No textbook yet? Use these Web sites instead:

USGS Radiometric Dating:

Smithsonian Foundational Concepts of Paleobiology:

USGS Age of the Earth:


Terms you should know:

isotope -
half-life -
fission tracks -

blocking temperature -

primordial lead ratios -
magnetic polarity -


Key Concepts and Questions:

The textbook is not necessarily set up in the same structure as the Reading Guides. You will probably have to pull concepts from different parts of the chapters to answer the questions below. Read the questions here BEFORE you start reading the chapter, then keep the Reading Guide beside you as you go through the chapter. Be sure to record important ideas under the appropriate question below as you read.

1. What is the difference between relative time and absolute time?

What methods did we use to determine relative time?


2. How does radioactive decay provide a clock for dating rocks?

How do you determine the age of a rock?


What must you measure in the rock?


What other information do you need?


What assumptions are you making?



3. What does the "date" obtained through isotopic dating actually measure?

For igneous rocks:


For sedimentary rocks:


For metamorphic rocks:


4. What factors may influence the accuracy of dating?

What processes can change the proportions of isotopes in minerals?



What are other potential sources of error in the age measured by isotopic dating?


5. How is Carbon-14 dating different than other isotopes used for dating?



6. Aside from isotopic dating, how else can we assign numeric ages to rocks?

List three other means of obtaining numbers:



7. How can we use magnetic reversals to tell time?



8. What is the age of the Earth?

What different kinds of isotopic evidence tell us about the early Earth, and what information does each technique yield?


8. You will memorize the Geologic Time Scale - you will recieve a copy of the time scale in class (the one in the book is not the currently recognized time scale. You must know all the eons, eras and periods, the epochs of the Cenozoic, and these numbers: origin of Earth, Archean/Proterozoic boundary, Proterozoic/Phanerozoic boundary, Paleozoic/Mesozoic boundary, Mesozoic/Cenozoic boundary. You will have a quiz on the time scale at the beginning of class each week until you complete three quizzes perfectly.



1. Radioactive decay

You should be able to:

2. Limits of isotopic dating

You should be able to: