EARTH: AN INTRODUCTION TO PHYSICAL GEOLOGY

Chapter 9: Geologic Time

Concept #5 Quiz

Choose the best possible answer to the following questions about Key Concept 5 "Determining ages of events – Radioactive dating."

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1. Why is radiometric dating the most reliable method of dating the geologic past? [Hint]

2.

Match the type of radioactive decay with the appropriate description:  

Using the pulldown boxes, match each item on the left to the corresponding item at right.

A. an electron combines with a proton and forms a neutron, leaving the mass number unchanged, but the atomic number decreases by 1
B. an electron is given off from the nucleus, leaving the mass number unchanged, but the atomic number increases by 1
C. two protons and two neutrons are emitted from the nucleus, reducing the mass and atomic numbers
 

3. Why is carbon-14 dating more appropriate for dating “young” events (younger than about 20,000 years old), but not for dating very old events (like the birth of the planet 4.5 billion years ago)? [Hint]

4. According to radioactive decay theory, how many half-lives have elapsed with the ratio of parent to daughter isotopes is 0.40 (meaning there is 40% of the parent isotope remaining) if there was 100% parent isotope originally? [Hint]

5. When does the “geochronological clock” begin for a mineral or fossil when the mineral grows or when the organism dies?  

6.

Assume a radioactive isotope with a half-life of 1 million years. Match the percent of "parent" material remaining indicated on the figure with the age of the material.  

For each item below, use the pull-down menu to select the letter that labels the correct part of the image.

The letters A through D appear on an image associated with this question.

7. Using uranium isotopes, what would be the calculated age of a mineral in a metamorphic rock if the half-life of uranium-238 is 4.5 billion years, and there has been just a half of 1 half-life elapsed? Assume that no daughter isotopes were present at the beginning, and that no gain or loss of parent or daughter has occurred. [Hint]

8. Why must a mineral not gain or lose any parent or daughter isotopes during its “life” in order to provide an accurate radiometric date? [Hint]

9. Which of the following are considered difficulties in assigning numerical ages to layers of sedimentary rock? [Hint]

Some questions in this exercise may have more than one correct answer. And answer choices in this exercise are randomized and will appear in a different order each time the page is loaded.
 




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