Radiometric Dating of Rock Samples
Use the decay reaction shown below to answer questions 1 -10.
Uranium-234  Thorium-230 + particles + energy
Half-life: 80,000 years
Directions: Select the correct word or phrase.
1. The radioactive isotope is: (Uranium – 234 / Thorium – 230)
2. The parent isotope is: (Uranium – 234 / Thorium – 230)
3. The daughter isotope is: (Uranium – 234 / Thorium – 230)
4. The larger unstable nucleus belong to: (Uranium – 234 / Thorium – 230)
5. The smaller stable nucleus belongs to: (Uranium – 234 / Thorium – 230)
6. The nucleus that breaks apart belongs to: (Uranium – 234 / Thorium – 230)
7. Describe what is happening in the decay reaction shown above. Summarize using as
many key terms as possible.
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8. A sample of basalt from a research excavation is analyzed using radiometric techniques.
Analysis reveals that the sample is 1/2 U-234 and 1/2 Th-230. How old is the sample?
9. If the sample is 1/4 U-234 and 3/4 Th-230. How old is the sample?
10. Describe how the ratio (amounts) of U-234: Th-230 change over time.
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Use the following decay reaction for questions.
Potassium – 40  Argon – 40 + particles + energy
Half-life: 1.3 billion years
11. A sample of granite had 60 grams of potassium-40 when it crystallized from magma. Fill
in the chart below to show how amounts of parent and daughter will change over time.
Potassium – 40
Argon – 40
Ratio (%) Ratio (fraction)
a.
1 half-life: _____________g
___________ g
50%:50%
½: ½
b.
2 half-lives: ____________ g
___________ g
________
_______
c.
3 half-lives: ____________ g
___________ g
________
_______
d.
4 half-lives: ____________ g
___________ g
________
_______
12. How old would the granite be when it is 50% potassium-40 and 50% argon-40?
13. How old would the granite be when it is 25% potassium-40 and 75% argon-40?
14. How old would the granite be when it is ¼ potassium-40 and ¾ argon-40?
15. How old would the granite be when it is 1/8 potassium-40 and 7/8 argon-40?
16. Explain why radiometric dating cannot be used on sedimentary rock layers.
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17. Explain why radiometric dating cannot be used on metamorphic rock layers.
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18. What methods can scientists use to determine when sedimentary rock layers formed?
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