4. Determining the Age of Rocks and Fossils
PURPOSE: This activity will help students to determine relative age of a geologically complex area using the concepts of radioactive decay and superposition AND to determine absolute ages by using parent and daughter isotope data.

5. To Find RELATIVE AGE
Principle of superposition: Younger sedimentary rocks are deposited on top of older sedimentary rocks. Principle of cross-cutting relationships: Any geologic feature is younger than anything else that it cuts across.

6. To find ABSOLUTE AGE:
Radiometric Dating

7. Procedure Part 2
Take the 100 pennies and place them HEADS UP on a piece of paper. This represents the PARENT isotope.
Pour the pennies into a container, shake them and pour them back onto the paper so they can be spread out, not piled. This represents one half life and all the pennies that landed TAILS UP represents a change to the DAUGHTER isotope
Pick up and set aside ONLY the daughter isotopes, then count the number of pennies left heads up. These are the PARENT isotopes that did not change during the first half life.
Record the number of parent isotopes after the first run in your data table.
Continue to repeat the same procedure of shaking and counting the “survivors” and filling in the next row on the decay table. This should be done 7 or 8 more times. Each time represents a half life.
Copy your data onto the CLASS data table and then copy the other teams’ data into your data table.

9. Procedure Part 1
Determine the relative age of each of the rock units in the block diagram (Figure 1).
List the rock layers in order at the bottom of Figure 1 on the limes provided from most recent at the top to oldest at the bottom.

10. DO NOW Don’t forget on-line fossil activity due tomorrow.
Answer these questions in the DO NOW of your binder:
How old is rock if it contains 50% parent material and 50% daughter material and the half life is 10 years?
What percent of a rock will be parent material if it is 20 years old and the half life is 10 years?

11. Procedure Part 2 continued
3 lines:
YOUR team’s data
CLASS AVERAGE data
MATHEMATIC-ALLY PREDICTED data
** Don’t forget a key!

12. Procedure Part 3 start After 1 half life 2 half lives _____ half lives
Fill in the data table for U-235 in Figure 5 for 5 half lives.
The actual half life of U-235 is 704 years, not 2 minutes. Complete the chart using the actual half-life.
start
After
1 half life
2 half lives
_____ half lives
____ half lives
U- 235
parent
128 64
Pb- 207
daughter
Ratio
U: Pb
1:1
Half lives 1
Years
704 M yrs

13. Procedure Part 3
start
After
1 half life
2 half lives
_____ half lives
____ half lives
U- 235
parent
128 64
Pb- 207
daughter
Ratio
U: Pb
1:1
Half lives 1
Years
704 M yrs
Go back to the block diagram (Figure 1) and figure out how old these rocks layers are using the following information.
The ratio of U-235:Pb-207 atoms in the pegmatite is 1:1, and their ratio in the granite is 3:1. Use what you learned in part 2b to determine how old the pegmatite and granite are. Put those ages in the lines in Figure 1.

14. Procedure Part 3 continued
Figure 4 is a special scale called a logarithmic scale—different than a regular graph. You can find the age of shale/siltstone layer and the basalt layer. Shale/siltstone contains 94% U-235. How old is it? Fill in the radiometric age in the line provided in Figure 1.
The ratio of U-2325 in basalt is 70%. How old is the basalt layer? Fill in the radiometric age of basalt on the diagram in Figure 1.

15. ANALYSIS QUESTIONS 1) What is the rock layer that has acritarchs and bacteria? Based on the available radiometric ages, can you determine the possible age of this rock? Why can't you say exactly what the age of the rock is? 2) Which rock contains trilobites? Based on the available radiometric ages, can you determine the possible age of this rock? Why can't you say exactly what the age of the rock is? 3) What is the age of the rock that contains the Triceratops fossils? Why can you be more precise about the age of this rock than you could about the ages of the rock that has the trilobites and the rock that contains acritarchs and bacteria?