1. Geologic Time and Earth History Part 2 – Absolute Age

2. Uniformitarianism
A principle that states that geologic processes that have been occurring today have been occurring since Earth formed.
Key to the science of geology and studying Earth’s past!

3. Essential Questions
How can the amount of parent material in a rock sample be determined by using half-lives?
How does the amount of parent material in a sample change as time increases?

4. Absolute Dating
Radioactive isotope content can be used to identify the amount of time passed since the rock formed.
Rocks that can be dated like this are mostly igneous, but sometimes metamorphic.
Sedimentary rocks cannot be dated this way.
The process is also known as radiometric analysis.

5. Radioactive Decay
Method that is used to determine the absolute age of a rock or fossil.
When an atom breaks down or decays over time until it is stable.

6. Half-Life
Half-life: The time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope.
Ranges from seconds to billions of years depending on the material.

7. Half-Life
For example, carbon-14 takes 5730 years to break down. So, if I have 100g of carbon-14, after 5730 years I will only have 50g. How much will be left after another 5730 years?
How much time would have passed?
Answer:
You would have 25g of carbon-14
11,460 years of total time would have passed.

8. Using Half-Life Parent: unstable radioactive isotope
Daughter: isotope resulting from the decay of the parent.
# of Half Lives
Parent Isotopes
Daughter Isotopes
100% 0% 1
50% 2
25%
75% 3
12.5%
87.5% 4
6.25%
93.75%

9. Parent and Daughter material in terms of half-life
By comparing the percentage of an original element (parent) to the percentage of the decay element (daughter), the age of rock can be calculated.
The ratio of the two atom types is a direct function of its age because when a rock was formed, it had all parent atoms and no daughters.

10. Calculating Half-Life
How old would a fossil be if it had only 1/8 of its original carbon-14 content remaining? Remember carbon-14 takes 5730 years to divide in half.
Create a table! Time, Amount of parent/daughter isotopes, and # of half-lives

11. Problem #1 Answer: 6.25 grams
The half-life of radon-222 is 3.8 days. How much of a 100g sample is left after 15.2 days?
Answer:
6.25 grams
Time
Amount
0 Days
100 grams
3.8
50 grams
7.6
25 grams
11.4
12.5 grams
15.2
6.25 grams

12. Problem #2 Answer: 8.75 grams
Carbon-14 has a half-life of 5730 years. If a sample contains 70g originally, how much is left after 17,190 years?
Time
Amount
0 years
70 grams
5730 years
35 grams
11,460 years
17.5 grams
17,190 years
8.75 grams
Answer:
8.75 grams

13. Problem #3 Answer: 15.625 grams
How much of a 500g sample of potassium-42 is left after 62 hours? The half-life of potassium-42 is 12.4 hours.
Time
Amount
0 hours
500 grams
12.4 hours
250 grams
24.8 hours
125 grams
37.2 hours
62.5 grams
49.6 hours
31.25 grams
62 hours
grams
Answer: grams