1. Radioactive Half-life
Absolute Dating
Radioactive Half-life

2. Superposition tells us who is older but not the actual ages.
To find the actual age of rocks we use radioactive elements.

3. Objectives Identify methods used for absolute dating
Describe the process of radioactive decay
Calculate the age of an material using known ½ life of a radioactive isotope

4. Absolute time A natural clock is necessary
-- Rates of Erosion or Deposition
Good for relatively small time frames
(100-10,000 years, but not reliable for large time frames)
Example: Niagara Falls…. We can measure how fast it erodes
--- Varves: Light/dark or course/fine layers of sediments that
vary annually (each year) due to summer and winter.
These mainly form in glacial lakes.
-- Dendrochrolonology: using tree rings to date
(only useful to about 10,000 years ago)
-- Radiometric dating (very accurate)
(nuclear clock: decay of radioactive isotopes) 4

5. Radioactive Decay
Isotope = atoms that have gained or lost neutrons and whose nucleus has become unstable
Decay = when the nucleus of an isotope releases energy and a particle (proton, neutron or both) and becomes a new element

6. Half-Life
The amount of time it takes for one half of a radioactive substance to decay into a nonradioactive substance.

7. Half-life Graph
# half-lives x half-life = age of the rock

8. Carbon 14 Dating
Constant generation of C-14 in the upper atmosphere by cosmic particle bombardment of N (nitrogen).
Nitrogen (N-15) emits a proton and becomes C-14. This is radioactive with a half-life of about 5,730 years.
Plants and animals ingest a constant amount of radioactive C-14. When they die, the ingestion stops, and the radioactive C-14 clock begins to count down. 8

9. After death the C-14 in an organisms body decays
After death the C-14 in an organisms body decays. After one half-life (5730 years), the amount of atoms gets cut in half. 9

10. One half-life (5730 years later) only one half of the C-14 is left10

11. Two half-lives (2 x 5730 = 11460 years later) only ¼ of the C -14 is left

12. Three half-lives (3 x 5730 = 17,190 years later) how much C-14 is left?
1/8 of the C-14 is left 12

13. Four half-lives (4 x 5730 = 22,920 years later)
1/16 of the C-14 is left 13

14. Carbon 14 dating is only useful to date objects 60,000 years or younger because too little is left to measure accurately past this point 14

15. Elements Used for ½ Life dating
Carbon-14 is in all living things but can only be used for things about 60,000 years old
To age the Earth we use elements with longer half lives.
Radioactive Parent
Stable Daughter
Half life
Potassium 40
Argon 40
1.25 billion yrs
Rubidium 87
Strontium 87
48.8 billion yrs
Thorium 232
Lead 208
14 billion years
Uranium 235
Lead 207
704 million years
Uranium 238
Lead 206
4.47 billion years

16. Radioactive Half Life Lab:
Read the information in the book about radioactive half-life and finding a date by decay (Page 193 – 196, also see figure 4).
The beans in this lab represent radioactive atoms. Put 100 beans into a cup. Shake the cup then dump and spread the beans out. Any bean with a painted side facing upward has radioactively decayed. Remove these beans from the pile and put them back in their bag. Record the number of beans remaining in the table on your handout
Put the remaining beans in the cup and repeat like this until all beans have decayed or until you have done 12 tosses.
Complete the graph and questions on the handout 16

17. Dating Igneous Rock
Some radioactive elements turn into a gas after they decay.
Any gases will escape from lava.
All remaining radioactive elements are trapped in crystals
To find out how many half lives remain we just have to compare the amount of gas trapped to the amount of radioactive atoms left

18. Half-life Questions
A rock has 100 radioactive atoms and 100 daughter atoms. How many half-lives have gone by?
Answer = 1
A rock has 25 radioactive atoms and 175 daughter atoms. How many half-lives have gone by?
Answer = 3
If the half-life of Carbon-14 is 5,700 years and three half-lives have gone by, how old is the rock?
Answer = 17,100 years

19. 1. An isotope has a half-life of 30 years. If a rock started with 1
1. An isotope has a half-life of 30 years. If a rock started with 1.0 grams and 90 years goes by, how many grams of the parent isotope will be left?
1.0g 0.5g0.3g0.15
2. A 2.5g sample of an isotope was formed by the explosion of an atomic bomb at Johnson Island in the Pacific ocean, in The half-life of this element is 28 years. How much of this sample will still be remaining in the year 2016? In the year 2072?
2.5g 1.3g.07g
0.07g0.04g0.02g