1. Absolute Dating
Radiometric dating

2. How do they measure the actual age of rock?

3. Count Tree rings
Dates trees back to 8000 years

4. In the summer when glaciers melt and retreat, sediment in the center of large glacial lakes is deposited in annual layers called varves
Count the layers
Dates back to the last glacial ice age 15,000 yrs. ago

5. Radiometric dating of rocks
Marie Curie 1900’s
Discovered Radioactive isotopes:
are Naturally occurring in rock
are unstable atoms
that decay
released energy
change into different element
Half-life – the time for half of the radioactive nuclei to decay

7. 5. Carbon-14 dating All living things contain carbon
Carbon-14 is the radioactive (“evil twin” of carbon-12)
It’s half-life decay rate is 5730 years.
Organic materials less than 70,000 yrs old can be dated this way.

8. The radioactive decay curve
“Parent” is the radioactive isotope that decays
“Daughter” is new atoms that have formed from the decay of the parent
First half life
2nd half life
Figure 11.17

9. Rubidium-87 47 billion years
Potassium ,000 years
Radiometric dating can be done, but only reliably on IGNEOUS ROCKS, volcanic ash

10. Each radioactive element has it’s own fixed half-life
Each radioactive element has it’s own fixed half-life. Each decays at a predictable, regular RATE

11. Radioactive Half-life (Candy) Lab
Parent
isotope
Total
Number
decayed
Start #
of atoms 48
1st half life
2nd half life
3rd half life
Percent
Parent
remaining
Age of rock
100%
________
If the half life of this radioactive isotope was 3000 years, how old is this rock?
50%
25%
12.5%

12. Practice A radioactive isotope has a half-life of 10,000years.
The percent of parent atoms left is 25%
How OLD is the rock containing the radioactive material?
1 half-life = 50% remains, age of rock is 10,000 years old
2 half-life = 25% remains rock is ANOTHER 10,000 yrs. old
So……. 20,000 years old!

13. Directions: determine the AGE of your rock using radioactive atoms in the rock
Remove any “mutant” atoms that do not have a letter on them.
Put radioactive “parent” atom’ in the box facing UP.
Count the number of atoms to start. Record on data table
Put lid on and gently toss to decay the atoms.
Remove the “daughter” atoms that have ‘flipped over’ (decayed). Count and record the number of remaining atoms in the box .
Continue until all the atoms have ‘decayed’
Determine the age of your rock using the half-life time.

14. Cross out 1st column, make your prediction in 2nd column
Half-lives that have occurred
Prediction of the # of “parent” (radioactive) atoms that will remain after the next half-life (toss)
Actual # of “parent” (radioactive) atoms that REMAINING after the half-life (toss)
Percent % of “parent” atoms remaining
Age of your rock 50
100% 1 2 3 4 5 6

15. How old are layers D and J?F B K N A J D M H C L G E
425 my
300 my
How old are layers D and J?

16. Dating sedimentary strata using radiometric dating
Which type of rock has been dated:
Igneous, sedimentary of metamorphic?
How old is the Dakota Sandstone?
How old is the
Wasatch Formation?
Figure 11.21

17. Percent of parent remaining
Number of
half-lives
Ratio of
Parent to daughter
(radioactive) : (stable) atoms
Percent of parent remaining
1:0
100% 1 2 3
1: %
1: %
1: % 4 5
1: %