Notepack 19

1. Notepack 19 AIM: How can we tell the age of rocks? Do Now: Regents Question: Put the layers of rock in order from oldest to youngest.

2. Geological Time • Geological Time refers to time as it relates to the Earth. • There are 2 types of time • Relative age – Time based on comparison • This rock is older than that rock • Absolute age – Time based with a number • This rock is 4 million years old

3. HOW CAN WE KNOW THE AGE OF ROCKS? Absolute age • Finding a number/date • radioactive decay data Relative age • Comparing ages • Superposition • *rock layers • index fossils – fossils of a known age found in a layer of rock

4. How do we find absolute age of rocks? • Scientist use radioactive dating to find an approximant age of rocks and fossils • Radioactive isotopes are atoms that have too many neutrons in their nucleus and decay over time! • Isotopes are very unstable! • Since everything is made up of atoms, scientist can use these isotope to measure the age of anything including rocks

5. The oldest known rock 4.3 billion years old

6. How do we find absolute age? cont. • Radioactive dating involves understanding how radioactive isotopes decay over time! • As isotopes decay (breakdown), the original atom changes into a different atom. • This decaying process occurs at a specific rate which scientist can measure; this is known as a half life.

7. Half -life • A half life refers to the time in which half of the amount of isotopes atoms (parent material) decays (changes) into a more stable atom (daughter material).

8. Here is 100 atoms of Carbon - 14. 100 carbon 14

9. After 5700 years (1 half life) 50 carbon 14 50 nitrogen

10. After another 5700 years – 11400 years total (2 half lives) 25 carbon 14 75 nitrogen

11. After another 5700 years – 17100 years total(3 half lives) 12.5 carbon 14 87.5 nitrogen

12. Each half-life, the amount of atoms gets cut in half. 100%

13. One half-life. 50%

14. Two half-lives. 25%

15. Three half-lives. 12.5%

16. Four half-lives. 6.25%

17. Which Isotope should you use to date something? • Each isotope has its own unique half-life. • Carbon-14 has the shortest half-life on the chart. (57,000 yrs) • Because fossils contain carbon and has been around in a relatively short time, scientist use Carbon-14 to determine the age. • Potassium-40 and Uranium-238 are used to date the Earth. • Rubidium-87 is used to date the universe. (49,000,000,000 years)

18. How to answer questions on the Regents? • If you are asked how many half-lives did something go through, just start with the original amount and divide by 2 until you get to the number that is needed. Each time you divide by 2 equals 1 half-life. • How many half-lives did carbon-14 go through if you started with 84 grams and ended with 21 grams? • 84/2= 42 (1 half-life), 42/2=21 (2nd half-life)

19. How to answer questions on the Regents? • If you are asked what the original amount of a substance was if it went through a number of half-lives, just start with the amount you are given and multiple it by 2 for each half-life that you are given. Each time you multiple by 2 equals 1 half-life. • What was the original amount of carbon-14 if it went through 4 half-lives and the you currently have 2 grams? • 2 x2= 4 (1 half-life), 4x2=8 (2nd half-life), 8x2=16 (3rd half-life), 16x2=32 (4th half-life)

20. How to answer questions on the Regents? • If you are given the amount of an isotope and its stable atom and were asked to calculate how many half-live did the sample go through, just add the amount of the isotope and the stable atom to get the total amount. Then divide this number by 2 until you get to the amount given for the isotope. Each time you divide by 2 equal one half-life. . • If you have 10 grams of Carbon-14 and 30 grams of nitrogen, how many half-lives occurred? • 10+30=40 • 40/2= 20 (1 half-life), 20/2=10 (2nd half-life)

21. 1. If a sample contains 50g of Carbon14 and 50g of Nitrogen14, how many half-lives has it undergone?

22. 1. If a sample contains 50g of Carbon14 and 50g of Nitrogen14, how many half-lives has it undergone? One half-life. After one have life 100g of C14 becomes 50 grams of C14

23. 2. If a sample contains 25g of Carbon14 and 75g of Nitrogen14, how many half-lives has it undergone?

24. 2. If a sample contains 25g of Carbon14 and 75g of Nitrogen14, how many half-lives has it undergone? • Two half-lives. After one half-life there is 50 g C14. after a second half-life there is 25 g C14

25. 3. If a sample contains 25g of Carbon14 and 175g of Nitrogen14, how many half-lives has it undergone?

26. 3. If a sample contains 25g of Carbon14 and 175g of Nitrogen14, how many half-lives has it undergone? 3! Total = 200 g C14. 1st half life – 100g 2nd half life – 50g 3rd half life – 25 g

27. 4. How old is a bone in which the Carbon14 in it has undergone 3 half-lives?

28. 4. How old is a bone in which the Carbon14 in it has undergone 3 half-lives? 5,700 + 5,700+ 5,700 = 17,100 years

29. 5. What happens to the amount of Nitrogen14 as the Carbon14 decays?

30. 5. What happens to the amount of Nitrogen14 as the Carbon14 decays? It increases

31. 6. If a 20g of Carbon14 has a half-life of 5,700 years, what would be the half-life of a 40g sample?

32. 6. If a 20g of Carbon14 has a half-life of 5,700 years, what would be the half-life of a 40g sample? 5,700 years Trick question – the half-life of C14 is always 5,700 years.