Relative Dating and Fossils Relative dating Relative dating Law of Superposition Law of Superposition Index Fossils Index Fossils What are fossils? What are fossils? Preserved remains of ancient organisms Preserved remains of ancient organisms How are fossils formed? How are fossils formed? Dead organisms are buried by sediments, if conditions are perfect, they are persevered Dead organisms are buried by sediments, if conditions are perfect, they are persevered Why is the fossil record incomplete? Why is the fossil record incomplete? It takes perfect conditions to make fossils, so many organisms were never persevered It takes perfect conditions to make fossils, so many organisms were never persevered

Radioactive Dating Use radioactive decay to find absolute ages of rocks and fossils Use radioactive decay to find absolute ages of rocks and fossils Half-life: the amount of time required for half the atoms in a sample to decay Half-life: the amount of time required for half the atoms in a sample to decay After 1 half-life: ½ atoms remain After 1 half-life: ½ atoms remain After 2 half-lives: ½ x ½ = ¼ atoms remain After 2 half-lives: ½ x ½ = ¼ atoms remain After 3 half-lives: ½x ½ x ½ = ⅛ atoms remain After 3 half-lives: ½x ½ x ½ = ⅛ atoms remain After n half-lives: ½ n atoms remain After n half-lives: ½ n atoms remain Different elements have different half-lives Different elements have different half-lives Carbon-14 ( 14 C) is very common and has a half life of about 5730 years Carbon-14 ( 14 C) is very common and has a half life of about 5730 years

Radioactive Decay “Cold” elements won’t harm you “Cold” elements won’t harm you “Hot” elements give off radiation (causing decay) which may harm you “Hot” elements give off radiation (causing decay) which may harm you Organic compounds have a mix of 14 C (hot) and 12 C (cold) Organic compounds have a mix of 14 C (hot) and 12 C (cold) 14 C decays into 14 N which is cold and can escape into the atmosphere 14 C decays into 14 N which is cold and can escape into the atmosphere Atomic number= protons Atomic number= protons (C= 6, N= 7) (C= 6, N= 7) Atomic mass= protons + neutrons Atomic mass= protons + neutrons 14= 6 protons + 8 neutrons (& 6e - ) 14= 6 protons + 8 neutrons (& 6e - ) One neutron decays into a proton and an electron One neutron decays into a proton and an electron 14= 7 protons + 7 neutrons (& 7e - ) 14= 7 protons + 7 neutrons (& 7e - ) As time goes by, the amount of 12 C increases compared to the amount of 14 C As time goes by, the amount of 12 C increases compared to the amount of 14 C

Sample Problem 14 C has a half-life of 5730 yrs, we can use it to date things back about 60,000 yrs 14 C has a half-life of 5730 yrs, we can use it to date things back about 60,000 yrs Remember: the amount of an element remaining= ½ n, n= number of half-lives passed Remember: the amount of an element remaining= ½ n, n= number of half-lives passed If you are made of 1% 14 C when you die, how much 14 C will be left after 5,730yrs? If you are made of 1% 14 C when you die, how much 14 C will be left after 5,730yrs? 5,730 yrs x 1 half-life/5,730yrs = 1 half-life 5,730 yrs x 1 half-life/5,730yrs = 1 half-life ½ 1 x 1% = 0.5% ½ 1 x 1% = 0.5% 11,460 yrs? 11,460 yrs? 11,460 yrs x 1 half-life/5,730yrs = 2 half-lives 11,460 yrs x 1 half-life/5,730yrs = 2 half-lives ½ 2 x 1% = 0.25% ½ 2 x 1% = 0.25%

Practice A fossil contains 12.5% of the 14 C that would have been present at the time of death. How old is the fossil? A fossil contains 12.5% of the 14 C that would have been present at the time of death. How old is the fossil? 0.5 x 0.5 x 0.5 = x 0.5 x 0.5 = half-lives have passed 3 half-lives have passed 3 half-lives x 5,730yrs/ 1 half-life = 17,190yrs 3 half-lives x 5,730yrs/ 1 half-life = 17,190yrs CHEATER METHOD! (USE AT YOUR OWN RISK!) CHEATER METHOD! (USE AT YOUR OWN RISK!) ½ n = ½ n = n log ½ = log n log ½ = log n = log / log ½ = 3 n = log / log ½ = 3 A fossil is tested and found to be 28,650 years old. How much of the original 14 C is still present? A fossil is tested and found to be 28,650 years old. How much of the original 14 C is still present? 28,650 yrs x 1 half-life/5,730yrs = 5 half-lives 28,650 yrs x 1 half-life/5,730yrs = 5 half-lives ½ 5 = or 3.125% ½ 5 = or 3.125%