Absolute Dating Radiometric dating

How do they measure the actual age of rock?

Count Tree rings Dates trees back to 8000 years

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

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

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.

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

Rubidium-87 47 billion years Potassium-40 50,000 years Radiometric dating can be done, but only reliably on IGNEOUS ROCKS, volcanic ash

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

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? 24 24 50% 12 36 25% 6 42 12.5%

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!

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.

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

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?

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

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 50% 1:3 25% 1:7 13% 4 5 1:15 6%