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Absolute Dating


I can use radiometric dating to calculate the absolute age of rocks and fossils.



What is radiometric dating?

How are radioactive elemets used to determine the age of rocks and fossils?



The absolute age of rocks and fossils can be measured with a unit called half-life.

Radioactive decay is a random process.

Radioactive decay can be related to probability.

The breakdown of a radioactive element into a decay element occurs at a constant rate.  By measuring the ratio of radioactive element to the decay element, the age of the fossil can be estimated.

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If you started with a sample of 600 radioactive nuclei, about how many would remain un-decayed after three half-lives?


600 divided by 2 = 300 nuclei (after 1 half-life)

300 divided by 2 = 150 nuclei (after 2 half-lives)

150 divided by 2 = 75 nuclei (after 3 half-lives)

The correct answer is 75 nuclei.



Carbon-14 has a half-life of about 5,730 years.  How many half-lives will it take a sample of 100.0 grams to decay to 12.50 grams?


100 grams divided by 2 = 50 grams (after 1 half -life)

50 grams divided by 2 = 25 grams (after 2 half-lives)

25 grams divided by 2 = 12.5 grams (after 3 half-lives)



Carbon-14 has a half life of about 5730 years.  An anthropologist finds some human bones containing only 3% un-decayed carbon-14 nuclei.  About how old are the hominid bones?

Solution:  (Use the radiometric dating table of carbon-14 to the right  of this page)

Locate 3% undecayed nuclei on the table.

The corresponding age of a set of bones containing 3% undecayed carbon-14 nuclei is about 28,000 years old



Radiometric Dating:  the method of obtaining a rock’s age by measuring the relative abundance of radioactive nuclei.

Half-life:  the amount of time required for half of an element’s radioactive nuclei to decay.

Radioactive Decay:  the spontaneous emission of radiation from the unstable nucleus of an atom.

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