Radiometric Dating

Continuation of Journal Entry #5

Radiometric Dating

• A technique used to date materials such as rocks based upon a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products using known rates of decay.

• Some elements have isotopes that predictably decay over time to more stable forms

Types of radioactive decay

• Alpha decay – Isotope emits 2 n0 and 2 p+

• Beta decay – Isotope emits 1 e-

• Neutron decay – Isotope emits 1 n0

Common radioactive isotopes and their products

• C-14 N-14 (beta decay)

• K-40 Ar-40 (beta decay)

• U-235 Pb-207 (all three forms of decay)

Radioactive Half-life

• The rate at which an isotope decays can be precisely measured in the lab and have been found to occur at a constant, predicable rate.

• An isotopes half-life is the amount of time necessary for ½ of the original amount of the radioactive isotope to decay to the more stable form.

Commonly used half-lives

• C-14 = 5,730 years• U-235 = 704 million years• K-40 = 1.25 billion years

• Question: If I found a bone with 12.5% of the expected amount of C-14, approximately how old is the bone?

• Question: If I found a bone with 12.5% of the expected amount of C-14, approximately how old is the bone?

• Answer: 5,730 years x 3 half-lives = 17,190 yrs

U-235 decay

• The geologic processes that form uranium are very different than those that form lead. Therefore, any lead found in a rock containing uranium must have been formed by decay. A measurement of the ratio of the two will reveal how many half-lives have passed – and thus will determine the age of the fossil.

Say hello to your lab partner!

Analysis Question

• If I found a fossil sample with 12 m&m’s remaining from the original sample, how old would it be if m&mdium has a half-life of 4.7 million years?

• #2 – Explain how an evolutionary biologist would use radiometric dating, relative dating, and fossils to determine something about how species have changed over time.

• #3 – Personal thoughts and reflections