GL 107 Earth Systems Science

K. Hannula

Study questions for 8/31/07

1.  What is the difference between relative and absolute dating?

Absolute dating (or numerical dating) refers to any method that can tell you the actual number of years ago that a geologic event occurred.

1.  How are parent and daughter isotopes related to one another?

Parent isotopes become transformed into daughter isotopes during radioactive decay.

2.  Define "half-life."

The half-life of a radioactive isotope is the amount of time necessary for half of the isotope to be transformed into its daughter.

3.  The half-life of isotope A is 100,000 years.  You analyze a rock sample and find that 1/4 of the parent remains, and that 3/4 has been transformed into the daughter isotope.  How old is the rock sample?

If 1/4 of the parent remains, that means that two half-lives have gone by.  (After one half life, half of the parent isotope remains; after two half lives, one half of 1/2 of the parent is left: one half of 1/2 is 1/4.)  Two half lives, in this case, is 2 time 100,000 years, or 200,000 years.

4.  What sorts of rocks could be dated using uranium-lead dating?  Potassium-argon dating?  Carbon-14 dating?

Uranium-lead dating is especially useful for dating igneous rocks.

Potassium-argon dating is most useful for dating volcanic igneous rocks.

Carbon-14 dating is used to date very young (thousands rather than millions of years) sediments and artifacts if they contain remnants of living organisms (charcoal, wood fragments, shells, etc.).  It can not be used to date very old fossils, because carbon-14 has a fairly short half-life.