Geology Reference
In-Depth Information
Scientists have discovered that decay rates of radioactive isotopes are constant: They
do not change in response to temperature, pressure, or other chemical factors. There-
fore, when the decay rate for a radioactive isotope is known, measuring the number of
parent and daughter atoms in a rock or mineral can be used to determine, in years, how
many half-lives have occurred since the mineral (or rock) was created.
For example, if a rock forms with 100 radioactive parent isotopes, after one half-life the
rock will have 50 parent isotopes and 50 daughter isotopes. After two half-lives it will
have 25 parent isotopes and 75 daughter isotopes.
Because half of the existing parent isotopes decay in each half-life period, there is a pre-
dictable ratio of the percentage of parent-daughter atoms that tells you how many half-
lives have passed:
When the rock is formed, it always has 100 percent of the parent isotope.
After one half-life, 50 percent of the parent isotope remains.
After two half-lives, 25 percent of the parent remains.
After three half-lives, 12.5 percent of the parent remains.
And so on until the amount is too small to measure or no parent atoms remain.
Common radioactive isotopes for geological dat-
ing
Not all radioactive isotopes are useful for dating. Some of them have half-lives that are
much too long or much too short for humans to measure. A good half-life range for dat-
ing earth materials is somewhere around several thousand years.
Choosing the best isotope to date a particular rock depends on how old you
think the rock may be, which minerals (and therefore elements) are in the rock,
and which type of rock you are dating. Table 16-1 lists and describes the most
common radioactive parent-daughter isotope pairs that are used in dating rocks.
