by Tas Walker A geologist works out the relative age of a rock by carefully studying where the rock is found in the field.The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.
The number of protons in the nucleus of an atom is called its atomic number.The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining.Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
His geological cross-section may look something like Figure 2.
When I have asked an audience this question they have looked at me incredulously and said, “Starting time?
” They realize that you cannot know how long the swimmer took unless you knew the time on the wristwatch when the race started.
Radioactive elements "decay" (that is, change into other elements) by "half lives." If a half life is equal to one year, then one half of the radioactive element will have decayed in the first year after the mineral was formed; one half of the remainder will decay in the next year (leaving one-fourth remaining), and so forth.
The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life (in other words raised to a power equal to the number of half-lives).
Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke intruded them.