Radioactive dating corals
In nearly all of the dating methods, except potassium-argon and the associated argon-argon method, there is always some amount of the daughter product already in the rock when it cools.
Using these methods is a little like trying to tell time from an hourglass that was turned over before all of the sand had fallen to the bottom.
This most often occurs if the rock experienced a high temperature usually a thousand degrees Fahrenheit or more at some point since its formation.
If that occurs, some of the argon gas moves around, and the analysis does not give a smooth plateau across the extraction temperature steps.
If the coral core layers contained radioactive elements with a known half-life, it would be possible to calculate almost exactly when each growth ring was made.
One can think of ways to correct for this in an hourglass: One could make a mark on the outside of the glass where the sand level started from and then repeat the interval with a stopwatch in the other hand to calibrate it.
Or if one is clever she or he could examine the hourglass' shape and determine what fraction of all the sand was at the top to start with.
“They got in touch with the local doctor and said, ‘Would you mind X-raying our coral slice?
’” When the coral slices were put in the X-ray scanner, a distinctive series of light and dark growth rings became visible, reflecting the density of the calcium carbonate that made up the coral skeleton.