Red Badger
Are you serious? How do they know the age of a rock? Do they have a birth certificate?
It was two miles down....................
>>How do they know the age of a rock? Do they have a birth certificate?<<
Yes. Yes they do.
You should see their antiquated filing system, though.
(more ignorance on FR? really?)
Some do. There are rocks in the south tip of Iceland we can date to the day and hour they solidified...
More to the point if you know the chemical composition of a crystal, and you know exactly what percentages of elements form that crystal, and some elements have a known rate of radioactive decay, and you know how much of those elements are missing from the crystal, you can estimate how long it took for that percentage of those atoms to go missing.
AND you can cross check your answer by measuring the excess of the elements they decay into that aren't part of the original crystalline structure that are trapped within the crystal.