The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Index fossils occur for a limited interval of time. Metamorphic Rock Facts: So, you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive. Rock Cycle Diagram.
The half-life of the uranium to lead is 4.
Some measure the decay of isotopes more indirectly. All amino acids except glycine the simplest one are optically activehaving an asymmetric carbon atom. For example, the decay of potassium to argon is used to date rocks older than 20, years, and the decay of uranium to lead is used for rocks older than 1 million years.