How are radioactive isotopes helpful for studying rocks and fossils?
Radioactive isotopes can be used to determine how old a rock or
fossil is. One method is based on 14C (or Carbon 14) which has a
half life of 5,730 years. Scientists know the ratio of 12C to 14C
in plant materials based on absorption of CO2 is relatively
constant1, but that the creation of 14C stops at death. With this
knowledge, if you measure the ratio of 12C to 14C you can estimate
how long ago the sample was alive.
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1Technically, the 14C levels have changed over the years due to
various geologic, astronomic, and technologic issues, so scientists
also compensate for that knowledge when they calibrate a particular
sample.