Geology Reference
In-Depth Information
Fission-track dating
is a method of counting the number of fission tracks in a mineral.
These are very small trails etched in a crystal when radioactive atoms of uranium decay.
As long as the minerals have not been exposed to high temperatures, the fission tracks
will be present. Scientists can count the fission tracks and calculate an age for the rock.
But if the minerals have been exposed to high temperatures, the trails will be erased,
and the calculations of any visible trails will provide an age younger than the actual age
of the rock.
Fission-track dating is most useful for rocks between 50,000 and 1.5 million
years old, but it can be used for younger samples (a few hundred years old) and
older samples (a few hundred million years old).
Radiocarbon
Another radioactive isotope that decays with a known half-life is the isotope of car-
bon-14. Maybe you've heard of
radiocarbon
or
C-14 dating
in television shows or movies.
This method, used also by archaeologists, is the most common method of dating events
within human history.
Carbon-14 provides the age of any material that used to be living and, therefore, has car-
bon in it. This includes bone, wood, shells, and paper. While carbon can be found in
some minerals, it is most common in living organisms, who take in carbon atoms from
the atmosphere while they are living. After the organism dies, the carbon-14 isotopes be-
gin to decay.
The half-life of C-14 is only 5,730 years, which means that it is useful for de-
termining the age of things between 100 and 70,000 years old. (In geologic time,
70,000 years is very, very recent.) This means that any sample older than 70,000
years will appear to be 70,000 years old — never older — either because there are
no carbon-14 atoms left or there are not enough to measure with current tech-
niques.
