Geoscience Reference
In-Depth Information
The success of radio-dating techniques is due in no small way to the power of the mass
spectrometer, an instrument which can virtually sort individual atoms by weight and so
give isotope ratios on trace constituents in very small samples. But it is only as good as
the assumptions that are made about the half-life, the original abundances of isotopes, and
the possible subsequent escape of decay products. The half-life of uranium isotopes makes
them good for dating the earliest rocks on Earth. Carbon 14 has a half-life of a mere 5,730
years. In the atmosphere it is constantly replenished by the action of cosmic rays. Once the
carbon is taken up by plants and the plants die, the isotope is no longer replenished and
the clock starts ticking as the carbon 14 decays. So it is very good for dating wood from
archaeological sites, for example. However, it turns out that the amount of carbon 14 in the
atmosphere has varied along with cosmic ray activity. It is only because it has been pos-
sible to build up an independent chronology by counting the annual growth rings in trees
that this came to light and corrections to carbon dating of up to 2,000 years could be made.
The geological column
Look at a section of sedimentary rocks in, for example, a cliff face and you will see that it is
made up of layers. Sometimes annual layers corresponding to floods and droughts are vis-
ible. More often, the layers represent occasional catastrophic events or slow but steady sed-
imentation across hundreds of thousands or even millions of years, followed by a change of
