Geoscience Reference
In-Depth Information
tope has a characteristic half-life, a time over which half of any given sample of the isotope
will have decayed. By itself, that's not much use unless you know the precise number of
atoms you start with. But, by measuring the ratios of different isotopes and their products
it is possible to get surprisingly accurate dates. Early in the 20th century, Ernest Rutherford
caused a sensation by announcing that a particular sample of a radioactive mineral called
pitchblende was 700 million years old, far older than many people thought the Earth to be
at that time. Later, Cambridge physicist R. J. Strutt showed, from the accumulation of he-
lium gas from the decay of thorium, that a mineral sample from Ceylon (now Sri Lanka)
was more than 2,400 million years old.
Uranium is a useful element for radio dating. It occurs naturally as two isotopes - forms
of the same element that differ only in their number of neutrons and hence atomic weight.
Uranium-238 decays via various intermediaries into lead-206 with a half-life of 4,510 mil-
lion years, whilst uranium-235 decays to lead-207 with a 713-million-year lifetime. Ana-
lysis of the ratios of all four in rocks, together with the accumulation of helium that comes
from the decay process, can give quite accurate ages and was used in 1913 by Arthur
Holmes to produce the first good estimate of the ages of the geological periods of the past
600 million years.
Some radioisotopes used for dating
