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Two approaches have been developed to circumvent these problems.The first involves sampling as much of the Earth's crust as possible and dating these rocks.This uniformity demonstrates that the principle is reliable.
The well-known carbon-14 method involves the conversion of radioactive carbon-14 to stable nitrogen at a rate of one-half about every 5700 years.
It can only be used to date organic matter, and is accurate only for materials younger than about 50 000 years (see ARCHAEOLOGY; GLACIATION).
These discussions were rendered obsolete by the discovery of radioactivity in 1896 by the French physicist Henri Becquerel.
The existence of radioactivities of various kinds in rocks has enabled earth scientists to determine the age of the Earth, the moon, meteorites, mountain chains and ocean basins, and to draw up a reasonably accurate time scale of evolution.
Rocks of almost this age have also been identified in other localities, including Labrador, Minnesota, Africa and India.
Many scientists are searching for rocks older than these, and in 1983 Australian scientists claimed to have discovered minute zircon crystals 4.2 billion years old.It has even been possible to work out a time scale of the reversals of the Earth's magnetic field.This "radiometric" approach has superseded all other techniques for determining absolute ages. Their nuclei tend to emit particles spontaneously - ie, they are radioactive.Since 1950, radiometric methods have been developed to a very sophisticated level in several countries, including Canada.It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age.They were found, however, in much younger sediment and it is not known where these zircons originated.