The Earth certainly must be older than the oldest terrestrial rocks found.
Samuel Bowring, now of the Massachussetts Institute of Technology, and his coworkers Ian Williams and William Compston of the Australian National University at Canberra have shown that a small area of metamorphic rock in northern Canada, known as the Acasta gneiss, is the oldest known intact solid piece of the Earth's crust.
Charles Darwin reinforced this idea by pointing to the time that must have been required for the EVOLUTION of advanced life from primitive forms.
On the other hand, the great physicist Lord Kelvin vehemently objected and suggested that the Earth might only be a few tens of millions of years old, based on his calculations of its cooling history.
A good example is rubidium-87, which changes to strontium-87 at a rate of one-half every 50 billion years.
Therefore, a rock can be dated by measuring how much of its original rubidium content has changed into strontium.
These results have been confirmed and agreement has been found among the rubidium-strontium, uranium-lead and samarium-neodymium methods.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.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).