Oldest Rocks

The proven age of the Earth is finally approaching the known age of meteorites, 4.5 billion years old. It's believed on astronomical grounds that meteorites were formed at the same time as the Earth. Samples of basement rock recovered in South Africa have been found by the uranium-lead dating method to be at least four billion years old. The oldest rock previously known was a specimen of mica, estimated to 3.4 billion years old, found near Murmansk in the U.S.S.R. in 1959.

The revised chronology pushes back several of the major subdivision of past scales. The beginning of the Cambrian period (the era of the earliest marine fossils) is now set at 600 million years ago, instead of 560 million, and the end of the Triassic (the age of the first Dinosaurs) has been fixed at 190 million years ago.

The new times were arrived at with the aid of improved procedures for measuring the radioactive decay of U238, Rb87 and K40.

The techniques now available can date suitable specimens of almost any period in the Earth's history with an error of a few per cent. The remaining difficulties are with samples between 50.000 and 1 million years old.