Dating fussen geological

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.

The vast majority of atoms (each composed of a nucleus surrounded by electrons) are stable. Because of this particle emission, the original radioactive parent atom changes its identity, becoming a different, stable daughter atom.

This change takes place at a known rate determined by the half-life; ie, the time required for one-half of the original number of radioactive atoms to convert to the stable daughter product.

The other key dating techniques involve uranium-235 transforming to lead-207 at a rate of one-half every 713 million years, uranium-238 becoming lead-206 at one-half every 4.5 billion years, potassium changing to argon (and calcium) at one-half every 1.3 billion years and samarium-147 becoming neodymium-143 at one-half every 106 billion years.

It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age.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been possible to work out a time scale of the reversals of the Earth's magnetic field.This "radiometric" approach has superseded all other techniques for determining absolute ages.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.For centuries people have argued about the age of the Earth; only recently has it been possible to come close to achieving reliable estimates.In the 19th century some geologists realized that the vast thicknesses of sedimentary rocks meant that the Earth must be at least hundreds of millions of years old.

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