Geological dating radioactive elements
It often proves possible to date even severely disturbed rocks.
To date the time of formation of a planet 12 740 km in diameter and 70.8% covered by water is not easy.
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.
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.
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.
It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age.
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).
They have clearly fallen to Earth from outside, often gouging out huge craters such as that called New Québec (61°17´ N, 73° 41' W).
Rubidium-strontium, potassium-argon, uranium-lead and samarium-neodymium dating all show that the meteorites formed about 4.6 billion years ago.