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Biology - Absolute dating

Absolute dating

Radiometric dating using radioactive isotopes: Any radioactive elements
in a rock are unstable and 'decay' at a steady, measurable rate. The rate
is referred to as its _half life,_ that is, the time it takes for half the
amount of the parent radioactive substance to decay to form the daughter
isotope. By measuring the amount of daughter isotope relative to parent
isotope the age of the material can be determined.

The oldest rocks on Earth have been dated at about 4.5 thousand million
years old.

Biological material such as bone, hair and skin is organic and can be
dated using the carbon dating method. During life the ratio of 14C to 12C
remains constant but after death the amount of 14C decreases due to
radio-active decay. The less the amount of 14C in relation to 12C the
longer ago the death of the organism. Carbon dating can be used on material
up to about 50 000 years old.

PARENT ISOTOPE
DAUGHTER ISOTOPE
HALF-LIFE

carbon - 14
nitrogen - 14
5 730 years

uranium - 235
lead - 207
700 million years

potassium - 40
argon - 40
1250 million years

uranium - 238
lead - 206
4 500 million years

rubidium - 87
strontium - 87
50 000 million years

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