What is radiometric dating of fossils

Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth.

In 1972 this assumption was shown to be highly questionable.

It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.

This activity on determining age of rocks and fossils is intended for 8th or 9th grade students.

After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.

To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.

For inorganic materials, such as rocks containing the radioactive isotope rubidium, the amount of the isotope in the object is compared to the amount of the isotope's decay products (in this case strontium).

Before 1955, ages for the Earth based on uranium/thorium/lead ratios were generally about a billion years younger than the currently popular 4.5 billion years. old Earth is reviewed and deficiencies of the uranium/lead method are discussed.

The basic theory of radiometric dating is briefly reviewed.

Some evidence is also presented to show that radiometric results that are in agreement with the accepted geological time scale are selectively published in preference to those results that are not in agreement.

The geological time scale and an age for the Earth of 4.5 b.y.

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