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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.

The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope.

Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.

Each of them typically exists in igneous rock, or rock made from cooled magma.

Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.

The energies involved are so large, and the nucleus is so small that physical conditions in the Earth (i.e. The rate of decay or rate of change of the number N of particles is proportional to the number present at any time, i.e.

magazine has been continuously published since 1978, we are publishing some of the articles from the archives for historical interest, such as this.

For teaching and sharing purposes, readers are advised to supplement these historic articles with more up-to-date ones suggested in the Related Articles and Further Reading below.

Despite this, the momentum gained in the two decades prior to 1972 has made 4.5 b.y.

a popularly accepted “universal constant” even though the foundations on which it was based have been virtually removed.

Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.

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