Dating a rock
Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.This information has also helped determine the age of the Earth itself.
If a zircon crystal originally crystallizes from a magma and remains a closed system (no loss or gain of U or Pb) from the time of crystallization to the present, then the Discordant dates will not fall on the Concordia curve.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*.Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.To determine the ages of these specimens, scientists need an isotope with a very long half-life.These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed.
By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them.
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.
But this sediment doesn't typically include the necessary isotopes in measurable amounts.