Vertebrate Paleontology

How do we know how old dinosaur bones are?
Answered by Science Channel
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    Science Channel

  1. The radiometric dating method used to determine the age of many artifacts doesn't work for dinosaur bones, because the fossils don't contain isotopes with a long enough half-life to make that approach workable. A half-life is basically how long it takes something to decay by half of its amount. When scientists know the half-life of a particular isotope, they can extrapolate the age of the thing containing the isotope. The most common form of radiometric dating measures the decay of carbon-14 isotopes. This is very useful for dating human artifacts, but it doesn't work for dinosaurs, because all the unstable atoms in the carbon will have decayed after about 50,000 years. So the decay happens much too fast (if 50,000 years can be called fast!). To date material that is millions of years old, scientists need to look for isotopes with a much longer half-life. Among those used are uranium-238, uranium-235 and potassium-40: all have half-lives of more than a million years. But there's still a problem with dinosaur fossils -- they don't contain any of these measurable isotopes!

    Fortunately, there is another way. Dinosaur fossils are found in sedimentary rock, while isotopes like uranium-238, uranium-235 and potassium-40 -- which, remember, have the required long half-lives of more than 1 million years -- are found in igneous (volcanic) rock. So scientists look for the layers of igneous rock (typically from volcanic ash) that "sandwich" the fossil-bearing layer. These igneous rocks -- the "bread" of the sandwich -- can be dated, leaving a bracketed period of time (in between the slices of bread) in which the sedimentary rocks with the dinosaur remains must have been formed. This allows scientists to affix a rough age range to dinosaur fossils, all thanks to the layers of time stacked upon each other in the earth around us.

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