Radioactive elements decay at a certain constant rate and this is the basis of radiometric dating. But, the decay elements need to be set, much like you would re-set a stop watch for a runner, to ensure an accurate measurement. When minerals get subducted into the Earth and come back as volcanic magmas or ash, this essential re-sets the radiometric clock back to zero and therefore a reliable age date is possible. Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence. For example, if you find a dinosaur bone in a sedimentary sequence and you find an ash layer 10 meter above the bone and another ash layer 20 meters below it, you can determine the age of the two ash layers.
And, at the same time as there are quite a few diverse fossils in a rock their man or woman ranges yield a constrained age span for that rock. A radioactive rock that would nicely be dated would lie upon a mattress that has no such radioactivity, yet is loaded with fossils. A mattress below that one would no longer have fossils nor radioactivity, yet when there's a mattress below that which will be easily dated then the intervening one has to have an intermediate age.
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Why is it difficult to date sedimentary rocks using radiometric dating techniques?
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The half-life of carbon is only 5, years, so carbon dating is only effective on samples that are less than 50, years old.
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.
Some of the isotopes used for this purpose are uranium, uranium and potassium , each of which has a half-life of more than a million years. Unfortunately, these elements don't exist in dinosaur fossils themselves.
Why cant radioactive dating be used to date sedimentary rocks
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. Fossils can't form in the igneous rock that usually does contain the isotopes. The extreme temperatures of the magma would just destroy the bones.