Absolute dating rock layers
To find their age, two major geological dating methods are used.These are called relative and absolute dating techniques.You can then infer that the dino must have lived some time between these two age dates.The most widely known form of radiometric dating is carbon-14 dating.This Science Struck post enlists the differences between the absolute and relative dating methods.Geological specimens that are unearthed need to be assigned an appropriate age.
While the oldest known rocks on Earth are about 3.5 billion years old, researchers have found zircon crystals that are 4.3 billion years old [source: USGS].
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.
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.