Absolute dating rock layers
So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash.
These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed.
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-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.
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.
This Science Struck post enlists the differences between the absolute and relative dating methods.
Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.
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.
This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.
Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.