The carbon in their bodies at the time of their death will remain in their bodies until they decompose, or if they become fossilized, then forever. This allows scientists to look at the amount of decay in a fossil’s radioactive carbon and determine a relative date.

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Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.

Radiometric dating (often called radioactive dating) is a way to find out how old something is.

The method compares the amount of a naturally occurring radioactive isotope and its decay products, in samples. It is the main way to learn the age of rocks and other geological features, including the age of the Earth itself.

Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.

In a way this field, called geochronology, is some of the purest detective work earth scientists do.

It may be used to date a wide range of natural and man-made materials.

Fossils may be dated by taking samples of rocks from above and below the fossil's original position.

The object's approximate age can then be figured out using the known rate of decay of the isotope.

For organic materials, the comparison is between the current ratio of a radioactive isotope to a stable isotope of the same element and the known ratio of the two isotopes in living organisms.

However, scientists can look at the decay of other elements in these objects allowing them to date them up to 2.2 billion years.