The most common methods for determining the age of fossils are biostratigraphy and radiometric dating.


Biostratigraphy is a way of determining the relative ages of different fossil species by looking at how layers, or strata, of sedimentary rocks are positioned relative to one another.

It is a form of relative dating.

Generally, we can assume that lower layers of sedimentary rocks are older than higher layers.

Paleontologists determine how old a fossil is by looking at the sedimentary rocks above and below it.

They know that that the organism lived before the organisms found in the rocks below it and after those found in the rocks above it.

Index fossils are often used in biostratigraphy.

These are fossils that are used to identify geological periods.

Index fossils are fossils from common, easily identifiable species that lived in wide ranges during restricted time periods.

Some index fossils are trilobites, ammonites, corals, sea urchins and brachiopods.

If two different sediments contain the same species of index fossil, we know that both of these sediments were deposited within a relatively short (geologically speaking) time frame.

Radiometric dating

Radiometric dating, a type of absolute dating, is the process of using radioactive isotopes to determine how old an object is.

Isotopes are atoms of the same element that have the same number of protons and electrons, but a different number of neutrons.

Because they have the same number of charged particles (protons and electrons), the chemical properties of different isotopes of the same element are almost identical.

Some isotopes are radioactive. This means that the nucleus (the part of the atom that includes the protons and neutrons) is unstable and breaks down over time. When the nucleus of a radioactive isotope decays, it releases particles from the nucleus as well as radiation.

Because the decay of the isotope changes the composition of the nucleus, the isotope is transformed into another isotope of the same element or into a different element. This new isotope or element is known as the decay product. Sometimes, the decay product can itself be radioactive, and break down to form another decay product. This can result in a decay chain, in which a sequence of radioactive decay products are formed until finally, a non-radioactive isotope is produced.

Different isotopes break down at different rates. If you have a particular quantity of a radioactive isotope, the amount of time it takes for half of it to break down is known as that isotope's half-life.

If you know the half-life of a radioactive isotope and you know the proportion of that isotope to its decay product in an object, you can calculate the age of that object. .

Radiometric dating can be used to determine the age of rocks near a fossil. This information can be used to find out the age of the fossil.

The half-life of an isotope determines the timescale in which radiometric dating is accurate.

Therefore, different radioactive isotopes are used to study the ages of objects from different time periods.

Carbon Dating

Carbon dating is a well known method of radiometric dating.

Willard Libby of the University of Chicago developed the technique of carbon dating in 1949. In 1960, he received the Nobel Prize in chemistry for his work.

The element carbon has two non-radioactive isotopes, carbon-12 and carbon-13. (The numbers 12 and 13 refer to the atomic weight of the isotope, which largely depends on the total number of protons and neutrons in the nucleus.)

Carbon also has a radioactive isotope, carbon-14, which exists in relatively small amounts on Earth. About one in one trillion carbon atoms on Earth is a carbon-14 atom.

Carbon-14 is constantly being formed in the Earth's atmosphere as the result of an interaction between cosmic rays and nitrogen in the atmosphere.

Atoms of carbon-14 combine with oxygen to form carbon dioxide. Plants absorb carbon dioxide, some of which has carbon-14 atoms, from the atmosphere. When animals, including human beings, eat plants, they take in these carbon-14 atoms. The animals that eat those animals also ingest carbon-14, and so on through the food chain.

The ratio of carbon-12 (the most common isotope of carbon) and carbon-14 in any living thing on Earth is practically constant. Even though carbon-14 atoms are always undergoing radioactive decay, they are constantly being replaced by new carbon-14 atoms.

As soon as a living thing dies, however, it stops taking in carbon. Therefore, the carbon-14 that decays is no longer replaced. This means that as time passes, the ratio of carbon-12 to carbon-14 increases.

By comparing the ratio of carbon-12 to carbon-14 to the ratio of those isotopes in a living thing, scientists can calculate when the organism died.

The half-life of carbon-14, which about 5,730 years, must be used in this calculation.

Because of the length of its half-life, carbon-14 can be used to date organic materials, such as bones, wood and lead, that are up to about 60,000 years old.

Uranium-Thorium Dating

Uranium-238 breaks down to form thorium-230.

Uranium-Thorium dating utilizes this process.

Uranium-238 and thorium-230 are both radioactive. The half-life of uranium-238 is about 4.5 billion years, while the half-life of thorium-230 is only about 75,000 years.

Uranium is soluble in water but thorium is not, so uranium-thorium dating is most useful for measuring the age of objects that come from the oceans.

Uranium-thorium dating can be used to find the ages of rocks, cave deposits, soil, coral, wood, and bones.

It can determine the ages of objects that are up to about 500,000 years old.

Potassium-Argon Dating

Potassium-argon dating is based on the decay of postasium-40 to argon-40.

Potassium-40 has a half-life of 1.3 billion years.

Potassium-argon dating can accurately date objects between about 100,000 years old and 4.5 billion years old.

Uranium-Lead Dating

Uranium-lead dating is based on two decay processes: The decay of uranium-235 to lead-207 and the decay of uranium-238 to lead-206.

Uranium-235 has a half-life of about 700 million years and uranium-238 has a half-life of about 4.5 billion years.

Uranium-lead dating is used for geologic samples that are between about 1 million years and 4.5 billion years old.

The mineral zircon, whose crystalline structure includes uranium but not lead, is usually used for uranium-lead dating. Any lead in the zircon must have been produced by the radioactive decay of uranium.