Determining the age of rocks and fossils.

Trilobite Sinespinaspis markhami from the Lower Silurian of the Forbes district, New South Wales. Source: Museum Victoria.

The Earth is about 4600 million years old, the oldest fossils are about 3500 million years old, and the oldest fossils with hard parts are about 545 million years old. How do we know this?

There are two methods by which we can estimate the age of rocks and the fossils contained in them:

Relative Dating is based on the study of rock strata and the order of appearance of the fossils contained in them.

Absolute (Radiometric) Dating is based on the rate of decay of radioactive elements in rocks.

Relative Dating

By examining quarries, cliffs, road cuttings and drill cores, geologists can plot the different layers of rock that make up the Earth's crust. These formations can be mapped across the country and may be named after an area where they are well displayed.

By assuming that any layer of rock is younger than the layer beneath it (except where the rock layers have been overturned by folding) ancient geological events were described sequentially using the names of the Geological Time Scale. Although the order of geological events could be interpreted, the age in years was unknown. For instance, the rocks at the bottom of a cliff were logically interpreted to be older than the rocks at the top. However, prior to radiometric dating, no one could guess the size of the time gap between the top and bottom layers of rock.

Absolute (Radiometric) Dating

Radioactive decay allows geologists and physicists to measure the age of ancient fossils, rocks and even the Earth. This process is called radiometric or radioactive dating.

Radiometric dating is possible because the radioactive decay of large numbers of radioactive atoms follows a predictable pattern. This predictability allows scientists to measure the age of an object (e.g. a mineral in a rock) if they can work out how many radioactive atoms were originally present. Two situations where we can do this involve Potassium-40 atoms and Carbon-14 atoms.

All radioactive atoms decay to become a more stable kind of atom. The original radioactive atom is known as a parent isotope, while the atom produced by the decay process is known as a daughter isotope.

Isotopes are different forms of the same element. For example Uranium-235 and Uranium-238 are both Uranium atoms with the same number of protons, but they have a different number of neutrons. The number used to identify the isotope refers to the total number of particles in the nucleus of each atom.

Some of the different kinds of radioactive atoms used to date objects are shown in the following table: