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 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:
| Geological Application |
Parent Isotope |
Half-life (years) |
Daughter Isotope |
| Fossil wood, shell, bone, fabric and ash between 1000 and 70 000 years old |
Carbon-14 |
5570 |
Nitrogen-14 |
| Uranium ores and granite rocks more than 110 000 years old |
Uranium-235 |
713 million |
Lead-207 |
| Potassium-bearing minerals more than 100 000 years old |
Potassium-40 |
1300 million |
Argon-40 |
| Uranium ores and granitic rocks more than 10 million years old |
Uranium-238 |
4510 million |
Lead-206 |
| Uranium ores and granitic rocks more than 50 million years old |
Thorium-232 |
13 900 million |
Lead-208 |
| Some granitic rocks, sandstones, igneous, sedimentary and metamorphic rocks more than 10 million years old |
Rubidium-87 |
47 000 million |
Strontium-87 |
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Potassium-Argon Dating
Potassium atoms are used to date rocks that have formed from molten rock. Radioactive Potassium decays into Argon. Argon is an inert gas—it does not chemically bond to other atoms. Argon in molten rock can just bubble out and escape. Once the rock cools and solidifies, Argon that is formed by radioactive decay is trapped inside. As no Argon was present in the rock when it first solidified, all Argon in the rock is due to the radioactive decay of Potassium. By measuring the amounts of Potassium and Argon present we can date volcanic rocks that are millions of years old.
Carbon Dating
Another important dating technique is Carbon-14 dating. This is used to date the remains of things that were once living. All living things absorb Carbon from the atmosphere into their bodies, and excrete Carbon back into the environment. A small percentage of this Carbon is the radioactive form, Carbon-14.
While an organism is alive this exchange of Carbon continues, and the percentage of Carbon-14 in its body is the same as the percentage of Carbon-14 in the atmosphere. Once the organism dies, this exchange stops and the Carbon-14 that decays is not replaced.
By measuring the percentage of Carbon-14 left, and comparing it to the percentage of Carbon-14 in the atmosphere, we can date organic specimens up to 70000 years old.
This involves knowing how much Carbon-14 was present in the atmosphere at the time the organism died. Scientists know that this has changed over the last few centuries with the burning of fossil fuels, but are confidant that no other significant changes have occurred in the last 100 000 years.
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 Volcanic rock can be dated
Organic specimens can be Carbon dated |
