Absolute Dating Throughout the centuries mankind has striven to try to determine the exact age of Earth. What these people were seeking was a numerical date of Earth’s age. Numerical dates pinpoint the time in history when an event took place, such as the extinction of the dinosaurs 65 million years ago.

Our current understanding of radioactivity allows scientists to use the natural radioactivity of certain elements in rocks to accurately determine their numerical dates. Therefore, we can obtain numeric dates of many rocks that represent important events in Earth’s distant past.

Radioactive Half-life and Decay To determine the absolute ages of fossils and rocks, scientists analyze isotopes of radioactive elements. Isotopes are atoms of the same element that have the same number of protons, but have different numbers of neutrons. Isotopes of the same element display the same chemical attributes, but sometimes display very different physical attributes. Most isotopes are stable, meaning that they stay in their original form. Isotopes that are unstable are radioactive. For instance carbon-12 is a stable isotope while carbon-14 is radioactive because it has two extra neutrons.

Radioactive isotopes have unstable atomic nuclei that have a tendency to change, or decay, over time. This process is called radioactive decay. The figure below shows an example of how radioactive decay can occur.

The starting form of the element is called the parent isotope and the form that it changes into is called the daughter isotope. Because radioactive decay occurs at a steady rate, scientists can use the relative amounts of stable and unstable isotopes present in a rock to determine the rock’s age.

If you know the rate of decay for a radioactive element in a rock, you can figure out the absolute age of the rock. Determining the absolute age of a sample, based on the ratio of parent material to daughter material, is called radiometric dating.

The half-life is the time that it takes one-half of a radioactive sample to decay. If you have a rock sample that contains an isotope with a half- life of 10,000 years, that means that in 10,000 years, half the parent material will have decayed and become daughter material. If you analyze the sample and find equal amounts of parent material and daughter material, this means that half the original radioactive isotope has decayed and that the sample must be about 10,000 years old. If you find that ¼ of your sample is parent material and ¾ is daughter material, it took 10,000 years for half the original sample to decay and another 10, 000 years for half of what remained to decay. The age of your sample would be 20,000 years.

Potassium - Argon Method One isotope that is used for radiometric dating is potassium-40. Potassium-40 has a half-life of 1.3 billion years, and it decays to argon and calcium. Geologists measure argon as the daughter material. This method is used primarily to date rocks older than 100,000 years.

Uranium - Lead Method Uranium-238 is a radioactive isotope that decays in a series of steps to lead-206. The half-life of uranium-238 is 4.5 billion years. The older the rock is, the more daughter material (lead-206) there will be in the rock. Uranium-lead dating can be used for rocks more than 10 million years old. Younger rocks do not contain enough daughter material to be accurately measured by this method.

Rubidium - Strontium Method Through radioactive decay, the unstable parent isotope rubidium-87 forms the stable daughter isotope strontium-87. The half-life of rubidium-87 is 49 billion years. This method is used to date rocks older than 10 million years.

Carbon-14 Method The element carbon is normally found in three forms, the stable isotopes carbon-12 and carbon- 13 and the radioactive isotope carbon-14. These carbon isotopes combine with oxygen to form the gas carbon dioxide, which is taken in by plants during photosynthesis. As long as a plant is alive, new carbon dioxide with a constant carbon-14 to carbon-12 ratio is continually taken in. Animals that eat plants contain the same ratio of carbon isotopes.

Once a plant or an animal dies, however, no new carbon is taken in. The amount of carbon-14 begins to decrease as the plant or animal decays, and the ratio of carbon-14 to carbon-12 decreases. This decrease can be measured in a laboratory. Because the half-life of carbon-14 is only 5,730 years, this dating method is used mainly for dating things that lived within the last 50,000 years.