Physical Geology 2002

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Carbon 14 Dating

As shown in the diagram above, the radioactive isotope carbon-14 originates in the Earth's atmosphere, is distributed among the living organisms on the surface, and ceases to replenish itself within an organism after that organism is dead. This means that lifeless organic matter is effectively a closed system, since no carbon-14 enters the organism after death, an occurrence that would affect accurate measurements. In radiometric dating, the decaying matter is called the parent isotope and the stable outcome of the decay is called the daughter product.

Since the half-life of carbon-14 is 5730 years, scientists can measure the age of a sample by determining how many times its original carbon-14 amount has been cut in half since the death of the organism. For example, an object with a quarter of its original amount (2x1/2) should be roughly 11,460 years old.

In all radiometric procedures there is a specific age range for when a technique can be used. If there is too much daughter product(in this case nitrogen-14), age is hard to determine since the half-life does not make up a significant percentage of the material's age. The range of practical use for carbon-14 dating is roughly a few hundred years to fifty thousand years.

The equation (called the 'age equation') below shows the relationship of parent/daughter atoms to half-lives in all types of radiometric dating:

Potassium-Argon Dating

The isotope potassium-40 (k-40) decays into a fixed ratio of calcium and argon (88.8 percent calcium, 11.2 percent argon). Since argon is a noble gas, it would have escaped the rock-formation process, and therefore any argon in a rock sample should have been formed as a result of k-40 decay. The half-life of this process is 1.25 billion years, meaning that it can date significantly older samples.

Rubidium-Strontium Dating

In rubidium-strontium dating a rubidium-87 isotope becomes the daughter product strontium-87. In an igneous rock formation, the entirety of the cooled rock will have the same ratio of strontium-87 and strontium-86 (another stable isotope). This means that as the rubidium-87 decays and more strontium-87 is formed, the ratio will change. The half-life of rubidium-87 is 48.8 billion years, meaning it can accurately measure rocks as old as the Earth itself.