

On this Site

Common Types of Radiometric Dating Carbon 14 Dating As shown in the diagram above, the radioactive isotope carbon14 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 carbon14 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 halflife of carbon14 is 5730 years, scientists can measure the age of a sample by determining how many times its original carbon14 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 nitrogen14), age is hard to determine since the halflife does not make up a significant percentage of the material's age. The range of practical use for carbon14 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 halflives in all types of radiometric dating: PotassiumArgon Dating The isotope potassium40 (k40) 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 rockformation process, and therefore any argon in a rock sample should have been formed as a result of k40 decay. The halflife of this process is 1.25 billion years, meaning that it can date significantly older samples. RubidiumStrontium Dating In rubidiumstrontium dating a rubidium87 isotope becomes the daughter product strontium87. In an igneous rock formation, the entirety of the cooled rock will have the same ratio of strontium87 and strontium86 (another stable isotope). This means that as the rubidium87 decays and more strontium87 is formed, the ratio will change. The halflife of rubidium87 is 48.8 billion years, meaning it can accurately measure rocks as old as the Earth itself. UraniumLead Dating Uraniumlead dating is one of the most complicated of all dating techniques. This is in part because uranium and lead are not retained in rocks as easily as some others, and in part because the parent isotopes and daughter products are not even directly related. For the isotopes uranium235 and uranium238 to respectively become lead207 and lead206, they must first undergo a serious of highly unstable transformations into isotopes with very short halflives. However, if one knows the scientific formula for interpreting these transitions, the results can be "highly precise" according to paleontologist Guy Narbonne (Kerr, 789). The halflife of uranium235 is 704 million years, while the halflife of uranium238 is 4.5 billion years. 

[Links to all class members sites will be inserted here.] This website
is part of a Geology 211 class project on Processes in Physical Geology.
Copyright ©2001 Earlham College. Revised 25 February 2002. Send corrections or comments to parkero@earlham.edu 