Scientists determine the age of fossils using radiometric dating.
Modern scientists determine the age of a fossil using a procedure called radiometric dating. This process requires the use of a mass spectrometer to determine the radioisotopes present in a rock sample. Radioisotopes are atoms that decay and become different atoms. One example is uranium-235, which eventually decays into lead. This decay occurs at a predictable rate, which allows a scientist to determine the age of a fossil based on the uranium-235-to-lead ratio in a rock sample. Fossils form in sedimentary rock, a type of rock that usually does not contain radioactive isotopes, so scientists must obtain rock samples that do from the same geologic layer where the fossil was found. The age of the fossil will be very close to that of the other rocks in its geologic layer.
Instructions
1. Locate a sample of igneous rock from the same rock-layer where you found your fossil. The closer the igneous rock to the original location of the fossil, the better. The igneous rock should be taken from a depth identical to the original depth of the fossil.
2. Run a sample of the igneous rock you collected through your mass spectrometer to determine the ratio of uranium-235-to-lead atoms. Be sure to follow the instructions that came with your mass spectrometer closely, as one can be very different from another. If you do not have your own mass spectrometer, you can send your sample of rock to a lab to have it tested.
3. Calculate the proportion of uranium-235 to lead atoms present in your igneous rock sample. A laboratory with a mass spectrometer can supply this information for you. Otherwise, use this equation:
Proportion = A / (A + B) x 100 percent
A = number of uranium-235 atoms
B = number of lead atoms
4. Use the chart in the first reference below to determine the age, in half-lives, of your fossil. The half-life of uranium-235 is 707 million years.
Example: If uranium-235 still represents 75 percent of the total uranium-235 and lead present in your sample, then the sample has an age of roughly 1/3 of a half-life, which is:
1/3 x 707 million years = 235.7 million years
Tags: igneous rock, mass spectrometer, million years, rock sample, your mass, your mass spectrometer
