Relative Age vs. Absolute Age

Interstate road cut through limestone and shale strata in East Tennessee Have you ever been driving down the road and seen rock layers exposed on a rock formation by the side of the road?

Often the layers of rocks have different colors and textures. Have you ever wondered how they formed? Remember in a previous unit you learned how sediment settles on top of sedimentary rock. Over time the sediment gets buried, hardens, and eventually forms sedimentary rock. At the exact same time the remains of organisms settle into the layers and may become fossils. Over time, the layers change as the landscape changes helping to provide a record of the Earth's geologic history.

If someone was to give you a fossil like the one pictured here you might wonder 'How old is that?"

how old is that?

There are two ways you can determine the age of the fossil. You can determine the relative age or the absolute age. Geologists in the 18^th and 19^th century studied rock layers and fossils and determined the relative age of the fossils. Relative age is the age compared to the age of other things, in this case rock layers. In the 20^th century, as scientists became aware of radioactive dating, the absolute age of rocks and fossils could be determined. Absolute age is the number of years since the rock has been formed. Now scientists were able to determine the actual ages of fossils.

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