What Do Geologists Use To Determine The Absolute Age Of Rocks & Fossils

Absolute age allows scientists to tell what era fossils are from.

When geologists and archaeologists measure the age of rocks and fossils, they can do so in either relative age or absolute age. Relative age determines whether something is older or younger than something else; absolute age allows you to assign a time (such as years, months or minutes) to the age of an object. Different methods are used for rocks and fossils because of their composition and the rate of decay of the isotopes in each one.

The Geologic Column

Using the results of detailed stratigraphic, paleontological and correlation studies, geologists have developed a geologic column. This provides era, periods and the relative and absolute ages of rocks from those periods.

Radiometric Dating

Radiometric dating relies on radioactive isotopes, which decay at a constant rate, and radiogenic isotopes, which are formed by the radioactive decay of the radioactive isotopes. The decay rate of radioactive isotopes is known as isotope’s half-life, which is the amount of time it takes one half of the radioactive isotope to decay to the radiogenic isotope.

Radiocarbon Dating

Radiocarbon dating can’t be used to find the absolute age of rocks, but it can be used to find the absolute age of organic material, meaning fossils such as bone or wood, because they have carbon. It is widely used in dating and was developed in 1949. Atmospheric nitrogen is broken down by radiation into Carbon 14, or C-14, an unstable carbon isotope. Through photosynthesis, C-14 becomes part of plants and is then eaten by animals. While an animal or plant is alive, the ratio of the C-14 stays the same as in the atmosphere, but when it dies, the C-14 begins to decrease. That halves itself every 5,730 years.

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Simple Counting

Sometimes scientists can use sediment layers to determine an artifact’s or rock’s absolute age. If known historic events mark the layer in which an artifact is found, scientists can mark the time it has come from. This strategy was used for dating the age of artifacts found in the Nile River.

Pattern Matching

In areas with glaciers, the ice melts every spring and summer. These waters carry sediment that was trapped in the ice. When these waters flow into lakes, that sediment is deposited on the lake floor. Two layers appear; the coarse sediment appears in a thick layer and the fine sediment that remains suspended in the water forms a thin layer on top. Counting these layers and comparing the patterns of the layers allows scientists to reveal the absolute age of any layer within the lake basin.