# Correlation (Geology)

## Absolute Vs. Relative Ages Of Strata

The correlational studies described so far allow scientists to estimate the relative ages of strata. If stratum B lies above stratum A, B is the younger of the two. However determining the actual, or absolute, age of strata (for example, 3.5 million years old) is often difficult since the age of a fossil cannot be determined directly. The most useful tool in dating strata is radiometric dating of materials. A radioactive **isotope** such as uranium-238 decays at a very regular and well-known **rate**. That rate is known as its **half-life**, the time it takes for one-half of a **sample** of the isotope to decay. The half-life of uranium-238, for example, is 4.5 billion years. By measuring the **concentration** of uranium-238 in comparison with the products of its decay (especially lead-206), a scientist can estimate the age of the rock in which the **uranium** was found. This kind of **radioactive dating** has made it possible to place specific dates on the ages of strata that have been studied and correlated by other means.

David E. Newton

## Additional topics

Science EncyclopediaScience & Philosophy: *Condensation* to *Cosh*Correlation (Geology) - The Nature Of Sedimentary Strata, Physical Correlation, Interpreting Earth History Within A Stratum, Fossil Correlation