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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

KEY TERMS

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Bedding plane

—The top of a layer of rock.

Deposition

—The accumulation of sediments after transport by wind, water, ice, or gravity.

Facies

—A body of sedimentary rock with distinctive characteristics.

Fossil correlation

—The matching of sedimentary strata based on fossils present in the strata.

Lateral continuity

—The principle that sedimentary strata are three-dimensional features that extend horizontally in all directions and that eventually terminate against the margin of other strata.

Physical correlation

—The matching of sedimentary strata based on the physical characteristics of rocks that make up the strata.

Radiometric dating

—A process by which the age of a rock can be determined by studying the relative concentrations of a radioactive isotope and the products formed by its decay.

Superposition

—The principle that a layer of rocks is older than any other layer that lies above it and younger than any other layer that lies below it.

Additional topics

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