Geologic Dating Definition

Geologic dating, or geochronology, is used to obtain the age of such specimens as rocks, fossils and sediments. Built into each dating method is a particular range of uncertainty; thus a specimen age doesn't always equal an exact age. Dating methods can be either absolute or relative.

Types

Dating geological specimens involves an interdisciplinary approach using more than one dating method and cross-validating the results. Absolute dating methods include radiometric, luminescence and incremental dating. Relative dating methods fall under the science of stratigraphy.

Radiometric Dating

Radiometric dating is based on the knowledge that certain naturally occurring radioactive isotopes decay, or transform into a different element, at known rates. Specimen age can then be determined by measuring the proportion of the radioactive materials remaining compared to the proportion of the "decayed" element. The half-life, or amount of time it takes for half of the measured isotope to decay, must agree with age of specimen. In other words, an isotope with a very short half-life can't be used to date very old fossils.

Radiometric dating includes carbon dating, used to date specimens up to about 75,000 years old, and potassium-argon and rubidium-strontium dating, which are used to date much older fossils. Using radiometric dating we know the earth is about 4.5 billion years old.

Luminescence Dating

Geologists and archeologists use luminescence dating by observing photons, or light, emitted from minerals such as quartz, diamond, feldspar and calcite. As radiation from photons is stored in sediment layers, age since last exposure to sunlight can be calculated from this information.

Incremental Dating

Incremental dating incorporates several techniques including dendrochronology, ice cores and varve analysis. Dendrochronology (tree-ring dating) is used to date wood fossils. Shallow ice cores are dated exactly by counting layers; each layer represents a year. Varve analysis is used to date archaeological specimens based on patterns of glacial deposit.

Stratigraphy

This method is based on the assumption that geological layers can provide relative ages for the specimens found within them, provided that the deeper rock layers formed earlier than the shallower ones. Sequences of rock layering reveal the general patterns that describe the geochronology of the planet. Because of the way fossils are formed, fossil specimens found in rock layers must be older than the surrounding rock. Support for evolutionary theories is found in part from the observation that rock layers of similar age contain fossils of similar flora and fauna.

References

• Biology: Concepts and Connections; Neil A. Campbell; 2009
• USGS
• Stratigraphy