Geochronology is the science of determining the age of rocks, fossils, and sediments. A variety of dating methods are used by geologists to achieve this.

Dating methods

• Radiometric techniques measure the decay of radioactive isotopes, and other radiogenic activity.
• Incremental techniques measure the regular addition of material to sediments or organisms.
• Correlation of marker horizons allow age-equivalence to be established between different sites.

Radiometric dating

By measuring the amount of radiocative decay of a radioactive isotope with a known half-life, geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.

• Radiocarbon dating. This technique measures the decay of Carbon-14 in organic material (e.g. plant macrofossils), and can be applied to samples younger than about 50,000 years.
• Uranium-lead dating. This technique measures the ratio of two lead isotopes (Pb-206 and Pb-207) to the amount of uranium in a mineral or rock. Often applied to the trace mineral zircon in igneous rocks, this method is one of the two most commonly used (along with argon-argon dating) for geologic dating. Uranium-lead dating is applied to samples older than about 1 million years.
• Uranium-thorium dating. This technique is used to date speleothems, corals, carbonates, and fossil bones. Its range is from a few years to about 700,000 years.
• Potassium-argon dating and argon-argon dating. These techniques are used to date igneous and volcanic rocks. They are also used to date volcanic ash layers within or overlying paleoanthropologic sites. The younger limit of the argon-argon method is a few thousand years.

Other radiogenic dating techniques include:

• Fission track dating
• Cosmogenic isotope dating
• Rubidium-strontium dating
• Samarium-neodymium dating
• Rhenium-osmium dating
• Lutetium-hafnium dating
• Paleomagnetic dating

Incremental dating

Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be fixed (i.e. linked to the present day and thus calendar or sidereal time) or floating.

• Dendrochronology
• Ice cores
• Lichenometry
• Varves