Archeologists use relative and absolute dating methods. They use these methods to determine the age of sites, artifacts, and the behaviors they represent.
- Relative dating arranges past events in a sequence, in relation to one another, for instance, earlier, later, more recent, and so forth.
- Absolute dating attempts to pinpoint a discrete, known interval in time such as a day, year, century, or millennia. Very few artifacts recovered from an archeological site can be absolutely dated.
Archeologists also use diagnostic artifacts to date sites. Diagnostic artifacts are indicative of a particular period or cultural group. They are placed into a sequence by style, manufacture, or other attributes to track change over time. Categories of diagnostic artifacts include stone tools, nails, glass bottles, and ceramics.
Relative Dating Examples
- Geologic Dating – Geologists develop dates for various geological stages by relating them to documented climate and geology events.
- Stratigraphy – The Law of Superposition holds that, under normal circumstances, deeper layers of archeological sites are older than the ones above them.
- Seriation – Seriation orders evidence of human behavior into a series. For example, we can order clothing or car styles over time based on style or other distinctive characteristics.
- Cross-dating – Cross-dating compares artifacts found in one place to other places to determine relative age. The artifacts might be within a site or at different sites. It allows archeologists to compare soil characteristics and artifacts within each stratum to determine their relationship relative to each other.
- TPQ Dating – Terminus post quem dating, often referred to as TPQ dating, is defined as the date after which a stratum, feature, or artifact must have been deposited. When several artifacts are recovered from a single stratum, the TPQ date corresponds with the first possible date that the latest-occurring artifact could have made its way into the ground.
- Horizon – In archeology, a horizon is a pattern characterized by widespread distribution of a complex of cultural traits that lasts a relatively short time. Events that might create the pattern of a horizon include a rapid military conquest or an effective religious mission.
Absolute Dating Examples
- Radiocarbon dating – Radiocarbon dating measures the decay of carbon. Carbon decays at a steady, predictable rate. It releases carbon-14 atoms with a half-life of 5,700 years. Scientists measure the carbon-14 to estimate age of organic objects, such as charcoal, shell, wood, bone, or hair.
- Obsidian hydration – Obsidian hydration measures the microscopic amount of water absorbed on freshly broken surfaces. The longer the surface is exposed, the thicker the hydration band will be.
- Thermoluminescence – Thermoluminescence dating is used for rocks, minerals, ceramics and burned features. It is based on the fact that almost all natural minerals are thermoluminescent—they emit light when heated. Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections. In the lab, samples are heated releasing the trapped electrons and producing light.
- Dendrochronology – Seasonal conditions affect annual tree growth, causing all trees of the same species within a given geographical region to have the same tree-ring pattern. Cross sections of cut or dead trees from a single region are compared and the tree-ring patterns are matched.
- Historical records – Government records, diaries, newspapers, or other historical records include precise dates of events.
- Mean ceramic dating – Mean ceramic dating establishes the average age of a ceramic assemblage. It is a function of known manufacturing dates and the frequency of ceramic types in the assemblage.