How Archaeologists Determine the Age of Fossils and Artifacts

When archaeologists say an artifact is '3,000 years old' or a fossil-bearing layer is '1.8 million years old', that number comes from one or more scientific dating techniques, not a single universal method. Dating methods fall into two categories: relative dating, which establishes whether one object or layer is older or younger than another, and absolute (or chronometric) dating, which estimates an actual age range using measurable physical or chemical processes, according to the U.S. National Park Service.

Relative dating: establishing order, not exact years

The oldest and simplest relative method is stratigraphy, based on the Law of Superposition: in an undisturbed site, deeper soil layers are older than the layers above them, the National Park Service explains. Excavators remove soil in the reverse order it was deposited, so the sequence of layers gives a relative timeline even before any scientific test is run.

Archaeologists also use seriation, which arranges pottery, tools or other artifact styles into a sequence based on how their design features changed over time, and the related idea of terminus post quem, which uses the most recent datable object in a layer to set the earliest possible date for everything found with it. Cross-dating compares finds between sites to link their relative chronologies, per the National Park Service.

Radiocarbon dating: the most widely used absolute method

Radiocarbon (carbon-14) dating works because living organisms continuously exchange carbon with the atmosphere, maintaining a steady ratio of radioactive carbon-14 to stable carbon-12, according to the University of Oxford's Research Laboratory for Archaeology and the History of Art, which operates a radiocarbon dating service. Once an organism dies, it stops absorbing carbon, and its carbon-14 decays at a known rate, allowing scientists to calculate elapsed time from the remaining ratio.

The method applies only to material that was once part of a living organism — bone, wood, charcoal, shell, hair or similar remains — not to stone or metal unless organic residue is attached, the Oxford lab and the National Park Service both note. Carbon-14's half-life is cited as roughly 5,730 years by National Geographic and about 5,700 years by the National Park Service.

On the usable range, sources give slightly different limits: the Oxford lab states the method spans from a few hundred years ago to about 50,000 years, while National Geographic reports that samples older than roughly 40,000 years become very difficult to date and those beyond 60,000 years cannot be dated at all. Raw results also need calibration because atmospheric carbon-14 levels have varied over time, and results can be skewed by soil contamination or by the 'reservoir effect' seen in marine samples, per the Oxford lab.

Dating rock, wood and fired clay: other absolute methods

For volcanic rock associated with archaeological or fossil layers, scientists use potassium-argon dating or its refined version, argon-argon (40Ar/39Ar) dating, which tracks the decay of potassium-40 into argon-40, according to the U.S. Geological Survey. The 40Ar/39Ar variant irradiates a sample to convert some potassium into argon-39, letting researchers measure two argon isotopes from a single sample rather than two separate elements, which improves accuracy. The USGS notes this approach works best on potassium-rich crystals such as sanidine and has been applied to eruptions from roughly 70,000 years ago to events within the last 12,000 years.

Wooden artifacts and structures can be dated through dendrochronology, or tree-ring dating. The University of Arizona's Laboratory of Tree-Ring Research explains that simply counting rings is unreliable because trees can produce locally absent rings or false bands that resemble true growth rings. Instead, specialists use crossdating — matching the pattern of wide and narrow rings, which reflects yearly growing conditions, across multiple wood samples from the same region — often through a technique called skeleton plotting, to assign each ring an exact calendar year.

Other absolute techniques listed by the National Park Service include thermoluminescence, which measures electrons trapped in fired clay or heated stone by background radiation and released as light when the material is reheated in a lab; obsidian hydration, which measures the thickness of a water-absorption layer on a freshly broken obsidian surface; and mean ceramic dating, which calculates an average site date from the known production spans of different pottery types found there.

Why archaeologists combine several methods

No single technique covers every material or every time span, which is why excavation teams typically pair relative methods such as stratigraphy with one or more absolute methods suited to the materials actually present — organic remains for radiocarbon, volcanic deposits for potassium-argon, wood for dendrochronology, or fired clay for thermoluminescence. Cross-checking results from different methods, and against historical records where they exist, helps archaeologists identify errors and narrow down the final age estimate reported for a site or object.