How Archaeologists Actually Date Things: Radiocarbon, Potassium-Argon, and Tree Rings Compared

Absolute versus relative dating

Archaeological dating methods split into two fundamentally different categories. Relative dating establishes sequence — this layer is older than that layer, this artefact style predates that one — without assigning calendar years to anything. Absolute (or "chronometric") dating assigns an actual age estimate, typically with an uncertainty range, based on some physical or chemical process that changes at a known, measurable rate. Radiometric methods are the largest family within absolute dating, and they all share the same underlying logic: measure how much of an unstable isotope has decayed into its stable daughter product, and use the known decay rate to work backward to elapsed time.

Radiocarbon dating: the workhorse for the last 50,000 years

Radiocarbon (carbon-14) dating works because living organisms continuously exchange carbon with the atmosphere while alive, maintaining a roughly constant ratio of radioactive carbon-14 to stable carbon-12 that matches the atmosphere's ratio. Once an organism dies, that exchange stops, and the carbon-14 already present begins decaying at a fixed rate, with a half-life of about 5,730 years — meaning half of any given sample's carbon-14 will have decayed after that period, half of the remainder after another 5,730 years, and so on. Measuring the remaining carbon-14 ratio in a sample of bone, charcoal, wood, or other organic material and comparing it to the known atmospheric ratio gives an age estimate.

The method is limited to organic material — it cannot date stone tools or ceramics directly, only organic material found in reliable association with them — and its useful range tops out around 50,000 years, because beyond roughly nine or ten half-lives the remaining carbon-14 becomes too sparse to measure reliably against background noise. Under good conditions, typical precision is in the range of ±50 to ±100 years, though raw radiocarbon ages also need calibration against independently dated records (most importantly tree rings) to correct for historical fluctuations in atmospheric carbon-14 concentration, which was not perfectly constant over time as the simple model assumes.

Potassium-argon dating: reaching back millions to billions of years

Potassium-40 decays into argon-40 with a half-life of about 1.25 billion years — enormously longer than carbon-14's — which makes potassium-argon dating suited to a completely different part of the timescale: from roughly 100,000 years ago back through billions of years, rather than the tens-of-thousands-of-years range where radiocarbon operates. It is applied to volcanic rock and ash, not organic material, because the method relies on argon gas (a decay product) being trapped within a rock's crystal structure once it solidifies from molten material — any argon present before solidification escapes as gas, effectively "zeroing the clock" at the moment of crystallisation.

In archaeology and palaeoanthropology this typically means dating volcanic ash layers found above or below the sediment layer containing hominid fossils or early stone tools, rather than dating the fossils or tools themselves — the fossil's age is then bracketed by the dated ash layers around it. This indirect approach is why potassium-argon dating has been central to establishing the age of early hominid sites in East Africa's Rift Valley, where volcanic activity left datable ash layers interspersed with fossil-bearing sediment.

Thermoluminescence and optically stimulated luminescence

Some materials — quartz, feldspar, and fired clay — accumulate energy from background radiation in their crystal lattice over time, in the form of trapped electrons. Heating the material (as happens when pottery is fired) or exposing it to sunlight (as happens to buried sediment grains before burial) releases that trapped energy and resets the accumulation to zero. From that reset point, the material begins accumulating trapped energy again at a rate that depends on the local background radiation level. Measuring how much energy has accumulated since the last reset — either by heating the sample in a lab and measuring the light emitted (thermoluminescence) or by exposing it to a controlled light source (optically stimulated luminescence) — gives an estimate of time elapsed since firing or burial.

This makes thermoluminescence particularly useful for dating fired pottery directly (rather than needing associated organic material, as radiocarbon does), and optically stimulated luminescence useful for dating buried sediment layers themselves — both methods reach usefully back to around 500,000 years under good conditions, filling part of the gap between radiocarbon's ~50,000-year ceiling and potassium-argon's much older effective range.

Dendrochronology: annual precision without any radioactivity at all

Tree-ring dating is the odd one out among these methods because it isn't radiometric at all — it relies on the fact that trees in seasonal climates add one growth ring per year, and ring width varies with that year's growing conditions (rainfall, temperature), producing a distinctive pattern shared by all trees growing in the same region during the same years. By overlapping the ring patterns of a living tree, an older dead tree, and progressively older timber recovered from archaeological sites, researchers can build a continuous reference chronology stretching back thousands of years in some regions, against which a new wood sample's ring pattern can be matched to assign it a specific calendar year, sometimes down to the exact year of felling.

Its precision — potentially single-year resolution — is unmatched by any radiometric method, but it's constrained to regions with strong seasonal growth signals and a long enough surviving timber record to build the reference chronology in the first place, and to wood as a material. Dendrochronology's other major role is as the primary calibration tool for radiocarbon dating: because tree rings can be dated by direct counting rather than by carbon-14 decay, radiocarbon measurements taken from wood of known dendrochronological age are what revealed and let researchers correct for the historical fluctuations in atmospheric carbon-14 mentioned earlier.