How can scientists know how old an archaeological site is?
(Lansing State Journal, November 24,1993)
Dendrochronologists study tree rings to determine the timing and extent of environmental changes. Such changes affect the way trees grow, and consequently the size, spacing and composition of the trees' annual growth rings. If a large piece of wood is found at an archaeological site, the pattern of the rings can be used to estimate its age. This is done by comparing it with an established master pattern from trees growing near the site. This method is useful for sites that are up to 3,000 years old.
Radiocarbon dating can be used if cloth, wood or plant material is found. The method takes advantage of two physical phenomena, the relatively stable ratio of carbon isotopes in the atmosphere and radioactive decay. In the atmosphere there is a very small amount of radioactive carbon 14 as compared to the stable isotope called carbon 12. As long as a plant or animal is alive it will take up the two isotopes in roughly the same proportion. Once it dies, the ratio of the two isotopes changes as the carbon 14 decays into nitrogen 14. Because the rate of this change is known, a sample's age can be estimated. This method is useful for dating sites that are 1,000 to 30,000 years old.
While these methods are useful, they rarely provide the final say on how old a site is. Generally, the stylistic and broader historical analysis of a variety of artifacts will be used with the above methods to get a more accurate estimate.