Radioactive dating volcanic ash
In the 1950s a new method of archaeological analysis, radiocarbon dating, allowed organic materials such as wood, shell or bone to be accurately dated.
All living organisms contain a radioactive form of carbon (carbon-14), which decays at a known rate.
Using logs recovered from old buildings and ancient ruins, scientists have been able to compare tree rings to create a continuous record of tree rings over the past 2,000 years.
This tree ring record has proven extremely useful in creating a record of climate change, and in finding the age of ancient structures. The thick, light-colored part of each ring represents rapid spring and summer growth.
The width of a series of growth rings can give clues to past climates and various disruptions such as forest fires.From the 1960s to the 1980s the oldest radiocarbon dates derived from analysis of settlement sites in New Zealand were 1000–1100 AD or even a little earlier.During the 1990s these dates were systematically re-assessed as the technique became better understood, and many problems were found with earlier dates.The thin, dark part of each ring represents slow autumn and winter growth.Several other processes result in the accumulation of distinct yearly layers that can be used for dating.