There are several ways to approximate an eruption date.
Historical excavations can reveal when a known settlement was covered by lava.
For prehistorical eruptions, some large ones near polar regions left a layer of ash trapped in the polar ice, and oxygen isotopes can be used to tell when the ice solidified around it.
However, dating charred wood or any kind of vegetation from close to the eruption site is the most common method.
Carbon dating relies on the rate of radioactive decay of one carbon isotope, carbon 14. It is used for eruptions that took place in the last forty thousand years, but more than two hundred years ago.
Living things take on carbon from the carbon dioxide in the atmosphere, and when they die, they cannot take on any more.
It is assumed that the percentage of carbon in the atmosphere has remained constant and that the radioactivity of carbon 14 has decayed at a constant rate, approximately by half in 5,700 years.
Charcoal from trees burned in an eruption is nearly pure carbon and so is ideal for tracing the minuscule amounts of carbon 14 present. Tree rings are not useful because it is hard to find a tree close to the site that did not burn, while ash and trees from farther out tend to wash away.
The dates are given in a range of plus or minus one hundred years, but it is sometimes possible to determine a more exact date. For example, the eruption at Sunset Crater, Arizona, has been placed at 1066 A.D.
Carbon dating put the year around 1065, and Indians’ oral historical records helped place the event in 1066.
Pottery that showed an eclipse of the sun in conjunction with an eruption helped confirm that date.