Tephrochronology
Tephrochronology is a geochronological technique for dating archaeological, geological and palaeoenvironmental sequences and events by their location between upper and lower layers of tephra (volcanic ejecta) of known date, and for correlating such sequences and events at separate locations between the same layers. The premise of the technique is that each volcanic event produces a "tephra horizon", a layer of ash with a unique chemical "fingerprint" that allows the deposit to be identified across the area affected by fallout.