In the fields of archaeology and paleoenvironmentology, tephrochronology is the use of discrete layers of tephra (volcanic ash), each produced by a single volcanic eruption, to create a chronological framework for dating archeological finds or historical events.
Tephrochronology relies upon the fact that each volcanic eruption, even from the same volcano, possesses a unique geochemical "fingerprint" which can be distinguished using an electron microprobe or techniques such as laser ablation inductively coupled plasma mass spectrometry (which is quite a mouthful, and thus usually abbreviated LA-ICP-MS).
Once a layer of tephra has been identified and positively linked to a known volcanic eruption, it can be used as a chronological marker for dating purposes wherever the layer is found.
Originally, tephrochronology had important but limited use as a tool for dating sites very close to a volcano. The tephra layers would first be identified with the naked eye and then subjected to chemical analysis. In the past decade, however, tephrochronology has made a quantum leap forward in usefulness, as scientists have developed techniques for identifying minute layers of tephra invisible to the human eye (so-called "cryptotephra"). This allows researchers to identify layers of tephra at sites located extremely far away from the volcano which originally emitted them (sometimes even on the other side of the world), and to identify large numbers of tephra layers at almost any location on Earth, allowing for much more precise dating.