Tephrochronology

Tephrochronology is a geochronological technique that uses discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which paleoenvironmental or archaeological records can be placed.

The premise of the technique is that each volcanic event produces ash with a unique chemical "fingerprint" that allows the deposit to be identified across the area affected by fallout.

[5] For example the often very explosive nature of rhyolytic eruptions will cause wider distribution, the higher potassium content of rhyolite allows more accurate time determinations, and the location of a deposit will influence its potential for chemical alteration after being laid down.

Minor volcanic events may also leave their fingerprint in the geological record: Hayes Volcano is responsible for a series of six major tephra layers in the Cook Inlet region of Alaska.

[7] Since the late 1990s, techniques developed by Chris S. M. Turney (QUB, Belfast; now University of Exeter) and others for extracting tephra horizons invisible to the naked eye ("cryptotephra")[8] have revolutionised the application of tephrochronology.

It has led to the first discovery of the Vedde ash on the mainland of Britain, in Sweden, in the Netherlands, in the Swiss Lake Soppensee and in two sites on the Karelian Isthmus of Baltic Russia.

[10] Since 2010 Bayesian age modelling built around ever-improving 14C-calibration curves and other age-related data,such as zircon double dating continues to better define tephrochronology.