Tephrochronology is a geochronological technique that uses discrete layers of tephra&mdash;volcanic ash from a single eruption&mdash;to create a chronological framework in which paleoenvironmental or archaeological records can be placed. Such an established event provides a "tephra horizon". 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. Thus, once the volcanic event has been independently dated, the tephra horizon will act as time marker. It is a variant of the basic geological technique of stratigraphy.
The main advantages of the technique are that the volcanic ash layers can be relatively easily identified in many sediments and that the tephra layers are deposited relatively instantaneously over a wide spatial area. This means they provide accurate temporal marker layers which can be used to verify or corroborate other dating techniques, linking sequences widely separated by location into a unified chronology that correlates climatic sequences and events. This results in "age-equivalent dating".
Effective tephrochronology requires accurate geochemical fingerprinting (usually via an electron microprobe). An important recent advance is the use of LA-ICP-MS (i.e. laser ablation ICP-MS) to measure trace-element abundances in individual tephra shards. One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras. Some tephra horizons and the use of zircon directed techniques are more useful than others in linking layers over wide areas and determining eruption details. 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. Zircon techniques applied to tephra and other samples from the same eruption, may allow magma sources, magma residence times and the geochemical conditions of the magma formation to be better understood with dating of more than just the eruption itself, but also when the magma first evolved separately, or incorporated other rocks.