Dating tephra layers

During the past year, Tephrabase has seen its largest expansion with new data being added, as well as rewriting of some of the code and a tidying up and cleaning of the data already held in the database. Tephrabase is an integral part of this multinational effort, which aims to link environmental, arhcaoelogical and historical sources in the North Atlantic region. Highlights include the addition over the past year of new sites, tephra layers and over geochemical analyses. The remaining sites are of distal cryotephras found in Europe. Tephrabase's growing data currently contains details of nearly tephra layers including over geochemical analyses at nearly sites.
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Geochronology/Tephra layers

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Ocean Drilling Program Leg Scientific Results: Chapter 20

Tephra is a general term for all the fragmental material erupted explosively from a volcano — ranging from fine dust called ash to car-size blocks. It is a Greek word meaning 'ashes', originally used by Aristotle to describe a volcanic eruption in the Aeolian Islands near Sicily, about BC. North Island volcanoes have blasted huge volumes of tephra into the air, to be blown over northern New Zealand and in some cases far out to sea, for more than 1, kilometres. This volcanism has deposited layer upon layer of tephras over the landscape. The layers have helped volcanologists work out the history of volcanoes and the distribution of their far-reaching airborne products. In many parts of the North Island, natural cliffs along terraces, river banks or at the coast, together with cuttings made during road construction or quarrying, reveal blanketing layers of tephra fallout from numerous eruptions. They drape the landscape on which they fall, generally following the contours of hills, terraces and valleys.
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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. 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.
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A tephra layer which bears its own unique chemistry and character may be used as a temporal marker horizon in archaeological and geological sites. The distribution of tephra following an eruption usually involves the largest boulders falling to the ground quickest and therefore closest to the vent, while smaller fragments travel further. Ash can often travel for thousands of miles, even circumglobal, as it can stay in the stratosphere for days to weeks following an eruption. When large amounts of tephra accumulate in the atmosphere from massive volcanic eruptions or from a multitude of smaller eruptions occurring simultaneously , they can reflect light and heat from the sun back through the atmosphere, in some cases causing the temperature to drop, resulting in a climate change: "volcanic winter". Tephra mixed in with precipitation can also be acidic and cause acid rain and snowfall.
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