Tephrochronology

'Tephra' derives from a Greek word tephra meaning 'ashes.' It is a collective term for the explosively erupted, loose fragmental (pyroclastic) products of a volcanic eruption, and encompasses all grain sizes. Tephra deposits have two special features: (1) they are erupted over geologically short time periods, typically only hours or days to perhaps weeks or months and (2) they can be spread widely over land and sea to form a thin blanket that has the same age wherever it occurs. Therefore, once identified by its mineralogical and geochemical properties, a tephra layer, unless reworked, provides a time-parallel marker bed or isochron. Tephrochronology is the use of tephra deposits as isochrons to correlate sequences in different places by providing precise chronostratigraphic tie-points, and to transfer relative or numerical ages to such sequences where the tephras have been dated by radiometric, incremental, or age-equivalent methods. Tephras are now routinely detected and dated in terrestrial, marine, and ice-core records throughout the world in both macroscopic and microscopic (cryptotephra) forms, and consequently are used in a diverse range of disciplinary fields including stratigraphy, geomorphology, glaciology, sedimentology, archaeology, hominid evolution, and paleoenvironmental reconstruction including paleoecology and paleoclimatology. Tephrochronology is also central to establishing (via tephrostratigraphy) the frequency/periodicity of major episodes of volcanic activity and the assessment of volcanic hazards.