Tephrochronology
Friday, 18. January 2008, 08:14:32
Chronology is the science of dating (past) events and their sequence, and an important issue in geology (chronostratigraphy). A lot of different methods are used for dating, a.o. depending on time scale. At some time I mentioned the use of lichens for dating in my post on Earthquakes and Lichenometry.
Tephra is solid material of all sizes (ash, bombs, cinders, etc.) explosively ejected from a volcano into the atmosphere, and dating by use of tephra is called tephrochronology. Tephrochronology is a geochronological technique that utilises discrete layers of tephra (or volcanic ash layers if you like) from a single eruption to create a chronological framework in which palaeoenvironmental records can be placed. Such an established event provides a "tephra horizon". The tephra layers are deposited relatively instantaneously over a wide spatial area. Because each volcanic event has a unique and identifiable chemical 'fingerprint', volcanic ash layers can be relatively easily identified in many sediments.
Until the 1960’s, tephrochronological studies in north-west Europe were restricted to Iceland, the only country with active volcanoes in the region. - Although I remember that we counted volcanic ash layers even before that time in sediments on the island of Fur in Denmark. More than 200 layers of volcanic ash of predominantly basaltic composition have been found within the Mo-clay of the Fur Formation of early Eocene age. 180 volcanic ash layers of the c. 60 m thick Fur Formation have been numbered. - Through new developments in geochemical analysis the scope of tephrochronology as a potential tool to investigate and precisely determine rates of climate change has been much enhanced. Much of the analysed material is extremely small - microscopic glass shards <100 µm in size referred to as ‘microtephras’ or ‘cryptotephras’.
A paper titled “Three new distal tephras in sediments spanning the Last Glacial-Interglacial Transition in Scotland” published in the Journal of Quaternary Science of September 2007 reports the results of tephrostratigraphic investigations at a number of Last Glacial-Interglacial Transition sites from the Inner Hebrides and Scottish mainland.
As they say at the Tephrochronological Database at http://www.tephrabase.org : “Tephra layers are now an invaluable tool in palaeoenvironmental studies. The data produced by such research can be difficult to handle and disseminate. Tephrabase is a database of tephra layers found in Iceland, north-west and northern Europe, Russia and central Mexico. Details on the location, name, age and geochemistry of tephra layers are stored in the database, as well as information about relevant volcanoes and volcanic systems. A comprehensive reference database is also included.“
• http://www3.interscience.wiley.com/cgi-bin/abstract/113521548/ABSTRACT?CRETRY=1&SRETRY=0
• http://en.wikipedia.org/wiki/Tephrochronology
• http://www.tephrabase.org/tephrochron.html
Tephra is solid material of all sizes (ash, bombs, cinders, etc.) explosively ejected from a volcano into the atmosphere, and dating by use of tephra is called tephrochronology. Tephrochronology is a geochronological technique that utilises discrete layers of tephra (or volcanic ash layers if you like) from a single eruption to create a chronological framework in which palaeoenvironmental records can be placed. Such an established event provides a "tephra horizon". The tephra layers are deposited relatively instantaneously over a wide spatial area. Because each volcanic event has a unique and identifiable chemical 'fingerprint', volcanic ash layers can be relatively easily identified in many sediments.
Until the 1960’s, tephrochronological studies in north-west Europe were restricted to Iceland, the only country with active volcanoes in the region. - Although I remember that we counted volcanic ash layers even before that time in sediments on the island of Fur in Denmark. More than 200 layers of volcanic ash of predominantly basaltic composition have been found within the Mo-clay of the Fur Formation of early Eocene age. 180 volcanic ash layers of the c. 60 m thick Fur Formation have been numbered. - Through new developments in geochemical analysis the scope of tephrochronology as a potential tool to investigate and precisely determine rates of climate change has been much enhanced. Much of the analysed material is extremely small - microscopic glass shards <100 µm in size referred to as ‘microtephras’ or ‘cryptotephras’.
A paper titled “Three new distal tephras in sediments spanning the Last Glacial-Interglacial Transition in Scotland” published in the Journal of Quaternary Science of September 2007 reports the results of tephrostratigraphic investigations at a number of Last Glacial-Interglacial Transition sites from the Inner Hebrides and Scottish mainland.
As they say at the Tephrochronological Database at http://www.tephrabase.org : “Tephra layers are now an invaluable tool in palaeoenvironmental studies. The data produced by such research can be difficult to handle and disseminate. Tephrabase is a database of tephra layers found in Iceland, north-west and northern Europe, Russia and central Mexico. Details on the location, name, age and geochemistry of tephra layers are stored in the database, as well as information about relevant volcanoes and volcanic systems. A comprehensive reference database is also included.“
• http://www3.interscience.wiley.com/cgi-bin/abstract/113521548/ABSTRACT?CRETRY=1&SRETRY=0
• http://en.wikipedia.org/wiki/Tephrochronology
• http://www.tephrabase.org/tephrochron.html
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