Jökull


Jökull - 01.12.1979, Page 36

Jökull - 01.12.1979, Page 36
are excellent time-stratigraphic markers for geology, geomorphology and archaeology. The identification of tephra layers in the field is achieved in many ways, such as by their strati- graphic relation, their colour, which is an indicator of their chemical composition, their thickness and grain sizes, which are, within certain limits, an ex- ponential function of their distance from their source. In the laboratory the tephra Iayers are identified by determination of their mineralogical, chemical and physical characteristics, microprobe analyses of their glasses being most useful. Various granu- lometric parameters reveal the type of explosive activity. As for age determination of tephra layers, those deposited in historical time can often be identified as resulting from eruptions mentioned in written records and can thus be dated more or less accu- rately according to the accuracy of the records concerned. In some cases they are exact even to the day of the tephra fa.ll. In Iceland eruptions dated by written records go back to about 900 A. D. Tephra layers from eruptions not mentioned in written records can often be more or less exactly dated by various means, such as by C14-dating of peat soil immediately above and beneath them or trees or shrubs embedded in them. They can also be dated by extrapolation from other exactly dated tephra layers which show the rate of sedimentation, by dendrochronology, pollenanalysis, and last but not least with the aid of archaeology. The conditions prerequisite for the establish- ment of a tephrochronological time scale in Iceland are in fact very good. With the reservations outlined in the previous chapter one can say that about 200 volcanoes have been active in the coun- try in Postglacial time, between 30 and 40 of which since the Nordic settlement began 11 centuries ago. The tephra layers are more numerous than one would expect in a volcanic area producing mainly basalt. This is due mainly to the fact that the ice covered basalt volcanoes, such as Grímsvötn and Katla, produce tephra instead of the lava they would produce under subaerial conditions. Up to about 150 Postglacial tephra layers have been detected in the field in a single profile. On the whole the tephra layers are more numerous and thicker E of the active volcanic zones than W of them, as the air currents in the higher strata of the troposphere are predominantly westerly. The tephra layers mainly used hitherato as key horizons in the Postglacial soils of Iceland are the acid ones, as they are light coloured and therefore easily traced in soil profiles. There are 12 widespread acid layers in the Postglacial soils. In Iceland the establishment of tephrochrono- logy by field studies is facilitated by the country being one of the few highly active volcanic areas in the world where peat formation and formation of Fig. 2. Map showing in which direction tephra was dispersed during the initial phase of each of Hekla’s 15 eruptions in historical time. The width of each arrow indicates roughly the relative size of the estimated volume of the layers. 34 JÖKULL 29. ÁR
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