Jökull


Jökull - 01.01.2016, Page 79

Jökull - 01.01.2016, Page 79
Grain characteristics of silicic Katla tephra layers geochemical composition seems an unlikely reason for SILK-A11’s differences. The small age difference between SILK-A11 and SILK-A12 eruptions (∼100 years) makes major changes in the glacier thickness unlikely. A different source area with different ice thickness and meltwater supply at the vents is a more likely explanation. Dellino and Volpe (1996) analyzed grain shapes with the purpose of identifying the origin of the grains, in order to establish if they were of hydro- magmatic or magmatic origin. Their work suggested that spherical shaped grains originate from hydro- magmatic eruptions. However, Eiríksson and Wigum (1989) state that grains from silicic magmas are more readily elongated than those from basaltic ones and magmatic grains tend to be more elongated than the hydromagmatic grains. According to the elon- gation values, the SILK grains are very elongated, which agrees with Eiríksson and Wigum (1989). The high proportion of fine grained material also supports phreatomagmatic activity (Self and Sparks, 1978). We believe that the elongated SILK grain shape is connected to phreatomagmatic explosive activity. The extreme elongate shape is the result of the break-up of viscous silicic magma with abundant elongate vesi- cles, as a result of a shock caused either by cooling contraction, a sudden expansion of steam, or both. In the case of the SILK-A11 tephra, the fragmentation occurred before the vesicles acquired an elongated form. We speculate that the ice at the eruption site was thicker and degassing was arrested by abundant meltwater before the magma froth was drawn out into the elongated vesicles that characterize other SILK- type tephras from this period. SUMMARY The younger SILK tephra layers appear to be coarser grained, i.e. have larger maximum grain size, while the older layers appear to be finer grained, except for SILK-A7. These changes occur between SILK- N1 (∼5800 year old) and SILK-A1 (∼6000 year old). However, the mean grain size does not clearly follow this trend. Grain shape results for the SILK-LN, SILK-N1, SILK-A8, SILK-A11 and SILK-A12 tephra layers show that SILK-A11 is significantly different from the other four layers. The main difference lies in the elongation values, as no systematic changes with time were observed in the shape parameters (elongation, ruggedness and circularity). Omitting SILK-A11 and adding the ∼3400 old SILK-LN to the measurements does not change this conclusion. The chemical composition of the SILK-A11 and SILK-A12 show higher SiO2 values and lower TiO2, MgO and CaO values compared to all other SILK lay- ers. Grain size and chemical composition appear to be correlated, where the SILK tephra layers with the low- est SiO2 and highest FeO and CaO tend to be coarser grained than the others, but there is no correlation be- tween grain shape (parameters measured in this study) and chemical composition. The results of this study do not support radical changes in the eruption environment. On the contrary, the results support a fairly stable eruption conditions during the period under investigation, and presence of ice cover throughout the Holocene as suggested by Dugmore (1989) and Óladóttir et al. (2007). Acknowledgements This work was supported by the International Civil Aviation Organization (ICAO) as a part of the Cata- logue of Icelandic Volcanoes Project of the Icelandic Meteorological Office and the Institute of Earth Sci- ences, University of Iceland. We are grateful to Bergrún A. Óladóttir, Johanne Schmith, Tinna Jóns- dóttir and Agnes Ö. Magnúsdóttir for assistance and good company in the field, and to Jón Eiríksson for assistance with the grain shape analyses. We thank the two reviewers for their constructive reviews. ÁGRIP Kornastærð og kornalögun súrrar til ísúrrar gjósku úr 12 Kötlugosum, sem urðu fyrir 2800 til 8100 árum, var rannsökuð með það að markmiði að kanna hvort kornaeinkenni gjóskunnar hefðu breyst með tíma og í slíku tilfelli hvort breytingarnar endurspegluðu breyt- ingar á umhverfi gosstöðvanna eða einhverjar aðrar ástæður, t.d. breytta efnasamsetningu. Kornastærðar- greiningarnar benda til að gjóska í yngri gosunum sé JÖKULL No. 66, 2016 79
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