How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit
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Tephra layers are frequently used to reconstruct past volcanic activity. Inferences made from tephra layers rely on the assumption that the preserved tephra layer is representative of the initial deposit. However, a great deal can happen to tephra after it is deposited; thus, tephra layer taphonomy is a crucial but poorly understood process. The overall goal of this research was to gain greater insight into the taphonomy of terrestrial tephra layers, specifically the extent to which deposit thickness is altered over time, with implications for tephra volume estimation. We approached this by a)conducting a new survey of the tephra layer from the recent, well-studied eruption of Mount St Helens on May 18th, 1980 (MSH1980); b)modelling the tephra layer thickness using a mathematical technique and c)comparing our results with an equivalent model based on measurements taken immediately after the eruption. In this way, we aimed to quantify any losses and transformations that have occurred. During our study, we collected measurements of tephra layer thickness from 86 locations ranging from <20 to >600 km from the vent. Geochemical analysis was used to identify tephra of uncertain origin. Our results indicated that the extant tephra layer at undisturbed sites was representative of the original deposit: overall, preservation in these locations (in terms of thickness, stratigraphy and geochemistry) had been remarkably good. However, the isopach maps generated from our measurements diverged from isopachs produced in the same way, but derived from the original survey data. Furthermore, our estimates of the quantity of tephra produced during the eruption greatly exceeded previous estimates of the fallout volume. In our study, inaccuracies in the modelled fallout arose from issues of sampling strategy, rather than taphonomy. Our results demonstrate the sensitivity of volcanological reconstructions to measurement location, and the great importance of reliably measured low/zero values in reconstructing tephra deposits.
Cutler , N A , Streeter , R T , Engwell , S L , Bolton , M S , Jensen , B J L & Dugmore , A J 2020 , ' How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit ' , Journal of Volcanology and Geothermal Research , vol. 399 , 106883 . https://doi.org/10.1016/j.jvolgeores.2020.106883
Journal of Volcanology and Geothermal Research
Copyright © 2020 Elsevier B.V. All rights reserved. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1016/j.jvolgeores.2020.106883
DescriptionThis research was supported by grants from the Royal Geographical Society (ref: SRG 01/18) and Quaternary Research Association to NAC.
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