How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit

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, 399, 106883.

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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. 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 an objective 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 verify the identity of 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, isopach maps generated from our measurements diverged from isopachs derived from the original survey data. Furthermore, our estimate of the quantity of tephra produced during eruption greatly exceeded previous estimates of the fallout volume. In this case, 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.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 03770273
Date made live: 24 Apr 2020 13:16 +0 (UTC)

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