Abstract
Volcanologists frequently use grain size distributions (GSDs) in tephra layers to infer eruption parameters. However, for long-past eruptions, the accuracy of the reconstruction depends upon the correspondence between the initial tephra deposit and preserved tephra layer on which inferences are based. We ask: how closely does the GSD of a decades-old tephra layer resemble the deposit from which it originated? We addressed this question with a study of the tephra layer produced by the eruption of Mount St Helens, USA, in May 1980. We compared grain size distributions from the fresh, undisturbed tephra with grain size measurements from the surviving tephra layer. We found that the overall grain size characteristics of the tephra layer were similar to the original deposit, and that distinctive features identified by earlier authors had been preserved. However, detailed analysis of our samples showed qualitative differences, specifically a loss of fine material (which we attributed to ‘winnowing’). Understanding how tephra deposits are transformed over time is critical to efforts to reconstruct past eruptions, but inherently difficult to study. We propose long-term, tephra application experiments as a potential way forward.
Highlights
The grain size characteristics of tephra deposits are influenced by volcanogenic processes and atmospheric conditions during eruptions (Bonadonna et al 2015; Bonadonna and Houghton 2005; Carey and Sparks 1986; Pyle 1989)
It is assumed that grain size distributions (GSDs) in tephra layers are representative of the original deposit, but this is not necessarily the case
If no contemporary observations of the MSH1980 eruption existed, it is likely that researchers attempting to infer plume dynamics from the tephra layer would reach similar conclusions to Durant et al (2009) and Eychenne et al (2015)
Summary
The grain size characteristics of tephra deposits are influenced by volcanogenic processes and atmospheric conditions during eruptions (Bonadonna et al 2015; Bonadonna and Houghton 2005; Carey and Sparks 1986; Pyle 1989). Editorial responsibility: K.V. Cashman currents, wind and/or column height changes, ash aggregation, etc.) eruptions for which there are no contemporary observations can be reconstructed from tephra layers preserved in the sedimentary record. Geochemical alteration of the buried tephra layer could continue indefinitely Despite these observations, temporal changes in tephra GSD are not well understood (Buckland et al 2020; Dugmore et al 2020). We addressed this knowledge gap by comparing recent measurements of tephra grain size with similar measurements made decades earlier, shortly after the tephra was deposited.
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