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.

火山学家经常使用火山灰层中的粒度分布 (GSD) 来推断喷发参数。然而，对于过去很长时间的火山喷发，重建的准确性取决于最初的火山灰沉积物与推断所依据的保存的火山灰层之间的对应关系。我们问：几十年前的火山灰层的 GSD 与它起源的矿床有多接近？我们通过对 1980 年 5 月美国圣海伦斯火山喷发产生的火山灰层的研究来解决这个问题。我们将新鲜、未受干扰的火山灰的粒度分布与幸存的火山灰层的粒度测量值进行了比较。我们发现火山灰层的整体粒度特征与原始矿床相似，早期作者确定的独特特征得到了保留。然而，对我们样品的详细分析显示出质量差异，特别是精细材料的损失（我们将其归因于“风选”）。了解火山灰沉积物如何随时间转变对于重建过去的火山喷发的努力至关重要，但本质上很难研究。我们建议将长期的 tephra 应用实验作为一种潜在的前进方式。