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Tephra transformations: unpicking the record of past volcanic eruptions

Tephra transformations: unpicking the record of past volcanic eruptions

Project aims

This study aims to determine how fine, pyroclastic fragments (tephra) that rain down during volcanic eruptions are transformed into tephra layers in the soil, in order to improve the interpretation of these features. The specific objectives of the project are to:

  1. To record the depth of tephra layers originating from the 1980 eruption of Mount St Helens;
  2. To compare the thickness of these layers with initial tephra depth, as recorded by researchers from the US Geological Survey (USGS) shortly after the eruption;
  3. To assess whether tephra layer thickness was influenced by factors such as slope position and vegetation type.

Background

This project was inspired by research conducted on tephra layers in Iceland. The previous project established that Icelandic vegetation plays a crucial role in the retention and stabilisation of tephra (and, by extension, the formation of tephra layers). This finding raised the question: does this relationship apply in other volcanic locations? And can knowledge of the factors influencing tephra layer formation improve our ability to reconstruct past volcanic eruptions?

Layers of tephra preserved in soils are often used to infer the characteristics of volcanic eruptions for which there are no direct observations. However, this approach is premised on a number of assumptions about the ways in which newly-deposited tephra are translated into the sedimentary record. This process, which has never been directly observed, is poorly understood. One way to address this knowledge gap is to compare the characteristics of a buried tephra layer (e.g. thickness) with equivalent measurements made shortly after an eruption. In this way, it should be possible to calibrate the degree of transformation that a tephra deposit undergoes as it is translated into the soil.

Study area and methods

The study focussed on the tephra layer deposited during the 1980 eruption of Mount St Helens, USA, because this eruption has been studied in great detail. Crucially, records of initial tephra depth are available. The eruption is sufficiently recent that many of the original researchers are still active, but sufficiently distant in time that the tephra should be fully incorporated into the soil. Tephra measurements were made in the Gifford Pinchot National Forest (proximal to Mt St Helens) and around Ritzville, WA, in August 2015 (Fig. 1). Areas where the initial deposit depth was moderate (< 5 cm) were selected i.e. locations where the tephra did not obliterate the vegetation cover present at the time of the eruption. The two sampling locations had contrasting vegetation types. The proximal location was characterised by closed coniferous forest (Fig. 2a), the distil location by sagebrush (Fig. 2c). We measured tephra thickness in both locations (Figs 2b, d), assessing the impact of vegetation type and slope, and collected samples in order to calculate the mass of tephra per unit area.

Figure 1

Fig. 1: Location plan, showing the proximal (forest) and distal (sagebrush) sampling areas in relation to Mount St Helens. The contours indicate the thickness of the tephra produced in the May 18 eruption; measurements are in centimetres. Adapted from Sarna-Wojcicki et al. (1981).

Figure 2

Fig. 2: Field sites showing a) a typical sampling location in the Gifford Pinchot National Forest. The trees are mainly mature Douglas fir (Pseudotsuga menziesii); b) a soil section from the forest. The tephra layer is an off-white colour. It sits on a reddish-brown layer of decayed wood (the 1980 forest floor) and is cover by a few centimetres of organic material; c) sagebrush (Artemisia sp.) vegetation close to Ritzville, WA; d) a soil section from the Ritzville area, showing the white 1980 tephra capped by a thin biocrust (the orange colouration on the surface is a moss).

Early findings and implications of research

The preservation of the 1980 tephra layer was remarkably good: much of the tephra that initially fell on the study areas was retained in situ (as evidenced by the preservation of delicate units in the deposit). This was unexpected, as the tephra a mostly fine-grained. The broad distribution of the deposit matched the spatial patterns recorded in the period immediately after the eruption and within-site variability was low. Some of the patterns observed were familiar from Iceland, e.g. a lack of variability in deposit depth with slope position. Others, notably the retention of tephra in areas lacking vascular vegetation (Fig. 3), were entirely new and worthy of further study. The role of biological soil crusts (biocrusts) in preserving tephra layers around Ritzville was surprising, and may have implications of the interpretation of tephra layers in other arid locations. Please refer to our 2015 fieldwork report for further details.

Figure 3

Fig. 3: Tephra depths under different vegetation types in the Ritzville area. Vegetation cover does not seem to influence the tephra retention, even though the tephra grains are fine and the Ritzville area is windy and arid in the summer. The tephra seem to be quickly and effectively stabilised by biocrusts.

Funding and collaborators

This project is funded by the National Science Foundation, via a grant to Prof Andrew Dugmore of the University of Edinburgh, and conducted in collaboration with Prof Dugmore and Dr Richard Streeter (University of St Andrews).

References

  • Sarna-Wojcicki, A. M., Shipley, S., Waitt, R. B., Dzurisin, D. & Wood, S. H. (1981) Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980. The 1980 eruptions of Mount St. Helens, Washington (eds P. W. Lipman & D. R. Mullineaux), pp. 577-600. U.S. Department of the Interior, USGS, Washington, D.C.