Preservation of thin tephra

Abstract

The preservation of thin (<300 mm thick) tephra falls was investigated at four sites in Papua New Guinea (PNG), Alaska and Washington, USA. Measurements of the variations in the thickness of: (i) Tibito Tephra 150 km downwind from the source, Long Island (PNG) erupted mid-seventeenth century; (ii) St Helens W tephra (erupted 1479-80 A.D.) on the slopes of the adjacent Mt. Rainier in Washington State; (iii) Novarupta (1912) tephra preserved on Kodiak Island (Alaska, USA); and (iv) an experimentally placed tephra at a site near Mt. Hagen (PNG) allow tentative conclusions to be drawn about the relative importance to tephra preservation of slope gradients, vegetation cover and soil faunal activity. Results for the experimental tephra suggest that compaction occurs rapidly post-deposition and that estimates of tephra thickness and bulk density need to indicate how long after deposition thickness measurements were made. These studies show that erosional reworking of thin tephra is not rapid even on steeper slopes in high rainfall environments. In Papua New Guinea a 350-year old tephra is rarely present under forest but is well-preserved under alpine grasslands. On Mt. Rainier 500-year old tephra is readily preserved under forest but absent under grasslands as a result of gopher activity. Despite the poor relationship between tephra thickness and slope steepness the thickness of thin tephras is highly variable. On Kodiak Island thickness variability across a few metres is similar to that observed across the whole northeast of the island. The measured variability reported here indicates large sample sizes are necessary to adequately estimate the mean thickness of these thin tephra. These results have implications for the preparation of isopach maps, estimation of tephra volumes and elaboration of the potential consequences of tephra falls.

The interpretative value of transformed tephra sequences

“…A limited number of such studies have taken place (e.g. Payne and Gehrels, 2010;Todd et al, 2014;Blong et al, 2017), but we suggest experiments which cover a wider range of environments and tephra characteristics may give valuable insights into the processes that alter tephra while it is on the surface.…”

“…Earth surface processes and bioturbation can affect tephras immediately after their initial deposition. Deposits will settle and become compact; people can clear field systems and settlements, people and animals can walk over deposits leaving footprints and churn surface layers; wind, and water in the form of precipitation, snowmelt or overland flow, will remobilise particular fractions or entire deposits (Houk et al, 2009;Wilson et al, 2013;Blong et al, 2017;Cutler et al, 2018). This phase of reworking may persist for as long as the tephra remains on the surface and can last for years to decades, particularly in parts of the world where vegetation cover is limited, such as Iceland (Liu et al, 2014) and elsewhere, e.g.…”

Thirty years of tephra erosion following the 1980 eruption of Mount St. Helens

“…Gabet et al, 2003), and the tendency through time for a tephra layer to homogenize or remain as a distinct horizon will vary generally with the mixing agents present in a given environment (Blong et al, 2017). Gabet et al, 2003), and the tendency through time for a tephra layer to homogenize or remain as a distinct horizon will vary generally with the mixing agents present in a given environment (Blong et al, 2017).…”

“…Various flora and fauna are effective soil mixing agents (for a review, see e.g. Gabet et al, 2003), and the tendency through time for a tephra layer to homogenize or remain as a distinct horizon will vary generally with the mixing agents present in a given environment (Blong et al, 2017). For example, in some parts of the area affected by the lateral blast at Mount St. Helens, the northern pocket gopher (Thomomys talpoides) brought buried soil to the tephra surface starting in the first few months following the eruption (Andersen and MacMahon, 1985).…”

A model to assess tephra clean-up requirements in urban environments

“…The bulk density of tephra could have an influence of the total volume of material that is disposed of and the disposal site requirements. Here we have used deposit thickness to estimate volume, but thickness of deposits can naturally compact up to 50% within just a few days and could further compact during the transportation in trucks, which would reduce the volume of material that is disposed (Blong et al 2011;Engwell et al 2013;Hayes et al 2015). Further, the surge deposits are likely to be relatively hot, which could create an issue around accessibility and clean-up of those areas in the short term.…”