Preservation of thin tephras

R. Blong; N.J. Enright

INTRODUCTION Numerous observations attest to the rapid erosion from hillslopes of recently emplaced tephras. A survey of the literature on Soufrière (St Vincent, 1902), Rabaul (Papua New Guinea, 1937), Paricutin (Mexico, 1943-1945), Irazu (Costa Rica, 1963-64), Usu (Japan, 1977) and Mt St Helens (USA, 1980) together with occasional comments about other tephra-producing eruptions suggest the following conclusions: 1. Deep rills and gullies are quickly cut in fresh tephra especially where the development of an impermeable surface crust increases runoff volumes (Cilento, 1937, p47-8; Huggins, 1902, p20; Waldron, 1967, p11; Higashi et al., 1978; Kadomura et al., 1978; Lowdermilk and Bailey, 1946, p286; Collins et al., 1983). 2. Rates of rill and gully erosion are amongst the highest recorded anywhere while sediment concentrations in mudflows or secondary lahars may be as much as 65% by weight (Ollier and Brown, 1971; Waldron, 1967; Higashi et al., 1978). 3. Erosion of tephra is frequently proportional to slope steepness with extensive redeposition on valley floors. Rill erosion and shallow landsliding of tephra are important processes on steeper slopes. Topographic position is also important in determining how much tephra remains at a site (Anderson and Flett, 1903, p437; Segerstrom, 1950; 1960; Collins et al., 1983; Lehre et al., 1983). 4. The amount of vegetation remaining on tephra-mantled slopes influences erosion rates (Collins et al., 1983). Tephra erosion facilitates recovery of surviving vegetation (Lawrence and Ripple, 2000) and vegetation regrowth influences the retention of tephra (Segerstrom, 1950; 1960). 5. As much as one third to one half of the tephra may be removed from the slopes within one year or less of emplacement (Anderson and Flett, 1903, p453; Waldron, 1967, p11), though more detailed studies at Mt St Helens suggest only 11% of tephra was removed in the first year (Collins et al., 1983) and that erosion rates declined dramatically with time (Collins and Dunne, 1986). 6. The decline in erosion rate is not produced by revegetation but by increased infiltration capacity, decreased erodibility of the tephra exposed and the development of a stable rill network (Collins and Dunne, 1986). 7. In the long term, stability of the underlying substrate is an important influence on erosional removal of the tephra mantle (Blong and Pain, 1978). The examples on which the above conclusions are based do not always specify the thickness of the tephra mantle but observations were generally made close to the volcanoes where the tephra was at least 300 mm deep, and sometimes considerably deeper. On the other hand, erosional reworking and/or survival of thin (i.e., 10-300 mm) seems to have not been reported in any detail; although emplacement of thin tephras is usually less destructive of the vegetation cover it is not clear whether erosion of thin tephras is similarly rapid or whether preservation is ensured. Most studies of thin tephras relate to their use as chronostratigraphic marker beds and are commonly based on tephras preserved in lakes and/or swamp deposits. By and large, preservation of thin tephras in other situations is poorly documented. Nonetheless, thin tephras are of considerable value in geomorphic, geologic and archaeologic investigations as they form obvious marker horizons and cover large areas. The present contributions sets out observations on the preservation of thin tephras at four sites: near Mt Hagen and in the Western Finisterre Ranges, Papua New Guinea, on the slopes of Mt Rainier, Washington, USA, and on Kodiak Island, Alaska, USA. Each of the four studies was of only limited duration and detail but, collectively, the results provide considerable data on erosion and survival of thin tephras and the factors that influence their preservation.

Preservation of thin tephras

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