Abstract

The far-reaching directed “blast” of Mount St. Helens and the devastating nuees ardentes of Mount Pelée produced deposits that in many places consist of three main layers: a basal gravelly or sandy layer 1, a massive or bedded ash layer 2, and a capping ash-cloud layer 3. These thin and in part landscape-mantling deposits are generally ascribed to pyroclastic surges, and the dune bedding seen in parts of layer 2 has reinforced this interpretation. There are several reasons, however, for preferring a pyroclastic-flow origin: (1) much of layer 2 is unequivocal pyroclastic flow; (2) the tripartite subdivision and landscape mantling habit is similar to that displayed by ignimbrites of the low-aspect-ratio type; (3) the deflation of a pyroclastic surge and the subsequent deposition of particulate material will tend not to conserve the fine ash and dust that are abundant in valley-ponded parts of layer 2 and cannot explain the observed tendency for layer 2 to show stronger fines depletion on ridges than in valley bottoms; and (4) the coarseness and the variance of grain size found within dune-bedded bed sets in Mount St. Helens layer 2 are like those of very weak pyroclastic surges, and the dune bedding most likely resulted from local minor turbulence (as could have been caused by surface roughness in a mountainous terrain littered with tree stumps and fallen trees) in a thin depositing pyroclastic flow. The directed blast and nuees ardentes at Mount St. Helens and Mount Pelée are interpreted to have been violently emplaced pyroclastic flows producing deposits of low-aspect-ratio type, the characteristics of which stem from an exceptionally high flow velocity.

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