Abstract

The so-called "lateral blast" associated with the May 18, 1980 eruption of Mount St. Helens was witnessed by two of us (C.H. and P.H.), and its deposits were later studied. Oriented samples were collected on a ridge 14 km northwest of the mountain. The thickness of the deposit at this locality ranges from 40 cm to more than 1 m. The surface of the deposit was sculpted by the flow into large elongate bedforms aligned parallel with the flow, with their positions being partly controlled by small features beneath the deposit. The deposit consists of a very poorly sorted basal unit that may locally be divided into a lower massive layer and an upper crudely lenticular laminated layer, overlain by a unit of accretionary lapilli ranging from a few millimetres to several centimetres in thickness. The lower layer contains elongate organic clasts aligned parallel to the flow. The rock fragments that make up the remainder of the layer are more equant and have no obvious preferred orientation on a macroscopic scale.Oriented samples of the lower, poorly sorted layer were resin-impregnated and thin sectioned for grain orientation analysis. The vector orientation data were analyzed both unweighted and weighted by the eccentricity and elongation of the grains. One sample was successfully impregnated and analysis has shown that the grains have a statistically significant orientation parallel to the flow direction as determined by blown down timber and abrasion of stumps. These results are consistent with the rapid accumulation of the deposit from a high velocity, moderate particle density, low fluid viscosity flow analogous to pyroclastic surges observed elsewhere on a smaller scale.

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