Complex proximal deposition during the Plinian eruptions of 1912 at Novarupta, Alaska

Abstract

Proximal (<3 km) deposits from episodes II and III of the 60-h-long Novarupta 1912 eruption exhibit a very complex stratigraphy, the result of at least four transport regimes and diverse depositional mechanisms. They contrast with the relatively simple stratigraphy (and inferred emplacement mechanisms) for the previously documented, better known, medial–distal fall deposits and the Valley of Ten Thousand Smokes ignimbrite. The proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta, and numerous thin complex deposits of pyroclastic density currents (PDCs) with no regional analogs. The locally dispersed component of the fall deposits forms sector-confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100–300 m (cf. several kilometers for the medial–distal fall deposits). This locally dispersed fall material (and many of the associated PDC deposits) is rich in andesitic and banded pumices and richer in shallow-derived wall-rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition. There are no marked contrasts in grain size in the near-vent deposits, however, between locally and widely dispersed beds, and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial–distal samples. Associated PDC deposits form a spectrum of facies from fines-poor, avalanched beds through thin-bedded, landscape-mantling beds to channelized lobes of pumice-block-rich ignimbrite. The origins of the Novarupta near-vent deposits are considered within a spectrum of four transport regimes: (1) sustained buoyant plume, (2) fountaining with co-current flow, (3) fountaining with counter-current flow, and (4) direct lateral ejection. The Novarupta deposits suggest a model where buoyant, stable, regime-1 plumes characterized most of episodes II and III, but were accompanied by transient and variable partitioning of clasts into the other three regimes. Only one short period of vent blockage and cessation of the Plinian plume occurred, separating episodes II and III, which was followed by a single PDC interpreted as an overpressured "blast" involving direct lateral ejection. In contrast, regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume, which were being strongly affected by short-lived instabilities. These instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent. Of the parameters that control explosive eruptive behavior, only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products.

Appendix: techniques

Samples were hand-sieved in the field down to the 8 or 4 mm size fractions at one-phi intervals, and a split of the finer fraction retained for particle size analysis at half-phi intervals in the laboratory. As the contents of lithics were generally low, weight percentages were calculated with respect to the original field (wet) weights of the samples.

Componentry was done on a weight percent basis by picking each size fraction down to the 4-mm sieve fraction. Juvenile material was allocated to andesite (A), dacite (D) or rhyolite (R) classes on the basis of color and crystal content (following Hildreth 1983; Fierstein and Hildreth 1992). Mingled pumices were allocated pro-rata to their component end-members, e.g., an A–D–R banded clast would have one third of its mass allocated to each of the three components. All percentages of A:D:R given in this paper are recalculated to total 100%. The greatest ambiguity in the juvenile components was with mid-toned gray-brown crystal-rich pumices. In some samples such clasts are transitional to paler dacite (i.e., the ambiguous clasts were interpreted as thermally colored dacite); in others to darker andesite (i.e., the ambiguous clasts were interpreted as bleached, or oxidized andesite). Wall-rock lithic proportions were also measured in the >4-mm fractions, likewise within three categories: fragments of vitrophyre (V) recycled from the Episode I tuffs, fragments of lava (L) underlying the Episode I deposits, and pieces of Naknek sedimentary basement (N).