Geology topics

Research about Novarupta

Source-linked reports with geographic coverage including Novarupta.

3 recordsLinked to original sources

USGS2004

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

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. Springer-Verlag 2003.

Alaska↗

USGS2002

Magma storage prior to the 1912 eruption at Novarupta, Alaska

New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H 2 O+CO 2 fluid saturation were determined for an andesite (58.7 wt% SiO 2 ) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H 2 O-saturated conditions (i.e., P H2O =P TOT ) in a negatively sloping region in T-P space from 930 ° C/100 MPa to 960 ° C/75 MPa with f O 2 ~N NO + 1. The H 2 O-saturated equilibration conditions of the dacite are constrained to a T-P region from 850 ° C/ 50 MPa to 880 ° C/25 MPa. If H 2 O-saturated, these magmas equilibrated at (and above) the level where coerupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H 2 O + CO 2 ) fluid for the same range in P H2O . Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H 2 O-saturated rhyolite is consistent with experimental findings only if the intermediates have X H2O fl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days.

Alaska↗

USGS2001

Shallow-storage conditions for the rhyolite of the 1912 eruption at Novarupta, Alaska

Recent studies have proposed contrasting models for the plumbing system that fed the 1912 eruption of Novarupta, Alaska. Here, we investigate the conditions under which the rhyolitic part of the erupted magma last resided in the crust prior to eruption. Geothermometry suggests that the rhyolite was held at ∼800-850 °C, and analyses of melt inclusions suggest that it was fluid saturated and contained ∼4 wt% water. Hydrothermal, water-saturated experiments on rhyolite pumice reveal that at those temperatures the rhyolite was stable between 40 and 100 MPa, or a depth of 1.8-4.4 km. These results suggest that pre-eruptive storage and crystal growth of the rhyolite were shallow; if the rhyolite ascended from greater depths, it did so slowly enough for unzoned phenocrysts to grow as it passed through the shallow crust.

Alaska↗