Research ArticleGEOLOGY

Explosive-effusive volcanic eruption transitions caused by sintering

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Science Advances  23 Sep 2020:
Vol. 6, no. 39, eaba7940
DOI: 10.1126/sciadv.aba7940

Figures

  • Fig. 1 The water content of erupted rhyolites worldwide.

    (A) The distribution of dissolved water measured in glass inclusions trapped in crystals for a range of rhyolite eruptions (213 data points) (1012). Taken to represent the initial, dominant volatile conditions for ascent, these values are similar in variance and mean value and do not vary substantially with eruptive style (we do not account for volatile loss after entrapment). (B) The distribution of dissolved total water content remaining in the groundmass glass of eruptive products, in a second global database of rhyolite compositions compiled here (7430 data points) (10, 1219, 46). Effusive eruption products have substantially lower water than explosive products; hybrid products fill the gap, forming a continuum. wt % is used to denote weight %.

  • Fig. 2 A new cryptic fragmentation model to explain the textural and chemical record of rhyolitic volcanism.

    (A to D) Schematic summaries of the cryptic fragmentation model for rhyolitic volcanism. We note that this schematic is necessarily a simplification of what may be a more complex three-dimensional (3D) picture. Hydrostatic considerations indicate that a pressure of just a few megapascals at the base of the aggrading mass (in C and D) is sufficient to drive effusion. Inset in (D) shows data for H2O from the shallow drilling of the Obsidian Dome (20), showing that shallowest apparently effusive eruptions are equilibrated below magmastatic conditions, at an approximate atmospheric pressure down to −50 m below the surface, a feature consistent with our model. (E to G) The textural record of fragmentation and welding in rhyolites. (E) Sintered fractures in the Mono Craters obsidian chips collected from fall deposits from a Plinian explosive volcanic plume. Photo credit: J. Gardner. (F) A partially welded tuffisite vein from a bomb at Volcán Chaitén. Photo reproduced with permission from (52). (G) Particles of <5 × 10−5 m plastered and sintered in an apparently effusive lava. Photo credit: H. Tuffen.

  • Fig. 3 A quantitative test of the cryptic fragmentation model.

    (A) The output velocity u of a magma mixture rising to the surface of Earth, from a one-dimensional volcanic conduit model (32) for conduit radii H = 10 m and H = 25 m. The arrow marks fragmentation (see Fig. 2) and labels the mixture dissolved water content at that point, termed as Ci, which is used to initialize the particle-scale model. (B and C) The results of the particle-scale model above fragmentation in which the averaged bulk water concentration remnant to a particle 〈C〉 traveling through the velocity and pressure field toward the surface is tracked [H = 10 m (B) and H = 25 m (C)] where the upper arrow denotes the equilibrium value at the surface. (D) The value of water concentration 〈C〉 at the point where the particle reaches the surface as a function of the spherical radius of the particle R. Large ash particles (R ≳ 10−4 m) preserve the water concentration at fragmentation and do not equilibrate during eruption, whereas small ash particles (R ≲ 10−5 m) thoroughly degas and are consistent with the mean water concentrations remnant in apparently effusive eruptions (histogram on right axis; see Fig. 1). Inset: the sintering time λ at the surface of Earth for particles with the 〈C〉 predicted, showing that, once captured, the smallest particles can produce dense lava in under 1 hour; a time scale far shorter than dome extrusion times.

  • Fig. 4 Ascent rates of subaerial rhyolites recorded by experimental petrology or direct observations of ascent, extrusion, or eruption rates (38, 48, 49).

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