Emma Liu

Ash mists and brown snow: Remobilization of volcanic ash from recent Icelandic eruptions

Recent eruptions in Iceland and Chile have demonstrated that volcanic ash problems persist long after an eruption. For this reason, ash dispersion models are being extended to include ash remobilization. Critical to these models is knowledge of the ash source and the particle sizes that can be mobilized under different wind and moisture conditions. Here we characterize the physical and chemical characteristics of ash deposited on new snow in Reykjavik, Iceland, following a blizzard on 6 March 2013. Morphological, textural, and compositional analyses indicate resuspension from multiple eruptive deposits, including both Grimsvotn (2011) and Eyjafjallajokull (2010) eruptions. Grain size measurements show a mode of 32–63 µm, with particles as large as 177 µm; there is little mass in the very fine fraction, ≤10 µm (PM10). We compare our observations to predictions using the Lagrangian particle dispersion model, NAME (UK Met Office). The model output is consistent with observations in that it forecasts resuspension from both Eyjafjallajokull and Grimsvotn source regions, and shows ash deposition coincident with the timing of observed deposition in Reykjavik. The modeled deposit in Reykjavik predicts, however, a substantially lower proportion of Grimsvotn ash than observed. This discrepancy has highlighted the need to reassess the assumptions used in the simulations, particularly regarding the source area and precipitation thresholds. Furthermore, we suggest that modification of ash deposits in the form of erosion, redeposition, compaction, or cementation may influence the dynamics of resuspension over time, thus influencing the ability of model simulations to accurately forecast remobilization events.

The role of bubbles in generating fine ash during hydromagmatic eruptions

The abundant fine ash produced in the 2011 subglacial eruption of Grimsvotn, Iceland, highlights the fragmentation efficiency of mafic hydromagmatic eruptions, which is considerably higher than for comparable “dry” eruptions. Ash from the 2011 eruption can be divided into three morphological components—vesicular particles, shards, and dense fragments—distinguished by the size and abundance of constituent vesicles. We use the vesicle characteristics to define a new shape factor, the concavity index, which provides an unbiased way to classify individual ash particles as either bubbly (vesicular particles and shards) or dense. The relative proportion of bubbly and dense particles varies systematically with grain size, with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter. Measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from Hawaiian fountains and suggest a comparable degassing history during magma ascent. Yet concordance between the size distributions of ash and of bubbles in the Grimsvotn samples stands in contrast to the size distributions in Hawaiian fountains, where pyroclasts are orders of magnitude larger than individual bubbles. We propose that the Grimsvotn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water. The strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation.

Contrasting mechanisms of magma fragmentation during coeval magmatic and hydromagmatic activity: the Hverfjall Fires fissure eruption, Iceland

Growing evidence for significant magmatic vesiculation prior to magma-water interaction (MWI) has brought into question the use of ‘diagnostic’ features, such as low vesicularities and blocky morphologies, to identify hydromagmatic pyroclasts. We address this question by quantifying co-variations in particle size, shape and texture in both magmatic and hydromagmatic deposits from the Hverfjall Fires fissure eruption, Iceland. Overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth, with substantial vesiculation prior to MWI. Hydromagmatic fragmentation occurred principally by brittle mechanisms, where the length scale and geometry of fracturing was controlled by the bubble population. This suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven, at least in part, by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling. In this way, the shape and size distributions of hydromagmatic pyroclasts, both critical input parameters for ash dispersion models, are strongly influenced by the dynamics of vesiculation prior to MWI. This result underlines the need to analyse multiple grain-size fractions to characterise the balance between magmatic and hydromagmatic processes. During the Hverfjall Fires eruption, the external water supply was sufficient to maintain MWI throughout the eruption, with no evidence for progressive exhaustion of a water reservoir. We suggest that both the longevity and the spatial distribution of MWI were determined by the pre-existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas.

A distinct metal fingerprint in arc volcanic emissions

As well as gases that regulate climate over geological time, volcanoes emit prodigious quantities of metals into the atmosphere, where they have key roles as catalysts, pollutants and nutrients. Here we compare measurements of arc basaltic volcano metal emissions with those from hotspot settings. As well as emitting higher fluxes of metals (similar to those building ore deposits), these arc emissions possess a distinct compositional fingerprint, particularly rich in tungsten, arsenic, thallium, antimony and lead when compared with those from hotspots. We propose that volcanic metal emissions are controlled by magmatic water content and redox: hydrous arc magmas that do not undergo sulfide saturation yield metal-rich, saline aqueous fluid; shallow degassing and resorption of late-stage sulfides feeds volcanic gases in Hawai’i and Iceland. Although global arc magma chemistries vary considerably, our findings suggest that volcanic emissions in arcs have a distinct fingerprint when compared with other settings. A shift in global volcanic metal emissions may have occurred in Earth’s past as more oxidized, water-rich magmas became prevalent, influencing the surface environment.Arc volcanism emits higher metal fluxes to Earth’s atmosphere than hotspot volcanism. The systems’ unique gas compositions are controlled by magmatic water content and redox state, as shown by a compilation of volcanic gas and aerosol metal data.