Ash aggregation during the 11 February 2010 partial dome collapse of the Soufrière Hills Volcano, Montserrat

Abstract

On 11 February 2010, Soufrière Hills volcano, Montserrat, underwent a partial dome collapse (~50 x 10 6 m 3) and a short-lived Vulcanian explosion towards the end. Three main pyroclastic units were identified N and NE of the volcano: dome-collapse pyroclastic density current (PDC) deposits, fountain-collapse PDC deposits formed by the Vulcanian explosion, and a tephra deposit associated with elutriation from the dome-collapse and fountain-collapse PDCs (i.e. co-PDC fallout deposit). The fallout associated with the Vulcanian explosion was mostly dispersed E and SE by high altitude winds. All units N and NE of the volcano contain variable amounts and types of particle aggregates, although the co-PDC fallout deposit is associated with the largest abundance (i.e. up to 24 wt%). The size of aggregates found in the co-PDC fallout deposit increases with distance from the volcano and proximity to the sea, reaching a maximum diameter of 12 mm ∼500m from the coast. The internal grain size of all aggregates have nearly identical distributions (with Mdφ ≈ 4-5), with particles in the size categories >3phi (i.e. <250µm) being distributed in similar proportions within the aggregates but in different proportions within distinct internal layers. In fact, most aggregates are characterized by a coarse grained central core occupying the main part of the aggregate, coated by a thin layer of finer ash (single-layer aggregates), while others have one or two additional layers accreted over the core (multiple-layer aggregates). Calculated aggregate porosity and settling velocity vary between 0.3-0.5 and 11-21 m/s, respectively. The aggregate size shows a clear correlation with both the core size and the size of the largest particles found in the core. The large abundance of aggregates in the co-PDC fallout deposits suggests that the buoyant plumes elutriated above PDCs represent an optimal environment for the formation (particle collision) and development (aggregate layering) of particle aggregates. However, specific conditions are required, including i) a large availability of water (in this case provided by the steam plumes associated with the entrance of PDCs into the ocean), ii) presence of plume regions with different grain-size features (i.e. both median size and sorting) that allows for the developments of multiple layers, iii) strong turbulence that permits both particle collision and the transition of the aggregates through different plume regions, iv) presence of hot

The fate of volcanic ash: premature or delayed sedimentation?

“… Generalized plot showing the potential for particle rafting for different types of aggregates. a Green zones indicate observed values of porosity ( ϕ A ) for PC1 18 , 19 , PC2 19 and PC3 17 , 19 , while the pink area indicates the typical values of porosity for AP1 and AP2 18 , 43 (see also table 16 .1 in Sparks et al 1997 44 ). Unfortunately, the few studies present in literature for PC1 do not allow a good constraint on the values of the associated porosities (indicated, therefore, as dashed lines).…”

Aggregation in particle rich environments: a textural study of examples from volcanic eruptions, meteorite impacts, and fluidized bed processing

“…Aggregates were sampled from the co-PDC fall deposits of the 11 February 2010 eruption (see Stinton et al 2014 ; Burns et al 2017 ). The sampled deposit consisted of a 10-cm-thick layer entirely composed of accretionary pellets.…”

“…Observations of aggregate fallout indicate that many aggregates do not survive transport and/or sedimentation (e.g., Taddeucci et al 2011 ; Bonadonna et al 2011 ; Bagheri et al 2016 ; Mueller et al 2017a ), and this has also been inferred from modeling and observations of secondary thickness maxima in ash deposits (Durant et al 2009 ). Larger and strongly bonded aggregates, such as accretionary lapilli, do survive transport and deposition processes and are commonly preserved in deposits as, for example, in deposits of the eruptions of Tungurahua volcano, Ecuador (Kueppers et al 2016 ), Soufriere Hills volcano, Montserrat (Burns et al 2017 ), and Volcán de Colima, Mexico (Reyes-Dávila et al 2016 ), as well as many others (e.g., Brown et al 2012 ).…”

Stability of volcanic ash aggregates and break-up processes

“…These experiments were designed to evaluate the effect of particle size distribution (PSD), shape and surface morphology of primary particles and binder concentration on aggregate stability. We performed these experiments on artificial aggregates only as young natural ash aggregates are rare and even the youngest 43 have likely already undergone further post-depositional (re-)crystallization, e.g. by growth of zeolites.…”