In situ granulation by thermal stress during subaqueous volcanic eruptions (original) (raw)
Related papers
A review of volcanic ash aggregation
Physics and Chemistry of the Earth, Parts A/B/C, 2012
Most volcanic ash particles with diameters <63 lm settle from eruption clouds as particle aggregates that cumulatively have larger sizes, lower densities, and higher terminal fall velocities than individual constituent particles. Particle aggregation reduces the atmospheric residence time of fine ash, which results in a proportional increase in fine ash fallout within 10-100 s km from the volcano and a reduction in airborne fine ash mass concentrations 1000 s km from the volcano. Aggregate characteristics vary with distance from the volcano: proximal aggregates are typically larger (up to cm size) with concentric structures, while distal aggregates are typically smaller (sub-millimetre size). Particles comprising ash aggregates are bound through hydro-bonds (liquid and ice water) and electrostatic forces, and the rate of particle aggregation correlates with cloud liquid water availability. Eruption source parameters (including initial particle size distribution, erupted mass, eruption column height, cloud water content and temperature) and the eruption plume temperature lapse rate, coupled with the environmental parameters, determines the type and spatiotemporal distribution of aggregates. Field studies, lab experiments and modelling investigations have already provided important insights on the process of particle aggregation. However, new integrated observations that combine remote sensing studies of ash clouds with field measurement and sampling, and lab experiments are required to fill current gaps in knowledge surrounding the theory of ash aggregate formation.
Journal of Volcanology and Geothermal Research, 2006
The goal of this paper is to determine the parameters that control the aggregation efficiency and the growth rate of volcanic particles within the eruption column. Numerical experiments are performed with the plume model ATHAM (Active Tracer High resolution Atmospheric Model). In this study we employ the parameterizations described in a companion paper (this issue). The presence of hydrometeors promotes the aggregation of ash particles, which strongly increases their fall velocities and thus their environmental impact. The tephra mass is about two orders of magnitude greater than that of hydrometeors during typical Plinian eruptions without interaction of external water. Ice is highly dominant in comparison to liquid water (N 99% by mass). This is caused by the fast column rise (N 100 m s À 1 on average) to very cold altitudes. Most particles occur in the form of frozen aggregates with low ice content.
Annals of Geophysics, 2018
The Secche di Lazzaro formation (ca. 6.2-7 kys BP) is a phreatomagmatic deposit situated in the southwestern part of the island of Stromboli (Aeolian Archipelago, Italy). The volcanic sequence is comprised of three main units. In the lower unit accretionary lapilli are particularly abundant and are characterized by strong cementation between the particles and an uncommon resistance to breakage. To understand the processes behind the formation of the Secche di Lazzaro (SdL) accretionary lapilli a multi-analytical approach was used on the lapilli Aggregate Tuff (AT), and on single Accretionary Lapilli (AL). We carried out granulometric analysis, Field Emission -Scanning Electron Microscopy (FE-SEM), Electron Microprobe Analysis (EMPA), X-ray powder diffraction (XRPD) and 3D imaging by X-ray micro-tomography (X-mCT). The granulometric data show that most particles in the AT have a diameter equal to Φ -1 corresponding to 2 mm. The EMPA, FE-SEM and XRPD analyses reveal the presence of different mineral phases, mainly plagioclase, K-feldspar, halite, and clinopyroxene, together with volcanic glass. From the X-mCT analysis, we constrained the particle distribution and estimated the porosity of AL. The results of the FE-SEM images provided the chemical distribution within individual lapilli allowing the identification of rim and core zoning as well as the presence of halite located both on the border of single lapilli and on the juncture between different lapilli. Moreover, halite occurs among different aggregates in single AL, thus acting as a binding agent, as well as within rim pores.
Granular disruption during explosive volcanic eruptions
2012
Explosive volcanic eruptions are among the most energetic events on Earth. The hazard to surrounding populations and aviation is controlled by the concentration and size of particles that exit the volcanic vent. The size distribution of volcanic particles is thought to be determined by the initial fragmentation process 1-4 , where bubbly magmatic mixtures transition to gas-particle flows. Here we show that collisional processes in the volcanic conduit after initial fragmentation can change the grain-size distribution of particles that leave the volcanic vent. We use experimental analysis of the breakup of natural volcanic rocks during collisions, as well as numerical simulations, to estimate the probability that particles pass through the volcanic conduit and survive intact. We find that breakup in the conduit is strongly controlled by the initial particle size and the location of the initial fragmentation: particles that measure more than 1 cm in diameter and those fragmented at great depths break up most frequently. Abundant large pumice clasts in volcanic deposits therefore imply shallow fragmentation that may be transient. In contrast, fragmentation events at depth will lead to enhanced ash production and greater atmospheric loading of long-residence, fine-grained ash.
Journal of Volcanology and Geothermal …, 2013
This study documents the processes and products of volcanic ash aggregation in phreatomagmatic phases of the 25.4 ka Oruanui supereruption from Taupo volcano, New Zealand. Detailed textural and stratigraphic relationships of aggregates are examined in six of the ten erupted units, which range from relatively dry styles of eruption and deposition (units 2, 5) to mixed (units 6, 7, 8) and dominantly wet (unit 3). Aggregate structures and grain size distributions shift abruptly over vertical scales of cm to dm, providing diagnostic features to identify deposits emplaced primarily as vertical fallout or pyroclastic density currents (PDCs). The six categories of ash aggregates documented here are used to infer distinct volcanic and meteorological interactions in the eruption cloud related to dispersal characteristics and mode of emplacement. Our field observations support the notion of Brown et al. (2010, Origin of accretionary lapilli within ground-hugging density currents: evidence from pyroclastic couplets on Tenerife. Geol. Soc. Am. Bull. 122, 305–320) that deposits bearing matrix-supported accretionary lapilli with concentric internal structure and abundant rim fragments are associated with emplacement of PDCs. However, on the basis of grain size distributions and field relationships, it is inferred that these types of ash aggregates formed their ultrafine ash (dominantly < 10 μm) outer layers in the buoyant plumes of fine ash lofted from PDCs, rather than during lateral transport in ground-hugging density currents. The propagation of voluminous PDCs beneath an overriding buoyant cloud – whether coignimbrite or vent-derived in origin – is proposed to generate the observed, concentrically structured accretionary lapilli by producing multiple updrafts of convectively unstable, ash-laden air. The apparent coarsening of mean grain size with distance from source, which is observed in aggregate-bearing fall facies, reflects a combination of multi-level plume transport and enhanced proximal fallout of fine ash (< 250 μm) by aggregation. Gravitational fallout and melting of abundant ice in the clouds was likely to have contributed a key source of liquid water for wet aggregation in near-source areas. In contrast, deposits from relatively drier eruption phases are aggregate-poor in proximal areas, yet develop loosely-bound particle clusters and mm-scale massive ash pellets > 100 km from vent. It is inferred that ambient meteorological conditions play a more important role in ash fallout in these cases. Entrainment of moist air, and distal subsidence and melting of ice carried by the plume, are both likely to have contributed to the observed features of late-stage aggregation in the drier phases of eruption. These observations suggest that proximal, column-influenced aggregation processes, which weaken with distance from source, are overprinted by secondary, later-stage aggregation mechanisms farther downwind.
Another look to the mechanisms of formation of ash aggregates in pyroclastic deposits
Rocha R., Pais J., Kullberg J., Finney S. (eds) STRATI 2013. Springer Geology. Springer, Cham, 2014
Ash aggregation has been a subject of great interest in volcanology, due to its importance in removing the finer-grained fraction of the fragmented material generated during explosive eruptions. In such events, the amount of ash (<2 mm) represents a large fraction of the total erupted mass, and is dispersed into the surrounding atmosphere by vertical plumes and/or pyroclastic density currents (PDCs). Aggregation enhances sedimentation, reducing the residence time of solid particles in the atmosphere; therefore, understanding the processes that govern particle accretion is of critical importance for hazard assessment. Observations and experimental studies to date indicate that water, either in liquid or solid states, is able, in certain proportions, to provide the strongest bonds between particles, which are necessary to form spherical to oblate aggregates able to survive impact with the ground and to be preserved in pyroclastic deposits. In contrast, electrostatic attraction between particles forms only dry, loosely bound aggregates, several hundreds of microns in size, which rapidly disintegrate. In general, aggregates are sub-mm to a few mm in size, even if maximum sizes of several centimetres are sometimes reported. Nevertheless, the individual accreted particles rarely exceed 1 mm. Several types of aggregates were described in the PDCs produced during the 1982 eruption of El Chichón volcano (Mexico), characterized by the injection of 8 million tons of SO2 into the atmosphere, and responsible for a 5–6 °C warming in the tropical lower stratosphere. In such aggregates, individual components are strongly cemented by an S-rich film, in which particles between 1 and a few mm in diameter are common. Even if not visible at the outcrop scale, they represent a consistent proportion of the deposits and are extremely resistant to disaggregation, as shown by their capacity to survive not only the impact with the ground after falling, but also collisions with other clasts. Their similarities with aggregates found in sulphur cones at Poás volcano suggest that liquid sulphur is the cementing material. The explosive ejection of sulphur may occur in volcanoes with active hydrothermal systems. The ability of liquid sulphur to cement particles larger than 1 mm in diameter indicates that size fractions of lapilli can be efficiently removed from eruptive clouds at distances of a few km from the vent, which has important implications for hazard assessment.
Scientific reports, 2017
Interactions with volcanic gases in eruption plumes produce soluble salt deposits on the surface of volcanic ash. While it has been postulated that saturation-driven precipitation of salts following the dissolution of ash surfaces by condensed acidic liquids is a primary mechanism of salt formation during an eruption, it is only recently that this mechanism has been subjected to detailed study. Here we spray water and HCl droplets into a suspension of salt-doped synthetic glass or volcanic ash particles, and produce aggregates. Deposition of acidic liquid droplets on ash particles promotes dissolution of existing salts and leaches cations from the underlying material surface. The flow of liquid, due to capillary forces, will be directed to particle-particle contact points where subsequent precipitation of salts will cement the aggregate. Our data suggest that volcanically-relevant loads of surface salts can be produced by acid condensation in eruptive settings. Several minor and tra...