The fluidization behaviour of ignimbrite at high temperature and with mechanical agitation

Experiments were carried out to study the fluidization behaviour of ignimbrite at high temperature and while being mechanically agitated. Geldart group C behaviour occurs up to 565 degreesC when the material is subjected to increasing gas flow ( without agitation) from the loosely packed state. In contrast, even gentle mechanical agitation inhibits channelling and results in group-A type behaviour with homogeneous (non-bubbling) expansions of up to 30 - 40%. Bed collapse tests exhibit group-C behaviour at room temperature, group-A behaviour at 200 - 565 degreesC, and transitional behaviour at 55 degreesC. Both elevated temperature and mechanical agitation greatly increase the fluidizability of ignimbrite. It is inferred that a combination of high temperature and shear during transport will promote Geldart group A behaviour in pyroclastic flows

Pre-explosive conduit conditions of the 1997 Vulcanian explosions at Soufrière Hills Volcano, Montserrat: II. Overpressure and depth distributions

International audienceA type example of Vulcanian eruptive dynamics is the series of 88 explosions that occurred between August and October 1997 at Soufrière Hills volcano on Montserrat Island. These explosions are interpreted to be caused by the pressurization of a conduit by a shallow highly crystalline and degassed magma plug. We test such an interpretation by combining the pressures and porosities of the pre-explosive magma column proposed by Burgisser et al. (2010, doi:10.1016/j.jvolgeores.2010.04.008) into a physical model that reconstructs a depth-referenced density profile of the column for four mechanisms of pressure buildup. Each mechanism yields a different overpressure profile: 1) gas accumulation, 2) conduit wall elasticity, 3) microlite crystallization, and 4) magma flowage. For the first three mechanisms, the three-part vertical layering of the conduit prior to explosion was spatially distributed as a dense cap atop the conduit with a thickness of a few tens of meters, a transition zone of 400–700 m with heterogeneous vesicularities, and, at greater depth, a more homogeneous, low-porosity zone that brings the total column length to ~ 3.5 km. A shorter column can be obtained with mechanism 4: a dense cap of less than a few meters, a heterogeneous zone of 200–500 m, and a total column length as low as 2.5 km. Inflation/deflation cycles linked to a periodic overpressure source offer a dataset that we use to constrain the four overpressure mechanisms. Magma flowage is sufficient to cause periodic edifice deformation through semi-rigid conduit walls and build overpressures able to trigger explosions. Gas accumulation below a shallow plug is also able to build such overpressures and can occur regardless of magma flowage. The concurrence of these three mechanisms offers the highest likelihood of building overpressures leading to the 1997 explosion series. We also explore the consequences of sudden (eruptive) overpressure release on our magmatic columns to assess the role of syn-explosive vesiculation and pre-fragmentation column expansion. We find that large shallow overpressures and efficient syn-explosive vesiculation cause the most dramatic pre-fragmentation expansion. This leads us to depict two end-member pictures of a Vulcanian explosion. The first case corresponds to the widely accepted view that the downward motion of a fragmentation front controls column evacuation. In the second case, syn-explosive column expansion just after overpressure release brings foamed-up magma up towards an essentially stationary and shallow fragmentation front

Quantitative textural analysis of Vulcanian pyroclasts (Montserrat) using multi-scale X-ray computed microtomography: comparison with results from 2D image analysis

International audienceX-ray computed microtomography (μCT) was carried out on four pyroclasts from the 1997 Vulcanian explosions of Soufrière Hills Volcano, Montserrat. Three-dimensional data from multiple image stacks with different spatial resolutions (0.37, 4-8, and 17.4 μm px−1) were combined to generate size distributions of vesicles, inter-vesicle throats, crystals, and Fe-Ti oxides over a 3.4-860-μm size range, and to compare the results with those obtained by 2D image analysis on the same samples. Qualitative textural observations are in good agreement with those made in 2D, but μCT provides better resolution of textural features and spatial relationships. Calculation of size distributions requires automated decoalescence of the connected vesicle network. Problems related to this process, in part due to the high porosity of pumice, result in potential artefacts in the calculated size distributions, which are discussed in detail. The main modes of the 3D vesicle volume distributions are systematically shifted to larger sizes compared with those of the 2D distributions. Sample total vesicularities obtained in 3D are within 13 vol.% of those found in 2D, and within 10 vol.% of those measured by He-pycnometry. Total number densities of vesicles and Fe-Ti oxides from the two methods are consistent only to the first order, 3D values ranging from 37% to 309% of those in 2D. Vesicle coalescence, investigated by examining inter-vesicle throat size distributions, occurred in all pyroclasts between neighbouring vesicles of many sizes. The larger the vesicle, the more connected it is

Bubble nucleation, growth and coalescence during the 1997 Vulcanian explosions of Soufrière Hills Volcano, Montserrat

International audienceSoufrière Hills Volcano had two periods of repetitive Vulcanian activity in 1997. Each explosion discharged the contents of the upper 0.5–2 km of the conduit as pyroclastic flows and fallout: frothy pumices from a deep, gas-rich zone, lava and breadcrust bombs from a degassed lava plug, and dense pumices from a transition zone. Vesicles constitute 1–66 vol.% of breadcrust bombs and 24–79% of pumices, all those larger than a few tens of µm being interconnected. Small vesicles ( few hundreds of µm) in pumices are interpreted as pre-dating explosion, implying pre-explosive conduit porosities up to 55%. About a sixth of large vesicles in pumices, and all those in breadcrust bombs, are angular voids formed by syn-explosive fracturing of amphibole phenocrysts. An intermediate-sized vesicle population formed by coalescence of the small syn-explosive bubbles. Bubble nucleation took place heterogeneously on titanomagnetite, number densities of which greatly exceed those of vesicles, and growth took place mainly by decompression. Development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion–decompression and surface quench in contact with air. Lava-plug fragments entered the air quickly after fragmentation (not, vert, similar 10 s), so the interiors continued to vesiculate once the rinds had quenched, forming breadcrust bombs. Deeper, gas-rich magma took longer (not, vert, similar 50 s) to reach the surface, and vesiculation of resulting pumice clasts was essentially complete prior to surface quench. This accounts for the absence of breadcrusting on pumice clasts, and for the textural similarity between pyroclastic flow and fallout pumices, despite different thermal histories after leaving the vent. It also allowed syn-explosive coalescence to proceed further in the pumices than in the breadcrust bombs. Uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn-explosive vesiculation even prior to magma fragmentation, probably in a zone of steep pressure gradient beneath the descending fragmentation front. Syn-explosive decompression rates estimated from vesicle number densities (> 0.3–6.5 MPa s− 1) are consistent with those predicted by previously published numerical models

Forecasting magma-chamber rupture at Santorini volcano, Greece

How much magma needs to be added to a shallow magma chamber to cause rupture, dyke injection, and a potential eruption? Models that yield reliable answers to this question are needed in order to facilitate eruption forecasting. Development of a long-lived shallow magma chamber requires periodic influx of magmas from a parental body at depth. This redistribution process does not necessarily cause an eruption but produces a net volume change that can be measured geodetically by inversion techniques. Using continuum-mechanics and fracture-mechanics principles, we calculate the amount of magma contained at shallow depth beneath Santorini volcano, Greece. We demonstrate through structural analysis of dykes exposed within the Santorini caldera, previously published data on the volume of recent eruptions, and geodetic measurements of the 2011–2012 unrest period, that the measured 0.02% increase in volume of Santorini’s shallow magma chamber was associated with magmatic excess pressure increase of around 1.1 MPa. This excess pressure was high enough to bring the chamber roof close to rupture and dyke injection. For volcanoes with known typical extrusion and intrusion (dyke) volumes, the new methodology presented here makes it possible to forecast the conditions for magma-chamber failure and dyke injection at any geodetically well-monitored volcano

The fast response of volcano-seismic activity to intense precipitation: Triggering of primary volcanic activity by rainfall at Soufrière Hills Volcano, Montserrat

One-minute resolution time series of rainfall and seismic data from the Soufriere Hills Volcano, Montserrat are analysed to explore the mechanism of external forcing of volcanic eruptions by rainfall over three years of activity. The real-time seismic amplitude (RSAM) shows a narrow, statistically significant, peak within 30 min after the start of intense rainfall events, and a much broader peak with a lag of 6?40 h. The classified seismic events indicate that the volcanic response to rainfall begins at the surface and gradually penetrates deeper into the dome, as there is an increase in the pseudo-magnitude of: surface rockfall events (including pyroclastic flows) with lags from the first 30 min to 40 h, long-period rockfalls (from shallow degassing) at lags of 4 and 14 h, and long-period and hybrid events (source depth approximately 1 km) with lags at 14 and 24 h after the start of rainfall events. There was no rainfall-related change in deeper, volcano-tectonic activity. There was no change in the frequency of any type of classified event, indicating that the rainfall acts to modulate existing, internal processes, rather than generating new events itself. These robust results are due to many (229) different rainfall events, and not just to a few, large magnitude cases. The rainfalltriggered volcanic activity examined here is consistent with a model of fast, shallow interactions with rainfall at the dome surface, after which, a deeper dome collapse follows

Eucalyptus cunninghamii (Myrtaceae) in the Blue Mountains of New South Wales – an Unexpected Connection between Botany and Geology

A three-year field study of the rare Eucalyptus cunninghamii Sweet (Myrtaceae), the Cliff Mallee Ash, uncovered a hitherto unrecognised connection between this taxon and its substrate. E cunninghamii has long been known to grow on cliff edges around two valleys of the upper Blue Mountains, the Grose and the Jamison, but details of that habitat were unexplored. We show that its preferred habitat in the upper Blue Mountains is on or just downslope of a clay layer, most often the Wentworth Falls Claystone Member (WFC), and that E cunninghamii is not found elsewhere (except for an anomalous outlier near Mittagong) because the WFC is not expressed stratigraphically except around the clifflines of the two valleys. This and other environmental features delimit the distribution of E cunninghamii to a very precise geospatial range which may cause problems in times of climate change

Observations on Average Trunk Diameters of Eucalyptus cunninghamii (Myrtaceae) in Relation to Elemental Concentrations of their Substrates

The rare Eucalyptus cunninghamii Sweet (Myrtaceae), the Cliff Mallee Ash, inhabits a geological niche associated with claystones, most often the Wentworth Falls Claystone Member of the Narrabeen Group. We show that some gross morphological attributes of E cunninghamii vary widely and that they are determined by elemental composition of its claystone substrate. We examined eight widely separated specimens and their substrates in the upper Blue Mountains of New South Wales and found that concentrations of thirteen elements in those substrates varied widely, mostly following a linear pattern. We also found that average trunk diameter of each specimen correlated strongly with most of the elemental concentrations in its substrate, and that average trunk diameter was therefore a good guide to the nutrient status of its substrate. Concentrations of potassium and phosphorus both varied by an order of magnitude