•Glass Mountain porous pyroclasts had water content of about 0.45 wt% immediately after eruption.•Meteoric water diffused in pyroclasts at a rate of 10−23.5±0.5 m2s−1 after the eruption.•Degassing ...occurs in closed-system until magma porosity reaches 65±5%, and in open-system beyond.
Volcanic eruptions of rhyolitic magma often show shifts from powerful (Vulcanian to Plinian) explosive episodes to a more gentle effusion of viscous lava forming obsidian flows. Another prevailing characteristic of these eruptions is the presence of pyroclastic obsidians intermingled with the explosive tephra. This dense, juvenile product is similar to the tephra and obsidian flow in composition, but is generally less degassed than its flow counterpart. The formation mechanism(s) of pyroclastic obsidians and the information they can provide concerning the extent to which magma degassing modulates the eruptive style of rhyolitic eruptions are currently subject to active research. Porous tephra and pyroclastic and flow obsidians from the 1060CE Glass Mountain rhyolitic eruption at Medicine Lake Volcano (California) were analyzed for their porosity, ϕ, water content, H2O, and hydrogen isotopic composition, δD. H2O in porous pyroclasts is correlated negatively with δD and positively with ϕ, indicating that the samples were affected by post-eruptive rehydration. Numerical modeling suggests that this rehydration occurred at an average rate of 10−23.5±0.5 m2s−1 during the ∼960 years since the eruption, causing some pyroclasts to gain up to 1 wt% of meteoric water. Pyroclastic and flow obsidians were not affected by rehydration due to their very low porosity. Comparison between modeled δD-H2O relationships in degassing magma and values measured in the Glass Mountain samples supports the idea that rhyolitic magma degasses in closed-system until its porosity reaches a value of about 65±5%, beyond which degassing occurs in open-system until quench. During the explosive phase, rapidly ascending magma fragments soon after it becomes permeable, creating porous lapilli and ash that continue to degas in open-system within an expanding gas phase. As suggested by recent studies, some ash may aggregate and sinter on the conduit sides at different depths above the fragmentation level, partly equilibrating with the continuously fluxing heavier magmatic vapor, explaining the wide range of H2O contents and high variability in δD measured in the pyroclastic obsidians. Using only H2O and δD, it is impossible to rule out the possibility that pyroclastic obsidians may also form by permeable foam collapse, either syn-explosively near the conduit sides below the fragmentation level or during more effusive periods interspersed in the explosive phase. During the final effusive phase of the eruption, slowly ascending magma degasses in open-system until it reaches the surface, creating flows with low H2O and δD. This study shows that H2O measured in highly porous pyroclasts of a few hundred years or more cannot be used to infer syn-eruptive magma degassing pathways, unless careful assessment of post-eruptive rehydration is first carried out. If their mechanism of formation can be better understood, detailed analysis of the variations in texture and volatile content of pyroclastic obsidians throughout the explosive phase may help decipher the reasons why rhyolitic eruptions commonly shift from explosive to effusive phases.
Landslide-generated tsunamis at Réunion Island Kelfoun, Karim; Giachetti, Thomas; Labazuy, Philippe
Journal of Geophysical Research: Earth Surface,
December 2010, Letnik:
115, Številka:
F4
Journal Article
Recenzirano
Odprti dostop
Landslides that occur on oceanic volcanoes can reach the sea and trigger catastrophic tsunamis. Réunion Island has been the location of numerous huge landslides involving tens to hundreds of cubic ...kilometers of material. We use a new two‐fluid (seawater and landslide) numerical model to estimate the wave amplitudes and the propagation of tsunamis associated with landslide events on Réunion Island. A 10 km3 landslide from the eastern flank of Piton de la Fournaise volcano would lift the water surface by about 150 m where it entered the sea. The wave thus generated would reach Saint‐Denis, the capital of Réunion Island (population of about 150,000 people), in only 12 min, with an amplitude of more than 10 m, and would reach Mauritius Island in 18 min. Although Mauritius is located about 175 km from the impact, waves reaching its coast would be greater than those for Réunion Island. This is due to the initial shape of the wave, and its propagation normal to the coast at Mauritius but generally coast‐parallel at Réunion Island. A submarine landslide of the coastal shelf of 2 km3, would trigger a ∼40 m high wave that would severely affect the proximal coast in the western part of Réunion Island. For a landslide of the shelf of only 0.5 km3, waves of about 2 m in amplitude would affect the proximal coast.
Magma‐water interaction can dramatically influence the explosivity of volcanic eruptions. However, syn‐ and post‐eruptive diffusion of external (non‐magmatic) water into volcanic glass remains poorly ...constrained and may bias interpretation of water in juvenile products. Hydrogen isotopes in ash from the 2009 eruption of Redoubt Volcano, Alaska, record syn‐eruptive hydration by vaporized glacial meltwater. Both ash aggregation and hydration occurred in the wettest regions of the plume, which resulted in the removal and deposition of the most hydrated ash in proximal areas <50 km from the vent. Diffusion models show that the high temperatures of pyroclast‐water interactions (>400°C) are more important than the cooling rate in facilitating hydration. These observations suggest that syn‐eruptive glass hydration occurred where meltwater was entrained at high temperature, in the plume margins near the vent. Ash in the drier plume interior remained insulated from entrained meltwater until it cooled sufficiently to avoid significant hydration.
Plain Language Summary
Explosive volcanic eruptions produce plumes of volcanic ash and gas that commonly mix with water from overlying seawater, glaciers, or hydrothermal systems. Within these plumes, volcanic glass (rapidly cooled magma) can lose its dissolved magmatic water or gain additional water from the surrounding environment. This study uses water concentrations in volcanic glass, and the hydrogen isotopes of that water, to identify if water was lost or gained in ash during the 2009 eruption of Redoubt Volcano, Alaska, USA. Results show that most of the magmatic water was lost, and some external water was gained in samples that fell closest to the volcano. Numerical models show that external water is most easily gained in glass at high temperatures even at the fastest cooling rates. These findings suggest external water was incorporated into the margins of the eruption plumes during the eruption. Ash hydration and aggregation occurred in these wet plume margins near the vent and preferentially deposited it closer to the vent. Ash in the hotter plume core that encounters water at cooler temperatures is erupted to higher altitudes and disperses the drier ash to further distances.
Key Points
Water contents and hydrogen isotopes in volcanic ash record syn‐eruptive hydration during the wet 2009 eruption of Redoubt Volcano, Alaska
The temperature of pyroclast‐water interaction, more than the pyroclast cooling rate, dictates the extent of syn‐eruptive glass hydration
More extensive hydration of proximal ashfall suggests wetter plume margins and drier, higher transport of the plume interiors
We document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass ...Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (<50–80 nm) to multidomain (>10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 1012 to 1020 m−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 1018 to 1019 m−3. These ranges agree well with TND determinations of 1018 to 1020 m−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation.
Plain Language Summary
We use magnetism experiments to prove that nanometer‐sized magnetic particles are present in volcanic rocks with low iron content and few visible crystals. Nanolites (particles between 30 and 1,000 nm) and ultra‐nanolites (particles smaller than 30 nm) are extremely difficult to detect in volcanic rocks composed mainly of glass using conventional methods such as optical and electron microscopy. Titanomagnetite nano‐particles may play a role in controlling the explosiveness of volcanic eruptions. The magnetic signatures of minerals can be used to determine their chemical composition, particle size range, and particle abundance. Pumice and obsidian contain the mineral titanomagnetite, with no evidence of prolonged crystallization at high oxygen levels at the Earth's surface. Observed magnetic behaviors are very similar to those of previously published studies of titanomagnetite in the 10–1,000 nm size range, and similar to mathematical models that simulate this size range. We find that pumice clasts have a magnetic fabric, suggesting that the nanolites and ultra‐nanolites were aligned in spatial patterns before the magma solidified, with stronger alignment coinciding with high degrees of vesicularity. Our results indicate that titanomagnetite crystals are highly abundant, and had crystallized in the magma chamber before the eruption.
Key Points
Magnetic methods document titanomagnetite nanolites in rhyolitic materials from Glass Mountain, Medicine Lake Volcano, California
Titanomagnetite number densities for pumice, obsidian, and vesicular obsidian span 1012 to 1020 m−3 of solid material
Titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation
Nucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate ...(heterogeneous nucleation, <50 MPa), and maximized when no nucleation substrate is present (homogeneous nucleation, >100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (<100 MPa) depths represents an apparent paradox that hints at the presence of a cryptic nucleation substrate. In a pair of studies focusing on Glass Mountain eruptive units from Medicine Lake, California, we characterize titanomagnetite nanolites and ultrananolites in pumice, obsidian, and vesicular obsidian (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336), calculate titanomagnetite crystal number densities, and compare titanomagnetite abundance with the physical properties of pumice to evaluate hypotheses on the timing of titanomagnetite crystallization. Titanomagnetite crystals with grain sizes of approximately 3–33 nm are identified in pumice samples from the thermal unblocking of low‐temperature thermoremanent magnetization. The titanomagnetite number densities for pumice are 1018 to 1020 m−3, comparable to number densities in pumice and obsidian obtained from room temperature methods (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336). This range exceeds reported bubble number densities (BND) within the pumice from the same eruptive units (average BND ∼4 × 1014 m−3). The similar abundances of nm‐scale titanomagnetite crystals in the effusive and explosive products of the same eruption, together with the lack of correlation between pumice permeability and titanomagnetite content, are consistent with titanomagnetite formation having preceded the bubble formation. Results suggest sub‐micron titanomagnetite crystals are responsible for heterogeneous bubble nucleation in this nominally aphyric rhyolite magma.
Key Points
Aphyric rhyolite eruptions staged from shallow magma reservoirs lack the overpressure needed for homogeneous bubble nucleation
Heterogeneous bubble nucleation may occur on sub‐µm titanomagnetite crystals, which are undetectable using standard analytical techniques
Sub‐µm titanomagnetite crystals can be detected and quantified with low temperature magnetic analyses
Pyroclastic density currents (PDCs) are the most lethal volcanic process on Earth. Forecasting their inundation area is essential to mitigate their risk, but existing models are limited by our poor ...understanding of their dynamics. Here, we explore the role of evolving grain-size distribution in controlling the runout of the most common PDCs, known as block-and-ash flows (BAFs). Through a combination of theory, analysis of deposits and experiments of natural mixtures, we show that rapid changes of the grain-size distribution transported in BAFs result in the reduction of pore volume (compaction) within the first kilometres of their runout. We then use a multiphase flow model to show how the compressibility of granular mixtures leads to fragmentation-induced fluidisation (FIF) and excess pore-fluid pressure in BAFs. This process dominates the first ~2 km of their runout, where the effective friction coefficient is progressively reduced. Beyond that distance, transport is modulated by diffusion of the excess pore pressure. Fragmentation-induced fluidisation provides a physical basis to explain the decades-long use of low effective friction coefficients used in depth-averaged simulations required to match observed flow inundation.
Numerical simulations of real-time volcanic ash dispersal forecasts and ensuing tephra hazard assessments rely on field-derived Eruption Source Parameters (ESPs) such as plume height, erupted volume, ...mass eruption rate and the Total Grain-Size Distribution (TGSD) of particles ejected from a volcano into the atmosphere. Here we calculate ESPs for the ∼7.7 ka Cleetwood eruption of Mount Mazama (Crater Lake/giiwas, Oregon, United States) that immediately preceded the caldera-forming eruption. We also introduce a novel approach to produce high-resolution grain-size distributions (GSDs) of individual samples over a wide range of particle sizes (0.00035–35 mm) by combining laser diffraction with dynamic image analysis. Detailed field analysis allows us to divide the Cleetwood eruptive sequence into a series of two distinct and consecutive VEI four eruptions: the lower (∼0.98 km
3
) and upper (∼0.20 km
3
) Cleetwood units. The lower Cleetwood was the most intense with a plume height of ∼19 km and an average mass discharge rate of ∼3.1×10
7
kg s
−1
. Its Total Grain-Size Distribution yields a fractal dimension D∼3.1, like other similar eruptions. All twelve high-resolution GSDs produced in this study exhibit two systematic breaks in slope from a power-law relationship at ∼0.125 mm and ∼0.510 mm. These breaks in slope create three segments: S1 (<0.125 mm), S2 (0.125–0.510 mm), and S3 (>0.510 mm) that can be fit by power-law relationships with fractal dimensions of D1=2.5 ± 0.2, D2=0.5 ± 0.1, and D3=3.6 ± 1.1, respectively. Together with ESPs and detailed componentry, D values at various locations give insight into magma fragmentation and tephra transport. We find that D1 values are positively correlated with the median grain-size and are similar to values found in rapid decompression magma fragmentation experiments. We infer that D1 values reflect the size distribution of the primary products of magma fragmentation and could thus be used to infer the potential energy at fragmentation. We interpret the relatively low values of D2 to an increase in dense components due to particle rafting. Our work shows that comparing high-resolution grain-size distributions at several locations on the dispersal axis can further constrain primary and secondary eruptive processes which prove crucial to improving tephra hazard assessments and dispersal forecasting.
Pyroclasts from explosive eruptions, such as the 1060 CE explosive Glass Mountain eruption of Medicine Lake volcano, California, contain large amounts of water. This may be the consequence of ...diffusive rehydration of the volcanic glass by meteoric (secondary) water after the eruption. Discriminating between magmatic and secondary water in the matrix glass of pyroclasts is important, because the degassing of magmatic water affects the intensity of volcanic eruptions. Such discrimination has remained a challenging problem, especially because some aspects of water diffusion in silicate glasses at low temperatures and atmospheric pressure remain poorly constrained. We used thermogravimetry to analyze the loss of water from natural volcanic glasses and glasses that were hydrated in the laboratory at magmatic temperatures and pressures. Numerical modeling of diffusive water loss during thermogravimetric analyses accounted for the interconversion of molecular water (H2Om) and hydroxyls groups (OH), and indicates that Glass Mountain pumices contain 0.2–0.5wt% primary water, but gained 1–2wt% of meteoric water by diffusive rehydration during the past 950years. These results confirm that the majority of magmatic water is lost from the magma during explosive eruptions. Furthermore, the integration of thermogravimetric analysis and numerical modeling facilitates discrimination between the magmatic and secondary water content of volcanic glasses.
Coalescence during bubble nucleation and growth in crystal‐free rhyolitic melt was experimentally investigated, and the percolation threshold, defined as the porosity at which the vesicular melt ...first becomes permeable, was estimated. Experiments with bubble number densities between 1014 and 1015 m−3 were compared to four suites of rhyolitic Plinian pumices, which have approximately equal bubble number densities. At the same total porosity, Plinian samples have a higher percentage of coalesced bubbles compared to their experimental counterparts. Percolation modeling of the experimental samples indicates that all of them are impermeable and have percolation thresholds of approximately 80–90%, irrespective of their porosity. Percolation modeling of the Plinian pumices, all of which have been shown to be permeable, gives a percolation threshold of approximately 60%. The experimental samples fall on a distinct trend in terms of connected versus total porosity relative to the Plinian samples, which also have a greater melt‐bubble structural complexity. The same holds true for experimental samples of lower bubble number densities. We interpret the comparatively higher coalescence within the Plinian samples to be a consequence of shear deformation of the erupting magma, together with an inherently greater structural complexity resulting from a more complex nucleation process.
Key Points
The value of percolation threshold is a consequence of geometric and topological properties set during bubble nucleation
Percolation threshold is positively correlated with the bubble number density and negatively correlated with the index of packing disorder
Expanding rhyolitic magma during Plinian eruptions becomes permeable at a porosity of approximately 60%
PDF
