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What factors control superficial lava dome explosivity?

Boudon G, Balcone-Boissard H, Villemant B, Morgan DJ - Sci Rep (2015)

Bottom Line: Superficial explosion of a growing lava dome may be promoted through porosity reduction caused by both vesicle flattening due to gas escape and syn-eruptive cristobalite precipitation.Explosive activity is thus more likely to occur at the onset of lava dome extrusion, in agreement with observations, as the likelihood of superficial lava dome explosions depends inversely on lava dome volume.This new result is of interest for the whole volcanological community and for risk management.

View Article: PubMed Central - PubMed

Affiliation: Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005, Paris, France.

ABSTRACT
Dome-forming eruption is a frequent eruptive style and a major hazard on numerous volcanoes worldwide. Lava domes are built by slow extrusion of degassed, viscous magma and may be destroyed by gravitational collapse or explosion. The triggering of lava dome explosions is poorly understood: here we propose a new model of superficial lava-dome explosivity based upon a textural and geochemical study (vesicularity, microcrystallinity, cristobalite distribution, residual water contents, crystal transit times) of clasts produced by key eruptions. Superficial explosion of a growing lava dome may be promoted through porosity reduction caused by both vesicle flattening due to gas escape and syn-eruptive cristobalite precipitation. Both processes generate an impermeable and rigid carapace allowing overpressurisation of the inner parts of the lava dome by the rapid input of vesiculated magma batches. The relative thickness of the cristobalite-rich carapace is an inverse function of the external lava dome surface area. Explosive activity is thus more likely to occur at the onset of lava dome extrusion, in agreement with observations, as the likelihood of superficial lava dome explosions depends inversely on lava dome volume. This new result is of interest for the whole volcanological community and for risk management.

No MeSH data available.


Related in: MedlinePlus

Residual H2O content (H2Or) vs Vesicularity glass of clasts from dome-forming eruptions; comparison with clasts from plinian and vulcanian eruptions.Magmatic H2Or is calculated from H2O content measured in bulk clast corrected from phenocrysts content (crystallinity). Lava domes, C-PDCs and D-PDCs: blue and red domains as in Fig. 4. Plinian and vulcanian clasts: Orange circle: 1902 plinian eruption of Santa Maria (Guatemala;2122); Open circle: plinian phase of the P1 eruption of Montagne Pelée (650 y. BP; Martinique;2122); Green triangle: vulcanian phase of Soufrière Hills (Montserrat;22). Lines refer to closed system degassing models from initial rhyolitic melts containing 6.5% H2O (orange line: plinian-type eruptions;2122) and 1% of H2O (green dotted line: vulcanian explosions,23) which fit the H2Or - vesicularity evolution of clasts. All C-PDC fragments plot below these two closed system evolution lines. D-PDC clasts of every studied eruption span a very large domain between H2O-poor, vesicle collapsed clasts and H2O-rich, highly vesiculated clasts from plinian eruptions. This extreme heterogeneity indicates that before explosion, the different parts of the lava dome have suffered highly variable degassing conditions varying between pure closed system degassing, likely at the center of the lava dome or as new intruding magma batch, to highly degassed in open system (extreme H2O loss and vesicle collapse), likely at the lava dome upper external carapace.
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f6: Residual H2O content (H2Or) vs Vesicularity glass of clasts from dome-forming eruptions; comparison with clasts from plinian and vulcanian eruptions.Magmatic H2Or is calculated from H2O content measured in bulk clast corrected from phenocrysts content (crystallinity). Lava domes, C-PDCs and D-PDCs: blue and red domains as in Fig. 4. Plinian and vulcanian clasts: Orange circle: 1902 plinian eruption of Santa Maria (Guatemala;2122); Open circle: plinian phase of the P1 eruption of Montagne Pelée (650 y. BP; Martinique;2122); Green triangle: vulcanian phase of Soufrière Hills (Montserrat;22). Lines refer to closed system degassing models from initial rhyolitic melts containing 6.5% H2O (orange line: plinian-type eruptions;2122) and 1% of H2O (green dotted line: vulcanian explosions,23) which fit the H2Or - vesicularity evolution of clasts. All C-PDC fragments plot below these two closed system evolution lines. D-PDC clasts of every studied eruption span a very large domain between H2O-poor, vesicle collapsed clasts and H2O-rich, highly vesiculated clasts from plinian eruptions. This extreme heterogeneity indicates that before explosion, the different parts of the lava dome have suffered highly variable degassing conditions varying between pure closed system degassing, likely at the center of the lava dome or as new intruding magma batch, to highly degassed in open system (extreme H2O loss and vesicle collapse), likely at the lava dome upper external carapace.

Mentions: To acquire a significant strength, the carapace must attain a thickness of few meters to several tens of meters in the absence of other consolidating effects29. We suggest that silicification may reduce the threshold thickness to the lower range. This impermeable and resistant carapace mechanically isolates the core of the lava dome, thus preventing further volatile exsolution and induced crystallization of the melt confined in this core. The D-PDC’s clasts span a wide range of vesicularity and H2Or content, from almost completely degassed, microcrystalline clasts to undegassed, microlite-free clasts (~2.5 wt% of H2Or for a glass vesicularity of ~80%; Figs 2d and 4b; Tables 1 and 2). The vesiculated, H2O-rich clasts display textural characteristics similar to those of plinian clasts33, contrary to the low vesiculated, H2O-poor clasts, which are highly microcrystalline and cristobalite-rich. Thus volatile-rich and vesiculated clasts may represent the magma stored within the core of the lava dome and that evolves in a closed system degassing, leading to overpressurization of the upper volcanic edifice (Fig. 6). By contrast, the less-vesiculated, H2O-poor clasts may represent magma that evolved in an open-system, degassing in the external part of the lava dome. Overpressures of 0.1–1 MPa are sufficient to surpass the tensile strength of the carapace and to trigger the explosive destruction of the upper part of the lava dome63435. The pressure distribution within the edifice may be significantly modified by the existence of shear stress at the conduit vent12: the largest gas overpressure may be located at the conduit wall (horizontal spreading at the vent) or at the center of the conduit (zero horizontal velocity as in the case of a pre-existing lava dome)36.


What factors control superficial lava dome explosivity?

Boudon G, Balcone-Boissard H, Villemant B, Morgan DJ - Sci Rep (2015)

Residual H2O content (H2Or) vs Vesicularity glass of clasts from dome-forming eruptions; comparison with clasts from plinian and vulcanian eruptions.Magmatic H2Or is calculated from H2O content measured in bulk clast corrected from phenocrysts content (crystallinity). Lava domes, C-PDCs and D-PDCs: blue and red domains as in Fig. 4. Plinian and vulcanian clasts: Orange circle: 1902 plinian eruption of Santa Maria (Guatemala;2122); Open circle: plinian phase of the P1 eruption of Montagne Pelée (650 y. BP; Martinique;2122); Green triangle: vulcanian phase of Soufrière Hills (Montserrat;22). Lines refer to closed system degassing models from initial rhyolitic melts containing 6.5% H2O (orange line: plinian-type eruptions;2122) and 1% of H2O (green dotted line: vulcanian explosions,23) which fit the H2Or - vesicularity evolution of clasts. All C-PDC fragments plot below these two closed system evolution lines. D-PDC clasts of every studied eruption span a very large domain between H2O-poor, vesicle collapsed clasts and H2O-rich, highly vesiculated clasts from plinian eruptions. This extreme heterogeneity indicates that before explosion, the different parts of the lava dome have suffered highly variable degassing conditions varying between pure closed system degassing, likely at the center of the lava dome or as new intruding magma batch, to highly degassed in open system (extreme H2O loss and vesicle collapse), likely at the lava dome upper external carapace.
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Related In: Results  -  Collection

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f6: Residual H2O content (H2Or) vs Vesicularity glass of clasts from dome-forming eruptions; comparison with clasts from plinian and vulcanian eruptions.Magmatic H2Or is calculated from H2O content measured in bulk clast corrected from phenocrysts content (crystallinity). Lava domes, C-PDCs and D-PDCs: blue and red domains as in Fig. 4. Plinian and vulcanian clasts: Orange circle: 1902 plinian eruption of Santa Maria (Guatemala;2122); Open circle: plinian phase of the P1 eruption of Montagne Pelée (650 y. BP; Martinique;2122); Green triangle: vulcanian phase of Soufrière Hills (Montserrat;22). Lines refer to closed system degassing models from initial rhyolitic melts containing 6.5% H2O (orange line: plinian-type eruptions;2122) and 1% of H2O (green dotted line: vulcanian explosions,23) which fit the H2Or - vesicularity evolution of clasts. All C-PDC fragments plot below these two closed system evolution lines. D-PDC clasts of every studied eruption span a very large domain between H2O-poor, vesicle collapsed clasts and H2O-rich, highly vesiculated clasts from plinian eruptions. This extreme heterogeneity indicates that before explosion, the different parts of the lava dome have suffered highly variable degassing conditions varying between pure closed system degassing, likely at the center of the lava dome or as new intruding magma batch, to highly degassed in open system (extreme H2O loss and vesicle collapse), likely at the lava dome upper external carapace.
Mentions: To acquire a significant strength, the carapace must attain a thickness of few meters to several tens of meters in the absence of other consolidating effects29. We suggest that silicification may reduce the threshold thickness to the lower range. This impermeable and resistant carapace mechanically isolates the core of the lava dome, thus preventing further volatile exsolution and induced crystallization of the melt confined in this core. The D-PDC’s clasts span a wide range of vesicularity and H2Or content, from almost completely degassed, microcrystalline clasts to undegassed, microlite-free clasts (~2.5 wt% of H2Or for a glass vesicularity of ~80%; Figs 2d and 4b; Tables 1 and 2). The vesiculated, H2O-rich clasts display textural characteristics similar to those of plinian clasts33, contrary to the low vesiculated, H2O-poor clasts, which are highly microcrystalline and cristobalite-rich. Thus volatile-rich and vesiculated clasts may represent the magma stored within the core of the lava dome and that evolves in a closed system degassing, leading to overpressurization of the upper volcanic edifice (Fig. 6). By contrast, the less-vesiculated, H2O-poor clasts may represent magma that evolved in an open-system, degassing in the external part of the lava dome. Overpressures of 0.1–1 MPa are sufficient to surpass the tensile strength of the carapace and to trigger the explosive destruction of the upper part of the lava dome63435. The pressure distribution within the edifice may be significantly modified by the existence of shear stress at the conduit vent12: the largest gas overpressure may be located at the conduit wall (horizontal spreading at the vent) or at the center of the conduit (zero horizontal velocity as in the case of a pre-existing lava dome)36.

Bottom Line: Superficial explosion of a growing lava dome may be promoted through porosity reduction caused by both vesicle flattening due to gas escape and syn-eruptive cristobalite precipitation.Explosive activity is thus more likely to occur at the onset of lava dome extrusion, in agreement with observations, as the likelihood of superficial lava dome explosions depends inversely on lava dome volume.This new result is of interest for the whole volcanological community and for risk management.

View Article: PubMed Central - PubMed

Affiliation: Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005, Paris, France.

ABSTRACT
Dome-forming eruption is a frequent eruptive style and a major hazard on numerous volcanoes worldwide. Lava domes are built by slow extrusion of degassed, viscous magma and may be destroyed by gravitational collapse or explosion. The triggering of lava dome explosions is poorly understood: here we propose a new model of superficial lava-dome explosivity based upon a textural and geochemical study (vesicularity, microcrystallinity, cristobalite distribution, residual water contents, crystal transit times) of clasts produced by key eruptions. Superficial explosion of a growing lava dome may be promoted through porosity reduction caused by both vesicle flattening due to gas escape and syn-eruptive cristobalite precipitation. Both processes generate an impermeable and rigid carapace allowing overpressurisation of the inner parts of the lava dome by the rapid input of vesiculated magma batches. The relative thickness of the cristobalite-rich carapace is an inverse function of the external lava dome surface area. Explosive activity is thus more likely to occur at the onset of lava dome extrusion, in agreement with observations, as the likelihood of superficial lava dome explosions depends inversely on lava dome volume. This new result is of interest for the whole volcanological community and for risk management.

No MeSH data available.


Related in: MedlinePlus