196,126 research outputs found
Constraining the dynamics of volcanic eruptions by characterization of pumice textures
No full textWe have characterized the textures of pumice clasts from Phlegraean Fields to gain insights into the conduit flow-dynamics of alkaline explosive eruptions. Vesicularities, vesicle number densities, and vesicle sizes and shapes were measured to obtain the bulk and groundmass properties of the juvenile fraction of Campanian Ignimbrite (CI) and Agnano Monte Spina (AMS) eruptions. The results report the coexistence of three end-member pumice types in the deposits of both eruptions, 1) microvesicular, 2) tube and 3) expanded, which differ according to clast morphology and the macro- to microscopic vesicle texture. Vesicularities (0.85-0.94 for CI, 0.51-0.91 for AMS) and vesicle number densities (2-4×105 cm-2 in CI, 3×105-106 cm-2 in AMS) span quite a wide range in all the three pumice types. Overall, tube pumices exhibit the highest bulk (0.89) and groundmass (CI 0.85, AMS 0.82) average vesicle volume fractions but the lowest average vesicle number densities (CI 2×105, AMS 4×105 cm-2). Comparison with textures of calc-alkaline pumices has revealed many similarities and points to a common origin and distribution of the products from both magma compositions within the volcanic conduit. In addition, the results of the textural analysis were interpreted in the light of the conduit flow modeling of Phlegraean Fields eruptions. The comparison of textural observations with results from simulations of conduit magma ascent has exhibited a good agreement between measured and numerically calculated vesicularities for both compositions, helping to constrain the overall dynamics of alkaline versus calc-alkaline eruptions
Textural heterogeneities in pumices from the climactic eruption of Mount Pinatubo, 15 June 1991, and implications for magma ascent dynamics
No full textThe Plinian phase of the Campanian Ignimbrite eruption (phlegrean fields, Italy): Evidence from density measurements and textural characterization of pumice
No full textTextural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content, size, shape, vesicle wall thickness and the degree of interconnectivity. The Plinian fallout deposit directly underlying the Campanian Ignimbrite (CI) eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater. The deposit consists of a lower (LFU) and upper (UFU) pumice lapilli bed generated by the sub-steady eruption of trachytic magma with 0.90, with vesicle number density ranging from 107-108 cm-3. The degree of vesicle coalescence is high for all pumice types, with interconnected vesicles generally representing more than 90% of the bulk vesicle population. The results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase, the different pumice types and also within each single pumice type fragment. The origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior. The conclusions of this study are that vesicle deformation, degree of coalescence and intense shear at the conduit walls play a major role on the degassing process, hence affecting the entire conduit dynamics
Editorial: Recent Advancements in X-Ray and Neutron Imaging of Dynamic Processes in Earth Sciences
No full textConstraining the dynamics of volcanic eruptions by characterization of pumice textures
No full textWe have characterized the textures of pumice clasts from Phlegraean Fields to gain insights into the conduit
flow-dynamics of alkaline explosive eruptions. Vesicularities, vesicle number densities, and vesicle sizes and
shapes were measured to obtain the bulk and groundmass properties of the juvenile fraction of Campanian Ignimbrite
(CI) and Agnano Monte Spina (AMS) eruptions. The results report the coexistence of three end-member
pumice types in the deposits of both eruptions, 1) microvesicular, 2) tube and 3) expanded, which differ according
to clast morphology and the macro- to microscopic vesicle texture. Vesicularities (0.85-0.94 for CI,
0.51-0.91 for AMS) and vesicle number densities (2-4×105 cm-2 in CI, 3×105-106 cm-2 in AMS) span quite a
wide range in all the three pumice types. Overall, tube pumices exhibit the highest bulk (0.89) and groundmass
(CI 0.85, AMS 0.82) average vesicle volume fractions but the lowest average vesicle number densities (CI
2×105, AMS 4×105 cm-2). Comparison with textures of calc-alkaline pumices has revealed many similarities and
points to a common origin and distribution of the products from both magma compositions within the volcanic
conduit. In addition, the results of the textural analysis were interpreted in the light of the conduit flow modeling
of Phlegraean Fields eruptions. The comparison of textural observations with results from simulations of
conduit magma ascent has exhibited a good agreement between measured and numerically calculated vesicularities
for both compositions, helping to constrain the overall dynamics of alkaline versus calc-alkaline eruptions.PublishedJCR Journalope
Microscale textural heterogeneity and tip-streaming instability in alkaline magmas: evidence in tube pumices from Campi Flegrei, Italy
No full textWe present a textural investigation of compositionally homogeneous alkaline tube pumices from the plinian fallouts of the Agnano Monte Spina eruption (ca. 4.1 ka), Campi Flegrei, Italy. The pumice clasts present strong textural complexity from hand specimen to microscopic scale of inspection. The clasts are characterized by large vesicle tubes that extend up to several millimetres along the clast main axis. They also exhibit small and elongated vesicles arranged in stretched bands around textural domains of larger, sub-spherical vesicles associated with pyroxene, feldspar, and biotite phenocrysts. Clasts, vesicle tubes, vesicle deformation, major axes of pyroxene phenocrysts and crystals with size ≤100 μm align in the same direction. In addition, we observe chains of gradually smaller stretched vesicles likely generated from break-up of a larger parent vesicle (i.e., tip-streaming phenomenon). We use image analysis to quantify the heterogeneity of vesicle sizes and shapes. Then, we deduce the Capillary number (Ca) of the each single vesicle, which represents the ratio between the deforming viscous stress on a bubble and the restoring stress supplied by surface tension, and obtain the spatial distribution of Ca values within pumice clasts. As Ca depends on strain-rate and viscosity, the spatial distribution of Ca values provides us with a snapshot of the sub-millimetre scale variation in magma strain-rate and rheology before fragmentation. Ca in highly stretched vesicles is close to values (0.45 to 0.6) required for bubble-breakup induced by tip-streaming. Instead, low Ca values (0.3 to 0.45) are calculated for large sub-spherical vesicles mostly distributed around phenocrysts. We suggest that sub-millimetre heterogeneities in vesicle sizes and shapes recorded in tube pumices are caused by a combination of tip-streaming, heterogeneous bubble nucleation and second-boiling processes prior to fragmentation. The heterogeneous spatial distribution of Ca values may be the result of local micrometric shear localisation and consequent deformation upon break-up experienced by magma while ascending in the conduit. This deformation suggests that a variation of magma strain-rate and viscosity occurred before fragmentation at the clast microscale and this is likely to affect magma dynamics and fragmentation. Our results claim for future experimental and numerical modelling studies that investigate and consider the effect of such micrometric heterogeneities on magma rheology and flow condition variations inside volcanic conduits
The 2002 January 17th Nyragongo eruption: insights from textural and rheological investigation
No full textExistence of molten peridotite in the early history of the Earth has long been the subject of debate and conjecture. Interest in the physical properties stems from a number of sources but was re-focussed in the wake of the proposal for the existence of a ̧Smagma ocean ¡T in the evolution of the moon, the Earth and other terrestrial planets. The application of phase equilibrium, buoyancy, thermodynamic and fluid dynamic constraints on the behaviour of molten mantle all rely on adequate characterisation ofthe properties of molten peridotite, largely lacking to date. The viscosity in particular, has received too little attention. A big experimental effort has been provided to obtain the dependence on temperature (T) of viscosity at ambient pressure (P) for the natural peridotite collected at Balmuccia, Italy. High-T measurements were performed by using concentric cylinder (CC). The high-T viscometry was started at 1600_C and proceeded at 10_C intervals, separated by cooling stages at 5_C/min, each one held for 1 hour. No measurements were possible below 1570 _C, because crystallization had occurred. All standard attempts to obtain a homogeneous glass failed. A new technique was therefore used. Small 1-2 mm chips were hung in Pt loops suspended from a long Pt wire and the loops lowered by hand into the high-T viscosity furnace until the chips fused into a bead of liquid held in the loop by surface tension. These samples were then left to quench and placed aside to be used in the splat-quenching device (SQD) (which allows quench rates on the order of 10exp4 _C/s) to finallyobtain a supercooled liquids by squeezing and rapidly quenching a falling liquid drop, through a joint action of a complex photoelectric-driven electromagnetic device. Electron microprobe analysis revealed that only a few vol% of the obtained glasses crystallized in isochemical crystals, whereas the homogeneity of the glassy matrix composition was found to be excellent. As the amount of glass obtained was too small to be used in the micropenetration technique we used differential scanning calorimetry (DSC) to derive the viscosity at low-T. DSC allowed us to unequivocally determine glass transition temperatures (Tg) for cooling/heating rates of 20, 15, 10, 8 and 5 K/min, as the peak of the Cp curves. At this point we used a recent method developed by [1] that, on the basis of the equivalence of the shear stress and the enthalpic relaxation time, allow to predict the low-T viscosity. The combined results obtained by using the different techniques above mentioned were fit by VFT equation with the high-T limiting value (viscosity value at infinite temperature) being fixed at a value of 10exp-4.31 Pas [2]. A comparison between the data obtained here with the recent model from [3] (calibrated with melts as basic as basanite), have shown that in the range 900 to 1600 _C, the viscosity calculated according to [3] is very similar to those measured or calculated by the VFT fit, if A = -4.31; the discrepancy becoming significant at T<900 _C. The very low T dependence of viscosity at superliquidus conditions obtained from the fitting here, indicates that at putative temperatures of the core-mantle boundary, near 5000 _C, the viscosity will decrease up to 10exp-3.5 Pa s. [1] J. Gottsmann et al. (2002), EPSL, 198, 417;. [2] J.K. Russell et al. (2003), Am. Mineral., 88, 1390;. [3] D. Giordano & D.B. Dingwell (2003), EPSL, 208, 337 and Errata Corrige, in press
Eruption dynamics of the 22–23 April 2015 calbuco volcano (Southern Chile): Analyses of tephra fall deposits
No full textAfter 54 years since its last major eruption in 1961, Calbuco Volcano (Ensenada, Southern Chile) reawakened with few hours of warning on 22 April 2015 at 18:05 local time. The main explosive eruption consisted of two eruption pulses (lasting ~1.5 and 6 h each one) on 22 and 23 April, producing stratospheric (>15 km height) eruption columns. The erupted materials correspond to porphyritic basaltic andesite (~55 wt.% of SiO2). The tephra fall affected mainly the area northeast of the volcano and the finest ash was deposited over Southern Chile and Patagonia Argentina. We studied the tephra fall deposits of both pulses in terms of stratigraphy, distribution, volume, emplacement dynamics and eruption source parameters. Here, we show field observations that have been made 5-470 km downwind and distinguish five layers (Layers A, B, B1, C and D) representing different stages of the eruption evolution: eruption onset (Layer A; pulse 1), followed by the first paroxysmal event (Layer B; pulse 1), in some places interbedded by layer B1, tentatively representing the sedimentation of a secondary plume during the end of pulse 1. We recognized a second paroxysm (Layer C; pulse 2) followed by the waning of the eruption (Layer D; pulse 2). The total calculated bulk tephra fall deposit volume is 0.27 ± 0.007 km3 (0.11-0.13 km3 dense rock equivalent), 38% of which was erupted during the first phase and 62% during the second pulse. This eruption was a magnitude 4.45 event (VEI 4 eruption) of subPlinian type.Fil: Romero, J. E.. Universidad de Atacama; ChileFil: Morgavi, D.. Università di Perugia; ItaliaFil: Arzilli, F.. University of Manchester; Reino UnidoFil: Daga, Romina Betiana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Caselli, Alberto Tomás. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Río Negro; ArgentinaFil: Reckziegel, Florencia Mabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; ArgentinaFil: Viramonte, Jose German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; ArgentinaFil: Díaz Alvarado, J.. Universidad de Atacama; ChileFil: Polacci, M.. University of Manchester; Reino UnidoFil: Burton, M.. University of Manchester; Reino UnidoFil: Perugini, D.. Università di Perugia; Itali
- …
