Pranabendu Moitra

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• Moitra, P. (2024). Rheological arrest vs. rapid growth of bubbles in crystal-rich magma. Earth and Planetary Science Letters, 646. doi:10.1016/j.epsl.2024.118984
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  Effusive to violently explosive eruptions of crystal-rich magmas are frequently found in volcanic records. The competing effects of rheological stiffening of magma in the presence of crystals and magma overpressure build-up in the presence of bubbles typically control the volcanic explosivity. The bubble growth exerts extensional stress on its wall, i.e., the melt+crystal matrix surrounding it. However, the rheology of crystal-rich magma under such extension along with the effect of crystals on bubble growth, are poorly understood. From analog experiments, this study finds that crystalline magma exhibits yield stress and power-law rheology with broadly comparable values under extensional and shear deformation. The pressure loss due to the presence of yield stress can significantly affect bubble growth in magma. Using bubble growth model in crystallizing magma, this study shows that the yield stress in melt+crystal matrix surrounding bubbles can exceed gas overpressure, preventing bubble growth. The model parameter search exhibits three regimes of bubble growth in crystallizing magmas for a wide range of magma decompression and crystallization rates during effusive to explosive volcanic eruptions. In the yield stress-limited regime, a complete halt in bubble growth can occur at a relatively small viscosity of crystal-rich basaltic magma (∼106 Pa s), and depending on the crystalline system, at a crystal volume as low as ∼30%. On the other hand, at relatively higher magma decompression rates, significant magma expansion associated with relatively rapid bubble growth, even at a relatively high normalized crystal content of >90%, could cause magma fragmentation and eruption explosivity. This study demonstrates that small changes in eruption conditions, such as magma decompression rates and crystallization rates, can cause significant changes in bubble growth dynamics with implications for transitions in volcanic eruption styles.
• Mallik, A., Moitra, P., Roy, A., & Schwinger, S. (2022).

  Controls on determining the bulk water content of the Moon

  . Meteoritics & Planetary Science, 57(12), 2143-2157. doi:10.1111/maps.13921
• Moitra, P., & Sonder, I. (2022).

  Vapor Bubbles and Velocity Control on the Cooling Rates of Lava and Pyroclasts During Submarine Eruptions

  . Journal of Geophysical Research: Solid Earth, 127(8). doi:10.1029/2022jb024665
• Sonder, I., & Moitra, P. (2022). Experimental constraints on the stability and oscillation of water vapor film—a precursor for phreatomagmatic and explosive submarine eruptions. Frontiers in Earth Science, 10. doi:10.3389/feart.2022.983112
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  Pre-mixing of magma and external water plays a key role in driving explosive phreatomagmatic and submarine volcanic eruptions. A thin film of water vapor forms at the magma–water interface as soon as hot magma comes in direct contact with the cold water (Leidenfrost effect). The presence of a stable vapor film drives efficient mixing and mingling between magma and water, as well as magma and wet and water-saturated sediments. Such mixing occurs before explosive molten fuel–coolant type interactions. Using high-temperature laboratory experiments, we investigate the effect of magma and water temperatures on the stability of vapor film, which has not been performed systematically for a magmatic heat source. The experiments were performed with re-melted volcanic rock material, from which spherically-shaped rock samples were produced. These samples were heated to 1,110°C and then submerged in a water pool with a constant temperature (3–93°C). The experiments were recorded on video, and, synchronously, sample and water temperatures were measured using thermocouples. The time-dependent thickness of the vapor film was measured from the video material. The vapor film tends to oscillate with time on the order of 102 Hz. We find that the vertical collapse rates of vapor films along the sample–water interfaces are 13.7 mm s−1 and 4.2 mm s−1 for water temperatures of 3.0°C and 65°C, respectively. For a given initial sample temperature, the thickness and stability time scales decrease with decreasing water temperature, which has implications for the efficiency of pre-mixing required for explosive eruptions. Using thermodynamics and previously measured material parameters, it is shown that a sudden collapse of the vapor film can start brittle fragmentation of the melt and thus serves as the starting point of thermohydraulic explosions.
• Horvath, D., Moitra, P., Hamilton, C., Craddock, R., & Andrews-Hanna, J. (2021). Evidence for geologically recent explosive volcanism in Elysium Planitia, Mars. Icarus, 365. doi:10.1016/j.icarus.2021.114499
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  Volcanic activity on Mars peaked during the Noachian and Hesperian periods but has continued since then in isolated locales. Elysium Planitia hosts numerous young, fissure-fed flood lavas with ages ranging from approximately 500 to 2.5 million years (Ma). We present evidence for a fine-grained unit that is atypical of aeolian deposits in the region and may be the youngest volcanic deposit yet documented on Mars. The unit has a low albedo, high thermal inertia, includes high‑calcium pyroxene-rich material, and is distributed symmetrically around a segment of the Cerberus Fossae fissure system in Elysium Planitia. This deposit is superficially similar to features interpreted as pyroclastic deposits on the Moon and Mercury. Unlike previously documented lava flows in Elysium Planitia, this feature is morphologically consistent with a fissure-fed pyroclastic deposit, mantling the surrounding lava flows with a thickness on the order of tens of cm over most of the deposit and a volume of 1.1–2.8 × 107 m3. Thickness and volume estimates are consistent with tephra deposits on Earth. Stratigraphic relationships indicate a relative age younger than the surrounding volcanic plains and the Zunil impact crater (~0.1–1 Ma), with crater counting suggesting that the deposit has an absolute model age of 53 ± 7 to 210 ± 12 ka. This young age implies that if this deposit is volcanic then the Cerberus Fossae region may not be extinct and that Mars may still be volcanically active. This interpretation is consistent with the identification of seismicity in this region by the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) lander, and has additional implications for astrobiology.
• Moitra, P., & Houghton, B. (2021). Porosity-permeability relationships in crystal-rich basalts from Plinian eruptions. Bulletin of Volcanology, 83(11). doi:10.1007/s00445-021-01496-7
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  Magma permeability allows release of exsolved and pressurized volatiles during magma ascent, potentially modulating explosive volcanic eruptions. While porosity-permeability relationships in the clasts from silicic Plinian eruptions have received considerable interests in recent years, knowledge on magma permeability during Plinian eruptions of basaltic magma is lacking. In this study, we investigate the porosity-permeability relationships in pyroclasts from the well-studied Plinian eruptions of basaltic magma at Mt. Tarawera, New Zealand (1886 CE) and Mt. Etna, Italy (122 BCE). We find that Darcian permeabilities in the studied clasts range between approximately 10−12 m2 and 10−10 m2 over a range of total porosity between 48 and 82%. The vesicles are well connected with values of 45–82% for connected porosity. Pyroclasts from the studied Plinian eruptions contain abundant microlites (∼ 60–90 vol. %) in the matrix surrounding vesicles. At a given total porosity, the measured permeabilities are somewhat higher than that measured in the crystal-poor lapilli from less explosive basaltic eruptions, and about 1–2 orders of magnitude higher than that in silicic Plinian clasts. The estimated percolation threshold is ∼34% for the two basaltic Plinian eruptions. Using modeling of ascent of multiphase magma through volcanic conduits during eruptions, we find that the relative timing of crystallization and the onset of percolation might have played a key role in the development of the measured porosity-permeability relationships. Using scaling analysis, we further show that rheological stiffening of basaltic melt due to a high crystal content just prior to magma fragmentation should have stabilized the vesicle networks, preserving textures reflecting the eruptive conditions with insignificant post-eruptive modification.
• Moitra, P., Horvath, D., & Andrews-Hanna, J. (2021). Investigating the roles of magmatic volatiles, ground ice and impact-triggering on a very recent and highly explosive volcanic eruption on Mars. Earth and Planetary Science Letters, 567. doi:10.1016/j.epsl.2021.116986
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  Volcanic activity on Mars has been dominantly effusive. The existence of a young (∼0.05-1 Ma) and well-preserved possible pyroclastic deposit along a segment of the Cerberus Fossae fissures, overlying the effusive lava flows making up the bulk of Elysium Planitia, provides the motivation and opportunity to explore the dynamics of explosive volcanic eruptions on Mars. Here we investigate the subsurface magmatic processes that may have led to magma fragmentation and the explosivity of the eruption forming the deposit. Using numerical models of magma ascent in a volcanic fissure, we show that the dissolved magmatic water with or without suspended crystals is capable of driving the inferred explosive magma fragmentation and the formation of the deposit. We also explore an alternative eruption scenario and show that an intruded dike explosively interacting with melted ground ice might also have generated the deposit. The close proximity of the proposed pyroclastic deposit (15-35 km) to the similarly aged Zunil impact crater suggests the possibility of an impact-triggered volcanic eruption scenario. Using scaling analysis, we find that the high seismic energy density associated with the impact may have been sufficient to trigger a volcanic eruption if a magma chamber was present in the subsurface. These findings have implications for the generation of similar explosive eruptions on Mars and other bodies, as well as the possibility of ongoing magmatic activity on Mars.
• Moitra, P., Sonder, I., & Valentine, G. A. (2020). The role of external water on rapid cooling and fragmentation of magma. Earth and Planetary Science Letters. doi:10.1016/j.epsl.2020.116194
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  The cooling rate of magma in the presence of external water during phreatomagmatic and submarine eruptions is one of the key parameters governing non-explosive to explosive magma fragmentation and eruption transitions, but remains poorly constrained. Combining results from laboratory experiments with realistic eruptive temperatures of magma cooling in ambient water of variable temperatures, and numerical modeling of transient heat transfer, we find that magma-to-water heat flux can be up to 4×106 W m−2. The experiments exhibit two distinct water boiling regimes: A film-boiling regime defined by the presence of a coherent water vapor film between magma surface and ambient water, and a nucleate boiling regime below a critical magma surface temperature (known as the Leidenfrost temperature), where the vapor film breaks and numerous bubbles form at the magma-water interface. In general the vapor film was stable in our experiments for time scales of ≤ 5 s, indicating that this might be a limiting factor in pre-explosion magma-water mixing for energetic molten fuel-coolant interaction and explosive volcanic eruptions. The time scale of vapor film stability increases and the Leidenfrost temperature (1223 to 948 K) decreases with increasing water temperature (276 to 366 K). We show that for the empirically obtained large heat flux to external water, the cooling rate of magma can reach up to 106 K s−1 at length scales of few microns, thus magma may undergo fine fragmentation due to quench-induced large thermal stresses. Our experimental and modeling results demonstrate that the time scales of various water boiling regimes, and erupting magma and ambient water temperatures determine the magma-to-water heat transfer rates, which in turn determine the transition to explosivity under subaqueous eruption conditions.
• Moitra, P., Gonnermann, H., Houghton, B., & Tiwary, C. (2018). Fragmentation and Plinian eruption of crystallizing basaltic magma. Earth and Planetary Science Letters, 500. doi:10.1016/j.epsl.2018.08.003
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  Basalt is the most ubiquitous magma on Earth, erupting typically at intensities ranging from quiescently effusive to mildly explosive. The discovery of highly explosive Plinian eruptions of basaltic magma has therefore spurred debate about their cause. Silicic eruptions of similar style are a consequence of brittle fragmentation, as magma deformation becomes progressively more viscoelastic. Magma eventually crosses the glass transition and fragments due to a positive feedback between water exsolution, viscosity and decompression rate. In contrast to silicic eruptions, the viscosity of basaltic magmas is thought to be too low to reach conditions for brittle fragmentation. Pyroclasts from several basaltic Plinian eruptions, however, contain abundant micron-size crystals that can increase magma viscosity substantially. We therefore hypothesize that magma crystallization led to brittle fragmentation during these eruptions. Using combined oscillatory and extensional rheometry of concentrated particle-liquid suspensions that are dynamically similar to microcrystalline basaltic magma, we show that high volume fractions of particles and extension rates of about 1 s−1 or greater result in viscoelastic deformation and brittle fracture. We further show that for experimentally observed crystallization rate, basaltic magma can reach the empirical failure conditions when erupting at high discharge rates.
• Moitra, P., Sonder, I., & Valentine, G. A. (2018).

  Effects of Size and Temperature‐Dependent Thermal Conductivity on the Cooling of Pyroclasts in Air

  . Geochemistry, Geophysics, Geosystems, 19(10), 3623-3636. doi:10.1029/2018gc007510
• Sonder, I., Harp, A., Graettinger, A., Moitra, P., Valentine, G., Zimanowski, B., & Büttner, R. (2018). Meter-Scale Experiments on Magma-Water Interaction. Journal of Geophysical Research: Solid Earth, 123(12). doi:10.1029/2018jb015682
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  Interaction of magma with groundwater or surface water can lead to explosive phreatomagmatic eruptions. Questions of this process center on effects of system geometry and length and time scales, and these necessitate experiments at larger scale than previously conducted in order to investigate the thermohydraulic escalation behavior of rapid heat transfer. Previous experimental work either realized melt-water interaction at similar meter scales, using a thermite-based magma analog in a confining vessel, or on smaller scale using ≃0.4 kg remelted volcanic rock in an open crucible, with controlled premix and a ≃5 J kinetic energy trigger event. The new setup uses 55 kg melt for interaction, and the timing and location of the magma-water premix can be controlled on a scale up to 1 m. A trigger mechanism is a falling hammer that drives a plunger into the melt (≃28 J kinetic energy). Results show intense interaction (L max ≥ 0.25m 2 at relatively low magma/water mass ratio. A video analysis quantifies rate and amount of melt ejection and compares results with those using the same melt in the smaller scale setup. Experiments show that on the meter scale intense interaction can start spontaneously without an external trigger if the melt column above the initial mixing location is larger than 0.3 m. Experiments suggest that buoyant rise of water domains in a melt column could promote explosive interaction. We assess interaction scenarios between introduced water domains and a magma column, some of which could result in eruptions of Strombolian style, promoting brecciation and incorporation of wall rock debris into a magma column.
• Lee, C. A., Morton, D. M., Farner, M. J., & Moitra, P. (2015). Field and model constraints on silicic melt segregation by compaction/hindered settling: The role of water and its effect on latent heat release. American Mineralogist. doi:10.2138/am-2015-5121
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  To investigate how large volumes of silicic melts segregate to form granitic plutons, we conducted a case study of a zoned pluton, in which SiO2 increases from intermediate (69 wt%) to highly silicic compositions (74 wt%) toward the contact with metasedimentary wallrock in the outer 25 m of the pluton. All other major, minor, and trace elements vary systematically with SiO2 and indicate that outward increasing SiO2 is due to a decrease in mafic elements and minerals. Whole-rock oxygen isotopes and elemental variation diagrams do not support mixing with wallrock as an explanation for the Si-rich boundary layer. Instead, mafic enclaves, which are common in the pluton, also decrease in abundance in the outer 25 m of the pluton, suggesting a mechanical origin for the Si-rich boundary layer. The coupling of mechanical and geochemical boundary layers, combined with geochemical modeling, indicate that the silica-rich, enclave-poor boundary layer formed by hindered settling or compaction of a crystal-rich (crystal fractions >60%) magmatic mush. Segregation of melts at high crystal fraction is known to be a slow process. However, petrography and Zr-based thermometry indicate that the residual Si-rich liquids were water-saturated. Water decreases melt viscosity, which helps expulsion, but equally importantly, water also delays much of the latent heat release to late in the thermal and crystallization history of a cooling magma. We show that the higher the water content, the longer the time interval over which a magma chamber resides at the stage when water-saturated, high-silica liquids form, allowing sufficient time for exfiltration of silicic liquids before the magma body freezes.
• Moitra, P., & Gonnermann, H. (2015). Effects of crystal shape- and size-modality on magma rheology. Geochemistry, Geophysics, Geosystems, 16(1). doi:10.1002/2014gc005554
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  Erupting magma often contains crystals over a wide range of sizes and shapes, potentially affecting magma viscosity over many orders of magnitude. A robust relation between viscosity and the modality of crystal sizes and shapes remains lacking, principally because of the dimensional complexity and size of the governing parameter space. We have performed a suite of shear viscosity measurements on liquid-particle suspensions of dynamical similarity to crystal-bearing magma. Our experiments encompass five suspension types, each consisting of unique mixtures of two different particle sizes and shapes. The experiments span two orthogonal subspaces of particle concentration, as well as particle size and shape for each suspension type, thereby providing insight into the topology of parameter space. For each suspension type, we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses. The results were fitted using a Herschel-Bulkley model and augment existing predictive capabilities. We demonstrate that our results are consistent with previous work, including friction-based constitutive laws for granular materials. We conclude that predictions for ascent rates of crystal-rich magmas must take the shear-rate dependence of viscosity into account. Shear-rate dependence depends first and foremost on the volume fraction of crystals, relative to the maximum packing fraction, which in turn depends on crystal size and shape distribution.
• Moitra, P., Gonnermann, H., Houghton, B., & Giachetti, T. (2013). Relating vesicle shapes in pyroclasts to eruption styles. Bulletin of Volcanology, 75(2). doi:10.1007/s00445-013-0691-8
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  Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor Ω, a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that Ω is smaller for eruptions with Ca ≪ 1 than for eruptions with Ca ≫ 1. Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have Ca ≪ 1, in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by Ca ≫ 1 and larger values of Ω than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation. © 2013 Springer-Verlag Berlin Heidelberg.