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Showing 1-18 of about 18 results.
Eruption dynamics and explosive-effusive transitions during the 1400 cal BP eruption of Opala volcano, Kamchatka, RussiaAndrews, Benjamin J.Dufek, JosefPonomareva, Vera2018DOI: info:10.1016/j.jvolgeores.2018.02.019Journal of Volcanology and Geothermal Researchv. 356316330316–3300377-0273
Andrews, Benjamin J., Dufek, Josef, and Ponomareva, Vera. 2018. "Eruption dynamics and explosive-effusive transitions during the 1400 cal BP eruption of Opala volcano, Kamchatka, Russia." Journal of Volcanology and Geothermal Research. 356:316–330. https://doi.org/10.1016/j.jvolgeores.2018.02.019
ID: 145720
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Deposits and pumice from the 1400 cal BP eruption of Opala volcano record activity that occurred at the explosive-effusive transition, resulting in intermittent, or stop-start, behavior, where explosive activity resumed following a pause. The eruption deposited distinctive, biotite-bearing rhyolite tephra across much of Kamchatka, and its stratigraphy consists of a lithic-rich pumice fall, overlain by pumice falls and pyroclastic density deposits, with the proportion of the latter increasing with height. This sequence repeats such that the middle of the total deposit is marked by a lithic-rich fall with abundant obsidian clasts. Notably, the eruptive pumice are poorly vesiculated, with vesicle textures that record fragmentation of a partially collapsed magmatic foam. The eruption vent, Baranii Amphitheater is filled with obsidian lavas of the same composition as the rhyolite tephra. Based upon the stratigraphic and compositional relations, we divide the eruption into four phases. Phase I initiated with eruption of a lithic-rich pumice fall, followed by eruption of Plinian falls and pyroclastic density currents. During Phase II, the eruption paused for at least 5-6 h; in this time, microlites nucleated and began to grow in the magma. Phase III essentially repeated the Phase I sequence. Obsidian lavas were emplaced during Phase IV. The pumice textures suggest that the magma ascended very near the threshold decompression rate for the transition between explosive (fast) and effusive (slow) behavior. The pause during Phase II likely occurred as decompression slowed enough for the magma to develop sufficient permeability for gas to escape resulting in collapse of the magmatic foam, stopping the eruption and temporarily sealing the conduit. After about 5-6 h, eruption resumed with, once again, magma decompressing very near the explosive-effusive transition. Phase III ended when the decompression rate slowed and lava dome emplacement began. Distributions of pumice and lithic clasts, and inclusion of data from previous workers, indicate minimum deposit volumes of ~0.75 and ~0.75-1.15 km3 (DRE) and eruption column heights of ~18 and ~20 km for Phases I and III, respectively. Phases I-III had a likely total duration of ~60-80 h, including a pause in activity of 5-6 h during Phase II. This study demonstrates that analysis of vesicle textures from numerous pumice combined with stratigraphic data can reveal syn-eruptive changes in and links between magma permeability, decompression rate, and eruption style.
ORCID(s): 0000-0001-9928-1225
Inferring the nature of pyroclastic density currents from tree damage: The 18 May 1980 blast surge of Mount St. Helens, USAGardner, James E.Nazworth, CarolineHelper, Mark A.Andrews, Benjamin J.2018DOI: info:10.1130/G45353.1Geologyv. 46No. 9795798BOULDER; PO BOX 9140, BOULDER, CO 80301-9140 USAGEOLOGICAL SOC AMER, INC795–7980091-7613
Gardner, James E., Nazworth, Caroline, Helper, Mark A., and Andrews, Benjamin J. 2018. "Inferring the nature of pyroclastic density currents from tree damage: The 18 May 1980 blast surge of Mount St. Helens, USA." Geology. 46 (9):795–798. https://doi.org/10.1130/G45353.1
ID: 148392
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences
Abstract: The laterally directed pyroclastic density current (blast surge) that erupted from Mount St. Helens (Washington State, USA) on 18 May 1980 devastated similar to 600 km(2) of thick conifer forest, with most large trees uprooted or broken off near the base. Throughout this area, at least 51 patches of trees were left standing, mostly where the blast surge had reached 95 /- 5% of its distance traveled. Analysis of post-eruption digital topography and topographic profiles aligned in the direction that the surge traveled (defined by orientations of locally toppled trees) show that the stands of trees are on lee sides of hills and, rarely, on downstream sides of plateaus. All hills are higher than neighboring (within 500 m) hills that lack tree stands. In all cases, toppled trees upstream are orientated toward the patches of standing trees, arguing that the surge was not blocked. The patches thus reflect where the base of the surge temporarily left the ground. Because the stands are only found near the end of the surge runout, yet where inferred speeds of the surge were as slow as 40 m s(-1) and as fast as 115 m s(-1), the most likely reason the surge briefly detached from the ground was that its density had locally decreased enough that it approached that of the atmosphere. Numerical modeling shows that the distance the surge traveled before reattaching to the ground is controlled by its density and trajectory (i.e., hill height and stoss-side rise). This study shows that the path of destruction left by pyroclastic density currents can provide clues about their internal mechanics.
ORCID(s): 0000-0001-9928-1225
Liftoff of the 18 May 1980 surge of Mount St. Helens (USA) and the deposits left behindGardner, James E.Andrews, Benjamin J.Dennen, Robert2017DOI: info:10.1007/s00445-016-1095-3Bulletin of Volcanologyv. 79No. 188New YorkSpringer8–80258-8900
Gardner, James E., Andrews, Benjamin J., and Dennen, Robert. 2017. "Liftoff of the 18 May 1980 surge of Mount St. Helens (USA) and the deposits left behind." Bulletin of Volcanology. 79 (1):8–8. https://doi.org/10.1007/s00445-016-1095-3
ID: 142297
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences
Abstract: The distance that ground-hugging pyroclastic density currents travel is limited partly by when they reverse buoyancy and liftoff into the atmosphere. It is not clear, however, what deposits are left behind by lofting flows. One current that was seen to liftoff was the surge erupted from Mount St. Helens on the morning of 18 May 1980. Before lofting, it had leveled a large area of thick forest (the blowdown zone). The outer edge of the devastated area-where trees were scorched but left standing (the scorched zone)-is where the surge is thought to have lifted off. Deposits in the outer parts of the blowdown and in the scorched zone were examined at 32 sites. The important finding is that the laterally moving surge traveled through the scorched zone, and hence, the change in tree damage does not mark the runout distance of the surge. Buoyancy reversal and liftoff are thus not preserved in the deposits where the surge lofted upwards. We propose, based on interpretation of eyewitness accounts and the impacts of the surge on trees and vehicles, that the surge consisted of a faster, dilute Bovercurrent" and a slower Bundercurrent," where most of the mass (and heat) was retained. Reasonable estimates for flow density and velocity show that dynamic pressure of the surge (i. e., its ability to topple trees) peaked near the base of the overcurrent. We propose that where the overcurrent began to liftoff, the height of peak dynamic pressure rose above the trees and stopped toppling them. The slower undercurrent continued forward, however, scorching trees, but lacked the dynamic pressure needed to topple them. Grain-size variations argue that it slowed from similar to 30 m s(-1) when it entered the scorched zone to similar to 3 m s(-1) at the far end.
ORCID(s): 0000-0001-9928-1225
Crystal-rich lava dome extrusion during vesiculation: An experimental studyPistone, MattiaWhittington, Alan G.Andrews, BenjaminCottrell, Elizabeth2017DOI: info:10.1016/j.jvolgeores.2017.06.018Journal of Volcanology and Geothermal Researchv. 3471141–140377-0273
Pistone, Mattia, Whittington, Alan G., Andrews, Benjamin, and Cottrell, Elizabeth. 2017. "Crystal-rich lava dome extrusion during vesiculation: An experimental study." Journal of Volcanology and Geothermal Research. 347:1–14. https://doi.org/10.1016/j.jvolgeores.2017.06.018
ID: 143242
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Lava dome-forming eruptions represent a common eruptive style and a major hazard at numerous active volcanoes worldwide. The extrusion mechanics of crystal-rich lava domes and the influence of volatiles on the transition from viscous to brittle behaviour during lava dome extrusion remain unclear. Understanding how gas exsolution and crystallinity control effusive versus explosive eruption behaviour is essential. Here, we present new experimental results on the rheology of synthesised, crystal-rich (50 to 80 vol% quartz crystals), hydrous (4.2 wt% H2O in the glass) dacite samples, which vesiculate from 5 to 27 vol% gas bubbles at high temperatures (from glass transition temperature to 797 °C) during deformation conducted in a parallel plate viscometer (constant stress at 0.63-0.64 MPa, and variable strain-rates ranging from 8.32·10- 8 to 3.58·10- 5 s- 1). The experiments reproduce certain aspects of lava dome deformation in volcanic conduits during vesiculation of the residual melt, instigated in the experiments by increasing temperature. During gas exsolution (i.e. nucleation and growth of gas-pressurised bubbles) and volume inflation, we find that the rheological lubrication of the system during deformation is strongly dictated by the initial crystallinity. At crystal contents < 60 vol%, gas bubbles form and coalesce during expansion and viscous deformation, favouring strain localisation and gas permeability within shear bands, which control the overall sample rheology. At crystallinities of 60 to 70 vol%, gas exsolution generates pressurisation (i.e. pore pressure increase) within the bubbles trapped in the solid crystal clusters, and embryonic formation of microscopic fractures through melt and crystals drives the system to a brittle behaviour. At higher crystallinity (80 vol%) vesiculation leads to large pressurisation, which then triggers extensive brittle fragmentation. Through macroscopic fractures, outgassing determines the rheological stalling of the system. In the light of these results we propose a rheological description of crystal-rich lava dome mechanics. The contrasting experimental behaviours at different crystallinities have implications for the style of slow-ascending dome-forming eruptions. All other factors being equal, our experiments suggest that crystal-poor magmas will undergo efficient outgassing, reducing the potential for an explosive eruption. Conversely, crystal-rich magmas may experience limited outgassing and larger gas overpressures during vesiculation, therefore increasing the potential for an explosive eruption.
ORCID(s): 0000-0001-9928-1225,0000-0001-7560-3146
Ash production and dispersal from sustained low-intensity Mono-Inyo eruptionsBlack, Benjamin A.Manga, MichaelAndrews, Benjamin J.2016DOI: info:10.1007/s00445-016-1053-0Bulletin of Volcanologyv. 78No. 85757NEW YORK; 233 SPRING ST, NEW YORK, NY 10013 USASPRINGER57–570258-8900
Black, Benjamin A., Manga, Michael, and Andrews, Benjamin J. 2016. "Ash production and dispersal from sustained low-intensity Mono-Inyo eruptions." Bulletin of Volcanology. 78 (8):57–57. https://doi.org/10.1007/s00445-016-1053-0
ID: 141703
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences
Abstract: Recent rhyolitic volcanism has demonstrated that prolonged low-intensity ash venting may accompany effusive dome formation. We examine the possibility and some consequences of episodes of extended, weak ash venting at the rhyolitic Mono-Inyo chain in Eastern California. We describe ash-filled cracks within one of the youngest domes, Panum Crater, which provide a textural record of ash venting during dome effusion. We use synchrotron-based X-ray computed tomography to characterize the particles in these tuffisites. Particle sizes in well-sorted tuffisite layers agree well with grain size distributions observed during weak ash venting at Soufricre Hills Volcano, Montserrat, and yield approximate upper and lower bounds on gas velocity and mass flux during the formation of those layers. We simulate ash dispersal with Ash3d to assess the consequences of long-lived Mono-Inyo ash venting for ash deposition and the accompanying volcanic hazards. Our results highlight the sensitivity of large-scale outcomes of volcanic eruptions to small-scale processes.
ORCID(s): 0000-0001-9928-1225
The role of superheating in the formation of Glass Mountain obsidians (Long Valley, CA) inferred through crystallization of sanidineWaters, Laura E.Andrews, Benjamin J.2016DOI: info:10.1007/s00410-016-1291-3Contributions to Mineralogy and Petrologyv. 171No. 1079Springer Nature790010-7999
Waters, Laura E. and Andrews, Benjamin J. 2016. "The role of superheating in the formation of Glass Mountain obsidians (Long Valley, CA) inferred through crystallization of sanidine." Contributions to Mineralogy and Petrology. 171 (10):79. https://doi.org/10.1007/s00410-016-1291-3
ID: 140286
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: The Glass Mountain obsidians (Long Valley, CA) are crystal poor (<8 vol%) and highly evolved (high SiO2, low Sr), and therefore, their formation required extremely efficient separation of melts from a crystal-rich source. A petrologic and experimental investigation of the mineral phases in Glass Mountain lavas identifies conditions under which phenocrysts grew and the driving mechanism for crystallization, which places constraints on the possible processes that generated the obsidians. The obsidian in this study (GM-11) is saturated in nine phases (sanidine + quartz + plagioclase + titanomagnetite + ilmenite + zircon + apatite + allanite + biotite), and results of high-resolution SEM compositional mapping and electron microprobe analysis reveal that individual sanidine crystals are normally zoned and span a range of compositions (Or40–78). Sanidines have a “granophyric” texture, characterized by intergrowths of quartz and sanidine. Mineral phases in the natural sample are compared to H2O-saturated phase equilibrium experiments conducted in cold-seal pressure vessels, over a range of conditions (700–850 °C; 75–225 MPa), and all are found to be plausible phenocrysts. Comparison of sanidine compositions from the natural sample with those grown in phase equilibrium experiments demonstrates that sanidine in the natural sample occurs in a reduced abundance. Further comparison with phase equilibrium experiments suggests that sanidine compositions track progressive loss of dissolved melt water (±cooling), suggesting that crystallization in the natural obsidian was driven predominantly by degassing resulting from decompression. It is paradoxical that an effusively (slowly) erupted lava should contain multiple phenocryst phases, including sanidine crystals that span a range of compositions with granophyric textures, and yet remain so crystal poor. To resolve this paradox, it is necessary that the solidification mechanism (degassing or cooling) that produced the sanidine crystals (and other mineral phases) must have an associated kinetic effect(s) that efficiently hinders crystal nucleation and growth. Decompression experiments conducted in this study and from the literature collectively demonstrate that the simplest way to inhibit nucleation during degassing-induced crystallization is to initiate degassing ± cooling from superliquidus conditions, and therefore, the Glass Mountain obsidians were superheated prior to crystallization.
ORCID(s): 0000-0001-9928-1225
Petrogenesis of antecryst-bearing arc basalts from the Trans-Mexican Volcanic Belt: Insights into along-arc variations in magma-mush ponding depths, H2O contents, and surface heat fluxZellmer, Georg F.Pistone, MattiaIizuka, YoshiyukiAndrews, Benjamin J.Gomez-Tuena, ArturoStraub, Susanne M.Cottrell, Elizabeth2016DOI: info:10.2138/am-2016-5701American Mineralogistv. 101No. 1124052422Chantilly, VirginiaMineralogical Society of America2405–24220003-004X
Zellmer, Georg F., Pistone, Mattia, Iizuka, Yoshiyuki, Andrews, Benjamin J., Gomez-Tuena, Arturo, Straub, Susanne M., and Cottrell, Elizabeth. 2016. "Petrogenesis of antecryst-bearing arc basalts from the Trans-Mexican Volcanic Belt: Insights into along-arc variations in magma-mush ponding depths, H2O contents, and surface heat flux." American Mineralogist. 101 (11):2405–2422. https://doi.org/10.2138/am-2016-5701
ID: 141425
Keywords: Peer-reviewed; NMNH; NH-Mineral Sciences
Abstract: The Trans-Mexican Volcanic Belt (TMVB) is known for the chemical diversity in its erupted products. We have analyzed the olivine, pyroxene, and plagioclase mineral chemistry of 30 geochemically well-characterized mafic eruptives from Isla Maria at the western end of the arc to Palma Sola in the east. The mineral major oxide data indicate the dominance of open system processes such as antecryst uptake, and the scarcity of mineral-mineral and mineral-melt equilibria suggests that apart from forming microlites, erupted melts do not significantly crystallize during ascent. A combination of plagioclase antecryst chemistry and MELTS thermodynamic modeling of H2O-saturated isobaric fractional crystallization was employed to develop a pressure sensor aimed at determining the ponding depths of the co-genetic magmas from which the erupted plagioclase crystal assemblage originates. We show that the depth of magma-mush reservoirs increase eastward along the TMVB. We suggest that magma ponding is triggered by degassing-induced crystallization during magma ascent, and that the pressure sensor can also be regarded as a degassing sensor, with more hydrous melts beginning to degas at greater depths. Modeled initial magma H2O contents at the Moho range from similar to 4 to similar to 9 wt%. Magma-mush ponding depth variations fully explain the observed westward increase of average surface heat flux along the TMVB, supporting a new model of mafic arc magma ascent, where rapidly rising, initially aphyric melts pick up their antecrystic crystal cargo from a restricted crustal depth range, in which small unerupted batches of previously risen co-genetic magmas typically stall and solidify. This implies that, globally, mafic arc magmas may be used to constrain the depths of degassing and mush zone formation, as well as the amount of H2O in the primary melts.
ORCID(s): 0000-0001-9928-1225,0000-0001-7560-3146
Thermal vesiculation during volcanic eruptionsLavallée, YanDingwell, Donald B.Johnson, Jeffrey B.Cimarelli, CorradoHornby, Adrian J.Kendrick, Jackie E.von Aulock, Felix W.Kennedy, Ben M.Andrews, Benjamin J.Wadsworth, Fabian B.Rhodes, EmmaChigna, Gustavo2015DOI: info:10.1038/nature16153Naturev. 528No. 7583544547544–5470028-0836
Lavallée, Yan, Dingwell, Donald B., Johnson, Jeffrey B., Cimarelli, Corrado, Hornby, Adrian J., Kendrick, Jackie E., von Aulock, Felix W., Kennedy, Ben M., Andrews, Benjamin J., Wadsworth, Fabian B., Rhodes, Emma, and Chigna, Gustavo. 2015. "Thermal vesiculation during volcanic eruptions." Nature. 528 (7583):544–547. https://doi.org/10.1038/nature16153
ID: 138542
Keywords: NH-Mineral Sciences; NMNH; si-federal; Peer-reviewed
ORCID(s): 0000-0001-9928-1225
Ten years of satellite observations reveal highly variable sulphur dioxide emissions at Anatahan Volcano, Mariana IslandsMcCormick, BrendanPopp, ChristophAndrews, Benjamin J.Cottrell, Elizabeth2015DOI: info:10.1002/2014JD022856Journal of Geophysical Research. D. Atmospheresv. 120No. 1472587282American Geophysical Union7258–72822169-8996
McCormick, Brendan, Popp, Christoph, Andrews, Benjamin J., and Cottrell, Elizabeth. 2015. "Ten years of satellite observations reveal highly variable sulphur dioxide emissions at Anatahan Volcano, Mariana Islands." Journal of Geophysical Research. D. Atmospheres. 120 (14):7258–7282. https://doi.org/10.1002/2014JD022856
ID: 137108
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Satellite remote sensing enables continuous multiyear observations of volcanic activity in remote settings. Anatahan (Mariana Islands) is a remote volcano in the western North Pacific. Available ground-based measurements of sulphur dioxide (SO2) gas emissions at Anatahan place it among thelargest volcanic SO2 sources worldwide. These ground-based measurements, however, are restricted to eruptive intervals. Anatahan's activity since 2003 has been dominated temporally by prolonged periods of quiescence. Using 10?years of satellite observations from OMI, AIRS, SCIAMACHY, and GOME-2, we report highly variable SO2 emissions within and between eruptive and quiescent intervals at Anatahan. We find close correspondence between levels of activity reported at the volcano and levels of SO2 emissions detected from space. Eruptive SO2 emission rates have a mean value of ~6400?t?d-1, but frequently are in excess of 20,000?t?d-1. Conversely, SO2 emissions during quiescent intervals are below the detection limit of space-based sensors and therefore are not likely to exceed ~300?t?d-1. We show that while Anatahan occupies a quiescent state for 85% of the past 10?years, only ~15% of total SO2 emissions over this interval occur during quiescence, with the remaining ~85% released in short duration but intense syn-eruptive degassing. We propose that the integration of multiyear satellite data sets and activity histories are a powerful complement to targeted ground-based campaign measurements in better describing the long-term degassing behavior of remote volcanoes.
ORCID(s): 0000-0001-9928-1225
Earth's Volcanoes and Their Eruptions: An OverviewSiebert, LeeCottrell, ElizabethVenzke, Edward A.Andrews, Benjamin J.Sigurdsson, Haraldur2015239255AmsterdamAcademic Press239–255978-0-12-385938-9
Siebert, Lee, Cottrell, Elizabeth, Venzke, Edward A., and Andrews, Benjamin J. 2015. "Earth's Volcanoes and Their Eruptions: An Overview." in The Encyclopedia of Volcanoes, edited by Sigurdsson, Haraldur., 2nd ed. 239–255. Amsterdam: Academic Press.
ID: 142098
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Volcanoes are not random phenomena, but owe their existence, location, morphology, and eruptive styles to tectonic plate motions. Volcanic landforms vary widely in size and morphology from the stereotypical towering glacier-clad stratovolcano. They range from small spatter cones to massive shield volcanoes several thousand cubic kilometers in volume or broad volcanic depressions, cinder cones being the most common volcanic construct. Their eruptions likewise span many orders of magnitude in volume. Evaluation of eruption magnitudes and frequencies contrasts the vast majority of mild-to-moderate eruptions (~80% with volcanic explosivity index, or VEI <=2) with the more infrequent larger, higher-impact events (5% with VEI >=4). Increased risk from population growth in proximity to volcanoes has been mitigated by enhanced monitoring and hazard-reduction efforts.
ORCID(s): 0000-0001-9928-1225
Rapid Crystallization of Plagioclase Phenocrysts in Silicic Melts during Fluid-saturated Ascent: Phase Equilibrium and Decompression ExperimentsWaters, Laura E.Andrews, Benjamin J.Lange, Rebecca A.2015DOI: info:10.1093/petrology/egv025Journal of Petrologyv. 56No. 59811006OxfordOxford University Press981–10060022-3530
Waters, Laura E., Andrews, Benjamin J., and Lange, Rebecca A. 2015. "Rapid Crystallization of Plagioclase Phenocrysts in Silicic Melts during Fluid-saturated Ascent: Phase Equilibrium and Decompression Experiments." Journal of Petrology. 56 (5):981–1006. https://doi.org/10.1093/petrology/egv025
ID: 136293
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Three crystal-poor obsidian samples (one dacite, 67 wt % SiO2; two rhyolites, 73 and 75 wt % SiO2), which erupted effusively from monogenetic vents, contain sparse (<2%) plagioclase phenocrysts that span a remarkably wide and continuous range in composition (=30 mol % An). Many, but not all, of the plagioclase crystals display diffusion-limited growth textures (e.g. swallow-tails, skeletal, vermiform). Hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include (1) incorporation of xenocrysts and/or magma mingling, (2) slow crystallization of plagioclase driven by slow cooling in a magma chamber, (3) slow crystallization of plagioclase followed by a resorption (e.g. heating) event, and (4) crystallization driven by rapid degassing (i.e. loss of melt H2O) ± rapid cooling during ascent. To test these hypotheses, a series of phase equilibrium experiments were conducted under pure-H2O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950°C. The results show that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An60–63) in one sample. The results additionally rule out slow cooling in a magma chamber, because this would lead to equilibrium abundances of plagioclase (=20%), which are far higher than what is observed in the samples (<2%). Nor can resorption (i.e. heating) explain the low abundance of plagioclase, because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed. The most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface; this is consistent with the results of isothermal (850°C) continuous decompression experiments (2·9, 1·0, 0·8, and 0·1 MPa h–1), under pure-H2O fluid-saturated conditions, which were performed on one of the rhyolites (MLV-36; 73 wt % SiO2) and quenched at PH2O = 89, 58 and 40 MPa. The four decompression rates correspond to degassing rates of 1·6, 0·56, 0·45 and 0·06 wt % H2O per day. Decompressions =1·0 MPa(PH2O) h–1, initiated above the liquidus, quenched to 100% glass at all final PH2O. Decompressions at 0·8 MPa(PH2O) h–1, also initiated above the liquidus, led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate, but initiated just below the plagioclase-in curve. It is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals, leading to larger crystals. Plagioclase growth rates from these experiments show that the largest phenocrysts (~1 mm) in the MLV-36 obsidian could have grown in <42 h. A cooling rate of ~1·2°C h–1 closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0·8 MPa(PH2O) h–1 decompression over the first 50 h. The combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing ± cooling during ascent to the surface of melts that were initially above their liquidus. The obsidian samples must have been efficiently segregated as nearly 100% liquids from their respective source regions at H2O-fluid undersaturated conditions to attain a degree of superheating upon ascent before reaching fluid saturation.
ORCID(s): 0000-0001-9928-1225
Dispersal and air entrainment in unconfined dilute pyroclastic density currentsAndrews, Benjamin J.2014DOI: info:10.1007/s00445-014-0852-4Bulletin of Volcanologyv. 76No. 9852852New York; 233 Spring ST, New York, NY 10013 USASpringer852–8520258-8900
Andrews, Benjamin J. 2014. "Dispersal and air entrainment in unconfined dilute pyroclastic density currents." Bulletin of Volcanology. 76 (9):852–852. https://doi.org/10.1007/s00445-014-0852-4
ID: 128787
Keywords: NMNH; NH-Mineral Sciences; Peer-reviewed
Abstract: Unconfined scaled laboratory experiments show that 3D structures control the behavior of dilute pyroclastic density currents (PDCs) during and after liftoff. Experiments comprise heated and ambient temperature 20 mu m talc powder turbulently suspended in air to form density currents within an unobstructed 8.5x6x2.6-m chamber. Comparisons of Richardson, thermal Richardson, Froude, Stokes, and settling numbers and buoyant thermal to kinetic energy densities show good agreement between experimental currents and dilute PDCs. The experimental Reynolds numbers are lower than those of PDCs, but the experiments are fully turbulent; thus, the large-scale dynamics are similar between the two systems. High-frequency, simultaneous observation in three orthogonal planes shows that the currents behave very differently than previous 2D (i.e., confined) currents. Specifically, whereas ambient temperature currents show radial dispersal patterns, buoyancy reversal, and liftoff of heated currents focuses dispersal along narrow axes beneath the rising plumes. The aspect ratios, defined as the current length divided by a characteristic width, are typically 2.5-3.5 in heated currents and 1.5-2.5 in ambient temperature currents, reflecting differences in dispersal between the two types of currents. Mechanisms of air entrainment differ greatly between the two currents: entrainment occurs primarily behind the heads and through the upper margins of ambient temperature currents, but heated currents entrain air through their lateral margins. That lateral entrainment is much more efficient than the vertical entrainment, >0.5 compared to similar to 0.1, where entrainment is defined as the ratio of cross-stream to streamwise velocity. These experiments suggest that generation of coignimbrite plumes should focus PDCs along narrow transport axes, resulting in elongate rather than radial deposits.
ORCID(s): 0000-0001-9928-1225
Magmatic storage conditions, decompression rate, and incipient caldera collapse of the 1902 eruption of Santa Maria Volcano, GuatemalaAndrews, Benjamin J.2014DOI: info:10.1016/j.jvolgeores.2014.06.009Journal of Volcanology and Geothermal Researchv. 282103114103–1140377-0273
Andrews, Benjamin J. 2014. "Magmatic storage conditions, decompression rate, and incipient caldera collapse of the 1902 eruption of Santa Maria Volcano, Guatemala." Journal of Volcanology and Geothermal Research. 282:103–114. https://doi.org/10.1016/j.jvolgeores.2014.06.009
ID: 127132
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Phase equilibria experiments and analysis of natural pumice and phenocryst compositions indicate the 1902 Santa Maria dacite was stored at ~ 140-170 MPa and 840-850 °C prior to eruption. H2O-saturated, cold-seal experiments conducted in vessels with an intrinsic log fO2 of NNO + 1 ± 0.5 show that the natural phase assemblage (melt + plagioclase + amphibole + orthopyroxene + Fe-Ti oxides + apatite) is stable from approximately 115-140 MPa at temperatures below ~ 825 °C, to ~ 840-860 °C at 150 MPa, to > 850 and 50 MPa and > 50 °C greater than experimental run conditions; precise estimates of magmatic conditions based solely upon amphibole composition are likely inaccurate. The experimental results and analysis of natural crystals suggest that although the natural amphiboles likely record a broad range of magmatic conditions, only the lower bounds of that range reflects pre-eruptive storage conditions. Comparison of Santa Maria microlite abundances with decompression experiments examining other silicic systems from the literature suggests that the 1902 dacite decompressed at a rate of ~ 0.005 to 0.01 MPa/s during the eruption. Applying the decompression rate with the previously described eruption rate of approximately 2-3 × 108 kg/s (Williams and Self, 1983; Carey and Sparks, 1986) to the conduit model CONFLOW reveals that the eruption conduit was dike-like with an along-strike length > 1 km. Despite depositing ~ 20 km3 of dacite tephra (equivalent to ~ 8.5 km3 magma), the 1902 eruption did not form an obvious caldera. This work suggests that collapse of the dike-like conduit terminated the eruption, preventing full caldera collapse.
ORCID(s): 0000-0001-9928-1225
Thermal and rheological controls on the formation of mafic enclaves or banded pumiceAndrews, Benjamin J.Manga, Michael2014DOI: info:10.1007/s00410-013-0961-7Contributions to Mineralogy and Petrologyv. 167No. 1Springer0010-7999
Andrews, Benjamin J. and Manga, Michael. 2014. "Thermal and rheological controls on the formation of mafic enclaves or banded pumice." Contributions to Mineralogy and Petrology. 167 (1):https://doi.org/10.1007/s00410-013-0961-7
ID: 118484
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
ORCID(s): 0000-0001-9928-1225
Explosive dome eruptions modulated by periodic gas-driven inflationJohnson, Jeffrey B.Lyons, J. J.Andrews, Benjamin J.Lees, J. M.2014DOI: info:10.1002/2014GL061310Geophysical Research Lettersv. 41No. 1966896697HobokenWiley-Blackwell6689–66970094-8276
Johnson, Jeffrey B., Lyons, J. J., Andrews, Benjamin J., and Lees, J. M. 2014. "Explosive dome eruptions modulated by periodic gas-driven inflation." Geophysical Research Letters. 41 (19):6689–6697. https://doi.org/10.1002/2014GL061310
ID: 127961
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Volcan Santiaguito (Guatemala) ‘breathes’ with extraordinary regularity as the edifice's conduit system accumulates free gas, which periodically vents to the atmosphere. Periodic pressurization controls explosion timing, which nearly always occurs at peak inflation, as detected with tiltmeters. Tilt cycles in January 2012 reveal regular 26 +/-6?min inflation/deflation cycles corresponding to at least ~101?kg/s of gas fluxing the system. Very long period (VLP) earthquakes presage explosions and occur during cycles when inflation rates are most rapid. VLPs locate ~300?m below the vent and indicate mobilization of volatiles, which ascend at ~50?m/s. Rapid gas ascent feeds pyroclast-laden eruptions lasting several minutes and rising to ~1?km. VLPs are not observed during less rapid inflation episodes; instead, gas vents passively through the conduit producing no infrasound and no explosion. These observations intimate that steady gas exsolution and accumulation in shallow reservoirs may drive inflation cycles at open-vent silicic volcanoes.
ORCID(s): 0000-0001-9928-1225
Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currentsAndrews, Benjamin J.Manga, Michael2012DOI: info:10.1016/j.jvolgeores.2012.02.011Journal of Volcanology and Geothermal Researchv. 225-226304430–440377-0273
Andrews, Benjamin J. and Manga, Michael. 2012. "Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currents." Journal of Volcanology and Geothermal Research. 225-226:30–44. https://doi.org/10.1016/j.jvolgeores.2012.02.011
ID: 110375
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Laboratory density currents comprising warm talc powder turbulently suspended in air simulate many aspects of dilute pyroclastic density currents (PDCs) and demonstrate links between bulk current behavior, sedimentation, and turbulent structures. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers match those of natural PDCs as does the ratio of thermal to kinetic energy density. The experimental currents have lower bulk Reynolds numbers than natural PDCs, but the experiments are fully turbulent. Consequently, the experiments are dynamically similar to the dilute portions of some natural currents. In general, currents traverse the floor of the experimental tank, sedimenting particles and turbulently entraining, heating, and thermally expanding air until all particles sediment or the currents become buoyant and lift off to form coignimbrite plumes. When plumes form, currents often undergo local flow reversals. Current runout distance and liftoff position decrease with increasing densimetric Richardson number and thermal energy density. As those parameters increase, total sedimentation decreases such that > 50% of initial current mass commonly fractionates into the plumes, in agreement with some observations of recent volcanic eruptions. Sedimentation profiles are best described by an entraining sedimentation model rather than the exponential fit resulting from non-entraining box models. Time series analysis shows that sedimentation is not a constant rate process in the experiments, but rather occurs as series of sedimentation-erosion couplets that propagate across the tank floor tracking current motion and behavior. During buoyant liftoff, sedimentation beneath the rising plumes often becomes less organized. Auto-correlation analysis of times series of particle concentration is used to characterize the turbulent structures of the currents and indicates that currents quickly partition into a slow-moving upper portion and faster, more concentrated, lower portion. Air entrainment occurs within the upper region. Turbulent structures within the lower region track sedimentation-erosion waves and indicate that eddies control deposition. Importantly, both eddies and sedimentation waves track reversals in flow direction that occur following buoyant liftoff. Further, these results suggest that individual laminations within PDC deposits may record passage of single eddies, thus the duration of individual PDCs may be estimated as the product of the number of laminations and the current's turbulent timescale.
ORCID(s): 0000-0001-9928-1225
Terrestrial smokers: Thermal springs due to hydrothermal convection of groundwater connected to surface waterCardenas, M. B.Lagmay, Alfredo Mahar F.Andrews, Benjamin J.Rodolfo, Raymond S.Cabria, Hillel B.Zamora, Peter B.Lapus, Mark R.2012DOI: info:10.1029/2011GL050475Geophysical Research Lettersv. 390094-8276
Cardenas, M. B., Lagmay, Alfredo Mahar F., Andrews, Benjamin J., Rodolfo, Raymond S., Cabria, Hillel B., Zamora, Peter B., and Lapus, Mark R. 2012. "Terrestrial smokers: Thermal springs due to hydrothermal convection of groundwater connected to surface water." Geophysical Research Letters. 39:https://doi.org/10.1029/2011GL050475
ID: 109836
Keywords: Peer-reviewed; NH-Mineral Sciences; NMNH; si-federal
ORCID(s): 0000-0001-9928-1225
Effects of topography on pyroclastic density current runout and formation of coignimbritesAndrews, Benjamin J.Manga, Michael2011DOI: info:10.1130/G32226.1Geologyv. 39No. 12109911021099–11020091-7613
Andrews, Benjamin J. and Manga, Michael. 2011. "Effects of topography on pyroclastic density current runout and formation of coignimbrites." Geology. 39 (12):1099–1102. https://doi.org/10.1130/G32226.1
ID: 107999
Keywords: NH-Mineral Sciences; NMNH; Peer-reviewed
Abstract: Laboratory experiments of dilute mixtures of warm talc powder in air simulate dilute pyroclastic density currents (PDCs) and show the effects of topography on current runout, buoyancy reversal, and liftoff into buoyant plumes. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers for our experiments match those of natural PDCs. The laboratory currents are fully turbulent, although the experiments have lower Reynolds numbers than PDCs. In sum, our experiments are dynamically similar to natural currents. Comparisons of currents traversing flat topography or encountering barriers show that runout distance is not significantly reduced for currents that traverse barriers with height <1.5 times the current thickness, but currents do not pass taller barriers. Buoyancy reversals occur in most currents, resulting in liftoff and generation of a buoyant plume. Liftoff occurs near the maximum runout distance for currents traveling over flat topography, but is focused near or above barriers for currents that encounter barriers. Notably, plume formation above barriers can result in reversal of flow direction downstream of the obstruction as portions of the current flow back and feed the rising plume.
ORCID(s): 0000-0001-9928-1225