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  • 1
    ISSN: 1432-0819
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Quantitative measurements of crystal size distributions (CSDs) have been used to obtain kinetic information on crystallization of industrial compounds (Randolph and Larson 1971) and more recently on Hawaiian basalts (Cashman and Marsh 1988). The technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizeL, i.e., at steady staten =n o exp(−L/itGτ), wheren o is nucleation density,G is the average crystal growth rate,τ is the average growth time, and the nucleation rateJ =n o G. CSD (Inn vsL) plots of plagioclase phenocrysts in 12 samples of Mount St. Helens “blast” dacite and 14 samples of dacite from the 1980–1986 Mount St. Helens dome are similar and averageGτ = 9.6 (± 1.1) × 10−3 cm andn o = 1−2 × 106 cm−4. Reproducibility of the measurements was tested by measuring CSDs of 12 sections cut from a single sample in three mutually perpendicular directions; precision of the size distributions is good in terms of relative, but not necessarily absolute values (± 10%). Growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets ofτ = 30–150 years; growth ratesG are 3−10 × 10−12cm/s, and nucleation ratesJ are 5−21 × 10−6/cm3 s.G andJ for Fe-Ti oxides calculated from CSD data areG = 2−13 ± 10−13 cm/sec andJ = 7−33 × 10−5/cm3 s, respectively. The higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase. Sizes of plagioclase microlites (〈0.01 mm) in the blast dacite groundmass have been measured from backscatter SEM photographs. Nucleation of these microlites was probably triggered by intrusion of material into the cone of Mount St. Helens in spring 1980. This residence time of 52 days gives minimum crystallization estimates ofG = 1−3 × 10−11 cm/s andJ = 9−16 × 1O3/cm3 s. The skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling (ΔT) relative to the phenocrysts. A comparison of nucleation and growth rates for the two crystal populations (phenocrysts vs microlites) suggests that while growth rate seems to be only slightly affected by changes inΔT, nucleation rate is a very strong function of undercooling. A comparison of plagioclase nucleation and growth rates measured in the Mount St. Helens dacite and in basalt from Makaopuhi lava lake in Hawaii suggests that plagioclase nucleation rates are not as dependent on composition. Groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit, in the dome, or after extrusion onto the surface. Most of this crystallization appears to be in the form of crystal growth (coarsening) of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals. This continuous crystallization in a shallow magmatic reservoir may provide the “overpressurization” needed to drive the continuing periodic domebuilding extrusions, which have been the pattern of activity at Mount St. Helens since December 1980.
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  • 2
    ISSN: 1432-0819
    Keywords: Key words Hawaiian basalt ; Pāhoehoe ; ‘a‘ā transition ; Textural characterization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Pāhoehoe and ‘a‘ā are the most common surface morphologies in basaltic lava flows, yet no predictive models exist to link physical parameters of flow emplacement to changes in their surface textures and rheological properties. We have characterized changes of vesicularity, vesicle deformation, and crystallinity across the pāhoehoe–‘a‘ā transition as preserved in two different eruptions, a brief, high-velocity effusion of lava from Kīlauea Volcano on 1 February 1996, and a small breakout from an ephemeral vent within a larger channel produced during the 1868 eruption of Mauna Loa. This allowed us to compare conditions leading to the pāhoehoe–‘a‘ā transition for both open channel flow (Kīlauea 1996) and reactivation of lava from an ephemeral vent (Mauna Loa 1868). Textural changes across the transition include (a) decrease in vesicularity and vesicle number density, (b) increase in microlite crystallinity and crystal number density, and (c) increase in vesicle deformation. The results support past qualitative descriptions of the transition and highlight the importance of plagioclase crystallinity in controlling lava rheology and surface morphology.
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  • 3
    ISSN: 1432-0819
    Keywords: Key words Volatiles ; Degassing ; Microlite textures ; Crystal size distribution ; Pulsatory subplinian eruptions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Dacite tephras produced by the 1991 pre-climactic eruptive sequence at Mt. Pinatubo display extreme heterogeneity in vesicularity, ranging in clast density from 700 to 2580 kg m–3. Observations of the 13 surge-producing blasts that preceded the climactic plinian event include radar-defined estimates of column heights and seismically defined eruptive and intra-eruptive durations. A comparison of the characteristics of erupted material, including microlite textures, chemical compositions, and H2O contents, with eruptive parameters suggests that devolatilization-induced crystallization of the magma occurred to a varying extent prior to at least nine of the explosive events. Although volatile loss progressed to the same approximate level in all of the clasts analyzed (weight percent H2O=1.26-1.73), microlite crystallization was extremely variable (0–22%). We infer that syn-eruptive volatile exsolution from magma in the conduit and intra-eruptive separation of the gas phase was facilitated by the development of permeability within magma residing in the conduit. Correlation of maximum microlite crystallinity with repose interval duration (28–262 min) suggests that crystallization occurred primarily intra-eruptively, in response to the reduction in dissolved H2O content that occurred during the preceding event. Detailed textural characterization, including determination of three-dimensional shapes and crystal size distributions (CSD), was conducted on a subset of clasts in order to determine rates of crystal nucleation and growth using repose interval as the time available for crystallization. Shape and size analysis suggests that crystallization proceeded in response to lessening degrees of feldspar supersaturation as repose interval durations increased. We thus propose that during repose intervals, a plug of highly viscous magma formed due to the collapse of vesicular magma that had exsolved volatiles during the previous explosive event. If plug thickness grew proportionally to the square root of time, and if magma pressurization increased during the eruptive sequence, the frequency of eruptive pulses may have been modulated by degassing of magma within the conduit. Dense clasts in surge deposits probably represent plug material entrained by each subsequent explosive event.
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  • 4
    Publication Date: 2017-04-29
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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  • 5
    Publication Date: 2017-02-08
    Description: Magma systems that supply volcanoes can extend throughout the crust and consist of mush (melt within a crystalline framework) together with ephemeral magma accumulations. Within a crystal-rich mush, slow processes of melt segregation and heat loss alternate with fast processes of destablisation and magma transport. Magma chambers form by two mechanisms: incremental magma intrusion into sub-solidus rocks or the segregation and rapid merging of melt-rich layers within mush regions. Three volcanic states reflect alternations of slow and fast processes: dormancy, unrest and eruption. Monitoring needs to detect processes of melt and fluid movements in the lower and middle crust during destabilisation to improve forecasting.
    Print ISSN: 1811-5209
    Electronic ISSN: 1811-5217
    Topics: Geosciences
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  • 6
    Publication Date: 2016-10-29
    Description: To assess the complexity of eruptive activity within mafic volcanic fields, we present a detailed geologic investigation of Holocene volcanism in the upper McKenzie River catchment in the central Oregon Cascades, United States. We focus on the Sand Mountain volcanic field, which covers 76 km 2 and consists of 23 vents, associated tephra deposits, and lava fields. We find that the Sand Mountain volcanic field was active for a few decades around 3 ka and involved at least 13 eruptive units. Despite the small total volume erupted (~1 km 3 dense rock equivalent [DRE]), Sand Mountain volcanic field lava geochemistry indicates that erupted magmas were derived from at least two, and likely three, different magma sources. Single units erupted from one or more vents, and field data provide evidence of both vent migration and reoccupation. Overall, our study shows that mafic volcanism was clustered in space and time, involved both explosive and effusive behavior, and tapped several magma sources. These observations provide important insights on possible future hazards from mafic volcanism in the central Oregon Cascades.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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  • 7
    Publication Date: 2015-02-25
    Description: The abundant fine ash produced in the 2011 subglacial eruption of Grímsvötn, Iceland, highlights the fragmentation efficiency of mafic hydromagmatic eruptions, which is considerably higher than for comparable "dry" eruptions. Ash from the 2011 eruption can be divided into three morphological components—vesicular particles, shards, and dense fragments—distinguished by the size and abundance of constituent vesicles. We use the vesicle characteristics to define a new shape factor, the concavity index, which provides an unbiased way to classify individual ash particles as either bubbly (vesicular particles and shards) or dense. The relative proportion of bubbly and dense particles varies systematically with grain size, with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter. Measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from Hawaiian fountains and suggest a comparable degassing history during magma ascent. Yet concordance between the size distributions of ash and of bubbles in the Grímsvötn samples stands in contrast to the size distributions in Hawaiian fountains, where pyroclasts are orders of magnitude larger than individual bubbles. We propose that the Grímsvötn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water. The strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 8
    Publication Date: 2015-09-17
    Description: Calderas are formed by the collapse of large magma reservoirs and are commonly elliptical in map view. The orientation of elliptical calderas is often used as an indicator of the local stress regime; but, in some rift settings, pre-existing structural trends may also influence the orientation. We investigated whether either of these two mechanisms controls the orientation of calderas in the Kenyan Rift. Satellite-based mapping was used to identify the rift border faults, intra-rift faults and orientation of the calderas to measure the stress orientations and pre-existing structural trends and to determine the extensional regime at each volcano. We found that extension in northern Kenya is orthogonal, whereas that in southern Kenya is oblique. Elliptical calderas in northern Kenya are orientated NW–SE, aligned with pre-existing structures and perpendicular to recent rift faults. In southern Kenya, the calderas are aligned NE–SW and lie oblique to recent rift faults, but are aligned with pre-existing structures. We conclude that, in oblique continental rifts, pre-existing structures control the development of elongated magma reservoirs. Our results highlight the structural control of magmatism at different crustal levels, where pre-existing structures control the storage and orientation of deeper magma reservoirs and the local stress regime controls intra-rift faulting and shallow magmatism. Supplementary material: Details of the Standard Deviation Ellipse function and statistical methods are available at http://www.geolsoc.org.uk/SUP18849
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 9
    Publication Date: 2015-12-10
    Description: Experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions. Interpretation of natural samples may be complicated, however, by both the decompression path and the composition of exsolving volatiles, which affect the time evolution of crystal textures. Here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of Mount St. Helens rhyodacite. Three families of experiments were employed to simulate varied P H 2 O– t trajectories: single-step, H 2 O-saturated decompression (SSD); continuous, H 2 O-saturated decompression (CD); continuous, H 2 O–CO 2 -saturated decompression. Quantitative textural data (crystal abundance, number density, and size) are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products. These are the first experiments to quantify feldspar nucleation and growth rates during H 2 O–CO 2 -saturated decompression. We find that reducing the initial melt water content through addition of CO 2 increases nucleation rates relative to the pure water case, an effect most pronounced at low d P /d t. Moreover, these early formed textural distinctions persist at the lowest pressures examined, suggesting that deep H 2 O–CO 2 fluids could leave a lasting textural ‘fingerprint’ on erupted magmas. Crystals formed prior to decompression during the annealing process also modulate sample textures, and growth on pre-existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions. The phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption. Finally, quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive–effusive transition at Mount St. Helens. This comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the May 18 Plinian eruption, such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories.
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
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  • 10
    Publication Date: 2013-12-04
    Description: Mafic lava flows are common; for this reason, they have long been a focus of volcanological studies. However, field studies of both older and active flows have been hampered by difficulties in field access; active flows are hot, whereas older flows have rough and jagged surfaces that are difficult to traverse. As a result, morphometric studies of lava flows have generally lagged behind theoretical studies of flow behavior. The advent of laser scanning (LS) (i.e., lidar, light detection and ranging) technologies, both airborne mapping (ALSM) and terrestrial (TLS), is promoting detailed studies of lava flows by generating data suitable for production of high-resolution digital elevation models (DEMs). These data are revolutionizing both the visual and quantitative analysis of lava flows. First and foremost, this technology allows accurate mapping of flow boundaries, particularly in vegetated areas where bare earth imaging dramatically improves mapping capabilities. Detailed imaging of flow surfaces permits mapping and measurement of flow components, such as channels, surface folds, cracks, blocks, and surface roughness. Differencing of preeruptive and posteruptive DEMs allows analysis of flow thickness variations, which can be related to the dynamics of lava emplacement. Multitemporal imaging of active flows provides information not only on the rates and locations of individual flow lobes, but also measurement of pulsed lava transport. Together these new measurement capabilities can be used to test proposed models of channel formation, lava tube formation, rates of flow advance, and flow conditions within lava channels; they also provide new ways to assess the hazard and risk posed by lava flow inundation. Early published studies illustrate the potential of applying lidar to volcanic terrain; it is clear, however, that the availability of high-resolution digital topography is poised to revolutionize the study of mafic lava flows.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
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