186 research outputs found
Improving fluid flow in geothermal reservoirs by thermal and mechanical stimulation:The case of Krafla volcano, Iceland
Impact interaction of in-flight high-energy molten volcanic ash droplets with jet engines
© 2019 The turbine technology incorporated in jet engines is inherently vulnerable to attack by environmental silicate debris. Amongst the various kinds of such debris, volcanic ash is a particular threat as its glass softens to a liquid at temperatures of 500â800 °C, far below jet engine operating temperatures of âŒ1500 °C. As a result, ingested re-molten droplets impact and form splats on the protective thermal barrier coatings (TBCs). Investigation of the damage to jet engines ensuing from this process has, to date been restricted to forensic observations after critical encounters. Here, we employ a thermal spray technology to recreate the âin-situâ generation of molten volcanic ash droplets and observe their morphological evolution and interaction with TBCs. The mechanism of splat formation is found to depend both on substrate topography and on in-flight droplet characteristics, whereby splat circularity increases with surface roughness and with the product of the Weber and Reynolds numbers. The experiments reveal that the molten ash droplet adhesion rate is dictated by droplet temperature and viscosity, ash concentration and substrate roughness. A new dimensionless number, S, is developed to quantify the molten ash droplet adhesion rate to both substrate topography and in-flight droplet characteristics. These findings provide a greatly improved basis for the quantification of the hazard potential of volcanic ash to jet engines and should be incorporated into protocols for operational aviation response during volcanic crises
a comparison of morphological and petrological methods
In planetary sciences, the emplacement of lava flows is commonly modelled
using a single rheological parameter (apparent viscosity or apparent yield
strength) calculated from morphological dimensions using Jeffreysʌ and Hulmeʌs
equations. The rheological parameter is then typically further interpreted in
terms of the nature and chemical composition of the lava (e.g., mafic or
felsic). Without the possibility of direct sampling of the erupted material,
the validity of this approach has remained largely untested. In modern
volcanology, the complex rheological behaviour of lavas is measured and
modelled as a function of chemical composition of the liquid phase, fractions
of crystals and bubbles, temperature and strain rate. Here, we test the
planetary approach using a terrestrial basaltic lava flow from the Western
Volcanic Zone in Iceland. The geometric parameters required to employ
Jeffreysʌ and Hulmeʌs equations are accurately estimated from high-resolution
HRSC-AX Digital Elevation Models. Samples collected along the lava flow are
used to constrain a detailed model of the transient rheology as a function of
cooling, crystallisation, and compositional evolution of the residual melt
during emplacement. We observe that the viscosity derived from the morphology
corresponds to the value estimated when significant crystallisation inhibits
viscous deformation, causing the flow to halt. As a consequence, the inferred
viscosity is highly dependent on the details of the crystallisation sequence
and crystal shapes, and as such, is neither uniquely nor simply related to the
bulk chemical composition of the erupted material. This conclusion, drawn for
a mafic lava flow where crystallisation is the primary process responsible for
the increase of the viscosity during emplacement, should apply to most of
martian, lunar, or mercurian volcanic landforms, which are dominated by
basaltic compositions. However, it may not apply to felsic lavas where
vitrification resulting from degassing and cooling may ultimately cause lava
flows to halt
Sintering of vesiculating pyroclasts
Hot volcanic pyroclasts can sinter, vesiculate, and outgas in concert â a combination of processes which remains poorly constrained. And yet this combination of processes can occur coincidently during deposition from pyroclastic density currents, in conduit-filling pyroclastic debris, and in tuffisites. In many of these settings, it is the sintering-driven evolution of permeability that is key to gas transport through the evolving deposit. Here, we experimentally and theoretically investigate the evolution of the permeable networks during sintering of hot fragmental volcanic systems, which are hydrous and oversaturated at the experimental conditions. Firstly, we find that vesiculation results in shutting of the inter-granular porous network as bubble growth drives expansion of the particles into one another, destroying interconnected pores. Secondly, we observe that degassing by diffusion out of the particle edge results in contraction of the vesicular particles, re-opening pore spaces between them. Therefore, we find that vesiculation, and diffusive outgassing compete to determine both the intra-fragment vesicularity and the permeability during sintering. The development of intra-fragment vesicularity directly impacts the inter-fragment pore space and its connectivity, which decreases during vesiculation and subsequently increases during diffusive outgassing, prompting complex, non-linear permeability evolution.The relative dominance of these processes is fragment size dependent; proportionally, fine fragments lose gas at a higher rate than coarser fragments during diffusive outgassing due to larger surface area to volume ratios. As the systems progress, larger fragments retain a higher proportion of gas and so attain greater vesicularities than finer ones â and therefore, the coarse fragmental pyroclasts experience a greater, yet transient, reduction in connected porosity and permeability. We suggest that where vesiculation is sufficient, it can lead to the complete loss of connected porosity and the sealing of permeable pathways much earlier than in a sintering-only system. Our results suggest that classical sintering models must be modified to account for these vesiculation and diffusive degassing processes, and that only a combined vesiculation, sintering, and diffusive outgassing model can resolve the evolution of permeability in hot clastic volcanic systems
Crystal plasticity as an indicator of the viscous-brittle transition in magmas
Understanding the flow of multi-phase (melt, crystals and bubbles) magmas is of great importance for interpreting eruption dynamics. Here we report the first observation of crystal plasticity, identified using electron backscatter diffraction, in plagioclase in andesite dome lavas from VolcĂĄn de Colima, Mexico. The same lavas, deformed experimentally at volcanic conduit temperature and load conditions, exhibit a further, systematic plastic response in the crystalline fraction, observable as a lattice misorientation. At higher stress, and higher crystal fraction, the amount of strain accommodated by crystal plasticity is larger. Crystal plastic distortion is highest in the intact segments of broken crystals, which have exceeded their plastic limit. We infer that crystal plasticity precludes failure and can punctuate the viscous-brittle transition in crystal-bearing magmas at certain shallow magmatic conditions. Since crystal plasticity varies systematically with imposed conditions, this raises the possibility that it may be used as a strain marker in well-constrained systems
Eruptive shearing of tube pumice: pure and simple
Abstract. Understanding the physico-chemical conditions extant and mechanisms operative during explosive volcanism is essential for reliable forecasting and mitigation of volcanic events. Rhyolitic pumices reflect highly vesiculated magma whose bubbles can serve as a strain indicator for inferring the state of stress operative immediately prior to eruptive fragmentation. Obtaining the full kinematic picture reflected in bubble population geometry has been extremely difficult, involving dissection of a small number of delicate samples. The advent of reliable high-resolution tomography has changed this situation radically. Here we demonstrate via the use of tomography how a statistically powerful picture of the shapes and connectivity of thousands of individual bubbles within a single sample of tube pumice emerges. The strain record of tube pumice is dominated by simple shear (not pure shear) in the late deformational history of vesicular magma before eruption. This constraint in turn implies that magma ascent is conditioned by a velocity gradient at the point of origin of tube pumice. Magma ascent accompanied by simple shear should enhance high eruption rates inferred independently for these highly viscous systems. </jats:p
Transcriptional Basis of Mouse and Human Dendritic Cell Heterogeneity
Dendritic cells (DCs) play a critical role in orchestrating adaptive immune responses due to their
unique ability to initiate T cell responses and direct
their differentiation into effector lineages. Classical
DCs have been divided into two subsets, cDC1 and
cDC2, based on phenotypic markers and their
distinct abilities to prime CD8 and CD4 T cells. While
the transcriptional regulation of the cDC1 subset has
been well characterized, cDC2 development and
function remain poorly understood. By combining
transcriptional and chromatin analyses with genetic
reporter expression, we identified two principal
cDC2 lineages defined by distinct developmental
pathways and transcriptional regulators, including
T-bet and RORgt, two key transcription factors
known to define innate and adaptive lymphocyte
subsets. These novel cDC2 lineages were characterized by distinct metabolic and functional programs. Extending our findings to humans revealed
conserved DC heterogeneity and the presence of
the newly defined cDC2 subsets in human cancer
Characterization of moderate ash-and-gas explosions at Santiaguito volcano, Guatemala, from infrasound waveform inversion and thermal infrared measurements
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