Using hydraulic equivalences to discriminate transport processes of volcanic flows.

We characterized stratified deposits from Upper Toluca Pumice at Toluca Volcano, Mexico, to distinguish the various modes of transport at play in their genesis. Using the concept of hydraulic equivalence, we determined that deposits resulted from a combination of suspended-load fallout, saltation, and rolling. In particular, some well-sorted coarse stratified beds have a single pumice mode most likely indicative of clasts having traveled through both the transport system and the traction bed. Such beds are likely remnants of the sorting operated within the large-scale transport system. Other coarse beds have pumice and lithic modes suggesting rolling in the traction bed. We propose that boundary layer processes control the sorting of those beds and all finer beds. By helping to discriminate between transport mechanisms, hydraulic equivalences have a general applicability in geophysical flows involving clasts of contrasted densities

Experimental Constraints on Degassing and Permeability in Volcanic Conduit Flow.

This study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions: degassing and permeability in magmas. We studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature. From the data collected, we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium. Equilibrium bubble growth occurs when the decompression rate is slower than 0.1 MPa s-1, while higher rates cause porosity to deviate rapidly from equilibrium, defining the first disequilibrium regime of degassing. If the deviation is large enough, a critical threshold of super-saturation is reached and bubble growth accelerates, defining the second disequilibrium regime. We studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions. Under these open conditions, we observed that bubbles start to coalesce at ~43 vol.% porosity, regardless of decompression rate. Coalescence profoundly affects bubble texture and size distributions, and induces the melt to become permeable. We parameterized and incorporated our experimental results into a 1D conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust. Compared to previous models that assume equilibrium degassing of the melt during ascent, the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by ~ 25 %, the acceleration at high porosities (> 50 vol.%) by a factor 5, and the associated decompression rate by an order of magnitude. The integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed, and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape

Eruption and deposition of the fisher tuff (Alaska) : evidence for the evolution of pyroclastic flows.

International audienceRecognition that the Fisher Tuff (Unimak Island, Alaska) was deposited on the leeside of ~500-700 m high mountain range (Tugamak Range) more than 10 km away from its source played a major role in defining pyroclastic flows as momentum-driven currents. We re-examined the Fisher Tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features. We studied the tuff at 89 sites, and sieved bulk samples from 27 of those sites. We find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume (22 km) that deposited ~0.2 km3 of dacite magma as a pyroclastic fall layer to erupting ~10-100 km3 of andesitic magma as scoria-rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera, including those in valleys within the Tugamak Range. The distribution of the flow deposits and their welding, internal stratification, and occurrence of lithic breccia, all suggest that the pyroclastic flows were fed from a fountaining column that vented from an inclined conduit, the first time such a conduit has been recognized during a large volume caldera eruption. Pyroclastic flow deposits before and after the mountain range, and thin veneer deposits high in the Range, are best explained by a flow that was stratified into a dense undercurrent and an over-riding dilute turbulent cloud, from which deposition before the range was mainly from the undercurrent. When the flow ran into the mountain range, however, the undercurrent was blocked, but the turbulent cloud continued on. As the flow continued north, it re-stratified, forming another undercurrent. The Fisher Tuff thus records the passing of a flow that was significantly higher (800-1100 m thick) than the mountain range, and thus did not require excessive momentum

Addressing Complexity in Laboratory Experiments: The Scaling of Dilute Multiphase Flows in Magmatic Systems.

The kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments. We present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles (crystal, bubble, or pyroclast) and the continuous phase (liquid or gas). We consider three canonical igneous systems: magma chambers, volcanic plumes, and pyroclastic surges, and we provide estimates of the proposed scaling relations for published experiments on those systems. Dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average (Eulerian) properties and instantaneous (Lagrangian) variables. The Eulerian properties of the mixtures yield the Reynolds number (Re), which indicates the level of unsteadiness in the continuous phase. The Lagrangian acceleration of particles is a function of the viscous drag and gravity forces, and from these two forces are derived the Stokes number (ST) and the stability number (ΣT), two dimensionless numbers that describe the dynamic behavior of the particles within the mixture. The compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition (Re>104), and for scaling ST and ΣT. Among the particle dynamic regimes present in surges and plumes, some deserve special attention, such as the role of mesoscale structures on transport and sedimentary processes, or the consequences of the transition to turbulence on particle gathering and dispersal. The compilation of 7 experimental studies relevant to magma bodies indicates that in the laminar regime, crystals mostly follow the motion of the melt, and thus the physical state of the system can be approximated as single phase. In the transition to turbulence, magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients. This heterogeneity has a strong potential for crystals sorting. In conclusion, the Re-ST-ΣT framework demonstrates that, despite numerous experimental studies on processes relevant to magmatic systems, some and perhaps most, geologically important parameter ranges still need to be addressed at the laboratory scale

Experimental and model constraints on degassing of magma during ascent and eruption.

Surface volcanic gases may reflect volatile budgets of magma and forecast impending eruptions, and their release to the atmosphere may affect climate. The dynamics of magma degassing is complicated, however, by differences in the solubility, partitioning, and diffusion of the various volatiles, all of which can vary with pressure, temperature, and melt composition. To constrain possible gas outputs, we carried out experiments to determine how Cl partitions between water bubbles and silicate melt, and decompression experiments to examine how Cl behaves during closed- and open-system degassing. We incorporate our findings and those from the literature for CO2 and S into a steady, isothermal, and homogeneous flow model to estimate fluxes of gases at the vent from ascending water-rich magma, assuming different scenarios for the onset and development of permeability in bubbly magma. We find that, for given permeability scenarios, total gas fluxes vary with magma flux, but ratios of gas species do not change. The S/Cl and SO2/CO2 ratios do change, however, depending on whether the magma is oxidized or reduced. After magma fragments into a Plinian eruption column gases continue to escape from cooling pumice in the plume, but here the rate of gas release is controlled by diffusion, which varies with temperature. Degassing of pumice and ash was modeled by linking a steady-state plume model, which gives the vertical variation of mean temperature and velocity of particles inside the plume, to a conductive cooling model of pumices, which controls diffusion of Cl, CO2, and S in pumice. We find that gas loss increases with column height (mass flux) and initial temperature, because in both cases pumices cool over a longer time period, allowing more gas to diffuse out of the matrix glass. The amount of gas released also depends on the size distribution of particles in the erupting mixture, with less being released for a finely skewed distribution

Platelet degranulation and bleeding phenotype in a large cohort of Von Willebrand disease patients

Von Willebrand disease (VWD) is a bleeding disorder caused by quantitative (type 1 or 3) or qualitative (type 2A/2B/2M/2N) defects of circulating von Willebrand factor (VWF). Circulating VWF levels not always fully explain bleeding phenotypes, suggesting a role for alternative factors, like platelets. Here, we investigated platelet factor 4 (PF4) in a large cohort of patients with VWD. PF4 levels were lower in type 2B and current bleeding phenotype was significantly associated with higher PF4 levels, particularly in type 1 VWD. Based on our findings we speculate that platelet degranulation and cargo release may play a role across VWD subtypes

Ambiguous Nucleotide Calls From Population-based Sequencing of HIV-1 are a Marker for Viral Diversity and the Age of Infection

The fraction of ambiguous nucleotide calls in bulk sequencing of human immunodeficiency virus type 1 (HIV-1) carries important information on viral diversity and the age of infection. In particular, a fraction of ambiguous nucleotides of >.5% provides evidence against a recent infection event <1 year ago

Estimation of ash injection in the atmosphere by basaltic volcanic plumes: the case of the Eyjafjallajökull 2010 eruption

During explosive eruptions, volcanic plumes inject ash into the atmosphere and may severely affect air traffic, as illustrated by the 2010 Eyjafjallajökull eruption. Quantitative estimates of ash injection can be deduced from the height reached by the volcanic plume on the basis of scaling laws inferred from models of powerful Plinian plumes. In less explosive basaltic eruptions, there is a partitioning of the magma influx between the atmospheric plume and an effusive lava flow on the ground. We link the height reached by the volcanic plume with the rate of ash injection in the atmosphere via a refined plume model that (1) includes a recently developed variable entrainment law and (2) accounts for mass partitioning between ground flow and plume. We compute the time evolution of the rate of injection of ash into the atmosphere for the Eyjafjallajökull eruption on the basis of satellite thermal images and plume heights and use the dispersion model of the Volcanic Ash Advisory Center of Toulouse to translate these numbers into hazard maps. The classical Plinian model would have overestimated ash injection by about 20% relative to the refined estimate, which does not jeopardize risk assessment. This small error was linked to effective fragmentation by intense interactions of magma with water derived from melting of ice and hence strong mass partitioning into the plume. For a less well fragmented basaltic dry eruption, the error may reach 1 order of magnitude and hence undermine the prediction of ash dispersion, which demonstrates the need to monitor both plume heights and ground flows during an explosive eruption