Magmas In Motion: Degassing In Volcanic Conduits And Fabrics Of Pyroclastic Density Current

Thesis (Ph.D.) University of Alaska Fairbanks, 2003Volcanoes are caused by the transport of magma batches from the Earth's crust to the surface. These magmas in motion undergo drastic changes of rheologic properties during their journey to the surface and this work explores how these changes affect volcanic eruptions. The first part of this study is devoted to the dynamic aspects of degassing and permeability in magmas with high pressure, high temperature experiments on natural volcanic rocks. Degassing is measured by the influence of decompression rate on the growth of the bubbles present in the magma while permeability is deduced from the temporal evolution of these bubbles. The parameterization of our results in a numerical model of volcanic conduit flow show that previous models based on equilibrium degassing overestimate the acceleration and the decompression rate of the magma. Assessing permeability effects derived form our results show that the transition between explosive and effusive eruptions is a strong function of the magma initial ascent rate. The second part of this work is a unification of two end-members of pyroclastic currents (highly concentrated pyroclastic flows and dilute, turbulent pyroclastic surges) using theoretical scaling arguments based on multiphase physics. Starting from the dynamics of the particle interactions with a fundamental eddy, we consider the full spectrum of eddies generated within a turbulent current. We demonstrate that the presence of particles with various sizes induces a density stratification of the current, leading to its segregation into a basal concentrated part overlain by a dilute cloud. To verify our predictions on the interactions of such a segregated pyroclastic current with its surroundings (hills and sea), we studied the products of the 2050 BP caldera-forming eruption of Okmok Volcano (Alaska). This field study allowed us to reconstruct the eruptive sequence and to validate the main aspects of our theoretical model, such as the superposition of a dense and dilute part, their decoupling at sea entrance and the characteristics of the particles they transport

Reconciling Pyroclastic Flow and Surge: the Multiphase Physics of Pyroclastic Density Currents.

Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number (ST), the Stability factor (ET) and the Dense-Dilute condition (DD). We recognize five types of particle behaviors within a fluid eddy as a function of ST and ET : (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or “fast-tracked” as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and DD determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents

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

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

Simulating the behavior of volatiles belonging tothe C-O-H-S system in silicate melts undermagmatic conditions with the software d-compress

International audienceModeling magmatic degassing, or how the volatile distribution between gas and meltchanges at pressure varies, is a complex task that involves a large number ofthermodynamical relationships and that requires dedicated software. This article presents thesoftware D-Compress, which computes the gas and melt volatile composition of five elementsets in magmatic systems (O-H, S-O-H, C-S-O-H, C-S-O-H-Fe, and C-O-H). It has beencalibrated so as to simulate the volatiles coexisting with three common types of silicate melts(basalt, phonolite, and rhyolite). Operational temperatures depend on melt composition andrange from 790 to 1400°C. A specificity of D-Compress is the calculation of volatilecomposition as pressure varies along a (de)compression path between atmospheric and 3000bars. This software was prepared so as to maximize versatility by proposing different sets ofinput parameters. In particular, whenever new solubility laws on specific melt compositionsare available, the model parameters can be easily tuned to run the code on that composition.Parameter gaps were minimized by including sets of chemical species for which calibrationdata were available over a wide range of pressure, temperature, and melt composition. A briefdescription of the model rationale is followed by the presentation of the software capabilities.Examples of use are then presented with outputs comparisons between D-Compress and othercurrently available thermodynamical models. The compiled software and the source code areavailable as electronic supplementary materials

Physical volcanology of the 2050 BP caldera-forming eruption of Okmok Volcano, Alaska.

In the Aleutian volcanic chain (USA), the 2050 ± 50 BP collapse of Okmok caldera generated pyroclasts that spread over 1000 km2 on Umnak Island. After expelling up to 0.25 km3 DRE of rhyodacitic Plinian air fall and 0.35 km3 DRE of andesitic phreatomagmatic tephra, the caldera collapsed and produced the 29 km3 DRE Okmok II scoria deposit, which is composed of valley-ponding, poorly sorted, massive facies and over-bank, stratified facies with planar and cross bedding. Geological and sedimentological data suggest that a single density current produced the Okmok II deposits by segregating into a highly concentrated base and an overriding dilute cloud. The dense base deposited massive facies, whereas the dilute cloud sedimented preferentially on hills as stratified deposits. The pyroclastic current spread around Okmok in an axisymmetric fashion, encountering topographic barriers on the southwest, and reaching Unalaska Island across an 8-km strait on the east, and reaching the shoreline of Umnak in the other directions. The kinematic model by Burgisser and Bergantz (2002, Earth Planet. Sci. Lett. 202:405-418) was used to show how decoupling of the pyroclastic current was triggered by both sea entrance and interaction with the topography. In the former case, the dense part of the current and the lithics transported by the dilute cloud went underwater. In the latter case, topographical barriers noticeably decelerated both parts of the decoupled current and favored sedimentation by partial or complete blocking. The resulting unloading of the dilute current drastically reduced the runout distance by triggering an early buoyant lift-off

Controls on magma permeability in the volcanic conduit during the climactic phase of the Kos Plateau Tuff eruption (Aegean Arc)

International audienceX-ray computed microtomography (μCT) was applied to pumices from the largest Quaternary explosive eruption of the active South Aegean Arc (the Kos Plateau Tuff; KPT) in order to better understand magma permeability within volcanic conduits. Two different types of pumices (one with highly elongated bubbles, tube pumice; and the other with near spherical bubbles, frothy pumice) produced synchronously and with identical chemical composition were selected for μCT imaging to obtain porosity, tortuosity, bubble size and throat size distributions. Tortuosity drops on average from 2.2 in frothy pumice to 1.5 in tube pumice. Bubble size and throat size distributions provide estimates for mean bubble size (~93– 98 μm) and mean throat size (~23–29 μm). Using a modified Kozeny-Carman equation, variations in porosity, tortuosity, and throat size observed in KPT pumices explain the spread found in laboratory measurements of the Darcian permeability. Measured difference in inertial permeability between tube and frothy pumices can also be partly explained by the same variables but require an additional parameter related to the internal roughness of the porous medium (friction factor f0). Constitutive equations for both types of permeability allow the quantification of laminar and turbulent gas escape during ascent of rhyolitic magma in volcanic conduits

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

Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption

Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) "frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5-10vol.% of the coarsest pyroclastic flow deposits, (2) dominantly "microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zon