We introduce a novel analytical expression that allows for fast assessment of mass flow rate of both vertically‐rising and bent‐over volcanic plumes as a function of their height, while first order ...physical insight is maintained. This relationship is compared with a one‐dimensional plume model to demonstrate its flexibility and then validated with observations of the 1980 Mount St. Helens and of the 2010 Eyjafjallajökull eruptions. The influence of wind on the dynamics of volcanic plumes is quantified by a new dimensionless parameter (Π) and it is shown how even vertically‐rising plumes, such as the one associated with the Mount St. Helens 1980 eruption, can be significantly affected by strong wind. Comparison between a one‐dimensional model and the analytical equation gives anR2‐value of 0.88, while existing expressions give negativeR2‐values due to their inability to adapt to different source and atmospheric conditions. Therefore, this new expression has important implications both for current strategies of real‐time forecasting of ash transport in the atmosphere and for the characterization of explosive eruptions based on the study of tephra deposits. In addition, this work provides a framework for the application of more complete three‐dimensional numerical models as it greatly reduces the parameter space that needs to be explored.
Key Points
New analytical expression to derive mass flow rate of volcanic plumes
New dimensionless parameter to assess influence of wind on plume height
Examination of mass flow rates associated with two important eruptions
Whether a magma body is able to produce eruptions and at what frequency remains a challenging problem in volcanology as it involves the nonlinear interplay of different processes acting over ...different time scales. Due to their complexity these are often considered independently in spite of their coupled nature. Here we consider an idealized model that focuses on the evolution of the thermodynamic state of the chamber (pressure, temperature, gas and crystal content) as new magma is injected into the chamber. The magma chamber cools in contact with the crust, which responds viscoelastically to the pressure accumulated during recharge and volatile exsolution. The magma is considered eruptible if the crystal volume fraction is smaller than 0.5. If a critical overpressure is reached, mass is released from the magma chamber until the lithostatic pressure is recovered. The setup of the model allows for rapid calculations that provide the opportunity to test the influence of competing processes on the evolution of the magma reservoir. We show how the frequency of eruptions depends on the timescale of injection, cooling, and viscous relaxation and develop a scaling law that relates these timescales to the eruption frequency. Based on these timescales we place different eruption triggering mechanisms (second boiling, mass injection, and buoyancy) in a coherent framework and evaluate the conditions needed to grow large magma reservoirs.
•A self-consistent model for the evolution of upper crustal silicic magma chambers is developed.•An expression for eruption frequency as a function of mass injection and radius is derived.•A coherent framework for eruption triggering mechanisms is provided.
The mechanics of shallow magma reservoir outgassing Parmigiani, A.; Degruyter, W.; Leclaire, S. ...
Geochemistry, geophysics, geosystems : G3,
August 2017, 2017-08-00, 20170801, Letnik:
18, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Magma degassing fundamentally controls the Earth's volatile cycles. The large amount of gas expelled into the atmosphere during volcanic eruptions (i.e., volcanic outgassing) is the most obvious ...display of magmatic volatile release. However, owing to the large intrusive:extrusive ratio, and considering the paucity of volatiles left in intrusive rocks after final solidification, volcanic outgassing likely constitutes only a small fraction of the overall mass of magmatic volatiles released to the Earth's surface. Therefore, as most magmas stall on their way to the surface, outgassing of uneruptible, crystal‐rich magma storage regions will play a dominant role in closing the balance of volatile element cycling between the mantle and the surface. We use a numerical approach to study the migration of a magmatic volatile phase (MVP) in crystal‐rich magma bodies (“mush zones”) at the pore scale. Our results suggest that buoyancy‐driven outgassing is efficient over crystal volume fractions between 0.4 and 0.7 (for mm‐sized crystals). We parameterize our pore‐scale results for MVP migration in a thermomechanical magma reservoir model to study outgassing under dynamical conditions where cooling controls the evolution of the proportion of crystal, gas, and melt phases and to investigate the role of the reservoir size and the temperature‐dependent viscoelastic response of the crust on outgassing efficiency. We find that buoyancy‐driven outgassing allows for a maximum of 40–50% volatiles to leave the reservoir over the 0.4–0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7) through veining and/or capillary fracturing.
Key Points
Outgassing potential of a magma reservoir is a strong function of its crystal content
Outgassing efficiency is also modulated by mechanical coupling between reservoir and crust
Simulations that consider both aspects reveal that the majority of exsolved volatiles is released at intermediate to high crystallinity
The eruption style of silicic magmas is affected by the loss of gas (outgassing) during ascent. We investigate outgassing using a numerical model for one-dimensional, two-phase, steady flow in a ...volcanic conduit. By implementing Forchheimer's equation rather than Darcy's equation for outgassing we are able to investigate the relative influence of Darcian and inertial permeability on the transition between effusive and explosive eruptions. These permeabilities are defined by constitutive equations obtained from textural analysis of pyroclasts and determined by bubble number density, throat–bubble size ratio, tortuosity, and roughness. The efficiency of outgassing as a function of these parameters can be quantified by two dimensionless quantities: the Stokes number, the ratio of the response time of the magma and the characteristic time of gas flow, and the Forchheimer number, the ratio of the viscous and inertial forces inside the bubble network. A small Stokes number indicates strong coupling between gas and magma and thus promotes explosive eruption. A large Forchheimer number signifies that gas escape from the bubble network is dominated by inertial effects, which leads to explosive behaviour. To provide context we compare model predictions to the May 18, 1980 Mount St. Helens and the August–September 1997 Soufrière Hills eruptions. We show that inertial effects dominate outgassing during both effusive and explosive eruptions, and that in this case the eruptive regime is determined by a new dimensionless quantity defined by the ratio of Stokes and Forchheimer number. Of the considered textural parameters, the bubble number density has the strongest influence on this quantity. This result has implications for permeability studies and conduit modelling.
► Modelling of outgassing during volcanic eruptions. ► Outgassing occurs in the turbulent regime. ► Effusive–explosive is determined by the ratio of Stokes and Forchheimer number. ► Inertial permeability should replace Darcian permeability.
The collapse of volcanic plumes has significant implications for eruption dynamics and associated hazards. We show how eruptive columns can collapse and generate pyroclastic density currents as a ...result of not only the source conditions, but also of the atmospheric environment. The ratio of the potential energy and the kinetic energy at the source quantified by the Richardson number, and the entrainment efficiency quantified by the radial entrainment coefficient have already been identified as key parameters in controlling the transition between a buoyant and collapsing plume. Here we quantify how this transition is affected by wind using scaling arguments in combination with a one-dimensional plume model. Air entrainment due to wind causes a volcanic plume to lower its density at a faster rate and therefore to favor buoyancy. We identify the conditions when wind entrainment becomes dominant over radial entrainment and quantify the effect of wind on column collapse. These findings are framed into a generalized regime diagram that also describes previous regime diagrams for the specific case of choked flows. Many observations confirm how bent-over plumes typically do not generate significant collapses. A quantitative comparison with the 1996 Ruapehu and the 2010 Eyjafjallajökull eruptions shows that the likelihood of collapse is reduced even at moderate wind speeds relative to the exit velocity at the vent.
•Wind prevents volcanic plumes from collapsing.•New regime diagram for collapse and buoyant plumes.•Comparison of the model predictions with the 1996 Ruapehu and the 2010 Eyjafjallajökull eruptions.
The two most recent eruptions of Volcán Quizapu (southern Andes, Chile), only 85 years apart, were both triggered by magma recharge and extruded the same volume (about 5 km3) of the same ...volatile‐rich dacitic magma, but showed a remarkable shift from effusive (1846–1847) to explosive (1932) behavior. We demonstrate, using a newly developed model, that the presence or absence of an exsolved volatile phase in the reservoir strongly influences its mechanical and thermal response to new inputs of magma. We propose that, prior to the 1846–1847 effusive eruption, gas bubbles damped the build‐up of excess pressure and allowed recharge of a significant volume of magma before triggering the 1846–1847 eruption. The strong temperature increase that resulted enhanced syneruptive outgassing leading to an effusive eruption. In contrast, during the repose period between the 1847 and 1932 eruptions, new recharges found a much less compressible host reservoir as the exsolved gas phase was largely removed in response to the prior eruption, yielding rapid pressurization, minor reheating, and comparatively less syneruptive outgassing. The combination of these effects culminated in an explosive eruption.
Key Points
The presence of exsolved volatiles changes the thermal evolution of a magma reservoir
Exsolved volatiles allow for a stronger temperature increase during recharge events leading up to eruption
The eruptive style at Volcán Quizapu is explained through changes in exsolved volatile content in the magma chamber
X-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
f
0
). Constitutive equations for both types of permeability allow the quantification of laminar and turbulent gas escape during ascent of rhyolitic magma in volcanic conduits.
Analogous to volcanic deposits on Earth, we can infer eruption characteristics on Enceladus from the relationship between particle size and distance from the vent. We develop a model in which ice ...particles feeding plumes are accelerated by the gas. We consider two cases: drag‐limited and collision‐limited acceleration, which link particle size to exit velocity. After being ejected at the vent, particles follow ballistic trajectories. We fit the model to observations of particle size on the surface inferred from modeled VIMS data collected by the Cassini spacecraft. We obtain a relationship between gas temperature and characteristic acceleration length, whereby lower gas temperatures require longer acceleration lengths. The model shows that the large size of particles on the surface is consistent with the size of particles observed with the CDA and VIMS instruments at heights of Cassini flybys, and the size of particles that reach escape velocity and are found in Saturn's E‐ring.
Key Points
The deposits on Enceladus are cryoclastic in origin
Link deposit to eruption temperature
Prediction of particle size with height
Explosive basaltic eruptions pose significant threats to local communities, regional infrastructures and international airspace. They produce tephra plumes that are often associated with a lava ...fountain, complicating their dynamics. Consequently, source parameters cannot be easily constrained using traditional formulations. Particularly, mass flow rates (MFRs) derived from height observations frequently differ from field deposit‐derived MFRs. Here, we investigate this discrepancy using a novel integral plume model that explicitly accounts for a lava fountain, which is represented as a hot, coarse‐grained inner plume co‐flowing with a finer‐grained outer plume. The new model shows that a plume associated with a lava fountain has higher variability in rise height than a standard plume for the same initial MFR depending on initial conditions. The initial grain‐size distribution and the relative size of the lava fountain compared to the surrounding plume are primary controls on the final plume height as they determine the strength of coupling between the two plumes. We apply the new model to the August 29, 2011 paroxysmal eruption of Mount Etna, Italy. The modeled MFR profile indicates that the field‐derived MFR does not correspond to that at the vent, but rather the MFR just above the lava fountain top. High fallout from the lava fountain results in much of the erupted solid material not reaching the top of the plume. This material deposits to form the proximal cone rather than dispersing in the atmosphere. With our novel model, discrepancies between the two types of observation‐derived MFR can be investigated and understood.
Key Points
We have developed a new integral model that accounts for the coupling between a tephra plume and a lava fountain
The initial grain‐size distribution and radius of a lava fountain control the rise height of the surrounding tephra plume
The model can explain the relationship between the tephra plume and cone deposits and the observed plume and lava fountain height
In order to understand outgassing during volcanic eruptions, we performed permeability measurements on trachy-phonolitic pyroclastic products from the Campanian Ignimbrite and Monte Nuovo, two ...explosive eruptions from the active Campi Flegrei caldera, Southern Italy. Viscous (Darcian) permeability spans a wide range between 1.22×10−14 and 9.31×10−11m2. Inertial (non-Darcian) permeability follows the same trend as viscous permeability: it increases as viscous permeability increases, highlighting the strong direct correlation between these two parameters. We observe that vesicularity does not exert a first order control on permeability: the Monte Nuovo scoria clasts are the most permeable samples but not the most vesicular; pumice clasts from the Campanian Ignimbrite proximal facies, whose vesicularity is comparable with that of Monte Nuovo scoriae, are instead the least permeable. In addition, we find that sample geometry exhibits permeability anisotropy as samples oriented parallel to vesicle elongation are more permeable than those oriented perpendicular. We compare our results with permeability values of volcanic products from effusive and explosive activity, and discuss the role of melt viscosity and crystallinity on magma permeability.
•We produce the first permeability data on the Campanian Ignimbrite and Monte Nuovo explosive eruptions from Campi Flegrei•In evolved alkaline magmas vesicularity does not exert a first order control on permeability•In evolved alkaline magmas sample geometry exhibits permeability anisotropy•We compare our data with permeability measurements in products from effusive and explosive eruptions•We make the first assessment of the role of viscosity on permeability in natural magmas.
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