Mechanics of Magma Chamber with the Implication of the Effect of CO2 Fluxing (original) (raw)
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Effect of stress fields on magma chamber stability and the formation of collapse calderas
Tectonics, 2003
The summits of many of the Earth's and other planets' larger volcanoes are occupied by calderas that formed by collapse into an evacuating, underlying magma chamber. These collapse calderas are typically several tens of square kilometers in area and are commonly elliptical in shape. We show that the long axes of late Quaternary collapse calderas in the Kenya rift valley, the western Basin and Range province, the Snake River-Yellowstone Plateau, and the Iceland rift zone are parallel to the upper crustal minimum horizontal stress direction (S h) as determined by independent criteria. We suggest that circular magma chambers beneath these volcanoes became elliptical by stress-induced spalling of their chamber walls, by a mechanism that is analogous to the formation of breakouts in boreholes and tunnels. In breakouts, the hole becomes elongate parallel to the far-field minimum stress. In the Kenya rift, Late Pleistocene caldera collapse was accompanied by a 45°rotation of S h and an increase in the magnitude of the maximum horizontal stress (S H). The breakout model predicts increasingly unstable caldera walls under these conditions, a possible explanation for the sudden appearance of so many collapse events in a volcanic setting that had never experienced them before. This mechanism of stress change-induced collapse may have played a role in other caldera settings.
Collapse calderas are common on Earth and some other solid planetary bodies, particularly on Io (a satellite of Jupiter), Mars and Venus. Caldera structures are generally similar on all these bodies, but the sizes vary considerably. Thus, on Mars the largest caldera diameter reaches 150 km, whereas the largest caldera diameters on Io and Venus approach 300 km. The arithmetic average diameter of calderas on Io is 41 km, on Mars 48 km, and on Venus 68 km. By contrast, the largest (multiple) caldera on Earth has a maximum diameter of about 80 km. On Earth, the arithmetic average maximum diameter is about 6 km for calderas associated with basaltic edifices and about 18 km for calderas associated with composite volcanoes.
Thermo-mechanical effects of magma chambers and caldera faults
2016
The subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies. All of these inferred structures need to be compared with actual ring faults so as to test the model implications. Here, we present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland. At the time of collapse caldera formation, over 200 m of vertical displacement was accommodated along a ring fault, which is exceptionally well exposed at a depth of approximately 1.2 km below the original surface of the volcano. There are abrupt changes in the ring fault attitude with depth, but its overall dip is steeply inward. Several inclined sheets within the caldera became arrested at the ring fault; other sheets became deflected up along the fault to form a multiple ring dike. We present numerical models showing stress fields that encourage sheet de...
The role of magma chamber-fault interaction in caldera forming eruptions
This paper examines the role of the position and orientation of a regional fault in the roof of a magma chamber on stress distribution, mechanical failure, and dyking using 2D finite element numerical simulations. The study pertains to the magma chamber behavior in the relatively short time intervals of several hundreds to thousand of years. The magma chamber is represented as an elliptical inclusion (eccentricity, a/b=0.12) at a relative depth, H/a, of 0.9. The fault has a 45° dip and is represented by a frictionless fracture. The temperature field in the host rock is calculated assuming a quasisteady-state thermal regime that develops through periodic episodes of magma supply. The rheology of the surrounding rocks is treated using viscoelasticity with temperature activated strain-rate dependent viscosity. Strain weakening of the rocks in the ductile zone is described within the frame of the Dynamic Power Law model . The magma pressure is coupled with the deformation of the rock mass hosting the chamber, including the fault. The variation of magma pressure in response to magma supply and chamber deformation is calculated in the elastic and viscoelastic regimes. The latter corresponds to slow filling, while the former represents a filling time much less than the viscous relaxation time scale. The resulting “equation of state” for the magma chamber couples the magma pressure with the chamber volume in the elastic regime, and with the filling rate for the viscoelastic regime. Analysis of stresses is used to predict dyke propagation conditions, and the mechanical failure of the chamber roof for different fault positions and magma overpressures. Results show that an outward dipping fault located on the periphery of the chamber roof hinders the propagation of dykes to the surface, causing magma to accumulate under the footwall of the fault. At high to moderate overpressures (30–40 MPa), the fault causes localized shear failure and chamber roof collapse that might lead to the first stage of a caldera-forming eruption.
Pressure evolution during explosive caldera-forming eruptions
Earth and Planetary Science Letters, 2000
Caldera-forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber. Field studies of caldera-forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions. Pressure evolution during explosive caldera-forming eruptions is investigated through a simple model that describes the first-order quantitative behaviour of the chamber. We consider a piston-like model that assumes a coherent block subsiding along circular, subvertical, ring faults into the magma chamber. This subsidence occurs after significant decompression of the chamber by an initial central vent eruption. We assume that the initial pressure distribution in the chamber is magmastatic. Once collapse has begun the chamber roof is supported by the magma, so that magma pressure at the chamber roof increases to lithostatic. We suggest that pressure variations during caldera-forming eruptions are mainly controlled by variations in magma volatile content. Regardless of what induces the formation of ring faults, the model suggests that the occurrence of explosive caldera-forming events depends on the strength of the chamber walls, and the depth, water content and aspect ratio of the magma chamber. No significant differences exist between model results for a cylindrical, or a more realistic elliptical magma chamber geometry of comparable aspect ratio. Assuming a constant strength of the host rock, the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges, for deep, gas-poor chambers, from a few percent up to 40% for shallow, gas-rich chambers. The model suggests that zoned chambers tend to collapse earlier than homogeneous chambers. Dike-shaped chambers will erupt less magma than sill-like chambers before caldera collapse initiates, although dike-like geometries are not associated with stress fields appropriate to create ring faults. The model suggests that once initiated, caldera collapse will tend to force out most or all of the volatile-rich magma from the chamber. For volatile-rich magma chambers, the total volume of erupted magma during caldera-forming event is of the same order as the chamber volume. The model also explains the variation in the erupted mass during the different phases of explosive caldera-forming eruptions, and is in good agreement with natural examples. ß
Physics of the Earth and Planetary Interiors, 2014
We present a stress-strain analysis using the Finite Element Method to investigate failure conditions of pressured magma chambers embedded in an inelastic domain. The pressure build-up induces variations in the stress field until failure conditions are reached. Therefore, the definition of the failure conditions could have a significant impact on the volcano hazard assessment. Using a numerical approach, we analyze the stresses in a gravitationally loaded model assuming a brittle failure criterion, to determine the favorable conditions for magma chamber failure in different source geometries, reference stress states, pore fluid pressures, rock rheologies and topographic profiles. The numerical results allow us to pinpoint the conditions promoting seismicity near the magma chamber. The methodology places a limit on the pressure that a magma chamber can sustain before failing and provides a quantitative estimate of the uplift expected at the ground surface. Thermally-activated ductile regimes, which may develop in the region surrounding a heated magma chamber, are also investigated. The stress relaxation in a ductile shell may prevent the wall rupture, favoring the growth of large overpressured chambers, which could lead to considerable deformation at the ground surface without significant seismicity. The numerical results suggest that a spherical source, compressive regime, gentle edifice topography, and growth of a ductile shell are important factors for the initial formation and the mechanical stability of magma storage systems. On the other hand, an elongated ellipsoidal source, extensional regime, steep volcano topography and high pore fluid pressure lower the overpressure necessary for inducing failure. These findings could help in gaining insights on the internal state of the volcano and, hence, in advancing the assessment of the likelihood of volcano unrest.
Magma chamber behavior beneath a volcanic edifice
Journal of Geophysical Research, 2003
1] The construction of a large volcanic edifice at Earth's surface generates stresses in the upper crust whose magnitude is comparable to those of tectonic stresses and overpressures within a magma chamber. We study how this affects eruption behavior. Analytical calculations are carried out in two dimensions for a cylindrical reservoir with an internal overpressure in an elastic half-space with an edifice at the surface. Different edifice shapes are considered, from shield volcanoes with gentle slopes to stratovolcanoes with steeper flanks. Without an edifice at the top, the hoop stress at the cavity walls reaches a maximum at two symmetrical points at some distance from the axis, away from the top of the chamber. With an edifice at the top, the maximum is reached at the top of the chamber, just beneath the edifice summit. This implies preferential failure of chamber walls at the axis and hence the focussing of volcanic activity through a central vent system. Tensile failure of the cavity walls occurs for a critical value of magma overpressure which depends on the dimensions of the edifice and on the depth and size of the cavity. For a small magma chamber beneath a large stratovolcano, the magmatic overpressure at the onset of eruption increases as the edifice grows and decreases following edifice destruction. These effects may explain why pressures recorded in phenocryst assemblages at Mount St. Helens, have varied over the past 4000 years as the edifice went through successive phases of growth and destruction.
Magma chamber stability in arc and continental crust
2010
The growth and thermo-mechanical stability of magma chambers in Earth's crust dictate the dynamics of volcanism at the surface, and the organization of volcanic plumbing at depth. We analyze a model of magma chamber evolution in which volumetric growth is governed by the mechanical focusing of rising dikes by the magma chamber, "magmatic lensing," as well as melting and assimilation of country rock. This modeling framework emphasizes the two-way coupling between chamber stresses and thermal evolution with specific compositions of intruding magma and country rock. We consider as end member compositional scenarios a "wet" environment magma chamber, in which basalt with 2 wt.% H 2 O intrudes an amphibolitic country rock, and a "dry" chamber consisting of anhydrous basalt intruding tonalitic country rocks. Magma chambers that erupt, freeze, or reach dynamic equilibrium in the crust occupy distinct regions of a parameter space that measures the relative importance of depth, chamber pressurization, wall rock viscoelastic rheology, and thermal viability. Lower crustal melt flux is the most important factor controlling chamber stability, but chamber depth and composition also help determine long-term dynamical behavior. In general, interactions between thermal and mechanical processes exert first-order control on chamber stability, defining four distinct regimes of magma chamber dynamics. In addition to thermally and mechanically unstable (freezing and eruptive) chambers, we find steady-state thermally viable chamber volumes are possible as well as a range of parameters for which chamber growth is roughly exponential in time and mechanically stable (no eruption occurs). Long-lived (N1 Ma) chambers generally result from lower crustal melt flux values that range from ∼ 10 − 4 to ∼ 10 − 1 m 3 /m 2 /yr for 20 and 40 km deep chambers and both compositional end members used in this study. However chambers become considerably less stable in cool shallow environments, particularly with anhydrous compositions of magma and country rock. Model predictions in this framework suggest that a range of observed intrusive structures in Earth's crust may be the result of magma chambers in different, clearly defined dynamical regimes.
Magma chambers: formation, local stresses, excess pressures, and compartments
An existing magma chamber is normally a necessary condition for the generation of a large volcanic edifice. Most magma chambers form through repeated magma injections, commonly sills, and gradually expand and change their shapes. Highly irregular magma-chamber shapes are thermo-mechanically unstable; common long-term equilibrium shapes are comparatively smooth and approximate those of ellipsoids of revolution. Some chambers, particularly small and sill-like, may be totally molten. Most chambers, however, are only partially molten, the main part of the chamber being crystal mush, a porous material. During an eruption, magma is drawn from the crystal mush towards a molten zone beneath the lower end of the feeder dyke. Magma transport to the feeder dyke, however, depends on the chamber's internal structure; in particular on whether the chamber contains pressure compartments that are, to a degree, isolated from other compartments. It is only during large drops in the hydraulic potential beneath the feeder dyke that other compartments become likely to supply magma to the erupting compartment, thereby contributing to its excess pressure (the pressure needed to rupture a magma chamber) and the duration of the eruption.
Relationship between caldera collapse and magma chamber withdrawal: an experimental approach
Collapse calderas have received considerable attention due to their link to Earth's ore deposits and geothermal energy resources, but also because of their tremendous destructive potential. Although calderas have been investigated through fieldwork, numerical models and experimental studies, some important aspects on their formation still remain poorly understood. One key issue concerns the volume of magmas involved in caldera-forming eruptions. We perform analogue experiments to correlate the structural evolution of a collapse with the erupted magma chamber volume fraction. The experimental device consists of a transparent box (60 × 60 × 40 cm) filled with dry quartz sand and a water-filled latex balloon as a magma chamber analogue. Evacuation of water through a pipe causes a progressive deflation of the balloon that leads to a collapse of the overlying structure. The experimental design allows to record the temporal evolution of the collapse and to track the evolution of fractures and faults. We study the appearance and development of specific brittle structures, such as surface fractures or internal reverse faults, and correlate each different structure with the corresponding removed magma chamber volume fraction. We also determine the critical conditions for caldera onset. Experimental results show that, at any stage of caldera developments, the experimental relationship between volume fraction and chamber roof aspect ratio fits a logarithmic curve. It implies that volume fractions required to trigger caldera collapse are lower for chambers with low aspect ratios (shallow and wide) than for chambers with high aspect ratios (deep and small). These results are in agreement with natural examples and previous theoretical studies.