1,721,021 research outputs found
Evidence that coupling to magma chambers controls the volume history and velocity of laterally propagating intrusions
No full textIn this paper, I present a simple analytical solution for the unsolved problem of a propagating dike coupled to a magma chamber. As recognized by Segall et al. (2001), the flow of the magma from a chamber to a dike is described by an ordinary differential equation for the unknown pressures of chamber and dike. The most intuitive assumption on magma exchange is that the volume gained by the intrusion equals the volume lost by the chamber. This constraint, however, implies a chamber so large that its pressure does not drop during the feeding process and an incompressible magma that does not expand or vesiculate as it intrudes. Models assuming constant driving pressure fail to explain observations. Here, mass conservation constrains pressure as magma flows from the chamber to the dike and the compressibilities of magma and sources control volume changes. These assumptions allow me to decouple the equations and solve the system analytically. The model predicts that chamber and dike volume change exponentially with time as V(t) = V∞[1 − exp(−t/τ)]. The asymptotic volume V∞ and the timescale τ are found to be functions of rock, magma, chamber and dike parameters and of the initial pressure conditions. Assuming that the intrusion is shaped as an elongating cuboid, its velocity is found to change as v = v0exp(−t/τ), where v0 is the initial dike velocity. The model is successfully tested against the best observations available for lateral dike propagation events: the volume history of chamber and dike during the 2000 Miyakejima intrusion (Japan) and dike velocity during the 1978 Krafla event (Iceland) and during some intrusions following the 2005 event in Afar (Ethiopia). This paper confirms and extends the results of a previous study by Rivalta and Segall (2008), who found the final ratio between dike volume and the volume withdrawn from the chamber to be rV = 1 + 4μβm/3 > 1, where μ is the host rock rigidity and βm is the magma compressibility. Here, I demonstrate that the formula for rV holds at any time during the intrusion, not just at the end. This model confirms that some magma chambers behave as stiff magma‐tanks, able to inflate large dikes as balloons, and demonstrates that high volume ratios are unlikely if the chamber has a pure sill shape. Fitting observations with the model allows us to estimate several dike, chamber, magma and rock parameters
Acceleration of buoyancy-driven fractures and magmatic dikes beneath the free surface
No full textBy performing buoyancy-driven fracture experiments in brittle gelatin we observe that the ascent velocity of a fracture containing a finite volume of fluid increases when approaching the free surface. We theoretically describe this free-surface effect and quantify it by introducing an effective depth-dependent fracture toughness and developing an ascent model on the basis of linear fracture mechanics. We develop a successful inversion approach and resolve the actual and critical fracture length and the ascent velocity far away from the free surface from the observation of the fracture tip migration alone. Other parameters, as the fluid volume included in the fracture and the in situ fracture toughness, can be derived. Applying the model and inversion to the 1998 eruption at Piton de la Fournaise, Reunion Island, reveals estimates of the length and critical length of the feeding magma batch, the magma batch volume and the in situ fracture toughness. It further indicates that the ascent velocity of the magma batch was probably much smaller at greater depths and that the batch might have been initiated several months or years before the eruption
Analytical Solutions for Gravity Changes Caused by Triaxial Volumetric Sources
Get PDFVolcanic crises are often associated with magmatic intrusions or the pressurization of magma chambers of various shapes. These volumetric sources deform the country rocks, changing their density, and cause surface uplift. Both the net mass of intruding magmatic fluids and these deformation effects contribute to surface gravity changes. Thus, to estimate the intrusion mass from gravity changes, the deformation effects must be accounted for. We develop analytical solutions and computer codes for the gravity changes caused by triaxial sources of expansion. This establishes coupled solutions for joint inversions of deformation and gravity changes. Such inversions can constrain both the intrusion mass and the deformation source parameters more accurately
Magma compressibility and the missing source for some dike intrusions
No full textDike intrusions are often accompanied by localized deflation, interpreted as depressurizing magma chambers feeding the dike. In some cases the inferred volume decrease is a factor of 4 or 5 less than the volume increase of the dike. Here we explore whether this discrepancy can be explained by compressibility of the magma combined with the fact that cracks are much more compliant than equidimensional magma chambers. If pressure changes are small, the magma compressibility βm is constant, and the dike ends up in hydrostatic equilibrium with an ellipsoidal magma chamber at the same depth, the ratio rV of the volume of the crack to the volume lost by the chamber is rV = 1 + 4μβm/3 > 1, where μ is the host rock rigidity. For gas poor magmas, βm = 0.6–2 · 10−10 Pa−1 and μ = 3–25 GPa, we find 1.2 < rV < 7.7. Large changes in magma compressibility due to gas exsolution increase rV
Explosive expansion of a slowly decompressed magma analogue: Evidence for delayed bubble nucleation
Get PDFWhile ascending in the plumbing system of volcanoes, magma undergoes decompression at rates spanning several orders of magnitude and set by a number of factors internal and external to the volcano. Slow decompression generally results in an effusive or mildly explosive expansion of the magma, but counterexamples of sudden switches from effusive to explosive eruptive behavior have been documented at various volcanoes worldwide. The mechanisms involved in this behavior are currently debated, in particular for basaltic magmas. Here, we explore the interplay between decompression rate and vesiculation vigor by decompressing a magma analogue obtained by dissolving pine resin into acetone in varying proportions. Analogue experiments allow direct observations of the processes of bubble nucleation and growth, flow dynamics, and fragmentation that is not currently possible with magmatic systems. Our mixtures contain solid particles, and upon decompression, nucleation of acetone bubbles is observed. We find that mixtures with a high acetone content, containing smaller and fewer solid particles, experience strong supersaturation and fragment under very slow decompressions, despite having low viscosity, while mixtures with lower acetone content, with more and larger solid particles, degas efficiently without fragmentation. We interpret our results in terms of delayed bubble nucleation due to a lack of efficient nucleation sites. We discuss how a similar mechanism might induce violent, explosive expansion in volatile‐rich and poorly crystalline low‐silica magmas, by analogy with the behavior of rhyolitic magmas
What drives the lateral vs vertical propagation of dikes? Insights from analogue models.
No full textSloshing of a bubbly magma reservoir as a mechanism of triggered eruptions
Get PDFLarge earthquakes sometimes activate volcanoes both in the near field as well as in the far field. One possible explanation is that shaking may increase the mobility of the volcanic gases stored in magma reservoirs and conduits. Here experimentally and theoretically we investigate how sloshing, the oscillatory motion of fluids contained in a shaking tank, may affect the presence and stability of bubbles and foams, with important implications for magma conduits and reservoirs. We adopt this concept from engineering: severe earthquakes are known to induce sloshing and damage petroleum tanks. Sloshing occurs in a partially filled tank or a fully filled tank with density-stratified fluids. These conditions are met at open summit conduits or at sealed magma reservoirs where a bubbly magma layer overlays a newly injected denser magma layer. We conducted sloshing experiments by shaking a rectangular tank partially filled with liquids, bubbly fluids (foams) and fully filled with density-stratified fluids; i.e., a foam layer overlying a liquid layer. In experiments with foams, we find that foam collapse occurs for oscillations near the resonance frequency of the fluid layer. Low viscosity and large bubble size favor foam collapse during sloshing. In the layered case, the collapsed foam mixes with the underlying liquid layer. Based on scaling considerations, we constrain the conditions for the occurrence of foam collapse in natural magma reservoirs. We find that seismic waves with lower frequencies < 1 Hz, usually excited by large earthquakes, can resonate with magma reservoirs whose width is > 0.5 m. Strong ground motion > 0.1 m s− 1 can excite sloshing with sufficient amplitude to collapse a magma foam in an open conduit or a foam overlying basaltic magma in a closed magma reservoir. The gas released from the collapsed foam may infiltrate the rock or diffuse through pores, enhancing heat transfer, or may generate a gas slug to cause a magmatic eruption. The overturn in the magma reservoir provides new nucleation sites which may help to prepare a following/delayed eruption. Mt. Fuji erupted 49 days after the large Hoei earthquake (1707) both dacitic and basaltic magmas. The eruption might have been triggered by magma mixing through sloshing
On precisely modelling surface deformation due to interacting magma chambers and dykes
Get PDFCombined data sets of InSAR and GPS allow us to observe surface deformation in volcanic settings. However, at the vast majority of volcanoes, a detailed 3-D structure that could guide the modelling of deformation sources is not available, due to the lack of tomography studies, for example. Therefore, volcano ground deformation due to magma movement in the subsurface is commonly modelled using simple point (Mogi) or dislocation (Okada) sources, embedded in a homogeneous, isotropic and elastic half-space. When data sets are too complex to be explained by a single deformation source, the magmatic system is often represented by a combination of these sources and their displacements fields are simply summed. By doing so, the assumption of homogeneity in the half-space is violated and the resulting interaction between sources is neglected. We have quantified the errors of such a simplification and investigated the limits in which the combination of analytical sources is justified. We have calculated the vertical and horizontal displacements for analytical models with adjacent deformation sources and have tested them against the solutions of corresponding 3-D finite element models, which account for the interaction between sources. We have tested various double-source configurations with either two spherical sources representing magma chambers, or a magma chamber and an adjacent dyke, modelled by a rectangular tensile dislocation or pressurized crack. For a tensile Okada source (representing an opening dyke) aligned or superposed to a Mogi source (magma chamber), we find the discrepancies with the numerical models to be insignificant (<'5 per cent) independently of the source separation. However, if aMogi source is placed side by side to an Okada source (in the strike-perpendicular direction), we find the discrepancies to become significant for a source separation less than four times the radius of the magma chamber. For horizontally or vertically aligned pressurized sources, the discrepancies are up to 20 per cent, which translates into surprisingly large errors when inverting deformation data for source parameters such as depth and volume change. Beyond 8 radii however, we demonstrate that the summation of analytical sources represents adjacent magma chambers correctly
Magmatic architecture within a rift segment: Articulate axial magma storage at Erta Ale volcano, Ethiopia
No full textUnderstanding the magmatic systems beneath rift volcanoes provides insights into the deeper processes associated with rift architecture and development. At the slow spreading Erta Ale segment (Afar, Ethiopia) transition from continental rifting to seafloor spreading is ongoing on land. A lava lake has been documented since the twentieth century at the summit of the Erta Ale volcano and acts as an indicator of the pressure of its magma reservoir. However, the structure of the plumbing system of the volcano feeding such persistent active lava lake and the mechanisms controlling the architecture of magma storage remain unclear. Here, we combine high-resolution satellite optical imagery and radar interferometry (InSAR) to infer the shape, location and orientation of the conduits feeding the 2017 Erta Ale eruption. We show that the lava lake was rooted in a vertical dike-shaped reservoir that had been inflating prior to the eruption. The magma was subsequently transferred into a shallower feeder dike. We also find a shallow, horizontal magma lens elongated along axis inflating beneath the volcano during the later period of the eruption. Edifice stress modeling suggests the hydraulically connected system of horizontal and vertical thin magmatic bodies able to open and close are arranged spatially according to stresses induced by loading and unloading due to topographic changes. Our combined approach may provide new constraints on the organization of magma plumbing systems beneath volcanoes in continental and marine settings
Spatio-temporal evolution of rift volcanism controlled top-down by a deepening graben
No full textVolcanism in continental rifts is generally observed to shift over time from the inside of the basin to its flanks
and vice versa, but the controls on these switches are still unclear. Here we use numerical simulations of dike
propagation to test the hypothesis that the spatio-temporal evolution of rift volcanism is controlled by the crustal
stresses produced during the development of the rift basin. We find that the progressive deepening of a rift rotates
the direction of the principal stresses under the basin, deflecting ascending dikes. This causes an early shift of
volcanism from the inside of the graben to its flanks. The intensification of this stress pattern, due to further
deepening of the basin, promotes the formation of lower crustal sill-like intrusions that can stack under the rift,
shallowing the depth at which dikes nucleate, eventually causing a late stage of in-rift axial volcanism. Given
the agreement between our model results and observations, we conclude that the temporal shifts in the location
of rift volcanism are controlled to first order by the elastic stresses developing in the crust as the rift matures.
We thereby suggest that geodynamic models should account for elasticity and the redistribution of surface loads
in order to effectively reproduce rift-related magmatism.Published118593OSV1: Verso la previsione dei fenomeni vulcanici pericolosiJCR Journa
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