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Melnik, O.
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- PublicationOpen AccessVolcanic Eruptions:Cyclicity during Lava Dome Growth(2008-07-04)
; ; ; ; ;Melnik, O.; Institute of Mechanics, Moscow State University, ;Sparks, R. S. J.; Earth Science Department, University of Bristol, ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Barmin, A.; Institute of Mechanics, Moscow State University,; ; ; We consider the process of slow extrusion of very viscous magma that forms lava domes. Dome-building eruptions are commonly associated with hazardous phenomena, in- cluding pyroclastic flows generated by dome collapses, explosive eruptions and volcanic blasts. These eruptions commonly display fairly regular alternations between pe- riods of high and low or no activity with time scales from hours to years. Usually hazardous phenomena are asso- ciated with periods of high magma discharge rate, thus, understanding the causes of pulsatory activity during ex- trusive eruptions is an important step towards forecasting volcanic behavior, especially the transition to explosive ac- tivity when magma discharge rate increases by a few orders of magnitude. In recent years the risks have increased be- cause the population density in the vicinity of many active volcanoes has increased.179 393 - PublicationRestrictedModelling ground deformation caused by oscillating overpressure in a dyke conduit at Soufrière Hills Volcano, Montserrat(2009)
; ; ; ; ; ; ;Hautmann, S.; Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom ;Gottsmann, J.; Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom ;Sparks, R. S. J.; Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Melnik, O.; Institute of Mechanics, Moscow State University, 1-112b Michurinsky prosp., Moscow, Russia ;Voight, B.; Department of Geosciences, Penn State University, 334A Deike Building, University Park, PA 16802, USA; ; ; ; ; A dyke conduit has been shown to be a realistic model for the shallow magmatic feeder system for the dome forming eruption at Soufrière Hills Volcano, Montserrat. Here we use a three-dimensional Finite Element model to examine the ground deformation that can be expected due to the pressurization of a dyke conduit. We find that the generated deformation has a bilateral symmetry with nearly no displacement in the direction of dyke strike, and a maximal ground deformation about 1 km away from the vent in the perpendicular direction. Resultant surface deformation is mainly triggered by the upper part of the feeder system, where the dyke opens into a cylindrical conduit. We apply our deformation model to investigate tilt data collected in 1997 in order to infer the orientation of the dyke. We obtain a best-fit for a NNW–SSE trending dyke, which matches observations of the ground displacement field obtained by differential GPS and the alignment of main structural geologic features of southern Montserrat.401 25 - PublicationRestrictedControls of conduit geometry and wallrock elasticity on lava dome eruptions(2007)
; ; ; ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Melnik, O.; Centre for Environmental and Geophysical Flows, Department of Earth Sciences, University of Bristol, Bristol, UK Institute of Mechanics, Moscow State University, Moscow, Russia ;Sparks, R. S. J; Centre for Environmental and Geophysical Flows, Department of Earth Sciences, University of Bristol, Bristol, UK; ; Many lava dome building eruptions show periodic to complex non-periodic pulsatory activity. Typical time-scales associated with this activity range from hours to decades. Previous studies modelled the ascent of magma using a set of transient 1-D transport equations, accounting for degassing induced crystallization kinetics, gas exsolution and viscosity increase due to crystal growth. These models assumed flow in a cylindrical conduit with a fixed cross-section area. Since several observations suggest that extrusions are mainly fed by dykes, with cylindrical geometries developing only at shallow levels, here we generalised the model to the flow geometry represented by an elliptical dyke with major and minor semi-axes changing with depth. Quasi-static elastic deformation of the dyke is accounted by an analytical solution that couples cross-section area with the magmatic overpressure. The effects of the main dyke geometrical parameters and boundary conditions on the eruption dynamics were investigated. The presence of a deformable dyke can lead to a more complex periodic behaviour with a wider range of time-scales and cyclicity patterns with respect to a uniform cylindrical conduit. There is a regime where the period of pulsations is controlled by the elasticity of the dyke and a regime where the period is controlled by the volume of the magma chamber. Intermediate regimes are possible. Periodic variations in discharge rate are also possible for both fixed pressure in dyke source region and fixed influx rate into the dyke. Our study emphasizes the strong nonlinearities and complex behaviours of lava dome eruptions. From a forecasting and hazard perspective, intrinsic uncertainties in governing parameters may make volcanic systems in some circumstances unpredictable. On the other hand, lava dome systems may also develop episodic and systematic behaviours so that behaviour becomes predictable for a while.181 25 - PublicationRestrictedThermal effects during magma ascent in conduits(2007-12-27)
; ; ; ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Melnik, O.; Centre for Environmental and Geophysical Flows, Department of Earth Sciences, University of Bristol, Bristol, UK ;Vedeneeva, E.; Institute of Mechanics, Moscow State University, Moscow, Russia; ; Because of strong coupling between viscosity and temperature, the dynamics of magma flows in conduits are drastically controlled by thermal effects due to heat generation by viscous dissipation and loss to the walls by conduction. Here we present analytical solutions and a practical procedure based on an order of magnitude analysis that permits the characterization of the regime and estimation of the main features of the flow. The ranges of validity of analytical and asymptotic solutions were bounded by using results from fully two-dimensional (2-D) numerical solutions of mass, momentum, and energy equations for magma flow inside a cylindrical conduit and the heat conduction in the surrounding host rocks. The results permitted the identification of three regimes: a conductive-heat-loss-dominated regime, an intermediate regime, and a viscous-heating-dominated regime. Some useful analytical parameterizations are proposed for estimating friction in simplified 1-D models. Temperature layering due to heat loss by conduction can lead to local crystal growth and magma solidification whereas heat generated by viscous dissipation can be responsible for crystal resorption and remelting of wall rocks282 358 - PublicationRestrictedA stress-controlled mechanism for the intensity of very large magnitude explosive eruptions(2011-08-15)
; ; ; ; ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Gottsmann, J.; Dep. Earth. Sci., University of Bristol, UK ;Melnik, O.; Inst. Mechanics, Moscow State University, Moskow, Russia ;Sparks, R. S. J.; Dep. Earth. Sci., University of Bristol, UK; ; ; Large magnitude explosive eruptions are the result of the rapid and large-scale transport of silicic magma stored in the Earth's crust, but the mechanics of erupting teratonnes of silicic magma remain poorly understood. Here, we demonstrate that the combined effect of local crustal extension and magma chamber overpressure can sustain linear dyke-fed explosive eruptions with mass fluxes in excess of 1010 kg/s from shallow-seated (4–6 km depth) chambers during moderate extensional stresses. Early eruption column collapse is facilitated with eruption duration of the order of few days with an intensity of at least one order of magnitude greater than the largest eruptions in the 20th century. The conditions explored in this study are one way in which high mass eruption rates can be achieved to feed large explosive eruptions. Our results corroborate geological and volcanological evidences from volcano-tectonic complexes such as the Sierra Madre Occidental (Mexico) and the Taupo Volcanic Zone (New Zealand).126 24 - PublicationRestrictedControl of magma flow in dykes on cyclic lava dome extrusion(2007)
; ; ; ; ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Melnik, O.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Sparks, R. S. J.; Department of Earth Sciences, University of Bristol, Bristol, UK. ;Voight, B.; Geosciences, Penn State University, University Park, Pennsylvania; ; ; Lava dome eruptions are commonly characterized by large fluctuations in discharge rate with cyclic behaviour on time-scales ranging from hours to decades. Examples include Bezymianny volcano (Russia), Merapi (Java), Santiaguito (Guatemala), Mt St Helens (USA), Mt Unzen (Japan), and Soufrie`re Hills volcano (Montserrat). Previous models have assumed simple cylindrical conduits for magma transport, but extrusions are mainly fed by dykes, with cylindrical geometries developing only at shallow levels. The widths of dykes embedded in an elastic medium are influenced by local magma pressure, affecting flow rates and system dynamics strongly. We develop a model for magma flow in dykes, which predicts intense pulsations of magma extrusion for the case of a constant source pressure. The period time scale is determined by the elastic deformation of the dyke walls and the length-to-width ratio of the dyke. The dyke acts like a volumetric capacitor, storing magma as pressure increases and then releasing magma in a pulse of extrusion. For the Soufrie`re Hills volcano, cyclic extrusions with time-scales of a few weeks are predicted for dykes 300–500 m long and 3–6 m wide, matching observations. The model explains the sharp onset of tilt pulsations and seismic swarms.307 27 - PublicationRestrictedCyclic extrusion of a lava dome based on a stick-slip mechanismLava dome eruptions are sometimes characterised by large periodic fluctuations in extrusion rate over periods of hours that may be accompanied by Vulcanian explosions and pyroclastic flows. We consider a simple system of nonlinear equations describing a 1D flow of lava extrusion through a deep elastic dyke feeding a shallower cylindrical conduit in order to simulate this short-period cyclicity. Stick-slip conditions depending on a critical shear stress are assumed at the wall boundary of the cylindrical conduit. By analogy with the behaviour of industrial polymers in a plastic extruder, the elastic dyke acts like a barrel and the shallower cylindrical portion of the conduit as a die for the flow of magma acting as a polymer. When we applied the model to the Soufriere Hills Volcano, Montserrat, for which the key parameters have been evaluated from previous studies, cyclic extrusions with periods from 3 to 30 h were readily simulated, matching observations. The model also reproduces the reduced period of cycles observed when a major unloading event occurs due to lava dome collapse.
57 6 - PublicationRestrictedEffects of wall-rock elasticity on magma flow in dykes during explosive eruptions(2009-11)
; ; ; ; ;Costa, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Sparks, R. S. J.; Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK ;Macedonio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Melnik, O.; Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK ; Institute of Mechanics, Moscow State University, Moscow, Russia; ; ; Magma flow during explosive volcanic eruptions has been described assuming rigid conduits with simple cylindrical or planar geometries. Here we study the dynamics of explosive volcanic flows to take account of the role of elastic deformation of the conduit influenced by local magmatic pressure. Three cases are investigated: a dyke with elliptical cross-section, a cylindrical conduit and a deep dyke connected to a shallow cylinder. The model CPIUC (Macedonio et al., 2005) was used for simulations and generalized to account for elastic deformations of the conduit cross-section area due to magmatic overpressure. Fragmentation level is typically deeper in a dyke than in a cylinder. For flows in wide dykes pressure at the fragmentation depth can be lower than the surrounding lithostatic pressure by several tens of MPa, indicating that the wall-rocks of the dyke will be unstable, constraining the dyke width and eventually blocking the eruption. On the other hand, when the fragmentation level is shallow the corresponding lithostatic pressure is not large enough to close the dyke and eruptions from wide dykes are possible. The behaviour changes drastically when we assume the conduit is a dyke at depth that evolves to a cylinder near the surface. In this case even very wide dykes can be stable because the fragmentation level moves into the cylindrical region where deformation is negligible.319 29