1,721,073 research outputs found
Rete multi-parametrica per lo studio e il monitoraggio dei rischi naturali nel canale d'Otranto e nel Mar Ionio
No full textChanges in lava effusion rate from a volcanic fissure due to pressure changes in the conduit
No full textWe calculate the change in effusion rate of lava from a volcanic fissure due to pressure changes in the volcanic conduit. The conduit is modelled as a cylinder with elliptical cross-section, embedded in an elastic medium. The elliptical shape can represent a wide range of cross-sections, according to the value of eccentricity, from almost circular vents to very long and narrow fissures. A 2-D problem is considered assuming invariance of pressure changes and conduit geometry with depth. The problem is solved analytically and expressions for the displacement and the stress fields in the elastic medium are provided. The displacement of the conduit wall is proportional to the ratio between the pressure change and the rigidity of surrounding rocks. The flow rate is a nonlinear function of the pressure change and increases with increasing pressure, due to the elastic deformation of the conduit wall.We consider flow rate oscillations with periods ranging from several minutes to hours, as are often observed during effusive eruptions. Assuming pressure oscillations with these periods, flow rate oscillations resulting from the elastic deformation of the conduit are calculated. The greatest oscillations in flow rate are obtained for very large values of the conduit eccentricity, corresponding to long and narrow volcanic fissures. For example, if a fissure is 100 m long and 2 m large, a pressure oscillation with an amplitude of 1 MPa yields a maximum displacement of the conduit wall equal to about 6 cm and an amplitude of flow rate oscillations of about 20 per cent
Viscous Newtonian laminar flow in a rectangular channel: application to Etna lava flows
No full textWe introduce a 3D model for near-vent channelized lava flows. We assume the lava to be an isothermal Newtonian liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated with an analytical steady-state solution of the Navier-Stokes equation. The surface velocity and the flow rate are calculated as functions of the flow thickness for different flow widths, and the results are compared with those of a 2D model. For typical Etna lava flow parameters, the influence of levees on the flow dynamics is significant when the flow width is less than 25 m. The model predicts the volume flow rate corresponding to the surface velocity, taking into account that both depend on flow thickness. The effusion rate is a critical parameter to evaluate lava flow hazard. We propose a model to calculate the effusion rate given the lava flow width, the topograhic slope, the lava density, the surface flow velocity, and either the lava viscosity or the flow thickness
The effect of crystallization on the rheology and dynamics of lava flows
No full textThe dynamics of a lava flow is studied by a two-dimensional model describing a viscous fluid with Bingham rheology, flowing down a slope. The temperature in the flow is calculated assuming that heat is transferred through the plug by conduction and is lost by radiation to the atmosphere at the top of the flow. Taken into account is that the increasing crystallization takes place in the flow as a consequence of cooling. The lava viscosity and yield stress are expressed as a function of crystallization degree as well as of temperature: in particular it is assumed that yield stress reaches a maximum value above the solidus temperature, according to experimental data. Dynamical variables, such as velocity and thickness of the flow, are calculated for different values of the maximum crystallization degree and the flow rate. The model shows how the lava flow dynamics is affected by cooling and crystallization. The cooling of the flow is controlled by the increase of yield stress, which produces a thicker plug and makes the heat loss slower. The increasing crystallization has two opposing effects on viscosity: it produces an increase of viscosity, but at the same time produces an increase of yield stress and hence reduces the heat loss and keeps the internal temperature high. As a consequence, lava flows are significantly affected by the dependence of yield stress on temperature and scarcely by the maximum crystallization degree
Complex events in a fault model with interacting asperities
No full textThe dynamics of a fault with heterogeneous friction is studied by employing a discrete fault model with two asperities of different strengths. The average values of stress, friction and slip on each asperity are considered and the state of the fault is described by the slip deficits of the asperities as functions of time. The fault has three different slipping modes, corresponding to the asperities slipping one at a time or simultaneously. Any seismic event produced by the fault is a sequence of n slipping modes. According to initial conditions, seismic events can be different sequences of slipping modes, implying different moment rates and seismic moments. Each event can be represented geometrically in the state space by an orbit that is the union of n damped Lissajous curves. We focus our interest on events that are sequences of two or more slipping modes: they show a complex stress interchange between the asperities and a complex temporal pattern of slip rate. The initial stress distribution producing these events is not uniform on the fault. We calculate the stress drop, the moment rate and the frequency spectrum of the events, showing how these quantities depend on initial conditions. These events have the greatest seismic moments that can be produced by fault slip. As an example, we model the moment rate of the 1992 Landers, California, earthquake that can be described as the consecutive failure of two asperities, one of which has a double strength than the other, and evaluate the evolution of stress distribution on the fault during the event
A three-dimensional Bingham model for channeled lava flows
No full textWe propose a three-dimensional (3-D) Bingham model for channeled lava flow. Unlike from the 3-D Newtonian models, this model can be applied also far from the vent where the Bingham rheology cannot be neglected as a consequence of the lava cooling. We assume the lava to be an isothermal Bingham liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated by solving semianalytically the steady state Navier-Stokes equation together with the 3-D Bingham constitutive equation. The flow vorticity is evaluated and used to define the plug shape and position for different flows: a completely filled conduit, a partially filled conduit, and an open channel. Each component of the flow vorticity vector satisfies the Laplace equation and has been evaluated by using the relaxation method. The mass flow rate is evaluated for different values of the yield stress; it appears that the Bingham rheology causes a significant reduction in flow rate as the yield stress increases. For the highest yield stress values the plug in the center of the flow welds with the plugs in the flow corners: suggesting a possible rheological mechanism for the lava tube formation
Magnetic anisotropy produced by magma flow: Theoretical model and experimental data from Ferrar dolerite sills (Antarctica)
No full textVolcanic rocks forming sills, dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement. We model a sill as a steady-state how of a Bingham fluid, driven by a pressure gradient in a horizontal conduit, The magma velocity as a function of depth is calculated from the motion and constitutive equations. Vorticity and strain rate are determined for a reference system moving with the fluid, The angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain. Magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations. It is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction, and that the magnetic foliation plane changes periodically from horizontal to vertical. The results are compared with the magnetic fabric of Ferrar dolerite sills (Victoria Land, East Antarctica) derived from low-field susceptibility measurements
A model for the formation of lava tubes by the growth of the crust from the levees
No full textWe propose a model to explain lava tube formation by the growth of crust slabs from the levees of a channel toward its center, involving solid surface fragments. The flow dynamics are described with the steady state solution of the Navier-Stokes equation in a rectangular channel, for a Newtonian, homogeneous, isotropic and incompressible fluid. The cooling of a lava flow is described by the mechanism of heat conduction into the atmosphere. The presence of levees is taken into account both in the dynamical and in the thermal model. As long as the flow cools, a solid layer forms at the surface, much thicker near the levees than at the center of the channel. The crust that has formed breaks under the effects of the applied stresses. Solid blocks slow the fluid down, the shear stress at the interface between crust and fluid lava increases and the flow thickens. Using the thin elastic plate approximation, we determine conditions allowing a crust to resist both the action of the shear stress due to the drag of the underlying fluid and the tensile stress due to the weight of the crust itself, detecting the required crust thickness and distances from the eruptive vent where the tube can form
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