Thermal effects during magma ascent in conduits (original) (raw)
Related papers
Thermal budget of magma flows in a conduit: effects of viscous heating and heat loss
2006
Viscosity of silicate melts is strongly temperature-dependent. Because of that energy and momentum equations in magma flows are strongly coupled. In order to avoid imposing arbitrary thermal boundary conditions at conduit walls, here we solve numerically mass, momentum and energy equations for magma flow inside a cylindrical conduit and heat conduction equation in the surrounding host rocks imposing local far field conditions for rocks temperature. Magma solidification and melting of country rock were neglected. Simulation results show that both effects of viscous heating and heat loss to the conduit walls are of pivotal importance in determining magma dynamics. When viscous heating is negligible (at low discharge rates or for low viscosity magmas) heat loss from conduit walls can be responsible of a significant local increase in magma viscosity near the boundary, i.e. an increase in the friction factor. When viscous heating is dominant, the heat generated by viscous friction produc...
Thermal effects during magma ascent in conduits 2 3
2007
Due to 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 2-D numerical solutions of mass, momentum and energy
Viscous heating in fluids with temperature-dependent viscosity: implications for magma flows
Arxiv preprint physics/0302100, 2003
Viscous heating plays an important role in the dy- namics of fluids with strongly temperature-dependent viscos- ity because of the coupling between the energy and momen- tum equations. The heat generated by viscous friction pro- duces a local temperature increase near the tube walls with a consequent decrease of the viscosity which may dramat- ically change the temperature and velocity profiles. These processes are mainly controlled by the Pecle ́t number, the Nahme number, the flow rate and the thermal boundary con- ditions. The problem of viscous heating in fluids was in- vestigated in the past for its practical interest in the polymer industry, and was invoked to explain some rheological be- haviours of silicate melts, but was not completely applied to study magma flows. In this paper we focus on the thermal and mechanical effects caused by viscous heating in tubes of finite lengths. We find that in magma flows at high Nahme number and typical flow rates, viscous heating is responsi- ble for the evolution from Poiseuille flow, with a uniform temperature distribution at the inlet, to a plug flow with a hotter layer near the walls. When the temperature gradients induced by viscous heating are very pronounced, local insta- bilities may occur and the triggering of secondary flows is possible. For completeness, this paper also describes magma flow in infinitely long tubes both at steady state and in tran- sient phase.
Viscous heating plays an important role in the dynamics of fluids with strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The heat generated by viscous friction produces a local temperature increase near the tube walls with a consequent decrease of the viscosity which may dramatically change the temperature and velocity profiles. These processes are mainly controlled by the Peclét number, the Nahme number, the flow rate and the thermal boundary conditions. The problem of viscous heating in fluids was investigated in the past for its practical interest in the polymer industry, and was invoked to explain some rheological behaviours of silicate melts, but was not completely applied to study magma flows. In this paper we focus on the thermal and mechanical effects caused by viscous heating in tubes of finite lengths. We find that in magma flows at high Nahme number and typical flow rates, viscous heating is responsible for the evolution from Poiseuille flow, with a uniform temperature distribution at the inlet, to a plug flow with a hotter layer near the walls. When the temperature gradients induced by viscous heating are very pronounced, local insta-bilities may occur and the triggering of secondary flows is possible. For completeness, this paper also describes magma flow in infinitely long tubes both at steady state and in transient phase.
Ascent and decompression of viscous vesicular magma in a volcanic conduit
During eruption, lava domes and flows may become unstable and generate dangerous explosions. Fossil lava-filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content. These observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressures: those of the gas and magma phases, and that of the exterior. To solve for these three different pressures, one must account for expansion in all directions and hence for both horizontal and vertical velocity components. We present a new two-dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, gas overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the ``exit'' boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lava flows spreading horizontally at the surface, the largest gas overpressures, and hence the smallest exsolved gas contents, are achieved at the conduit walls. For zero horizontal velocity at the vent, corresponding to a plug-like eruption through a preexisting lava dome or to spine growth, gas overpressures are largest at the center of the vent. The magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth-dependent radii.
Instability of Magma Flow from Volatile-Dependent Viscosity
Science, 1999
crystalline magma) or to fluid systems with spatially and time-ya~ying viscosity as deyeloped by th~s report. We ass~uned that the conduit 1s a yert~cally oriented cylindrical tube. Most of the pressure Volatiles dissolved in silicic magma at depth exsolve as the magma nears the b o p along a co~lduit occurs 111 approx~mately surface and cause an increase in viscosity of the magma. A model of a volcanic the top half ltilometer because s111cic magma conduit within an elastic medium and a viscosity dependent on the volatile ascending toward the surface becomes heavily content of the magma produces oscillatory magma flow for a critical range of degassed and therefore has larger yiscosity (13steady input flow rates. Oscillatory flow is recognized as a fundamental mode 15). With this in mind. we divided the conduit of behavior at silicic volcanoes, and understanding it allows improved short-into tsvo distinct parts. The upper portion is term forecasting of timing and eruption style.
A heat pipe model for vertical, magma-filled conduits
Journal of Volcanology and Geothermal Research, 1983
. A heat pipe model for vertical, magma-filled conduits. J. Volcanol. Geotherm. Res., 16:279--298 A 5-m radius magma-filled conduit will solidify in much less than one year if heat losses to the conduit wall are not offset by some form of forced or free convection of magma from some source body through the conduit. If the forced convection of magma from a source through the conduit is either too weak or is prevented by closure of the conduit at the end nearest the surface, only free convective circulations between the source chamber and conduit are available to balance the wall heat loss. Using an integral approach, the efficiency of free convection is investigated for conduits emplaced in both conductive and hydrothermally convective host rock environments. The results of the model strongly suggest that free circulations within conduits of large aspect ratio provide an efficient mechanism for offsetting heat losses to the conduit wall. The model provides a possible explanation for the occurrence of periodic eruptions from a conduit when the periodicity greatly exceeds the time scale for the cooling of a quiescent conduit by heat loss through the wall.
Some effects of viscosity on the dynamics of replenished magma chambers
1984
Some aspects of the dynamical experiments are also described: the release of behavior of magma chambers, replenished from gas by a chemical reaction, to model the release below with hotter but •denser magma, have been of volatiles following an overturning event in a modeled in a series o$ laboratory experiments. magma chamber; the effect of a cold, immiscible In previously reported work the fluids used were layer above the cooling crystallizing fluid; the aqueous solutions of comparable viscosity, and influence of two viscous layers with a density thus the results should be applicable to basaltic step between them; and the constraining effects magma chambers, in which the magmas do not vary of a density (with corresponding viscosity) gragreatly in viscosity. In that case, the lower dient in the upper region. The experiments indilayer cools by convective heat transfer to the cate that whatever the strmtificmtion, whether it fluid above, and crystallization causes the den-be in layers or continuous, the form of the inisity of the residual liquid in the lower layer to tial motion in the upper fluid is determined by decrease. When the density becomes equal to that the viscosity ratio between the two fluids in the upper layer, sudden overturning and inti-immediately adjacent to the interface. Geologimate mixing take place. The present paper cal applications are not examined in detail in reports experimental results that allow us to this paper, but the experiments suggest that both extend the application to systems in which there sudden overturning (characteristic of magmas of is a large viscosity ratio between the resident nearly equal viscosity) and continuous release and the injected fluid, for example, to calcalka-(when the upper ]myer is much more viscous) are line magmas, where magma viscosity can vary by as viable mechanisms for magma mixing in the much as 5 orders of magnitude. The largest appropriate circumstances. viscosity ratio in our experiments (about 3000) was achieved using cold glycerine for the upper 1. We have concentrated so far on two experimental Copyright 1984 by the American Geophysical Union. configurations. First, the phenomena of layering and differentiation have been studied in a rec-Effects %n Replenished Magma Chambers 6869
Earth Planetary Science Letters, 2019
Unravelling the rheological behaviour of magmas is fundamental for hazard assessment. At shallow depth the combined effects of degassing, vesiculation and crystallization are likely to produce dramatic changes in the rheology, hence modulating flow dynamics and eruptive style. The rheological evolution from a low viscosity crystal-poor, bubble-free, water-rich melt to a highly viscous crystal-rich, vesicular magma containing a water-poor melt often occurs in the conduit. To clarify the viscous flow dynamics of rheologically-layered volcanic conduits, we performed decompression experiments using a magma analogue system characterized by a low-viscous Layer L (10 Pa s) at the bottom and a high-viscous particle-bearing Layer H (≥1000 Pa s) at the top. Silicone oils and spherical glass beads are employed as magma and crystal analogues, respectively. Three sets of experiments address the effects of: 1) decompression rate (ca. 10 −2 and 10 4 MPa/s); 2) crystal content in the high viscosity magma (0, 10, 30 and 70 vol.%); and 3) volume ratio of the two rheological layers (0.6 or 0.3). Our results indicate that decompression rate exerts the most dramatic role, yielding changes in timescale of outgassing up to two orders of magnitude, and affecting the style of decompression response (permeable outgassing or fragmentation). The solid fraction 1) strongly modulates gas mobility, 2) influences the pervasiveness of fragmentation and 3) affects the extent of mingling in the experimental conduit. These results demonstrate that the properties of a shallow, partially-crystallized portion of the magmatic column and its response to varying ascent rate are primary controls on eruptive style.
Temporal evolution of flow conditions in sustained magmatic explosive eruptions
The temporal evolution of fundamental flow conditions in the magma chamber plus conduit system–such as pressure, velocity, mass flow-rate, erupted mass, etc.–during sustained magmatic explosive eruptions was investigated. To this aim, simplified one-dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together. The chamber model assumed an homogeneous composition of magma and a vertical profile of water content. The chamber could have a cylindrical, elliptical or spherical rigid geometry. Inside the chamber, magma was assumed to be in hydrostatic equilibrium both before and during the eruption. Since the timescale of pressure variations at the conduit inlet–of the order of hours–is much longer than the travel time of magma in the conduit–of the order of a few minutes–the flow in the conduit was assumed as at steady-state. The one-dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked-flow conditions at the exit. Bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law. Above the nucleation level, bubbles and liquid magma were considered in mechanical equilibrium. The same equilibrium assumption was made above the fragmentation level between gas and pyroclasts. Due to the hydrostatic hypothesis, the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and, through the conduit model, as a function of time. Simulation results pertaining to rhyolitic and basaltic magmas defined at the Volcanic Eruption Mechanism Modeling Workshops (Durham, NH, 2002; Nice, France, 2003) are presented. Important flow variables, such as pressure, density, velocity, shear stress in the chamber and conduit, are discussed as a function of time and magma chamber and conduit properties. Results indicate that vent variables react in different ways to the pressure variation of the chamber. Pressure, density and mass flow-rate show relative variations of the same order of magnitude as the conduit inlet pressure, whereas velocity is more constant in time. Sill-like chambers produce also significantly longer and more voluminous eruptions than dike-like chambers. Water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption