Granular flow and viscous fluctuations in low Bagnold number granitic magmas

Petford, Nick; Koenders, M. A.

doi:10.1144/gsjgs.155.5.0873

Cited by 29 publications

(21 citation statements)

References 42 publications

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“…The behaviour of natural magmas may be described by granular flow theory and, until the magma has stopped, deviation from simple Newtonian flow is unlikely (Petford & Koenders 1998). Gravity-controlled crystals accumulation.…”

Section: The Processes Operating During Formation Of Magmatic Layeringmentioning

confidence: 99%

The origin of magmatic layering in the High Tatra granite, Central Western Carpathians – implications for the formation of granitoid plutons

Gawęda

Szopa

2012

Earth and Environmental Science Transactions of the Royal Socie

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The High Tatra granite intrusion is an example of a Variscan syn-tectonic, tongue-shaped intrusion. In some portions of the intrusion, structures occur which appear to be of sedimentary origin. These include structures similar to graded bedding, cross-bedding, troughs and flame structures, K-feldspar-rich cumulates and magmatic breccias. Formation of these structures might be related to changing magma properties, including crystal fraction, development of a crystal mush and a decrease in magma viscosity, stimulated by influx of mafic magma and high volatile content. The suggested processes in operation are: gravity-controlled separation, magma flow segregation, deposition on the magma-chamber floor, filter pressing and density currents stimulated by tectonic activity.%The formation of the sedimentary structures was also aided by the presence of large numbers of xenoliths that acted as a heat sink and influenced the thermal field in the intrusion, stimulating rapid cooling and crystal nucleation. Sinking xenoliths deformed the layering and, to some extent, protected the unconsolidated crystal mush from erosion by magma flowing past.%Areas with well-developed sedimentary magmatic structures can be viewed as having involved magma rich in crystals locally forming closely-packed networks from which residual melt was extracted by filter pressing, and preserved in leucocratic pods and dykes. Interleaved, non-layered granite may be interpreted to have formed from the magma with initially low crystal fractions.%It is suggested that the intrusion was formed from numerous magma injections representing different stages in the mixing and mingling of felsic and mafic sources. It solidified by gravitation-driven crystal accumulation and flow sorting on the magma chamber floor and on the surfaces of large numbers of xenoliths. Shear stress acting during intrusion might have influenced the formation of magmatic structures.

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Section: The Processes Operating During Formation Of Magmatic Layeringmentioning

confidence: 99%

The origin of magmatic layering in the High Tatra granite, Central Western Carpathians – implications for the formation of granitoid plutons

Gawęda

Szopa

2012

Earth and Environmental Science Transactions of the Royal Socie

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“…Pinkerton and Stevenson (1992) suggested that yield strength was only developed in materials containing at least 30 vol.% crystals. Petford and Koenders (1998) consider that a Bingham model is inappropriate for millimeter to centimeter grain size, which applies to most intermediate to silicic magmas. This observation that we have no definitive evidence for the existence of a yield strength at magmatic temperatures for about 30 vol.% crystals or more is in conflict with many other studies on crystal-bearing melts (Petford and Koenders 1998;Hoover et al 2001;Petford 2003), but it may be due to the difficulties of measuring yield strength (Nguyen and Boger 1992;Moller et al 2006;Ishibashi and Sato 2007) and it can also be due to the extreme sensitivity of this parameter to the crystal shape (Hoover et al 2001), volume fraction, and alignment (Walsh and Saar 2008).…”

Section: Yield Strength Of Dacitic Lava At Magmatic Temperaturesmentioning

confidence: 99%

Rheology of arc dacite lavas: experimental determination at low strain rates

Avard

Whittington

2012

Bull Volcanol

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Andesitic-dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315 K, at low shear stress (0.085 to 0.42 MPa), low strain rate (between 1.1×10 −8 and 1.9×10 −5 s −1 ), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate-and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226 K and strain rates lower than 1.8×10 −4 s −1 , is log η app ¼ À0:738 þ 9:24 Â 10 3 =T ðKÞ À 0:654 Á log _ " where η app is apparent viscosity and _ " is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances.

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“…The latter have been derived for a smooth boundary (McTigue and Jenkins, 1992), but not for rough boundaries, a more realistic natural situation where magma is flowing in dykes and sills with irregular walls. A solution to this problem was obtained for the first time by Petford and Koenders (1998) for granitic magmas flowing vertically in a dyke. They showed that the accompanying fluctuation in velocity intensity resulted in a granular velocity fluctuation intensity that was greatest closest to the dyke walls, as expected from theory.…”

Section: Shear-aided Particle Migrationmentioning

confidence: 99%

Which effective viscosity?

Petford

2009

Mineral. mag.

111

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Magmas undergoing shear are prime examples of flows that involve the transport of solids and gases by a separate (silicate melt) carrier phase. Such flows are called multiphase, and have attracted much attention due to their important range of engineering applications. Where the volume fraction of the dispersed phase (crystals) is large, the influence of particles on the fluid motion becomes significant and must be taken into account in any explanation of the bulk behaviour of the mixture. For congested magma deforming well in excess of the dilute limit (particle concentrations >40% by volume), sudden changes in the effective or relative viscosity can be expected. The picture is complicated further by the fact that the melt phase is temperature-and shear-rate-dependent. In the absence of a constitutive law for the flow of congested magma under an applied force, it is far from clear which of the many hundreds of empirical formulae devised to predict the rheology of suspensions as the particle fraction increases with time are best suited. Some of the more commonly used expressions in geology and engineering are reviewed with an aim to home in on those variables key to an improved understanding of magma rheology. These include a temperature, compositional and shear-rate dependency of viscosity of the melt phase with the shear-rate dependency of the crystal (particle) packing arrangement. Building on previous formulations, a new expression for the effective (relative) viscosity of magma is proposed that gives users the option to define a packing fraction range as a function of shear stress. Comparison is drawn between processes (segregation, clustering, jamming), common in industrial slurries, and structures seen preserved in igneous rocks. An equivalence is made such that congested magma, viewed in purely mechanical terms as a high-temperature slurry, is an inherently nonequilibrium material where flow at large Péclet numbers may result in shear thinning and spontaneous development of layering.KEYWORDS: viscosity, rheology, fluid motion, magma, engineering, shear rate.Introduction WHILE significant progress has been made in quantifying the flow properties of silicate melt in terms of viscosity and density over a wide compositional range (see Shaw, 1965;Giordano et al., 2008 amongst many others), a fundamental treatment that captures simultaneously the rheology and flow behaviour of magma as a fluid-particle suspension remains elusive. This lack of understanding of what may appear at first glance a relatively trivial problem arises from the fact that magmas are complex multiphase flows with time-and rate-dependent properties (Fig. 1). Especially problematic is that factors that govern the flow of magma, including heat transfer, phase transitions, coupled deformation of both solid and fluid phases, seepage phenomena and chemical processes, occur simultaneously and interdependently. Of critical importance is that for multiphase materials, the viscosity is not a singlevalued function nor necessarily isotropic, but is i...

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Granular flow and viscous fluctuations in low Bagnold number granitic magmas

Cited by 29 publications

References 42 publications

The origin of magmatic layering in the High Tatra granite, Central Western Carpathians – implications for the formation of granitoid plutons

The origin of magmatic layering in the High Tatra granite, Central Western Carpathians – implications for the formation of granitoid plutons

Rheology of arc dacite lavas: experimental determination at low strain rates

Which effective viscosity?

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