Thermal properties of vesicular rhyolite

Thermal diffusivity of rhyolite melt and rhyolite foam (70–80% porosity) has been measured using the radial heat transfer method. Cylindrical samples (length 50–55 mm, diameter 22 mm) of rhyolite melt and foam have been derived by heating samples of Little Glass Mountain obsidian. Using available data on heat capacity and density of rhyolite melt, the thermal conductivity of samples has been determined. The difference in thermal conductivity between rhyolite melt and foam at igneous temperatures ( 1000°C) is about one order of magnitude. The effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

Frequency Dependent Rheology of Vesicular Rhyolite

Frequency dependent rheology of magmas may result from the presence of inclusions (bubbles, crystals) in the melt and/or from viscoelastic behavior of the melt itself. With the addition of deformable inclusions to a melt possessing viscoelastic properties one might expect changes in the relaxation spectrum of the shear stresses of the material (e.g., broadening of the relaxation spectrum) resulting from the viscously deformable geometry of the second phase. We have begun to investigate the effect of bubbles on the frequency dependent rheology of rhyolite melt. The present study deals with the rheology of bubble-free and vesicular rhyolite melts containing spherical voids of 10 and 30 vol %. We used a sinusoidal torsion deformation device. Vesicular rhyolite melts were generated by the melting (at 1 bar) of an Armenian obsidian (Dry Fountain, Erevan, Armenia) and Little Glass Mountain obsidian (California). The real and imaginary parts of shear viscosity and shear modulus have been determined in a frequency range of 0.005–10 Hz and temperature range of 600°–900°C. The relaxed shear viscosities of samples obtained at low frequencies and high temperatures compare well with data previously obtained by parallel plate viscometry. The relaxed shear viscosity of vesicular rhyolites decreases progressively with increasing bubble content. The relaxation spectrum for rhyolite melt without bubbles has an asymmetric form and fits an extended exponent relaxation. The presence of deformable bubbles results in an imaginary component of the shear modulus that becomes more symmetrical and extends into the low-frequency/high-temperature range. The internal friction Q −1 is unaffected in the high-frequency/low-temperature range by the presence of bubbles and depends on the bubble content in the high-temperature/low-frequency range. The present work, in combination with the previous study of Stein and Spera (1992), illustrates that magma viscosity can either increase or decrease with bubble content, depending upon the rate of style of strain during magmatic flow

Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions

This work is a contribution to the Natural Environment Research Council (NERC) funded RiftVolc project (NE/L013932/1, Rift volcanism: past, present and future) through which several of the authors are supported. In addition, Clarke was funded by a NERC doctoral training partnership grant (NE/L002558/1).Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. The dynamics of peralkaline rhyolite eruptions remain elusive with no direct observations recorded, significantly hindering the assessment of hazard and risk. Here we describe uniquely-preserved, fluidal-shaped pyroclasts found within pumice cone deposits at Aluto, a peralkaline rhyolite caldera in the Main Ethiopian Rift. We use a combination of field-observations, geochemistry, X-ray computed microtomography (XCT) and thermal-modelling to investigate how these pyroclasts are formed. We find that they deform during flight and, depending on size, quench prior to deposition or continue to inflate then quench in-situ. These findings reveal important characteristics of the eruptions that gave rise to them: that despite the relatively low viscosity of these magmas, and similarities to basaltic scoria-cone deposits, moderate to intense, unstable, eruption columns are developed; meaning that such eruptions can generate extensive tephra-fall and pyroclastic density currents.Publisher PDFPeer reviewe

Viscoelastic behaviour of basaltic lavas.

The rheological properties of basaltic lavas from Etna, Hawai′i and Vesuvius have been investigated at temperatures between 500 and 1150°C using a small-strain oscillatory shear. The viscoelastic response of the lavas to small, forced, sinusoidal torques (109 Pa s) could be measured at strain rates less than 10−2–10−1 s−1. At 800°C, temporal variations in complex shear modulus and internal friction suggest that, over durations of up to 120 h, structural adjustments were occurring within some of the samples. This time-varying behaviour of lava samples may be attributed to the slow closing (healing) of microcracks and small pore spaces, resulting in the apparent stiffening of lava samples under annealing. Thus, those parts of lava flows that undergo slow cooling will have more elastic properties. Regions that cool faster possess smaller shear moduli and higher internal friction due to thermal microcracking and less cohesion between crystals and the bulk glassy matrix

A review and investigation of the Non-Newtonian properties of lavas based on laboratory experiments with analogue materials.

Realistic lava flow models require a comprehensive understanding of the rheological properties of lava under a range of stress conditions. Previous measurements have shown that at typical eruption temperatures lavas are non-Newtonian. This is commonly attributed to the formation and destruction of crystal networks. In the present study, the effects of bubbles on the time-dependent, non-Newtonian properties of vesicular melts are investigated experimentally using analogue materials. The shear-thinning behaviour of bubbly liquids is shown to be dependent on the previous shearing history. This thixotropic behaviour, which was investigated using a rotational vane-viscometer, is caused by delayed bubble deformation and recovery when subjected to changes in shear stress. The viscoelastic transition and the transient flow behaviour of analogue fluids were studied using both a rotational vane-viscometer and oscillatory shear apparatus. These experiments have shown that vesicular suspensions are viscoelastic fluids with a yield strength, power law rheology, and a non-zero shear modulus. These properties are also found in polymer fluids commonly used as analogue materials for lava such as gum rosin. We show that, when materials with this rheology are accelerated in channels, they may be fragmented, and when they flow through a narrowing conduit, pulsating flow can develop as a consequence of a transition from slip to non-slip conditions at the conduit wall. This has important implications both for effusive and explosive volcanic eruptions

Vesiculation processes in a water-rich calc-alkaline obsidian

The effect of temperature and viscosity on the kinetics of bubble growth was measured for a natural water-rich rhyolite melt. The change in volume of a natural crystal-poor calc-alkaline rhyolitic obsidian with an initial water content of = 1.8 wt% H,O was determined in the temperature interval 520-624°C. Shear viscosity was measured on the natural sample using the micropenetration method in the temperature interval 450-590°C and water contents before and after viscometry were monitored by FTIR. The time-dependence of the volume increase as a result of vesiculation approximates the Avrami equation: AV(t) = 1 - exp[ -(t/t)°]. At temperatures close to the glass transition temperature (Ts), growing bubbles ruptured the surrounding melt. During diffusion of water from the hydrous melt with = 1.8 wt% H,O into a bubble, the deltaTg of the melt at the bubble wall increases by = 315°C. In addition, the viscosity of the melt at the bubble wall increases by 5-6 orders of magnitude. The increasing elastic component of shear stress on the bubble wall as a result of bubble growth at about Tg may exceed the bubble wall yield strength, resulting in cracking. No time lag in the onset of bubble growth occurred for this water-rich rhyolite. In those parts of volcanic edifices where water contents are of several weight percent (e.g., within the upper parts of volcanic conduits), the probability of melt fracturing due to the degassing of water at about Tg increases

A model for the rheology of particle-bearing suspensions and partially molten rocks

An edited version of this paper was published by AGU. Copyright (2009) American Geophysical UnionThis contribution presents a semiempirical model describing the effective relative viscosity of crystalbearing magmas as function of crystal fraction and strain rate. The model was applied to an extensive data set of magmatic suspensions and partially molten rocks providing a range of values for the fitting parameters that control the behavior of the relative viscosity curves as a function of the crystal fraction in an intermediate range of crystallinity (30–80 vol % crystals). The analysis of the results and of the materials used in the experiments allows for evaluating the physical meaning of the parameters of the proposed model. We show that the model, by varying the parameters within the ranges obtained during the fitting procedure, is able to describe satisfactory the effective relative viscosity as a function of crystal fraction and strain rate for suspensions having different geometrical characteristics of the suspended solid fraction.PublishedQ030102.3. TTC - Laboratori di chimica e fisica delle rocce3.6. Fisica del vulcanismoJCR Journalreserve

A model for the rheology of particle-bearing suspensions and partially molten rocks

This contribution presents a semiempirical model describing the effective relative viscosity of crystalbearing magmas as function of crystal fraction and strain rate. The model was applied to an extensive data set of magmatic suspensions and partially molten rocks providing a range of values for the fitting parameters that control the behavior of the relative viscosity curves as a function of the crystal fraction in an intermediate range of crystallinity (30–80 vol % crystals). The analysis of the results and of the materials used in the experiments allows for evaluating the physical meaning of the parameters of the proposed model. We show that the model, by varying the parameters within the ranges obtained during the fitting procedure, is able to describe satisfactory the effective relative viscosity as a function of crystal fraction and strain rate for suspensions having different geometrical characteristics of the suspended solid fraction

The influence of trace amount of water on the viscosity of rhyolites

As a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas. This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650 degrees C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300 degrees C, 3 kbar) obsidians. Ten natural rhyolitic obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained <0.2 wt.% H2O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts. In addition, a marked nonlinear decrease in eta exists with increasing water content. In contrast to the case for peralkaline rhyolites, 0.1-0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity, largely based on synthetic systems