Experimental Rheology of Magmatic Emulsions and Foams: Effects of Composition, Temperature, Shear Rate and Bubble Content

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9972922SperaThe rheological properties of bubble-bearing magmas are of interest for fundamental as well as practical reasons. Because bubbles are deformable, bubble shapes change during flow and this gives rise to a shear-rate dependent viscosity for the emulsion (melt + vapor). At low shear rate, bubbles retain a surface tension -dominated spherical shape and the ratio of the emulsion viscosity to the bubble-free melt viscosity increases as the bubble content increases. At high rates of shear just the opposite occurs; the viscosity of the emulsion decreases relative to the melt as the bubble content increases. Because small amounts of volatile components such as H2O and CO2 are ubiquitous in magmas and because these compounds are sparingly soluble, virtually all magmas become volatile-saturated (hence bubbly) at low pressure and upon eruption. Quantification of the rheological properties of magma emulsions and froths as a function of temperature, shear rate, bubble-content and melt composition using a high torque precision concentric cylinder viscometer specifically fabricated for this purpose will provide basic information concerning the rheological properties of bubble-beraing magmas. These rheological data will be summarized by developing constitutive relations that can be used in physical volcanological modelling studies and will fillan important gap in our knowledge regarding the viscometric properties of magma.