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The Effects of Shape on Crystal Settling and on the Rheology of Magmas

The effects of the shape of phenocrysts on their settling velocity and on the rheology of magmas are discussed. Spherical crystals fall at a velocity given by Stokes law and need to be present in large concentrations before they have a marked effect on the viscosity of a suspension. Elongate crystals fall at a much smaller velocity, which may be determined from the formulas and graphs given herein. Such elongate crystals can have a dramatic effect on the viscosity of a suspension even at low volume fractions. Suspended crystals and bubbles will not impart a yield stress to the magma unless they form a touching network across the entire suspension. It is suggested, therefore, that most previous inferences of a yield stress in subliquidus magmas are erroneous. Measurements of the subliquidus rheology of magmas need to be accompanied by careful characterization of the microstructure and distribution of the solid phase, on which such rheologies critically depend.