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The viscosities of granitic melts play a crucial role in controlling the kinetics and dynamics of magma transport including generation, segregation, ascent, differentiation and emplacement. No general theory of liquid viscosity has been successfully applied to date to the description of granitic melt viscosities. The approach in the earth sciences so far has been empirical and experiments designed to measure the viscosity of granitic melts have made considerable progress in improving our picture of the composition-, temperature- and pressure-dependence of granitic melt viscosities. This chapter summarizes the recent progress in parameterizing the viscosity of granitic melts with respect to the most important compositional variables such as alkali/aluminium ratio, silica content, water content, anorthite content. The pressure-dependence of granitic melt viscosities is very slight. The temperature-dependence, in contrast, is a very strong and non-linear function of the reciprocal absolute temperature (i.e. non-Arrhenian). Indirect determinations of the viscosity of granitic melts have also contributed substantial information on the composition-, pressure- and temperature-dependence of melt viscosity. These measurements are particularly useful where direct viscosity determinations are difficult or impossible to perform. The outlook for a more complete picture of granitic melt viscosities in the next years is excellent. The combination of several factors, including conceptual advances in the quantitative understanding of relaxation in silicate melts, experimental advances in the measurement of viscosity of silicate melts, and the identification in the experimental studies to date of the areas of investigation where the effort should be concentrated in the future, give reason to expect a more or less complete picture, fully adequate for the purposes of modelling the dynamics of granitic magmas, in the near future.

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