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 (30β80 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