Further, density controls the degree to which magmas can mix as well the extent of compositional change as the result of magmatic differentiation. The density of the magma is determined by dividing the molar volume into the molecular weight of the magma. Viscosity Viscosity is literally defined as resistance to flow. It is important because it controls the extrusion rate of magmas as well as the ultimate shapes of the resultant rock bodies. More effectively viscosity can be defined as If a large shear stress results in slow deformation the liquid is said to have a high viscosity as for example molasses or honey.
Water is an example of a liquid of low viscosity.
As the magnitude of the shear stress increases so does the rate of strain. For many liquids this relationship is linear. These are referred to as Newtonian liquids . Viscosity is commonly given in units of Pa s. What causes this vast range of difference in the viscosities of typical magmas.