Magma composition ?

Major elements in igneous rocks = O, Si, Al, Fe, Ca, Mg, Na, K, and S. Composition plus temperature, pressure, and H2O content characterize magma and determine what minerals form. Compositions range from silicic (or felsic or sialic) to intermediate, mafic and ultramafic. Major differences in composition for common igneous rocks = silica (SiO2) content (felsic = highest, ultramafic = lowest), proportions of Al, K, and Na (felsic = highest, ultramafic = lowest), and proportions of Ca, Fe, and Mg (felsic = lowest, ultramafic = highest). Alkalic rocks = rich in alkali elements (Na and K). - Other important differences = temperature of crystallization (felsic forms at lowest temperatures, ~600 - 800°C; ultramafic forms at highest temperatures, >1,200°C) and viscosity or resistance to flow (felsic is most viscous, i.e., resists flowing, and ultramafic is least viscous, i.e., flows easily). What are 2 reasons for difference in viscosity?