Silica Content and Viscosity

The most important factors controlling the viscosity of silicate melts are melt composition and temperature. Often, the compositional variable controlling viscosity is equated to SiO2 content. Although important, the SiO2 factor does not completely describe the compositional control on viscosity. The photograph shows a trachyte, the Highway Flow, which contains approximately 60% SiO2 (Stout, et al., 1994). The form of the flow suggests its viscosity is less than the viscosity of the melts that formed the rhyolite domes. The rhyolite obsidians that form the domes in the Snake River Plain contain 68% to 75% SiO2. The trachytes carry phenocrysts of flourapatite, suggesting that fluorine may contribute to the lowering of the viscosity of the trachyte melts.